Squashed 'external/entt/entt/' content from commit fef92113

git-subtree-dir: external/entt/entt
git-subtree-split: fef921132cae7588213d0f9bcd2fb9c8ffd8b7fc

.clang-format

@@ -0,0 +1,41 @@
+BasedOnStyle: llvm
+---
+AccessModifierOffset: -4
+AlignEscapedNewlines: DontAlign
+AllowShortBlocksOnASingleLine: Empty
+AllowShortEnumsOnASingleLine: true
+AllowShortFunctionsOnASingleLine: Empty
+AllowShortIfStatementsOnASingleLine: WithoutElse
+AllowShortLoopsOnASingleLine: true
+AlwaysBreakTemplateDeclarations: Yes
+BreakBeforeBinaryOperators: NonAssignment
+BreakBeforeTernaryOperators: true
+ColumnLimit: 0
+DerivePointerAlignment: false
+IncludeCategories:
+  - Regex: '<[[:alnum:]_]+>'
+    Priority: 1
+  - Regex: '<(gtest|gmock)/'
+    Priority: 2
+  - Regex: '<[[:alnum:]_./]+>'
+    Priority: 3
+  - Regex: '<entt/'
+    Priority: 4
+  - Regex: '.*'
+    Priority: 5
+IndentPPDirectives: AfterHash
+IndentWidth: 4
+KeepEmptyLinesAtTheStartOfBlocks: false
+Language: Cpp
+PointerAlignment: Right
+SpaceAfterCStyleCast: false
+SpaceAfterTemplateKeyword: false
+SpaceAroundPointerQualifiers: After
+SpaceBeforeCaseColon: false
+SpaceBeforeCtorInitializerColon: false
+SpaceBeforeInheritanceColon: false
+SpaceBeforeParens: Never
+SpaceBeforeRangeBasedForLoopColon: false
+Standard: Latest
+TabWidth: 4
+UseTab: Never

.github/FUNDING.yml

@@ -0,0 +1,4 @@
+# These are supported funding model platforms
+
+github: skypjack
+custom: https://www.paypal.me/skypjack

.github/workflows/analyzer.yml

@@ -0,0 +1,55 @@
+name: analyzer
+
+on:
+  push:
+    branches:
+      - master
+      - wip
+
+jobs:
+
+  iwyu:
+    timeout-minutes: 30
+
+    env:
+      IWYU: 0.18
+      LLVM: 14
+
+    runs-on: ubuntu-latest
+    continue-on-error: true
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install llvm/clang
+      # see: https://apt.llvm.org/
+      run: |
+        wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+        sudo add-apt-repository "deb http://apt.llvm.org/focal/ llvm-toolchain-focal-$LLVM main"
+        sudo apt update
+        sudo apt remove -y "llvm*"
+        sudo apt remove -y "libclang-dev*"
+        sudo apt remove -y "clang*"
+        sudo apt install -y llvm-$LLVM-dev
+        sudo apt install -y libclang-$LLVM-dev
+        sudo apt install -y clang-$LLVM
+    - name: Compile iwyu
+      # see: https://github.com/include-what-you-use/include-what-you-use
+      working-directory: build
+      run: |
+        git clone https://github.com/include-what-you-use/include-what-you-use.git --branch $IWYU --depth 1
+        mkdir include-what-you-use/build
+        cd include-what-you-use/build
+        cmake -DCMAKE_C_COMPILER=clang-$LLVM -DCMAKE_CXX_COMPILER=clang++-$LLVM -DCMAKE_INSTALL_PREFIX=./ ..
+        make -j4
+        bin/include-what-you-use --version
+    - name: Compile tests
+      working-directory: build
+      run: |
+        export PATH=$PATH:${GITHUB_WORKSPACE}/build/include-what-you-use/build/bin
+        cmake -DENTT_BUILD_TESTING=ON \
+              -DENTT_BUILD_BENCHMARK=ON \
+              -DENTT_BUILD_EXAMPLE=ON \
+              -DENTT_BUILD_LIB=ON \
+              -DENTT_BUILD_SNAPSHOT=ON \
+              -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE="include-what-you-use;-Xiwyu;--mapping_file=${GITHUB_WORKSPACE}/entt.imp;-Xiwyu;--no_fwd_decls;-Xiwyu;--verbose=1" ..
+        make -j4

.github/workflows/build.yml

@@ -0,0 +1,169 @@
+name: build
+
+on: [push, pull_request]
+
+jobs:
+
+  linux:
+    timeout-minutes: 15
+
+    strategy:
+      matrix:
+        compiler:
+          - pkg: g++-7
+            exe: g++-7
+          - pkg: g++-8
+            exe: g++-8
+          - pkg: g++-9
+            exe: g++-9
+          - pkg: g++-10
+            exe: g++-10
+          - pkg: clang-8
+            exe: clang++-8
+          - pkg: clang-9
+            exe: clang++-9
+          - pkg: clang-10
+            exe: clang++-10
+          - pkg: clang-11
+            exe: clang++-11
+          - pkg: clang-12
+            exe: clang++-12
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install compiler
+      run: |
+        sudo apt update
+        sudo apt install -y ${{ matrix.compiler.pkg }}
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXX: ${{ matrix.compiler.exe }}
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4
+
+  linux-extra:
+    timeout-minutes: 15
+
+    strategy:
+      matrix:
+        compiler: [g++, clang++]
+        id_type: ["std::uint32_t", "std::uint64_t"]
+        cxx_std: [cxx_std_17, cxx_std_20]
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXX: ${{ matrix.compiler }}
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_CXX_STD=${{ matrix.cxx_std }} -DENTT_ID_TYPE=${{ matrix.id_type }} ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4
+
+  windows:
+    timeout-minutes: 15
+
+    strategy:
+      matrix:
+        toolset: [default, v141, v142, clang-cl]
+        include:
+          - toolset: v141
+            toolset_option: -T"v141"
+          - toolset: v142
+            toolset_option: -T"v142"
+          - toolset: clang-cl
+            toolset_option: -T"ClangCl"
+
+    runs-on: windows-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ${{ matrix.toolset_option }} ..
+        cmake --build . -j 4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4
+
+  windows-extra:
+    timeout-minutes: 15
+
+    strategy:
+      matrix:
+        id_type: ["std::uint32_t", "std::uint64_t"]
+        cxx_std: [cxx_std_17, cxx_std_20]
+
+    runs-on: windows-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_CXX_STD=${{ matrix.cxx_std }} -DENTT_ID_TYPE=${{ matrix.id_type }} ..
+        cmake --build . -j 4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4
+
+  macos:
+    timeout-minutes: 15
+    runs-on: macOS-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4
+
+  macos-extra:
+    timeout-minutes: 15
+
+    strategy:
+      matrix:
+        id_type: ["std::uint32_t", "std::uint64_t"]
+        cxx_std: [cxx_std_17, cxx_std_20]
+
+    runs-on: macOS-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_CXX_STD=${{ matrix.cxx_std }} -DENTT_ID_TYPE=${{ matrix.id_type }} ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4

.github/workflows/coverage.yml

@@ -0,0 +1,38 @@
+name: coverage
+
+on: [push, pull_request]
+
+jobs:
+
+  codecov:
+    timeout-minutes: 15
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXXFLAGS: "--coverage -fno-inline"
+        CXX: g++
+      run: |
+        cmake -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4
+    - name: Collect data
+      working-directory: build
+      run: |
+        sudo apt install lcov
+        lcov -c -d . -o coverage.info
+        lcov -l coverage.info
+    - name: Upload coverage to Codecov
+      uses: codecov/codecov-action@v2
+      with:
+        token: ${{ secrets.CODECOV_TOKEN }}
+        files: build/coverage.info
+        name: EnTT
+        fail_ci_if_error: true

.github/workflows/deploy.yml

@@ -0,0 +1,39 @@
+name: deploy
+
+on:
+  release:
+    types: published
+
+jobs:
+
+  homebrew-entt:
+    timeout-minutes: 5
+    runs-on: ubuntu-latest
+
+    env:
+      GH_REPO: homebrew-entt
+      FORMULA: entt.rb
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Clone repository
+      working-directory: build
+      env:
+        PERSONAL_ACCESS_TOKEN: ${{ secrets.PERSONAL_ACCESS_TOKEN }}
+      run: git clone https://$GITHUB_ACTOR:$PERSONAL_ACCESS_TOKEN@github.com/$GITHUB_ACTOR/$GH_REPO.git
+    - name: Prepare formula
+      working-directory: build
+      run: |
+        cd $GH_REPO
+        curl "https://github.com/${{ github.repository }}/archive/${{ github.ref }}.tar.gz" --location --fail --silent --show-error --output archive.tar.gz
+        sed -i -e '/url/s/".*"/"'$(echo "https://github.com/${{ github.repository }}/archive/${{ github.ref }}.tar.gz" | sed -e 's/[\/&]/\\&/g')'"/' $FORMULA
+        sed -i -e '/sha256/s/".*"/"'$(openssl sha256 archive.tar.gz | cut -d " " -f 2)'"/' $FORMULA
+    - name: Update remote
+      working-directory: build
+      run: |
+        cd $GH_REPO
+        git config --local user.email "action@github.com"
+        git config --local user.name "$GITHUB_ACTOR"
+        git add $FORMULA
+        git commit -m "Update to ${{ github.ref }}"
+        git push origin master

.github/workflows/sanitizer.yml

@@ -0,0 +1,31 @@
+name: sanitizer
+
+on: [push, pull_request]
+
+jobs:
+
+  clang:
+    timeout-minutes: 15
+
+    strategy:
+      matrix:
+        compiler: [clang++]
+        id_type: ["std::uint32_t", "std::uint64_t"]
+        cxx_std: [cxx_std_17, cxx_std_20]
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Compile tests
+      working-directory: build
+      env:
+        CXX: ${{ matrix.compiler }}
+      run: |
+        cmake -DENTT_USE_SANITIZER=ON -DENTT_BUILD_TESTING=ON -DENTT_BUILD_LIB=ON -DENTT_BUILD_EXAMPLE=ON -DENTT_CXX_STD=${{ matrix.cxx_std }} -DENTT_ID_TYPE=${{ matrix.id_type }} ..
+        make -j4
+    - name: Run tests
+      working-directory: build
+      env:
+        CTEST_OUTPUT_ON_FAILURE: 1
+      run: ctest --timeout 30 -C Debug -j4

.gitignore

@@ -0,0 +1,13 @@
+# Conan
+conan/test_package/build
+
+# IDEs
+*.user
+.idea
+.vscode
+.vs
+CMakeSettings.json
+cpp.hint
+
+# Bazel
+/bazel-*

AUTHORS

@@ -0,0 +1,56 @@
+# Author
+
+skypjack
+
+# Contributors
+
+alexames
+BenediktConze
+bjadamson
+ceeac
+ColinH
+corystegel
+Croydon
+cschreib
+cugone
+dbacchet
+dBagrat
+djarek
+DNKpp
+DonKult
+drglove
+eliasdaler
+erez-o
+eugeneko
+gale83
+ghost
+grdowns
+Green-Sky
+Innokentiy-Alaytsev
+Kerndog73
+Koward
+Lawrencemm
+markand
+mhammerc
+Milerius
+Minimonium
+morbo84
+m-waka
+netpoetica
+NixAJ
+Oortonaut
+Paolo-Oliverio
+pgruenbacher
+prowolf
+Qix-
+stefanofiorentino
+suVrik
+szunhammer
+The5-1
+vblanco20-1
+willtunnels
+WizardIke
+WoLfulus 
+w1th0utnam3
+xissburg
+zaucy

BUILD.bazel

@@ -0,0 +1,14 @@
+_msvc_copts = ["/std:c++17"]	
+_gcc_copts = ["-std=c++17"]
+
+cc_library(
+    name = "entt",
+    visibility = ["//visibility:public"],
+    strip_include_prefix = "src",
+    hdrs = glob(["src/**/*.h", "src/**/*.hpp"]),
+    copts = select({
+      "@bazel_tools//src/conditions:windows": _msvc_copts,
+      "@bazel_tools//src/conditions:windows_msvc": _msvc_copts,
+      "//conditions:default": _gcc_copts,
+    }),
+)

CMakeLists.txt

@@ -0,0 +1,325 @@
+#
+# EnTT
+#
+
+cmake_minimum_required(VERSION 3.12.4)
+
+#
+# Read project version
+#
+
+set(ENTT_VERSION_REGEX "#define ENTT_VERSION_.*[ \t]+(.+)")
+file(STRINGS "${CMAKE_CURRENT_SOURCE_DIR}/src/entt/config/version.h" ENTT_VERSION REGEX ${ENTT_VERSION_REGEX})
+list(TRANSFORM ENTT_VERSION REPLACE ${ENTT_VERSION_REGEX} "\\1")
+string(JOIN "." ENTT_VERSION ${ENTT_VERSION})
+
+#
+# Project configuration
+#
+
+project(
+    EnTT
+    VERSION ${ENTT_VERSION}
+    DESCRIPTION "Gaming meets modern C++ - a fast and reliable entity-component system (ECS) and much more"
+    HOMEPAGE_URL "https://github.com/skypjack/entt"
+    LANGUAGES C CXX
+)
+
+if(NOT CMAKE_BUILD_TYPE)
+    set(CMAKE_BUILD_TYPE Debug)
+endif()
+
+message(VERBOSE "*")
+message(VERBOSE "* ${PROJECT_NAME} v${PROJECT_VERSION} (${CMAKE_BUILD_TYPE})")
+message(VERBOSE "* Copyright (c) 2017-2022 Michele Caini <michele.caini@gmail.com>")
+message(VERBOSE "*")
+
+#
+# CMake stuff
+#
+
+list(INSERT CMAKE_MODULE_PATH 0 ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules)
+
+#
+# Compiler stuff
+#
+
+option(ENTT_USE_LIBCPP "Use libc++ by adding -stdlib=libc++ flag if available." OFF)
+option(ENTT_USE_SANITIZER "Enable sanitizers by adding -fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined flags if available." OFF)
+
+if(ENTT_USE_LIBCPP)
+    if(NOT WIN32)
+        include(CheckCXXSourceCompiles)
+        include(CMakePushCheckState)
+
+        cmake_push_check_state()
+
+        set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -stdlib=libc++")
+
+        check_cxx_source_compiles("
+            #include<type_traits>
+            int main() { return std::is_same_v<int, char>; }
+        " ENTT_HAS_LIBCPP)
+
+        cmake_pop_check_state()
+    endif()
+
+    if(NOT ENTT_HAS_LIBCPP)
+        message(VERBOSE "The option ENTT_USE_LIBCPP is set but libc++ is not available. The flag will not be added to the target.")
+    endif()
+endif()
+
+if(ENTT_USE_SANITIZER)
+    if(CMAKE_CXX_COMPILER_ID MATCHES "Clang|GNU")
+        set(ENTT_HAS_SANITIZER TRUE CACHE BOOL "" FORCE)
+        mark_as_advanced(ENTT_HAS_SANITIZER)
+    endif()
+
+    if(NOT ENTT_HAS_SANITIZER)
+        message(VERBOSE "The option ENTT_USE_SANITIZER is set but sanitizer support is not available. The flags will not be added to the target.")
+    endif()
+endif()
+
+#
+# Add EnTT target
+#
+
+option(ENTT_INCLUDE_HEADERS "Add all EnTT headers to the EnTT target." OFF)
+option(ENTT_INCLUDE_NATVIS "Add EnTT natvis files to the EnTT target." OFF)
+
+if(ENTT_INCLUDE_NATVIS)
+    if(MSVC)
+        set(ENTT_HAS_NATVIS TRUE CACHE BOOL "" FORCE)
+        mark_as_advanced(ENTT_HAS_NATVIS)
+    endif()
+
+    if(NOT ENTT_HAS_NATVIS)
+        message(VERBOSE "The option ENTT_INCLUDE_NATVIS is set but natvis files are not supported. They will not be added to the target.")
+    endif()
+endif()
+
+include(GNUInstallDirs)
+
+add_library(EnTT INTERFACE)
+add_library(EnTT::EnTT ALIAS EnTT)
+
+target_include_directories(
+    EnTT
+    INTERFACE
+        $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src>
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+)
+
+target_compile_features(EnTT INTERFACE cxx_std_17)
+
+if(ENTT_INCLUDE_HEADERS)
+    target_sources(
+        EnTT
+        INTERFACE
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/config/config.h>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/config/macro.h>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/config/version.h>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/container/dense_map.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/container/dense_set.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/container/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/algorithm.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/any.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/attribute.h>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/compressed_pair.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/enum.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/family.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/hashed_string.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/ident.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/iterator.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/memory.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/monostate.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/tuple.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/type_info.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/type_traits.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/core/utility.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/component.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/entity.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/group.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/handle.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/helper.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/observer.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/organizer.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/registry.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/runtime_view.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/snapshot.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/sparse_set.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/storage.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/storage_mixin.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entity/view.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/adjacency_matrix.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/dot.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/flow.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/graph/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/locator/locator.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/adl_pointer.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/container.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/context.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/factory.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/meta.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/node.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/pointer.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/policy.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/range.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/resolve.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/template.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/type_traits.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/meta/utility.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/platform/android-ndk-r17.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/poly/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/poly/poly.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/process/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/process/process.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/process/scheduler.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/cache.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/loader.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/resource/resource.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/delegate.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/dispatcher.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/emitter.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/fwd.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/signal/sigh.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/entt.hpp>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/src/entt/fwd.hpp>
+    )
+endif()
+
+if(ENTT_HAS_NATVIS)
+    target_sources(
+        EnTT
+        INTERFACE
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/config.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/container.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/core.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/entity.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/graph.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/locator.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/meta.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/platform.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/poly.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/process.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/resource.natvis>
+            $<BUILD_INTERFACE:${EnTT_SOURCE_DIR}/natvis/entt/signal.natvis>
+    )
+endif()
+
+if(ENTT_HAS_SANITIZER)
+    target_compile_options(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined>)
+    target_link_libraries(EnTT INTERFACE $<$<CONFIG:Debug>:-fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined>)
+endif()
+
+if(ENTT_HAS_LIBCPP)
+    target_compile_options(EnTT BEFORE INTERFACE -stdlib=libc++)
+endif()
+
+#
+# Install pkg-config file
+#
+
+include(JoinPaths)
+
+set(EnTT_PKGCONFIG ${CMAKE_CURRENT_BINARY_DIR}/entt.pc)
+
+join_paths(EnTT_PKGCONFIG_INCLUDEDIR "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
+
+configure_file(
+    ${EnTT_SOURCE_DIR}/cmake/in/entt.pc.in
+    ${EnTT_PKGCONFIG}
+    @ONLY
+)
+
+install(
+    FILES ${EnTT_PKGCONFIG}
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+)
+
+#
+# Install EnTT
+#
+
+include(CMakePackageConfigHelpers)
+
+install(
+    TARGETS EnTT
+    EXPORT EnTTTargets
+    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+)
+
+write_basic_package_version_file(
+    EnTTConfigVersion.cmake
+    VERSION ${PROJECT_VERSION}
+    COMPATIBILITY AnyNewerVersion
+)
+
+configure_package_config_file(
+    ${EnTT_SOURCE_DIR}/cmake/in/EnTTConfig.cmake.in
+    EnTTConfig.cmake
+    INSTALL_DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
+)
+
+export(
+    EXPORT EnTTTargets
+    FILE ${CMAKE_CURRENT_BINARY_DIR}/EnTTTargets.cmake
+    NAMESPACE EnTT::
+)
+
+install(
+    EXPORT EnTTTargets
+    FILE EnTTTargets.cmake
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
+    NAMESPACE EnTT::
+)
+
+install(
+    FILES
+        ${PROJECT_BINARY_DIR}/EnTTConfig.cmake
+        ${PROJECT_BINARY_DIR}/EnTTConfigVersion.cmake
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/EnTT/cmake
+)
+
+install(DIRECTORY src/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+
+export(PACKAGE EnTT)
+
+#
+# Tests
+#
+
+option(ENTT_BUILD_TESTING "Enable building tests." OFF)
+
+if(ENTT_BUILD_TESTING)
+    option(ENTT_FIND_GTEST_PACKAGE "Enable finding gtest package." OFF)
+    option(ENTT_BUILD_BENCHMARK "Build benchmark." OFF)
+    option(ENTT_BUILD_EXAMPLE "Build examples." OFF)
+    option(ENTT_BUILD_LIB "Build lib tests." OFF)
+    option(ENTT_BUILD_SNAPSHOT "Build snapshot test with Cereal." OFF)
+
+    set(ENTT_ID_TYPE std::uint32_t CACHE STRING "Type of identifiers to use for the tests")
+    set(ENTT_CXX_STD cxx_std_17 CACHE STRING "C++ standard revision to use for the tests")
+
+    include(CTest)
+    enable_testing()
+    add_subdirectory(test)
+endif()
+
+#
+# Documentation
+#
+
+option(ENTT_BUILD_DOCS "Enable building with documentation." OFF)
+
+if(ENTT_BUILD_DOCS)
+    find_package(Doxygen 1.8)
+
+    if(DOXYGEN_FOUND)
+        add_subdirectory(docs)
+    endif()
+endif()

CONTRIBUTING.md

@@ -0,0 +1,43 @@
+# Contributing
+
+First of all, thank you very much for taking the time to contribute to the
+`EnTT` library.<br/>
+How to do it mostly depends on the type of contribution:
+
+* If you have a question, **please** ensure there isn't already an answer for
+  you by searching on GitHub under
+  [issues](https://github.com/skypjack/entt/issues). Do not forget to search
+  also through the closed ones. If you are unable to find a proper answer, feel
+  free to [open a new issue](https://github.com/skypjack/entt/issues/new).
+  Usually, questions are marked as such and closed in a few days.
+
+* If you want to fix a typo in the inline documentation or in the README file,
+  if you want to add some new sections or if you want to help me with the
+  language by reviewing what I wrote so far (I'm not a native speaker after
+  all), **please** open a new
+  [pull request](https://github.com/skypjack/entt/pulls) with your changes.
+
+* If you found a bug, **please** ensure there isn't already an answer for you by
+  searching on GitHub under [issues](https://github.com/skypjack/entt/issues).
+  If you are unable to find an open issue addressing the problem, feel free to
+  [open a new one](https://github.com/skypjack/entt/issues/new). **Please**, do
+  not forget to carefully describe how to reproduce the problem, then add all
+  the information about the system on which you are experiencing it and point
+  out the version of `EnTT` you are using (tag or commit).
+
+* If you found a bug and you wrote a patch to fix it, open a new
+  [pull request](https://github.com/skypjack/entt/pulls) with your code.
+  **Please**, add some tests to avoid regressions in future if possible, it
+  would be really appreciated. Note that the `EnTT` library has a
+  [coverage at 100%](https://coveralls.io/github/skypjack/entt?branch=master)
+  (at least it was at 100% at the time I wrote this file) and this is the reason
+  for which you can be confident with using it in a production environment.
+
+* If you want to propose a new feature and you know how to code it, **please**
+  do not issue directly a pull request. Before to do it,
+  [create a new issue](https://github.com/skypjack/entt/issues/new) to discuss
+  your proposal. Other users could be interested in your idea and the discussion
+  that will follow can refine it and therefore give us a better solution.
+
+* If you want to request a new feature, I'm available for hiring. Take a look at
+  [my profile](https://github.com/skypjack) and feel free to write me.

LICENSE

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2017-2022 Michele Caini, author of EnTT
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copy of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copy or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

@@ -0,0 +1,431 @@
+![EnTT: Gaming meets modern C++](https://user-images.githubusercontent.com/1812216/103550016-90752280-4ea8-11eb-8667-12ed2219e137.png)
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+[![Build Status](https://github.com/skypjack/entt/workflows/build/badge.svg)](https://github.com/skypjack/entt/actions)
+[![Coverage](https://codecov.io/gh/skypjack/entt/branch/master/graph/badge.svg)](https://codecov.io/gh/skypjack/entt)
+[![Try online](https://img.shields.io/badge/try-online-brightgreen)](https://godbolt.org/z/zxW73f)
+[![Documentation](https://img.shields.io/badge/docs-docsforge-blue)](http://entt.docsforge.com/)
+[![Gitter chat](https://badges.gitter.im/skypjack/entt.png)](https://gitter.im/skypjack/entt)
+[![Discord channel](https://img.shields.io/discord/707607951396962417?logo=discord)](https://discord.gg/5BjPWBd)
+[![Donate](https://img.shields.io/badge/donate-paypal-blue.svg)](https://www.paypal.me/skypjack)
+
+> `EnTT` has been a dream so far, we haven't found a single bug to date and it's
+> super easy to work with
+>
+> -- Every EnTT User Ever
+
+`EnTT` is a header-only, tiny and easy to use library for game programming and
+much more written in **modern C++**.<br/>
+[Among others](https://github.com/skypjack/entt/wiki/EnTT-in-Action), it's used
+in [**Minecraft**](https://minecraft.net/en-us/attribution/) by Mojang, the
+[**ArcGIS Runtime SDKs**](https://developers.arcgis.com/arcgis-runtime/) by Esri
+and the amazing [**Ragdoll**](https://ragdolldynamics.com/).<br/>
+If you don't see your project in the list, please open an issue, submit a PR or
+add the [#entt](https://github.com/topics/entt) tag to your _topics_! :+1:
+
+---
+
+Do you want to **keep up with changes** or do you have a **question** that
+doesn't require you to open an issue?<br/>
+Join the [gitter channel](https://gitter.im/skypjack/entt) and the
+[discord server](https://discord.gg/5BjPWBd), meet other users like you. The
+more we are, the better for everyone.<br/>
+Don't forget to check the
+[FAQs](https://github.com/skypjack/entt/wiki/Frequently-Asked-Questions) and the
+[wiki](https://github.com/skypjack/entt/wiki) too. Your answers may already be
+there.
+
+Do you want to support `EnTT`? Consider becoming a
+[**sponsor**](https://github.com/users/skypjack/sponsorship).
+Many thanks to [these people](https://skypjack.github.io/sponsorship/) and
+**special** thanks to:
+
+[![mojang](https://user-images.githubusercontent.com/1812216/106253145-67ca1980-6217-11eb-9c0b-d93561b37098.png)](https://mojang.com)
+[![imgly](https://user-images.githubusercontent.com/1812216/106253726-271ed000-6218-11eb-98e0-c9c681925770.png)](https://img.ly/)
+
+# Table of Contents
+
+* [Introduction](#introduction)
+  * [Code Example](#code-example)
+  * [Motivation](#motivation)
+  * [Performance](#performance)
+* [Integration](#integration)
+  * [Requirements](#requirements)
+  * [CMake](#cmake)
+  * [Natvis support](#natvis-support)
+  * [Packaging Tools](#packaging-tools)
+  * [pkg-config](#pkg-config)
+* [Documentation](#documentation)
+* [Tests](#tests)
+* [EnTT in Action](#entt-in-action)
+* [Contributors](#contributors)
+* [License](#license)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+The entity-component-system (also known as _ECS_) is an architectural pattern
+used mostly in game development. For further details:
+
+* [Entity Systems Wiki](http://entity-systems.wikidot.com/)
+* [Evolve Your Hierarchy](http://cowboyprogramming.com/2007/01/05/evolve-your-heirachy/)
+* [ECS on Wikipedia](https://en.wikipedia.org/wiki/Entity%E2%80%93component%E2%80%93system)
+
+This project started off as a pure entity-component system. Over time the
+codebase has grown as more and more classes and functionalities were added.<br/>
+Here is a brief, yet incomplete list of what it offers today:
+
+* Built-in **RTTI system** mostly similar to the standard one.
+* A `constexpr` utility for human-readable **resource names**.
+* Minimal **configuration system** built using the monostate pattern.
+* Incredibly fast **entity-component system** with its own _pay for what you
+  use_ policy, unconstrained component types with optional pointer stability and
+  hooks for storage customization.
+* Views and groups to iterate entities and components and allow different access
+  patterns, from **perfect SoA** to fully random.
+* A lot of **facilities** built on top of the entity-component system to help
+  the users and avoid reinventing the wheel.
+* General purpose **execution graph builder** for optimal scheduling.
+* The smallest and most basic implementation of a **service locator** ever seen.
+* A built-in, non-intrusive and macro-free runtime **reflection system**.
+* **Static polymorphism** made simple and within everyone's reach.
+* A few homemade containers, like a sparse set based **hash map**.
+* A **cooperative scheduler** for processes of any type.
+* All that is needed for **resource management** (cache, loaders, handles).
+* Delegates, **signal handlers** and a tiny event dispatcher.
+* A general purpose **event emitter** as a CRTP idiom based class template.
+* And **much more**! Check out the
+  [**wiki**](https://github.com/skypjack/entt/wiki).
+
+Consider this list a work in progress as well as the project. The whole API is
+fully documented in-code for those who are brave enough to read it.<br/>
+Please, do note that all tools are also DLL-friendly now and run smoothly across
+boundaries.
+
+One thing known to most is that `EnTT` is also used in **Minecraft**.<br/>
+Given that the game is available literally everywhere, I can confidently say 
+that the library has been sufficiently tested on every platform that can come to 
+mind.
+
+## Code Example
+
+```cpp
+#include <entt/entt.hpp>
+
+struct position {
+    float x;
+    float y;
+};
+
+struct velocity {
+    float dx;
+    float dy;
+};
+
+void update(entt::registry &registry) {
+    auto view = registry.view<const position, velocity>();
+
+    // use a callback
+    view.each([](const auto &pos, auto &vel) { /* ... */ });
+
+    // use an extended callback
+    view.each([](const auto entity, const auto &pos, auto &vel) { /* ... */ });
+
+    // use a range-for
+    for(auto [entity, pos, vel]: view.each()) {
+        // ...
+    }
+
+    // use forward iterators and get only the components of interest
+    for(auto entity: view) {
+        auto &vel = view.get<velocity>(entity);
+        // ...
+    }
+}
+
+int main() {
+    entt::registry registry;
+
+    for(auto i = 0u; i < 10u; ++i) {
+        const auto entity = registry.create();
+        registry.emplace<position>(entity, i * 1.f, i * 1.f);
+        if(i % 2 == 0) { registry.emplace<velocity>(entity, i * .1f, i * .1f); }
+    }
+
+    update(registry);
+}
+```
+
+## Motivation
+
+I started developing `EnTT` for the _wrong_ reason: my goal was to design an
+entity-component system to beat another well known open source library both in
+terms of performance and possibly memory usage.<br/>
+In the end, I did it, but it wasn't very satisfying. Actually it wasn't
+satisfying at all. The fastest and nothing more, fairly little indeed. When I
+realized it, I tried hard to keep intact the great performance of `EnTT` and to
+add all the features I wanted to see in *my own library* at the same time.
+
+Nowadays, `EnTT` is finally what I was looking for: still faster than its
+_competitors_, lower memory usage in the average case, a really good API and an
+amazing set of features. And even more, of course.
+
+## Performance
+
+The proposed entity-component system is incredibly fast to iterate entities and
+components, this is a fact. Some compilers make a lot of optimizations because
+of how `EnTT` works, some others aren't that good. In general, if we consider
+real world cases, `EnTT` is somewhere between a bit and much faster than many of
+the other solutions around, although I couldn't check them all for obvious
+reasons.
+
+If you are interested, you can compile the `benchmark` test in release mode (to
+enable compiler optimizations, otherwise it would make little sense) by setting
+the `ENTT_BUILD_BENCHMARK` option of `CMake` to `ON`, then evaluate yourself
+whether you're satisfied with the results or not.
+
+Honestly I got tired of updating the README file whenever there is an
+improvement.<br/>
+There are already a lot of projects out there that use `EnTT` as a basis for
+comparison (this should already tell you a lot). Many of these benchmarks are
+completely wrong, many others are simply incomplete, good at omitting some
+information and using the wrong function to compare a given feature. Certainly
+there are also good ones but they age quickly if nobody updates them, especially
+when the library they are dealing with is actively developed.
+
+The choice to use `EnTT` should be based on its carefully designed API, its
+set of features and the general performance, **not** because some single
+benchmark shows it to be the fastest tool available.
+
+In the future I'll likely try to get even better performance while still adding
+new features, mainly for fun.<br/>
+If you want to contribute and/or have suggestions, feel free to make a PR or
+open an issue to discuss your idea.
+
+# Integration
+
+`EnTT` is a header-only library. This means that including the `entt.hpp` header
+is enough to include the library as a whole and use it. For those who are
+interested only in the entity-component system, consider to include the sole
+`entity/registry.hpp` header instead.<br/>
+It's a matter of adding the following line to the top of a file:
+
+```cpp
+#include <entt/entt.hpp>
+```
+
+Use the line below to include only the entity-component system instead:
+
+```cpp
+#include <entt/entity/registry.hpp>
+```
+
+Then pass the proper `-I` argument to the compiler to add the `src` directory to
+the include paths.
+
+## Requirements
+
+To be able to use `EnTT`, users must provide a full-featured compiler that
+supports at least C++17.<br/>
+The requirements below are mandatory to compile the tests and to extract the
+documentation:
+
+* `CMake` version 3.7 or later.
+* `Doxygen` version 1.8 or later.
+
+Alternatively, [Bazel](https://bazel.build) is also supported as a build system
+(credits to [zaucy](https://github.com/zaucy) who offered to maintain it).<br/>
+In the documentation below I'll still refer to `CMake`, this being the official
+build system of the library.
+
+## CMake
+
+To use `EnTT` from a `CMake` project, just link an existing target to the
+`EnTT::EnTT` alias.<br/>
+The library offers everything you need for locating (as in `find_package`),
+embedding (as in `add_subdirectory`), fetching (as in `FetchContent`) or using
+it in many of the ways that you can think of and that involve `CMake`.<br/>
+Covering all possible cases would require a treaty and not a simple README file,
+but I'm confident that anyone reading this section also knows what it's about
+and can use `EnTT` from a `CMake` project without problems.
+
+## Natvis support
+
+When using `CMake`, just enable the option `ENTT_INCLUDE_NATVIS` and enjoy
+it.<br/>
+Otherwise, most of the tools are covered via Natvis and all files can be found
+in the `natvis` directory, divided by module.<br/>
+If you spot errors or have suggestions, any contribution is welcome!
+
+## Packaging Tools
+
+`EnTT` is available for some of the most known packaging tools. In particular:
+
+* [`Conan`](https://github.com/conan-io/conan-center-index), the C/C++ Package
+  Manager for Developers.
+
+* [`vcpkg`](https://github.com/Microsoft/vcpkg), Microsoft VC++ Packaging
+  Tool.<br/>
+  You can download and install `EnTT` in just a few simple steps:
+
+  ```
+  $ git clone https://github.com/Microsoft/vcpkg.git
+  $ cd vcpkg
+  $ ./bootstrap-vcpkg.sh
+  $ ./vcpkg integrate install
+  $ vcpkg install entt
+  ```
+
+  Or you can use the `experimental` feature to test the latest changes:
+
+  ```
+  vcpkg install entt[experimental] --head
+  ```
+
+  The `EnTT` port in `vcpkg` is kept up to date by Microsoft team members and
+  community contributors.<br/>
+  If the version is out of date, please
+  [create an issue or pull request](https://github.com/Microsoft/vcpkg) on the
+  `vcpkg` repository.
+
+* [`Homebrew`](https://github.com/skypjack/homebrew-entt), the missing package
+  manager for macOS.<br/>
+  Available as a homebrew formula. Just type the following to install it:
+
+  ```
+  brew install skypjack/entt/entt
+  ```
+
+* [`build2`](https://build2.org), build toolchain for developing and packaging C
+  and C++ code.<br/>
+  In order to use the [`entt`](https://cppget.org/entt) package in a `build2`
+  project, add the following line or a similar one to the `manifest` file:
+
+  ```
+  depends: entt ^3.0.0
+  ```
+
+  Also check that the configuration refers to a valid repository, so that the
+  package can be found by `build2`:
+
+  * [`cppget.org`](https://cppget.org), the open-source community central
+    repository, accessible as `https://pkg.cppget.org/1/stable`.
+
+  * [Package source repository](https://github.com/build2-packaging/entt):
+    accessible as either `https://github.com/build2-packaging/entt.git` or
+    `ssh://git@github.com/build2-packaging/entt.git`.
+    Feel free to [report issues](https://github.com/build2-packaging/entt) with
+    this package.
+
+  Both can be used with `bpkg add-repo` or added in a project
+  `repositories.manifest`. See the official
+  [documentation](https://build2.org/build2-toolchain/doc/build2-toolchain-intro.xhtml#guide-repositories)
+  for more details.
+
+Consider this list a work in progress and help me to make it longer if you like.
+
+## pkg-config
+
+`EnTT` also supports `pkg-config` (for some definition of _supports_ at least).
+A suitable file called `entt.pc` is generated and installed in a proper
+directory when running `CMake`.<br/>
+This should also make it easier to use with tools such as `Meson` or similar.
+
+# Documentation
+
+The documentation is based on [doxygen](http://www.doxygen.nl/). To build it:
+
+    $ cd build
+    $ cmake .. -DENTT_BUILD_DOCS=ON
+    $ make
+
+The API reference will be created in HTML format within the directory
+`build/docs/html`. To navigate it with your favorite browser:
+
+    $ cd build
+    $ your_favorite_browser docs/html/index.html
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+The same version is also available [online](https://skypjack.github.io/entt/)
+for the latest release, that is the last stable tag. If you are looking for
+something more pleasing to the eye, consider reading the nice-looking version
+available on [docsforge](https://entt.docsforge.com/): same documentation, much
+more pleasant to read.<br/>
+Moreover, there exists a [wiki](https://github.com/skypjack/entt/wiki) dedicated
+to the project where users can find all related documentation pages.
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Tests
+
+To compile and run the tests, `EnTT` requires *googletest*.<br/>
+`cmake` will download and compile the library before compiling anything else.
+In order to build the tests, set the `CMake` option `ENTT_BUILD_TESTING` to
+`ON`.
+
+To build the most basic set of tests:
+
+* `$ cd build`
+* `$ cmake -DENTT_BUILD_TESTING=ON ..`
+* `$ make`
+* `$ make test`
+
+Note that benchmarks are not part of this set.
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# EnTT in Action
+
+`EnTT` is widely used in private and commercial applications. I cannot even
+mention most of them because of some signatures I put on some documents time
+ago. Fortunately, there are also people who took the time to implement open
+source projects based on `EnTT` and did not hold back when it came to
+documenting them.
+
+[Here](https://github.com/skypjack/entt/wiki/EnTT-in-Action) you can find an
+incomplete list of games, applications and articles that can be used as a
+reference.
+
+If you know of other resources out there that are about `EnTT`, feel free to
+open an issue or a PR and I'll be glad to add them to the list.
+
+# Contributors
+
+Requests for features, PRs, suggestions ad feedback are highly appreciated.
+
+If you find you can help and want to contribute to the project with your
+experience or you do want to get part of the project for some other reason, feel
+free to contact me directly (you can find the mail in the
+[profile](https://github.com/skypjack)).<br/>
+I can't promise that each and every contribution will be accepted, but I can
+assure that I'll do my best to take them all as soon as possible.
+
+If you decide to participate, please see the guidelines for
+[contributing](CONTRIBUTING.md) before to create issues or pull
+requests.<br/>
+Take also a look at the
+[contributors list](https://github.com/skypjack/entt/blob/master/AUTHORS) to
+know who has participated so far.
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# License
+
+Code and documentation Copyright (c) 2017-2022 Michele Caini.<br/>
+Colorful logo Copyright (c) 2018-2021 Richard Caseres.
+
+Code released under
+[the MIT license](https://github.com/skypjack/entt/blob/master/LICENSE).<br/>
+Documentation released under
+[CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).<br/>
+All logos released under
+[CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).

TODO

@@ -0,0 +1,27 @@
+* debugging tools (#60): the issue online already contains interesting tips on this, look at it
+* work stealing job system (see #100) + mt scheduler based on const awareness for types
+
+EXAMPLES
+* filter on runtime values/variables (not only types)
+* support to polymorphic types (see #859)
+
+DOC:
+* storage<void>
+* custom storage/view
+* examples (and credits) from @alanjfs :)
+* update entity doc when the storage based model is in place
+
+TODO (high prio):
+* remove the static storage from the const assure in the registry
+
+WIP:
+* get rid of observers, storage based views made them pointless - document alternatives
+* add storage getter for filters to views and groups
+* exploit the tombstone mechanism to allow enabling/disabling entities (see bump, compact and clear for further details)
+* basic_storage::bind for cross-registry setups (see and remove todo from entity_copy.cpp)
+* process scheduler: reviews, use free lists internally
+* dedicated entity storage, in-place O(1) release/destroy for non-orphaned entities, out-of-sync model
+* entity-only and exclude-only views (both solved with entity storage and storage<void>)
+* custom allocators all over (registry, ...)
+* add test for maximum number of entities reached
+* deprecate non-owning groups in favor of owning views and view packs, introduce lazy owning views

WORKSPACE

@@ -0,0 +1 @@
+workspace(name = "com_github_skypjack_entt")

build/.gitignore

@@ -0,0 +1,2 @@
+*
+!.gitignore

cmake/in/EnTTConfig.cmake.in

@@ -0,0 +1,5 @@
+@PACKAGE_INIT@
+
+set(EnTT_VERSION "@PROJECT_VERSION@")
+include("${CMAKE_CURRENT_LIST_DIR}/EnTTTargets.cmake")
+check_required_components("@PROJECT_NAME@")

cmake/in/entt.pc.in

@@ -0,0 +1,8 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+includedir=@EnTT_PKGCONFIG_INCLUDEDIR@
+
+Name: EnTT
+Description: Gaming meets modern C++
+Url: https://github.com/skypjack/entt
+Version: @ENTT_VERSION@
+Cflags: -I${includedir}

cmake/modules/JoinPaths.cmake

@@ -0,0 +1,23 @@
+# This module provides function for joining paths
+# known from most languages
+#
+# SPDX-License-Identifier: (MIT OR CC0-1.0)
+# Copyright 2020 Jan Tojnar
+# https://github.com/jtojnar/cmake-snips
+#
+# Modelled after Python’s os.path.join
+# https://docs.python.org/3.7/library/os.path.html#os.path.join
+# Windows not supported
+function(join_paths joined_path first_path_segment)
+    set(temp_path "${first_path_segment}")
+    foreach(current_segment IN LISTS ARGN)
+        if(NOT ("${current_segment}" STREQUAL ""))
+            if(IS_ABSOLUTE "${current_segment}")
+                set(temp_path "${current_segment}")
+            else()
+                set(temp_path "${temp_path}/${current_segment}")
+            endif()
+        endif()
+    endforeach()
+    set(${joined_path} "${temp_path}" PARENT_SCOPE)
+endfunction()

conan/build.py

@@ -0,0 +1,37 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+from cpt.packager import ConanMultiPackager
+import os
+
+if __name__ == "__main__":
+    username = os.getenv("GITHUB_ACTOR")
+    tag_version = os.getenv("GITHUB_REF")
+    tag_package = os.getenv("GITHUB_REPOSITORY")
+    login_username = os.getenv("CONAN_LOGIN_USERNAME")
+    package_version = tag_version.replace("refs/tags/v", "")
+    package_name = tag_package.replace("skypjack/", "")
+    reference = "{}/{}".format(package_name, package_version)
+    channel = os.getenv("CONAN_CHANNEL", "stable")
+    upload = os.getenv("CONAN_UPLOAD")
+    stable_branch_pattern = os.getenv("CONAN_STABLE_BRANCH_PATTERN", r"v\d+\.\d+\.\d+.*")
+    test_folder = os.getenv("CPT_TEST_FOLDER", os.path.join("conan", "test_package"))
+    upload_only_when_stable = os.getenv("CONAN_UPLOAD_ONLY_WHEN_STABLE", True)
+    disable_shared = os.getenv("CONAN_DISABLE_SHARED_BUILD", "False")
+
+    builder = ConanMultiPackager(username=username,
+                                 reference=reference,
+                                 channel=channel,
+                                 login_username=login_username,
+                                 upload=upload,
+                                 stable_branch_pattern=stable_branch_pattern,
+                                 upload_only_when_stable=upload_only_when_stable,
+                                 test_folder=test_folder)
+    builder.add()
+
+    filtered_builds = []
+    for settings, options, env_vars, build_requires, reference in builder.items:
+        if disable_shared == "False" or not options["{}:shared".format(package_name)]:
+             filtered_builds.append([settings, options, env_vars, build_requires])
+    builder.builds = filtered_builds
+
+    builder.run()

conan/ci/build.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+set -e
+set -x
+
+conan user
+python conan/build.py

conan/ci/install.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+set -e
+set -x
+
+pip install -U conan_package_tools conan

conan/test_package/CMakeLists.txt

@@ -0,0 +1,13 @@
+cmake_minimum_required(VERSION 3.7.2)
+project(test_package)
+
+set(CMAKE_VERBOSE_MAKEFILE TRUE)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+include(${CMAKE_BINARY_DIR}/conanbuildinfo.cmake)
+conan_basic_setup()
+
+add_executable(${PROJECT_NAME} test_package.cpp)
+target_link_libraries(${PROJECT_NAME} ${CONAN_LIBS})

conan/test_package/conanfile.py

@@ -0,0 +1,19 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+from conans import ConanFile, CMake
+import os
+
+
+class TestPackageConan(ConanFile):
+    settings = "os", "compiler", "build_type", "arch"
+    generators = "cmake"
+
+    def build(self):
+        cmake = CMake(self)
+        cmake.configure()
+        cmake.build()
+
+    def test(self):
+        bin_path = os.path.join("bin", "test_package")
+        self.run(bin_path, run_environment=True)

conan/test_package/test_package.cpp

@@ -0,0 +1,56 @@
+#include <entt/entt.hpp>
+#include <cstdint>
+
+struct position {
+    float x;
+    float y;
+};
+
+struct velocity {
+    float dx;
+    float dy;
+};
+
+void update(entt::registry &registry) {
+    auto view = registry.view<position, velocity>();
+
+    for(auto entity: view) {
+        // gets only the components that are going to be used ...
+
+        auto &vel = view.get<velocity>(entity);
+
+        vel.dx = 0.;
+        vel.dy = 0.;
+
+        // ...
+    }
+}
+
+void update(std::uint64_t dt, entt::registry &registry) {
+    registry.view<position, velocity>().each([dt](auto &pos, auto &vel) {
+        // gets all the components of the view at once ...
+
+        pos.x += vel.dx * dt;
+        pos.y += vel.dy * dt;
+
+        // ...
+    });
+}
+
+int main() {
+    entt::registry registry;
+    std::uint64_t dt = 16;
+
+    for(auto i = 0; i < 10; ++i) {
+        auto entity = registry.create();
+        registry.emplace<position>(entity, i * 1.f, i * 1.f);
+        if(i % 2 == 0) { registry.emplace<velocity>(entity, i * .1f, i * .1f); }
+    }
+
+    update(dt, registry);
+    update(registry);
+
+    // ...
+
+    return EXIT_SUCCESS;
+}

conanfile.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+from conans import ConanFile
+
+
+class EnttConan(ConanFile):
+    name = "entt"
+    description = "Gaming meets modern C++ - a fast and reliable entity-component system (ECS) and much more "
+    topics = ("conan," "entt", "gaming", "entity", "ecs")
+    url = "https://github.com/skypjack/entt"
+    homepage = url
+    author = "Michele Caini <michele.caini@gmail.com>"
+    license = "MIT"
+    exports = ["LICENSE"]
+    exports_sources = ["src/*"]
+    no_copy_source = True
+
+    def package(self):
+        self.copy(pattern="LICENSE", dst="licenses")
+        self.copy(pattern="*", dst="include", src="src", keep_path=True)
+
+    def package_info(self):
+        if not self.in_local_cache:
+            self.cpp_info.includedirs = ["src"]
+
+    def package_id(self):
+        self.info.header_only()

docs/CMakeLists.txt

@@ -0,0 +1,40 @@
+#
+# Doxygen configuration (documentation)
+#
+
+set(DOXY_DEPS_DIRECTORY ${EnTT_SOURCE_DIR}/deps)
+set(DOXY_SOURCE_DIRECTORY ${EnTT_SOURCE_DIR}/src)
+set(DOXY_DOCS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
+set(DOXY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+
+configure_file(doxy.in doxy.cfg @ONLY)
+
+add_custom_target(
+    docs ALL
+    COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/doxy.cfg
+    WORKING_DIRECTORY ${EnTT_SOURCE_DIR}
+    VERBATIM
+    SOURCES
+        dox/extra.dox
+        md/config.md
+        md/container.md
+        md/core.md
+        md/entity.md
+        md/faq.md
+        md/lib.md
+        md/links.md
+        md/locator.md
+        md/meta.md
+        md/poly.md
+        md/process.md
+        md/reference.md
+        md/resource.md
+        md/signal.md
+        md/unreal.md
+        doxy.in
+)
+
+install(
+    DIRECTORY ${DOXY_OUTPUT_DIRECTORY}/html
+    DESTINATION share/${PROJECT_NAME}-${PROJECT_VERSION}/
+)

docs/dox/extra.dox

@@ -0,0 +1,5 @@
+/**
+ * @namespace entt
+ *
+ * @brief `EnTT` default namespace.
+ */

docs/md/config.md

@@ -0,0 +1,121 @@
+# Crash Course: configuration
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Definitions](#definitions)
+  * [ENTT_NOEXCEPTION](#entt_noexception)
+  * [ENTT_USE_ATOMIC](#entt_use_atomic)
+  * [ENTT_ID_TYPE](#entt_id_type)
+  * [ENTT_SPARSE_PAGE](#entt_sparse_page)
+  * [ENTT_PACKED_PAGE](#entt_packed_page)
+  * [ENTT_ASSERT](#entt_assert)
+    * [ENTT_ASSERT_CONSTEXPR](#entt_assert_constexpr)
+    * [ENTT_DISABLE_ASSERT](#entt_disable_assert)
+  * [ENTT_NO_ETO](#entt_no_eto)
+  * [ENTT_STANDARD_CPP](#entt_standard_cpp)
+
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+`EnTT` has become almost completely customizable over time, in many
+respects. These variables are just one of the many ways to customize how it
+works.<br/>
+In the vast majority of cases, users will have no interest in changing the
+default parameters. For all other cases, the list of possible configurations
+with which it's possible to adjust the behavior of the library at runtime can be
+found below.
+
+# Definitions
+
+All options are intended as parameters to the compiler (or user-defined macros
+within the compilation units, if preferred).<br/>
+Each parameter can result in internal library definitions. It's not recommended
+to try to also modify these definitions, since there is no guarantee that they
+will remain stable over time unlike the options below.
+
+## ENTT_NOEXCEPTION
+
+Define this variable without assigning any value to it to turn off exception
+handling in `EnTT`.<br/>
+This is roughly equivalent to setting the compiler flag `-fno-exceptions` but is
+also limited to this library only.
+
+## ENTT_USE_ATOMIC
+
+In general, `EnTT` doesn't offer primitives to support multi-threading. Many of
+the features can be split over multiple threads without any explicit control and
+the user is the one who knows if a synchronization point is required.<br/>
+However, some features aren't easily accessible to users and are made
+thread-safe by means of this definition.
+
+## ENTT_ID_TYPE
+
+`entt::id_type` is directly controlled by this definition and widely used within
+the library.<br/>
+By default, its type is `std::uint32_t`. However, users can define a different
+default type if necessary.
+
+## ENTT_SPARSE_PAGE
+
+It's known that the ECS module of `EnTT` is based on _sparse sets_. What is less
+known perhaps is that the sparse arrays are paged to reduce memory usage.<br/>
+Default size of pages (that is, the number of elements they contain) is 4096 but
+users can adjust it if appropriate. In all case, the chosen value **must** be a
+power of 2.
+
+## ENTT_PACKED_PAGE
+
+As it happens with sparse arrays, packed arrays are also paginated. However, in
+this case the aim isn't to reduce memory usage but to have pointer stability
+upon component creation.<br/>
+Default size of pages (that is, the number of elements they contain) is 1024 but
+users can adjust it if appropriate. In all case, the chosen value **must** be a
+power of 2.
+
+## ENTT_ASSERT
+
+For performance reasons, `EnTT` doesn't use exceptions or any other control
+structures. In fact, it offers many features that result in undefined behavior
+if not used correctly.<br/>
+To get around this, the library relies on a lot of asserts for the purpose of
+detecting errors in debug builds. By default, it uses `assert` internally. Users
+are allowed to overwrite its behavior by setting this variable.
+
+### ENTT_ASSERT_CONSTEXPR
+
+Usually, an assert within a `constexpr` function isn't a big deal. However, in
+case of extreme customizations, it might be useful to differentiate.<br/>
+For this purpose, `EnTT` introduces an admittedly badly named variable to make
+the job easier in this regard. By default, this variable forwards its arguments
+to `ENTT_ASSERT`.
+
+### ENTT_DISABLE_ASSERT
+
+Assertions may in turn affect performance to an extent when enabled. Whether
+`ENTT_ASSERT` and `ENTT_ASSERT_CONSTEXPR` are redefined or not, all asserts can
+be disabled at once by means of this definition.<br/>
+Note that `ENTT_DISABLE_ASSERT` takes precedence over the redefinition of the
+other variables and is therefore meant to disable all controls no matter what.
+
+## ENTT_NO_ETO
+
+In order to reduce memory consumption and increase performance, empty types are
+never instantiated nor stored by the ECS module of `EnTT`.<br/>
+Use this variable to treat these types like all others and therefore to create a
+dedicated storage for them.
+
+## ENTT_STANDARD_CPP
+
+`EnTT` mixes non-standard language features with others that are perfectly
+compliant to offer some of its functionalities.<br/>
+This definition prevents the library from using non-standard techniques, that
+is, functionalities that aren't fully compliant with the standard C++.<br/>
+While there are no known portability issues at the time of this writing, this
+should make the library fully portable anyway if needed.

docs/md/container.md

@@ -0,0 +1,67 @@
+# Crash Course: containers
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Containers](#containers)
+  * [Dense map](#dense-map)
+  * [Dense set](#dense-set)
+
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+The standard C++ library offers a wide range of containers and it's really
+difficult to do better (although it's very easy to do worse, as many examples
+available online demonstrate).<br/>
+`EnTT` doesn't try in any way to replace what is offered by the standard. Quite
+the opposite, given the widespread use that is made of standard containers.<br/>
+However, the library also tries to fill a gap in features and functionality by
+making available some containers initially developed for internal use.
+
+This section of the library is likely to grow larger over time. However, for the
+moment it's quite small and mainly aimed at satisfying some internal needs.<br/>
+For all containers made available, full test coverage and stability over time is
+guaranteed as usual.
+
+# Containers
+
+## Dense map
+
+The dense map made available in `EnTT` is a hash map that aims to return a
+packed array of elements, so as to reduce the number of jumps in memory during
+iterations.<br/>
+The implementation is based on _sparse sets_ and each bucket is identified by an
+implicit list within the packed array itself.
+
+The interface is very close to its counterpart in the standard library, that is,
+`std::unordered_map`.<br/>
+However, both local and non-local iterators returned by a dense map belong to
+the input iterator category although they respectively model the concepts of a
+_forward iterator_ type and a _random access iterator_ type.<br/>
+This is because they return a pair of references rather than a reference to a
+pair. In other words, dense maps return a so called _proxy iterator_ the value
+type of which is:
+
+* `std::pair<const Key &, Type &>` for non-const iterator types.
+* `std::pair<const Key &, const Type &>` for const iterator types.
+
+This is quite different from what any standard library map returns and should be
+taken into account when looking for a drop-in replacement.
+
+## Dense set
+
+The dense set made available in `EnTT` is a hash set that aims to return a
+packed array of elements, so as to reduce the number of jumps in memory during
+iterations.<br/>
+The implementation is based on _sparse sets_ and each bucket is identified by an
+implicit list within the packed array itself.
+
+The interface is in all respects similar to its counterpart in the standard
+library, that is, `std::unordered_set`.<br/>
+Therefore, there is no need to go into the API description.

docs/md/faq.md

@@ -0,0 +1,215 @@
+# Frequently Asked Questions
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [FAQ](#faq)
+  * [Why is my debug build on Windows so slow?](#why-is-my-debug-build-on-windows-so-slow)
+  * [How can I represent hierarchies with my components?](#how-can-i-represent-hierarchies-with-my-components)
+  * [Custom entity identifiers: yay or nay?](#custom-entity-identifiers-yay-or-nay)
+  * [Warning C4307: integral constant overflow](#warning-C4307-integral-constant-overflow)
+  * [Warning C4003: the min, the max and the macro](#warning-C4003-the-min-the-max-and-the-macro)
+  * [The standard and the non-copyable types](#the-standard-and-the-non-copyable-types)
+  * [Which functions trigger which signals](#which-functions-trigger-which-signals)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+This is a constantly updated section where I'm trying to put the answers to the
+most frequently asked questions.<br/>
+If you don't find your answer here, there are two cases: nobody has done it yet
+or this section needs updating. In both cases, you can
+[open a new issue](https://github.com/skypjack/entt/issues/new) or enter either
+the [gitter channel](https://gitter.im/skypjack/entt) or the
+[discord server](https://discord.gg/5BjPWBd) to ask for help.<br/>
+Probably someone already has an answer for you and we can then integrate this
+part of the documentation.
+
+# FAQ
+
+## Why is my debug build on Windows so slow?
+
+`EnTT` is an experimental project that I also use to keep me up-to-date with the
+latest revision of the language and the standard library. For this reason, it's
+likely that some classes you're working with are using standard containers under
+the hood.<br/>
+Unfortunately, it's known that the standard containers aren't particularly
+performing in debugging (the reasons for this go beyond this document) and are
+even less so on Windows apparently. Fortunately this can also be mitigated a
+lot, achieving good results in many cases.
+
+First of all, there are two things to do in a Windows project:
+
+* Disable the [`/JMC`](https://docs.microsoft.com/cpp/build/reference/jmc)
+  option (_Just My Code_ debugging), available starting with Visual Studio 2017
+  version 15.8.
+
+* Set the [`_ITERATOR_DEBUG_LEVEL`](https://docs.microsoft.com/cpp/standard-library/iterator-debug-level)
+  macro to 0. This will disable checked iterators and iterator debugging.
+
+Moreover, set the `ENTT_DISABLE_ASSERT` variable or redefine the `ENTT_ASSERT`
+macro to disable internal debug checks in `EnTT`:
+
+```cpp
+#define ENTT_ASSERT(...) ((void)0)
+```
+
+These asserts are introduced to help the users but they require to access to the
+underlying containers and therefore risk ruining the performance in some cases.
+
+With these changes, debug performance should increase enough in most cases. If
+you want something more, you can also switch to an optimization level `O0` or
+preferably `O1`.
+
+## How can I represent hierarchies with my components?
+
+This is one of the first questions that anyone makes when starting to work with
+the entity-component-system architectural pattern.<br/>
+There are several approaches to the problem and the best one depends mainly on
+the real problem one is facing. In all cases, how to do it doesn't strictly
+depend on the library in use, but the latter certainly allows or not different
+techniques depending on how the data are laid out.
+
+I tried to describe some of the approaches that fit well with the model of
+`EnTT`. [This](https://skypjack.github.io/2019-06-25-ecs-baf-part-4/) is the
+first post of a series that tries to _explore_ the problem. More will probably
+come in future.<br/>
+In addition, `EnTT` also offers the possibility to create stable storage types
+and therefore have pointer stability for one, all or some components. This is by
+far the most convenient solution when it comes to creating hierarchies and
+whatnot. See the documentation for the ECS part of the library and in particular
+what concerns the `component_traits` class for further details.
+
+## Custom entity identifiers: yay or nay?
+
+Custom entity identifiers are definitely a good idea in two cases at least:
+
+* If `std::uint32_t` isn't large enough for your purposes, since this is the
+  underlying type of `entt::entity`.
+
+* If you want to avoid conflicts when using multiple registries.
+
+Identifiers can be defined through enum classes and class types that define an
+`entity_type` member of type `std::uint32_t` or `std::uint64_t`.<br/>
+In fact, this is a definition equivalent to that of `entt::entity`:
+
+```cpp
+enum class entity: std::uint32_t {};
+```
+
+There is no limit to the number of identifiers that can be defined.
+
+## Warning C4307: integral constant overflow
+
+According to [this](https://github.com/skypjack/entt/issues/121) issue, using a
+hashed string under VS (toolset v141) could generate a warning.<br/>
+First of all, I want to reassure you: it's expected and harmless. However, it
+can be annoying.
+
+To suppress it and if you don't want to suppress all the other warnings as well,
+here is a workaround in the form of a macro:
+
+```cpp
+#if defined(_MSC_VER)
+#define HS(str) __pragma(warning(suppress:4307)) entt::hashed_string{str}
+#else
+#define HS(str) entt::hashed_string{str}
+#endif
+```
+
+With an example of use included:
+
+```cpp
+constexpr auto identifier = HS("my/resource/identifier");
+```
+
+Thanks to [huwpascoe](https://github.com/huwpascoe) for the courtesy.
+
+## Warning C4003: the min, the max and the macro
+
+On Windows, a header file defines two macros `min` and `max` which may result in
+conflicts with their counterparts in the standard library and therefore in
+errors during compilation.
+
+It's a pretty big problem but fortunately it's not a problem of `EnTT` and there
+is a fairly simple solution to it.<br/>
+It consists in defining the `NOMINMAX` macro before including any other header
+so as to get rid of the extra definitions:
+
+```cpp
+#define NOMINMAX
+```
+
+Please refer to [this](https://github.com/skypjack/entt/issues/96) issue for
+more details.
+
+## The standard and the non-copyable types
+
+`EnTT` uses internally the trait `std::is_copy_constructible_v` to check if a
+component is actually copyable. However, this trait doesn't really check whether
+a type is actually copyable. Instead, it just checks that a suitable copy
+constructor and copy operator exist.<br/>
+This can lead to surprising results due to some idiosyncrasies of the standard.
+
+For example, `std::vector` defines a copy constructor that is conditionally
+enabled depending on whether the value type is copyable or not. As a result,
+`std::is_copy_constructible_v` returns true for the following specialization:
+
+```cpp
+struct type {
+    std::vector<std::unique_ptr<action>> vec;
+};
+```
+
+However, the copy constructor is effectively disabled upon specialization.
+Therefore, trying to assign an instance of this type to an entity may trigger a
+compilation error.<br/>
+As a workaround, users can mark the type explicitly as non-copyable. This also
+suppresses the implicit generation of the move constructor and operator, which
+will therefore have to be defaulted accordingly:
+
+```cpp
+struct type {
+    type(const type &) = delete;
+    type(type &&) = default;
+
+    type & operator=(const type &) = delete;
+    type & operator=(type &&) = default;
+
+    std::vector<std::unique_ptr<action>> vec;
+};
+```
+
+Note that aggregate initialization is also disabled as a consequence.<br/>
+Fortunately, this type of trick is quite rare. The bad news is that there is no
+way to deal with it at the library level, this being due to the design of the
+language. On the other hand, the fact that the language itself also offers a way
+to mitigate the problem makes it manageable.
+
+## Which functions trigger which signals
+
+The `registry` class offers three signals that are emitted following specific
+operations. Maybe not everyone knows what these operations are, though.<br/>
+If this isn't clear, below you can find a _vademecum_ for this purpose:
+
+* `on_created` is invoked when a component is first added (neither modified nor 
+  replaced) to an entity.
+
+* `on_update` is called whenever an existing component is modified or replaced.
+
+* `on_destroyed` is called when a component is explicitly or implicitly removed 
+  from an entity.
+
+Among the most controversial functions can be found `emplace_or_replace` and
+`destroy`. However, following the above rules, it's quite simple to know what 
+will happen.<br/>
+In the first case, `on_created` is invoked if the entity has not the component,
+otherwise the latter is replaced and therefore `on_update` is triggered. As for
+the second case, components are removed from their entities and thus freed when
+they are recycled. It means that `on_destroyed` is triggered for every component 
+owned by the entity that is destroyed.

docs/md/graph.md

@@ -0,0 +1,299 @@
+# Crash Course: graph
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Data structures](#data-structures)
+  * [Adjacency matrix](#adjacency-matrix)
+  * [Graphviz dot language](#graphviz-dot-language)
+* [Flow builder](#flow-builder)
+  * [Tasks and resources](#tasks-and-resources)
+  * [Fake resources and order of execution](#fake-resources-and-order-of-execution)
+  * [Sync points](#sync-points)
+  * [Execution graph](#execution-graph)
+
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+`EnTT` doesn't aim to offer everything one needs to work with graphs. Therefore,
+anyone looking for this in the _graph_ submodule will be disappointed.<br/>
+Quite the opposite is true. This submodule is minimal and contains only the data
+structures and algorithms strictly necessary for the development of some tools
+such as the _flow builder_.
+
+# Data structures
+
+As anticipated in the introduction, the aim isn't to offer all possible data
+structures suitable for representing and working with graphs. Many will likely
+be added or refined over time, however I want to discourage anyone expecting
+tight scheduling on the subject.<br/>
+The data structures presented in this section are mainly useful for the
+development and support of some tools which are also part of the same submodule.
+
+## Adjacency matrix
+
+The adjacency matrix is designed to represent either a directed or an undirected
+graph:
+
+```cpp
+entt::adjacency_matrix<entt::directed_tag> adjacency_matrix{};
+```
+
+The `directed_tag` type _creates_ the graph as directed. There is also an
+`undirected_tag` counterpart which creates it as undirected.<br/>
+The interface deviates slightly from the typical double indexing of C and offers
+an API that is perhaps more familiar to a C++ programmer. Therefore, the access
+and modification of an element will take place via the `contains`, `insert` and
+`erase` functions rather than a double call to an `operator[]`:
+
+```cpp
+if(adjacency_matrix.contains(0u, 1u)) {
+    adjacency_matrix.erase(0u, 1u);
+} else {
+    adjacency_matrix.insert(0u, 1u);
+}
+```
+
+Both `insert` and` erase` are idempotent functions which have no effect if the
+element already exists or has already been deleted.<br/>
+The first one returns an `std::pair` containing the iterator to the element and
+a boolean value indicating whether the element has been inserted or was already
+present. The second one instead returns the number of deleted elements (0 or 1).
+
+An adjacency matrix must be initialized with the number of elements (vertices)
+when constructing it but can also be resized later using the `resize` function:
+
+```cpp
+entt::adjacency_matrix<entt::directed_tag> adjacency_matrix{3u};
+```
+
+To visit all vertices, the class offers a function named `vertices` that returns
+an iterable object suitable for the purpose:
+
+```cpp
+for(auto &&vertex: adjacency_matrix.vertices()) {
+    // ...
+}
+```
+
+Note that the same result can be obtained with the following snippet, since the
+vertices are unsigned integral values:
+
+```cpp
+for(auto last = adjacency_matrix.size(), pos = {}; pos < last; ++pos) {
+    // ...
+}
+```
+
+As for visiting the edges, a few functions are available.<br/>
+When the purpose is to visit all the edges of a given adjacency matrix, the
+`edges` function returns an iterable object that can be used to get them as
+pairs of vertices:
+
+```cpp
+for(auto [lhs, rhs]: adjacency_matrix.edges()) {
+    // ...
+}
+```
+
+On the other hand, if the goal is to visit all the in- or out-edges of a given
+vertex, the `in_edges` and `out_edges` functions are meant for that:
+
+```cpp
+for(auto [lhs, rhs]: adjacency_matrix.out_edges(3u)) {
+    // ...
+}
+```
+
+As might be expected, these functions expect the vertex to visit (that is, to
+return the in- or out-edges for) as an argument.<br/>
+Finally, the adjacency matrix is an allocator-aware container and offers most of
+the functionality one would expect from this type of containers, such as `clear`
+or 'get_allocator` and so on.
+
+## Graphviz dot language
+
+As it's one of the most popular formats, the library offers minimal support for
+converting a graph to a Graphviz dot snippet.<br/>
+The simplest way is to pass both an output stream and a graph to the `dot`
+function:
+
+```cpp
+std::ostringstream output{};
+entt::dot(output, adjacency_matrix);
+```
+
+However, there is also the option of providing a callback to which the vertices
+are passed and which can be used to add (`dot`) properties to the output from
+time to time:
+
+```cpp
+std::ostringstream output{};
+entt::dot(output, adjacency_matrix, [](auto &output, auto vertex) {
+    out << "label=\"v\"" << vertex << ",shape=\"box\"";
+});
+```
+
+This second mode is particularly convenient when the user wants to associate
+data managed externally to the graph being converted.
+
+# Flow builder
+
+A flow builder is used to create execution graphs from tasks and resources.<br/>
+The implementation is as generic as possible and doesn't bind to any other part
+of the library.
+
+This class is designed as a sort of _state machine_ to which a specific task is
+attached for which the resources accessed in read-only or read-write mode are
+specified.<br/>
+Most of the functions in the API also return the flow builder itself, according
+to what is the common sense API when it comes to builder classes.
+
+Once all tasks have been registered and resources assigned to them, an execution
+graph in the form of an adjacency matrix is returned to the user.<br/>
+This graph contains all the tasks assigned to the flow builder in the form of
+_vertices_. The _vertex_ itself can be used as an index to get the identifier
+passed during registration.
+
+## Tasks and resources
+
+Although these terms are used extensively in the documentation, the flow builder
+has no real concept of tasks and resources.<br/>
+This class works mainly with _identifiers_, that is, values of type `id_type`.
+That is, both tasks and resources are identified by integral values.<br/>
+This allows not to couple the class itself to the rest of the library or to any
+particular data structure. On the other hand, it requires the user to keep track
+of the association between identifiers and operations or actual data.
+
+Once a flow builder has been created (which requires no constructor arguments),
+the first thing to do is to bind a task. This will indicate to the builder who
+intends to consume the resources that will be specified immediately after:
+
+```cpp
+entt::flow builder{};
+builder.bind("task_1"_hs);
+```
+
+Note that the example uses the `EnTT` hashed string to generate an identifier
+for the task.<br/>
+Indeed, the use of `id_type` as an identifier type is not by accident. In fact,
+it matches well with the internal hashed string class. Moreover, it's also the
+same type returned by the hash function of the internal RTTI system, in case the
+user wants to rely on that.<br/>
+However, being an integral value, it leaves the user full freedom to rely on his
+own tools if he deems it necessary.
+
+Once a task has been associated with the flow builder, it can be assigned
+read-only or read-write resources, as appropriate:
+
+```cpp
+builder
+    .bind("task_1"_hs)
+        .ro("resource_1"_hs)
+        .ro("resource_2"_hs)
+    .bind("task_2"_hs)
+        .rw("resource_2"_hs)
+```
+
+As mentioned, many functions return the builder itself and it's therefore easy
+to concatenate the different calls.<br/>
+Also in the case of resources, these are identified by numeric values of type
+`id_type`. As above, the choice is not entirely random. This goes well with the
+tools offered by the library while leaving room for maximum flexibility.
+
+Finally, both the `ro` and` rw` functions also offer an overload that accepts a
+pair of iterators, so that one can pass a range of resources in one go.
+
+## Fake resources and order of execution
+
+The flow builder doesn't offer the ability to specify when a task should execute
+before or after another task.<br/>
+In fact, the order of _registration_ on the resources also determines the order
+in which the tasks are processed during the generation of the execution graph.
+
+However, there is a way to force the execution order of two processes.<br/>
+Briefly, since accessing a resource in opposite modes requires sequential rather
+than parallel scheduling, it's possible to make use of fake resources to force
+the order execution:
+
+```cpp
+builder
+    .bind("task_1"_hs)
+        .ro("resource_1"_hs)
+        .rw("fake"_hs)
+    .bind("task_2"_hs)
+        .ro("resource_2"_hs)
+        .ro("fake"_hs)
+    .bind("task_3"_hs)
+        .ro("resource_2"_hs)
+        .ro("fake"_hs)
+```
+
+This snippet forces the execution of `task_2` and `task_3` **after** `task_1`.
+This is due to the fact that the latter sets a read-write requirement on a fake
+resource that the other tasks also want to access in read-only mode.<br/>
+Similarly, it's possible to force a task to run after a certain group:
+
+```cpp
+builder
+    .bind("task_1"_hs)
+        .ro("resource_1"_hs)
+        .ro("fake"_hs)
+    .bind("task_2"_hs)
+        .ro("resource_1"_hs)
+        .ro("fake"_hs)
+    .bind("task_3"_hs)
+        .ro("resource_2"_hs)
+        .rw("fake"_hs)
+```
+
+In this case, since there are a number of processes that want to read a specific
+resource, they will do so in parallel by forcing `task_3` to run after all the
+others tasks.
+
+## Sync points
+
+Sometimes it's useful to assign the role of _sync point_ to a node.<br/>
+Whether it accesses new resources or is simply a watershed, the procedure for
+assigning this role to a vertex is always the same: first it's tied to the flow
+builder, then the `sync` function is invoked:
+
+```cpp
+builder.bind("sync_point"_hs).sync();
+```
+
+The choice to assign an _identity_ to this type of nodes lies in the fact that,
+more often than not, they also perform operations on resources.<br/>
+If this isn't the case, it will still be possible to create no-op vertices to
+which empty tasks are assigned.
+
+## Execution graph
+
+Once both the resources and their consumers have been properly registered, the
+purpose of this tool is to generate an execution graph that takes into account
+all specified constraints to return the best scheduling for the vertices:
+
+```cpp
+entt::adjacency_matrix<entt::directed_tag> graph = builder.graph();
+```
+
+The search for the main vertices, that is those without in-edges, is usually the
+first thing required:
+
+```cpp
+for(auto &&vertex: graph) {
+    if(auto in_edges = graph.in_edges(vertex); in_edges.begin() == in_edges.end()) {
+        // ...
+    }
+}
+```
+
+Starting from them, using the other functions appropriately (such as `out_edges`
+to retrieve the children of a given task or `edges` to access their identifiers)
+it will be possible to instantiate an execution graph.

docs/md/lib.md

@@ -0,0 +1,99 @@
+# Push EnTT across boundaries
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Working across boundaries](#working-across-boundaries)
+  * [Smooth until proven otherwise](#smooth-until-proven-otherwise)
+  * [Meta context](#meta-context)
+  * [Memory management](#memory-management)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Working across boundaries
+
+`EnTT` has historically had a limit when used across boundaries on Windows in
+general and on GNU/Linux when default visibility was set to hidden. The
+limitation was mainly due to a custom utility used to assign unique, sequential
+identifiers with different types.<br/>
+Fortunately, nowadays using `EnTT` across boundaries is much easier.
+
+## Smooth until proven otherwise
+
+Many classes in `EnTT` make extensive use of type erasure for their purposes.
+This isn't a problem on itself (in fact, it's the basis of an API so convenient
+to use). However, a way is needed to recognize the objects whose type has been
+erased on the other side of a boundary.<br/>
+The `type_hash` class template is how identifiers are generated and thus made
+available to the rest of the library. In general, this class doesn't arouse much
+interest. The only exception is when a conflict between identifiers occurs
+(definitely uncommon though) or when the default solution proposed by `EnTT`
+isn't suitable for the user's purposes.<br/>
+The section dedicated to `type_info` contains all the details to get around the
+issue in a concise and elegant way. Please refer to the specific documentation.
+
+When working with linked libraries, compile definitions `ENTT_API_EXPORT` and
+`ENTT_API_IMPORT` can be used where there is a need to import or export symbols,
+so as to make everything work nicely across boundaries.<br/>
+On the other hand, everything should run smoothly when working with plugins or
+shared libraries that don't export any symbols.
+
+For anyone who needs more details, the test suite contains multiple examples
+covering the most common cases (see the `lib` directory for all details).<br/>
+It goes without saying that it's impossible to cover **all** possible cases.
+However, what is offered should hopefully serve as a basis for all of them.
+
+## Meta context
+
+The runtime reflection system deserves a special mention when it comes to using
+it across boundaries.<br/>
+Since it's linked already to a static context to which the elements are attached
+and different contexts don't relate to each other, they must be _shared_ to
+allow the use of meta types across boundaries.
+
+Fortunately, sharing a context is also trivial to do. First of all, the local
+one is acquired in the main space:
+
+```cpp
+auto handle = entt::locator<entt::meta_ctx>::handle();
+```
+
+Then, it's passed to the receiving space that sets it as its default context,
+thus discarding or storing aside the local one:
+
+```cpp
+entt::locator<entt::meta_ctx>::reset(handle);
+```
+
+From now on, both spaces refer to the same context and on it are attached all
+new meta types, no matter where they are created.<br/>
+Note that resetting the main context doesn't also propagate changes across
+boundaries. In other words, resetting a context results in the decoupling of the
+two sides and therefore a divergence in the contents.
+
+## Memory Management
+
+There is another subtle problem due to memory management that can lead to
+headaches.<br/>
+It can occur where there are pools of objects (such as components or events)
+dynamically created on demand. This is usually not a problem when working with
+linked libraries that rely on the same dynamic runtime. However, it can occur in
+the case of plugins or statically linked runtimes.
+
+As an example, imagine creating an instance of `registry` in the main executable
+and sharing it with a plugin. If the latter starts working with a component that
+is unknown to the former, a dedicated pool is created within the registry on
+first use.<br/>
+As one can guess, this pool is instantiated on a different side of the boundary
+from the `registry`. Therefore, the instance is now managing memory from
+different spaces and this can quickly lead to crashes if not properly addressed.
+
+To overcome the risk, it's recommended to use well-defined interfaces that make
+fundamental types pass through the boundaries, isolating the instances of the
+`EnTT` classes from time to time and as appropriate.<br/>
+Refer to the test suite for some examples, read the documentation available
+online about this type of issues or consult someone who has already had such
+experiences to avoid problems.

docs/md/links.md

@@ -0,0 +1,266 @@
+# EnTT in Action
+
+`EnTT` is widely used in private and commercial applications. I cannot even
+mention most of them because of some signatures I put on some documents time
+ago. Fortunately, there are also people who took the time to implement open
+source projects based on `EnTT` and didn't hold back when it came to documenting
+them.
+
+Below an incomplete list of games, applications and articles that can be used as
+a reference. Where I put the word _apparently_ means that the use of `EnTT` is
+documented but the authors didn't make explicit announcements or contacted me
+directly.
+
+I hope this list can grow much more in the future:
+
+* Games:
+  * [Minecraft](https://minecraft.net/en-us/attribution/) by
+    [Mojang](https://mojang.com/): of course, **that** Minecraft, see the
+    open source attributions page for more details.
+  * [Minecraft Earth](https://www.minecraft.net/en-us/about-earth) by
+    [Mojang](https://mojang.com/): an augmented reality game for mobile, that
+    lets users bring Minecraft into the real world.
+  * [Ember Sword](https://embersword.com/): a modern Free-to-Play MMORPG with a
+    player-driven economy, a classless combat system, and scarce, tradable
+    cosmetic collectibles.
+  * Apparently [Diablo II: Resurrected](https://diablo2.blizzard.com/) by
+    [Blizzard](https://www.blizzard.com/): monsters, heroes, items, spells, all
+    resurrected. Thanks unknown insider.
+  * [Apparently](https://www.youtube.com/watch?v=P8xvOA3ikrQ&t=1105s)
+    [Call of Duty: Vanguard](https://www.callofduty.com/vanguard) by
+    [Sledgehammer Games](https://www.sledgehammergames.com/): I can neither
+    confirm nor deny but there is a license I know in the credits.
+  * Apparently [D&D Dark Alliance](https://darkalliance.wizards.com) by
+    [Wizards of the Coast](https://company.wizards.com): your party, their
+    funeral.
+  * [TiltedOnline](https://github.com/tiltedphoques/TiltedOnline) by
+    [Tilted Phoques](https://github.com/tiltedphoques): Skyrim and Fallout 4 mod
+    to play online.
+  * [Antkeeper](https://github.com/antkeeper/antkeeper-source): an ant colony
+    simulation [game](https://antkeeper.com/).
+  * [Openblack](https://github.com/openblack/openblack): open source
+    reimplementation of the game _Black & White_ (2001).
+  * [Land of the Rair](https://github.com/LandOfTheRair/core2): the new backend
+    of [a retro-style MUD](https://rair.land/) for the new age.
+  * [Face Smash](https://play.google.com/store/apps/details?id=com.gamee.facesmash):
+    a game to play with your face.
+  * [EnTT Pacman](https://github.com/Kerndog73/EnTT-Pacman): an example of how
+    to make Pacman with `EnTT`.
+  * [Wacman](https://github.com/carlfindahl/wacman): a pacman clone with OpenGL.
+  * [Classic Tower Defence](https://github.com/kerndog73/Classic-Tower-Defence):
+    a tiny little tower defence game featuring a homemade font.
+    [Check it out](https://indi-kernick.itch.io/classic-tower-defence).
+  * [The Machine](https://github.com/Kerndog73/The-Machine): a box pushing
+    puzzler with logic gates and other cool stuff.
+    [Check it out](https://indi-kernick.itch.io/the-machine-web-version).
+  * [EnTTPong](https://github.com/DomRe/EnttPong): a basic game made to showcase
+    different parts of `EnTT` and C++17.
+  * [Randballs](https://github.com/gale93/randballs): simple `SFML` and `EnTT`
+    playground.
+  * [EnTT Tower Defense](https://github.com/Daivuk/tddod): a data oriented tower
+    defense example.
+  * [EnTT Breakout](https://github.com/vblanco20-1/entt-breakout): simple
+    example of a breakout game, using `SDL` and `EnTT`.
+  * [Arcade puzzle game with EnTT](https://github.com/MasonRG/ArcadePuzzleGame):
+    arcade puzzle game made in C++ using the `SDL2` and `EnTT` libraries.
+  * [Snake with EnTT](https://github.com/MasonRG/SnakeGame): simple snake game
+    made in C++ with the `SDL2` and `EnTT` libraries.
+  * [Mirrors lasers and robots](https://github.com/guillaume-haerinck/imac-tower-defense):
+    a small tower defense game based on mirror orientation.
+  * [PopHead](https://github.com/SPC-Some-Polish-Coders/PopHead/): 2D, Zombie,
+    RPG game made from scratch in C++.
+  * [Robotligan](https://github.com/Trisslotten/robotligan): multiplayer
+    football game.
+  * [DungeonSlayer](https://github.com/alohaeee/DungeonSlayer): 2D game made
+    from scratch in C++.
+  * [3DGame](https://github.com/kwarkGorny/3DGame): 2.5D top-down space shooter.
+  * [Pulcher](https://github.com/AODQ/pulcher): 2D cross-platform game inspired
+    by Quake.
+  * [Destroid](https://github.com/tyrannicaltoucan/destroid): _one-bazillionth_
+    arcade game about shooting dirty rocks in space, inspired by Asteroids.
+  * [Wanderer](https://github.com/albin-johansson/wanderer): a 2D exploration
+    based indie game.
+  * [Spelunky® Classic remake](https://github.com/dbeef/spelunky-psp): a truly
+    multiplatform experience with a rewrite from scratch.
+  * [CubbyTower](https://github.com/utilForever/CubbyTower): a simple tower
+    defense game using C++ with Entity Component System (ECS).
+  * [Runeterra](https://github.com/utilForever/Runeterra): Legends of Runeterra
+    simulator using C++ with some reinforcement learning.
+  * [Black Sun](https://store.steampowered.com/app/1670930/Black_Sun/): fly your
+    space ship through a large 2D open world.
+  * [PokeMaster](https://github.com/utilForever/PokeMaster): Pokemon Battle
+    simulator using C++ with some reinforcement learning.
+  * [HomeHearth](https://youtu.be/GrEWl8npL9Y): choose your hero, protect the
+    town, before it's too late.
+  * [City Builder Game](https://github.com/PhiGei2000/CityBuilderGame): a simple
+    city-building game using C++ and OpenGL.
+  * [BattleSub](https://github.com/bfeldpw/battlesub): two player 2D submarine
+    game with some fluid dynamics.
+  * [Crimson Rush](https://github.com/WilKam01/LuaCGame): a dungeon-crawler and
+    rougelike inspired game about exploring and surviving as long as possible.
+  * [Space Fight](https://github.com/cholushkin/SpaceFight): one screen
+    multi-player arcade shooter game prototype.
+  * [Confetti Party](https://github.com/hexerei/entt-confetti): C++ sample
+    application as a starting point using `EnTT` and `SDL2`.
+
+* Engines and the like:
+  * [Aether Engine](https://hadean.com/spatial-simulation/)
+    [v1.1+](https://docs.hadean.com/v1.1/Licenses/) by
+    [Hadean](https://hadean.com/): a library designed for spatially partitioning
+    agent-based simulations.
+  * [Fling Engine](https://github.com/flingengine/FlingEngine): a Vulkan game
+    engine with a focus on data oriented design.
+  * [NovusCore](https://github.com/novuscore/NovusCore): a modern take on World
+    of Warcraft emulation.
+  * [Chrysalis](https://github.com/ivanhawkes/Chrysalis): action RPG SDK for
+    CRYENGINE games.
+  * [LM-Engine](https://github.com/Lawrencemm/LM-Engine): the Vim of game
+    engines.
+  * [Edyn](https://github.com/xissburg/edyn): a real-time physics engine
+    organized as an ECS.
+  * [MushMachine](https://github.com/MadeOfJelly/MushMachine): engine...
+    vrooooommm.
+  * [Antara Gaming SDK](https://github.com/KomodoPlatform/antara-gaming-sdk):
+    the Komodo Gaming Software Development Kit.
+  * [XVP](https://ravingbots.com/xvp-expansive-vehicle-physics-for-unreal-engine/):
+    [_eXpansive Vehicle Physics_](https://github.com/raving-bots/xvp/wiki/Plugin-integration-guide)
+    plugin for Unreal Engine.
+  * [Apparently](https://teamwisp.github.io/credits/)
+    [Wisp](https://teamwisp.github.io/product/) by
+    [Team Wisp](https://teamwisp.github.io/): an advanced real-time ray tracing
+    renderer built for the demands of video game artists.
+  * [shiva](https://github.com/Milerius/shiva): modern C++ engine with
+    modularity.
+  * [ImGui/EnTT editor](https://github.com/Green-Sky/imgui_entt_entity_editor):
+    a drop-in, single-file entity editor for `EnTT` that uses `ImGui` as
+    graphical backend (with
+    [demo code](https://github.com/Green-Sky/imgui_entt_entity_editor_demo)).
+  * [SgOgl](https://github.com/stwe/SgOgl): a game engine library for OpenGL
+    developed for educational purposes.
+  * [Lumos](https://github.com/jmorton06/Lumos): game engine written in C++
+    using OpenGL and Vulkan.
+  * [Silvanus](https://github.com/hobbyistmaker/silvanus): Silvanus Fusion 360
+    Box Generator.
+  * [Lina Engine](https://github.com/inanevin/LinaEngine): an open-source,
+    modular, tiny and fast C++ game engine, aimed to develop 3D desktop games.
+  * [Spike](https://github.com/FahimFuad/Spike): a powerful game engine which
+    can run on a toaster.
+  * [Helena Framework](https://github.com/NIKEA-SOFT/HelenaFramework): a modern
+    framework in C++17 for backend development.
+  * [Unity/EnTT](https://github.com/TongTungGiang/unity-entt): tech demo of a
+    native simulation layer using `EnTT` and `Unity` as a rendering engine.
+  * [OverEngine](https://github.com/OverShifted/OverEngine): an over-engineered
+    game engine.
+  * [Electro](https://github.com/Electro-Technologies/Electro): high performance
+    3D game engine with a high emphasis on rendering.
+  * [Kawaii](https://github.com/Mathieu-Lala/Kawaii_Engine): a modern data
+    oriented game engine.
+  * [Becketron](https://github.com/Doctor-Foxling/Becketron): a game engine
+    written mostly in C++.
+  * [Spatial Engine](https://github.com/luizgabriel/Spatial.Engine): a
+    cross-platform engine created on top of google's filament rendering engine.
+  * [Kaguya](https://github.com/KaiH0717/Kaguya): D3D12 Rendering Engine.
+  * [OpenAWE](https://github.com/OpenAWE-Project/OpenAWE): open implementation
+    of the Alan Wake Engine.
+  * [Nazara Engine](https://github.com/DigitalPulseSoftware/NazaraEngine): fast,
+    cross-platform, object-oriented API to help in daily developer life.
+  * [Billy Engine](https://github.com/billy4479/BillyEngine): some kind of a 2D
+    engine based on `SDL2` and `EnTT`.
+  * [Ducktape](https://github.com/DucktapeEngine/Ducktape): an open source C++
+    2D & 3D game engine that focuses on being fast and powerful.
+
+* Articles, videos and blog posts:
+  * [Some posts](https://skypjack.github.io/tags/#entt) on my personal
+    [blog](https://skypjack.github.io/) are about `EnTT`, for those who want to
+    know **more** on this project.
+  * [Game Engine series](https://www.youtube.com/c/TheChernoProject/videos) by
+    [The Cherno](https://github.com/TheCherno) (not only about `EnTT` but also
+    on the use of an ECS in general):
+    - [Intro to EnTT](https://www.youtube.com/watch?v=D4hz0wEB978).
+    - [Entities and Components](https://www.youtube.com/watch?v=-B1iu4QJTUc).
+    - [The ENTITY Class](https://www.youtube.com/watch?v=GfSzeAcsBb0).
+    - [Camera Systems](https://www.youtube.com/watch?v=ubZn7BlrnTU).
+    - [Scene Camera](https://www.youtube.com/watch?v=UKVFRRufKzo).
+    - [Native Scripting](https://www.youtube.com/watch?v=iIUhg88MK5M).
+    - [Native Scripting (now with virtual functions!)](https://www.youtube.com/watch?v=1cHEcrIn8IQ).
+    - [Scene Hierarchy Panel](https://www.youtube.com/watch?v=wziDnE8guvI).
+    - [Properties Panel](https://www.youtube.com/watch?v=NBpB0qscF3E).
+    - [Camera Component UI](https://www.youtube.com/watch?v=RIMt_6agUiU).
+    - [Drawing Component UI](https://www.youtube.com/watch?v=u3yq8s3KuSE).
+    - [Transform Component UI](https://www.youtube.com/watch?v=8JqcXYbzPJc).
+    - [Adding/Removing Entities and Components UI](https://www.youtube.com/watch?v=PsyGmsIgp9M).
+    - [Saving and Loading Scenes](https://www.youtube.com/watch?v=IEiOP7Y-Mbc).
+    - ... And so on.
+      [Check out](https://www.youtube.com/channel/UCQ-W1KE9EYfdxhL6S4twUNw) the
+      _Game Engine Series_ by The Cherno for more videos.
+  * [Space Battle: Huge edition](http://victor.madtriangles.com/code%20experiment/2018/06/11/post-ecs-battle-huge.html):
+    huge space battle built entirely from scratch.
+  * [Space Battle](https://github.com/vblanco20-1/ECS_SpaceBattle): huge space
+    battle built on `UE4`.
+  * [Experimenting with ECS in UE4](http://victor.madtriangles.com/code%20experiment/2018/03/25/post-ue4-ecs-battle.html):
+    interesting article about `UE4` and `EnTT`.
+  * [Implementing ECS architecture in UE4](https://forums.unrealengine.com/development-discussion/c-gameplay-programming/1449913-implementing-ecs-architecture-in-ue4-giant-space-battle):
+    giant space battle.
+  * [Conan Adventures (SFML and EnTT in C++)](https://leinnan.github.io/blog/conan-adventuressfml-and-entt-in-c.html):
+    create projects in modern C++ using `SFML`, `EnTT`, `Conan` and `CMake`.
+  * [Adding EnTT ECS to Chrysalis](https://www.tauradius.com/post/adding-an-ecs-to-chrysalis/):
+    a blog entry (and its 
+    [follow-up](https://www.tauradius.com/post/chrysalis-update-2020-08-02/)) 
+    about the integration of `EnTT` into `Chrysalis`, an action RPG SDK for
+    CRYENGINE games.
+  * [Creating Minecraft in One Week with C++ and Vulkan](https://vazgriz.com/189/creating-minecraft-in-one-week-with-c-and-vulkan/):
+    a crack at recreating Minecraft in one week using a custom C++ engine and
+    Vulkan ([code included](https://github.com/vazgriz/VoxelGame)).
+  * [Ability Creator](https://www.erichildebrand.net/blog/ability-creator-project-retrospect):
+    project retrospect by [Eric Hildebrand](https://www.erichildebrand.net/).
+  * [EnTT Entity Component System Gaming Library](https://gamefromscratch.com/entt-entity-component-system-gaming-library/):
+    `EnTT` on GameFromScratch.com.
+  * [Custom C++ server for UE5](https://youtu.be/fbXZVNCOvjM) optimized for
+    MMO(RPG)s and its [follow-up](https://youtu.be/yGlZeopx2hU) episode about
+    player bots and full external ECS: a series definitely worth looking at.
+
+* Any Other Business:
+  * [ArcGIS Runtime SDKs](https://developers.arcgis.com/arcgis-runtime/) by
+    [Esri](https://www.esri.com/): they use `EnTT` for the internal ECS and the
+    cross platform C++ rendering engine. The SDKs are utilized by a lot of
+    enterprise custom apps, as well as by Esri for its own public applications
+    such as
+    [Explorer](https://play.google.com/store/apps/details?id=com.esri.explorer),
+    [Collector](https://play.google.com/store/apps/details?id=com.esri.arcgis.collector)
+    and
+    [Navigator](https://play.google.com/store/apps/details?id=com.esri.navigator).
+  * [FASTSUITE Edition 2](https://www.fastsuite.com/en_EN/fastsuite/fastsuite-edition-2.html)
+    by [Cenit](http://www.cenit.com/en_EN/about-us/overview.html): they use
+    `EnTT` to drive their simulation, that is, the communication between robot
+    controller emulator and renderer.
+  * [Ragdoll](https://ragdolldynamics.com/): real-time physics for Autodesk Maya
+    2020.
+  * [Project Lagrange](https://github.com/adobe/lagrange): a robust geometry
+    processing library by [Adobe](https://github.com/adobe).
+  * [AtomicDEX](https://github.com/KomodoPlatform/atomicDEX-Desktop): a secure
+    wallet and non-custodial decentralized exchange rolled into one application.
+  * [Apparently](https://www.linkedin.com/in/skypjack/)
+    [NIO](https://www.nio.io/): there was a collaboration to make some changes
+    to `EnTT`, at the time used for internal projects.
+  * [Apparently](https://www.linkedin.com/jobs/view/architekt-c%2B%2B-at-tieto-1219512333/)
+    [Tieto](https://www.tieto.com/): they published a job post where `EnTT` was
+    listed on their software stack.
+  * [Sequentity](https://github.com/alanjfs/sequentity): A MIDI-like
+    sequencer/tracker for C++ and `ImGui` (with `Magnum` and `EnTT`).
+  * [EnTT meets Sol2](https://github.com/skaarj1989/entt-meets-sol2): freely
+    available examples of how to combine `EnTT` and `Sol2`.
+  * [Godot meets EnTT](https://github.com/portaloffreedom/godot_entt_example/):
+    a simple example on how to use `EnTT` within
+    [`Godot`](https://godotengine.org/).
+  * [Godot and GameNetworkingSockets meet EnTT](https://github.com/portaloffreedom/godot_entt_net_example):
+    a simple example on how to use `EnTT` and
+    [`GameNetworkingSockets`](https://github.com/ValveSoftware/GameNetworkingSockets)
+    within [`Godot`](https://godotengine.org/).
+  * [MatchOneEntt](https://github.com/mhaemmerle/MatchOneEntt): port of
+    [Match One](https://github.com/sschmid/Match-One) for `Entitas-CSharp`.
+  * GitHub contains also
+    [many other examples](https://github.com/search?o=desc&q=%22skypjack%2Fentt%22&s=indexed&type=Code)
+    of use of `EnTT` from which to take inspiration if interested.
+
+If you know of other resources out there that are about `EnTT`, feel free to
+open an issue or a PR and I'll be glad to add them to this page.

docs/md/locator.md

@@ -0,0 +1,88 @@
+# Crash Course: service locator
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Service locator](#service-locator)
+  * [Opaque handles](#opaque-handles)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Usually, service locators are tightly bound to the services they expose and it's
+hard to define a general purpose solution.<br/>
+This tiny class tries to fill the gap and gets rid of the burden of defining a
+different specific locator for each application.
+
+# Service locator
+
+The service locator API tries to mimic that of `std::optional` and adds some
+extra functionalities on top of it such as allocator support.<br/>
+There are a couple of functions to set up a service, namely `emplace` and
+`allocate_emplace`:
+
+```cpp
+entt::locator<interface>::emplace<service>(argument);
+entt::locator<interface>::allocate_emplace<service>(allocator, argument);
+```
+
+The difference is that the latter expects an allocator as the first argument and
+uses it to allocate the service itself.<br/>
+Once a service is set up, it's retrieved using the `value` function:
+
+```cpp
+interface &service = entt::locator<interface>::value();
+```
+
+Since the service may not be set (and therefore this function may result in an
+undefined behavior), the `has_value` and `value_or` functions are also available
+to test a service locator and to get a fallback service in case there is none:
+
+```cpp
+if(entt::locator<interface>::has_value()) {
+    // ...
+}
+
+interface &service = entt::locator<interface>::value_or<fallback_impl>(argument);
+```
+
+All arguments are used only if necessary, that is, if a service doesn't already
+exist and therefore the fallback service is constructed and returned. In all
+other cases, they are discarded.<br/>
+Finally, to reset a service, use the `reset` function.
+
+## Opaque handles
+
+Sometimes it's useful to _transfer_ a copy of a service to another locator. For
+example, when working across boundaries it's common to _share_ a service with a
+dynamically loaded module.<br/>
+Options aren't much in this case. Among these is the possibility of _exporting_
+services and assigning them to a different locator.
+
+This is what the `handle` and `reset` functions are meant for.<br/>
+The former returns an opaque object useful for _exporting_ (or rather, obtaining
+a reference to) a service. The latter also accepts an optional argument to a
+handle which then allows users to reset a service by initializing it with an
+opaque handle:
+
+```cpp
+auto handle = entt::locator<interface>::handle();
+entt::locator<interface>::reset(handle);
+```
+
+It's worth noting that it's possible to get handles for uninitialized services
+and use them with other locators. Of course, all a user will get is to have an
+uninitialized service elsewhere as well.
+
+Note that exporting a service allows users to _share_ the object currently set
+in a locator. Replacing it won't replace the element even where a service has
+been configured with a handle to the previous item.<br/>
+In other words, if an audio service is replaced with a null object to silence an
+application and the original service was shared, this operation won't propagate
+to the other locators. Therefore, a module that share the ownership of the
+original audio service is still able to emit sounds.

docs/md/poly.md

@@ -0,0 +1,359 @@
+# Crash Course: poly
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+  * [Other libraries](#other-libraries)
+* [Concept and implementation](#concept-and-implementation)
+  * [Deduced interface](#deduced-interface)
+  * [Defined interface](#defined-interface)
+  * [Fulfill a concept](#fulfill-a-concept)
+* [Inheritance](#inheritance)
+* [Static polymorphism in the wild](#static-polymorphism-in-the-wild)
+* [Storage size and alignment requirement](#storage-size-and-alignment-requirement)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Static polymorphism is a very powerful tool in C++, albeit sometimes cumbersome
+to obtain.<br/>
+This module aims to make it simple and easy to use.
+
+The library allows to define _concepts_ as interfaces to fulfill with concrete
+classes without having to inherit from a common base.<br/>
+This is, among others, one of the advantages of static polymorphism in general
+and of a generic wrapper like that offered by the `poly` class template in
+particular.<br/>
+What users get is an object that can be passed around as such and not through a
+reference or a pointer, as happens when it comes to working with dynamic
+polymorphism.
+
+Since the `poly` class template makes use of `entt::any` internally, it also
+supports most of its feature. Among the most important, the possibility to
+create aliases to existing and thus unmanaged objects. This allows users to
+exploit the static polymorphism while maintaining ownership of objects.<br/>
+Likewise, the `poly` class template also benefits from the small buffer
+optimization offered by the `entt::any` class and therefore minimizes the number
+of allocations, avoiding them altogether where possible.
+
+## Other libraries
+
+There are some very interesting libraries regarding static polymorphism.<br/>
+Among all, the two that I prefer are:
+
+* [`dyno`](https://github.com/ldionne/dyno): runtime polymorphism done right.
+* [`Poly`](https://github.com/facebook/folly/blob/master/folly/docs/Poly.md):
+  a class template that makes it easy to define a type-erasing polymorphic
+  object wrapper.
+
+The former is admittedly an experimental library, with many interesting ideas.
+I've some doubts about the usefulness of some feature in real world projects,
+but perhaps my lack of experience comes into play here. In my opinion, its only
+flaw is the API which I find slightly more cumbersome than other solutions.<br/>
+The latter was undoubtedly a source of inspiration for this module, although I
+opted for different choices in the implementation of both the final API and some
+feature.
+
+Either way, the authors are gurus of the C++ community, people I only have to
+learn from.
+
+# Concept and implementation
+
+The first thing to do to create a _type-erasing polymorphic object wrapper_ (to
+use the terminology introduced by Eric Niebler) is to define a _concept_ that
+types will have to adhere to.<br/>
+For this purpose, the library offers a single class that supports both deduced
+and fully defined interfaces. Although having interfaces deduced automatically
+is convenient and allows users to write less code in most cases, this has some
+limitations and it's therefore useful to be able to get around the deduction by
+providing a custom definition for the static virtual table.
+
+Once the interface is defined, it will be sufficient to provide a generic
+implementation to fulfill the concept.<br/>
+Also in this case, the library allows customizations based on types or families
+of types, so as to be able to go beyond the generic case where necessary.
+
+## Deduced interface
+
+This is how a concept with a deduced interface is introduced:
+
+```cpp
+struct Drawable: entt::type_list<> {
+    template<typename Base>
+    struct type: Base {
+        void draw() { this->template invoke<0>(*this); }
+    };
+
+    // ...
+};
+```
+
+It's recognizable by the fact that it inherits from an empty type list.<br/>
+Functions can also be const, accept any number of parameters and return a type
+other than `void`:
+
+```cpp
+struct Drawable: entt::type_list<> {
+    template<typename Base>
+    struct type: Base {
+        bool draw(int pt) const { return this->template invoke<0>(*this, pt); }
+    };
+
+    // ...
+};
+```
+
+In this case, all parameters must be passed to `invoke` after the reference to
+`this` and the return value is whatever the internal call returns.<br/>
+As for `invoke`, this is a name that is injected into the _concept_ through
+`Base`, from which one must necessarily inherit. Since it's also a dependent
+name, the `this-> template` form is unfortunately necessary due to the rules of
+the language. However, there exists also an alternative that goes through an
+external call:
+
+```cpp
+struct Drawable: entt::type_list<> {
+    template<typename Base>
+    struct type: Base {
+        void draw() const { entt::poly_call<0>(*this); }
+    };
+
+    // ...
+};
+```
+
+Once the _concept_ is defined, users must provide a generic implementation of it
+in order to tell the system how any type can satisfy its requirements. This is
+done via an alias template within the concept itself.<br/>
+The index passed as a template parameter to either `invoke` or `poly_call`
+refers to how this alias is defined.
+
+## Defined interface
+
+A fully defined concept is no different to one for which the interface is
+deduced, with the only difference that the list of types is not empty this time:
+
+```cpp
+struct Drawable: entt::type_list<void()> {
+    template<typename Base>
+    struct type: Base {
+        void draw() { entt::poly_call<0>(*this); }
+    };
+
+    // ...
+};
+```
+
+Again, parameters and return values other than `void` are allowed. Also, the
+function type must be const when the method to bind to it is const:
+
+```cpp
+struct Drawable: entt::type_list<bool(int) const> {
+    template<typename Base>
+    struct type: Base {
+        bool draw(int pt) const { return entt::poly_call<0>(*this, pt); }
+    };
+
+    // ...
+};
+```
+
+Why should a user fully define a concept if the function types are the same as
+the deduced ones?<br>
+Because, in fact, this is exactly the limitation that can be worked around by
+manually defining the static virtual table.
+
+When things are deduced, there is an implicit constraint.<br/>
+If the concept exposes a member function called `draw` with function type
+`void()`, a concept can be satisfied:
+
+* Either by a class that exposes a member function with the same name and the
+  same signature.
+
+* Or through a lambda that makes use of existing member functions from the
+  interface itself.
+
+In other words, it's not possible to make use of functions not belonging to the
+interface, even if they are present in the types that fulfill the concept.<br/>
+Similarly, it's not possible to deduce a function in the static virtual table
+with a function type different from that of the associated member function in
+the interface itself.
+
+Explicitly defining a static virtual table suppresses the deduction step and
+allows maximum flexibility when providing the implementation for a concept.
+
+## Fulfill a concept
+
+The `impl` alias template of a concept is used to define how it's fulfilled:
+
+```cpp
+struct Drawable: entt::type_list<> {
+    // ...
+
+    template<typename Type>
+    using impl = entt::value_list<&Type::draw>;
+};
+```
+
+In this case, it's stated that the `draw` method of a generic type will be
+enough to satisfy the requirements of the `Drawable` concept.<br/>
+Both member functions and free functions are supported to fulfill concepts:
+
+```cpp
+template<typename Type>
+void print(Type &self) { self.print(); }
+
+struct Drawable: entt::type_list<void()> {
+    // ...
+
+    template<typename Type>
+    using impl = entt::value_list<&print<Type>>;
+};
+```
+
+Likewise, as long as the parameter types and return type support conversions to
+and from those of the function type referenced in the static virtual table, the
+actual implementation may differ in its function type since it's erased
+internally.<br/>
+Moreover, the `self` parameter isn't strictly required by the system and can be
+left out for free functions if not required.
+
+Refer to the inline documentation for more details.
+
+# Inheritance
+
+_Concept inheritance_ is straightforward due to how poly looks like in `EnTT`.
+Therefore, it's quite easy to build hierarchies of concepts if necessary.<br/>
+The only constraint is that all concepts in a hierarchy must belong to the same
+_family_, that is, they must be either all deduced or all defined.
+
+For a deduced concept, inheritance is achieved in a few steps:
+
+```cpp
+struct DrawableAndErasable: entt::type_list<> {
+    template<typename Base>
+    struct type: typename Drawable::template type<Base> {
+        static constexpr auto base = std::tuple_size_v<typename entt::poly_vtable<Drawable>::type>;
+        void erase() { entt::poly_call<base + 0>(*this); }
+    };
+
+    template<typename Type>
+    using impl = entt::value_list_cat_t<
+        typename Drawable::impl<Type>,
+        entt::value_list<&Type::erase>
+    >;
+};
+```
+
+The static virtual table is empty and must remain so.<br/>
+On the other hand, `type` no longer inherits from `Base` and instead forwards
+its template parameter to the type exposed by the _base class_. Internally, the
+size of the static virtual table of the base class is used as an offset for the
+local indexes.<br/>
+Finally, by means of the `value_list_cat_t` utility, the implementation consists
+in appending the new functions to the previous list.
+
+As for a defined concept instead, also the list of types must be extended, in a
+similar way to what is shown for the implementation of the above concept.<br/>
+To do this, it's useful to declare a function that allows to convert a _concept_
+into its underlying `type_list` object:
+
+```cpp
+template<typename... Type>
+entt::type_list<Type...> as_type_list(const entt::type_list<Type...> &);
+```
+
+The definition isn't strictly required, since the function will only be used
+through a `decltype` as it follows:
+
+```cpp
+struct DrawableAndErasable: entt::type_list_cat_t<
+    decltype(as_type_list(std::declval<Drawable>())),
+    entt::type_list<void()>
+> {
+    // ...
+};
+```
+
+Similar to above, `type_list_cat_t` is used to concatenate the underlying static
+virtual table with the new function types.<br/>
+Everything else is the same as already shown instead.
+
+# Static polymorphism in the wild
+
+Once the _concept_ and implementation have been introduced, it will be possible
+to use the `poly` class template to contain instances that meet the
+requirements:
+
+```cpp
+using drawable = entt::poly<Drawable>;
+
+struct circle {
+    void draw() { /* ... */ }
+};
+
+struct square {
+    void draw() { /* ... */ }
+};
+
+// ...
+
+drawable instance{circle{}};
+instance->draw();
+
+instance = square{};
+instance->draw();
+```
+
+The `poly` class template offers a wide range of constructors, from the default
+one (which will return an uninitialized `poly` object) to the copy and move
+constructors, as well as the ability to create objects in-place.<br/>
+Among others, there is also a constructor that allows users to wrap unmanaged
+objects in a `poly` instance (either const or non-const ones):
+
+```cpp
+circle shape;
+drawable instance{std::in_place_type<circle &>, shape};
+```
+
+Similarly, it's possible to create non-owning copies of `poly` from an existing
+object:
+
+```cpp
+drawable other = instance.as_ref();
+```
+
+In both cases, although the interface of the `poly` object doesn't change, it
+won't construct any element or take care of destroying the referenced objects.
+
+Note also how the underlying concept is accessed via a call to `operator->` and
+not directly as `instance.draw()`.<br/>
+This allows users to decouple the API of the wrapper from that of the concept.
+Therefore, where `instance.data()` will invoke the `data` member function of the
+poly object, `instance->data()` will map directly to the functionality exposed
+by the underlying concept.
+
+# Storage size and alignment requirement
+
+Under the hood, the `poly` class template makes use of `entt::any`. Therefore,
+it can take advantage of the possibility of defining at compile-time the size of
+the storage suitable for the small buffer optimization as well as the alignment
+requirements:
+
+```cpp
+entt::basic_poly<Drawable, sizeof(double[4]), alignof(double[4])>
+```
+
+The default size is `sizeof(double[2])`, which seems like a good compromise
+between a buffer that is too large and one unable to hold anything larger than
+an integer. The alignment requirement is optional instead and by default such
+that it's the most stringent (the largest) for any object whose size is at most
+equal to the one provided.<br/>
+It's worth noting that providing a size of 0 (which is an accepted value in all
+respects) will force the system to dynamically allocate the contained objects in
+all cases.

docs/md/process.md

@@ -0,0 +1,212 @@
+# Crash Course: cooperative scheduler
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [The process](#the-process)
+  * [Adaptor](#adaptor)
+* [The scheduler](#the-scheduler)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Sometimes processes are a useful tool to work around the strict definition of a
+system and introduce logic in a different way, usually without resorting to the
+introduction of other components.
+
+`EnTT` offers a minimal support to this paradigm by introducing a few classes
+that users can use to define and execute cooperative processes.
+
+# The process
+
+A typical process must inherit from the `process` class template that stays true
+to the CRTP idiom. Moreover, derived classes must specify what's the intended
+type for elapsed times.
+
+A process should expose publicly the following member functions whether needed
+(note that it isn't required to define a function unless the derived class wants
+to _override_ the default behavior):
+
+* `void update(Delta, void *);`
+
+  It's invoked once per tick until a process is explicitly aborted or it
+  terminates either with or without errors. Even though it's not mandatory to
+  declare this member function, as a rule of thumb each process should at
+  least define it to work properly. The `void *` parameter is an opaque pointer
+  to user data (if any) forwarded directly to the process during an update.
+
+* `void init();`
+
+  It's invoked when the process joins the running queue of a scheduler. This
+  happens as soon as it's attached to the scheduler if the process is a top
+  level one, otherwise when it replaces its parent if the process is a
+  continuation.
+
+* `void succeeded();`
+
+  It's invoked in case of success, immediately after an update and during the
+  same tick.
+
+* `void failed();`
+
+  It's invoked in case of errors, immediately after an update and during the
+  same tick.
+
+* `void aborted();`
+
+  It's invoked only if a process is explicitly aborted. There is no guarantee
+  that it executes in the same tick, this depends solely on whether the
+  process is aborted immediately or not.
+
+Derived classes can also change the internal state of a process by invoking
+`succeed` and `fail`, as well as `pause` and `unpause` the process itself. All
+these are protected member functions made available to be able to manage the
+life cycle of a process from a derived class.
+
+Here is a minimal example for the sake of curiosity:
+
+```cpp
+struct my_process: entt::process<my_process, std::uint32_t> {
+    using delta_type = std::uint32_t;
+
+    my_process(delta_type delay)
+        : remaining{delay}
+    {}
+
+    void update(delta_type delta, void *) {
+        remaining -= std::min(remaining, delta);
+
+        // ...
+
+        if(!remaining) {
+            succeed();
+        }
+    }
+
+private:
+    delta_type remaining;
+};
+```
+
+## Adaptor
+
+Lambdas and functors can't be used directly with a scheduler for they are not
+properly defined processes with managed life cycles.<br/>
+This class helps in filling the gap and turning lambdas and functors into
+full-featured processes usable by a scheduler.
+
+The function call operator has a signature similar to the one of the `update`
+function of a process but for the fact that it receives two extra arguments to
+call whenever a process is terminated with success or with an error:
+
+```cpp
+void(Delta delta, void *data, auto succeed, auto fail);
+```
+
+Parameters have the following meaning:
+
+* `delta` is the elapsed time.
+* `data` is an opaque pointer to user data if any, `nullptr` otherwise.
+* `succeed` is a function to call when a process terminates with success.
+* `fail` is a function to call when a process terminates with errors.
+
+Both `succeed` and `fail` accept no parameters at all.
+
+Note that usually users shouldn't worry about creating adaptors at all. A
+scheduler creates them internally each and every time a lambda or a functor is
+used as a process.
+
+# The scheduler
+
+A cooperative scheduler runs different processes and helps managing their life
+cycles.
+
+Each process is invoked once per tick. If it terminates, it's removed
+automatically from the scheduler and it's never invoked again. Otherwise it's
+a good candidate to run one more time the next tick.<br/>
+A process can also have a child. In this case, the parent process is replaced
+with its child when it terminates and only if it returns with success. In case
+of errors, both the parent process and its child are discarded. This way, it's
+easy to create chain of processes to run sequentially.
+
+Using a scheduler is straightforward. To create it, users must provide only the
+type for the elapsed times and no arguments at all:
+
+```cpp
+entt::scheduler<std::uint32_t> scheduler;
+```
+
+It has member functions to query its internal data structures, like `empty` or
+`size`, as well as a `clear` utility to reset it to a clean state:
+
+```cpp
+// checks if there are processes still running
+const auto empty = scheduler.empty();
+
+// gets the number of processes still running
+entt::scheduler<std::uint32_t>::size_type size = scheduler.size();
+
+// resets the scheduler to its initial state and discards all the processes
+scheduler.clear();
+```
+
+To attach a process to a scheduler there are mainly two ways:
+
+* If the process inherits from the `process` class template, it's enough to
+  indicate its type and submit all the parameters required to construct it to
+  the `attach` member function:
+
+  ```cpp
+  scheduler.attach<my_process>(1000u);
+  ```
+
+* Otherwise, in case of a lambda or a functor, it's enough to provide an
+  instance of the class to the `attach` member function:
+
+  ```cpp
+  scheduler.attach([](auto...){ /* ... */ });
+  ```
+
+In both cases, the return value is an opaque object that offers a `then` member
+function to use to create chains of processes to run sequentially.<br/>
+As a minimal example of use:
+
+```cpp
+// schedules a task in the form of a lambda function
+scheduler.attach([](auto delta, void *, auto succeed, auto fail) {
+    // ...
+})
+// appends a child in the form of another lambda function
+.then([](auto delta, void *, auto succeed, auto fail) {
+    // ...
+})
+// appends a child in the form of a process class
+.then<my_process>(1000u);
+```
+
+To update a scheduler and therefore all its processes, the `update` member
+function is the way to go:
+
+```cpp
+// updates all the processes, no user data are provided
+scheduler.update(delta);
+
+// updates all the processes and provides them with custom data
+scheduler.update(delta, &data);
+```
+
+In addition to these functions, the scheduler offers an `abort` member function
+that can be used to discard all the running processes at once:
+
+```cpp
+// aborts all the processes abruptly ...
+scheduler.abort(true);
+
+// ... or gracefully during the next tick
+scheduler.abort();
+```

docs/md/reference.md

@@ -0,0 +1,75 @@
+# Similar projects
+
+There are many projects similar to `EnTT`, both open source and not.<br/>
+Some even borrowed some ideas from this library and expressed them in different
+languages.<br/>
+Others developed different architectures from scratch and therefore offer
+alternative solutions with their pros and cons.
+
+Below an incomplete list of those that I've come across so far.<br/>
+If some terms or designs aren't clear, I recommend referring to the
+[_ECS Back and Forth_](https://skypjack.github.io/tags/#ecs) series for all the
+details.
+
+I hope this list can grow much more in the future:
+
+* C:
+  * [destral_ecs](https://github.com/roig/destral_ecs): a single-file ECS based
+    on sparse sets.
+  * [Diana](https://github.com/discoloda/Diana): an ECS that uses sparse sets to
+    keep track of entities in systems.
+  * [Flecs](https://github.com/SanderMertens/flecs): a multithreaded archetype
+    ECS based on semi-contiguous arrays rather than chunks.
+  * [lent](https://github.com/nem0/lent): the Donald Trump of the ECS libraries.
+
+* C++:
+  * [decs](https://github.com/vblanco20-1/decs): a chunk based archetype ECS.
+  * [ecst](https://github.com/SuperV1234/ecst): a multithreaded compile-time
+    ECS that uses sparse sets to keep track of entities in systems.
+  * [EntityX](https://github.com/alecthomas/entityx): a bitset based ECS that
+    uses a single large matrix of components indexed with entities.
+  * [Gaia-ECS](https://github.com/richardbiely/gaia-ecs): a chunk based
+    archetype ECS.
+  * [Polypropylene](https://github.com/pmbittner/Polypropylene): a hybrid
+    solution between an ECS and dynamic mixins.
+
+* C#
+  * [Entitas](https://github.com/sschmid/Entitas-CSharp): the ECS framework for
+    C# and Unity, where _reactive systems_ were invented.
+  * [LeoECS](https://github.com/Leopotam/ecs): simple lightweight C# Entity
+    Component System framework.
+  * [Svelto.ECS](https://github.com/sebas77/Svelto.ECS): a very interesting
+    platform agnostic and table based ECS framework.
+
+* Go:
+  * [gecs](https://github.com/tutumagi/gecs): a sparse sets based ECS inspired 
+    by `EnTT`.
+
+* Javascript:
+  * [\@javelin/ecs](https://github.com/3mcd/javelin/tree/master/packages/ecs):
+    an archetype ECS in TypeScript.
+  * [ecsy](https://github.com/MozillaReality/ecsy): I haven't had the time to
+    investigate the underlying design of `ecsy` but it looks cool anyway.
+
+* Perl:
+  * [Game::Entities](https://gitlab.com/jjatria/perl-game-entities): a simple
+    entity registry for ECS designs inspired by `EnTT`.
+
+* Raku:
+  * [Game::Entities](https://gitlab.com/jjatria/raku-game-entities): a simple
+    entity registry for ECS designs inspired by `EnTT`.
+
+* Rust:
+  * [Legion](https://github.com/TomGillen/legion): a chunk based archetype ECS.
+  * [Shipyard](https://github.com/leudz/shipyard): it borrows some ideas from
+    `EnTT` and offers a sparse sets based ECS with grouping functionalities.
+  * [Sparsey](https://github.com/LechintanTudor/sparsey): sparse set based ECS
+    written in Rust.
+  * [Specs](https://github.com/amethyst/specs): a parallel ECS based mainly on
+    hierarchical bitsets that allows different types of storage as needed.
+
+* Zig
+  * [zig-ecs](https://github.com/prime31/zig-ecs): a _zig-ification_ of `EnTT`.
+
+If you know of other resources out there that can be of interest for the reader,
+feel free to open an issue or a PR and I'll be glad to add them to this page.

docs/md/resource.md

@@ -0,0 +1,191 @@
+# Crash Course: resource management
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [The resource, the loader and the cache](#the-resource-the-loader-and-the-cache)
+  * [Resource handle](#resource-handle)
+  * [Loaders](#loader)
+  * [The cache class](#the-cache)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Resource management is usually one of the most critical parts of a game.
+Solutions are often tuned to the particular application. There exist several
+approaches and all of them are perfectly fine as long as they fit the
+requirements of the piece of software in which they are used.<br/>
+Examples are loading everything on start, loading on request, predictive
+loading, and so on.
+
+`EnTT` doesn't pretend to offer a _one-fits-all_ solution for the different
+cases.<br/>
+Instead, the library comes with a minimal, general purpose resource cache that
+might be useful in many cases.
+
+# The resource, the loader and the cache
+
+Resource, loader and cache are the three main actors for the purpose.<br/>
+The _resource_ is an image, an audio, a video or any other type:
+
+```cpp
+struct my_resource { const int value; };
+```
+
+The _loader_ is a callable type the aim of which is to load a specific resource:
+
+```cpp
+struct my_loader final {
+    using result_type = std::shared_ptr<my_resource>;
+
+    result_type operator()(int value) const {
+        // ...
+        return std::make_shared<my_resource>(value);
+    }
+};
+```
+
+Its function operator can accept any arguments and should return a value of the
+declared result type (`std::shared_ptr<my_resource>` in the example).<br/>
+A loader can also overload its function call operator to make it possible to
+construct the same or another resource from different lists of arguments.
+
+Finally, a cache is a specialization of a class template tailored to a specific
+resource and (optionally) a loader:
+
+```cpp
+using my_cache = entt::resource_cache<my_resource, my_loader>;
+
+// ...
+
+my_cache cache{};
+```
+
+The class is designed to create different caches for different resource types
+and to manage each one independently in the most appropriate way.<br/>
+As a (very) trivial example, audio tracks can survive in most of the scenes of
+an application while meshes can be associated with a single scene only, then
+discarded when a player leaves it.
+
+## Resource handle
+
+Resources aren't returned directly to the caller. Instead, they are wrapped in a
+_resource handle_, an instance of the `entt::resource` class template.<br/>
+For those who know the _flyweight design pattern_ already, that's exactly what
+it is. To all others, this is the time to brush up on some notions instead.
+
+A shared pointer could have been used as a resource handle. In fact, the default
+implementation mostly maps the interface of its standard counterpart and only
+adds a few things on top of it.<br/>
+However, the handle in `EnTT` is designed as a standalone class template. This
+is due to the fact that specializing a class in the standard library is often
+undefined behavior while having the ability to specialize the handle for one,
+more or all resource types could help over time.
+
+## Loaders
+
+A loader is responsible for _loading_ resources (quite obviously).<br/>
+By default, it's just a callable object that forwards its arguments to the
+resource itself. That is, a _passthrough type_. All the work is demanded to the
+constructor(s) of the resource itself.<br/>
+Loaders also are fully customizable as expected.
+
+A custom loader is a class with at least one function call operator and a member
+type named `result_type`.<br/>
+The loader isn't required to return a resource handle. As long as `return_type`
+is suitable for constructing a handle, that's fine.
+
+When using the default handle, it expects a resource type which is convertible
+to or suitable for constructing an `std::shared_ptr<Type>` (where `Type` is the
+actual resource type).<br/>
+In other terms, the loader should return shared pointers to the given resource
+type. However, this isn't mandatory. Users can easily get around this constraint
+by specializing both the handle and the loader.
+
+A cache forwards all its arguments to the loader if required. This means that
+loaders can also support tag dispatching to offer different loading policies:
+
+```cpp
+struct my_loader {
+    using result_type = std::shared_ptr<my_resource>;
+
+    struct from_disk_tag{};
+    struct from_network_tag{};
+
+    template<typename Args>
+    result_type operator()(from_disk_tag, Args&&... args) {
+        // ...
+        return std::make_shared<my_resource>(std::forward<Args>(args)...);
+    }
+
+    template<typename Args>
+    result_type operator()(from_network_tag, Args&&... args) {
+        // ...
+        return std::make_shared<my_resource>(std::forward<Args>(args)...);
+    }
+}
+```
+
+This makes the whole loading logic quite flexible and easy to extend over time.
+
+## The cache class
+
+The cache is the class that is asked to _connect the dots_.<br/>
+It loads the resources, stores them aside and returns handles as needed:
+
+```cpp
+entt::resource_cache<my_resource, my_loader> cache{};
+```
+
+Under the hood, a cache is nothing more than a map where the key value has type
+`entt::id_type` while the mapped value is whatever type its loader returns.<br/>
+For this reason, it offers most of the functionalities a user would expect from
+a map, such as `empty` or `size` and so on. Similarly, it's an iterable type
+that also supports indexing by resource id:
+
+```cpp
+for(auto [id, res]: cache) {
+    // ...
+}
+
+if(entt::resource<my_resource> res = cache["resource/id"_hs]; res) {
+    // ...
+}
+```
+
+Please, refer to the inline documentation for all the details about the other
+functions (such as `contains` or `erase`).
+
+Set aside the part of the API that this class _shares_ with a map, it also adds
+something on top of it in order to address the most common requirements of a
+resource cache.<br/>
+In particular, it doesn't have an `emplace` member function which is replaced by
+`load` and `force_load` instead (where the former loads a new resource only if
+not present while the second triggers a forced loading in any case):
+
+```cpp
+auto ret = cache.load("resource/id"_hs);
+
+// true only if the resource was not already present
+const bool loaded = ret.second;
+
+// takes the resource handle pointed to by the returned iterator
+entt::resource<my_resource> res = ret.first->second;
+```
+
+Note that the hashed string is used for convenience in the example above.<br/>
+Resource identifiers are nothing more than integral values. Therefore, plain
+numbers as well as non-class enum value are accepted.
+
+It's worth mentioning that the iterators of a cache as well as its indexing
+operators return resource handles rather than instances of the mapped type.<br/>
+Since the cache has no control over the loader and a resource isn't required to
+also be convertible to bool, these handles can be invalid. This usually means an
+error in the user logic but it may also be an _expected_ event.<br/>
+It's therefore recommended to verify handles validity with a check in debug (for
+example, when loading) or an appropriate logic in retail.

docs/md/signal.md

@@ -0,0 +1,549 @@
+# Crash Course: events, signals and everything in between
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Introduction](#introduction)
+* [Delegate](#delegate)
+  * [Runtime arguments](#runtime-arguments)
+  * [Lambda support](#lambda-support)
+* [Signals](#signals)
+* [Event dispatcher](#event-dispatcher)
+  * [Named queues](#named-queues)
+* [Event emitter](#event-emitter)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+# Introduction
+
+Signals are more often than not a core part of games and software architectures
+in general.<br/>
+They help to decouple the various parts of a system while allowing them to
+communicate with each other somehow.
+
+The so called _modern C++_ comes with a tool that can be useful in this regard,
+the `std::function`. As an example, it can be used to create delegates.<br/>
+However, there is no guarantee that an `std::function` doesn't perform
+allocations under the hood and this could be problematic sometimes. Furthermore,
+it solves a problem but may not adapt well to other requirements that may arise
+from time to time.
+
+In case that the flexibility and power of an `std::function` isn't required or
+if the price to pay for them is too high, `EnTT` offers a complete set of
+lightweight classes to solve the same and many other problems.
+
+# Delegate
+
+A delegate can be used as a general purpose invoker with no memory overhead for
+free functions and member functions provided along with an instance on which to
+invoke them.<br/>
+It doesn't claim to be a drop-in replacement for an `std::function`, so don't
+expect to use it whenever an `std::function` fits well. That said, it's most
+likely even a better fit than an `std::function` in a lot of cases, so expect to
+use it quite a lot anyway.
+
+The interface is trivial. It offers a default constructor to create empty
+delegates:
+
+```cpp
+entt::delegate<int(int)> delegate{};
+```
+
+What is needed to create an instance is to specify the type of the function the
+delegate _accepts_, that is the signature of the functions it models.<br/>
+However, attempting to use an empty delegate by invoking its function call
+operator results in undefined behavior or most likely a crash.
+
+There exist a few overloads of the `connect` member function to initialize a
+delegate:
+
+```cpp
+int f(int i) { return i; }
+
+struct my_struct {
+    int f(const int &i) const { return i; }
+};
+
+// bind a free function to the delegate
+delegate.connect<&f>();
+
+// bind a member function to the delegate
+my_struct instance;
+delegate.connect<&my_struct::f>(instance);
+```
+
+The delegate class also accepts data members, if needed. In this case, the
+function type of the delegate is such that the parameter list is empty and the
+value of the data member is at least convertible to the return type.
+
+Free functions having type equivalent to `void(T &, args...)` are accepted as
+well. The first argument `T &` is considered a payload and the function will
+receive it back every time it's invoked. In other terms, this works just fine
+with the above definition:
+
+```cpp
+void g(const char &c, int i) { /* ... */ }
+const char c = 'c';
+
+delegate.connect<&g>(c);
+delegate(42);
+```
+
+The function `g` is invoked with a reference to `c` and `42`. However, the
+function type of the delegate is still `void(int)`. This is also the signature
+of its function call operator.<br/>
+Another interesting aspect of the delegate class is that it accepts functions
+with a list of parameters that is shorter than that of its function type:
+
+```cpp
+void g() { /* ... */ }
+delegate.connect<&g>();
+delegate(42);
+```
+
+Where the function type of the delegate is `void(int)` as above. It goes without
+saying that the extra arguments are silently discarded internally.<br/>
+This is a nice-to-have feature in a lot of cases, as an example when the
+`delegate` class is used as a building block of a signal-slot system.
+
+To create and initialize a delegate at once, there are a few specialized
+constructors. Because of the rules of the language, the listener is provided by
+means of the `entt::connect_arg` variable template:
+
+```cpp
+entt::delegate<int(int)> func{entt::connect_arg<&f>};
+```
+
+Aside `connect`, a `disconnect` counterpart isn't provided. Instead, there
+exists a `reset` member function to use to clear a delegate.<br/>
+To know if a delegate is empty, it can be used explicitly in every conditional
+statement:
+
+```cpp
+if(delegate) {
+    // ...
+}
+```
+
+Finally, to invoke a delegate, the function call operator is the way to go as
+already shown in the examples above:
+
+```cpp
+auto ret = delegate(42);
+```
+
+In all cases, listeners don't have to strictly follow the signature of the
+delegate. As long as a listener can be invoked with the given arguments to yield
+a result that is convertible to the given result type, everything works just
+fine.
+
+As a side note, members of classes may or may not be associated with instances.
+If they are not, the first argument of the function type must be that of the
+class on which the members operate and an instance of this class must obviously
+be passed when invoking the delegate:
+
+```cpp
+entt::delegate<void(my_struct &, int)> delegate;
+delegate.connect<&my_struct::f>();
+
+my_struct instance;
+delegate(instance, 42);
+```
+
+In this case, it's not possible to _deduce_ the function type since the first
+argument doesn't necessarily have to be a reference (for example, it can be a
+pointer, as well as a const reference).<br/>
+Therefore, the function type must be declared explicitly for unbound members.
+
+## Runtime arguments
+
+The `delegate` class is meant to be used primarily with template arguments.
+However, as a consequence of its design, it also offers minimal support for
+runtime arguments.<br/>
+When used like this, some features aren't supported though. In particular:
+
+* Curried functions aren't accepted.
+* Functions with an argument list that differs from that of the delegate aren't
+  supported.
+* Return type and types of arguments **must** coincide with those of the
+  delegate and _being at least convertible_ isn't enough anymore.
+
+Moreover, for a given function type `Ret(Args...)`, the signature of the
+functions connected at runtime must necessarily be `Ret(const void *, Args...)`.
+
+Runtime arguments can be passed both to the constructor of a delegate and to the
+`connect` member function. An optional parameter is also accepted in both cases.
+This argument is used to pass arbitrary user data back and forth as a
+`const void *` upon invocation.<br/>
+To connect a function to a delegate _in the hard way_:
+
+```cpp
+int func(const void *ptr, int i) { return *static_cast<const int *>(ptr) * i; }
+const int value = 42;
+
+// use the constructor ...
+entt::delegate delegate{&func, &value};
+
+// ... or the connect member function
+delegate.connect(&func, &value);
+```
+
+The type of the delegate is deduced from the function if possible. In this case,
+since the first argument is an implementation detail, the deduced function type
+is `int(int)`.<br/>
+Invoking a delegate built in this way follows the same rules as previously
+explained.
+
+## Lambda support
+
+In general, the `delegate` class doesn't fully support lambda functions in all
+their nuances. The reason is pretty simple: a `delegate` isn't a drop-in
+replacement for an `std::function`. Instead, it tries to overcome the problems
+with the latter.<br/>
+That being said, non-capturing lambda functions are supported, even though some
+features aren't available in this case.
+
+This is a logical consequence of the support for connecting functions at
+runtime. Therefore, lambda functions undergo the same rules and
+limitations.<br/>
+In fact, since non-capturing lambda functions decay to pointers to functions,
+they can be used with a `delegate` as if they were _normal functions_ with
+optional payload:
+
+```cpp
+my_struct instance;
+
+// use the constructor ...
+entt::delegate delegate{+[](const void *ptr, int value) {
+    return static_cast<const my_struct *>(ptr)->f(value);
+}, &instance};
+
+// ... or the connect member function
+delegate.connect([](const void *ptr, int value) {
+    return static_cast<const my_struct *>(ptr)->f(value);
+}, &instance);
+```
+
+As above, the first parameter (`const void *`) isn't part of the function type
+of the delegate and is used to dispatch arbitrary user data back and forth. In
+other terms, the function type of the delegate above is `int(int)`.
+
+# Signals
+
+Signal handlers work with references to classes, function pointers and pointers
+to members. Listeners can be any kind of objects and users are in charge of
+connecting and disconnecting them from a signal to avoid crashes due to
+different lifetimes. On the other side, performance shouldn't be affected that
+much by the presence of such a signal handler.<br/>
+Signals make use of delegates internally and therefore they undergo the same
+rules and offer similar functionalities. It may be a good idea to consult the
+documentation of the `delegate` class for further information.
+
+A signal handler is can be used as a private data member without exposing any
+_publish_ functionality to the clients of a class.<br/>
+The basic idea is to impose a clear separation between the signal itself and the
+`sink` class, that is a tool to be used to connect and disconnect listeners on
+the fly.
+
+The API of a signal handler is straightforward. If a collector is supplied to
+the signal when something is published, all the values returned by its listeners
+are literally _collected_ and used later by the caller. Otherwise, the class
+works just like a plain signal that emits events from time to time.<br/>
+To create instances of signal handlers it's sufficient to provide the type of
+function to which they refer:
+
+```cpp
+entt::sigh<void(int, char)> signal;
+```
+
+Signals offer all the basic functionalities required to know how many listeners
+they contain (`size`) or if they contain at least a listener (`empty`), as well
+as a function to use to swap handlers (`swap`).
+
+Besides them, there are member functions to use both to connect and disconnect
+listeners in all their forms by means of a sink:
+
+```cpp
+void foo(int, char) { /* ... */ }
+
+struct listener {
+    void bar(const int &, char) { /* ... */ }
+};
+
+// ...
+
+entt::sink sink{signal};
+listener instance;
+
+sink.connect<&foo>();
+sink.connect<&listener::bar>(instance);
+
+// ...
+
+// disconnects a free function
+sink.disconnect<&foo>();
+
+// disconnect a member function of an instance
+sink.disconnect<&listener::bar>(instance);
+
+// disconnect all member functions of an instance, if any
+sink.disconnect(instance);
+
+// discards all listeners at once
+sink.disconnect();
+```
+
+As shown above, listeners don't have to strictly follow the signature of the
+signal. As long as a listener can be invoked with the given arguments to yield a
+result that is convertible to the given return type, everything works just
+fine.<br/>
+It's also possible to connect a listener before other elements already contained
+by the signal. The `before` function returns a `sink` object that is correctly
+initialized for the purpose and can be used to connect one or more listeners in
+order and in the desired position:
+
+```cpp
+sink.before<&foo>().connect<&listener::bar>(instance);
+```
+
+In all cases, the `connect` member function returns by default a `connection`
+object to be used as an alternative to break a connection by means of its
+`release` member function.<br/>
+A `scoped_connection` can also be created from a connection. In this case, the
+link is broken automatically as soon as the object goes out of scope.
+
+Once listeners are attached (or even if there are no listeners at all), events
+and data in general are published through a signal by means of the `publish`
+member function:
+
+```cpp
+signal.publish(42, 'c');
+```
+
+To collect data, the `collect` member function is used instead:
+
+```cpp
+int f() { return 0; }
+int g() { return 1; }
+
+// ...
+
+entt::sigh<int()> signal;
+entt::sink sink{signal};
+
+sink.connect<&f>();
+sink.connect<&g>();
+
+std::vector<int> vec{};
+signal.collect([&vec](int value) { vec.push_back(value); });
+
+assert(vec[0] == 0);
+assert(vec[1] == 1);
+```
+
+A collector must expose a function operator that accepts as an argument a type
+to which the return type of the listeners can be converted. Moreover, it can
+optionally return a boolean value that is true to stop collecting data, false
+otherwise. This way one can avoid calling all the listeners in case it isn't
+necessary.<br/>
+Functors can also be used in place of a lambda. Since the collector is copied
+when invoking the `collect` member function, `std::ref` is the way to go in this
+case:
+
+```cpp
+struct my_collector {
+    std::vector<int> vec{};
+
+    bool operator()(int v) {
+        vec.push_back(v);
+        return true;
+    }
+};
+
+// ...
+
+my_collector collector;
+signal.collect(std::ref(collector));
+```
+
+# Event dispatcher
+
+The event dispatcher class allows users to trigger immediate events or to queue
+and publish them all together later.<br/>
+This class lazily instantiates its queues. Therefore, it's not necessary to
+_announce_ the event types in advance:
+
+```cpp
+// define a general purpose dispatcher
+entt::dispatcher dispatcher{};
+```
+
+A listener registered with a dispatcher is such that its type offers one or more
+member functions that take arguments of type `Event &` for any type of event,
+regardless of the return value.<br/>
+These functions are linked directly via `connect` to a _sink_:
+
+```cpp
+struct an_event { int value; };
+struct another_event {};
+
+struct listener {
+    void receive(const an_event &) { /* ... */ }
+    void method(const another_event &) { /* ... */ }
+};
+
+// ...
+
+listener listener;
+dispatcher.sink<an_event>().connect<&listener::receive>(listener);
+dispatcher.sink<another_event>().connect<&listener::method>(listener);
+```
+
+Note that connecting listeners within event handlers can result in undefined
+behavior.<br/>
+The `disconnect` member function is used to remove one listener at a time or all
+of them at once:
+
+```cpp
+dispatcher.sink<an_event>().disconnect<&listener::receive>(listener);
+dispatcher.sink<another_event>().disconnect(listener);
+```
+
+The `trigger` member function serves the purpose of sending an immediate event
+to all the listeners registered so far:
+
+```cpp
+dispatcher.trigger(an_event{42});
+dispatcher.trigger<another_event>();
+```
+
+Listeners are invoked immediately, order of execution isn't guaranteed. This
+method can be used to push around urgent messages like an _is terminating_
+notification on a mobile app.
+
+On the other hand, the `enqueue` member function queues messages together and
+helps to maintain control over the moment they are sent to listeners:
+
+```cpp
+dispatcher.enqueue<an_event>(42);
+dispatcher.enqueue(another_event{});
+```
+
+Events are stored aside until the `update` member function is invoked:
+
+```cpp
+// emits all the events of the given type at once
+dispatcher.update<an_event>();
+
+// emits all the events queued so far at once
+dispatcher.update();
+```
+
+This way users can embed the dispatcher in a loop and literally dispatch events
+once per tick to their systems.
+
+## Named queues
+
+All queues within a dispatcher are associated by default with an event type and
+then retrieved from it.<br/>
+However, it's possible to create queues with different _names_ (and therefore
+also multiple queues for a single type). In fact, more or less all functions
+also take an additional parameter. As an example:
+
+```cpp
+dispatcher.sink<an_event>("custom"_hs).connect<&listener::receive>(listener);
+```
+
+In this case, the term _name_ is misused as these are actual numeric identifiers
+of type `id_type`.<br/>
+An exception to this rule is the `enqueue` function. There is no additional
+parameter for it but rather a different function:
+
+```cpp
+dispatcher.enqueue_hint<an_event>("custom"_hs, 42);
+```
+
+This is mainly due to the template argument deduction rules and unfortunately
+there is no real (elegant) way to avoid it.
+
+# Event emitter
+
+A general purpose event emitter thought mainly for those cases where it comes to
+working with asynchronous stuff.<br/>
+Originally designed to fit the requirements of
+[`uvw`](https://github.com/skypjack/uvw) (a wrapper for `libuv` written in
+modern C++), it was adapted later to be included in this library.
+
+To create an emitter type, derived classes must inherit from the base as:
+
+```cpp
+struct my_emitter: emitter<my_emitter> {
+    // ...
+}
+```
+
+Handlers for the different events are created internally on the fly. It's not
+required to specify in advance the full list of accepted events.<br/>
+Moreover, whenever an event is published, an emitter also passes a reference
+to itself to its listeners.
+
+To create new instances of an emitter, no arguments are required:
+
+```cpp
+my_emitter emitter{};
+```
+
+Listeners are movable and callable objects (free functions, lambdas, functors,
+`std::function`s, whatever) whose function type is compatible with:
+
+```cpp
+void(Type &, my_emitter &)
+```
+
+Where `Type` is the type of event they want to receive.<br/>
+To attach a listener to an emitter, there exists the `on` member function:
+
+```cpp
+emitter.on<my_event>([](const my_event &event, my_emitter &emitter) {
+    // ...
+});
+```
+
+Similarly, the `reset` member function is used to disconnect listeners given a
+type while `clear` is used to disconnect all listeners at once:
+
+```cpp
+// resets the listener for my_event
+emitter.erase<my_event>();
+
+// resets all listeners
+emitter.clear()
+```
+
+To send an event to the listener registered on a given type, the `publish`
+function is the way to go:
+
+```cpp
+struct my_event { int i; };
+
+// ...
+
+emitter.publish(my_event{42});
+```
+
+Finally, the `empty` member function tests if there exists at least a listener
+registered with the event emitter while `contains` is used to check if a given
+event type is associated with a valid listener:
+
+```cpp
+if(emitter.contains<my_event>()) {
+    // ...
+}
+```
+
+This class introduces a _nice-to-have_ model based on events and listeners.<br/>
+More in general, it's a handy tool when the derived classes _wrap_ asynchronous
+operations but it's not limited to such uses.

docs/md/unreal.md

@@ -0,0 +1,107 @@
+# EnTT and Unreal Engine
+
+<!--
+@cond TURN_OFF_DOXYGEN
+-->
+# Table of Contents
+
+* [Enable Cpp17](#enable-cpp17)
+* [EnTT as a third party module](#entt-as-a-third-party-module)
+* [Include EnTT](#include-entt)
+<!--
+@endcond TURN_OFF_DOXYGEN
+-->
+
+## Enable Cpp17
+
+As of writing (Unreal Engine v4.25), the default C++ standard of Unreal Engine
+is C++14.<br/>
+On the other hand, note that `EnTT` requires C++17 to compile. To enable it, in
+the main module of the project there should be a `<Game Name>.Build.cs` file,
+the constructor of which must contain the following lines:
+
+```cs
+PCHUsage = PCHUsageMode.NoSharedPCHs;
+PrivatePCHHeaderFile = "<PCH filename>.h";
+CppStandard = CppStandardVersion.Cpp17;
+```
+
+Replace `<PCH filename>.h` with the name of the already existing PCH header
+file, if any.<br/>
+In case the project doesn't already contain a file of this type, it's possible
+to create one with the following content:
+
+```cpp
+#pragma once
+#include "CoreMinimal.h"
+```
+
+Remember to remove any old `PCHUsage = <...>` line that was previously there. At
+this point, C++17 support should be in place.<br/>
+Try to compile the project to ensure it works as expected before following
+further steps.
+
+Note that updating a *project* to C++17 doesn't necessarily mean that the IDE in
+use will also start to recognize its syntax.<br/>
+If the plan is to use C++17 in the project too, check the specific instructions
+for the IDE in use.
+
+## EnTT as a third party module
+
+Once this point is reached, the `Source` directory should look like this:
+
+```
+Source
+|  MyGame.Target.cs
+|  MyGameEditor.Target.cs
+|
++---MyGame
+|  |  MyGame.Build.cs
+|  |  MyGame.h (PCH Header file)
+|
+\---ThirdParty
+   \---EnTT
+      |   EnTT.Build.cs
+      |
+      \---entt (GitHub repository content inside)
+```
+
+To make this happen, create the folder `ThirdParty` under `Source` if it doesn't
+exist already. Then, add an `EnTT` folder under `ThirdParty`.<br/>
+Within the latter, create a new file `EnTT.Build.cs` with the following content:
+
+```cs
+using System.IO;
+using UnrealBuildTool;
+
+public class EnTT: ModuleRules {
+    public EnTT(ReadOnlyTargetRules Target) : base(Target) {
+        Type = ModuleType.External;
+        PublicIncludePaths.Add(Path.Combine(ModuleDirectory, "entt", "src", "entt"));
+    }
+}
+```
+
+The last line indicates that the actual files will be found in the directory
+`EnTT/entt/src/entt`.<br/>
+Download the repository for `EnTT` and place it next to `EnTT.Build.cs` or
+update the path above accordingly.
+
+Finally, open the `<Game Name>.Build.cs` file and add `EnTT` as a dependency at
+the end of the list:
+
+```cs
+PublicDependencyModuleNames.AddRange(new[] {
+    "Core", "CoreUObject", "Engine", "InputCore", [...], "EnTT"
+});
+```
+
+Note that some IDEs might require a restart to start recognizing the new module
+for code-highlighting features and such.
+
+## Include EnTT
+
+In any source file of the project, add `#include "entt.hpp"` or any other path
+to the file from `EnTT` to use it.<br/>
+Try to create a registry as `entt::registry registry;` to make sure everything
+compiles fine.

entt.imp

@@ -0,0 +1,34 @@
+[
+  { "include": [ "@<gtest/internal/.*>", "private", "<gtest/gtest.h>", "public" ] },
+  { "include": [ "@<gtest/gtest-.*>", "private", "<gtest/gtest.h>", "public" ] },
+  { "include": [ "@[\"<].*/container/fwd.hpp[\">]", "private", "<entt/container/dense_map.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/container/fwd.hpp[\">]", "private", "<entt/container/dense_set.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd.hpp[\">]", "private", "<entt/core/any.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd.hpp[\">]", "private", "<entt/core/family.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd.hpp[\">]", "private", "<entt/core/hashed_string.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd.hpp[\">]", "private", "<entt/core/ident.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd.hpp[\">]", "private", "<entt/core/monostate.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd.hpp[\">]", "private", "<entt/core/type_info.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/core/fwd.hpp[\">]", "private", "<entt/core/type_traits.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/entity.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/group.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/handle.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/helper.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/observer.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/organizer.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/registry.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/runtime_view.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/snapshot.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/sparse_set.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/storage.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/entity/fwd.hpp[\">]", "private", "<entt/entity/view.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/meta/fwd.hpp[\">]", "private", "<entt/meta/meta.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/poly/fwd.hpp[\">]", "private", "<entt/poly/poly.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/resource/fwd.hpp[\">]", "private", "<entt/resource/cache.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/resource/fwd.hpp[\">]", "private", "<entt/resource/loader.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/resource/fwd.hpp[\">]", "private", "<entt/resource/resource.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/signal/fwd.hpp[\">]", "private", "<entt/signal/delegate.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/signal/fwd.hpp[\">]", "private", "<entt/signal/dispatcher.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/signal/fwd.hpp[\">]", "private", "<entt/signal/emitter.hpp>", "public" ] },
+  { "include": [ "@[\"<].*/signal/fwd.hpp[\">]", "private", "<entt/signal/sigh.hpp>", "public" ] }
+]

natvis/entt/config.natvis

@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+</AutoVisualizer>

natvis/entt/container.natvis

@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::dense_map&lt;*&gt;">
+		<Intrinsic Name="size" Expression="packed.first_base::value.size()"/>
+		<Intrinsic Name="bucket_count" Expression="sparse.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">packed.first_base::value.capacity()</Item>
+			<Item Name="[bucket_count]" ExcludeView="simple">bucket_count()</Item>
+			<Item Name="[load_factor]" ExcludeView="simple">(float)size() / (float)bucket_count()</Item>
+			<Item Name="[max_load_factor]" ExcludeView="simple">threshold</Item>
+			<IndexListItems>
+				<Size>size()</Size>
+				<ValueNode>packed.first_base::value[$i].element</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::dense_set&lt;*&gt;">
+		<Intrinsic Name="size" Expression="packed.first_base::value.size()"/>
+		<Intrinsic Name="bucket_count" Expression="sparse.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">packed.first_base::value.capacity()</Item>
+			<Item Name="[bucket_count]" ExcludeView="simple">bucket_count()</Item>
+			<Item Name="[load_factor]" ExcludeView="simple">(float)size() / (float)bucket_count()</Item>
+			<Item Name="[max_load_factor]" ExcludeView="simple">threshold</Item>
+			<IndexListItems>
+				<Size>size()</Size>
+				<ValueNode>packed.first_base::value[$i].second</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+</AutoVisualizer>

natvis/entt/core.natvis

@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+    <Type Name="entt::basic_any&lt;*&gt;">
+		<DisplayString>{{ type={ info->alias,na }, policy={ mode,en } }}</DisplayString>
+    </Type>
+	<Type Name="entt::compressed_pair&lt;*&gt;">
+		<Intrinsic Name="first" Optional="true" Expression="((first_base*)this)->value"/>
+		<Intrinsic Name="first" Optional="true" Expression="*(first_base::base_type*)this"/>
+		<Intrinsic Name="second" Optional="true" Expression="((second_base*)this)->value"/>
+		<Intrinsic Name="second" Optional="true" Expression="*(second_base::base_type*)this"/>
+		<DisplayString >({ first() }, { second() })</DisplayString>
+		<Expand>
+			<Item Name="[first]">first()</Item>
+			<Item Name="[second]">second()</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_hashed_string&lt;*&gt;">
+		<DisplayString Condition="base_type::repr != nullptr">{{ hash={ base_type::hash } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[data]">base_type::repr,na</Item>
+			<Item Name="[length]">base_type::length</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::type_info">
+		<DisplayString>{{ name={ alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[hash]">identifier</Item>
+			<Item Name="[index]">seq</Item>
+		</Expand>
+	</Type>
+</AutoVisualizer>

natvis/entt/entity.natvis

@@ -0,0 +1,153 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::basic_registry&lt;*&gt;">
+		<Intrinsic Name="pools_size" Expression="pools.packed.first_base::value.size()"/>
+		<Intrinsic Name="vars_size" Expression="vars.ctx.packed.first_base::value.size()"/>
+		<Intrinsic Name="to_entity" Expression="*((entity_traits::entity_type *)&amp;entity) &amp; entity_traits::entity_mask">
+			<Parameter Name="entity" Type="entity_traits::value_type &amp;"/>
+		</Intrinsic>
+		<DisplayString>{{ size={ epool.size() } }}</DisplayString>
+		<Expand>
+			<Item IncludeView="simple" Name="[epool]">epool,view(simple)nr</Item>
+			<Synthetic Name="[epool]" ExcludeView="simple">
+				<DisplayString>{ epool.size() }</DisplayString>
+				<Expand>
+					<CustomListItems>
+						<Variable Name="pos" InitialValue="0" />
+						<Variable Name="last" InitialValue="epool.size()"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<If Condition="to_entity(epool[pos]) == pos">
+								<Item Name="[{ pos }]">epool[pos]</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[destroyed]" ExcludeView="simple">
+				<DisplayString>{ to_entity(free_list) != entity_traits::entity_mask }</DisplayString>
+				<Expand>
+					<CustomListItems>
+						<Variable Name="it" InitialValue="to_entity(free_list)" />
+						<Loop>
+							<Break Condition="it == entity_traits::entity_mask"/>
+							<Item Name="[{ it }]">epool[it]</Item>
+							<Exec>it = to_entity(epool[it])</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[pools]">
+				<DisplayString>{ pools_size() }</DisplayString>
+				<Expand>
+					<IndexListItems ExcludeView="simple">
+						<Size>pools_size()</Size>
+						<ValueNode>*pools.packed.first_base::value[$i].element.second</ValueNode>
+					</IndexListItems>
+					<IndexListItems IncludeView="simple">
+						<Size>pools_size()</Size>
+						<ValueNode>*pools.packed.first_base::value[$i].element.second,view(simple)</ValueNode>
+					</IndexListItems>
+				</Expand>
+			</Synthetic>
+			<Item Name="[groups]" ExcludeView="simple">groups.size()</Item>
+			<Synthetic Name="[vars]">
+				<DisplayString>{ vars_size() }</DisplayString>
+				<Expand>
+					<IndexListItems>
+						<Size>vars_size()</Size>
+						<ValueNode>vars.ctx.packed.first_base::value[$i].element.second</ValueNode>
+					</IndexListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_sparse_set&lt;*&gt;">
+		<DisplayString>{{ size={ packed.size() }, type={ info->alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">packed.capacity()</Item>
+			<Item Name="[policy]">mode,en</Item>
+			<Synthetic Name="[sparse]">
+				<DisplayString>{ sparse.size() * entity_traits::page_size }</DisplayString>
+				<Expand>
+					<ExpandedItem IncludeView="simple">sparse,view(simple)</ExpandedItem>
+					<CustomListItems ExcludeView="simple">
+						<Variable Name="pos" InitialValue="0"/>
+						<Variable Name="page" InitialValue="0"/>
+						<Variable Name="offset" InitialValue="0"/>
+						<Variable Name="last" InitialValue="sparse.size() * entity_traits::page_size"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<Exec>page = pos / entity_traits::page_size</Exec>
+							<Exec>offset = pos &amp; (entity_traits::page_size - 1)</Exec>
+							<If Condition="sparse[page] &amp;&amp; (*((entity_traits::entity_type *)&amp;sparse[page][offset]) &lt; ~entity_traits::entity_mask)">
+								<Item Name="[{ pos }]">*((entity_traits::entity_type *)&amp;sparse[page][offset]) &amp; entity_traits::entity_mask</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[packed]">
+				<DisplayString>{ packed.size() }</DisplayString>
+				<Expand>
+					<ExpandedItem IncludeView="simple">packed,view(simple)</ExpandedItem>
+					<CustomListItems ExcludeView="simple">
+						<Variable Name="pos" InitialValue="0"/>
+						<Variable Name="last" InitialValue="packed.size()"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<If Condition="*((entity_traits::entity_type *)&amp;packed[pos]) &lt; ~entity_traits::entity_mask">
+								<Item Name="[{ pos }]">packed[pos]</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_storage&lt;*&gt;">
+		<DisplayString>{{ size={ base_type::packed.size() }, type={ base_type::info->alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" Optional="true" ExcludeView="simple">packed.first_base::value.capacity() * comp_traits::page_size</Item>
+			<Item Name="[page size]" Optional="true" ExcludeView="simple">comp_traits::page_size</Item>
+			<Item Name="[base]" ExcludeView="simple">(base_type*)this,nand</Item>
+			<Item Name="[base]" IncludeView="simple">(base_type*)this,view(simple)nand</Item>
+			<!-- having SFINAE-like techniques in natvis is priceless :) -->
+			<CustomListItems Condition="packed.first_base::value.size() != 0" Optional="true">
+				<Variable Name="pos" InitialValue="0" />
+				<Variable Name="last" InitialValue="base_type::packed.size()"/>
+				<Loop>
+					<Break Condition="pos == last"/>
+					<If Condition="*((base_type::entity_traits::entity_type *)&amp;base_type::packed[pos]) &lt; ~base_type::entity_traits::entity_mask">
+						<Item Name="[{ pos }:{ base_type::packed[pos] }]">packed.first_base::value[pos / comp_traits::page_size][pos &amp; (comp_traits::page_size - 1)]</Item>
+					</If>
+					<Exec>++pos</Exec>
+				</Loop>
+			</CustomListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_view&lt;*&gt;">
+		<DisplayString>{{ size_hint={ view->packed.size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[pools]">pools,na</Item>
+			<Item Name="[filter]">filter,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_runtime_view&lt;*&gt;">
+		<DisplayString Condition="pools.size() != 0u">{{ size_hint={ pools[0]->packed.size() } }}</DisplayString>
+		<DisplayString>{{ size_hint=0 }}</DisplayString>
+		<Expand>
+			<Item Name="[pools]">pools,na</Item>
+			<Item Name="[filter]">filter,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::null_t">
+		<DisplayString>&lt;null&gt;</DisplayString>
+	</Type>
+	<Type Name="entt::tombstone_t">
+		<DisplayString>&lt;tombstone&gt;</DisplayString>
+	</Type>
+</AutoVisualizer>

natvis/entt/graph.natvis

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+    <Type Name="entt::adjacency_matrix&lt;*&gt;">
+		<DisplayString>{{ size={ vert } }}</DisplayString>
+        <Expand>
+			<CustomListItems>
+				<Variable Name="pos" InitialValue="0" />
+				<Variable Name="last" InitialValue="vert * vert"/>
+				<Loop>
+					<Break Condition="pos == last"/>
+					<If Condition="matrix[pos] != 0u">
+						<Item Name="{pos / vert}">pos % vert</Item>
+					</If>
+					<Exec>++pos</Exec>
+				</Loop>
+			</CustomListItems>
+        </Expand>
+    </Type>
+</AutoVisualizer>

natvis/entt/locator.natvis

@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+</AutoVisualizer>

natvis/entt/meta.natvis

@@ -0,0 +1,121 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::internal::meta_base_node">
+		<DisplayString>{{}}</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_conv_node">
+		<DisplayString>{{}}</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_ctor_node">
+		<DisplayString>{{ arity={ arity } }}</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_data_node">
+		<Intrinsic Name="has_property" Expression="!!(traits &amp; property)">
+			<Parameter Name="property" Type="int"/>
+		</Intrinsic>
+		<DisplayString>{{ arity={ arity } }}</DisplayString>
+		<Expand>
+			<Item Name="[arity]">arity</Item>
+			<Item Name="[is_const]">has_property(entt::internal::meta_traits::is_const)</Item>
+			<Item Name="[is_static]">has_property(entt::internal::meta_traits::is_static)</Item>
+			<Item Name="[prop]">prop</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_func_node"	>
+		<Intrinsic Name="has_property" Expression="!!(traits &amp; property)">
+			<Parameter Name="property" Type="int"/>
+		</Intrinsic>
+		<DisplayString>{{ arity={ arity } }}</DisplayString>
+		<Expand>
+			<Item Name="[is_const]">has_property(entt::internal::meta_traits::is_const)</Item>
+			<Item Name="[is_static]">has_property(entt::internal::meta_traits::is_static)</Item>
+			<Item Name="[next]" Condition="next != nullptr">*next</Item>
+			<Item Name="[prop]">prop</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_prop_node">
+		<DisplayString>{ value }</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_template_node">
+		<DisplayString>{{ arity={ arity } }}</DisplayString>
+	</Type>
+	<Type Name="entt::internal::meta_type_node">
+		<Intrinsic Name="has_property" Expression="!!(traits &amp; property)">
+			<Parameter Name="property" Type="int"/>
+		</Intrinsic>
+		<DisplayString Condition="info != nullptr">{{ type={ info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[id]">id</Item>
+			<Item Name="[sizeof]">size_of</Item>
+			<Item Name="[is_arithmetic]">has_property(entt::internal::meta_traits::is_arithmetic)</Item>
+			<Item Name="[is_integral]">has_property(entt::internal::meta_traits::is_integral)</Item>
+			<Item Name="[is_signed]">has_property(entt::internal::meta_traits::is_signed)</Item>
+			<Item Name="[is_array]">has_property(entt::internal::meta_traits::is_array)</Item>
+			<Item Name="[is_enum]">has_property(entt::internal::meta_traits::is_enum)</Item>
+			<Item Name="[is_class]">has_property(entt::internal::meta_traits::is_class)</Item>
+			<Item Name="[is_meta_pointer_like]">has_property(entt::internal::meta_traits::is_meta_pointer_like)</Item>
+			<Item Name="[is_meta_sequence_container]">has_property(entt::internal::meta_traits::is_meta_sequence_container)</Item>
+			<Item Name="[is_meta_associative_container]">has_property(entt::internal::meta_traits::is_meta_associative_container)</Item>
+			<Item Name="[default_constructor]">default_constructor != nullptr</Item>
+			<Item Name="[conversion_helper]">conversion_helper != nullptr</Item>
+			<Item Name="[from_void]">from_void != nullptr</Item>
+			<Item Name="[template_info]">templ</Item>
+			<Item Name="[details]" Condition="details != nullptr">*details</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_any">
+		<DisplayString Condition="node.info != nullptr">{{ type={ node.info->alias,na }, policy={ storage.mode,en } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem>node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_handle">
+		<DisplayString>{ any }</DisplayString>
+	</Type>
+	<Type Name="entt::meta_associative_container">
+		<DisplayString>{ storage }</DisplayString>
+		<Expand>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_sequence_container">
+		<DisplayString>{ storage }</DisplayString>
+		<Expand>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_data">
+		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_func">
+		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_prop">
+		<DisplayString Condition="node != nullptr">{ *node }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_type">
+		<DisplayString>{ node }</DisplayString>
+		<Expand>
+			<ExpandedItem>node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx->value</Item>
+		</Expand>
+	</Type>
+</AutoVisualizer>

natvis/entt/platform.natvis

@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+</AutoVisualizer>

natvis/entt/poly.natvis

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::basic_poly&lt;*&gt;">
+		<DisplayString>{ storage }</DisplayString>
+	</Type>
+</AutoVisualizer>

natvis/entt/process.natvis

@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+</AutoVisualizer>

natvis/entt/resource.natvis

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::resource&lt;*&gt;">
+		<DisplayString>{ value }</DisplayString>
+		<Expand>
+			<ExpandedItem>value</ExpandedItem>
+		</Expand>
+	</Type>
+	<Type Name="entt::resource_cache&lt;*&gt;">
+		<DisplayString>{ pool.first_base::value }</DisplayString>
+		<Expand>
+			<ExpandedItem>pool.first_base::value</ExpandedItem>
+		</Expand>
+	</Type>
+</AutoVisualizer>

natvis/entt/signal.natvis

@@ -0,0 +1,56 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::delegate&lt;*&gt;">
+		<DisplayString>{{ type={ "$T1" } }}</DisplayString>
+		<Expand>
+			<Item Name="[empty]">fn == nullptr</Item>
+			<Item Name="[data]">instance</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_dispatcher&lt;*&gt;">
+		<Intrinsic Name="size" Expression="pools.first_base::value.packed.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
+		<Expand>
+			<Synthetic Name="[pools]">
+				<DisplayString>{ size() }</DisplayString>
+				<Expand>
+					<IndexListItems>
+						<Size>size()</Size>
+						<ValueNode>*pools.first_base::value.packed.first_base::value[$i].element.second</ValueNode>
+					</IndexListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::dispatcher_handler&lt;*&gt;">
+		<DisplayString>{{ size={ events.size() }, event={ "$T1" } }}</DisplayString>
+		<Expand>
+			<Item Name="[signal]">signal</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::emitter&lt;*&gt;">
+		<DisplayString>{{ size={ handlers.first_base::value.packed.first_base::value.size() } }}</DisplayString>
+	</Type>
+	<Type Name="entt::connection">
+		<DisplayString>{{ bound={ signal != nullptr } }}</DisplayString>
+	</Type>
+	<Type Name="entt::scoped_connection">
+		<DisplayString>{ conn }</DisplayString>
+	</Type>
+	<Type Name="entt::sigh&lt;*&gt;">
+		<DisplayString>{{ size={ calls.size() }, type={ "$T1" } }}</DisplayString>
+		<Expand>
+			<IndexListItems>
+				<Size>calls.size()</Size>
+				<ValueNode>calls[$i]</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::sink&lt;*&gt;">
+		<DisplayString>{{ type={ "$T1" } }}</DisplayString>
+		<Expand>
+			<Item Name="[signal]">signal,na</Item>
+			<Item Name="[offset]">offset</Item>
+		</Expand>
+	</Type>
+</AutoVisualizer>

scripts/amalgamate.py

@@ -0,0 +1,299 @@
+#!/usr/bin/env python
+# coding=utf-8
+
+# amalgamate.py - Amalgamate C source and header files.
+# Copyright (c) 2012, Erik Edlund <erik.edlund@32767.se>
+# 
+# Redistribution and use in source and binary forms, with or without modification,
+# are permitted provided that the following conditions are met:
+# 
+#  * Redistributions of source code must retain the above copyright notice,
+#  this list of conditions and the following disclaimer.
+# 
+#  * Redistributions in binary form must reproduce the above copyright notice,
+#  this list of conditions and the following disclaimer in the documentation
+#  and/or other materials provided with the distribution.
+# 
+#  * Neither the name of Erik Edlund, nor the names of its contributors may
+#  be used to endorse or promote products derived from this software without
+#  specific prior written permission.
+# 
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+# ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+# (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+# ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+# SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import argparse
+import datetime
+import json
+import os
+import re
+
+
+class Amalgamation(object):
+
+    # Prepends self.source_path to file_path if needed.
+    def actual_path(self, file_path):
+        if not os.path.isabs(file_path):
+            file_path = os.path.join(self.source_path, file_path)
+        return file_path
+
+    # Search included file_path in self.include_paths and
+    # in source_dir if specified.
+    def find_included_file(self, file_path, source_dir):
+        search_dirs = self.include_paths[:]
+        if source_dir:
+            search_dirs.insert(0, source_dir)
+
+        for search_dir in search_dirs:
+            search_path = os.path.join(search_dir, file_path)
+            if os.path.isfile(self.actual_path(search_path)):
+                return search_path
+        return None
+
+    def __init__(self, args):
+        with open(args.config, 'r') as f:
+            config = json.loads(f.read())
+            for key in config:
+                setattr(self, key, config[key])
+
+            self.verbose = args.verbose == "yes"
+            self.prologue = args.prologue
+            self.source_path = args.source_path
+            self.included_files = []
+
+    # Generate the amalgamation and write it to the target file.
+    def generate(self):
+        amalgamation = ""
+
+        if self.prologue:
+            with open(self.prologue, 'r') as f:
+                amalgamation += datetime.datetime.now().strftime(f.read())
+
+        if self.verbose:
+            print("Config:")
+            print(" target        = {0}".format(self.target))
+            print(" working_dir   = {0}".format(os.getcwd()))
+            print(" include_paths = {0}".format(self.include_paths))
+        print("Creating amalgamation:")
+        for file_path in self.sources:
+            # Do not check the include paths while processing the source
+            # list, all given source paths must be correct.
+            # actual_path = self.actual_path(file_path)
+            print(" - processing \"{0}\"".format(file_path))
+            t = TranslationUnit(file_path, self, True)
+            amalgamation += t.content
+
+        with open(self.target, 'w') as f:
+            f.write(amalgamation)
+
+        print("...done!\n")
+        if self.verbose:
+            print("Files processed: {0}".format(self.sources))
+            print("Files included: {0}".format(self.included_files))
+        print("")
+
+
+def _is_within(match, matches):
+    for m in matches:
+        if match.start() > m.start() and \
+                match.end() < m.end():
+            return True
+    return False
+
+
+class TranslationUnit(object):
+    # // C++ comment.
+    cpp_comment_pattern = re.compile(r"//.*?\n")
+
+    # /* C comment. */
+    c_comment_pattern = re.compile(r"/\*.*?\*/", re.S)
+
+    # "complex \"stri\\\ng\" value".
+    string_pattern = re.compile("[^']" r'".*?(?<=[^\\])"', re.S)
+
+    # Handle simple include directives. Support for advanced
+    # directives where macros and defines needs to expanded is
+    # not a concern right now.
+    include_pattern = re.compile(
+        r'#\s*include\s+(<|")(?P<path>.*?)("|>)', re.S)
+
+    # #pragma once
+    pragma_once_pattern = re.compile(r'#\s*pragma\s+once', re.S)
+
+    # Search for pattern in self.content, add the match to
+    # contexts if found and update the index accordingly.
+    def _search_content(self, index, pattern, contexts):
+        match = pattern.search(self.content, index)
+        if match:
+            contexts.append(match)
+            return match.end()
+        return index + 2
+
+    # Return all the skippable contexts, i.e., comments and strings
+    def _find_skippable_contexts(self):
+        # Find contexts in the content in which a found include
+        # directive should not be processed.
+        skippable_contexts = []
+
+        # Walk through the content char by char, and try to grab
+        # skippable contexts using regular expressions when found.
+        i = 1
+        content_len = len(self.content)
+        while i < content_len:
+            j = i - 1
+            current = self.content[i]
+            previous = self.content[j]
+
+            if current == '"':
+                # String value.
+                i = self._search_content(j, self.string_pattern,
+                                         skippable_contexts)
+            elif current == '*' and previous == '/':
+                # C style comment.
+                i = self._search_content(j, self.c_comment_pattern,
+                                         skippable_contexts)
+            elif current == '/' and previous == '/':
+                # C++ style comment.
+                i = self._search_content(j, self.cpp_comment_pattern,
+                                         skippable_contexts)
+            else:
+                # Skip to the next char.
+                i += 1
+
+        return skippable_contexts
+
+    # Returns True if the match is within list of other matches
+
+    # Removes pragma once from content
+    def _process_pragma_once(self):
+        content_len = len(self.content)
+        if content_len < len("#include <x>"):
+            return 0
+
+        # Find contexts in the content in which a found include
+        # directive should not be processed.
+        skippable_contexts = self._find_skippable_contexts()
+
+        pragmas = []
+        pragma_once_match = self.pragma_once_pattern.search(self.content)
+        while pragma_once_match:
+            if not _is_within(pragma_once_match, skippable_contexts):
+                pragmas.append(pragma_once_match)
+
+            pragma_once_match = self.pragma_once_pattern.search(self.content,
+                                                                pragma_once_match.end())
+
+        # Handle all collected pragma once directives.
+        prev_end = 0
+        tmp_content = ''
+        for pragma_match in pragmas:
+            tmp_content += self.content[prev_end:pragma_match.start()]
+            prev_end = pragma_match.end()
+        tmp_content += self.content[prev_end:]
+        self.content = tmp_content
+
+    # Include all trivial #include directives into self.content.
+    def _process_includes(self):
+        content_len = len(self.content)
+        if content_len < len("#include <x>"):
+            return 0
+
+        # Find contexts in the content in which a found include
+        # directive should not be processed.
+        skippable_contexts = self._find_skippable_contexts()
+
+        # Search for include directives in the content, collect those
+        # which should be included into the content.
+        includes = []
+        include_match = self.include_pattern.search(self.content)
+        while include_match:
+            if not _is_within(include_match, skippable_contexts):
+                include_path = include_match.group("path")
+                search_same_dir = include_match.group(1) == '"'
+                found_included_path = self.amalgamation.find_included_file(
+                    include_path, self.file_dir if search_same_dir else None)
+                if found_included_path:
+                    includes.append((include_match, found_included_path))
+
+            include_match = self.include_pattern.search(self.content,
+                                                        include_match.end())
+
+        # Handle all collected include directives.
+        prev_end = 0
+        tmp_content = ''
+        for include in includes:
+            include_match, found_included_path = include
+            tmp_content += self.content[prev_end:include_match.start()]
+            tmp_content += "// {0}\n".format(include_match.group(0))
+            if found_included_path not in self.amalgamation.included_files:
+                t = TranslationUnit(found_included_path, self.amalgamation, False)
+                tmp_content += t.content
+            prev_end = include_match.end()
+        tmp_content += self.content[prev_end:]
+        self.content = tmp_content
+
+        return len(includes)
+
+    # Make all content processing
+    def _process(self):
+        if not self.is_root:
+            self._process_pragma_once()
+        self._process_includes()
+
+    def __init__(self, file_path, amalgamation, is_root):
+        self.file_path = file_path
+        self.file_dir = os.path.dirname(file_path)
+        self.amalgamation = amalgamation
+        self.is_root = is_root
+
+        self.amalgamation.included_files.append(self.file_path)
+
+        actual_path = self.amalgamation.actual_path(file_path)
+        if not os.path.isfile(actual_path):
+            raise IOError("File not found: \"{0}\"".format(file_path))
+        with open(actual_path, 'r') as f:
+            self.content = f.read()
+            self._process()
+
+
+def main():
+    description = "Amalgamate C source and header files."
+    usage = " ".join([
+        "amalgamate.py",
+        "[-v]",
+        "-c path/to/config.json",
+        "-s path/to/source/dir",
+        "[-p path/to/prologue.(c|h)]"
+    ])
+    argsparser = argparse.ArgumentParser(
+        description=description, usage=usage)
+
+    argsparser.add_argument("-v", "--verbose", dest="verbose",
+                            choices=["yes", "no"], metavar="", help="be verbose")
+
+    argsparser.add_argument("-c", "--config", dest="config",
+                            required=True, metavar="", help="path to a JSON config file")
+
+    argsparser.add_argument("-s", "--source", dest="source_path",
+                            required=True, metavar="", help="source code path")
+
+    argsparser.add_argument("-p", "--prologue", dest="prologue",
+                            required=False, metavar="", help="path to a C prologue file")
+
+    amalgamation = Amalgamation(argsparser.parse_args())
+    amalgamation.generate()
+
+
+if __name__ == "__main__":
+    main()

scripts/config.json

@@ -0,0 +1,8 @@
+{
+	"project": "entt",
+	"target": "single_include/entt/entt.hpp",
+	"sources": [
+		"src/entt/entt.hpp"
+	],
+	"include_paths": ["src"]
+}

scripts/update_homebrew.sh

@@ -0,0 +1,60 @@
+#!/bin/sh
+
+# only argument should be the version to upgrade to
+if [ $# != 1 ]
+then
+  echo "Expected a version tag like v2.7.1"
+  exit 1
+fi
+
+VERSION="$1"
+URL="https://github.com/skypjack/entt/archive/$VERSION.tar.gz"
+FORMULA="entt.rb"
+
+echo "Updating homebrew package to $VERSION"
+
+echo "Cloning..."
+git clone https://github.com/skypjack/homebrew-entt.git
+if [ $? != 0 ]
+then
+  exit 1
+fi
+cd homebrew-entt
+
+# download the repo at the version
+# exit with error messages if curl fails
+echo "Curling..."
+curl "$URL" --location --fail --silent --show-error --output archive.tar.gz
+if [ $? != 0 ]
+then
+  exit 1
+fi
+
+# compute sha256 hash
+echo "Hashing..."
+HASH="$(openssl sha256 archive.tar.gz | cut -d " " -f 2)"
+
+# delete the archive
+rm archive.tar.gz
+
+echo "Sedding..."
+
+# change the url in the formula file
+# the slashes in the URL must be escaped
+ESCAPED_URL="$(echo "$URL" | sed -e 's/[\/&]/\\&/g')"
+sed -i -e '/url/s/".*"/"'$ESCAPED_URL'"/' $FORMULA
+
+# change the hash in the formula file
+sed -i -e '/sha256/s/".*"/"'$HASH'"/' $FORMULA
+
+# delete temporary file created by sed
+rm -rf "$FORMULA-e"
+
+# update remote repo
+echo "Gitting..."
+git add entt.rb
+git commit -m "Update to $VERSION"
+git push origin master
+
+# out of homebrew-entt dir
+cd ..

src/entt/config/config.h

@@ -0,0 +1,81 @@
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+#include "version.h"
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NOEXCEPTION)
+#    define ENTT_CONSTEXPR
+#    define ENTT_THROW throw
+#    define ENTT_TRY try
+#    define ENTT_CATCH catch(...)
+#else
+#    define ENTT_CONSTEXPR constexpr // use only with throwing functions (waiting for C++20)
+#    define ENTT_THROW
+#    define ENTT_TRY if(true)
+#    define ENTT_CATCH if(false)
+#endif
+
+#ifdef ENTT_USE_ATOMIC
+#    include <atomic>
+#    define ENTT_MAYBE_ATOMIC(Type) std::atomic<Type>
+#else
+#    define ENTT_MAYBE_ATOMIC(Type) Type
+#endif
+
+#ifndef ENTT_ID_TYPE
+#    include <cstdint>
+#    define ENTT_ID_TYPE std::uint32_t
+#endif
+
+#ifndef ENTT_SPARSE_PAGE
+#    define ENTT_SPARSE_PAGE 4096
+#endif
+
+#ifndef ENTT_PACKED_PAGE
+#    define ENTT_PACKED_PAGE 1024
+#endif
+
+#ifdef ENTT_DISABLE_ASSERT
+#    undef ENTT_ASSERT
+#    define ENTT_ASSERT(condition, msg) (void(0))
+#elif !defined ENTT_ASSERT
+#    include <cassert>
+#    define ENTT_ASSERT(condition, msg) assert(condition)
+#endif
+
+#ifdef ENTT_DISABLE_ASSERT
+#    undef ENTT_ASSERT_CONSTEXPR
+#    define ENTT_ASSERT_CONSTEXPR(condition, msg) (void(0))
+#elif !defined ENTT_ASSERT_CONSTEXPR
+#    define ENTT_ASSERT_CONSTEXPR(condition, msg) ENTT_ASSERT(condition, msg)
+#endif
+
+#ifdef ENTT_NO_ETO
+#    define ENTT_ETO_TYPE(Type) void
+#else
+#    define ENTT_ETO_TYPE(Type) Type
+#endif
+
+#ifdef ENTT_STANDARD_CPP
+#    define ENTT_NONSTD false
+#else
+#    define ENTT_NONSTD true
+#    if defined __clang__ || defined __GNUC__
+#        define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#        define ENTT_PRETTY_FUNCTION_PREFIX '='
+#        define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#        define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#        define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#        define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#    endif
+#endif
+
+#if defined _MSC_VER
+#    pragma detect_mismatch("entt.version", ENTT_VERSION)
+#    pragma detect_mismatch("entt.noexcept", ENTT_XSTR(ENTT_TRY))
+#    pragma detect_mismatch("entt.id", ENTT_XSTR(ENTT_ID_TYPE))
+#    pragma detect_mismatch("entt.nonstd", ENTT_XSTR(ENTT_NONSTD))
+#endif
+
+#endif

src/entt/config/macro.h

@@ -0,0 +1,7 @@
+#ifndef ENTT_CONFIG_MACRO_H
+#define ENTT_CONFIG_MACRO_H
+
+#define ENTT_STR(arg) #arg
+#define ENTT_XSTR(arg) ENTT_STR(arg)
+
+#endif

src/entt/config/version.h

@@ -0,0 +1,14 @@
+#ifndef ENTT_CONFIG_VERSION_H
+#define ENTT_CONFIG_VERSION_H
+
+#include "macro.h"
+
+#define ENTT_VERSION_MAJOR 3
+#define ENTT_VERSION_MINOR 11
+#define ENTT_VERSION_PATCH 1
+
+#define ENTT_VERSION \
+    ENTT_XSTR(ENTT_VERSION_MAJOR) \
+    "." ENTT_XSTR(ENTT_VERSION_MINOR) "." ENTT_XSTR(ENTT_VERSION_PATCH)
+
+#endif

src/entt/container/dense_set.hpp

@@ -0,0 +1,895 @@
+#ifndef ENTT_CONTAINER_DENSE_SET_HPP
+#define ENTT_CONTAINER_DENSE_SET_HPP
+
+#include <cmath>
+#include <cstddef>
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../core/compressed_pair.hpp"
+#include "../core/memory.hpp"
+#include "../core/type_traits.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename It>
+class dense_set_iterator final {
+    template<typename>
+    friend class dense_set_iterator;
+
+public:
+    using value_type = typename It::value_type::second_type;
+    using pointer = const value_type *;
+    using reference = const value_type &;
+    using difference_type = std::ptrdiff_t;
+    using iterator_category = std::random_access_iterator_tag;
+
+    constexpr dense_set_iterator() noexcept
+        : it{} {}
+
+    constexpr dense_set_iterator(const It iter) noexcept
+        : it{iter} {}
+
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    constexpr dense_set_iterator(const dense_set_iterator<Other> &other) noexcept
+        : it{other.it} {}
+
+    constexpr dense_set_iterator &operator++() noexcept {
+        return ++it, *this;
+    }
+
+    constexpr dense_set_iterator operator++(int) noexcept {
+        dense_set_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    constexpr dense_set_iterator &operator--() noexcept {
+        return --it, *this;
+    }
+
+    constexpr dense_set_iterator operator--(int) noexcept {
+        dense_set_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    constexpr dense_set_iterator &operator+=(const difference_type value) noexcept {
+        it += value;
+        return *this;
+    }
+
+    constexpr dense_set_iterator operator+(const difference_type value) const noexcept {
+        dense_set_iterator copy = *this;
+        return (copy += value);
+    }
+
+    constexpr dense_set_iterator &operator-=(const difference_type value) noexcept {
+        return (*this += -value);
+    }
+
+    constexpr dense_set_iterator operator-(const difference_type value) const noexcept {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
+        return it[value].second;
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return std::addressof(it->second);
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return *operator->();
+    }
+
+    template<typename ILhs, typename IRhs>
+    friend constexpr std::ptrdiff_t operator-(const dense_set_iterator<ILhs> &, const dense_set_iterator<IRhs> &) noexcept;
+
+    template<typename ILhs, typename IRhs>
+    friend constexpr bool operator==(const dense_set_iterator<ILhs> &, const dense_set_iterator<IRhs> &) noexcept;
+
+    template<typename ILhs, typename IRhs>
+    friend constexpr bool operator<(const dense_set_iterator<ILhs> &, const dense_set_iterator<IRhs> &) noexcept;
+
+private:
+    It it;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr std::ptrdiff_t operator-(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) noexcept {
+    return lhs.it - rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator==(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) noexcept {
+    return lhs.it == rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator!=(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator<(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) noexcept {
+    return lhs.it < rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator>(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) noexcept {
+    return rhs < lhs;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator<=(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) noexcept {
+    return !(lhs > rhs);
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator>=(const dense_set_iterator<ILhs> &lhs, const dense_set_iterator<IRhs> &rhs) noexcept {
+    return !(lhs < rhs);
+}
+
+template<typename It>
+class dense_set_local_iterator final {
+    template<typename>
+    friend class dense_set_local_iterator;
+
+public:
+    using value_type = typename It::value_type::second_type;
+    using pointer = const value_type *;
+    using reference = const value_type &;
+    using difference_type = std::ptrdiff_t;
+    using iterator_category = std::forward_iterator_tag;
+
+    constexpr dense_set_local_iterator() noexcept
+        : it{},
+          offset{} {}
+
+    constexpr dense_set_local_iterator(It iter, const std::size_t pos) noexcept
+        : it{iter},
+          offset{pos} {}
+
+    template<typename Other, typename = std::enable_if_t<!std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
+    constexpr dense_set_local_iterator(const dense_set_local_iterator<Other> &other) noexcept
+        : it{other.it},
+          offset{other.offset} {}
+
+    constexpr dense_set_local_iterator &operator++() noexcept {
+        return offset = it[offset].first, *this;
+    }
+
+    constexpr dense_set_local_iterator operator++(int) noexcept {
+        dense_set_local_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return std::addressof(it[offset].second);
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return *operator->();
+    }
+
+    [[nodiscard]] constexpr std::size_t index() const noexcept {
+        return offset;
+    }
+
+private:
+    It it;
+    std::size_t offset;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator==(const dense_set_local_iterator<ILhs> &lhs, const dense_set_local_iterator<IRhs> &rhs) noexcept {
+    return lhs.index() == rhs.index();
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator!=(const dense_set_local_iterator<ILhs> &lhs, const dense_set_local_iterator<IRhs> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Associative container for unique objects of a given type.
+ *
+ * Internally, elements are organized into buckets. Which bucket an element is
+ * placed into depends entirely on its hash. Elements with the same hash code
+ * appear in the same bucket.
+ *
+ * @tparam Type Value type of the associative container.
+ * @tparam Hash Type of function to use to hash the values.
+ * @tparam KeyEqual Type of function to use to compare the values for equality.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Hash, typename KeyEqual, typename Allocator>
+class dense_set {
+    static constexpr float default_threshold = 0.875f;
+    static constexpr std::size_t minimum_capacity = 8u;
+
+    using node_type = std::pair<std::size_t, Type>;
+    using alloc_traits = std::allocator_traits<Allocator>;
+    static_assert(std::is_same_v<typename alloc_traits::value_type, Type>, "Invalid value type");
+    using sparse_container_type = std::vector<std::size_t, typename alloc_traits::template rebind_alloc<std::size_t>>;
+    using packed_container_type = std::vector<node_type, typename alloc_traits::template rebind_alloc<node_type>>;
+
+    template<typename Other>
+    [[nodiscard]] std::size_t value_to_bucket(const Other &value) const noexcept {
+        return fast_mod(static_cast<size_type>(sparse.second()(value)), bucket_count());
+    }
+
+    template<typename Other>
+    [[nodiscard]] auto constrained_find(const Other &value, std::size_t bucket) {
+        for(auto it = begin(bucket), last = end(bucket); it != last; ++it) {
+            if(packed.second()(*it, value)) {
+                return begin() + static_cast<typename iterator::difference_type>(it.index());
+            }
+        }
+
+        return end();
+    }
+
+    template<typename Other>
+    [[nodiscard]] auto constrained_find(const Other &value, std::size_t bucket) const {
+        for(auto it = cbegin(bucket), last = cend(bucket); it != last; ++it) {
+            if(packed.second()(*it, value)) {
+                return cbegin() + static_cast<typename iterator::difference_type>(it.index());
+            }
+        }
+
+        return cend();
+    }
+
+    template<typename Other>
+    [[nodiscard]] auto insert_or_do_nothing(Other &&value) {
+        const auto index = value_to_bucket(value);
+
+        if(auto it = constrained_find(value, index); it != end()) {
+            return std::make_pair(it, false);
+        }
+
+        packed.first().emplace_back(sparse.first()[index], std::forward<Other>(value));
+        sparse.first()[index] = packed.first().size() - 1u;
+        rehash_if_required();
+
+        return std::make_pair(--end(), true);
+    }
+
+    void move_and_pop(const std::size_t pos) {
+        if(const auto last = size() - 1u; pos != last) {
+            size_type *curr = sparse.first().data() + value_to_bucket(packed.first().back().second);
+            packed.first()[pos] = std::move(packed.first().back());
+            for(; *curr != last; curr = &packed.first()[*curr].first) {}
+            *curr = pos;
+        }
+
+        packed.first().pop_back();
+    }
+
+    void rehash_if_required() {
+        if(size() > (bucket_count() * max_load_factor())) {
+            rehash(bucket_count() * 2u);
+        }
+    }
+
+public:
+    /*! @brief Key type of the container. */
+    using key_type = Type;
+    /*! @brief Value type of the container. */
+    using value_type = Type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Type of function to use to hash the elements. */
+    using hasher = Hash;
+    /*! @brief Type of function to use to compare the elements for equality. */
+    using key_equal = KeyEqual;
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Random access iterator type. */
+    using iterator = internal::dense_set_iterator<typename packed_container_type::iterator>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator = internal::dense_set_iterator<typename packed_container_type::const_iterator>;
+    /*! @brief Forward iterator type. */
+    using local_iterator = internal::dense_set_local_iterator<typename packed_container_type::iterator>;
+    /*! @brief Constant forward iterator type. */
+    using const_local_iterator = internal::dense_set_local_iterator<typename packed_container_type::const_iterator>;
+
+    /*! @brief Default constructor. */
+    dense_set()
+        : dense_set{minimum_capacity} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_set(const allocator_type &allocator)
+        : dense_set{minimum_capacity, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator and user
+     * supplied minimal number of buckets.
+     * @param cnt Minimal number of buckets.
+     * @param allocator The allocator to use.
+     */
+    dense_set(const size_type cnt, const allocator_type &allocator)
+        : dense_set{cnt, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function and user supplied minimal number of buckets.
+     * @param cnt Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param allocator The allocator to use.
+     */
+    dense_set(const size_type cnt, const hasher &hash, const allocator_type &allocator)
+        : dense_set{cnt, hash, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function, compare function and user supplied minimal number of buckets.
+     * @param cnt Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param equal Compare function to use.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_set(const size_type cnt, const hasher &hash = hasher{}, const key_equal &equal = key_equal{}, const allocator_type &allocator = allocator_type{})
+        : sparse{allocator, hash},
+          packed{allocator, equal},
+          threshold{default_threshold} {
+        rehash(cnt);
+    }
+
+    /*! @brief Default copy constructor. */
+    dense_set(const dense_set &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    dense_set(const dense_set &other, const allocator_type &allocator)
+        : sparse{std::piecewise_construct, std::forward_as_tuple(other.sparse.first(), allocator), std::forward_as_tuple(other.sparse.second())},
+          packed{std::piecewise_construct, std::forward_as_tuple(other.packed.first(), allocator), std::forward_as_tuple(other.packed.second())},
+          threshold{other.threshold} {}
+
+    /*! @brief Default move constructor. */
+    dense_set(dense_set &&) noexcept(std::is_nothrow_move_constructible_v<compressed_pair<sparse_container_type, hasher>> &&std::is_nothrow_move_constructible_v<compressed_pair<packed_container_type, key_equal>>) = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    dense_set(dense_set &&other, const allocator_type &allocator)
+        : sparse{std::piecewise_construct, std::forward_as_tuple(std::move(other.sparse.first()), allocator), std::forward_as_tuple(std::move(other.sparse.second()))},
+          packed{std::piecewise_construct, std::forward_as_tuple(std::move(other.packed.first()), allocator), std::forward_as_tuple(std::move(other.packed.second()))},
+          threshold{other.threshold} {}
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This container.
+     */
+    dense_set &operator=(const dense_set &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This container.
+     */
+    dense_set &operator=(dense_set &&) noexcept(std::is_nothrow_move_assignable_v<compressed_pair<sparse_container_type, hasher>> &&std::is_nothrow_move_assignable_v<compressed_pair<packed_container_type, key_equal>>) = default;
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return sparse.first().get_allocator();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the internal array.
+     * If the array is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal array.
+     */
+    [[nodiscard]] const_iterator cbegin() const noexcept {
+        return packed.first().begin();
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const noexcept {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() noexcept {
+        return packed.first().begin();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the internal array. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the element following the last instance of the
+     * internal array.
+     */
+    [[nodiscard]] const_iterator cend() const noexcept {
+        return packed.first().end();
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const noexcept {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() noexcept {
+        return packed.first().end();
+    }
+
+    /**
+     * @brief Checks whether a container is empty.
+     * @return True if the container is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return packed.first().empty();
+    }
+
+    /**
+     * @brief Returns the number of elements in a container.
+     * @return Number of elements in a container.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return packed.first().size();
+    }
+
+    /**
+     * @brief Returns the maximum possible number of elements.
+     * @return Maximum possible number of elements.
+     */
+    [[nodiscard]] size_type max_size() const noexcept {
+        return packed.first().max_size();
+    }
+
+    /*! @brief Clears the container. */
+    void clear() noexcept {
+        sparse.first().clear();
+        packed.first().clear();
+        rehash(0u);
+    }
+
+    /**
+     * @brief Inserts an element into the container, if it does not exist.
+     * @param value An element to insert into the container.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    std::pair<iterator, bool> insert(const value_type &value) {
+        return insert_or_do_nothing(value);
+    }
+
+    /*! @copydoc insert */
+    std::pair<iterator, bool> insert(value_type &&value) {
+        return insert_or_do_nothing(std::move(value));
+    }
+
+    /**
+     * @brief Inserts elements into the container, if they do not exist.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     */
+    template<typename It>
+    void insert(It first, It last) {
+        for(; first != last; ++first) {
+            insert(*first);
+        }
+    }
+
+    /**
+     * @brief Constructs an element in-place, if it does not exist.
+     *
+     * The element is also constructed when the container already has the key,
+     * in which case the newly constructed object is destroyed immediately.
+     *
+     * @tparam Args Types of arguments to forward to the constructor of the
+     * element.
+     * @param args Arguments to forward to the constructor of the element.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    template<typename... Args>
+    std::pair<iterator, bool> emplace(Args &&...args) {
+        if constexpr(((sizeof...(Args) == 1u) && ... && std::is_same_v<std::decay_t<Args>, value_type>)) {
+            return insert_or_do_nothing(std::forward<Args>(args)...);
+        } else {
+            auto &node = packed.first().emplace_back(std::piecewise_construct, std::make_tuple(packed.first().size()), std::forward_as_tuple(std::forward<Args>(args)...));
+            const auto index = value_to_bucket(node.second);
+
+            if(auto it = constrained_find(node.second, index); it != end()) {
+                packed.first().pop_back();
+                return std::make_pair(it, false);
+            }
+
+            std::swap(node.first, sparse.first()[index]);
+            rehash_if_required();
+
+            return std::make_pair(--end(), true);
+        }
+    }
+
+    /**
+     * @brief Removes an element from a given position.
+     * @param pos An iterator to the element to remove.
+     * @return An iterator following the removed element.
+     */
+    iterator erase(const_iterator pos) {
+        const auto diff = pos - cbegin();
+        erase(*pos);
+        return begin() + diff;
+    }
+
+    /**
+     * @brief Removes the given elements from a container.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     * @return An iterator following the last removed element.
+     */
+    iterator erase(const_iterator first, const_iterator last) {
+        const auto dist = first - cbegin();
+
+        for(auto from = last - cbegin(); from != dist; --from) {
+            erase(packed.first()[from - 1u].second);
+        }
+
+        return (begin() + dist);
+    }
+
+    /**
+     * @brief Removes the element associated with a given value.
+     * @param value Value of an element to remove.
+     * @return Number of elements removed (either 0 or 1).
+     */
+    size_type erase(const value_type &value) {
+        for(size_type *curr = sparse.first().data() + value_to_bucket(value); *curr != (std::numeric_limits<size_type>::max)(); curr = &packed.first()[*curr].first) {
+            if(packed.second()(packed.first()[*curr].second, value)) {
+                const auto index = *curr;
+                *curr = packed.first()[*curr].first;
+                move_and_pop(index);
+                return 1u;
+            }
+        }
+
+        return 0u;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given container.
+     * @param other Container to exchange the content with.
+     */
+    void swap(dense_set &other) {
+        using std::swap;
+        swap(sparse, other.sparse);
+        swap(packed, other.packed);
+        swap(threshold, other.threshold);
+    }
+
+    /**
+     * @brief Returns the number of elements matching a value (either 1 or 0).
+     * @param key Key value of an element to search for.
+     * @return Number of elements matching the key (either 1 or 0).
+     */
+    [[nodiscard]] size_type count(const value_type &key) const {
+        return find(key) != end();
+    }
+
+    /**
+     * @brief Returns the number of elements matching a key (either 1 or 0).
+     * @tparam Other Type of the key value of an element to search for.
+     * @param key Key value of an element to search for.
+     * @return Number of elements matching the key (either 1 or 0).
+     */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, size_type>>
+    count(const Other &key) const {
+        return find(key) != end();
+    }
+
+    /**
+     * @brief Finds an element with a given value.
+     * @param value Value of an element to search for.
+     * @return An iterator to an element with the given value. If no such
+     * element is found, a past-the-end iterator is returned.
+     */
+    [[nodiscard]] iterator find(const value_type &value) {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /*! @copydoc find */
+    [[nodiscard]] const_iterator find(const value_type &value) const {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /**
+     * @brief Finds an element that compares _equivalent_ to a given value.
+     * @tparam Other Type of an element to search for.
+     * @param value Value of an element to search for.
+     * @return An iterator to an element with the given value. If no such
+     * element is found, a past-the-end iterator is returned.
+     */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, iterator>>
+    find(const Other &value) {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /*! @copydoc find */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, const_iterator>>
+    find(const Other &value) const {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /**
+     * @brief Returns a range containing all elements with a given value.
+     * @param value Value of an element to search for.
+     * @return A pair of iterators pointing to the first element and past the
+     * last element of the range.
+     */
+    [[nodiscard]] std::pair<iterator, iterator> equal_range(const value_type &value) {
+        const auto it = find(value);
+        return {it, it + !(it == end())};
+    }
+
+    /*! @copydoc equal_range */
+    [[nodiscard]] std::pair<const_iterator, const_iterator> equal_range(const value_type &value) const {
+        const auto it = find(value);
+        return {it, it + !(it == cend())};
+    }
+
+    /**
+     * @brief Returns a range containing all elements that compare _equivalent_
+     * to a given value.
+     * @tparam Other Type of an element to search for.
+     * @param value Value of an element to search for.
+     * @return A pair of iterators pointing to the first element and past the
+     * last element of the range.
+     */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, std::pair<iterator, iterator>>>
+    equal_range(const Other &value) {
+        const auto it = find(value);
+        return {it, it + !(it == end())};
+    }
+
+    /*! @copydoc equal_range */
+    template<class Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, std::pair<const_iterator, const_iterator>>>
+    equal_range(const Other &value) const {
+        const auto it = find(value);
+        return {it, it + !(it == cend())};
+    }
+
+    /**
+     * @brief Checks if the container contains an element with a given value.
+     * @param value Value of an element to search for.
+     * @return True if there is such an element, false otherwise.
+     */
+    [[nodiscard]] bool contains(const value_type &value) const {
+        return (find(value) != cend());
+    }
+
+    /**
+     * @brief Checks if the container contains an element that compares
+     * _equivalent_ to a given value.
+     * @tparam Other Type of an element to search for.
+     * @param value Value of an element to search for.
+     * @return True if there is such an element, false otherwise.
+     */
+    template<typename Other>
+    [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>, std::conditional_t<false, Other, bool>>
+    contains(const Other &value) const {
+        return (find(value) != cend());
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator cbegin(const size_type index) const {
+        return {packed.first().begin(), sparse.first()[index]};
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator begin(const size_type index) const {
+        return cbegin(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] local_iterator begin(const size_type index) {
+        return {packed.first().begin(), sparse.first()[index]};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator cend([[maybe_unused]] const size_type index) const {
+        return {packed.first().begin(), (std::numeric_limits<size_type>::max)()};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator end(const size_type index) const {
+        return cend(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] local_iterator end([[maybe_unused]] const size_type index) {
+        return {packed.first().begin(), (std::numeric_limits<size_type>::max)()};
+    }
+
+    /**
+     * @brief Returns the number of buckets.
+     * @return The number of buckets.
+     */
+    [[nodiscard]] size_type bucket_count() const {
+        return sparse.first().size();
+    }
+
+    /**
+     * @brief Returns the maximum number of buckets.
+     * @return The maximum number of buckets.
+     */
+    [[nodiscard]] size_type max_bucket_count() const {
+        return sparse.first().max_size();
+    }
+
+    /**
+     * @brief Returns the number of elements in a given bucket.
+     * @param index The index of the bucket to examine.
+     * @return The number of elements in the given bucket.
+     */
+    [[nodiscard]] size_type bucket_size(const size_type index) const {
+        return static_cast<size_type>(std::distance(begin(index), end(index)));
+    }
+
+    /**
+     * @brief Returns the bucket for a given element.
+     * @param value The value of the element to examine.
+     * @return The bucket for the given element.
+     */
+    [[nodiscard]] size_type bucket(const value_type &value) const {
+        return value_to_bucket(value);
+    }
+
+    /**
+     * @brief Returns the average number of elements per bucket.
+     * @return The average number of elements per bucket.
+     */
+    [[nodiscard]] float load_factor() const {
+        return size() / static_cast<float>(bucket_count());
+    }
+
+    /**
+     * @brief Returns the maximum average number of elements per bucket.
+     * @return The maximum average number of elements per bucket.
+     */
+    [[nodiscard]] float max_load_factor() const {
+        return threshold;
+    }
+
+    /**
+     * @brief Sets the desired maximum average number of elements per bucket.
+     * @param value A desired maximum average number of elements per bucket.
+     */
+    void max_load_factor(const float value) {
+        ENTT_ASSERT(value > 0.f, "Invalid load factor");
+        threshold = value;
+        rehash(0u);
+    }
+
+    /**
+     * @brief Reserves at least the specified number of buckets and regenerates
+     * the hash table.
+     * @param cnt New number of buckets.
+     */
+    void rehash(const size_type cnt) {
+        auto value = cnt > minimum_capacity ? cnt : minimum_capacity;
+        const auto cap = static_cast<size_type>(size() / max_load_factor());
+        value = value > cap ? value : cap;
+
+        if(const auto sz = next_power_of_two(value); sz != bucket_count()) {
+            sparse.first().resize(sz);
+
+            for(auto &&elem: sparse.first()) {
+                elem = std::numeric_limits<size_type>::max();
+            }
+
+            for(size_type pos{}, last = size(); pos < last; ++pos) {
+                const auto index = value_to_bucket(packed.first()[pos].second);
+                packed.first()[pos].first = std::exchange(sparse.first()[index], pos);
+            }
+        }
+    }
+
+    /**
+     * @brief Reserves space for at least the specified number of elements and
+     * regenerates the hash table.
+     * @param cnt New number of elements.
+     */
+    void reserve(const size_type cnt) {
+        packed.first().reserve(cnt);
+        rehash(static_cast<size_type>(std::ceil(cnt / max_load_factor())));
+    }
+
+    /**
+     * @brief Returns the function used to hash the elements.
+     * @return The function used to hash the elements.
+     */
+    [[nodiscard]] hasher hash_function() const {
+        return sparse.second();
+    }
+
+    /**
+     * @brief Returns the function used to compare elements for equality.
+     * @return The function used to compare elements for equality.
+     */
+    [[nodiscard]] key_equal key_eq() const {
+        return packed.second();
+    }
+
+private:
+    compressed_pair<sparse_container_type, hasher> sparse;
+    compressed_pair<packed_container_type, key_equal> packed;
+    float threshold;
+};
+
+} // namespace entt
+
+#endif

src/entt/container/fwd.hpp

@@ -0,0 +1,26 @@
+#ifndef ENTT_CONTAINER_FWD_HPP
+#define ENTT_CONTAINER_FWD_HPP
+
+#include <functional>
+#include <memory>
+
+namespace entt {
+
+template<
+    typename Key,
+    typename Type,
+    typename = std::hash<Key>,
+    typename = std::equal_to<Key>,
+    typename = std::allocator<std::pair<const Key, Type>>>
+class dense_map;
+
+template<
+    typename Type,
+    typename = std::hash<Type>,
+    typename = std::equal_to<Type>,
+    typename = std::allocator<Type>>
+class dense_set;
+
+} // namespace entt
+
+#endif

src/entt/core/algorithm.hpp

@@ -0,0 +1,137 @@
+#ifndef ENTT_CORE_ALGORITHM_HPP
+#define ENTT_CORE_ALGORITHM_HPP
+
+#include <algorithm>
+#include <functional>
+#include <iterator>
+#include <utility>
+#include <vector>
+#include "utility.hpp"
+
+namespace entt {
+
+/**
+ * @brief Function object to wrap `std::sort` in a class type.
+ *
+ * Unfortunately, `std::sort` cannot be passed as template argument to a class
+ * template or a function template.<br/>
+ * This class fills the gap by wrapping some flavors of `std::sort` in a
+ * function object.
+ */
+struct std_sort {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given binary comparison function.
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Args Types of arguments to forward to the sort function.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     * @param args Arguments to forward to the sort function, if any.
+     */
+    template<typename It, typename Compare = std::less<>, typename... Args>
+    void operator()(It first, It last, Compare compare = Compare{}, Args &&...args) const {
+        std::sort(std::forward<Args>(args)..., std::move(first), std::move(last), std::move(compare));
+    }
+};
+
+/*! @brief Function object for performing insertion sort. */
+struct insertion_sort {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given binary comparison function.
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Compare Type of comparison function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     */
+    template<typename It, typename Compare = std::less<>>
+    void operator()(It first, It last, Compare compare = Compare{}) const {
+        if(first < last) {
+            for(auto it = first + 1; it < last; ++it) {
+                auto value = std::move(*it);
+                auto pre = it;
+
+                for(; pre > first && compare(value, *(pre - 1)); --pre) {
+                    *pre = std::move(*(pre - 1));
+                }
+
+                *pre = std::move(value);
+            }
+        }
+    }
+};
+
+/**
+ * @brief Function object for performing LSD radix sort.
+ * @tparam Bit Number of bits processed per pass.
+ * @tparam N Maximum number of bits to sort.
+ */
+template<std::size_t Bit, std::size_t N>
+struct radix_sort {
+    static_assert((N % Bit) == 0, "The maximum number of bits to sort must be a multiple of the number of bits processed per pass");
+
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given _getter_ to access the
+     * actual data to be sorted.
+     *
+     * This implementation is inspired by the online book
+     * [Physically Based Rendering](http://www.pbr-book.org/3ed-2018/Primitives_and_Intersection_Acceleration/Bounding_Volume_Hierarchies.html#RadixSort).
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Getter Type of _getter_ function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param getter A valid _getter_ function object.
+     */
+    template<typename It, typename Getter = identity>
+    void operator()(It first, It last, Getter getter = Getter{}) const {
+        if(first < last) {
+            static constexpr auto mask = (1 << Bit) - 1;
+            static constexpr auto buckets = 1 << Bit;
+            static constexpr auto passes = N / Bit;
+
+            using value_type = typename std::iterator_traits<It>::value_type;
+            std::vector<value_type> aux(std::distance(first, last));
+
+            auto part = [getter = std::move(getter)](auto from, auto to, auto out, auto start) {
+                std::size_t index[buckets]{};
+                std::size_t count[buckets]{};
+
+                for(auto it = from; it != to; ++it) {
+                    ++count[(getter(*it) >> start) & mask];
+                }
+
+                for(std::size_t pos{}, end = buckets - 1u; pos < end; ++pos) {
+                    index[pos + 1u] = index[pos] + count[pos];
+                }
+
+                for(auto it = from; it != to; ++it) {
+                    out[index[(getter(*it) >> start) & mask]++] = std::move(*it);
+                }
+            };
+
+            for(std::size_t pass = 0; pass < (passes & ~1); pass += 2) {
+                part(first, last, aux.begin(), pass * Bit);
+                part(aux.begin(), aux.end(), first, (pass + 1) * Bit);
+            }
+
+            if constexpr(passes & 1) {
+                part(first, last, aux.begin(), (passes - 1) * Bit);
+                std::move(aux.begin(), aux.end(), first);
+            }
+        }
+    }
+};
+
+} // namespace entt
+
+#endif

src/entt/core/any.hpp

@@ -0,0 +1,510 @@
+#ifndef ENTT_CORE_ANY_HPP
+#define ENTT_CORE_ANY_HPP
+
+#include <cstddef>
+#include <memory>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "../core/utility.hpp"
+#include "fwd.hpp"
+#include "type_info.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+enum class any_operation : std::uint8_t {
+    copy,
+    move,
+    transfer,
+    assign,
+    destroy,
+    compare,
+    get
+};
+
+enum class any_policy : std::uint8_t {
+    owner,
+    ref,
+    cref
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief A SBO friendly, type-safe container for single values of any type.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+template<std::size_t Len, std::size_t Align>
+class basic_any {
+    using operation = internal::any_operation;
+    using policy = internal::any_policy;
+    using vtable_type = const void *(const operation, const basic_any &, const void *);
+
+    struct storage_type {
+        alignas(Align) std::byte data[Len + !Len];
+    };
+
+    template<typename Type>
+    static constexpr bool in_situ = Len && alignof(Type) <= Align && sizeof(Type) <= Len &&std::is_nothrow_move_constructible_v<Type>;
+
+    template<typename Type>
+    static const void *basic_vtable(const operation op, const basic_any &value, const void *other) {
+        static_assert(!std::is_same_v<Type, void> && std::is_same_v<std::remove_cv_t<std::remove_reference_t<Type>>, Type>, "Invalid type");
+        const Type *element = nullptr;
+
+        if constexpr(in_situ<Type>) {
+            element = value.owner() ? reinterpret_cast<const Type *>(&value.storage) : static_cast<const Type *>(value.instance);
+        } else {
+            element = static_cast<const Type *>(value.instance);
+        }
+
+        switch(op) {
+        case operation::copy:
+            if constexpr(std::is_copy_constructible_v<Type>) {
+                static_cast<basic_any *>(const_cast<void *>(other))->initialize<Type>(*element);
+            }
+            break;
+        case operation::move:
+            if constexpr(in_situ<Type>) {
+                if(value.owner()) {
+                    return new(&static_cast<basic_any *>(const_cast<void *>(other))->storage) Type{std::move(*const_cast<Type *>(element))};
+                }
+            }
+
+            return (static_cast<basic_any *>(const_cast<void *>(other))->instance = std::exchange(const_cast<basic_any &>(value).instance, nullptr));
+        case operation::transfer:
+            if constexpr(std::is_move_assignable_v<Type>) {
+                *const_cast<Type *>(element) = std::move(*static_cast<Type *>(const_cast<void *>(other)));
+                return other;
+            }
+            [[fallthrough]];
+        case operation::assign:
+            if constexpr(std::is_copy_assignable_v<Type>) {
+                *const_cast<Type *>(element) = *static_cast<const Type *>(other);
+                return other;
+            }
+            break;
+        case operation::destroy:
+            if constexpr(in_situ<Type>) {
+                element->~Type();
+            } else if constexpr(std::is_array_v<Type>) {
+                delete[] element;
+            } else {
+                delete element;
+            }
+            break;
+        case operation::compare:
+            if constexpr(!std::is_function_v<Type> && !std::is_array_v<Type> && is_equality_comparable_v<Type>) {
+                return *element == *static_cast<const Type *>(other) ? other : nullptr;
+            } else {
+                return (element == other) ? other : nullptr;
+            }
+        case operation::get:
+            return element;
+        }
+
+        return nullptr;
+    }
+
+    template<typename Type, typename... Args>
+    void initialize([[maybe_unused]] Args &&...args) {
+        info = &type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+
+        if constexpr(!std::is_void_v<Type>) {
+            vtable = basic_vtable<std::remove_cv_t<std::remove_reference_t<Type>>>;
+
+            if constexpr(std::is_lvalue_reference_v<Type>) {
+                static_assert(sizeof...(Args) == 1u && (std::is_lvalue_reference_v<Args> && ...), "Invalid arguments");
+                mode = std::is_const_v<std::remove_reference_t<Type>> ? policy::cref : policy::ref;
+                instance = (std::addressof(args), ...);
+            } else if constexpr(in_situ<std::remove_cv_t<std::remove_reference_t<Type>>>) {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<std::remove_cv_t<std::remove_reference_t<Type>>>) {
+                    new(&storage) std::remove_cv_t<std::remove_reference_t<Type>>{std::forward<Args>(args)...};
+                } else {
+                    new(&storage) std::remove_cv_t<std::remove_reference_t<Type>>(std::forward<Args>(args)...);
+                }
+            } else {
+                if constexpr(sizeof...(Args) != 0u && std::is_aggregate_v<std::remove_cv_t<std::remove_reference_t<Type>>>) {
+                    instance = new std::remove_cv_t<std::remove_reference_t<Type>>{std::forward<Args>(args)...};
+                } else {
+                    instance = new std::remove_cv_t<std::remove_reference_t<Type>>(std::forward<Args>(args)...);
+                }
+            }
+        }
+    }
+
+    basic_any(const basic_any &other, const policy pol) noexcept
+        : instance{other.data()},
+          info{other.info},
+          vtable{other.vtable},
+          mode{pol} {}
+
+public:
+    /*! @brief Size of the internal storage. */
+    static constexpr auto length = Len;
+    /*! @brief Alignment requirement. */
+    static constexpr auto alignment = Align;
+
+    /*! @brief Default constructor. */
+    constexpr basic_any() noexcept
+        : basic_any{std::in_place_type<void>} {}
+
+    /**
+     * @brief Constructs a wrapper by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_any(std::in_place_type_t<Type>, Args &&...args)
+        : instance{},
+          info{},
+          vtable{},
+          mode{policy::owner} {
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Constructs a wrapper from a given value.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type, typename = std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>>>
+    basic_any(Type &&value)
+        : basic_any{std::in_place_type<std::decay_t<Type>>, std::forward<Type>(value)} {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    basic_any(const basic_any &other)
+        : basic_any{} {
+        if(other.vtable) {
+            other.vtable(operation::copy, other, this);
+        }
+    }
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_any(basic_any &&other) noexcept
+        : instance{},
+          info{other.info},
+          vtable{other.vtable},
+          mode{other.mode} {
+        if(other.vtable) {
+            other.vtable(operation::move, other, this);
+        }
+    }
+
+    /*! @brief Frees the internal storage, whatever it means. */
+    ~basic_any() {
+        if(vtable && owner()) {
+            vtable(operation::destroy, *this, nullptr);
+        }
+    }
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This any object.
+     */
+    basic_any &operator=(const basic_any &other) {
+        reset();
+
+        if(other.vtable) {
+            other.vtable(operation::copy, other, this);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This any object.
+     */
+    basic_any &operator=(basic_any &&other) noexcept {
+        reset();
+
+        if(other.vtable) {
+            other.vtable(operation::move, other, this);
+            info = other.info;
+            vtable = other.vtable;
+            mode = other.mode;
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    std::enable_if_t<!std::is_same_v<std::decay_t<Type>, basic_any>, basic_any &>
+    operator=(Type &&value) {
+        emplace<std::decay_t<Type>>(std::forward<Type>(value));
+        return *this;
+    }
+
+    /**
+     * @brief Returns the object type if any, `type_id<void>()` otherwise.
+     * @return The object type if any, `type_id<void>()` otherwise.
+     */
+    [[nodiscard]] const type_info &type() const noexcept {
+        return *info;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void *data() const noexcept {
+        return vtable ? vtable(operation::get, *this, nullptr) : nullptr;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @param req Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void *data(const type_info &req) const noexcept {
+        return *info == req ? data() : nullptr;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] void *data() noexcept {
+        return mode == policy::cref ? nullptr : const_cast<void *>(std::as_const(*this).data());
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @param req Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] void *data(const type_info &req) noexcept {
+        return mode == policy::cref ? nullptr : const_cast<void *>(std::as_const(*this).data(req));
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&...args) {
+        reset();
+        initialize<Type>(std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Assigns a value to the contained object without replacing it.
+     * @param other The value to assign to the contained object.
+     * @return True in case of success, false otherwise.
+     */
+    bool assign(const basic_any &other) {
+        if(vtable && mode != policy::cref && *info == *other.info) {
+            return (vtable(operation::assign, *this, other.data()) != nullptr);
+        }
+
+        return false;
+    }
+
+    /*! @copydoc assign */
+    bool assign(basic_any &&other) {
+        if(vtable && mode != policy::cref && *info == *other.info) {
+            if(auto *val = other.data(); val) {
+                return (vtable(operation::transfer, *this, val) != nullptr);
+            } else {
+                return (vtable(operation::assign, *this, std::as_const(other).data()) != nullptr);
+            }
+        }
+
+        return false;
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        if(vtable && owner()) {
+            vtable(operation::destroy, *this, nullptr);
+        }
+
+        // unnecessary but it helps to detect nasty bugs
+        ENTT_ASSERT((instance = nullptr) == nullptr, "");
+        info = &type_id<void>();
+        vtable = nullptr;
+        mode = policy::owner;
+    }
+
+    /**
+     * @brief Returns false if a wrapper is empty, true otherwise.
+     * @return False if the wrapper is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return vtable != nullptr;
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return False if the two objects differ in their content, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const basic_any &other) const noexcept {
+        if(vtable && *info == *other.info) {
+            return (vtable(operation::compare, *this, other.data()) != nullptr);
+        }
+
+        return (!vtable && !other.vtable);
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return True if the two objects differ in their content, false otherwise.
+     */
+    [[nodiscard]] bool operator!=(const basic_any &other) const noexcept {
+        return !(*this == other);
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] basic_any as_ref() noexcept {
+        return basic_any{*this, (mode == policy::cref ? policy::cref : policy::ref)};
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_any as_ref() const noexcept {
+        return basic_any{*this, policy::cref};
+    }
+
+    /**
+     * @brief Returns true if a wrapper owns its object, false otherwise.
+     * @return True if the wrapper owns its object, false otherwise.
+     */
+    [[nodiscard]] bool owner() const noexcept {
+        return (mode == policy::owner);
+    }
+
+private:
+    union {
+        const void *instance;
+        storage_type storage;
+    };
+    const type_info *info;
+    vtable_type *vtable;
+    policy mode;
+};
+
+/**
+ * @brief Performs type-safe access to the contained object.
+ * @tparam Type Type to which conversion is required.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param data Target any object.
+ * @return The element converted to the requested type.
+ */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(const basic_any<Len, Align> &data) noexcept {
+    const auto *const instance = any_cast<std::remove_reference_t<Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &data) noexcept {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto *const instance = any_cast<std::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type any_cast(basic_any<Len, Align> &&data) noexcept {
+    if constexpr(std::is_copy_constructible_v<std::remove_cv_t<std::remove_reference_t<Type>>>) {
+        if(auto *const instance = any_cast<std::remove_reference_t<Type>>(&data); instance) {
+            return static_cast<Type>(std::move(*instance));
+        } else {
+            return any_cast<Type>(data);
+        }
+    } else {
+        auto *const instance = any_cast<std::remove_reference_t<Type>>(&data);
+        ENTT_ASSERT(instance, "Invalid instance");
+        return static_cast<Type>(std::move(*instance));
+    }
+}
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+const Type *any_cast(const basic_any<Len, Align> *data) noexcept {
+    const auto &info = type_id<std::remove_cv_t<Type>>();
+    return static_cast<const Type *>(data->data(info));
+}
+
+/*! @copydoc any_cast */
+template<typename Type, std::size_t Len, std::size_t Align>
+Type *any_cast(basic_any<Len, Align> *data) noexcept {
+    if constexpr(std::is_const_v<Type>) {
+        // last attempt to make wrappers for const references return their values
+        return any_cast<Type>(&std::as_const(*data));
+    } else {
+        const auto &info = type_id<std::remove_cv_t<Type>>();
+        return static_cast<Type *>(data->data(info));
+    }
+}
+
+/**
+ * @brief Constructs a wrapper from a given type, passing it all arguments.
+ * @tparam Type Type of object to use to initialize the wrapper.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Args Types of arguments to use to construct the new instance.
+ * @param args Parameters to use to construct the instance.
+ * @return A properly initialized wrapper for an object of the given type.
+ */
+template<typename Type, std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename... Args>
+basic_any<Len, Align> make_any(Args &&...args) {
+    return basic_any<Len, Align>{std::in_place_type<Type>, std::forward<Args>(args)...};
+}
+
+/**
+ * @brief Forwards its argument and avoids copies for lvalue references.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Type Type of argument to use to construct the new instance.
+ * @param value Parameter to use to construct the instance.
+ * @return A properly initialized and not necessarily owning wrapper.
+ */
+template<std::size_t Len = basic_any<>::length, std::size_t Align = basic_any<Len>::alignment, typename Type>
+basic_any<Len, Align> forward_as_any(Type &&value) {
+    return basic_any<Len, Align>{std::in_place_type<Type &&>, std::forward<Type>(value)};
+}
+
+} // namespace entt
+
+#endif

src/entt/core/attribute.h

@@ -0,0 +1,30 @@
+#ifndef ENTT_CORE_ATTRIBUTE_H
+#define ENTT_CORE_ATTRIBUTE_H
+
+#ifndef ENTT_EXPORT
+#    if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#        define ENTT_EXPORT __declspec(dllexport)
+#        define ENTT_IMPORT __declspec(dllimport)
+#        define ENTT_HIDDEN
+#    elif defined __GNUC__ && __GNUC__ >= 4
+#        define ENTT_EXPORT __attribute__((visibility("default")))
+#        define ENTT_IMPORT __attribute__((visibility("default")))
+#        define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#    else /* Unsupported compiler */
+#        define ENTT_EXPORT
+#        define ENTT_IMPORT
+#        define ENTT_HIDDEN
+#    endif
+#endif
+
+#ifndef ENTT_API
+#    if defined ENTT_API_EXPORT
+#        define ENTT_API ENTT_EXPORT
+#    elif defined ENTT_API_IMPORT
+#        define ENTT_API ENTT_IMPORT
+#    else /* No API */
+#        define ENTT_API
+#    endif
+#endif
+
+#endif

src/entt/core/compressed_pair.hpp

@@ -0,0 +1,279 @@
+#ifndef ENTT_CORE_COMPRESSED_PAIR_HPP
+#define ENTT_CORE_COMPRESSED_PAIR_HPP
+
+#include <cstddef>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type, std::size_t, typename = void>
+struct compressed_pair_element {
+    using reference = Type &;
+    using const_reference = const Type &;
+
+    template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<Type>>>
+    constexpr compressed_pair_element() noexcept(std::is_nothrow_default_constructible_v<Type>)
+        : value{} {}
+
+    template<typename Arg, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Arg>>, compressed_pair_element>>>
+    constexpr compressed_pair_element(Arg &&arg) noexcept(std::is_nothrow_constructible_v<Type, Arg>)
+        : value{std::forward<Arg>(arg)} {}
+
+    template<typename... Args, std::size_t... Index>
+    constexpr compressed_pair_element(std::tuple<Args...> args, std::index_sequence<Index...>) noexcept(std::is_nothrow_constructible_v<Type, Args...>)
+        : value{std::forward<Args>(std::get<Index>(args))...} {}
+
+    [[nodiscard]] constexpr reference get() noexcept {
+        return value;
+    }
+
+    [[nodiscard]] constexpr const_reference get() const noexcept {
+        return value;
+    }
+
+private:
+    Type value;
+};
+
+template<typename Type, std::size_t Tag>
+struct compressed_pair_element<Type, Tag, std::enable_if_t<is_ebco_eligible_v<Type>>>: Type {
+    using reference = Type &;
+    using const_reference = const Type &;
+    using base_type = Type;
+
+    template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<base_type>>>
+    constexpr compressed_pair_element() noexcept(std::is_nothrow_default_constructible_v<base_type>)
+        : base_type{} {}
+
+    template<typename Arg, typename = std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_reference_t<Arg>>, compressed_pair_element>>>
+    constexpr compressed_pair_element(Arg &&arg) noexcept(std::is_nothrow_constructible_v<base_type, Arg>)
+        : base_type{std::forward<Arg>(arg)} {}
+
+    template<typename... Args, std::size_t... Index>
+    constexpr compressed_pair_element(std::tuple<Args...> args, std::index_sequence<Index...>) noexcept(std::is_nothrow_constructible_v<base_type, Args...>)
+        : base_type{std::forward<Args>(std::get<Index>(args))...} {}
+
+    [[nodiscard]] constexpr reference get() noexcept {
+        return *this;
+    }
+
+    [[nodiscard]] constexpr const_reference get() const noexcept {
+        return *this;
+    }
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief A compressed pair.
+ *
+ * A pair that exploits the _Empty Base Class Optimization_ (or _EBCO_) to
+ * reduce its final size to a minimum.
+ *
+ * @tparam First The type of the first element that the pair stores.
+ * @tparam Second The type of the second element that the pair stores.
+ */
+template<typename First, typename Second>
+class compressed_pair final
+    : internal::compressed_pair_element<First, 0u>,
+      internal::compressed_pair_element<Second, 1u> {
+    using first_base = internal::compressed_pair_element<First, 0u>;
+    using second_base = internal::compressed_pair_element<Second, 1u>;
+
+public:
+    /*! @brief The type of the first element that the pair stores. */
+    using first_type = First;
+    /*! @brief The type of the second element that the pair stores. */
+    using second_type = Second;
+
+    /**
+     * @brief Default constructor, conditionally enabled.
+     *
+     * This constructor is only available when the types that the pair stores
+     * are both at least default constructible.
+     *
+     * @tparam Dummy Dummy template parameter used for internal purposes.
+     */
+    template<bool Dummy = true, typename = std::enable_if_t<Dummy && std::is_default_constructible_v<first_type> && std::is_default_constructible_v<second_type>>>
+    constexpr compressed_pair() noexcept(std::is_nothrow_default_constructible_v<first_base> &&std::is_nothrow_default_constructible_v<second_base>)
+        : first_base{},
+          second_base{} {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    constexpr compressed_pair(const compressed_pair &other) noexcept(std::is_nothrow_copy_constructible_v<first_base> &&std::is_nothrow_copy_constructible_v<second_base>) = default;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    constexpr compressed_pair(compressed_pair &&other) noexcept(std::is_nothrow_move_constructible_v<first_base> &&std::is_nothrow_move_constructible_v<second_base>) = default;
+
+    /**
+     * @brief Constructs a pair from its values.
+     * @tparam Arg Type of value to use to initialize the first element.
+     * @tparam Other Type of value to use to initialize the second element.
+     * @param arg Value to use to initialize the first element.
+     * @param other Value to use to initialize the second element.
+     */
+    template<typename Arg, typename Other>
+    constexpr compressed_pair(Arg &&arg, Other &&other) noexcept(std::is_nothrow_constructible_v<first_base, Arg> &&std::is_nothrow_constructible_v<second_base, Other>)
+        : first_base{std::forward<Arg>(arg)},
+          second_base{std::forward<Other>(other)} {}
+
+    /**
+     * @brief Constructs a pair by forwarding the arguments to its parts.
+     * @tparam Args Types of arguments to use to initialize the first element.
+     * @tparam Other Types of arguments to use to initialize the second element.
+     * @param args Arguments to use to initialize the first element.
+     * @param other Arguments to use to initialize the second element.
+     */
+    template<typename... Args, typename... Other>
+    constexpr compressed_pair(std::piecewise_construct_t, std::tuple<Args...> args, std::tuple<Other...> other) noexcept(std::is_nothrow_constructible_v<first_base, Args...> &&std::is_nothrow_constructible_v<second_base, Other...>)
+        : first_base{std::move(args), std::index_sequence_for<Args...>{}},
+          second_base{std::move(other), std::index_sequence_for<Other...>{}} {}
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This compressed pair object.
+     */
+    constexpr compressed_pair &operator=(const compressed_pair &other) noexcept(std::is_nothrow_copy_assignable_v<first_base> &&std::is_nothrow_copy_assignable_v<second_base>) = default;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This compressed pair object.
+     */
+    constexpr compressed_pair &operator=(compressed_pair &&other) noexcept(std::is_nothrow_move_assignable_v<first_base> &&std::is_nothrow_move_assignable_v<second_base>) = default;
+
+    /**
+     * @brief Returns the first element that a pair stores.
+     * @return The first element that a pair stores.
+     */
+    [[nodiscard]] constexpr first_type &first() noexcept {
+        return static_cast<first_base &>(*this).get();
+    }
+
+    /*! @copydoc first */
+    [[nodiscard]] constexpr const first_type &first() const noexcept {
+        return static_cast<const first_base &>(*this).get();
+    }
+
+    /**
+     * @brief Returns the second element that a pair stores.
+     * @return The second element that a pair stores.
+     */
+    [[nodiscard]] constexpr second_type &second() noexcept {
+        return static_cast<second_base &>(*this).get();
+    }
+
+    /*! @copydoc second */
+    [[nodiscard]] constexpr const second_type &second() const noexcept {
+        return static_cast<const second_base &>(*this).get();
+    }
+
+    /**
+     * @brief Swaps two compressed pair objects.
+     * @param other The compressed pair to swap with.
+     */
+    constexpr void swap(compressed_pair &other) noexcept(std::is_nothrow_swappable_v<first_type> &&std::is_nothrow_swappable_v<second_type>) {
+        using std::swap;
+        swap(first(), other.first());
+        swap(second(), other.second());
+    }
+
+    /**
+     * @brief Extracts an element from the compressed pair.
+     * @tparam Index An integer value that is either 0 or 1.
+     * @return Returns a reference to the first element if `Index` is 0 and a
+     * reference to the second element if `Index` is 1.
+     */
+    template<std::size_t Index>
+    constexpr decltype(auto) get() noexcept {
+        if constexpr(Index == 0u) {
+            return first();
+        } else {
+            static_assert(Index == 1u, "Index out of bounds");
+            return second();
+        }
+    }
+
+    /*! @copydoc get */
+    template<std::size_t Index>
+    constexpr decltype(auto) get() const noexcept {
+        if constexpr(Index == 0u) {
+            return first();
+        } else {
+            static_assert(Index == 1u, "Index out of bounds");
+            return second();
+        }
+    }
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Type Type of value to use to initialize the first element.
+ * @tparam Other Type of value to use to initialize the second element.
+ */
+template<typename Type, typename Other>
+compressed_pair(Type &&, Other &&) -> compressed_pair<std::decay_t<Type>, std::decay_t<Other>>;
+
+/**
+ * @brief Swaps two compressed pair objects.
+ * @tparam First The type of the first element that the pairs store.
+ * @tparam Second The type of the second element that the pairs store.
+ * @param lhs A valid compressed pair object.
+ * @param rhs A valid compressed pair object.
+ */
+template<typename First, typename Second>
+inline constexpr void swap(compressed_pair<First, Second> &lhs, compressed_pair<First, Second> &rhs) {
+    lhs.swap(rhs);
+}
+
+} // namespace entt
+
+// disable structured binding support for clang 6, it messes when specializing tuple_size
+#if !defined __clang_major__ || __clang_major__ > 6
+namespace std {
+
+/**
+ * @brief `std::tuple_size` specialization for `compressed_pair`s.
+ * @tparam First The type of the first element that the pair stores.
+ * @tparam Second The type of the second element that the pair stores.
+ */
+template<typename First, typename Second>
+struct tuple_size<entt::compressed_pair<First, Second>>: integral_constant<size_t, 2u> {};
+
+/**
+ * @brief `std::tuple_element` specialization for `compressed_pair`s.
+ * @tparam Index The index of the type to return.
+ * @tparam First The type of the first element that the pair stores.
+ * @tparam Second The type of the second element that the pair stores.
+ */
+template<size_t Index, typename First, typename Second>
+struct tuple_element<Index, entt::compressed_pair<First, Second>>: conditional<Index == 0u, First, Second> {
+    static_assert(Index < 2u, "Index out of bounds");
+};
+
+} // namespace std
+#endif
+
+#endif

src/entt/core/enum.hpp

@@ -0,0 +1,97 @@
+#ifndef ENTT_CORE_ENUM_HPP
+#define ENTT_CORE_ENUM_HPP
+
+#include <type_traits>
+
+namespace entt {
+
+/**
+ * @brief Enable bitmask support for enum classes.
+ * @tparam Type The enum type for which to enable bitmask support.
+ */
+template<typename Type, typename = void>
+struct enum_as_bitmask: std::false_type {};
+
+/*! @copydoc enum_as_bitmask */
+template<typename Type>
+struct enum_as_bitmask<Type, std::void_t<decltype(Type::_entt_enum_as_bitmask)>>: std::is_enum<Type> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The enum class type for which to enable bitmask support.
+ */
+template<typename Type>
+inline constexpr bool enum_as_bitmask_v = enum_as_bitmask<Type>::value;
+
+} // namespace entt
+
+/**
+ * @brief Operator available for enums for which bitmask support is enabled.
+ * @tparam Type Enum class type.
+ * @param lhs The first value to use.
+ * @param rhs The second value to use.
+ * @return The result of invoking the operator on the underlying types of the
+ * two values provided.
+ */
+template<typename Type>
+[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
+operator|(const Type lhs, const Type rhs) noexcept {
+    return static_cast<Type>(static_cast<std::underlying_type_t<Type>>(lhs) | static_cast<std::underlying_type_t<Type>>(rhs));
+}
+
+/*! @copydoc operator| */
+template<typename Type>
+[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
+operator&(const Type lhs, const Type rhs) noexcept {
+    return static_cast<Type>(static_cast<std::underlying_type_t<Type>>(lhs) & static_cast<std::underlying_type_t<Type>>(rhs));
+}
+
+/*! @copydoc operator| */
+template<typename Type>
+[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
+operator^(const Type lhs, const Type rhs) noexcept {
+    return static_cast<Type>(static_cast<std::underlying_type_t<Type>>(lhs) ^ static_cast<std::underlying_type_t<Type>>(rhs));
+}
+
+/**
+ * @brief Operator available for enums for which bitmask support is enabled.
+ * @tparam Type Enum class type.
+ * @param value The value to use.
+ * @return The result of invoking the operator on the underlying types of the
+ * value provided.
+ */
+template<typename Type>
+[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type>
+operator~(const Type value) noexcept {
+    return static_cast<Type>(~static_cast<std::underlying_type_t<Type>>(value));
+}
+
+/*! @copydoc operator~ */
+template<typename Type>
+[[nodiscard]] constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, bool>
+operator!(const Type value) noexcept {
+    return !static_cast<std::underlying_type_t<Type>>(value);
+}
+
+/*! @copydoc operator| */
+template<typename Type>
+constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type &>
+operator|=(Type &lhs, const Type rhs) noexcept {
+    return (lhs = (lhs | rhs));
+}
+
+/*! @copydoc operator| */
+template<typename Type>
+constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type &>
+operator&=(Type &lhs, const Type rhs) noexcept {
+    return (lhs = (lhs & rhs));
+}
+
+/*! @copydoc operator| */
+template<typename Type>
+constexpr std::enable_if_t<entt::enum_as_bitmask_v<Type>, Type &>
+operator^=(Type &lhs, const Type rhs) noexcept {
+    return (lhs = (lhs ^ rhs));
+}
+
+#endif

src/entt/core/family.hpp

@@ -0,0 +1,32 @@
+#ifndef ENTT_CORE_FAMILY_HPP
+#define ENTT_CORE_FAMILY_HPP
+
+#include "../config/config.h"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Dynamic identifier generator.
+ *
+ * Utility class template that can be used to assign unique identifiers to types
+ * at runtime. Use different specializations to create separate sets of
+ * identifiers.
+ */
+template<typename...>
+class family {
+    inline static ENTT_MAYBE_ATOMIC(id_type) identifier{};
+
+public:
+    /*! @brief Unsigned integer type. */
+    using value_type = id_type;
+
+    /*! @brief Statically generated unique identifier for the given type. */
+    template<typename... Type>
+    // at the time I'm writing, clang crashes during compilation if auto is used instead of family_type
+    inline static const value_type value = identifier++;
+};
+
+} // namespace entt
+
+#endif

src/entt/core/fwd.hpp

@@ -0,0 +1,20 @@
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+#include <cstddef>
+#include "../config/config.h"
+
+namespace entt {
+
+template<std::size_t Len = sizeof(double[2]), std::size_t = alignof(double[2])>
+class basic_any;
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+} // namespace entt
+
+#endif

src/entt/core/hashed_string.hpp

@@ -0,0 +1,334 @@
+#ifndef ENTT_CORE_HASHED_STRING_HPP
+#define ENTT_CORE_HASHED_STRING_HPP
+
+#include <cstddef>
+#include <cstdint>
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename>
+struct fnv1a_traits;
+
+template<>
+struct fnv1a_traits<std::uint32_t> {
+    using type = std::uint32_t;
+    static constexpr std::uint32_t offset = 2166136261;
+    static constexpr std::uint32_t prime = 16777619;
+};
+
+template<>
+struct fnv1a_traits<std::uint64_t> {
+    using type = std::uint64_t;
+    static constexpr std::uint64_t offset = 14695981039346656037ull;
+    static constexpr std::uint64_t prime = 1099511628211ull;
+};
+
+template<typename Char>
+struct basic_hashed_string {
+    using value_type = Char;
+    using size_type = std::size_t;
+    using hash_type = id_type;
+
+    const value_type *repr;
+    size_type length;
+    hash_type hash;
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Zero overhead unique identifier.
+ *
+ * A hashed string is a compile-time tool that allows users to use
+ * human-readable identifiers in the codebase while using their numeric
+ * counterparts at runtime.<br/>
+ * Because of that, a hashed string can also be used in constant expressions if
+ * required.
+ *
+ * @warning
+ * This class doesn't take ownership of user-supplied strings nor does it make a
+ * copy of them.
+ *
+ * @tparam Char Character type.
+ */
+template<typename Char>
+class basic_hashed_string: internal::basic_hashed_string<Char> {
+    using base_type = internal::basic_hashed_string<Char>;
+    using hs_traits = internal::fnv1a_traits<id_type>;
+
+    struct const_wrapper {
+        // non-explicit constructor on purpose
+        constexpr const_wrapper(const Char *str) noexcept
+            : repr{str} {}
+
+        const Char *repr;
+    };
+
+    // Fowler–Noll–Vo hash function v. 1a - the good
+    [[nodiscard]] static constexpr auto helper(const Char *str) noexcept {
+        base_type base{str, 0u, hs_traits::offset};
+
+        for(; str[base.length]; ++base.length) {
+            base.hash = (base.hash ^ static_cast<hs_traits::type>(str[base.length])) * hs_traits::prime;
+        }
+
+        return base;
+    }
+
+    // Fowler–Noll–Vo hash function v. 1a - the good
+    [[nodiscard]] static constexpr auto helper(const Char *str, const std::size_t len) noexcept {
+        base_type base{str, len, hs_traits::offset};
+
+        for(size_type pos{}; pos < len; ++pos) {
+            base.hash = (base.hash ^ static_cast<hs_traits::type>(str[pos])) * hs_traits::prime;
+        }
+
+        return base;
+    }
+
+public:
+    /*! @brief Character type. */
+    using value_type = typename base_type::value_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename base_type::size_type;
+    /*! @brief Unsigned integer type. */
+    using hash_type = typename base_type::hash_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifier.
+     * @param len Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, const size_type len) noexcept {
+        return basic_hashed_string{str, len};
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @tparam N Number of characters of the identifier.
+     * @param str Human-readable identifier.
+     * @return The numeric representation of the string.
+     */
+    template<std::size_t N>
+    [[nodiscard]] static constexpr hash_type value(const value_type (&str)[N]) noexcept {
+        return basic_hashed_string{str};
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const_wrapper wrapper) noexcept {
+        return basic_hashed_string{wrapper};
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr basic_hashed_string() noexcept
+        : base_type{} {}
+
+    /**
+     * @brief Constructs a hashed string from a string view.
+     * @param str Human-readable identifier.
+     * @param len Length of the string to hash.
+     */
+    constexpr basic_hashed_string(const value_type *str, const size_type len) noexcept
+        : base_type{helper(str, len)} {}
+
+    /**
+     * @brief Constructs a hashed string from an array of const characters.
+     * @tparam N Number of characters of the identifier.
+     * @param str Human-readable identifier.
+     */
+    template<std::size_t N>
+    constexpr basic_hashed_string(const value_type (&str)[N]) noexcept
+        : base_type{helper(str)} {}
+
+    /**
+     * @brief Explicit constructor on purpose to avoid constructing a hashed
+     * string directly from a `const value_type *`.
+     *
+     * @warning
+     * The lifetime of the string is not extended nor is it copied.
+     *
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     */
+    explicit constexpr basic_hashed_string(const_wrapper wrapper) noexcept
+        : base_type{helper(wrapper.repr)} {}
+
+    /**
+     * @brief Returns the size a hashed string.
+     * @return The size of the hashed string.
+     */
+    [[nodiscard]] constexpr size_type size() const noexcept {
+        return base_type::length;
+    }
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the hashed string.
+     */
+    [[nodiscard]] constexpr const value_type *data() const noexcept {
+        return base_type::repr;
+    }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the hashed string.
+     */
+    [[nodiscard]] constexpr hash_type value() const noexcept {
+        return base_type::hash;
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] constexpr operator const value_type *() const noexcept {
+        return data();
+    }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the hashed string.
+     */
+    [[nodiscard]] constexpr operator hash_type() const noexcept {
+        return value();
+    }
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Char Character type.
+ * @param str Human-readable identifier.
+ * @param len Length of the string to hash.
+ */
+template<typename Char>
+basic_hashed_string(const Char *str, const std::size_t len) -> basic_hashed_string<Char>;
+
+/**
+ * @brief Deduction guide.
+ * @tparam Char Character type.
+ * @tparam N Number of characters of the identifier.
+ * @param str Human-readable identifier.
+ */
+template<typename Char, std::size_t N>
+basic_hashed_string(const Char (&str)[N]) -> basic_hashed_string<Char>;
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the two hashed strings are identical, false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator==(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
+    return lhs.value() == rhs.value();
+}
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the two hashed strings differ, false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator!=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the first element is less than the second, false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator<(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
+    return lhs.value() < rhs.value();
+}
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the first element is less than or equal to the second, false
+ * otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator<=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
+    return !(rhs < lhs);
+}
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the first element is greater than the second, false
+ * otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator>(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
+    return rhs < lhs;
+}
+
+/**
+ * @brief Compares two hashed strings.
+ * @tparam Char Character type.
+ * @param lhs A valid hashed string.
+ * @param rhs A valid hashed string.
+ * @return True if the first element is greater than or equal to the second,
+ * false otherwise.
+ */
+template<typename Char>
+[[nodiscard]] constexpr bool operator>=(const basic_hashed_string<Char> &lhs, const basic_hashed_string<Char> &rhs) noexcept {
+    return !(lhs < rhs);
+}
+
+/*! @brief Aliases for common character types. */
+using hashed_string = basic_hashed_string<char>;
+
+/*! @brief Aliases for common character types. */
+using hashed_wstring = basic_hashed_string<wchar_t>;
+
+inline namespace literals {
+
+/**
+ * @brief User defined literal for hashed strings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed string.
+ */
+[[nodiscard]] constexpr hashed_string operator"" _hs(const char *str, std::size_t) noexcept {
+    return hashed_string{str};
+}
+
+/**
+ * @brief User defined literal for hashed wstrings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed wstring.
+ */
+[[nodiscard]] constexpr hashed_wstring operator"" _hws(const wchar_t *str, std::size_t) noexcept {
+    return hashed_wstring{str};
+}
+
+} // namespace literals
+
+} // namespace entt
+
+#endif

src/entt/core/ident.hpp

@@ -0,0 +1,35 @@
+#ifndef ENTT_CORE_IDENT_HPP
+#define ENTT_CORE_IDENT_HPP
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+#include "fwd.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @brief Type integral identifiers.
+ * @tparam Type List of types for which to generate identifiers.
+ */
+template<typename... Type>
+class ident {
+    template<typename Curr, std::size_t... Index>
+    [[nodiscard]] static constexpr id_type get(std::index_sequence<Index...>) noexcept {
+        static_assert((std::is_same_v<Curr, Type> || ...), "Invalid type");
+        return (0 + ... + (std::is_same_v<Curr, type_list_element_t<Index, type_list<std::decay_t<Type>...>>> ? id_type{Index} : id_type{}));
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using value_type = id_type;
+
+    /*! @brief Statically generated unique identifier for the given type. */
+    template<typename Curr>
+    static constexpr value_type value = get<std::decay_t<Curr>>(std::index_sequence_for<Type...>{});
+};
+
+} // namespace entt
+
+#endif

src/entt/core/iterator.hpp

@@ -0,0 +1,197 @@
+#ifndef ENTT_CORE_ITERATOR_HPP
+#define ENTT_CORE_ITERATOR_HPP
+
+#include <iterator>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+namespace entt {
+
+/**
+ * @brief Helper type to use as pointer with input iterators.
+ * @tparam Type of wrapped value.
+ */
+template<typename Type>
+struct input_iterator_pointer final {
+    /*! @brief Value type. */
+    using value_type = Type;
+    /*! @brief Pointer type. */
+    using pointer = Type *;
+    /*! @brief Reference type. */
+    using reference = Type &;
+
+    /**
+     * @brief Constructs a proxy object by move.
+     * @param val Value to use to initialize the proxy object.
+     */
+    constexpr input_iterator_pointer(value_type &&val) noexcept(std::is_nothrow_move_constructible_v<value_type>)
+        : value{std::move(val)} {}
+
+    /**
+     * @brief Access operator for accessing wrapped values.
+     * @return A pointer to the wrapped value.
+     */
+    [[nodiscard]] constexpr pointer operator->() noexcept {
+        return std::addressof(value);
+    }
+
+    /**
+     * @brief Dereference operator for accessing wrapped values.
+     * @return A reference to the wrapped value.
+     */
+    [[nodiscard]] constexpr reference operator*() noexcept {
+        return value;
+    }
+
+private:
+    Type value;
+};
+
+/**
+ * @brief Plain iota iterator (waiting for C++20).
+ * @tparam Type Value type.
+ */
+template<typename Type>
+class iota_iterator final {
+    static_assert(std::is_integral_v<Type>, "Not an integral type");
+
+public:
+    /*! @brief Value type, likely an integral one. */
+    using value_type = Type;
+    /*! @brief Invalid pointer type. */
+    using pointer = void;
+    /*! @brief Non-reference type, same as value type. */
+    using reference = value_type;
+    /*! @brief Difference type. */
+    using difference_type = std::ptrdiff_t;
+    /*! @brief Iterator category. */
+    using iterator_category = std::input_iterator_tag;
+
+    /*! @brief Default constructor. */
+    constexpr iota_iterator() noexcept
+        : current{} {}
+
+    /**
+     * @brief Constructs an iota iterator from a given value.
+     * @param init The initial value assigned to the iota iterator.
+     */
+    constexpr iota_iterator(const value_type init) noexcept
+        : current{init} {}
+
+    /**
+     * @brief Pre-increment operator.
+     * @return This iota iterator.
+     */
+    constexpr iota_iterator &operator++() noexcept {
+        return ++current, *this;
+    }
+
+    /**
+     * @brief Post-increment operator.
+     * @return This iota iterator.
+     */
+    constexpr iota_iterator operator++(int) noexcept {
+        iota_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    /**
+     * @brief Dereference operator.
+     * @return The underlying value.
+     */
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return current;
+    }
+
+private:
+    value_type current;
+};
+
+/**
+ * @brief Comparison operator.
+ * @tparam Type Value type of the iota iterator.
+ * @param lhs A properly initialized iota iterator.
+ * @param rhs A properly initialized iota iterator.
+ * @return True if the two iterators are identical, false otherwise.
+ */
+template<typename Type>
+[[nodiscard]] constexpr bool operator==(const iota_iterator<Type> &lhs, const iota_iterator<Type> &rhs) noexcept {
+    return *lhs == *rhs;
+}
+
+/**
+ * @brief Comparison operator.
+ * @tparam Type Value type of the iota iterator.
+ * @param lhs A properly initialized iota iterator.
+ * @param rhs A properly initialized iota iterator.
+ * @return True if the two iterators differ, false otherwise.
+ */
+template<typename Type>
+[[nodiscard]] constexpr bool operator!=(const iota_iterator<Type> &lhs, const iota_iterator<Type> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+/**
+ * @brief Utility class to create an iterable object from a pair of iterators.
+ * @tparam It Type of iterator.
+ * @tparam Sentinel Type of sentinel.
+ */
+template<typename It, typename Sentinel = It>
+struct iterable_adaptor final {
+    /*! @brief Value type. */
+    using value_type = typename std::iterator_traits<It>::value_type;
+    /*! @brief Iterator type. */
+    using iterator = It;
+    /*! @brief Sentinel type. */
+    using sentinel = Sentinel;
+
+    /*! @brief Default constructor. */
+    constexpr iterable_adaptor() noexcept(std::is_nothrow_default_constructible_v<iterator> &&std::is_nothrow_default_constructible_v<sentinel>)
+        : first{},
+          last{} {}
+
+    /**
+     * @brief Creates an iterable object from a pair of iterators.
+     * @param from Begin iterator.
+     * @param to End iterator.
+     */
+    constexpr iterable_adaptor(iterator from, sentinel to) noexcept(std::is_nothrow_move_constructible_v<iterator> &&std::is_nothrow_move_constructible_v<sentinel>)
+        : first{std::move(from)},
+          last{std::move(to)} {}
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     * @return An iterator to the first element of the range.
+     */
+    [[nodiscard]] constexpr iterator begin() const noexcept {
+        return first;
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last element of the
+     * range.
+     */
+    [[nodiscard]] constexpr sentinel end() const noexcept {
+        return last;
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] constexpr iterator cbegin() const noexcept {
+        return begin();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] constexpr sentinel cend() const noexcept {
+        return end();
+    }
+
+private:
+    It first;
+    Sentinel last;
+};
+
+} // namespace entt
+
+#endif

src/entt/core/memory.hpp

@@ -0,0 +1,289 @@
+#ifndef ENTT_CORE_MEMORY_HPP
+#define ENTT_CORE_MEMORY_HPP
+
+#include <cstddef>
+#include <limits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+
+namespace entt {
+
+/**
+ * @brief Checks whether a value is a power of two or not.
+ * @param value A value that may or may not be a power of two.
+ * @return True if the value is a power of two, false otherwise.
+ */
+[[nodiscard]] inline constexpr bool is_power_of_two(const std::size_t value) noexcept {
+    return value && ((value & (value - 1)) == 0);
+}
+
+/**
+ * @brief Computes the smallest power of two greater than or equal to a value.
+ * @param value The value to use.
+ * @return The smallest power of two greater than or equal to the given value.
+ */
+[[nodiscard]] inline constexpr std::size_t next_power_of_two(const std::size_t value) noexcept {
+    ENTT_ASSERT_CONSTEXPR(value < (std::size_t{1u} << (std::numeric_limits<std::size_t>::digits - 1)), "Numeric limits exceeded");
+    std::size_t curr = value - (value != 0u);
+
+    for(int next = 1; next < std::numeric_limits<std::size_t>::digits; next = next * 2) {
+        curr |= curr >> next;
+    }
+
+    return ++curr;
+}
+
+/**
+ * @brief Fast module utility function (powers of two only).
+ * @param value A value for which to calculate the modulus.
+ * @param mod _Modulus_, it must be a power of two.
+ * @return The common remainder.
+ */
+[[nodiscard]] inline constexpr std::size_t fast_mod(const std::size_t value, const std::size_t mod) noexcept {
+    ENTT_ASSERT_CONSTEXPR(is_power_of_two(mod), "Value must be a power of two");
+    return value & (mod - 1u);
+}
+
+/**
+ * @brief Unwraps fancy pointers, does nothing otherwise (waiting for C++20).
+ * @tparam Type Pointer type.
+ * @param ptr Fancy or raw pointer.
+ * @return A raw pointer that represents the address of the original pointer.
+ */
+template<typename Type>
+[[nodiscard]] constexpr auto to_address(Type &&ptr) noexcept {
+    if constexpr(std::is_pointer_v<std::decay_t<Type>>) {
+        return ptr;
+    } else {
+        return to_address(std::forward<Type>(ptr).operator->());
+    }
+}
+
+/**
+ * @brief Utility function to design allocation-aware containers.
+ * @tparam Allocator Type of allocator.
+ * @param lhs A valid allocator.
+ * @param rhs Another valid allocator.
+ */
+template<typename Allocator>
+constexpr void propagate_on_container_copy_assignment([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) noexcept {
+    if constexpr(std::allocator_traits<Allocator>::propagate_on_container_copy_assignment::value) {
+        lhs = rhs;
+    }
+}
+
+/**
+ * @brief Utility function to design allocation-aware containers.
+ * @tparam Allocator Type of allocator.
+ * @param lhs A valid allocator.
+ * @param rhs Another valid allocator.
+ */
+template<typename Allocator>
+constexpr void propagate_on_container_move_assignment([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) noexcept {
+    if constexpr(std::allocator_traits<Allocator>::propagate_on_container_move_assignment::value) {
+        lhs = std::move(rhs);
+    }
+}
+
+/**
+ * @brief Utility function to design allocation-aware containers.
+ * @tparam Allocator Type of allocator.
+ * @param lhs A valid allocator.
+ * @param rhs Another valid allocator.
+ */
+template<typename Allocator>
+constexpr void propagate_on_container_swap([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) noexcept {
+    if constexpr(std::allocator_traits<Allocator>::propagate_on_container_swap::value) {
+        using std::swap;
+        swap(lhs, rhs);
+    } else {
+        ENTT_ASSERT_CONSTEXPR(lhs == rhs, "Cannot swap the containers");
+    }
+}
+
+/**
+ * @brief Deleter for allocator-aware unique pointers (waiting for C++20).
+ * @tparam Args Types of arguments to use to construct the object.
+ */
+template<typename Allocator>
+struct allocation_deleter: private Allocator {
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Pointer type. */
+    using pointer = typename std::allocator_traits<Allocator>::pointer;
+
+    /**
+     * @brief Inherited constructors.
+     * @param alloc The allocator to use.
+     */
+    constexpr allocation_deleter(const allocator_type &alloc) noexcept(std::is_nothrow_copy_constructible_v<allocator_type>)
+        : Allocator{alloc} {}
+
+    /**
+     * @brief Destroys the pointed object and deallocates its memory.
+     * @param ptr A valid pointer to an object of the given type.
+     */
+    constexpr void operator()(pointer ptr) noexcept(std::is_nothrow_destructible_v<typename allocator_type::value_type>) {
+        using alloc_traits = typename std::allocator_traits<Allocator>;
+        alloc_traits::destroy(*this, to_address(ptr));
+        alloc_traits::deallocate(*this, ptr, 1u);
+    }
+};
+
+/**
+ * @brief Allows `std::unique_ptr` to use allocators (waiting for C++20).
+ * @tparam Type Type of object to allocate for and to construct.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ * @tparam Args Types of arguments to use to construct the object.
+ * @param allocator The allocator to use.
+ * @param args Parameters to use to construct the object.
+ * @return A properly initialized unique pointer with a custom deleter.
+ */
+template<typename Type, typename Allocator, typename... Args>
+ENTT_CONSTEXPR auto allocate_unique(Allocator &allocator, Args &&...args) {
+    static_assert(!std::is_array_v<Type>, "Array types are not supported");
+
+    using alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<Type>;
+    using allocator_type = typename alloc_traits::allocator_type;
+
+    allocator_type alloc{allocator};
+    auto ptr = alloc_traits::allocate(alloc, 1u);
+
+    ENTT_TRY {
+        alloc_traits::construct(alloc, to_address(ptr), std::forward<Args>(args)...);
+    }
+    ENTT_CATCH {
+        alloc_traits::deallocate(alloc, ptr, 1u);
+        ENTT_THROW;
+    }
+
+    return std::unique_ptr<Type, allocation_deleter<allocator_type>>{ptr, alloc};
+}
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type>
+struct uses_allocator_construction {
+    template<typename Allocator, typename... Params>
+    static constexpr auto args([[maybe_unused]] const Allocator &allocator, Params &&...params) noexcept {
+        if constexpr(!std::uses_allocator_v<Type, Allocator> && std::is_constructible_v<Type, Params...>) {
+            return std::forward_as_tuple(std::forward<Params>(params)...);
+        } else {
+            static_assert(std::uses_allocator_v<Type, Allocator>, "Ill-formed request");
+
+            if constexpr(std::is_constructible_v<Type, std::allocator_arg_t, const Allocator &, Params...>) {
+                return std::tuple<std::allocator_arg_t, const Allocator &, Params &&...>{std::allocator_arg, allocator, std::forward<Params>(params)...};
+            } else {
+                static_assert(std::is_constructible_v<Type, Params..., const Allocator &>, "Ill-formed request");
+                return std::forward_as_tuple(std::forward<Params>(params)..., allocator);
+            }
+        }
+    }
+};
+
+template<typename Type, typename Other>
+struct uses_allocator_construction<std::pair<Type, Other>> {
+    using type = std::pair<Type, Other>;
+
+    template<typename Allocator, typename First, typename Second>
+    static constexpr auto args(const Allocator &allocator, std::piecewise_construct_t, First &&first, Second &&second) noexcept {
+        return std::make_tuple(
+            std::piecewise_construct,
+            std::apply([&allocator](auto &&...curr) { return uses_allocator_construction<Type>::args(allocator, std::forward<decltype(curr)>(curr)...); }, std::forward<First>(first)),
+            std::apply([&allocator](auto &&...curr) { return uses_allocator_construction<Other>::args(allocator, std::forward<decltype(curr)>(curr)...); }, std::forward<Second>(second)));
+    }
+
+    template<typename Allocator>
+    static constexpr auto args(const Allocator &allocator) noexcept {
+        return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::tuple<>{}, std::tuple<>{});
+    }
+
+    template<typename Allocator, typename First, typename Second>
+    static constexpr auto args(const Allocator &allocator, First &&first, Second &&second) noexcept {
+        return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(std::forward<First>(first)), std::forward_as_tuple(std::forward<Second>(second)));
+    }
+
+    template<typename Allocator, typename First, typename Second>
+    static constexpr auto args(const Allocator &allocator, const std::pair<First, Second> &value) noexcept {
+        return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(value.first), std::forward_as_tuple(value.second));
+    }
+
+    template<typename Allocator, typename First, typename Second>
+    static constexpr auto args(const Allocator &allocator, std::pair<First, Second> &&value) noexcept {
+        return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(std::move(value.first)), std::forward_as_tuple(std::move(value.second)));
+    }
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Uses-allocator construction utility (waiting for C++20).
+ *
+ * Primarily intended for internal use. Prepares the argument list needed to
+ * create an object of a given type by means of uses-allocator construction.
+ *
+ * @tparam Type Type to return arguments for.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ * @tparam Args Types of arguments to use to construct the object.
+ * @param allocator The allocator to use.
+ * @param args Parameters to use to construct the object.
+ * @return The arguments needed to create an object of the given type.
+ */
+template<typename Type, typename Allocator, typename... Args>
+constexpr auto uses_allocator_construction_args(const Allocator &allocator, Args &&...args) noexcept {
+    return internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...);
+}
+
+/**
+ * @brief Uses-allocator construction utility (waiting for C++20).
+ *
+ * Primarily intended for internal use. Creates an object of a given type by
+ * means of uses-allocator construction.
+ *
+ * @tparam Type Type of object to create.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ * @tparam Args Types of arguments to use to construct the object.
+ * @param allocator The allocator to use.
+ * @param args Parameters to use to construct the object.
+ * @return A newly created object of the given type.
+ */
+template<typename Type, typename Allocator, typename... Args>
+constexpr Type make_obj_using_allocator(const Allocator &allocator, Args &&...args) {
+    return std::make_from_tuple<Type>(internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...));
+}
+
+/**
+ * @brief Uses-allocator construction utility (waiting for C++20).
+ *
+ * Primarily intended for internal use. Creates an object of a given type by
+ * means of uses-allocator construction at an uninitialized memory location.
+ *
+ * @tparam Type Type of object to create.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ * @tparam Args Types of arguments to use to construct the object.
+ * @param value Memory location in which to place the object.
+ * @param allocator The allocator to use.
+ * @param args Parameters to use to construct the object.
+ * @return A pointer to the newly created object of the given type.
+ */
+template<typename Type, typename Allocator, typename... Args>
+constexpr Type *uninitialized_construct_using_allocator(Type *value, const Allocator &allocator, Args &&...args) {
+    return std::apply([value](auto &&...curr) { return new(value) Type(std::forward<decltype(curr)>(curr)...); }, internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...));
+}
+
+} // namespace entt
+
+#endif

src/entt/core/monostate.hpp

@@ -0,0 +1,56 @@
+#ifndef ENTT_CORE_MONOSTATE_HPP
+#define ENTT_CORE_MONOSTATE_HPP
+
+#include "../config/config.h"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Minimal implementation of the monostate pattern.
+ *
+ * A minimal, yet complete configuration system built on top of the monostate
+ * pattern. Thread safe by design, it works only with basic types like `int`s or
+ * `bool`s.<br/>
+ * Multiple types and therefore more than one value can be associated with a
+ * single key. Because of this, users must pay attention to use the same type
+ * both during an assignment and when they try to read back their data.
+ * Otherwise, they can incur in unexpected results.
+ */
+template<id_type>
+struct monostate {
+    /**
+     * @brief Assigns a value of a specific type to a given key.
+     * @tparam Type Type of the value to assign.
+     * @param val User data to assign to the given key.
+     */
+    template<typename Type>
+    void operator=(Type val) const noexcept {
+        value<Type> = val;
+    }
+
+    /**
+     * @brief Gets a value of a specific type for a given key.
+     * @tparam Type Type of the value to get.
+     * @return Stored value, if any.
+     */
+    template<typename Type>
+    operator Type() const noexcept {
+        return value<Type>;
+    }
+
+private:
+    template<typename Type>
+    inline static ENTT_MAYBE_ATOMIC(Type) value{};
+};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Value Value used to differentiate between different variables.
+ */
+template<id_type Value>
+inline monostate<Value> monostate_v = {};
+
+} // namespace entt
+
+#endif

src/entt/core/tuple.hpp

@@ -0,0 +1,103 @@
+#ifndef ENTT_CORE_TUPLE_HPP
+#define ENTT_CORE_TUPLE_HPP
+
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename>
+struct is_tuple_impl: std::false_type {};
+
+template<typename... Args>
+struct is_tuple_impl<std::tuple<Args...>>: std::true_type {};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is a
+ * tuple, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+struct is_tuple: internal::is_tuple_impl<std::remove_cv_t<Type>> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_tuple_v = is_tuple<Type>::value;
+
+/**
+ * @brief Utility function to unwrap tuples of a single element.
+ * @tparam Type Tuple type of any sizes.
+ * @param value A tuple object of the given type.
+ * @return The tuple itself if it contains more than one element, the first
+ * element otherwise.
+ */
+template<typename Type>
+constexpr decltype(auto) unwrap_tuple(Type &&value) noexcept {
+    if constexpr(std::tuple_size_v<std::remove_reference_t<Type>> == 1u) {
+        return std::get<0>(std::forward<Type>(value));
+    } else {
+        return std::forward<Type>(value);
+    }
+}
+
+/**
+ * @brief Utility class to forward-and-apply tuple objects.
+ * @tparam Func Type of underlying invocable object.
+ */
+template<typename Func>
+struct forward_apply: private Func {
+    /**
+     * @brief Constructs a forward-and-apply object.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<class... Args>
+    constexpr forward_apply(Args &&...args) noexcept(std::is_nothrow_constructible_v<Func, Args...>)
+        : Func{std::forward<Args>(args)...} {}
+
+    /**
+     * @brief Forwards and applies the arguments with the underlying function.
+     * @tparam Type Tuple-like type to forward to the underlying function.
+     * @param args Parameters to forward to the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template<class Type>
+    constexpr decltype(auto) operator()(Type &&args) noexcept(noexcept(std::apply(std::declval<Func &>(), args))) {
+        return std::apply(static_cast<Func &>(*this), std::forward<Type>(args));
+    }
+
+    /*! @copydoc operator()() */
+    template<class Type>
+    constexpr decltype(auto) operator()(Type &&args) const noexcept(noexcept(std::apply(std::declval<const Func &>(), args))) {
+        return std::apply(static_cast<const Func &>(*this), std::forward<Type>(args));
+    }
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Type of underlying invocable object.
+ */
+template<typename Func>
+forward_apply(Func) -> forward_apply<std::remove_reference_t<std::remove_cv_t<Func>>>;
+
+} // namespace entt
+
+#endif

src/entt/core/type_info.hpp

@@ -0,0 +1,274 @@
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+#include <string_view>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "../core/attribute.h"
+#include "fwd.hpp"
+#include "hashed_string.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+struct ENTT_API type_index final {
+    [[nodiscard]] static id_type next() noexcept {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() noexcept {
+#if defined ENTT_PRETTY_FUNCTION
+    std::string_view pretty_function{ENTT_PRETTY_FUNCTION};
+    auto first = pretty_function.find_first_not_of(' ', pretty_function.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX) + 1);
+    auto value = pretty_function.substr(first, pretty_function.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return std::string_view{""};
+#endif
+}
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr std::string_view type_name(int) noexcept {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+template<typename Type>
+[[nodiscard]] static std::string_view type_name(char) noexcept {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] static constexpr id_type type_hash(int) noexcept {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+template<typename Type>
+[[nodiscard]] static id_type type_hash(char) noexcept {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type, typename = void>
+struct ENTT_API type_index final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() noexcept {
+        static const id_type value = internal::type_index::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const noexcept {
+        return value();
+    }
+};
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type, typename = void>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() noexcept {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() noexcept {
+        return type_index<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const noexcept {
+        return value();
+    }
+};
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type, typename = void>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr std::string_view value() noexcept {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator std::string_view() const noexcept {
+        return value();
+    }
+};
+
+/*! @brief Implementation specific information about a type. */
+struct type_info final {
+    /**
+     * @brief Constructs a type info object for a given type.
+     * @tparam Type Type for which to construct a type info object.
+     */
+    template<typename Type>
+    constexpr type_info(std::in_place_type_t<Type>) noexcept
+        : seq{type_index<std::remove_cv_t<std::remove_reference_t<Type>>>::value()},
+          identifier{type_hash<std::remove_cv_t<std::remove_reference_t<Type>>>::value()},
+          alias{type_name<std::remove_cv_t<std::remove_reference_t<Type>>>::value()} {}
+
+    /**
+     * @brief Type index.
+     * @return Type index.
+     */
+    [[nodiscard]] constexpr id_type index() const noexcept {
+        return seq;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] constexpr id_type hash() const noexcept {
+        return identifier;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] constexpr std::string_view name() const noexcept {
+        return alias;
+    }
+
+private:
+    id_type seq;
+    id_type identifier;
+    std::string_view alias;
+};
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two type info objects are identical, false otherwise.
+ */
+[[nodiscard]] inline constexpr bool operator==(const type_info &lhs, const type_info &rhs) noexcept {
+    return lhs.hash() == rhs.hash();
+}
+
+/**
+ * @brief Compares the contents of two type info objects.
+ * @param lhs A type info object.
+ * @param rhs A type info object.
+ * @return True if the two type info objects differ, false otherwise.
+ */
+[[nodiscard]] inline constexpr bool operator!=(const type_info &lhs, const type_info &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+/**
+ * @brief Compares two type info objects.
+ * @param lhs A valid type info object.
+ * @param rhs A valid type info object.
+ * @return True if the first element is less than the second, false otherwise.
+ */
+[[nodiscard]] constexpr bool operator<(const type_info &lhs, const type_info &rhs) noexcept {
+    return lhs.index() < rhs.index();
+}
+
+/**
+ * @brief Compares two type info objects.
+ * @param lhs A valid type info object.
+ * @param rhs A valid type info object.
+ * @return True if the first element is less than or equal to the second, false
+ * otherwise.
+ */
+[[nodiscard]] constexpr bool operator<=(const type_info &lhs, const type_info &rhs) noexcept {
+    return !(rhs < lhs);
+}
+
+/**
+ * @brief Compares two type info objects.
+ * @param lhs A valid type info object.
+ * @param rhs A valid type info object.
+ * @return True if the first element is greater than the second, false
+ * otherwise.
+ */
+[[nodiscard]] constexpr bool operator>(const type_info &lhs, const type_info &rhs) noexcept {
+    return rhs < lhs;
+}
+
+/**
+ * @brief Compares two type info objects.
+ * @param lhs A valid type info object.
+ * @param rhs A valid type info object.
+ * @return True if the first element is greater than or equal to the second,
+ * false otherwise.
+ */
+[[nodiscard]] constexpr bool operator>=(const type_info &lhs, const type_info &rhs) noexcept {
+    return !(lhs < rhs);
+}
+
+/**
+ * @brief Returns the type info object associated to a given type.
+ *
+ * The returned element refers to an object with static storage duration.<br/>
+ * The type doesn't need to be a complete type. If the type is a reference, the
+ * result refers to the referenced type. In all cases, top-level cv-qualifiers
+ * are ignored.
+ *
+ * @tparam Type Type for which to generate a type info object.
+ * @return A reference to a properly initialized type info object.
+ */
+template<typename Type>
+[[nodiscard]] const type_info &type_id() noexcept {
+    if constexpr(std::is_same_v<Type, std::remove_cv_t<std::remove_reference_t<Type>>>) {
+        static type_info instance{std::in_place_type<Type>};
+        return instance;
+    } else {
+        return type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+    }
+}
+
+/*! @copydoc type_id */
+template<typename Type>
+[[nodiscard]] const type_info &type_id(Type &&) noexcept {
+    return type_id<std::remove_cv_t<std::remove_reference_t<Type>>>();
+}
+
+} // namespace entt
+
+#endif

src/entt/core/type_traits.hpp

@@ -0,0 +1,758 @@
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+struct choice_t
+    // Unfortunately, doxygen cannot parse such a construct.
+    : /*! @cond TURN_OFF_DOXYGEN */ choice_t<N - 1> /*! @endcond */
+{};
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<std::size_t N>
+inline constexpr choice_t<N> choice{};
+
+/**
+ * @brief Identity type trait.
+ *
+ * Useful to establish non-deduced contexts in template argument deduction
+ * (waiting for C++20) or to provide types through function arguments.
+ *
+ * @tparam Type A type.
+ */
+template<typename Type>
+struct type_identity {
+    /*! @brief Identity type. */
+    using type = Type;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type.
+ */
+template<typename Type>
+using type_identity_t = typename type_identity<Type>::type;
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ * @tparam The size of the type if `sizeof` accepts it, 0 otherwise.
+ */
+template<typename Type, typename = void>
+struct size_of: std::integral_constant<std::size_t, 0u> {};
+
+/*! @copydoc size_of */
+template<typename Type>
+struct size_of<Type, std::void_t<decltype(sizeof(Type))>>
+    : std::integral_constant<std::size_t, sizeof(Type)> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<typename Type>
+inline constexpr std::size_t size_of_v = size_of<Type>::value;
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_type = Type;
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_value = Value;
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = std::integral_constant<decltype(Value), Value>;
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct type_list_element;
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam First First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<std::size_t Index, typename First, typename... Other>
+struct type_list_element<Index, type_list<First, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>> {};
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam First First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename First, typename... Other>
+struct type_list_element<0u, type_list<First, Other...>> {
+    /*! @brief Searched type. */
+    using type = First;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<std::size_t Index, typename List>
+using type_list_element_t = typename type_list_element<Index, List>::type;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename, typename>
+struct type_list_index;
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ * @tparam First First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename First, typename... Other>
+struct type_list_index<Type, type_list<First, Other...>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 1u + type_list_index<Type, type_list<Other...>>::value;
+};
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_index<Type, type_list<Type, Other...>> {
+    static_assert(type_list_index<Type, type_list<Other...>>::value == sizeof...(Other), "Non-unique type");
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ */
+template<typename Type>
+struct type_list_index<Type, type_list<>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = std::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for and for which to return the index.
+ */
+template<typename Type, typename List>
+inline constexpr std::size_t type_list_index_v = type_list_index<Type, List>::value;
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+constexpr type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) {
+    return {};
+}
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = typename type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = typename type_list_cat<List...>::type;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct type_list_unique;
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam Type One of the types provided by the given type list.
+ * @tparam Other The other types provided by the given type list.
+ */
+template<typename Type, typename... Other>
+struct type_list_unique<type_list<Type, Other...>> {
+    /*! @brief A type list without duplicate types. */
+    using type = std::conditional_t<
+        (std::is_same_v<Type, Other> || ...),
+        typename type_list_unique<type_list<Other...>>::type,
+        type_list_cat_t<type_list<Type>, typename type_list_unique<type_list<Other...>>::type>>;
+};
+
+/*! @brief Removes duplicates types from a type list. */
+template<>
+struct type_list_unique<type_list<>> {
+    /*! @brief A type list without duplicate types. */
+    using type = type_list<>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Type A type list.
+ */
+template<typename Type>
+using type_list_unique_t = typename type_list_unique<Type>::type;
+
+/**
+ * @brief Provides the member constant `value` to true if a type list contains a
+ * given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>: std::disjunction<std::is_same<Type, Other>...> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<std::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = typename type_list_diff<List...>::type;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename, template<typename...> class>
+struct type_list_transform;
+
+/**
+ * @brief Applies a given _function_ to a type list and generate a new list.
+ * @tparam Type Types provided by the type list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename... Type, template<typename...> class Op>
+struct type_list_transform<type_list<Type...>, Op> {
+    /*! @brief Resulting type list after applying the transform function. */
+    using type = type_list<typename Op<Type>::type...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Type list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename List, template<typename...> class Op>
+using type_list_transform_t = typename type_list_transform<List, Op>::type;
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+/*! @brief Primary template isn't defined on purpose. */
+template<std::size_t, typename>
+struct value_list_element;
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<std::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>> {};
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<std::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+constexpr value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) {
+    return {};
+}
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = typename value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = typename value_list_cat<List...>::type;
+
+/*! @brief Same as std::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: std::false_type {};
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: std::is_invocable<Func, Args...> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+/*! @brief Same as std::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: std::false_type {};
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, std::tuple<Args...>>: std::is_invocable_r<Ret, Func, Args...> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_complete: std::false_type {};
+
+/*! @copydoc is_complete */
+template<typename Type>
+struct is_complete<Type, std::void_t<decltype(sizeof(Type))>>: std::true_type {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is an
+ * iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_iterator: std::false_type {};
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename, typename = void>
+struct has_iterator_category: std::false_type {};
+
+template<typename Type>
+struct has_iterator_category<Type, std::void_t<typename std::iterator_traits<Type>::iterator_category>>: std::true_type {};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/*! @copydoc is_iterator */
+template<typename Type>
+struct is_iterator<Type, std::enable_if_t<!std::is_same_v<std::remove_cv_t<std::remove_pointer_t<Type>>, void>>>
+    : internal::has_iterator_category<Type> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is both
+ * an empty and non-final class, false otherwise.
+ * @tparam Type The type to test
+ */
+template<typename Type>
+struct is_ebco_eligible
+    : std::conjunction<std::is_empty<Type>, std::negation<std::is_final<Type>>> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_ebco_eligible_v = is_ebco_eligible<Type>::value;
+
+/**
+ * @brief Provides the member constant `value` to true if `Type::is_transparent`
+ * is valid and denotes a type, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_transparent: std::false_type {};
+
+/*! @copydoc is_transparent */
+template<typename Type>
+struct is_transparent<Type, std::void_t<typename Type::is_transparent>>: std::true_type {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_transparent_v = is_transparent<Type>::value;
+
+/**
+ * @brief Provides the member constant `value` to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type, typename = void>
+struct is_equality_comparable: std::false_type {};
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename, typename = void>
+struct has_tuple_size_value: std::false_type {};
+
+template<typename Type>
+struct has_tuple_size_value<Type, std::void_t<decltype(std::tuple_size<const Type>::value)>>: std::true_type {};
+
+template<typename Type, std::size_t... Index>
+[[nodiscard]] constexpr bool unpack_maybe_equality_comparable(std::index_sequence<Index...>) {
+    return (is_equality_comparable<std::tuple_element_t<Index, Type>>::value && ...);
+}
+
+template<typename>
+[[nodiscard]] constexpr bool maybe_equality_comparable(choice_t<0>) {
+    return true;
+}
+
+template<typename Type>
+[[nodiscard]] constexpr auto maybe_equality_comparable(choice_t<1>) -> decltype(std::declval<typename Type::value_type>(), bool{}) {
+    if constexpr(is_iterator_v<Type>) {
+        return true;
+    } else if constexpr(std::is_same_v<typename Type::value_type, Type>) {
+        return maybe_equality_comparable<Type>(choice<0>);
+    } else {
+        return is_equality_comparable<typename Type::value_type>::value;
+    }
+}
+
+template<typename Type>
+[[nodiscard]] constexpr std::enable_if_t<is_complete_v<std::tuple_size<std::remove_cv_t<Type>>>, bool> maybe_equality_comparable(choice_t<2>) {
+    if constexpr(has_tuple_size_value<Type>::value) {
+        return unpack_maybe_equality_comparable<Type>(std::make_index_sequence<std::tuple_size<Type>::value>{});
+    } else {
+        return maybe_equality_comparable<Type>(choice<1>);
+    }
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/*! @copydoc is_equality_comparable */
+template<typename Type>
+struct is_equality_comparable<Type, std::void_t<decltype(std::declval<Type>() == std::declval<Type>())>>
+    : std::bool_constant<internal::maybe_equality_comparable<Type>(choice<2>)> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = std::remove_const_t<To>;
+};
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = const To;
+};
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = typename constness_as<To, From>::type;
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+class member_class {
+    static_assert(std::is_member_pointer_v<Member>, "Invalid pointer type to non-static member object or function");
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class *clazz(Ret (Class::*)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class *clazz(Ret (Class::*)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class *clazz(Type Class::*);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = std::remove_pointer_t<decltype(clazz(std::declval<Member>()))>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = typename member_class<Member>::type;
+
+/**
+ * @brief Extracts the n-th argument of a given function or member function.
+ * @tparam Index The index of the argument to extract.
+ * @tparam Candidate A valid function, member function or data member.
+ */
+template<std::size_t Index, auto Candidate>
+class nth_argument {
+    template<typename Ret, typename... Args>
+    static constexpr type_list<Args...> pick_up(Ret (*)(Args...));
+
+    template<typename Ret, typename Class, typename... Args>
+    static constexpr type_list<Args...> pick_up(Ret (Class ::*)(Args...));
+
+    template<typename Ret, typename Class, typename... Args>
+    static constexpr type_list<Args...> pick_up(Ret (Class ::*)(Args...) const);
+
+    template<typename Type, typename Class>
+    static constexpr type_list<Type> pick_up(Type Class ::*);
+
+public:
+    /*! @brief N-th argument of the given function or member function. */
+    using type = type_list_element_t<Index, decltype(pick_up(Candidate))>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Index The index of the argument to extract.
+ * @tparam Candidate A valid function, member function or data member.
+ */
+template<std::size_t Index, auto Candidate>
+using nth_argument_t = typename nth_argument<Index, Candidate>::type;
+
+} // namespace entt
+
+#endif

src/entt/core/utility.hpp

@@ -0,0 +1,101 @@
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+#include <type_traits>
+#include <utility>
+
+namespace entt {
+
+/*! @brief Identity function object (waiting for C++20). */
+struct identity {
+    /*! @brief Indicates that this is a transparent function object. */
+    using is_transparent = void;
+
+    /**
+     * @brief Returns its argument unchanged.
+     * @tparam Type Type of the argument.
+     * @param value The actual argument.
+     * @return The submitted value as-is.
+     */
+    template<class Type>
+    [[nodiscard]] constexpr Type &&operator()(Type &&value) const noexcept {
+        return std::forward<Type>(value);
+    }
+};
+
+/**
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+[[nodiscard]] constexpr auto overload(Type Class::*member) noexcept {
+    return member;
+}
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) noexcept {
+    return func;
+}
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<class... Func>
+overloaded(Func...) -> overloaded<Func...>;
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<class Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    constexpr y_combinator(Func recursive) noexcept(std::is_nothrow_move_constructible_v<Func>)
+        : func{std::move(recursive)} {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template<class... Args>
+    constexpr decltype(auto) operator()(Args &&...args) const noexcept(std::is_nothrow_invocable_v<Func, const y_combinator &, Args...>) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template<class... Args>
+    constexpr decltype(auto) operator()(Args &&...args) noexcept(std::is_nothrow_invocable_v<Func, y_combinator &, Args...>) {
+        return func(*this, std::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/component.hpp

@@ -0,0 +1,64 @@
+#ifndef ENTT_ENTITY_COMPONENT_HPP
+#define ENTT_ENTITY_COMPONENT_HPP
+
+#include <cstddef>
+#include <type_traits>
+#include "../config/config.h"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type, typename = void>
+struct in_place_delete: std::bool_constant<!(std::is_move_constructible_v<Type> && std::is_move_assignable_v<Type>)> {};
+
+template<typename Type>
+struct in_place_delete<Type, std::enable_if_t<Type::in_place_delete>>
+    : std::true_type {};
+
+template<typename Type, typename = void>
+struct page_size: std::integral_constant<std::size_t, !std::is_empty_v<ENTT_ETO_TYPE(Type)> * ENTT_PACKED_PAGE> {};
+
+template<typename Type>
+struct page_size<Type, std::enable_if_t<std::is_convertible_v<decltype(Type::page_size), std::size_t>>>
+    : std::integral_constant<std::size_t, Type::page_size> {};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Common way to access various properties of components.
+ * @tparam Type Type of component.
+ */
+template<typename Type, typename = void>
+struct component_traits {
+    static_assert(std::is_same_v<std::decay_t<Type>, Type>, "Unsupported type");
+
+    /*! @brief Component type. */
+    using type = Type;
+
+    /*! @brief Pointer stability, default is `false`. */
+    static constexpr bool in_place_delete = internal::in_place_delete<Type>::value;
+    /*! @brief Page size, default is `ENTT_PACKED_PAGE` for non-empty types. */
+    static constexpr std::size_t page_size = internal::page_size<Type>::value;
+};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Type of component.
+ */
+template<class Type>
+inline constexpr bool ignore_as_empty_v = (std::is_void_v<Type> || component_traits<Type>::page_size == 0u);
+
+} // namespace entt
+
+#endif

src/entt/entity/entity.hpp

@@ -0,0 +1,339 @@
+#ifndef ENTT_ENTITY_ENTITY_HPP
+#define ENTT_ENTITY_ENTITY_HPP
+
+#include <cstddef>
+#include <cstdint>
+#include <type_traits>
+#include "../config/config.h"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename, typename = void>
+struct entt_traits;
+
+template<typename Type>
+struct entt_traits<Type, std::enable_if_t<std::is_enum_v<Type>>>
+    : entt_traits<std::underlying_type_t<Type>> {};
+
+template<typename Type>
+struct entt_traits<Type, std::enable_if_t<std::is_class_v<Type>>>
+    : entt_traits<typename Type::entity_type> {};
+
+template<>
+struct entt_traits<std::uint32_t> {
+    using entity_type = std::uint32_t;
+    using version_type = std::uint16_t;
+
+    static constexpr entity_type entity_mask = 0xFFFFF;
+    static constexpr entity_type version_mask = 0xFFF;
+    static constexpr std::size_t entity_shift = 20u;
+};
+
+template<>
+struct entt_traits<std::uint64_t> {
+    using entity_type = std::uint64_t;
+    using version_type = std::uint32_t;
+
+    static constexpr entity_type entity_mask = 0xFFFFFFFF;
+    static constexpr entity_type version_mask = 0xFFFFFFFF;
+    static constexpr std::size_t entity_shift = 32u;
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Entity traits.
+ * @tparam Type Type of identifier.
+ */
+template<typename Type>
+class entt_traits: internal::entt_traits<Type> {
+    using base_type = internal::entt_traits<Type>;
+
+public:
+    /*! @brief Value type. */
+    using value_type = Type;
+    /*! @brief Underlying entity type. */
+    using entity_type = typename base_type::entity_type;
+    /*! @brief Underlying version type. */
+    using version_type = typename base_type::version_type;
+    /*! @brief Reserved identifier. */
+    static constexpr entity_type reserved = base_type::entity_mask | (base_type::version_mask << base_type::entity_shift);
+    /*! @brief Page size, default is `ENTT_SPARSE_PAGE`. */
+    static constexpr auto page_size = ENTT_SPARSE_PAGE;
+
+    /**
+     * @brief Converts an entity to its underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the given value.
+     */
+    [[nodiscard]] static constexpr entity_type to_integral(const value_type value) noexcept {
+        return static_cast<entity_type>(value);
+    }
+
+    /**
+     * @brief Returns the entity part once converted to the underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the entity part.
+     */
+    [[nodiscard]] static constexpr entity_type to_entity(const value_type value) noexcept {
+        return (to_integral(value) & base_type::entity_mask);
+    }
+
+    /**
+     * @brief Returns the version part once converted to the underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the version part.
+     */
+    [[nodiscard]] static constexpr version_type to_version(const value_type value) noexcept {
+        return (to_integral(value) >> base_type::entity_shift);
+    }
+
+    /**
+     * @brief Constructs an identifier from its parts.
+     *
+     * If the version part is not provided, a tombstone is returned.<br/>
+     * If the entity part is not provided, a null identifier is returned.
+     *
+     * @param entity The entity part of the identifier.
+     * @param version The version part of the identifier.
+     * @return A properly constructed identifier.
+     */
+    [[nodiscard]] static constexpr value_type construct(const entity_type entity, const version_type version) noexcept {
+        return value_type{(entity & base_type::entity_mask) | (static_cast<entity_type>(version) << base_type::entity_shift)};
+    }
+
+    /**
+     * @brief Combines two identifiers in a single one.
+     *
+     * The returned identifier is a copy of the first element except for its
+     * version, which is taken from the second element.
+     *
+     * @param lhs The identifier from which to take the entity part.
+     * @param rhs The identifier from which to take the version part.
+     * @return A properly constructed identifier.
+     */
+    [[nodiscard]] static constexpr value_type combine(const entity_type lhs, const entity_type rhs) noexcept {
+        constexpr auto mask = (base_type::version_mask << base_type::entity_shift);
+        return value_type{(lhs & base_type::entity_mask) | (rhs & mask)};
+    }
+};
+
+/**
+ * @copydoc entt_traits<Entity>::to_integral
+ * @tparam Entity The value type.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_integral(const Entity value) noexcept {
+    return entt_traits<Entity>::to_integral(value);
+}
+
+/**
+ * @copydoc entt_traits<Entity>::to_entity
+ * @tparam Entity The value type.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr typename entt_traits<Entity>::entity_type to_entity(const Entity value) noexcept {
+    return entt_traits<Entity>::to_entity(value);
+}
+
+/**
+ * @copydoc entt_traits<Entity>::to_version
+ * @tparam Entity The value type.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr typename entt_traits<Entity>::version_type to_version(const Entity value) noexcept {
+    return entt_traits<Entity>::to_version(value);
+}
+
+/*! @brief Null object for all identifiers.  */
+struct null_t {
+    /**
+     * @brief Converts the null object to identifiers of any type.
+     * @tparam Entity Type of identifier.
+     * @return The null representation for the given type.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr operator Entity() const noexcept {
+        using entity_traits = entt_traits<Entity>;
+        return entity_traits::combine(entity_traits::reserved, entity_traits::reserved);
+    }
+
+    /**
+     * @brief Compares two null objects.
+     * @param other A null object.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const null_t other) const noexcept {
+        return true;
+    }
+
+    /**
+     * @brief Compares two null objects.
+     * @param other A null object.
+     * @return False in all cases.
+     */
+    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const null_t other) const noexcept {
+        return false;
+    }
+
+    /**
+     * @brief Compares a null object and an identifier of any type.
+     * @tparam Entity Type of identifier.
+     * @param entity Identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const noexcept {
+        using entity_traits = entt_traits<Entity>;
+        return entity_traits::to_entity(entity) == entity_traits::to_entity(*this);
+    }
+
+    /**
+     * @brief Compares a null object and an identifier of any type.
+     * @tparam Entity Type of identifier.
+     * @param entity Identifier with which to compare.
+     * @return True if the two elements differ, false otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator!=(const Entity entity) const noexcept {
+        return !(entity == *this);
+    }
+};
+
+/**
+ * @brief Compares a null object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
+ * @param other A null object yet to be converted.
+ * @return False if the two elements differ, true otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator==(const Entity entity, const null_t other) noexcept {
+    return other.operator==(entity);
+}
+
+/**
+ * @brief Compares a null object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
+ * @param other A null object yet to be converted.
+ * @return True if the two elements differ, false otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator!=(const Entity entity, const null_t other) noexcept {
+    return !(other == entity);
+}
+
+/*! @brief Tombstone object for all identifiers.  */
+struct tombstone_t {
+    /**
+     * @brief Converts the tombstone object to identifiers of any type.
+     * @tparam Entity Type of identifier.
+     * @return The tombstone representation for the given type.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr operator Entity() const noexcept {
+        using entity_traits = entt_traits<Entity>;
+        return entity_traits::combine(entity_traits::reserved, entity_traits::reserved);
+    }
+
+    /**
+     * @brief Compares two tombstone objects.
+     * @param other A tombstone object.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const tombstone_t other) const noexcept {
+        return true;
+    }
+
+    /**
+     * @brief Compares two tombstone objects.
+     * @param other A tombstone object.
+     * @return False in all cases.
+     */
+    [[nodiscard]] constexpr bool operator!=([[maybe_unused]] const tombstone_t other) const noexcept {
+        return false;
+    }
+
+    /**
+     * @brief Compares a tombstone object and an identifier of any type.
+     * @tparam Entity Type of identifier.
+     * @param entity Identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const noexcept {
+        using entity_traits = entt_traits<Entity>;
+        return entity_traits::to_version(entity) == entity_traits::to_version(*this);
+    }
+
+    /**
+     * @brief Compares a tombstone object and an identifier of any type.
+     * @tparam Entity Type of identifier.
+     * @param entity Identifier with which to compare.
+     * @return True if the two elements differ, false otherwise.
+     */
+    template<typename Entity>
+    [[nodiscard]] constexpr bool operator!=(const Entity entity) const noexcept {
+        return !(entity == *this);
+    }
+};
+
+/**
+ * @brief Compares a tombstone object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
+ * @param other A tombstone object yet to be converted.
+ * @return False if the two elements differ, true otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator==(const Entity entity, const tombstone_t other) noexcept {
+    return other.operator==(entity);
+}
+
+/**
+ * @brief Compares a tombstone object and an identifier of any type.
+ * @tparam Entity Type of identifier.
+ * @param entity Identifier with which to compare.
+ * @param other A tombstone object yet to be converted.
+ * @return True if the two elements differ, false otherwise.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr bool operator!=(const Entity entity, const tombstone_t other) noexcept {
+    return !(other == entity);
+}
+
+/**
+ * @brief Compile-time constant for null entities.
+ *
+ * There exist implicit conversions from this variable to identifiers of any
+ * allowed type. Similarly, there exist comparison operators between the null
+ * entity and any other identifier.
+ */
+inline constexpr null_t null{};
+
+/**
+ * @brief Compile-time constant for tombstone entities.
+ *
+ * There exist implicit conversions from this variable to identifiers of any
+ * allowed type. Similarly, there exist comparison operators between the
+ * tombstone entity and any other identifier.
+ */
+inline constexpr tombstone_t tombstone{};
+
+} // namespace entt
+
+#endif

src/entt/entity/fwd.hpp

@@ -0,0 +1,206 @@
+#ifndef ENTT_ENTITY_FWD_HPP
+#define ENTT_ENTITY_FWD_HPP
+
+#include <memory>
+#include <type_traits>
+#include "../core/fwd.hpp"
+#include "../core/type_traits.hpp"
+
+namespace entt {
+
+/*! @brief Default entity identifier. */
+enum class entity : id_type {};
+
+template<typename Entity = entity, typename = std::allocator<Entity>>
+class basic_sparse_set;
+
+template<typename Type, typename = entity, typename = std::allocator<Type>, typename = void>
+class basic_storage;
+
+template<typename Type>
+class sigh_storage_mixin;
+
+/**
+ * @brief Provides a common way to define storage types.
+ * @tparam Type Storage value type.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Entity = entity, typename Allocator = std::allocator<Type>, typename = void>
+struct storage_type {
+    /*! @brief Type-to-storage conversion result. */
+    using type = sigh_storage_mixin<basic_storage<Type, Entity, Allocator>>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Args Arguments to forward.
+ */
+template<typename... Args>
+using storage_type_t = typename storage_type<Args...>::type;
+
+/**
+ * Type-to-storage conversion utility that preserves constness.
+ * @tparam Type Storage value type, eventually const.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Entity = entity, typename Allocator = std::allocator<std::remove_const_t<Type>>>
+struct storage_for {
+    /*! @brief Type-to-storage conversion result. */
+    using type = constness_as_t<storage_type_t<std::remove_const_t<Type>, Entity, Allocator>, Type>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Args Arguments to forward.
+ */
+template<typename... Args>
+using storage_for_t = typename storage_for<Args...>::type;
+
+template<typename Entity = entity, typename = std::allocator<Entity>>
+class basic_registry;
+
+template<typename, typename, typename = void>
+class basic_view;
+
+template<typename Type, typename = std::allocator<Type *>>
+class basic_runtime_view;
+
+template<typename, typename, typename>
+class basic_group;
+
+template<typename>
+class basic_observer;
+
+template<typename>
+class basic_organizer;
+
+template<typename, typename...>
+struct basic_handle;
+
+template<typename>
+class basic_snapshot;
+
+template<typename>
+class basic_snapshot_loader;
+
+template<typename>
+class basic_continuous_loader;
+
+/**
+ * @brief Alias for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+using exclude_t = type_list<Type...>;
+
+/**
+ * @brief Variable template for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+inline constexpr exclude_t<Type...> exclude{};
+
+/**
+ * @brief Alias for lists of observed components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+using get_t = type_list<Type...>;
+
+/**
+ * @brief Variable template for lists of observed components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+inline constexpr get_t<Type...> get{};
+
+/**
+ * @brief Alias for lists of owned components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+using owned_t = type_list<Type...>;
+
+/**
+ * @brief Variable template for lists of owned components.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+inline constexpr owned_t<Type...> owned{};
+
+/*! @brief Alias declaration for the most common use case. */
+using sparse_set = basic_sparse_set<>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Type Type of objects assigned to the entities.
+ */
+template<typename Type>
+using storage = basic_storage<Type>;
+
+/*! @brief Alias declaration for the most common use case. */
+using registry = basic_registry<>;
+
+/*! @brief Alias declaration for the most common use case. */
+using observer = basic_observer<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using organizer = basic_organizer<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using handle = basic_handle<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using const_handle = basic_handle<const registry>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using handle_view = basic_handle<registry, Args...>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using const_handle_view = basic_handle<const registry, Args...>;
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot = basic_snapshot<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot_loader = basic_snapshot_loader<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using continuous_loader = basic_continuous_loader<registry>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Get Types of storage iterated by the view.
+ * @tparam Exclude Types of storage used to filter the view.
+ */
+template<typename Get, typename Exclude = exclude_t<>>
+using view = basic_view<type_list_transform_t<Get, storage_for>, type_list_transform_t<Exclude, storage_for>>;
+
+/*! @brief Alias declaration for the most common use case. */
+using runtime_view = basic_runtime_view<sparse_set>;
+
+/*! @brief Alias declaration for the most common use case. */
+using const_runtime_view = basic_runtime_view<const sparse_set>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Owned Types of storage _owned_ by the group.
+ * @tparam Get Types of storage _observed_ by the group.
+ * @tparam Exclude Types of storage used to filter the group.
+ */
+template<typename Owned, typename Get, typename Exclude>
+using group = basic_group<type_list_transform_t<Owned, storage_for>, type_list_transform_t<Get, storage_for>, type_list_transform_t<Exclude, storage_for>>;
+
+} // namespace entt
+
+#endif

src/entt/entity/group.hpp

@@ -0,0 +1,852 @@
+#ifndef ENTT_ENTITY_GROUP_HPP
+#define ENTT_ENTITY_GROUP_HPP
+
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "../core/iterator.hpp"
+#include "../core/type_traits.hpp"
+#include "component.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+#include "sparse_set.hpp"
+#include "storage.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename, typename, typename>
+class extended_group_iterator;
+
+template<typename It, typename... Owned, typename... Get>
+class extended_group_iterator<It, owned_t<Owned...>, get_t<Get...>> {
+    template<typename Type>
+    auto index_to_element([[maybe_unused]] Type &cpool) const {
+        if constexpr(ignore_as_empty_v<typename Type::value_type>) {
+            return std::make_tuple();
+        } else {
+            return std::forward_as_tuple(cpool.rbegin()[it.index()]);
+        }
+    }
+
+public:
+    using difference_type = std::ptrdiff_t;
+    using value_type = decltype(std::tuple_cat(std::make_tuple(*std::declval<It>()), std::declval<Owned>().get_as_tuple({})..., std::declval<Get>().get_as_tuple({})...));
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using iterator_category = std::input_iterator_tag;
+
+    constexpr extended_group_iterator()
+        : it{},
+          pools{} {}
+
+    extended_group_iterator(It from, const std::tuple<Owned *..., Get *...> &cpools)
+        : it{from},
+          pools{cpools} {}
+
+    extended_group_iterator &operator++() noexcept {
+        return ++it, *this;
+    }
+
+    extended_group_iterator operator++(int) noexcept {
+        extended_group_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] reference operator*() const noexcept {
+        return std::tuple_cat(std::make_tuple(*it), index_to_element(*std::get<Owned *>(pools))..., std::get<Get *>(pools)->get_as_tuple(*it)...);
+    }
+
+    [[nodiscard]] pointer operator->() const noexcept {
+        return operator*();
+    }
+
+    template<typename... Lhs, typename... Rhs>
+    friend constexpr bool operator==(const extended_group_iterator<Lhs...> &, const extended_group_iterator<Rhs...> &) noexcept;
+
+private:
+    It it;
+    std::tuple<Owned *..., Get *...> pools;
+};
+
+template<typename... Lhs, typename... Rhs>
+[[nodiscard]] constexpr bool operator==(const extended_group_iterator<Lhs...> &lhs, const extended_group_iterator<Rhs...> &rhs) noexcept {
+    return lhs.it == rhs.it;
+}
+
+template<typename... Lhs, typename... Rhs>
+[[nodiscard]] constexpr bool operator!=(const extended_group_iterator<Lhs...> &lhs, const extended_group_iterator<Rhs...> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Group.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename, typename, typename>
+class basic_group;
+
+/**
+ * @brief Non-owning group.
+ *
+ * A non-owning group returns all entities and only the entities that are at
+ * least in the given storage. Moreover, it's guaranteed that the entity list is
+ * tightly packed in memory for fast iterations.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New elements are added to the storage.
+ * * The entity currently pointed is modified (for example, components are added
+ *   or removed from it).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the pools iterated by the group in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @tparam Get Types of storage _observed_ by the group.
+ * @tparam Exclude Types of storage used to filter the group.
+ */
+template<typename... Get, typename... Exclude>
+class basic_group<owned_t<>, get_t<Get...>, exclude_t<Exclude...>> {
+    using underlying_type = std::common_type_t<typename Get::entity_type..., typename Exclude::entity_type...>;
+    using basic_common_type = std::common_type_t<typename Get::base_type..., typename Exclude::base_type...>;
+
+    template<typename Type>
+    static constexpr std::size_t index_of = type_list_index_v<std::remove_const_t<Type>, type_list<typename Get::value_type...>>;
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = underlying_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Common type among all storage types. */
+    using base_type = basic_common_type;
+    /*! @brief Random access iterator type. */
+    using iterator = typename base_type::iterator;
+    /*! @brief Reversed iterator type. */
+    using reverse_iterator = typename base_type::reverse_iterator;
+    /*! @brief Iterable group type. */
+    using iterable = iterable_adaptor<internal::extended_group_iterator<iterator, owned_t<>, get_t<Get...>>>;
+
+    /*! @brief Default constructor to use to create empty, invalid groups. */
+    basic_group() noexcept
+        : handler{} {}
+
+    /**
+     * @brief Constructs a group from a set of storage classes.
+     * @param ref The actual entities to iterate.
+     * @param gpool Storage types to iterate _observed_ by the group.
+     */
+    basic_group(basic_common_type &ref, Get &...gpool) noexcept
+        : handler{&ref},
+          pools{&gpool...} {}
+
+    /**
+     * @brief Returns a const reference to the underlying handler.
+     * @return A const reference to the underlying handler.
+     */
+    [[nodiscard]] const base_type &handle() const noexcept {
+        return *handler;
+    }
+
+    /**
+     * @brief Returns the storage for a given component type.
+     * @tparam Type Type of component of which to return the storage.
+     * @return The storage for the given component type.
+     */
+    template<typename Type>
+    [[nodiscard]] decltype(auto) storage() const noexcept {
+        return storage<index_of<Type>>();
+    }
+
+    /**
+     * @brief Returns the storage for a given index.
+     * @tparam Index Index of the storage to return.
+     * @return The storage for the given index.
+     */
+    template<std::size_t Index>
+    [[nodiscard]] decltype(auto) storage() const noexcept {
+        return *std::get<Index>(pools);
+    }
+
+    /**
+     * @brief Returns the number of entities that are part of the group.
+     * @return Number of entities that are part of the group.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return *this ? handler->size() : size_type{};
+    }
+
+    /**
+     * @brief Returns the number of elements that a group has currently
+     * allocated space for.
+     * @return Capacity of the group.
+     */
+    [[nodiscard]] size_type capacity() const noexcept {
+        return *this ? handler->capacity() : size_type{};
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        if(*this) {
+            handler->shrink_to_fit();
+        }
+    }
+
+    /**
+     * @brief Checks whether a group is empty.
+     * @return True if the group is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return !*this || handler->empty();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the group.
+     *
+     * The returned iterator points to the first entity of the group. If the
+     * group is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the group.
+     */
+    [[nodiscard]] iterator begin() const noexcept {
+        return *this ? handler->begin() : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the group. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * group.
+     */
+    [[nodiscard]] iterator end() const noexcept {
+        return *this ? handler->end() : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed group.
+     *
+     * The returned iterator points to the first entity of the reversed group.
+     * If the group is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed group.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const noexcept {
+        return *this ? handler->rbegin() : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed group. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * reversed group.
+     */
+    [[nodiscard]] reverse_iterator rend() const noexcept {
+        return *this ? handler->rend() : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns the first entity of the group, if any.
+     * @return The first entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const noexcept {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the group, if any.
+     * @return The last entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const noexcept {
+        const auto it = rbegin();
+        return it != rend() ? *it : null;
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const noexcept {
+        const auto it = *this ? handler->find(entt) : iterator{};
+        return it != end() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group is properly initialized.
+     * @return True if the group is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return handler != nullptr;
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const noexcept {
+        return *this && handler->contains(entt);
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its counterpart.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.
+     *
+     * @tparam Type Types of components to get.
+     * @param entt A valid identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Type>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        if constexpr(sizeof...(Type) == 0) {
+            return std::apply([entt](auto *...curr) { return std::tuple_cat(curr->get_as_tuple(entt)...); }, pools);
+        } else if constexpr(sizeof...(Type) == 1) {
+            return (std::get<index_of<Type>>(pools)->get(entt), ...);
+        } else {
+            return std::tuple_cat(std::get<index_of<Type>>(pools)->get_as_tuple(entt)...);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to non-empty components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &...);
+     * void(Type &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(const auto entt: *this) {
+            if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_group>().get({})))>) {
+                std::apply(func, std::tuple_cat(std::make_tuple(entt), get(entt)));
+            } else {
+                std::apply(func, get(entt));
+            }
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ a group.
+     *
+     * The iterable object returns tuples that contain the current entity and a
+     * set of references to its non-empty components. The _constness_ of the
+     * components is as requested.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @return An iterable object to use to _visit_ the group.
+     */
+    [[nodiscard]] iterable each() const noexcept {
+        return iterable{{begin(), pools}, {end(), pools}};
+    }
+
+    /**
+     * @brief Sort a group according to the given comparison function.
+     *
+     * Sort the group so that iterating it with a couple of iterators returns
+     * entities and components in the expected order. See `begin` and `end` for
+     * more details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(std::tuple<Type &...>, std::tuple<Type &...>);
+     * bool(const Type &..., const Type &...);
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Where `Type` are such that they are iterated by the group.<br/>
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function object must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @tparam Type Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename... Type, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&...args) {
+        if(*this) {
+            if constexpr(sizeof...(Type) == 0) {
+                static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>, "Invalid comparison function");
+                handler->sort(std::move(compare), std::move(algo), std::forward<Args>(args)...);
+            } else {
+                auto comp = [this, &compare](const entity_type lhs, const entity_type rhs) {
+                    if constexpr(sizeof...(Type) == 1) {
+                        return compare((std::get<index_of<Type>>(pools)->get(lhs), ...), (std::get<index_of<Type>>(pools)->get(rhs), ...));
+                    } else {
+                        return compare(std::forward_as_tuple(std::get<index_of<Type>>(pools)->get(lhs)...), std::forward_as_tuple(std::get<index_of<Type>>(pools)->get(rhs)...));
+                    }
+                };
+
+                handler->sort(std::move(comp), std::move(algo), std::forward<Args>(args)...);
+            }
+        }
+    }
+
+    /**
+     * @brief Sort the shared pool of entities according to the given component.
+     *
+     * Non-owning groups of the same type share with the registry a pool of
+     * entities with its own order that doesn't depend on the order of any pool
+     * of components. Users can order the underlying data structure so that it
+     * respects the order of the pool of the given component.
+     *
+     * @note
+     * The shared pool of entities and thus its order is affected by the changes
+     * to each and every pool that it tracks. Therefore changes to those pools
+     * can quickly ruin the order imposed to the pool of entities shared between
+     * the non-owning groups.
+     *
+     * @tparam Type Type of component to use to impose the order.
+     */
+    template<typename Type>
+    void sort() const {
+        if(*this) {
+            handler->respect(*std::get<index_of<Type>>(pools));
+        }
+    }
+
+private:
+    base_type *const handler;
+    const std::tuple<Get *...> pools;
+};
+
+/**
+ * @brief Owning group.
+ *
+ * Owning groups returns all entities and only the entities that are at
+ * least in the given storage. Moreover:
+ *
+ * * It's guaranteed that the entity list is tightly packed in memory for fast
+ *   iterations.
+ * * It's guaranteed that all components in the owned storage are tightly packed
+ *   in memory for even faster iterations and to allow direct access.
+ * * They stay true to the order of the owned storage and all instances have the
+ *   same order in memory.
+ *
+ * The more types of storage are owned, the faster it is to iterate a group.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New elements are added to the storage.
+ * * The entity currently pointed is modified (for example, components are added
+ *   or removed from it).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the pools iterated by the group in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @tparam Owned Types of storage _owned_ by the group.
+ * @tparam Get Types of storage _observed_ by the group.
+ * @tparam Exclude Types of storage used to filter the group.
+ */
+template<typename... Owned, typename... Get, typename... Exclude>
+class basic_group<owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>> {
+    using underlying_type = std::common_type_t<typename Owned::entity_type..., typename Get::entity_type..., typename Exclude::entity_type...>;
+    using basic_common_type = std::common_type_t<typename Owned::base_type..., typename Get::base_type..., typename Exclude::base_type...>;
+
+    template<typename Type>
+    static constexpr std::size_t index_of = type_list_index_v<std::remove_const_t<Type>, type_list<typename Owned::value_type..., typename Get::value_type...>>;
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = underlying_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Common type among all storage types. */
+    using base_type = basic_common_type;
+    /*! @brief Random access iterator type. */
+    using iterator = typename base_type::iterator;
+    /*! @brief Reversed iterator type. */
+    using reverse_iterator = typename base_type::reverse_iterator;
+    /*! @brief Iterable group type. */
+    using iterable = iterable_adaptor<internal::extended_group_iterator<iterator, owned_t<Owned...>, get_t<Get...>>>;
+
+    /*! @brief Default constructor to use to create empty, invalid groups. */
+    basic_group() noexcept
+        : length{} {}
+
+    /**
+     * @brief Constructs a group from a set of storage classes.
+     * @param extent The actual number of entities to iterate.
+     * @param opool Storage types to iterate _owned_ by the group.
+     * @param gpool Storage types to iterate _observed_ by the group.
+     */
+    basic_group(const std::size_t &extent, Owned &...opool, Get &...gpool) noexcept
+        : pools{&opool..., &gpool...},
+          length{&extent} {}
+
+    /**
+     * @brief Returns the storage for a given component type.
+     * @tparam Type Type of component of which to return the storage.
+     * @return The storage for the given component type.
+     */
+    template<typename Type>
+    [[nodiscard]] decltype(auto) storage() const noexcept {
+        return storage<index_of<Type>>();
+    }
+
+    /**
+     * @brief Returns the storage for a given index.
+     * @tparam Index Index of the storage to return.
+     * @return The storage for the given index.
+     */
+    template<std::size_t Index>
+    [[nodiscard]] decltype(auto) storage() const noexcept {
+        return *std::get<Index>(pools);
+    }
+
+    /**
+     * @brief Returns the number of entities that that are part of the group.
+     * @return Number of entities that that are part of the group.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return *this ? *length : size_type{};
+    }
+
+    /**
+     * @brief Checks whether a group is empty.
+     * @return True if the group is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return !*this || !*length;
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the group.
+     *
+     * The returned iterator points to the first entity of the group. If the
+     * group is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the group.
+     */
+    [[nodiscard]] iterator begin() const noexcept {
+        return *this ? (std::get<0>(pools)->base_type::end() - *length) : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the group. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * group.
+     */
+    [[nodiscard]] iterator end() const noexcept {
+        return *this ? std::get<0>(pools)->base_type::end() : iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed group.
+     *
+     * The returned iterator points to the first entity of the reversed group.
+     * If the group is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed group.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const noexcept {
+        return *this ? std::get<0>(pools)->base_type::rbegin() : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * group.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed group. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * reversed group.
+     */
+    [[nodiscard]] reverse_iterator rend() const noexcept {
+        return *this ? (std::get<0>(pools)->base_type::rbegin() + *length) : reverse_iterator{};
+    }
+
+    /**
+     * @brief Returns the first entity of the group, if any.
+     * @return The first entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const noexcept {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the group, if any.
+     * @return The last entity of the group if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const noexcept {
+        const auto it = rbegin();
+        return it != rend() ? *it : null;
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const noexcept {
+        const auto it = *this ? std::get<0>(pools)->find(entt) : iterator{};
+        return it != end() && it >= begin() && *it == entt ? it : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Checks if a group is properly initialized.
+     * @return True if the group is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return length != nullptr;
+    }
+
+    /**
+     * @brief Checks if a group contains an entity.
+     * @param entt A valid identifier.
+     * @return True if the group contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const noexcept {
+        return *this && std::get<0>(pools)->contains(entt) && (std::get<0>(pools)->index(entt) < (*length));
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * Prefer this function instead of `registry::get` during iterations. It has
+     * far better performance than its counterpart.
+     *
+     * @warning
+     * Attempting to use an invalid component type results in a compilation
+     * error. Attempting to use an entity that doesn't belong to the group
+     * results in undefined behavior.
+     *
+     * @tparam Type Types of components to get.
+     * @param entt A valid identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Type>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        if constexpr(sizeof...(Type) == 0) {
+            return std::apply([entt](auto *...curr) { return std::tuple_cat(curr->get_as_tuple(entt)...); }, pools);
+        } else if constexpr(sizeof...(Type) == 1) {
+            return (std::get<index_of<Type>>(pools)->get(entt), ...);
+        } else {
+            return std::tuple_cat(std::get<index_of<Type>>(pools)->get_as_tuple(entt)...);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to non-empty components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &...);
+     * void(Type &...);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(auto args: each()) {
+            if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_group>().get({})))>) {
+                std::apply(func, args);
+            } else {
+                std::apply([&func](auto, auto &&...less) { func(std::forward<decltype(less)>(less)...); }, args);
+            }
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ a group.
+     *
+     * The iterable object returns tuples that contain the current entity and a
+     * set of references to its non-empty components. The _constness_ of the
+     * components is as requested.
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @return An iterable object to use to _visit_ the group.
+     */
+    [[nodiscard]] iterable each() const noexcept {
+        return {{begin(), pools}, {end(), pools}};
+    }
+
+    /**
+     * @brief Sort a group according to the given comparison function.
+     *
+     * Sort the group so that iterating it with a couple of iterators returns
+     * entities and components in the expected order. See `begin` and `end` for
+     * more details.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to one of the following:
+     *
+     * @code{.cpp}
+     * bool(std::tuple<Type &...>, std::tuple<Type &...>);
+     * bool(const Type &, const Type &);
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Where `Type` are either owned types or not but still such that they are
+     * iterated by the group.<br/>
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function object must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @tparam Type Optional types of components to compare.
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename... Type, typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&...args) const {
+        if constexpr(sizeof...(Type) == 0) {
+            static_assert(std::is_invocable_v<Compare, const entity_type, const entity_type>, "Invalid comparison function");
+            std::get<0>(pools)->sort_n(*length, std::move(compare), std::move(algo), std::forward<Args>(args)...);
+        } else {
+            auto comp = [this, &compare](const entity_type lhs, const entity_type rhs) {
+                if constexpr(sizeof...(Type) == 1) {
+                    return compare((std::get<index_of<Type>>(pools)->get(lhs), ...), (std::get<index_of<Type>>(pools)->get(rhs), ...));
+                } else {
+                    return compare(std::forward_as_tuple(std::get<index_of<Type>>(pools)->get(lhs)...), std::forward_as_tuple(std::get<index_of<Type>>(pools)->get(rhs)...));
+                }
+            };
+
+            std::get<0>(pools)->sort_n(*length, std::move(comp), std::move(algo), std::forward<Args>(args)...);
+        }
+
+        std::apply([this](auto *head, auto *...other) {
+            for(auto next = *length; next; --next) {
+                const auto pos = next - 1;
+                [[maybe_unused]] const auto entt = head->data()[pos];
+                (other->swap_elements(other->data()[pos], entt), ...);
+            }
+        },
+                   pools);
+    }
+
+private:
+    const std::tuple<Owned *..., Get *...> pools;
+    const size_type *const length;
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/handle.hpp

@@ -0,0 +1,382 @@
+#ifndef ENTT_ENTITY_HANDLE_HPP
+#define ENTT_ENTITY_HANDLE_HPP
+
+#include <iterator>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "../core/iterator.hpp"
+#include "../core/type_traits.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename It>
+class handle_storage_iterator final {
+    template<typename Other>
+    friend class handle_storage_iterator;
+
+    using underlying_type = std::remove_reference_t<typename It::value_type::second_type>;
+    using entity_type = typename underlying_type::entity_type;
+
+public:
+    using value_type = typename std::iterator_traits<It>::value_type;
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using difference_type = std::ptrdiff_t;
+    using iterator_category = std::input_iterator_tag;
+
+    constexpr handle_storage_iterator() noexcept
+        : entt{null},
+          it{},
+          last{} {}
+
+    constexpr handle_storage_iterator(entity_type value, It from, It to) noexcept
+        : entt{value},
+          it{from},
+          last{to} {
+        while(it != last && !it->second.contains(entt)) { ++it; }
+    }
+
+    constexpr handle_storage_iterator &operator++() noexcept {
+        while(++it != last && !it->second.contains(entt)) {}
+        return *this;
+    }
+
+    constexpr handle_storage_iterator operator++(int) noexcept {
+        handle_storage_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return *it;
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return operator*();
+    }
+
+    template<typename ILhs, typename IRhs>
+    friend constexpr bool operator==(const handle_storage_iterator<ILhs> &, const handle_storage_iterator<IRhs> &) noexcept;
+
+private:
+    entity_type entt;
+    It it;
+    It last;
+};
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator==(const handle_storage_iterator<ILhs> &lhs, const handle_storage_iterator<IRhs> &rhs) noexcept {
+    return lhs.it == rhs.it;
+}
+
+template<typename ILhs, typename IRhs>
+[[nodiscard]] constexpr bool operator!=(const handle_storage_iterator<ILhs> &lhs, const handle_storage_iterator<IRhs> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Non-owning handle to an entity.
+ *
+ * Tiny wrapper around a registry and an entity.
+ *
+ * @tparam Registry Basic registry type.
+ * @tparam Scope Types to which to restrict the scope of a handle.
+ */
+template<typename Registry, typename... Scope>
+struct basic_handle {
+    /*! @brief Type of registry accepted by the handle. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename registry_type::entity_type;
+    /*! @brief Underlying version type. */
+    using version_type = typename registry_type::version_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = typename registry_type::size_type;
+
+    /*! @brief Constructs an invalid handle. */
+    basic_handle() noexcept
+        : reg{},
+          entt{null} {}
+
+    /**
+     * @brief Constructs a handle from a given registry and entity.
+     * @param ref An instance of the registry class.
+     * @param value A valid identifier.
+     */
+    basic_handle(registry_type &ref, entity_type value) noexcept
+        : reg{&ref},
+          entt{value} {}
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ a handle.
+     *
+     * The iterable object returns a pair that contains the name and a reference
+     * to the current storage.<br/>
+     * Returned storage are those that contain the entity associated with the
+     * handle.
+     *
+     * @return An iterable object to use to _visit_ the handle.
+     */
+    [[nodiscard]] auto storage() const noexcept {
+        auto iterable = reg->storage();
+        using iterator_type = internal::handle_storage_iterator<typename decltype(iterable)::iterator>;
+        return iterable_adaptor{iterator_type{entt, iterable.begin(), iterable.end()}, iterator_type{entt, iterable.end(), iterable.end()}};
+    }
+
+    /**
+     * @brief Constructs a const handle from a non-const one.
+     * @tparam Other A valid entity type (see entt_traits for more details).
+     * @tparam Args Scope of the handle to construct.
+     * @return A const handle referring to the same registry and the same
+     * entity.
+     */
+    template<typename Other, typename... Args>
+    operator basic_handle<Other, Args...>() const noexcept {
+        static_assert(std::is_same_v<Other, Registry> || std::is_same_v<std::remove_const_t<Other>, Registry>, "Invalid conversion between different handles");
+        static_assert((sizeof...(Scope) == 0 || ((sizeof...(Args) != 0 && sizeof...(Args) <= sizeof...(Scope)) && ... && (type_list_contains_v<type_list<Scope...>, Args>))), "Invalid conversion between different handles");
+
+        return reg ? basic_handle<Other, Args...>{*reg, entt} : basic_handle<Other, Args...>{};
+    }
+
+    /**
+     * @brief Converts a handle to its underlying entity.
+     * @return The contained identifier.
+     */
+    [[nodiscard]] operator entity_type() const noexcept {
+        return entity();
+    }
+
+    /**
+     * @brief Checks if a handle refers to non-null registry pointer and entity.
+     * @return True if the handle refers to non-null registry and entity, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return reg && reg->valid(entt);
+    }
+
+    /**
+     * @brief Checks if a handle refers to a valid entity or not.
+     * @return True if the handle refers to a valid entity, false otherwise.
+     */
+    [[nodiscard]] bool valid() const {
+        return reg->valid(entt);
+    }
+
+    /**
+     * @brief Returns a pointer to the underlying registry, if any.
+     * @return A pointer to the underlying registry, if any.
+     */
+    [[nodiscard]] registry_type *registry() const noexcept {
+        return reg;
+    }
+
+    /**
+     * @brief Returns the entity associated with a handle.
+     * @return The entity associated with the handle.
+     */
+    [[nodiscard]] entity_type entity() const noexcept {
+        return entt;
+    }
+
+    /*! @brief Destroys the entity associated with a handle. */
+    void destroy() {
+        reg->destroy(entt);
+    }
+
+    /**
+     * @brief Destroys the entity associated with a handle.
+     * @param version A desired version upon destruction.
+     */
+    void destroy(const version_type version) {
+        reg->destroy(entt, version);
+    }
+
+    /**
+     * @brief Assigns the given component to a handle.
+     * @tparam Component Type of component to create.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the newly created component.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) emplace(Args &&...args) const {
+        static_assert(((sizeof...(Scope) == 0) || ... || std::is_same_v<Component, Scope>), "Invalid type");
+        return reg->template emplace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Assigns or replaces the given component for a handle.
+     * @tparam Component Type of component to assign or replace.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the newly created component.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) emplace_or_replace(Args &&...args) const {
+        static_assert(((sizeof...(Scope) == 0) || ... || std::is_same_v<Component, Scope>), "Invalid type");
+        return reg->template emplace_or_replace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Patches the given component for a handle.
+     * @tparam Component Type of component to patch.
+     * @tparam Func Types of the function objects to invoke.
+     * @param func Valid function objects.
+     * @return A reference to the patched component.
+     */
+    template<typename Component, typename... Func>
+    decltype(auto) patch(Func &&...func) const {
+        static_assert(((sizeof...(Scope) == 0) || ... || std::is_same_v<Component, Scope>), "Invalid type");
+        return reg->template patch<Component>(entt, std::forward<Func>(func)...);
+    }
+
+    /**
+     * @brief Replaces the given component for a handle.
+     * @tparam Component Type of component to replace.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param args Parameters to use to initialize the component.
+     * @return A reference to the component being replaced.
+     */
+    template<typename Component, typename... Args>
+    decltype(auto) replace(Args &&...args) const {
+        static_assert(((sizeof...(Scope) == 0) || ... || std::is_same_v<Component, Scope>), "Invalid type");
+        return reg->template replace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Removes the given components from a handle.
+     * @tparam Component Types of components to remove.
+     * @return The number of components actually removed.
+     */
+    template<typename... Component>
+    size_type remove() const {
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Component> && ...), "Invalid type");
+        return reg->template remove<Component...>(entt);
+    }
+
+    /**
+     * @brief Erases the given components from a handle.
+     * @tparam Component Types of components to erase.
+     */
+    template<typename... Component>
+    void erase() const {
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Component> && ...), "Invalid type");
+        reg->template erase<Component...>(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has all the given components.
+     * @tparam Component Components for which to perform the check.
+     * @return True if the handle has all the components, false otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) all_of() const {
+        return reg->template all_of<Component...>(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has at least one of the given components.
+     * @tparam Component Components for which to perform the check.
+     * @return True if the handle has at least one of the given components,
+     * false otherwise.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) any_of() const {
+        return reg->template any_of<Component...>(entt);
+    }
+
+    /**
+     * @brief Returns references to the given components for a handle.
+     * @tparam Component Types of components to get.
+     * @return References to the components owned by the handle.
+     */
+    template<typename... Component>
+    [[nodiscard]] decltype(auto) get() const {
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Component> && ...), "Invalid type");
+        return reg->template get<Component...>(entt);
+    }
+
+    /**
+     * @brief Returns a reference to the given component for a handle.
+     * @tparam Component Type of component to get.
+     * @tparam Args Types of arguments to use to construct the component.
+     * @param args Parameters to use to initialize the component.
+     * @return Reference to the component owned by the handle.
+     */
+    template<typename Component, typename... Args>
+    [[nodiscard]] decltype(auto) get_or_emplace(Args &&...args) const {
+        static_assert(((sizeof...(Scope) == 0) || ... || std::is_same_v<Component, Scope>), "Invalid type");
+        return reg->template get_or_emplace<Component>(entt, std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Returns pointers to the given components for a handle.
+     * @tparam Component Types of components to get.
+     * @return Pointers to the components owned by the handle.
+     */
+    template<typename... Component>
+    [[nodiscard]] auto try_get() const {
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Component> && ...), "Invalid type");
+        return reg->template try_get<Component...>(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has components assigned.
+     * @return True if the handle has no components assigned, false otherwise.
+     */
+    [[nodiscard]] bool orphan() const {
+        return reg->orphan(entt);
+    }
+
+private:
+    registry_type *reg;
+    entity_type entt;
+};
+
+/**
+ * @brief Compares two handles.
+ * @tparam Args Scope of the first handle.
+ * @tparam Other Scope of the second handle.
+ * @param lhs A valid handle.
+ * @param rhs A valid handle.
+ * @return True if both handles refer to the same registry and the same
+ * entity, false otherwise.
+ */
+template<typename... Args, typename... Other>
+[[nodiscard]] bool operator==(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) noexcept {
+    return lhs.registry() == rhs.registry() && lhs.entity() == rhs.entity();
+}
+
+/**
+ * @brief Compares two handles.
+ * @tparam Args Scope of the first handle.
+ * @tparam Other Scope of the second handle.
+ * @param lhs A valid handle.
+ * @param rhs A valid handle.
+ * @return False if both handles refer to the same registry and the same
+ * entity, true otherwise.
+ */
+template<typename... Args, typename... Other>
+[[nodiscard]] bool operator!=(const basic_handle<Args...> &lhs, const basic_handle<Other...> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+} // namespace entt
+
+#endif

src/entt/entity/helper.hpp

@@ -0,0 +1,144 @@
+#ifndef ENTT_ENTITY_HELPER_HPP
+#define ENTT_ENTITY_HELPER_HPP
+
+#include <memory>
+#include <type_traits>
+#include "../core/fwd.hpp"
+#include "../core/type_traits.hpp"
+#include "../signal/delegate.hpp"
+#include "component.hpp"
+#include "fwd.hpp"
+#include "group.hpp"
+#include "view.hpp"
+
+namespace entt {
+
+/**
+ * @brief Converts a registry to a view.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class as_view {
+    template<typename... Get, typename... Exclude>
+    auto dispatch(get_t<Get...>, exclude_t<Exclude...>) const {
+        return reg.template view<constness_as_t<typename Get::value_type, Get>...>(exclude_t<constness_as_t<typename Exclude::value_type, Exclude>...>{});
+    }
+
+public:
+    /*! @brief Type of registry to convert. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = std::remove_const_t<typename registry_type::entity_type>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_view(registry_type &source) noexcept
+        : reg{source} {}
+
+    /**
+     * @brief Conversion function from a registry to a view.
+     * @tparam Get Type of storage used to construct the view.
+     * @tparam Exclude Types of storage used to filter the view.
+     * @return A newly created view.
+     */
+    template<typename Get, typename Exclude>
+    operator basic_view<Get, Exclude>() const {
+        return dispatch(Get{}, Exclude{});
+    }
+
+private:
+    registry_type &reg;
+};
+
+/**
+ * @brief Converts a registry to a group.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class as_group {
+    template<typename... Owned, typename... Get, typename... Exclude>
+    auto dispatch(owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>) const {
+        if constexpr(std::is_const_v<registry_type>) {
+            return reg.template group_if_exists<typename Owned::value_type...>(get_t<typename Get::value_type...>{}, exclude_t<typename Exclude::value_type...>{});
+        } else {
+            return reg.template group<constness_as_t<typename Owned::value_type, Owned>...>(get_t<constness_as_t<typename Get::value_type, Get>...>{}, exclude_t<constness_as_t<typename Exclude::value_type, Exclude>...>{});
+        }
+    }
+
+public:
+    /*! @brief Type of registry to convert. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = std::remove_const_t<typename registry_type::entity_type>;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_group(registry_type &source) noexcept
+        : reg{source} {}
+
+    /**
+     * @brief Conversion function from a registry to a group.
+     * @tparam Owned Types of _owned_ by the group.
+     * @tparam Get Types of storage _observed_ by the group.
+     * @tparam Exclude Types of storage used to filter the group.
+     * @return A newly created group.
+     */
+    template<typename Owned, typename Get, typename Exclude>
+    operator basic_group<Owned, Get, Exclude>() const {
+        return dispatch(Owned{}, Get{}, Exclude{});
+    }
+
+private:
+    registry_type &reg;
+};
+
+/**
+ * @brief Helper to create a listener that directly invokes a member function.
+ * @tparam Member Member function to invoke on a component of the given type.
+ * @tparam Registry Basic registry type.
+ * @param reg A registry that contains the given entity and its components.
+ * @param entt Entity from which to get the component.
+ */
+template<auto Member, typename Registry = std::decay_t<nth_argument_t<0u, Member>>>
+void invoke(Registry &reg, const typename Registry::entity_type entt) {
+    static_assert(std::is_member_function_pointer_v<decltype(Member)>, "Invalid pointer to non-static member function");
+    delegate<void(Registry &, const typename Registry::entity_type)> func;
+    func.template connect<Member>(reg.template get<member_class_t<decltype(Member)>>(entt));
+    func(reg, entt);
+}
+
+/**
+ * @brief Returns the entity associated with a given component.
+ *
+ * @warning
+ * Currently, this function only works correctly with the default pool as it
+ * makes assumptions about how the components are laid out.
+ *
+ * @tparam Registry Basic registry type.
+ * @tparam Component Type of component.
+ * @param reg A registry that contains the given entity and its components.
+ * @param instance A valid component instance.
+ * @return The entity associated with the given component.
+ */
+template<typename Registry, typename Component>
+typename Registry::entity_type to_entity(const Registry &reg, const Component &instance) {
+    const auto &storage = reg.template storage<Component>();
+    const typename Registry::base_type &base = storage;
+    const auto *addr = std::addressof(instance);
+
+    for(auto it = base.rbegin(), last = base.rend(); it < last; it += component_traits<Component>::page_size) {
+        if(const auto dist = (addr - std::addressof(storage.get(*it))); dist >= 0 && dist < static_cast<decltype(dist)>(component_traits<Component>::page_size)) {
+            return *(it + dist);
+        }
+    }
+
+    return null;
+}
+
+} // namespace entt
+
+#endif

src/entt/entity/observer.hpp

@@ -0,0 +1,432 @@
+#ifndef ENTT_ENTITY_OBSERVER_HPP
+#define ENTT_ENTITY_OBSERVER_HPP
+
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <type_traits>
+#include <utility>
+#include "../core/type_traits.hpp"
+#include "../signal/delegate.hpp"
+#include "fwd.hpp"
+#include "storage.hpp"
+
+namespace entt {
+
+/*! @brief Grouping matcher. */
+template<typename...>
+struct matcher {};
+
+/**
+ * @brief Collector.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename...>
+struct basic_collector;
+
+/**
+ * @brief Collector.
+ *
+ * A collector contains a set of rules (literally, matchers) to use to track
+ * entities.<br/>
+ * Its main purpose is to generate a descriptor that allows an observer to know
+ * how to connect to a registry.
+ */
+template<>
+struct basic_collector<> {
+    /**
+     * @brief Adds a grouping matcher to the collector.
+     * @tparam AllOf Types of components tracked by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto group(exclude_t<NoneOf...> = {}) noexcept {
+        return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>>{};
+    }
+
+    /**
+     * @brief Adds an observing matcher to the collector.
+     * @tparam AnyOf Type of component for which changes should be detected.
+     * @return The updated collector.
+     */
+    template<typename AnyOf>
+    static constexpr auto update() noexcept {
+        return basic_collector<matcher<type_list<>, type_list<>, AnyOf>>{};
+    }
+};
+
+/**
+ * @brief Collector.
+ * @copydetails basic_collector<>
+ * @tparam Reject Untracked types used to filter out entities.
+ * @tparam Require Untracked types required by the matcher.
+ * @tparam Rule Specific details of the current matcher.
+ * @tparam Other Other matchers.
+ */
+template<typename... Reject, typename... Require, typename... Rule, typename... Other>
+struct basic_collector<matcher<type_list<Reject...>, type_list<Require...>, Rule...>, Other...> {
+    /*! @brief Current matcher. */
+    using current_type = matcher<type_list<Reject...>, type_list<Require...>, Rule...>;
+
+    /**
+     * @brief Adds a grouping matcher to the collector.
+     * @tparam AllOf Types of components tracked by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto group(exclude_t<NoneOf...> = {}) noexcept {
+        return basic_collector<matcher<type_list<>, type_list<>, type_list<NoneOf...>, AllOf...>, current_type, Other...>{};
+    }
+
+    /**
+     * @brief Adds an observing matcher to the collector.
+     * @tparam AnyOf Type of component for which changes should be detected.
+     * @return The updated collector.
+     */
+    template<typename AnyOf>
+    static constexpr auto update() noexcept {
+        return basic_collector<matcher<type_list<>, type_list<>, AnyOf>, current_type, Other...>{};
+    }
+
+    /**
+     * @brief Updates the filter of the last added matcher.
+     * @tparam AllOf Types of components required by the matcher.
+     * @tparam NoneOf Types of components used to filter out entities.
+     * @return The updated collector.
+     */
+    template<typename... AllOf, typename... NoneOf>
+    static constexpr auto where(exclude_t<NoneOf...> = {}) noexcept {
+        using extended_type = matcher<type_list<Reject..., NoneOf...>, type_list<Require..., AllOf...>, Rule...>;
+        return basic_collector<extended_type, Other...>{};
+    }
+};
+
+/*! @brief Variable template used to ease the definition of collectors. */
+inline constexpr basic_collector<> collector{};
+
+/**
+ * @brief Observer.
+ *
+ * An observer returns all the entities and only the entities that fit the
+ * requirements of at least one matcher. Moreover, it's guaranteed that the
+ * entity list is tightly packed in memory for fast iterations.<br/>
+ * In general, observers don't stay true to the order of any set of components.
+ *
+ * Observers work mainly with two types of matchers, provided through a
+ * collector:
+ *
+ * * Observing matcher: an observer will return at least all the living entities
+ *   for which one or more of the given components have been updated and not yet
+ *   destroyed.
+ * * Grouping matcher: an observer will return at least all the living entities
+ *   that would have entered the given group if it existed and that would have
+ *   not yet left it.
+ *
+ * If an entity respects the requirements of multiple matchers, it will be
+ * returned once and only once by the observer in any case.
+ *
+ * Matchers support also filtering by means of a _where_ clause that accepts
+ * both a list of types and an exclusion list.<br/>
+ * Whenever a matcher finds that an entity matches its requirements, the
+ * condition of the filter is verified before to register the entity itself.
+ * Moreover, a registered entity isn't returned by the observer if the condition
+ * set by the filter is broken in the meantime.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @warning
+ * Lifetime of an observer doesn't necessarily have to overcome that of the
+ * registry to which it is connected. However, the observer must be disconnected
+ * from the registry before being destroyed to avoid crashes due to dangling
+ * pointers.
+ *
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class basic_observer: private basic_storage<std::uint32_t, typename Registry::entity_type> {
+    using base_type = basic_storage<std::uint32_t, typename Registry::entity_type>;
+
+    template<typename>
+    struct matcher_handler;
+
+    template<typename... Reject, typename... Require, typename AnyOf>
+    struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, AnyOf>> {
+        template<std::size_t Index>
+        static void maybe_valid_if(basic_observer &obs, Registry &reg, const typename Registry::entity_type entt) {
+            if(reg.template all_of<Require...>(entt) && !reg.template any_of<Reject...>(entt)) {
+                if(!obs.contains(entt)) {
+                    obs.emplace(entt);
+                }
+
+                obs.get(entt) |= (1 << Index);
+            }
+        }
+
+        template<std::size_t Index>
+        static void discard_if(basic_observer &obs, Registry &, const typename Registry::entity_type entt) {
+            if(obs.contains(entt) && !(obs.get(entt) &= (~(1 << Index)))) {
+                obs.erase(entt);
+            }
+        }
+
+        template<std::size_t Index>
+        static void connect(basic_observer &obs, Registry &reg) {
+            (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
+            reg.template on_update<AnyOf>().template connect<&maybe_valid_if<Index>>(obs);
+            reg.template on_destroy<AnyOf>().template connect<&discard_if<Index>>(obs);
+        }
+
+        static void disconnect(basic_observer &obs, Registry &reg) {
+            (reg.template on_destroy<Require>().disconnect(obs), ...);
+            (reg.template on_construct<Reject>().disconnect(obs), ...);
+            reg.template on_update<AnyOf>().disconnect(obs);
+            reg.template on_destroy<AnyOf>().disconnect(obs);
+        }
+    };
+
+    template<typename... Reject, typename... Require, typename... NoneOf, typename... AllOf>
+    struct matcher_handler<matcher<type_list<Reject...>, type_list<Require...>, type_list<NoneOf...>, AllOf...>> {
+        template<std::size_t Index, typename... Ignore>
+        static void maybe_valid_if(basic_observer &obs, Registry &reg, const typename Registry::entity_type entt) {
+            auto condition = [&reg, entt]() {
+                if constexpr(sizeof...(Ignore) == 0) {
+                    return reg.template all_of<AllOf..., Require...>(entt) && !reg.template any_of<NoneOf..., Reject...>(entt);
+                } else {
+                    return reg.template all_of<AllOf..., Require...>(entt) && ((std::is_same_v<Ignore..., NoneOf> || !reg.template any_of<NoneOf>(entt)) && ...) && !reg.template any_of<Reject...>(entt);
+                }
+            };
+
+            if(condition()) {
+                if(!obs.contains(entt)) {
+                    obs.emplace(entt);
+                }
+
+                obs.get(entt) |= (1 << Index);
+            }
+        }
+
+        template<std::size_t Index>
+        static void discard_if(basic_observer &obs, Registry &, const typename Registry::entity_type entt) {
+            if(obs.contains(entt) && !(obs.get(entt) &= (~(1 << Index)))) {
+                obs.erase(entt);
+            }
+        }
+
+        template<std::size_t Index>
+        static void connect(basic_observer &obs, Registry &reg) {
+            (reg.template on_destroy<Require>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<Reject>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<AllOf>().template connect<&maybe_valid_if<Index>>(obs), ...);
+            (reg.template on_destroy<NoneOf>().template connect<&maybe_valid_if<Index, NoneOf>>(obs), ...);
+            (reg.template on_destroy<AllOf>().template connect<&discard_if<Index>>(obs), ...);
+            (reg.template on_construct<NoneOf>().template connect<&discard_if<Index>>(obs), ...);
+        }
+
+        static void disconnect(basic_observer &obs, Registry &reg) {
+            (reg.template on_destroy<Require>().disconnect(obs), ...);
+            (reg.template on_construct<Reject>().disconnect(obs), ...);
+            (reg.template on_construct<AllOf>().disconnect(obs), ...);
+            (reg.template on_destroy<NoneOf>().disconnect(obs), ...);
+            (reg.template on_destroy<AllOf>().disconnect(obs), ...);
+            (reg.template on_construct<NoneOf>().disconnect(obs), ...);
+        }
+    };
+
+    template<typename... Matcher>
+    static void disconnect(Registry &reg, basic_observer &obs) {
+        (matcher_handler<Matcher>::disconnect(obs, reg), ...);
+    }
+
+    template<typename... Matcher, std::size_t... Index>
+    void connect(Registry &reg, std::index_sequence<Index...>) {
+        static_assert(sizeof...(Matcher) < std::numeric_limits<typename base_type::value_type>::digits, "Too many matchers");
+        (matcher_handler<Matcher>::template connect<Index>(*this, reg), ...);
+        release.template connect<&basic_observer::disconnect<Matcher...>>(reg);
+    }
+
+public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename registry_type::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Random access iterator type. */
+    using iterator = typename registry_type::base_type::iterator;
+
+    /*! @brief Default constructor. */
+    basic_observer()
+        : release{} {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_observer(const basic_observer &) = delete;
+    /*! @brief Default move constructor, deleted on purpose. */
+    basic_observer(basic_observer &&) = delete;
+
+    /**
+     * @brief Creates an observer and connects it to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    basic_observer(registry_type &reg, basic_collector<Matcher...>)
+        : basic_observer{} {
+        connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
+    }
+
+    /*! @brief Default destructor. */
+    ~basic_observer() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This observer.
+     */
+    basic_observer &operator=(const basic_observer &) = delete;
+
+    /**
+     * @brief Default move assignment operator, deleted on purpose.
+     * @return This observer.
+     */
+    basic_observer &operator=(basic_observer &&) = delete;
+
+    /**
+     * @brief Connects an observer to a given registry.
+     * @tparam Matcher Types of matchers to use to initialize the observer.
+     * @param reg A valid reference to a registry.
+     */
+    template<typename... Matcher>
+    void connect(registry_type &reg, basic_collector<Matcher...>) {
+        disconnect();
+        connect<Matcher...>(reg, std::index_sequence_for<Matcher...>{});
+        base_type::clear();
+    }
+
+    /*! @brief Disconnects an observer from the registry it keeps track of. */
+    void disconnect() {
+        if(release) {
+            release(*this);
+            release.reset();
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements in an observer.
+     * @return Number of elements.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return base_type::size();
+    }
+
+    /**
+     * @brief Checks whether an observer is empty.
+     * @return True if the observer is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return base_type::empty();
+    }
+
+    /**
+     * @brief Direct access to the list of entities of the observer.
+     *
+     * The returned pointer is such that range `[data(), data() + size())` is
+     * always a valid range, even if the container is empty.
+     *
+     * @note
+     * Entities are in the reverse order as returned by the `begin`/`end`
+     * iterators.
+     *
+     * @return A pointer to the array of entities.
+     */
+    [[nodiscard]] const entity_type *data() const noexcept {
+        return base_type::data();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the observer.
+     *
+     * The returned iterator points to the first entity of the observer. If the
+     * container is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the observer.
+     */
+    [[nodiscard]] iterator begin() const noexcept {
+        return base_type::base_type::begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the observer.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the observer. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * observer.
+     */
+    [[nodiscard]] iterator end() const noexcept {
+        return base_type::base_type::end();
+    }
+
+    /*! @brief Clears the underlying container. */
+    void clear() noexcept {
+        base_type::clear();
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them.
+     *
+     * The function object is invoked for each entity.<br/>
+     * The signature of the function must be equivalent to the following form:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(const auto entity: *this) {
+            func(entity);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them,
+     * then clears the observer.
+     *
+     * @sa each
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) {
+        std::as_const(*this).each(std::move(func));
+        clear();
+    }
+
+private:
+    delegate<void(basic_observer &)> release;
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/organizer.hpp

@@ -0,0 +1,412 @@
+#ifndef ENTT_ENTITY_ORGANIZER_HPP
+#define ENTT_ENTITY_ORGANIZER_HPP
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../core/type_info.hpp"
+#include "../core/type_traits.hpp"
+#include "../core/utility.hpp"
+#include "../graph/adjacency_matrix.hpp"
+#include "../graph/flow.hpp"
+#include "fwd.hpp"
+#include "helper.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename>
+struct is_view: std::false_type {};
+
+template<typename... Args>
+struct is_view<basic_view<Args...>>: std::true_type {};
+
+template<typename Type>
+inline constexpr bool is_view_v = is_view<Type>::value;
+
+template<typename Type, typename Override>
+struct unpack_type {
+    using ro = std::conditional_t<
+        type_list_contains_v<Override, const Type> || (std::is_const_v<Type> && !type_list_contains_v<Override, std::remove_const_t<Type>>),
+        type_list<std::remove_const_t<Type>>,
+        type_list<>>;
+
+    using rw = std::conditional_t<
+        type_list_contains_v<Override, std::remove_const_t<Type>> || (!std::is_const_v<Type> && !type_list_contains_v<Override, const Type>),
+        type_list<Type>,
+        type_list<>>;
+};
+
+template<typename... Args, typename... Override>
+struct unpack_type<basic_registry<Args...>, type_list<Override...>> {
+    using ro = type_list<>;
+    using rw = type_list<>;
+};
+
+template<typename... Args, typename... Override>
+struct unpack_type<const basic_registry<Args...>, type_list<Override...>>
+    : unpack_type<basic_registry<Args...>, type_list<Override...>> {};
+
+template<typename... Get, typename... Exclude, typename... Override>
+struct unpack_type<basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {
+    using ro = type_list_cat_t<type_list<typename Exclude::value_type...>, typename unpack_type<constness_as_t<typename Get::value_type, Get>, type_list<Override...>>::ro...>;
+    using rw = type_list_cat_t<typename unpack_type<constness_as_t<typename Get::value_type, Get>, type_list<Override...>>::rw...>;
+};
+
+template<typename... Get, typename... Exclude, typename... Override>
+struct unpack_type<const basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>>
+    : unpack_type<basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {};
+
+template<typename, typename>
+struct resource_traits;
+
+template<typename... Args, typename... Req>
+struct resource_traits<type_list<Args...>, type_list<Req...>> {
+    using args = type_list<std::remove_const_t<Args>...>;
+    using ro = type_list_cat_t<typename unpack_type<Args, type_list<Req...>>::ro..., typename unpack_type<Req, type_list<>>::ro...>;
+    using rw = type_list_cat_t<typename unpack_type<Args, type_list<Req...>>::rw..., typename unpack_type<Req, type_list<>>::rw...>;
+};
+
+template<typename... Req, typename Ret, typename... Args>
+resource_traits<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> free_function_to_resource_traits(Ret (*)(Args...));
+
+template<typename... Req, typename Ret, typename Type, typename... Args>
+resource_traits<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (*)(Type &, Args...));
+
+template<typename... Req, typename Ret, typename Class, typename... Args>
+resource_traits<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (Class::*)(Args...));
+
+template<typename... Req, typename Ret, typename Class, typename... Args>
+resource_traits<type_list<std::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (Class::*)(Args...) const);
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Utility class for creating a static task graph.
+ *
+ * This class offers minimal support (but sufficient in many cases) for creating
+ * an execution graph from functions and their requirements on resources.<br/>
+ * Note that the resulting tasks aren't executed in any case. This isn't the
+ * goal of the tool. Instead, they are returned to the user in the form of a
+ * graph that allows for safe execution.
+ *
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class basic_organizer final {
+    using callback_type = void(const void *, Registry &);
+    using prepare_type = void(Registry &);
+    using dependency_type = std::size_t(const bool, const type_info **, const std::size_t);
+
+    struct vertex_data final {
+        std::size_t ro_count{};
+        std::size_t rw_count{};
+        const char *name{};
+        const void *payload{};
+        callback_type *callback{};
+        dependency_type *dependency;
+        prepare_type *prepare{};
+        const type_info *info{};
+    };
+
+    template<typename Type>
+    [[nodiscard]] static decltype(auto) extract(Registry &reg) {
+        if constexpr(std::is_same_v<Type, Registry>) {
+            return reg;
+        } else if constexpr(internal::is_view_v<Type>) {
+            return as_view{reg};
+        } else {
+            return reg.ctx().template emplace<std::remove_reference_t<Type>>();
+        }
+    }
+
+    template<typename... Args>
+    [[nodiscard]] static auto to_args(Registry &reg, type_list<Args...>) {
+        return std::tuple<decltype(extract<Args>(reg))...>(extract<Args>(reg)...);
+    }
+
+    template<typename... Type>
+    static std::size_t fill_dependencies(type_list<Type...>, [[maybe_unused]] const type_info **buffer, [[maybe_unused]] const std::size_t count) {
+        if constexpr(sizeof...(Type) == 0u) {
+            return {};
+        } else {
+            const type_info *info[sizeof...(Type)]{&type_id<Type>()...};
+            const auto length = count < sizeof...(Type) ? count : sizeof...(Type);
+
+            for(std::size_t pos{}; pos < length; ++pos) {
+                buffer[pos] = info[pos];
+            }
+
+            return length;
+        }
+    }
+
+    template<typename... RO, typename... RW>
+    void track_dependencies(std::size_t index, const bool requires_registry, type_list<RO...>, type_list<RW...>) {
+        builder.bind(static_cast<id_type>(index));
+        builder.set(type_hash<Registry>::value(), requires_registry || (sizeof...(RO) + sizeof...(RW) == 0u));
+        (builder.ro(type_hash<RO>::value()), ...);
+        (builder.rw(type_hash<RW>::value()), ...);
+    }
+
+public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename registry_type::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Raw task function type. */
+    using function_type = callback_type;
+
+    /*! @brief Vertex type of a task graph defined as an adjacency list. */
+    struct vertex {
+        /**
+         * @brief Constructs a vertex of the task graph.
+         * @param vtype True if the vertex is a top-level one, false otherwise.
+         * @param data The data associated with the vertex.
+         * @param edges The indices of the children in the adjacency list.
+         */
+        vertex(const bool vtype, vertex_data data, std::vector<std::size_t> edges)
+            : is_top_level{vtype},
+              node{std::move(data)},
+              reachable{std::move(edges)} {}
+
+        /**
+         * @brief Fills a buffer with the type info objects for the writable
+         * resources of a vertex.
+         * @param buffer A buffer pre-allocated by the user.
+         * @param length The length of the user-supplied buffer.
+         * @return The number of type info objects written to the buffer.
+         */
+        size_type ro_dependency(const type_info **buffer, const std::size_t length) const noexcept {
+            return node.dependency(false, buffer, length);
+        }
+
+        /**
+         * @brief Fills a buffer with the type info objects for the read-only
+         * resources of a vertex.
+         * @param buffer A buffer pre-allocated by the user.
+         * @param length The length of the user-supplied buffer.
+         * @return The number of type info objects written to the buffer.
+         */
+        size_type rw_dependency(const type_info **buffer, const std::size_t length) const noexcept {
+            return node.dependency(true, buffer, length);
+        }
+
+        /**
+         * @brief Returns the number of read-only resources of a vertex.
+         * @return The number of read-only resources of the vertex.
+         */
+        size_type ro_count() const noexcept {
+            return node.ro_count;
+        }
+
+        /**
+         * @brief Returns the number of writable resources of a vertex.
+         * @return The number of writable resources of the vertex.
+         */
+        size_type rw_count() const noexcept {
+            return node.rw_count;
+        }
+
+        /**
+         * @brief Checks if a vertex is also a top-level one.
+         * @return True if the vertex is a top-level one, false otherwise.
+         */
+        bool top_level() const noexcept {
+            return is_top_level;
+        }
+
+        /**
+         * @brief Returns a type info object associated with a vertex.
+         * @return A properly initialized type info object.
+         */
+        const type_info &info() const noexcept {
+            return *node.info;
+        }
+
+        /**
+         * @brief Returns a user defined name associated with a vertex, if any.
+         * @return The user defined name associated with the vertex, if any.
+         */
+        const char *name() const noexcept {
+            return node.name;
+        }
+
+        /**
+         * @brief Returns the function associated with a vertex.
+         * @return The function associated with the vertex.
+         */
+        function_type *callback() const noexcept {
+            return node.callback;
+        }
+
+        /**
+         * @brief Returns the payload associated with a vertex, if any.
+         * @return The payload associated with the vertex, if any.
+         */
+        const void *data() const noexcept {
+            return node.payload;
+        }
+
+        /**
+         * @brief Returns the list of nodes reachable from a given vertex.
+         * @return The list of nodes reachable from the vertex.
+         */
+        const std::vector<std::size_t> &children() const noexcept {
+            return reachable;
+        }
+
+        /**
+         * @brief Prepares a registry and assures that all required resources
+         * are properly instantiated before using them.
+         * @param reg A valid registry.
+         */
+        void prepare(registry_type &reg) const {
+            node.prepare ? node.prepare(reg) : void();
+        }
+
+    private:
+        bool is_top_level;
+        vertex_data node;
+        std::vector<std::size_t> reachable;
+    };
+
+    /**
+     * @brief Adds a free function to the task list.
+     * @tparam Candidate Function to add to the task list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @param name Optional name to associate with the task.
+     */
+    template<auto Candidate, typename... Req>
+    void emplace(const char *name = nullptr) {
+        using resource_type = decltype(internal::free_function_to_resource_traits<Req...>(Candidate));
+        constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, registry_type>;
+
+        callback_type *callback = +[](const void *, registry_type &reg) {
+            std::apply(Candidate, to_args(reg, typename resource_type::args{}));
+        };
+
+        vertex_data vdata{
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            nullptr,
+            callback,
+            +[](const bool rw, const type_info **buffer, const std::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
+            +[](registry_type &reg) { void(to_args(reg, typename resource_type::args{})); },
+            &type_id<std::integral_constant<decltype(Candidate), Candidate>>()};
+
+        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
+        vertices.push_back(std::move(vdata));
+    }
+
+    /**
+     * @brief Adds a free function with payload or a member function with an
+     * instance to the task list.
+     * @tparam Candidate Function or member to add to the task list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @param name Optional name to associate with the task.
+     */
+    template<auto Candidate, typename... Req, typename Type>
+    void emplace(Type &value_or_instance, const char *name = nullptr) {
+        using resource_type = decltype(internal::constrained_function_to_resource_traits<Req...>(Candidate));
+        constexpr auto requires_registry = type_list_contains_v<typename resource_type::args, registry_type>;
+
+        callback_type *callback = +[](const void *payload, registry_type &reg) {
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            std::apply(Candidate, std::tuple_cat(std::forward_as_tuple(*curr), to_args(reg, typename resource_type::args{})));
+        };
+
+        vertex_data vdata{
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            &value_or_instance,
+            callback,
+            +[](const bool rw, const type_info **buffer, const std::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
+            +[](registry_type &reg) { void(to_args(reg, typename resource_type::args{})); },
+            &type_id<std::integral_constant<decltype(Candidate), Candidate>>()};
+
+        track_dependencies(vertices.size(), requires_registry, typename resource_type::ro{}, typename resource_type::rw{});
+        vertices.push_back(std::move(vdata));
+    }
+
+    /**
+     * @brief Adds an user defined function with optional payload to the task
+     * list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @param func Function to add to the task list.
+     * @param payload User defined arbitrary data.
+     * @param name Optional name to associate with the task.
+     */
+    template<typename... Req>
+    void emplace(function_type *func, const void *payload = nullptr, const char *name = nullptr) {
+        using resource_type = internal::resource_traits<type_list<>, type_list<Req...>>;
+        track_dependencies(vertices.size(), true, typename resource_type::ro{}, typename resource_type::rw{});
+
+        vertex_data vdata{
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            payload,
+            func,
+            +[](const bool rw, const type_info **buffer, const std::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
+            nullptr,
+            &type_id<void>()};
+
+        vertices.push_back(std::move(vdata));
+    }
+
+    /**
+     * @brief Generates a task graph for the current content.
+     * @return The adjacency list of the task graph.
+     */
+    std::vector<vertex> graph() {
+        std::vector<vertex> adjacency_list{};
+        adjacency_list.reserve(vertices.size());
+        auto adjacency_matrix = builder.graph();
+
+        for(auto curr: adjacency_matrix.vertices()) {
+            const auto iterable = adjacency_matrix.in_edges(curr);
+            std::vector<std::size_t> reachable{};
+
+            for(auto &&edge: adjacency_matrix.out_edges(curr)) {
+                reachable.push_back(edge.second);
+            }
+
+            adjacency_list.emplace_back(iterable.cbegin() == iterable.cend(), vertices[curr], std::move(reachable));
+        }
+
+        return adjacency_list;
+    }
+
+    /*! @brief Erases all elements from a container. */
+    void clear() {
+        builder.clear();
+        vertices.clear();
+    }
+
+private:
+    std::vector<vertex_data> vertices;
+    flow builder;
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/runtime_view.hpp

@@ -0,0 +1,335 @@
+#ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
+#define ENTT_ENTITY_RUNTIME_VIEW_HPP
+
+#include <algorithm>
+#include <cstddef>
+#include <iterator>
+#include <utility>
+#include <vector>
+#include "entity.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Set>
+class runtime_view_iterator final {
+    using iterator_type = typename Set::iterator;
+
+    [[nodiscard]] bool valid() const {
+        return (!tombstone_check || *it != tombstone)
+               && std::all_of(++pools->begin(), pools->end(), [entt = *it](const auto *curr) { return curr->contains(entt); })
+               && std::none_of(filter->cbegin(), filter->cend(), [entt = *it](const auto *curr) { return curr && curr->contains(entt); });
+    }
+
+public:
+    using difference_type = typename iterator_type::difference_type;
+    using value_type = typename iterator_type::value_type;
+    using pointer = typename iterator_type::pointer;
+    using reference = typename iterator_type::reference;
+    using iterator_category = std::bidirectional_iterator_tag;
+
+    constexpr runtime_view_iterator() noexcept
+        : pools{},
+          filter{},
+          it{},
+          tombstone_check{} {}
+
+    runtime_view_iterator(const std::vector<Set *> &cpools, const std::vector<Set *> &ignore, iterator_type curr) noexcept
+        : pools{&cpools},
+          filter{&ignore},
+          it{curr},
+          tombstone_check{pools->size() == 1u && (*pools)[0u]->policy() == deletion_policy::in_place} {
+        if(it != (*pools)[0]->end() && !valid()) {
+            ++(*this);
+        }
+    }
+
+    runtime_view_iterator &operator++() {
+        while(++it != (*pools)[0]->end() && !valid()) {}
+        return *this;
+    }
+
+    runtime_view_iterator operator++(int) {
+        runtime_view_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    runtime_view_iterator &operator--() {
+        while(--it != (*pools)[0]->begin() && !valid()) {}
+        return *this;
+    }
+
+    runtime_view_iterator operator--(int) {
+        runtime_view_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    [[nodiscard]] pointer operator->() const noexcept {
+        return it.operator->();
+    }
+
+    [[nodiscard]] reference operator*() const noexcept {
+        return *operator->();
+    }
+
+    [[nodiscard]] constexpr bool operator==(const runtime_view_iterator &other) const noexcept {
+        return it == other.it;
+    }
+
+    [[nodiscard]] constexpr bool operator!=(const runtime_view_iterator &other) const noexcept {
+        return !(*this == other);
+    }
+
+private:
+    const std::vector<Set *> *pools;
+    const std::vector<Set *> *filter;
+    iterator_type it;
+    bool tombstone_check;
+};
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Generic runtime view.
+ *
+ * Runtime views iterate over those entities that have at least all the given
+ * components in their bags. During initialization, a runtime view looks at the
+ * number of entities available for each component and picks up a reference to
+ * the smallest set of candidate entities in order to get a performance boost
+ * when iterate.<br/>
+ * Order of elements during iterations are highly dependent on the order of the
+ * underlying data structures. See sparse_set and its specializations for more
+ * details.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New instances of the given components are created and assigned to entities.
+ * * The entity currently pointed is modified (as an example, if one of the
+ *   given components is removed from the entity to which the iterator points).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all the other cases, modifying the pools of the given components in any
+ * way invalidates all the iterators and using them results in undefined
+ * behavior.
+ *
+ * @note
+ * Views share references to the underlying data structures of the registry that
+ * generated them. Therefore any change to the entities and to the components
+ * made by means of the registry are immediately reflected by the views, unless
+ * a pool was missing when the view was built (in this case, the view won't
+ * have a valid reference and won't be updated accordingly).
+ *
+ * @warning
+ * Lifetime of a view must not overcome that of the registry that generated it.
+ * In any other case, attempting to use a view results in undefined behavior.
+ *
+ * @tparam Type Common base type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Allocator>
+class basic_runtime_view {
+    using alloc_traits = std::allocator_traits<Allocator>;
+    static_assert(std::is_same_v<typename alloc_traits::value_type, Type *>, "Invalid value type");
+    using container_type = std::vector<Type *, Allocator>;
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename Type::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Common type among all storage types. */
+    using base_type = Type;
+    /*! @brief Bidirectional iterator type. */
+    using iterator = internal::runtime_view_iterator<base_type>;
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_runtime_view() noexcept
+        : basic_runtime_view{allocator_type{}} {}
+
+    /**
+     * @brief Constructs an empty, invalid view with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_runtime_view(const allocator_type &allocator)
+        : pools{allocator},
+          filter{allocator} {}
+
+    /*! @brief Default copy constructor. */
+    basic_runtime_view(const basic_runtime_view &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    basic_runtime_view(const basic_runtime_view &other, const allocator_type &allocator)
+        : pools{other.pools, allocator},
+          filter{other.filter, allocator} {}
+
+    /*! @brief Default move constructor. */
+    basic_runtime_view(basic_runtime_view &&) noexcept(std::is_nothrow_move_constructible_v<container_type>) = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_runtime_view(basic_runtime_view &&other, const allocator_type &allocator)
+        : pools{std::move(other.pools), allocator},
+          filter{std::move(other.filter), allocator} {}
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This container.
+     */
+    basic_runtime_view &operator=(const basic_runtime_view &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This container.
+     */
+    basic_runtime_view &operator=(basic_runtime_view &&) noexcept(std::is_nothrow_move_assignable_v<container_type>) = default;
+
+    /**
+     * @brief Exchanges the contents with those of a given view.
+     * @param other View to exchange the content with.
+     */
+    void swap(basic_runtime_view &other) {
+        using std::swap;
+        swap(pools, other.pools);
+        swap(filter, other.filter);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return pools.get_allocator();
+    }
+
+    /*! @brief Clears the view. */
+    void clear() {
+        pools.clear();
+        filter.clear();
+    }
+
+    /**
+     * @brief Appends an opaque storage object to a runtime view.
+     * @param base An opaque reference to a storage object.
+     * @return This runtime view.
+     */
+    basic_runtime_view &iterate(base_type &base) {
+        if(pools.empty() || !(base.size() < pools[0u]->size())) {
+            pools.push_back(&base);
+        } else {
+            pools.push_back(std::exchange(pools[0u], &base));
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Adds an opaque storage object as a filter of a runtime view.
+     * @param base An opaque reference to a storage object.
+     * @return This runtime view.
+     */
+    basic_runtime_view &exclude(base_type &base) {
+        filter.push_back(&base);
+        return *this;
+    }
+
+    /**
+     * @brief Estimates the number of entities iterated by the view.
+     * @return Estimated number of entities iterated by the view.
+     */
+    [[nodiscard]] size_type size_hint() const {
+        return pools.empty() ? size_type{} : pools.front()->size();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity that has the given
+     * components.
+     *
+     * The returned iterator points to the first entity that has the given
+     * components. If the view is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @return An iterator to the first entity that has the given components.
+     */
+    [[nodiscard]] iterator begin() const {
+        return pools.empty() ? iterator{} : iterator{pools, filter, pools[0]->begin()};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given components.
+     *
+     * The returned iterator points to the entity following the last entity that
+     * has the given components. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity that has the
+     * given components.
+     */
+    [[nodiscard]] iterator end() const {
+        return pools.empty() ? iterator{} : iterator{pools, filter, pools[0]->end()};
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entt A valid identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return !pools.empty()
+               && std::all_of(pools.cbegin(), pools.cend(), [entt](const auto *curr) { return curr->contains(entt); })
+               && std::none_of(filter.cbegin(), filter.cend(), [entt](const auto *curr) { return curr && curr->contains(entt); });
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them.
+     *
+     * The function object is invoked for each entity. It is provided only with
+     * the entity itself. To get the components, users can use the registry with
+     * which the view was built.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(const auto entity: *this) {
+            func(entity);
+        }
+    }
+
+private:
+    container_type pools;
+    container_type filter;
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/snapshot.hpp

@@ -0,0 +1,567 @@
+#ifndef ENTT_ENTITY_SNAPSHOT_HPP
+#define ENTT_ENTITY_SNAPSHOT_HPP
+
+#include <array>
+#include <cstddef>
+#include <iterator>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../container/dense_map.hpp"
+#include "../core/type_traits.hpp"
+#include "component.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+#include "view.hpp"
+
+namespace entt {
+
+/**
+ * @brief Utility class to create snapshots from a registry.
+ *
+ * A _snapshot_ can be either a dump of the entire registry or a narrower
+ * selection of components of interest.<br/>
+ * This type can be used in both cases if provided with a correctly configured
+ * output archive.
+ *
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class basic_snapshot {
+    using entity_traits = entt_traits<typename Registry::entity_type>;
+
+    template<typename Component, typename Archive, typename It>
+    void get(Archive &archive, std::size_t sz, It first, It last) const {
+        const auto view = reg->template view<const Component>();
+        archive(typename entity_traits::entity_type(sz));
+
+        while(first != last) {
+            const auto entt = *(first++);
+
+            if(reg->template all_of<Component>(entt)) {
+                std::apply(archive, std::tuple_cat(std::make_tuple(entt), view.get(entt)));
+            }
+        }
+    }
+
+    template<typename... Component, typename Archive, typename It, std::size_t... Index>
+    void component(Archive &archive, It first, It last, std::index_sequence<Index...>) const {
+        std::array<std::size_t, sizeof...(Index)> size{};
+        auto begin = first;
+
+        while(begin != last) {
+            const auto entt = *(begin++);
+            ((reg->template all_of<Component>(entt) ? ++size[Index] : 0u), ...);
+        }
+
+        (get<Component>(archive, size[Index], first, last), ...);
+    }
+
+public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename registry_type::entity_type;
+
+    /**
+     * @brief Constructs an instance that is bound to a given registry.
+     * @param source A valid reference to a registry.
+     */
+    basic_snapshot(const registry_type &source) noexcept
+        : reg{&source} {}
+
+    /*! @brief Default move constructor. */
+    basic_snapshot(basic_snapshot &&) noexcept = default;
+
+    /*! @brief Default move assignment operator. @return This snapshot. */
+    basic_snapshot &operator=(basic_snapshot &&) noexcept = default;
+
+    /**
+     * @brief Puts aside all the entities from the underlying registry.
+     *
+     * Entities are serialized along with their versions. Destroyed entities are
+     * taken in consideration as well by this function.
+     *
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename Archive>
+    const basic_snapshot &entities(Archive &archive) const {
+        const auto sz = reg->size();
+
+        archive(typename entity_traits::entity_type(sz + 1u));
+        archive(reg->released());
+
+        for(auto first = reg->data(), last = first + sz; first != last; ++first) {
+            archive(*first);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Puts aside the given components.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Component Types of components to serialize.
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename... Component, typename Archive>
+    const basic_snapshot &component(Archive &archive) const {
+        if constexpr(sizeof...(Component) == 1u) {
+            const auto view = reg->template view<const Component...>();
+            (component<Component>(archive, view.rbegin(), view.rend()), ...);
+            return *this;
+        } else {
+            (component<Component>(archive), ...);
+            return *this;
+        }
+    }
+
+    /**
+     * @brief Puts aside the given components for the entities in a range.
+     *
+     * Each instance is serialized together with the entity to which it belongs.
+     * Entities are serialized along with their versions.
+     *
+     * @tparam Component Types of components to serialize.
+     * @tparam Archive Type of output archive.
+     * @tparam It Type of input iterator.
+     * @param archive A valid reference to an output archive.
+     * @param first An iterator to the first element of the range to serialize.
+     * @param last An iterator past the last element of the range to serialize.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename... Component, typename Archive, typename It>
+    const basic_snapshot &component(Archive &archive, It first, It last) const {
+        component<Component...>(archive, first, last, std::index_sequence_for<Component...>{});
+        return *this;
+    }
+
+private:
+    const registry_type *reg;
+};
+
+/**
+ * @brief Utility class to restore a snapshot as a whole.
+ *
+ * A snapshot loader requires that the destination registry be empty and loads
+ * all the data at once while keeping intact the identifiers that the entities
+ * originally had.<br/>
+ * An example of use is the implementation of a save/restore utility.
+ *
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class basic_snapshot_loader {
+    using entity_traits = entt_traits<typename Registry::entity_type>;
+
+    template<typename Component, typename Archive>
+    void assign(Archive &archive) const {
+        typename entity_traits::entity_type length{};
+        entity_type entt;
+
+        archive(length);
+
+        if constexpr(ignore_as_empty_v<Component>) {
+            while(length--) {
+                archive(entt);
+                const auto entity = reg->valid(entt) ? entt : reg->create(entt);
+                ENTT_ASSERT(entity == entt, "Entity not available for use");
+                reg->template emplace<Component>(entt);
+            }
+        } else {
+            Component instance;
+
+            while(length--) {
+                archive(entt, instance);
+                const auto entity = reg->valid(entt) ? entt : reg->create(entt);
+                ENTT_ASSERT(entity == entt, "Entity not available for use");
+                reg->template emplace<Component>(entt, std::move(instance));
+            }
+        }
+    }
+
+public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename registry_type::entity_type;
+
+    /**
+     * @brief Constructs an instance that is bound to a given registry.
+     * @param source A valid reference to a registry.
+     */
+    basic_snapshot_loader(registry_type &source) noexcept
+        : reg{&source} {
+        // restoring a snapshot as a whole requires a clean registry
+        ENTT_ASSERT(reg->empty(), "Registry must be empty");
+    }
+
+    /*! @brief Default move constructor. */
+    basic_snapshot_loader(basic_snapshot_loader &&) noexcept = default;
+
+    /*! @brief Default move assignment operator. @return This loader. */
+    basic_snapshot_loader &operator=(basic_snapshot_loader &&) noexcept = default;
+
+    /**
+     * @brief Restores entities that were in use during serialization.
+     *
+     * This function restores the entities that were in use during serialization
+     * and gives them the versions they originally had.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename Archive>
+    const basic_snapshot_loader &entities(Archive &archive) const {
+        typename entity_traits::entity_type length{};
+
+        archive(length);
+        std::vector<entity_type> all(length);
+
+        for(std::size_t pos{}; pos < length; ++pos) {
+            archive(all[pos]);
+        }
+
+        reg->assign(++all.cbegin(), all.cend(), all[0u]);
+
+        return *this;
+    }
+
+    /**
+     * @brief Restores components and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the component is
+     * assigned doesn't exist yet, the loader will take care to create it with
+     * the version it originally had.
+     *
+     * @tparam Component Types of components to restore.
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename... Component, typename Archive>
+    const basic_snapshot_loader &component(Archive &archive) const {
+        (assign<Component>(archive), ...);
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have no components.
+     *
+     * In case all the entities were serialized but only part of the components
+     * was saved, it could happen that some of the entities have no components
+     * once restored.<br/>
+     * This functions helps to identify and destroy those entities.
+     *
+     * @return A valid loader to continue restoring data.
+     */
+    const basic_snapshot_loader &orphans() const {
+        reg->each([this](const auto entt) {
+            if(reg->orphan(entt)) {
+                reg->release(entt);
+            }
+        });
+
+        return *this;
+    }
+
+private:
+    registry_type *reg;
+};
+
+/**
+ * @brief Utility class for _continuous loading_.
+ *
+ * A _continuous loader_ is designed to load data from a source registry to a
+ * (possibly) non-empty destination. The loader can accommodate in a registry
+ * more than one snapshot in a sort of _continuous loading_ that updates the
+ * destination one step at a time.<br/>
+ * Identifiers that entities originally had are not transferred to the target.
+ * Instead, the loader maps remote identifiers to local ones while restoring a
+ * snapshot.<br/>
+ * An example of use is the implementation of a client-server applications with
+ * the requirement of transferring somehow parts of the representation side to
+ * side.
+ *
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class basic_continuous_loader {
+    using entity_traits = entt_traits<typename Registry::entity_type>;
+
+    void destroy(typename Registry::entity_type entt) {
+        if(const auto it = remloc.find(entt); it == remloc.cend()) {
+            const auto local = reg->create();
+            remloc.emplace(entt, std::make_pair(local, true));
+            reg->destroy(local);
+        }
+    }
+
+    void restore(typename Registry::entity_type entt) {
+        const auto it = remloc.find(entt);
+
+        if(it == remloc.cend()) {
+            const auto local = reg->create();
+            remloc.emplace(entt, std::make_pair(local, true));
+        } else {
+            if(!reg->valid(remloc[entt].first)) {
+                remloc[entt].first = reg->create();
+            }
+
+            // set the dirty flag
+            remloc[entt].second = true;
+        }
+    }
+
+    template<typename Container>
+    auto update(int, Container &container) -> decltype(typename Container::mapped_type{}, void()) {
+        // map like container
+        Container other;
+
+        for(auto &&pair: container) {
+            using first_type = std::remove_const_t<typename std::decay_t<decltype(pair)>::first_type>;
+            using second_type = typename std::decay_t<decltype(pair)>::second_type;
+
+            if constexpr(std::is_same_v<first_type, entity_type> && std::is_same_v<second_type, entity_type>) {
+                other.emplace(map(pair.first), map(pair.second));
+            } else if constexpr(std::is_same_v<first_type, entity_type>) {
+                other.emplace(map(pair.first), std::move(pair.second));
+            } else {
+                static_assert(std::is_same_v<second_type, entity_type>, "Neither the key nor the value are of entity type");
+                other.emplace(std::move(pair.first), map(pair.second));
+            }
+        }
+
+        using std::swap;
+        swap(container, other);
+    }
+
+    template<typename Container>
+    auto update(char, Container &container) -> decltype(typename Container::value_type{}, void()) {
+        // vector like container
+        static_assert(std::is_same_v<typename Container::value_type, entity_type>, "Invalid value type");
+
+        for(auto &&entt: container) {
+            entt = map(entt);
+        }
+    }
+
+    template<typename Component, typename Other, typename Member>
+    void update([[maybe_unused]] Component &instance, [[maybe_unused]] Member Other::*member) {
+        if constexpr(!std::is_same_v<Component, Other>) {
+            return;
+        } else if constexpr(std::is_same_v<Member, entity_type>) {
+            instance.*member = map(instance.*member);
+        } else {
+            // maybe a container? let's try...
+            update(0, instance.*member);
+        }
+    }
+
+    template<typename Component>
+    void remove_if_exists() {
+        for(auto &&ref: remloc) {
+            const auto local = ref.second.first;
+
+            if(reg->valid(local)) {
+                reg->template remove<Component>(local);
+            }
+        }
+    }
+
+    template<typename Component, typename Archive, typename... Other, typename... Member>
+    void assign(Archive &archive, [[maybe_unused]] Member Other::*...member) {
+        typename entity_traits::entity_type length{};
+        entity_type entt;
+
+        archive(length);
+
+        if constexpr(ignore_as_empty_v<Component>) {
+            while(length--) {
+                archive(entt);
+                restore(entt);
+                reg->template emplace_or_replace<Component>(map(entt));
+            }
+        } else {
+            Component instance;
+
+            while(length--) {
+                archive(entt, instance);
+                (update(instance, member), ...);
+                restore(entt);
+                reg->template emplace_or_replace<Component>(map(entt), std::move(instance));
+            }
+        }
+    }
+
+public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename registry_type::entity_type;
+
+    /**
+     * @brief Constructs an instance that is bound to a given registry.
+     * @param source A valid reference to a registry.
+     */
+    basic_continuous_loader(registry_type &source) noexcept
+        : reg{&source} {}
+
+    /*! @brief Default move constructor. */
+    basic_continuous_loader(basic_continuous_loader &&) = default;
+
+    /*! @brief Default move assignment operator. @return This loader. */
+    basic_continuous_loader &operator=(basic_continuous_loader &&) = default;
+
+    /**
+     * @brief Restores entities that were in use during serialization.
+     *
+     * This function restores the entities that were in use during serialization
+     * and creates local counterparts for them if required.
+     *
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @return A non-const reference to this loader.
+     */
+    template<typename Archive>
+    basic_continuous_loader &entities(Archive &archive) {
+        typename entity_traits::entity_type length{};
+        entity_type entt{};
+
+        archive(length);
+        // discards the head of the list of destroyed entities
+        archive(entt);
+
+        for(std::size_t pos{}, last = length - 1u; pos < last; ++pos) {
+            archive(entt);
+
+            if(const auto entity = entity_traits::to_entity(entt); entity == pos) {
+                restore(entt);
+            } else {
+                destroy(entt);
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Restores components and assigns them to the right entities.
+     *
+     * The template parameter list must be exactly the same used during
+     * serialization. In the event that the entity to which the component is
+     * assigned doesn't exist yet, the loader will take care to create a local
+     * counterpart for it.<br/>
+     * Members can be either data members of type entity_type or containers of
+     * entities. In both cases, the loader will visit them and update the
+     * entities by replacing each one with its local counterpart.
+     *
+     * @tparam Component Type of component to restore.
+     * @tparam Archive Type of input archive.
+     * @tparam Other Types of components to update with local counterparts.
+     * @tparam Member Types of members to update with their local counterparts.
+     * @param archive A valid reference to an input archive.
+     * @param member Members to update with their local counterparts.
+     * @return A non-const reference to this loader.
+     */
+    template<typename... Component, typename Archive, typename... Other, typename... Member>
+    basic_continuous_loader &component(Archive &archive, Member Other::*...member) {
+        (remove_if_exists<Component>(), ...);
+        (assign<Component>(archive, member...), ...);
+        return *this;
+    }
+
+    /**
+     * @brief Helps to purge entities that no longer have a conterpart.
+     *
+     * Users should invoke this member function after restoring each snapshot,
+     * unless they know exactly what they are doing.
+     *
+     * @return A non-const reference to this loader.
+     */
+    basic_continuous_loader &shrink() {
+        auto it = remloc.begin();
+
+        while(it != remloc.cend()) {
+            const auto local = it->second.first;
+            bool &dirty = it->second.second;
+
+            if(dirty) {
+                dirty = false;
+                ++it;
+            } else {
+                if(reg->valid(local)) {
+                    reg->destroy(local);
+                }
+
+                it = remloc.erase(it);
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have no components.
+     *
+     * In case all the entities were serialized but only part of the components
+     * was saved, it could happen that some of the entities have no components
+     * once restored.<br/>
+     * This functions helps to identify and destroy those entities.
+     *
+     * @return A non-const reference to this loader.
+     */
+    basic_continuous_loader &orphans() {
+        reg->each([this](const auto entt) {
+            if(reg->orphan(entt)) {
+                reg->release(entt);
+            }
+        });
+
+        return *this;
+    }
+
+    /**
+     * @brief Tests if a loader knows about a given entity.
+     * @param entt A valid identifier.
+     * @return True if `entity` is managed by the loader, false otherwise.
+     */
+    [[nodiscard]] bool contains(entity_type entt) const noexcept {
+        return (remloc.find(entt) != remloc.cend());
+    }
+
+    /**
+     * @brief Returns the identifier to which an entity refers.
+     * @param entt A valid identifier.
+     * @return The local identifier if any, the null entity otherwise.
+     */
+    [[nodiscard]] entity_type map(entity_type entt) const noexcept {
+        const auto it = remloc.find(entt);
+        entity_type other = null;
+
+        if(it != remloc.cend()) {
+            other = it->second.first;
+        }
+
+        return other;
+    }
+
+private:
+    dense_map<entity_type, std::pair<entity_type, bool>> remloc;
+    registry_type *reg;
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/sparse_set.hpp

@@ -0,0 +1,976 @@
+#ifndef ENTT_ENTITY_SPARSE_SET_HPP
+#define ENTT_ENTITY_SPARSE_SET_HPP
+
+#include <cstddef>
+#include <iterator>
+#include <memory>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../core/algorithm.hpp"
+#include "../core/any.hpp"
+#include "../core/memory.hpp"
+#include "../core/type_info.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Container>
+struct sparse_set_iterator final {
+    using value_type = typename Container::value_type;
+    using pointer = typename Container::const_pointer;
+    using reference = typename Container::const_reference;
+    using difference_type = typename Container::difference_type;
+    using iterator_category = std::random_access_iterator_tag;
+
+    constexpr sparse_set_iterator() noexcept
+        : packed{},
+          offset{} {}
+
+    constexpr sparse_set_iterator(const Container &ref, const difference_type idx) noexcept
+        : packed{std::addressof(ref)},
+          offset{idx} {}
+
+    constexpr sparse_set_iterator &operator++() noexcept {
+        return --offset, *this;
+    }
+
+    constexpr sparse_set_iterator operator++(int) noexcept {
+        sparse_set_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    constexpr sparse_set_iterator &operator--() noexcept {
+        return ++offset, *this;
+    }
+
+    constexpr sparse_set_iterator operator--(int) noexcept {
+        sparse_set_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    constexpr sparse_set_iterator &operator+=(const difference_type value) noexcept {
+        offset -= value;
+        return *this;
+    }
+
+    constexpr sparse_set_iterator operator+(const difference_type value) const noexcept {
+        sparse_set_iterator copy = *this;
+        return (copy += value);
+    }
+
+    constexpr sparse_set_iterator &operator-=(const difference_type value) noexcept {
+        return (*this += -value);
+    }
+
+    constexpr sparse_set_iterator operator-(const difference_type value) const noexcept {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
+        return packed->data()[index() - value];
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return packed->data() + index();
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return *operator->();
+    }
+
+    [[nodiscard]] constexpr difference_type index() const noexcept {
+        return offset - 1;
+    }
+
+private:
+    const Container *packed;
+    difference_type offset;
+};
+
+template<typename Type, typename Other>
+[[nodiscard]] constexpr std::ptrdiff_t operator-(const sparse_set_iterator<Type> &lhs, const sparse_set_iterator<Other> &rhs) noexcept {
+    return rhs.index() - lhs.index();
+}
+
+template<typename Type, typename Other>
+[[nodiscard]] constexpr bool operator==(const sparse_set_iterator<Type> &lhs, const sparse_set_iterator<Other> &rhs) noexcept {
+    return lhs.index() == rhs.index();
+}
+
+template<typename Type, typename Other>
+[[nodiscard]] constexpr bool operator!=(const sparse_set_iterator<Type> &lhs, const sparse_set_iterator<Other> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+template<typename Type, typename Other>
+[[nodiscard]] constexpr bool operator<(const sparse_set_iterator<Type> &lhs, const sparse_set_iterator<Other> &rhs) noexcept {
+    return lhs.index() > rhs.index();
+}
+
+template<typename Type, typename Other>
+[[nodiscard]] constexpr bool operator>(const sparse_set_iterator<Type> &lhs, const sparse_set_iterator<Other> &rhs) noexcept {
+    return lhs.index() < rhs.index();
+}
+
+template<typename Type, typename Other>
+[[nodiscard]] constexpr bool operator<=(const sparse_set_iterator<Type> &lhs, const sparse_set_iterator<Other> &rhs) noexcept {
+    return !(lhs > rhs);
+}
+
+template<typename Type, typename Other>
+[[nodiscard]] constexpr bool operator>=(const sparse_set_iterator<Type> &lhs, const sparse_set_iterator<Other> &rhs) noexcept {
+    return !(lhs < rhs);
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/*! @brief Sparse set deletion policy. */
+enum class deletion_policy : std::uint8_t {
+    /*! @brief Swap-and-pop deletion policy. */
+    swap_and_pop = 0u,
+    /*! @brief In-place deletion policy. */
+    in_place = 1u
+};
+
+/**
+ * @brief Basic sparse set implementation.
+ *
+ * Sparse set or packed array or whatever is the name users give it.<br/>
+ * Two arrays: an _external_ one and an _internal_ one; a _sparse_ one and a
+ * _packed_ one; one used for direct access through contiguous memory, the other
+ * one used to get the data through an extra level of indirection.<br/>
+ * This is largely used by the registry to offer users the fastest access ever
+ * to the components. Views and groups in general are almost entirely designed
+ * around sparse sets.
+ *
+ * This type of data structure is widely documented in the literature and on the
+ * web. This is nothing more than a customized implementation suitable for the
+ * purpose of the framework.
+ *
+ * @note
+ * Internal data structures arrange elements to maximize performance. There are
+ * no guarantees that entities are returned in the insertion order when iterate
+ * a sparse set. Do not make assumption on the order in any case.
+ *
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Entity, typename Allocator>
+class basic_sparse_set {
+    using alloc_traits = std::allocator_traits<Allocator>;
+    static_assert(std::is_same_v<typename alloc_traits::value_type, Entity>, "Invalid value type");
+    using sparse_container_type = std::vector<typename alloc_traits::pointer, typename alloc_traits::template rebind_alloc<typename alloc_traits::pointer>>;
+    using packed_container_type = std::vector<Entity, Allocator>;
+    using entity_traits = entt_traits<Entity>;
+
+    [[nodiscard]] auto sparse_ptr(const Entity entt) const {
+        const auto pos = static_cast<size_type>(entity_traits::to_entity(entt));
+        const auto page = pos / entity_traits::page_size;
+        return (page < sparse.size() && sparse[page]) ? (sparse[page] + fast_mod(pos, entity_traits::page_size)) : nullptr;
+    }
+
+    [[nodiscard]] auto &sparse_ref(const Entity entt) const {
+        ENTT_ASSERT(sparse_ptr(entt), "Invalid element");
+        const auto pos = static_cast<size_type>(entity_traits::to_entity(entt));
+        return sparse[pos / entity_traits::page_size][fast_mod(pos, entity_traits::page_size)];
+    }
+
+    [[nodiscard]] auto &assure_at_least(const Entity entt) {
+        const auto pos = static_cast<size_type>(entity_traits::to_entity(entt));
+        const auto page = pos / entity_traits::page_size;
+
+        if(!(page < sparse.size())) {
+            sparse.resize(page + 1u, nullptr);
+        }
+
+        if(!sparse[page]) {
+            auto page_allocator{packed.get_allocator()};
+            sparse[page] = alloc_traits::allocate(page_allocator, entity_traits::page_size);
+            std::uninitialized_fill(sparse[page], sparse[page] + entity_traits::page_size, null);
+        }
+
+        auto &elem = sparse[page][fast_mod(pos, entity_traits::page_size)];
+        ENTT_ASSERT(elem == null, "Slot not available");
+        return elem;
+    }
+
+    void release_sparse_pages() {
+        auto page_allocator{packed.get_allocator()};
+
+        for(auto &&page: sparse) {
+            if(page != nullptr) {
+                std::destroy(page, page + entity_traits::page_size);
+                alloc_traits::deallocate(page_allocator, page, entity_traits::page_size);
+                page = nullptr;
+            }
+        }
+    }
+
+private:
+    virtual const void *get_at(const std::size_t) const {
+        return nullptr;
+    }
+
+    virtual void swap_at(const std::size_t, const std::size_t) {}
+    virtual void move_element(const std::size_t, const std::size_t) {}
+
+protected:
+    /*! @brief Random access iterator type. */
+    using basic_iterator = internal::sparse_set_iterator<packed_container_type>;
+
+    /**
+     * @brief Erases an entity from a sparse set.
+     * @param it An iterator to the element to pop.
+     */
+    void swap_and_pop(const basic_iterator it) {
+        ENTT_ASSERT(mode == deletion_policy::swap_and_pop, "Deletion policy mismatched");
+        auto &self = sparse_ref(*it);
+        const auto entt = entity_traits::to_entity(self);
+        sparse_ref(packed.back()) = entity_traits::combine(entt, entity_traits::to_integral(packed.back()));
+        packed[static_cast<size_type>(entt)] = packed.back();
+        // unnecessary but it helps to detect nasty bugs
+        ENTT_ASSERT((packed.back() = null, true), "");
+        // lazy self-assignment guard
+        self = null;
+        packed.pop_back();
+    }
+
+    /**
+     * @brief Erases an entity from a sparse set.
+     * @param it An iterator to the element to pop.
+     */
+    void in_place_pop(const basic_iterator it) {
+        ENTT_ASSERT(mode == deletion_policy::in_place, "Deletion policy mismatched");
+        const auto entt = entity_traits::to_entity(std::exchange(sparse_ref(*it), null));
+        packed[static_cast<size_type>(entt)] = std::exchange(free_list, entity_traits::combine(entt, entity_traits::reserved));
+    }
+
+protected:
+    /**
+     * @brief Erases entities from a sparse set.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    virtual void pop(basic_iterator first, basic_iterator last) {
+        if(mode == deletion_policy::swap_and_pop) {
+            for(; first != last; ++first) {
+                swap_and_pop(first);
+            }
+        } else {
+            for(; first != last; ++first) {
+                in_place_pop(first);
+            }
+        }
+    }
+
+    /**
+     * @brief Assigns an entity to a sparse set.
+     * @param entt A valid identifier.
+     * @param force_back Force back insertion.
+     * @return Iterator pointing to the emplaced element.
+     */
+    virtual basic_iterator try_emplace(const Entity entt, const bool force_back, const void * = nullptr) {
+        ENTT_ASSERT(!contains(entt), "Set already contains entity");
+
+        if(auto &elem = assure_at_least(entt); free_list == null || force_back) {
+            packed.push_back(entt);
+            elem = entity_traits::combine(static_cast<typename entity_traits::entity_type>(packed.size() - 1u), entity_traits::to_integral(entt));
+            return begin();
+        } else {
+            const auto pos = static_cast<size_type>(entity_traits::to_entity(free_list));
+            elem = entity_traits::combine(entity_traits::to_integral(free_list), entity_traits::to_integral(entt));
+            free_list = std::exchange(packed[pos], entt);
+            return --(end() - pos);
+        }
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename entity_traits::value_type;
+    /*! @brief Underlying version type. */
+    using version_type = typename entity_traits::version_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Pointer type to contained entities. */
+    using pointer = typename packed_container_type::const_pointer;
+    /*! @brief Random access iterator type. */
+    using iterator = basic_iterator;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator = iterator;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    /*! @brief Constant reverse iterator type. */
+    using const_reverse_iterator = reverse_iterator;
+
+    /*! @brief Default constructor. */
+    basic_sparse_set()
+        : basic_sparse_set{type_id<void>()} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_sparse_set(const allocator_type &allocator)
+        : basic_sparse_set{type_id<void>(), deletion_policy::swap_and_pop, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with the given policy and allocator.
+     * @param pol Type of deletion policy.
+     * @param allocator The allocator to use (possibly default-constructed).
+     */
+    explicit basic_sparse_set(deletion_policy pol, const allocator_type &allocator = {})
+        : basic_sparse_set{type_id<void>(), pol, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with the given value type, policy
+     * and allocator.
+     * @param value Returned value type, if any.
+     * @param pol Type of deletion policy.
+     * @param allocator The allocator to use (possibly default-constructed).
+     */
+    explicit basic_sparse_set(const type_info &value, deletion_policy pol = deletion_policy::swap_and_pop, const allocator_type &allocator = {})
+        : sparse{allocator},
+          packed{allocator},
+          info{&value},
+          free_list{tombstone},
+          mode{pol} {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_sparse_set(basic_sparse_set &&other) noexcept
+        : sparse{std::move(other.sparse)},
+          packed{std::move(other.packed)},
+          info{other.info},
+          free_list{std::exchange(other.free_list, tombstone)},
+          mode{other.mode} {}
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_sparse_set(basic_sparse_set &&other, const allocator_type &allocator) noexcept
+        : sparse{std::move(other.sparse), allocator},
+          packed{std::move(other.packed), allocator},
+          info{other.info},
+          free_list{std::exchange(other.free_list, tombstone)},
+          mode{other.mode} {
+        ENTT_ASSERT(alloc_traits::is_always_equal::value || packed.get_allocator() == other.packed.get_allocator(), "Copying a sparse set is not allowed");
+    }
+
+    /*! @brief Default destructor. */
+    virtual ~basic_sparse_set() {
+        release_sparse_pages();
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This sparse set.
+     */
+    basic_sparse_set &operator=(basic_sparse_set &&other) noexcept {
+        ENTT_ASSERT(alloc_traits::is_always_equal::value || packed.get_allocator() == other.packed.get_allocator(), "Copying a sparse set is not allowed");
+
+        release_sparse_pages();
+        sparse = std::move(other.sparse);
+        packed = std::move(other.packed);
+        info = other.info;
+        free_list = std::exchange(other.free_list, tombstone);
+        mode = other.mode;
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given sparse set.
+     * @param other Sparse set to exchange the content with.
+     */
+    void swap(basic_sparse_set &other) {
+        using std::swap;
+        swap(sparse, other.sparse);
+        swap(packed, other.packed);
+        swap(info, other.info);
+        swap(free_list, other.free_list);
+        swap(mode, other.mode);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return packed.get_allocator();
+    }
+
+    /**
+     * @brief Returns the deletion policy of a sparse set.
+     * @return The deletion policy of the sparse set.
+     */
+    [[nodiscard]] deletion_policy policy() const noexcept {
+        return mode;
+    }
+
+    /**
+     * @brief Increases the capacity of a sparse set.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    virtual void reserve(const size_type cap) {
+        packed.reserve(cap);
+    }
+
+    /**
+     * @brief Returns the number of elements that a sparse set has currently
+     * allocated space for.
+     * @return Capacity of the sparse set.
+     */
+    [[nodiscard]] virtual size_type capacity() const noexcept {
+        return packed.capacity();
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    virtual void shrink_to_fit() {
+        packed.shrink_to_fit();
+    }
+
+    /**
+     * @brief Returns the extent of a sparse set.
+     *
+     * The extent of a sparse set is also the size of the internal sparse array.
+     * There is no guarantee that the internal packed array has the same size.
+     * Usually the size of the internal sparse array is equal or greater than
+     * the one of the internal packed array.
+     *
+     * @return Extent of the sparse set.
+     */
+    [[nodiscard]] size_type extent() const noexcept {
+        return sparse.size() * entity_traits::page_size;
+    }
+
+    /**
+     * @brief Returns the number of elements in a sparse set.
+     *
+     * The number of elements is also the size of the internal packed array.
+     * There is no guarantee that the internal sparse array has the same size.
+     * Usually the size of the internal sparse array is equal or greater than
+     * the one of the internal packed array.
+     *
+     * @return Number of elements.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return packed.size();
+    }
+
+    /**
+     * @brief Checks whether a sparse set is empty.
+     * @return True if the sparse set is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return packed.empty();
+    }
+
+    /**
+     * @brief Direct access to the internal packed array.
+     * @return A pointer to the internal packed array.
+     */
+    [[nodiscard]] pointer data() const noexcept {
+        return packed.data();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first entity of the internal packed
+     * array. If the sparse set is empty, the returned iterator will be equal to
+     * `end()`.
+     *
+     * @return An iterator to the first entity of the sparse set.
+     */
+    [[nodiscard]] const_iterator begin() const noexcept {
+        const auto pos = static_cast<typename iterator::difference_type>(packed.size());
+        return iterator{packed, pos};
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] const_iterator cbegin() const noexcept {
+        return begin();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last entity in
+     * a sparse set. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the element following the last entity of a sparse
+     * set.
+     */
+    [[nodiscard]] iterator end() const noexcept {
+        return iterator{packed, {}};
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] const_iterator cend() const noexcept {
+        return end();
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * The returned iterator points to the first entity of the reversed internal
+     * packed array. If the sparse set is empty, the returned iterator will be
+     * equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed internal packed
+     * array.
+     */
+    [[nodiscard]] const_reverse_iterator rbegin() const noexcept {
+        return std::make_reverse_iterator(end());
+    }
+
+    /*! @copydoc rbegin */
+    [[nodiscard]] const_reverse_iterator crbegin() const noexcept {
+        return rbegin();
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     *
+     * The returned iterator points to the element following the last entity in
+     * the reversed sparse set. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the element following the last entity of the
+     * reversed sparse set.
+     */
+    [[nodiscard]] reverse_iterator rend() const noexcept {
+        return std::make_reverse_iterator(begin());
+    }
+
+    /*! @copydoc rend */
+    [[nodiscard]] const_reverse_iterator crend() const noexcept {
+        return rend();
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const noexcept {
+        return contains(entt) ? --(end() - index(entt)) : end();
+    }
+
+    /**
+     * @brief Checks if a sparse set contains an entity.
+     * @param entt A valid identifier.
+     * @return True if the sparse set contains the entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const noexcept {
+        const auto elem = sparse_ptr(entt);
+        constexpr auto cap = entity_traits::to_entity(null);
+        // testing versions permits to avoid accessing the packed array
+        return elem && (((~cap & entity_traits::to_integral(entt)) ^ entity_traits::to_integral(*elem)) < cap);
+    }
+
+    /**
+     * @brief Returns the contained version for an identifier.
+     * @param entt A valid identifier.
+     * @return The version for the given identifier if present, the tombstone
+     * version otherwise.
+     */
+    [[nodiscard]] version_type current(const entity_type entt) const noexcept {
+        const auto elem = sparse_ptr(entt);
+        constexpr auto fallback = entity_traits::to_version(tombstone);
+        return elem ? entity_traits::to_version(*elem) : fallback;
+    }
+
+    /**
+     * @brief Returns the position of an entity in a sparse set.
+     *
+     * @warning
+     * Attempting to get the position of an entity that doesn't belong to the
+     * sparse set results in undefined behavior.
+     *
+     * @param entt A valid identifier.
+     * @return The position of the entity in the sparse set.
+     */
+    [[nodiscard]] size_type index(const entity_type entt) const noexcept {
+        ENTT_ASSERT(contains(entt), "Set does not contain entity");
+        return static_cast<size_type>(entity_traits::to_entity(sparse_ref(entt)));
+    }
+
+    /**
+     * @brief Returns the entity at specified location, with bounds checking.
+     * @param pos The position for which to return the entity.
+     * @return The entity at specified location if any, a null entity otherwise.
+     */
+    [[nodiscard]] entity_type at(const size_type pos) const noexcept {
+        return pos < packed.size() ? packed[pos] : null;
+    }
+
+    /**
+     * @brief Returns the entity at specified location, without bounds checking.
+     * @param pos The position for which to return the entity.
+     * @return The entity at specified location.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const noexcept {
+        ENTT_ASSERT(pos < packed.size(), "Position is out of bounds");
+        return packed[pos];
+    }
+
+    /**
+     * @brief Returns the element assigned to an entity, if any.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the sparse set results
+     * in undefined behavior.
+     *
+     * @param entt A valid identifier.
+     * @return An opaque pointer to the element assigned to the entity, if any.
+     */
+    [[nodiscard]] const void *get(const entity_type entt) const noexcept {
+        return get_at(index(entt));
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] void *get(const entity_type entt) noexcept {
+        return const_cast<void *>(std::as_const(*this).get(entt));
+    }
+
+    /**
+     * @brief Assigns an entity to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.
+     *
+     * @param entt A valid identifier.
+     * @param value Optional opaque value to forward to mixins, if any.
+     * @return Iterator pointing to the emplaced element in case of success, the
+     * `end()` iterator otherwise.
+     */
+    iterator emplace(const entity_type entt, const void *value = nullptr) {
+        return try_emplace(entt, false, value);
+    }
+
+    /**
+     * @brief Bump the version number of an entity.
+     *
+     * @warning
+     * Attempting to bump the version of an entity that doesn't belong to the
+     * sparse set results in undefined behavior.
+     *
+     * @param entt A valid identifier.
+     */
+    void bump(const entity_type entt) {
+        auto &entity = sparse_ref(entt);
+        ENTT_ASSERT(entt != tombstone && entity != null, "Cannot set the required version");
+        entity = entity_traits::combine(entity_traits::to_integral(entity), entity_traits::to_integral(entt));
+        packed[static_cast<size_type>(entity_traits::to_entity(entity))] = entt;
+    }
+
+    /**
+     * @brief Assigns one or more entities to a sparse set.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the sparse set
+     * results in undefined behavior.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @return Iterator pointing to the first element inserted in case of
+     * success, the `end()` iterator otherwise.
+     */
+    template<typename It>
+    iterator insert(It first, It last) {
+        for(auto it = first; it != last; ++it) {
+            try_emplace(*it, true);
+        }
+
+        return first == last ? end() : find(*first);
+    }
+
+    /**
+     * @brief Erases an entity from a sparse set.
+     *
+     * @warning
+     * Attempting to erase an entity that doesn't belong to the sparse set
+     * results in undefined behavior.
+     *
+     * @param entt A valid identifier.
+     */
+    void erase(const entity_type entt) {
+        const auto it = --(end() - index(entt));
+        pop(it, it + 1u);
+    }
+
+    /**
+     * @brief Erases entities from a set.
+     *
+     * @sa erase
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It>
+    void erase(It first, It last) {
+        if constexpr(std::is_same_v<It, basic_iterator>) {
+            pop(first, last);
+        } else {
+            for(; first != last; ++first) {
+                erase(*first);
+            }
+        }
+    }
+
+    /**
+     * @brief Removes an entity from a sparse set if it exists.
+     * @param entt A valid identifier.
+     * @return True if the entity is actually removed, false otherwise.
+     */
+    bool remove(const entity_type entt) {
+        return contains(entt) && (erase(entt), true);
+    }
+
+    /**
+     * @brief Removes entities from a sparse set if they exist.
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @return The number of entities actually removed.
+     */
+    template<typename It>
+    size_type remove(It first, It last) {
+        size_type count{};
+
+        for(; first != last; ++first) {
+            count += remove(*first);
+        }
+
+        return count;
+    }
+
+    /*! @brief Removes all tombstones from the packed array of a sparse set. */
+    void compact() {
+        size_type from = packed.size();
+        for(; from && packed[from - 1u] == tombstone; --from) {}
+
+        for(auto *it = &free_list; *it != null && from; it = std::addressof(packed[entity_traits::to_entity(*it)])) {
+            if(const size_type to = entity_traits::to_entity(*it); to < from) {
+                --from;
+                move_element(from, to);
+
+                using std::swap;
+                swap(packed[from], packed[to]);
+
+                const auto entity = static_cast<typename entity_traits::entity_type>(to);
+                sparse_ref(packed[to]) = entity_traits::combine(entity, entity_traits::to_integral(packed[to]));
+                *it = entity_traits::combine(static_cast<typename entity_traits::entity_type>(from), entity_traits::reserved);
+                for(; from && packed[from - 1u] == tombstone; --from) {}
+            }
+        }
+
+        free_list = tombstone;
+        packed.resize(from);
+    }
+
+    /**
+     * @brief Swaps two entities in a sparse set.
+     *
+     * For what it's worth, this function affects both the internal sparse array
+     * and the internal packed array. Users should not care of that anyway.
+     *
+     * @warning
+     * Attempting to swap entities that don't belong to the sparse set results
+     * in undefined behavior.
+     *
+     * @param lhs A valid identifier.
+     * @param rhs A valid identifier.
+     */
+    void swap_elements(const entity_type lhs, const entity_type rhs) {
+        ENTT_ASSERT(contains(lhs) && contains(rhs), "Set does not contain entities");
+
+        auto &entt = sparse_ref(lhs);
+        auto &other = sparse_ref(rhs);
+
+        const auto from = entity_traits::to_entity(entt);
+        const auto to = entity_traits::to_entity(other);
+
+        // basic no-leak guarantee (with invalid state) if swapping throws
+        swap_at(static_cast<size_type>(from), static_cast<size_type>(to));
+        entt = entity_traits::combine(to, entity_traits::to_integral(packed[from]));
+        other = entity_traits::combine(from, entity_traits::to_integral(packed[to]));
+
+        using std::swap;
+        swap(packed[from], packed[to]);
+    }
+
+    /**
+     * @brief Sort the first count elements according to the given comparison
+     * function.
+     *
+     * The comparison function object must return `true` if the first element
+     * is _less_ than the second one, `false` otherwise. The signature of the
+     * comparison function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * bool(const Entity, const Entity);
+     * @endcode
+     *
+     * Moreover, the comparison function object shall induce a
+     * _strict weak ordering_ on the values.
+     *
+     * The sort function object must offer a member function template
+     * `operator()` that accepts three arguments:
+     *
+     * * An iterator to the first element of the range to sort.
+     * * An iterator past the last element of the range to sort.
+     * * A comparison function to use to compare the elements.
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param length Number of elements to sort.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort_n(const size_type length, Compare compare, Sort algo = Sort{}, Args &&...args) {
+        ENTT_ASSERT(!(length > packed.size()), "Length exceeds the number of elements");
+        ENTT_ASSERT(free_list == null, "Partial sorting with tombstones is not supported");
+
+        algo(packed.rend() - length, packed.rend(), std::move(compare), std::forward<Args>(args)...);
+
+        for(size_type pos{}; pos < length; ++pos) {
+            auto curr = pos;
+            auto next = index(packed[curr]);
+
+            while(curr != next) {
+                const auto idx = index(packed[next]);
+                const auto entt = packed[curr];
+
+                swap_at(next, idx);
+                const auto entity = static_cast<typename entity_traits::entity_type>(curr);
+                sparse_ref(entt) = entity_traits::combine(entity, entity_traits::to_integral(packed[curr]));
+                curr = std::exchange(next, idx);
+            }
+        }
+    }
+
+    /**
+     * @brief Sort all elements according to the given comparison function.
+     *
+     * @sa sort_n
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Sort Type of sort function object.
+     * @tparam Args Types of arguments to forward to the sort function object.
+     * @param compare A valid comparison function object.
+     * @param algo A valid sort function object.
+     * @param args Arguments to forward to the sort function object, if any.
+     */
+    template<typename Compare, typename Sort = std_sort, typename... Args>
+    void sort(Compare compare, Sort algo = Sort{}, Args &&...args) {
+        compact();
+        sort_n(packed.size(), std::move(compare), std::move(algo), std::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Sort entities according to their order in another sparse set.
+     *
+     * Entities that are part of both the sparse sets are ordered internally
+     * according to the order they have in `other`. All the other entities goes
+     * to the end of the list and there are no guarantees on their order.<br/>
+     * In other terms, this function can be used to impose the same order on two
+     * sets by using one of them as a master and the other one as a slave.
+     *
+     * Iterating the sparse set with a couple of iterators returns elements in
+     * the expected order after a call to `respect`. See `begin` and `end` for
+     * more details.
+     *
+     * @param other The sparse sets that imposes the order of the entities.
+     */
+    void respect(const basic_sparse_set &other) {
+        compact();
+
+        const auto to = other.end();
+        auto from = other.begin();
+
+        for(size_type pos = packed.size() - 1; pos && from != to; ++from) {
+            if(contains(*from)) {
+                if(*from != packed[pos]) {
+                    // basic no-leak guarantee (with invalid state) if swapping throws
+                    swap_elements(packed[pos], *from);
+                }
+
+                --pos;
+            }
+        }
+    }
+
+    /*! @brief Clears a sparse set. */
+    void clear() {
+        if(const auto last = end(); free_list == null) {
+            pop(begin(), last);
+        } else {
+            for(auto &&entity: *this) {
+                // tombstone filter on itself
+                if(const auto it = find(entity); it != last) {
+                    pop(it, it + 1u);
+                }
+            }
+        }
+
+        free_list = tombstone;
+        packed.clear();
+    }
+
+    /**
+     * @brief Returned value type, if any.
+     * @return Returned value type, if any.
+     */
+    const type_info &type() const noexcept {
+        return *info;
+    }
+
+    /*! @brief Forwards variables to derived classes, if any. */
+    virtual void bind(any) noexcept {}
+
+private:
+    sparse_container_type sparse;
+    packed_container_type packed;
+    const type_info *info;
+    entity_type free_list;
+    deletion_policy mode;
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/storage.hpp

@@ -0,0 +1,903 @@
+#ifndef ENTT_ENTITY_STORAGE_HPP
+#define ENTT_ENTITY_STORAGE_HPP
+
+#include <cstddef>
+#include <iterator>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <vector>
+#include "../config/config.h"
+#include "../core/compressed_pair.hpp"
+#include "../core/iterator.hpp"
+#include "../core/memory.hpp"
+#include "../core/type_info.hpp"
+#include "component.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+#include "sparse_set.hpp"
+#include "storage_mixin.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Container>
+class storage_iterator final {
+    friend storage_iterator<const Container>;
+
+    using container_type = std::remove_const_t<Container>;
+    using alloc_traits = std::allocator_traits<typename container_type::allocator_type>;
+    using comp_traits = component_traits<std::remove_pointer_t<typename container_type::value_type>>;
+
+    using iterator_traits = std::iterator_traits<std::conditional_t<
+        std::is_const_v<Container>,
+        typename alloc_traits::template rebind_traits<typename std::pointer_traits<typename container_type::value_type>::element_type>::const_pointer,
+        typename alloc_traits::template rebind_traits<typename std::pointer_traits<typename container_type::value_type>::element_type>::pointer>>;
+
+public:
+    using value_type = typename iterator_traits::value_type;
+    using pointer = typename iterator_traits::pointer;
+    using reference = typename iterator_traits::reference;
+    using difference_type = typename iterator_traits::difference_type;
+    using iterator_category = std::random_access_iterator_tag;
+
+    constexpr storage_iterator() noexcept = default;
+
+    constexpr storage_iterator(Container *ref, const difference_type idx) noexcept
+        : packed{ref},
+          offset{idx} {}
+
+    template<bool Const = std::is_const_v<Container>, typename = std::enable_if_t<Const>>
+    constexpr storage_iterator(const storage_iterator<std::remove_const_t<Container>> &other) noexcept
+        : storage_iterator{other.packed, other.offset} {}
+
+    constexpr storage_iterator &operator++() noexcept {
+        return --offset, *this;
+    }
+
+    constexpr storage_iterator operator++(int) noexcept {
+        storage_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    constexpr storage_iterator &operator--() noexcept {
+        return ++offset, *this;
+    }
+
+    constexpr storage_iterator operator--(int) noexcept {
+        storage_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    constexpr storage_iterator &operator+=(const difference_type value) noexcept {
+        offset -= value;
+        return *this;
+    }
+
+    constexpr storage_iterator operator+(const difference_type value) const noexcept {
+        storage_iterator copy = *this;
+        return (copy += value);
+    }
+
+    constexpr storage_iterator &operator-=(const difference_type value) noexcept {
+        return (*this += -value);
+    }
+
+    constexpr storage_iterator operator-(const difference_type value) const noexcept {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
+        const auto pos = index() - value;
+        return (*packed)[pos / comp_traits::page_size][fast_mod(pos, comp_traits::page_size)];
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        const auto pos = index();
+        return (*packed)[pos / comp_traits::page_size] + fast_mod(pos, comp_traits::page_size);
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return *operator->();
+    }
+
+    [[nodiscard]] constexpr difference_type index() const noexcept {
+        return offset - 1;
+    }
+
+private:
+    Container *packed;
+    difference_type offset;
+};
+
+template<typename CLhs, typename CRhs>
+[[nodiscard]] constexpr std::ptrdiff_t operator-(const storage_iterator<CLhs> &lhs, const storage_iterator<CRhs> &rhs) noexcept {
+    return rhs.index() - lhs.index();
+}
+
+template<typename CLhs, typename CRhs>
+[[nodiscard]] constexpr bool operator==(const storage_iterator<CLhs> &lhs, const storage_iterator<CRhs> &rhs) noexcept {
+    return lhs.index() == rhs.index();
+}
+
+template<typename CLhs, typename CRhs>
+[[nodiscard]] constexpr bool operator!=(const storage_iterator<CLhs> &lhs, const storage_iterator<CRhs> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+template<typename CLhs, typename CRhs>
+[[nodiscard]] constexpr bool operator<(const storage_iterator<CLhs> &lhs, const storage_iterator<CRhs> &rhs) noexcept {
+    return lhs.index() > rhs.index();
+}
+
+template<typename CLhs, typename CRhs>
+[[nodiscard]] constexpr bool operator>(const storage_iterator<CLhs> &lhs, const storage_iterator<CRhs> &rhs) noexcept {
+    return lhs.index() < rhs.index();
+}
+
+template<typename CLhs, typename CRhs>
+[[nodiscard]] constexpr bool operator<=(const storage_iterator<CLhs> &lhs, const storage_iterator<CRhs> &rhs) noexcept {
+    return !(lhs > rhs);
+}
+
+template<typename CLhs, typename CRhs>
+[[nodiscard]] constexpr bool operator>=(const storage_iterator<CLhs> &lhs, const storage_iterator<CRhs> &rhs) noexcept {
+    return !(lhs < rhs);
+}
+
+template<typename It, typename... Other>
+class extended_storage_iterator final {
+    template<typename Iter, typename... Args>
+    friend class extended_storage_iterator;
+
+public:
+    using value_type = decltype(std::tuple_cat(std::make_tuple(*std::declval<It>()), std::forward_as_tuple(*std::declval<Other>()...)));
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using difference_type = std::ptrdiff_t;
+    using iterator_category = std::input_iterator_tag;
+
+    constexpr extended_storage_iterator()
+        : it{} {}
+
+    constexpr extended_storage_iterator(It base, Other... other)
+        : it{base, other...} {}
+
+    template<typename... Args, typename = std::enable_if_t<(!std::is_same_v<Other, Args> && ...) && (std::is_constructible_v<Other, Args> && ...)>>
+    constexpr extended_storage_iterator(const extended_storage_iterator<It, Args...> &other)
+        : it{other.it} {}
+
+    constexpr extended_storage_iterator &operator++() noexcept {
+        return ++std::get<It>(it), (++std::get<Other>(it), ...), *this;
+    }
+
+    constexpr extended_storage_iterator operator++(int) noexcept {
+        extended_storage_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return operator*();
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return {*std::get<It>(it), *std::get<Other>(it)...};
+    }
+
+    template<typename... CLhs, typename... CRhs>
+    friend constexpr bool operator==(const extended_storage_iterator<CLhs...> &, const extended_storage_iterator<CRhs...> &) noexcept;
+
+private:
+    std::tuple<It, Other...> it;
+};
+
+template<typename... CLhs, typename... CRhs>
+[[nodiscard]] constexpr bool operator==(const extended_storage_iterator<CLhs...> &lhs, const extended_storage_iterator<CRhs...> &rhs) noexcept {
+    return std::get<0>(lhs.it) == std::get<0>(rhs.it);
+}
+
+template<typename... CLhs, typename... CRhs>
+[[nodiscard]] constexpr bool operator!=(const extended_storage_iterator<CLhs...> &lhs, const extended_storage_iterator<CRhs...> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief Basic storage implementation.
+ *
+ * Internal data structures arrange elements to maximize performance. There are
+ * no guarantees that objects are returned in the insertion order when iterate
+ * a storage. Do not make assumption on the order in any case.
+ *
+ * @warning
+ * Empty types aren't explicitly instantiated. Therefore, many of the functions
+ * normally available for non-empty types will not be available for empty ones.
+ *
+ * @tparam Type Type of objects assigned to the entities.
+ * @tparam Entity A valid entity type (see entt_traits for more details).
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Entity, typename Allocator, typename>
+class basic_storage: public basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>> {
+    using alloc_traits = std::allocator_traits<Allocator>;
+    static_assert(std::is_same_v<typename alloc_traits::value_type, Type>, "Invalid value type");
+    using underlying_type = basic_sparse_set<Entity, typename alloc_traits::template rebind_alloc<Entity>>;
+    using container_type = std::vector<typename alloc_traits::pointer, typename alloc_traits::template rebind_alloc<typename alloc_traits::pointer>>;
+    using comp_traits = component_traits<Type>;
+
+    static constexpr bool is_pinned_type_v = !(std::is_move_constructible_v<Type> && std::is_move_assignable_v<Type>);
+
+    [[nodiscard]] auto &element_at(const std::size_t pos) const {
+        return packed.first()[pos / comp_traits::page_size][fast_mod(pos, comp_traits::page_size)];
+    }
+
+    auto assure_at_least(const std::size_t pos) {
+        auto &&container = packed.first();
+        const auto idx = pos / comp_traits::page_size;
+
+        if(!(idx < container.size())) {
+            auto curr = container.size();
+            container.resize(idx + 1u, nullptr);
+
+            ENTT_TRY {
+                for(const auto last = container.size(); curr < last; ++curr) {
+                    container[curr] = alloc_traits::allocate(packed.second(), comp_traits::page_size);
+                }
+            }
+            ENTT_CATCH {
+                container.resize(curr);
+                ENTT_THROW;
+            }
+        }
+
+        return container[idx] + fast_mod(pos, comp_traits::page_size);
+    }
+
+    template<typename... Args>
+    auto emplace_element(const Entity entt, const bool force_back, Args &&...args) {
+        const auto it = base_type::try_emplace(entt, force_back);
+
+        ENTT_TRY {
+            auto elem = assure_at_least(static_cast<size_type>(it.index()));
+            entt::uninitialized_construct_using_allocator(to_address(elem), packed.second(), std::forward<Args>(args)...);
+        }
+        ENTT_CATCH {
+            base_type::pop(it, it + 1u);
+            ENTT_THROW;
+        }
+
+        return it;
+    }
+
+    void shrink_to_size(const std::size_t sz) {
+        for(auto pos = sz, length = base_type::size(); pos < length; ++pos) {
+            if constexpr(comp_traits::in_place_delete) {
+                if(base_type::at(pos) != tombstone) {
+                    std::destroy_at(std::addressof(element_at(pos)));
+                }
+            } else {
+                std::destroy_at(std::addressof(element_at(pos)));
+            }
+        }
+
+        auto &&container = packed.first();
+        auto page_allocator{packed.second()};
+        const auto from = (sz + comp_traits::page_size - 1u) / comp_traits::page_size;
+
+        for(auto pos = from, last = container.size(); pos < last; ++pos) {
+            alloc_traits::deallocate(page_allocator, container[pos], comp_traits::page_size);
+        }
+
+        container.resize(from);
+    }
+
+private:
+    const void *get_at(const std::size_t pos) const final {
+        return std::addressof(element_at(pos));
+    }
+
+    void swap_at([[maybe_unused]] const std::size_t lhs, [[maybe_unused]] const std::size_t rhs) final {
+        // use a runtime value to avoid compile-time suppression that drives the code coverage tool crazy
+        ENTT_ASSERT((lhs + 1u) && !is_pinned_type_v, "Pinned type");
+
+        if constexpr(!is_pinned_type_v) {
+            using std::swap;
+            swap(element_at(lhs), element_at(rhs));
+        }
+    }
+
+    void move_element([[maybe_unused]] const std::size_t from, [[maybe_unused]] const std::size_t to) final {
+        // use a runtime value to avoid compile-time suppression that drives the code coverage tool crazy
+        ENTT_ASSERT((from + 1u) && !is_pinned_type_v, "Pinned type");
+
+        if constexpr(!is_pinned_type_v) {
+            auto &elem = element_at(from);
+            entt::uninitialized_construct_using_allocator(to_address(assure_at_least(to)), packed.second(), std::move(elem));
+            std::destroy_at(std::addressof(elem));
+        }
+    }
+
+protected:
+    /*! @brief Random access iterator type. */
+    using basic_iterator = typename underlying_type::basic_iterator;
+
+    /**
+     * @brief Erases entities from a sparse set.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    void pop(basic_iterator first, basic_iterator last) override {
+        for(; first != last; ++first) {
+            // cannot use first.index() because it would break with cross iterators
+            auto &elem = element_at(base_type::index(*first));
+
+            if constexpr(comp_traits::in_place_delete) {
+                base_type::in_place_pop(first);
+                std::destroy_at(std::addressof(elem));
+            } else {
+                auto &other = element_at(base_type::size() - 1u);
+                // destroying on exit allows reentrant destructors
+                [[maybe_unused]] auto unused = std::exchange(elem, std::move(other));
+                std::destroy_at(std::addressof(other));
+                base_type::swap_and_pop(first);
+            }
+        }
+    }
+
+    /**
+     * @brief Assigns an entity to a storage.
+     * @param entt A valid identifier.
+     * @param value Optional opaque value.
+     * @param force_back Force back insertion.
+     * @return Iterator pointing to the emplaced element.
+     */
+    basic_iterator try_emplace([[maybe_unused]] const Entity entt, [[maybe_unused]] const bool force_back, const void *value) override {
+        if(value) {
+            if constexpr(std::is_copy_constructible_v<value_type>) {
+                return emplace_element(entt, force_back, *static_cast<const value_type *>(value));
+            } else {
+                return base_type::end();
+            }
+        } else {
+            if constexpr(std::is_default_constructible_v<value_type>) {
+                return emplace_element(entt, force_back);
+            } else {
+                return base_type::end();
+            }
+        }
+    }
+
+public:
+    /*! @brief Base type. */
+    using base_type = underlying_type;
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Type of the objects assigned to entities. */
+    using value_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Pointer type to contained elements. */
+    using pointer = typename container_type::pointer;
+    /*! @brief Constant pointer type to contained elements. */
+    using const_pointer = typename alloc_traits::template rebind_traits<typename alloc_traits::const_pointer>::const_pointer;
+    /*! @brief Random access iterator type. */
+    using iterator = internal::storage_iterator<container_type>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator = internal::storage_iterator<const container_type>;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    /*! @brief Constant reverse iterator type. */
+    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+    /*! @brief Extended iterable storage proxy. */
+    using iterable = iterable_adaptor<internal::extended_storage_iterator<typename base_type::iterator, iterator>>;
+    /*! @brief Constant extended iterable storage proxy. */
+    using const_iterable = iterable_adaptor<internal::extended_storage_iterator<typename base_type::const_iterator, const_iterator>>;
+
+    /*! @brief Default constructor. */
+    basic_storage()
+        : basic_storage{allocator_type{}} {}
+
+    /**
+     * @brief Constructs an empty storage with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_storage(const allocator_type &allocator)
+        : base_type{type_id<value_type>(), deletion_policy{comp_traits::in_place_delete}, allocator},
+          packed{container_type{allocator}, allocator} {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_storage(basic_storage &&other) noexcept
+        : base_type{std::move(other)},
+          packed{std::move(other.packed)} {}
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_storage(basic_storage &&other, const allocator_type &allocator) noexcept
+        : base_type{std::move(other), allocator},
+          packed{container_type{std::move(other.packed.first()), allocator}, allocator} {
+        ENTT_ASSERT(alloc_traits::is_always_equal::value || packed.second() == other.packed.second(), "Copying a storage is not allowed");
+    }
+
+    /*! @brief Default destructor. */
+    ~basic_storage() override {
+        shrink_to_size(0u);
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This storage.
+     */
+    basic_storage &operator=(basic_storage &&other) noexcept {
+        ENTT_ASSERT(alloc_traits::is_always_equal::value || packed.second() == other.packed.second(), "Copying a storage is not allowed");
+
+        shrink_to_size(0u);
+        base_type::operator=(std::move(other));
+        packed.first() = std::move(other.packed.first());
+        propagate_on_container_move_assignment(packed.second(), other.packed.second());
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given storage.
+     * @param other Storage to exchange the content with.
+     */
+    void swap(basic_storage &other) {
+        using std::swap;
+        underlying_type::swap(other);
+        propagate_on_container_swap(packed.second(), other.packed.second());
+        swap(packed.first(), other.packed.first());
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return allocator_type{packed.second()};
+    }
+
+    /**
+     * @brief Increases the capacity of a storage.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) override {
+        if(cap != 0u) {
+            base_type::reserve(cap);
+            assure_at_least(cap - 1u);
+        }
+    }
+
+    /**
+     * @brief Returns the number of elements that a storage has currently
+     * allocated space for.
+     * @return Capacity of the storage.
+     */
+    [[nodiscard]] size_type capacity() const noexcept override {
+        return packed.first().size() * comp_traits::page_size;
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() override {
+        base_type::shrink_to_fit();
+        shrink_to_size(base_type::size());
+    }
+
+    /**
+     * @brief Direct access to the array of objects.
+     * @return A pointer to the array of objects.
+     */
+    [[nodiscard]] const_pointer raw() const noexcept {
+        return packed.first().data();
+    }
+
+    /*! @copydoc raw */
+    [[nodiscard]] pointer raw() noexcept {
+        return packed.first().data();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the internal array.
+     * If the storage is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal array.
+     */
+    [[nodiscard]] const_iterator cbegin() const noexcept {
+        const auto pos = static_cast<typename iterator::difference_type>(base_type::size());
+        return const_iterator{&packed.first(), pos};
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const noexcept {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() noexcept {
+        const auto pos = static_cast<typename iterator::difference_type>(base_type::size());
+        return iterator{&packed.first(), pos};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the internal array. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the element following the last instance of the
+     * internal array.
+     */
+    [[nodiscard]] const_iterator cend() const noexcept {
+        return const_iterator{&packed.first(), {}};
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const noexcept {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() noexcept {
+        return iterator{&packed.first(), {}};
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * The returned iterator points to the first instance of the reversed
+     * internal array. If the storage is empty, the returned iterator will be
+     * equal to `rend()`.
+     *
+     * @return An iterator to the first instance of the reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crbegin() const noexcept {
+        return std::make_reverse_iterator(cend());
+    }
+
+    /*! @copydoc crbegin */
+    [[nodiscard]] const_reverse_iterator rbegin() const noexcept {
+        return crbegin();
+    }
+
+    /*! @copydoc rbegin */
+    [[nodiscard]] reverse_iterator rbegin() noexcept {
+        return std::make_reverse_iterator(end());
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     *
+     * The returned iterator points to the element following the last instance
+     * of the reversed internal array. Attempting to dereference the returned
+     * iterator results in undefined behavior.
+     *
+     * @return An iterator to the element following the last instance of the
+     * reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crend() const noexcept {
+        return std::make_reverse_iterator(cbegin());
+    }
+
+    /*! @copydoc crend */
+    [[nodiscard]] const_reverse_iterator rend() const noexcept {
+        return crend();
+    }
+
+    /*! @copydoc rend */
+    [[nodiscard]] reverse_iterator rend() noexcept {
+        return std::make_reverse_iterator(begin());
+    }
+
+    /**
+     * @brief Returns the object assigned to an entity.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.
+     *
+     * @param entt A valid identifier.
+     * @return The object assigned to the entity.
+     */
+    [[nodiscard]] const value_type &get(const entity_type entt) const noexcept {
+        return element_at(base_type::index(entt));
+    }
+
+    /*! @copydoc get */
+    [[nodiscard]] value_type &get(const entity_type entt) noexcept {
+        return const_cast<value_type &>(std::as_const(*this).get(entt));
+    }
+
+    /**
+     * @brief Returns the object assigned to an entity as a tuple.
+     * @param entt A valid identifier.
+     * @return The object assigned to the entity as a tuple.
+     */
+    [[nodiscard]] std::tuple<const value_type &> get_as_tuple(const entity_type entt) const noexcept {
+        return std::forward_as_tuple(get(entt));
+    }
+
+    /*! @copydoc get_as_tuple */
+    [[nodiscard]] std::tuple<value_type &> get_as_tuple(const entity_type entt) noexcept {
+        return std::forward_as_tuple(get(entt));
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid identifier.
+     * @param args Parameters to use to construct an object for the entity.
+     * @return A reference to the newly created object.
+     */
+    template<typename... Args>
+    value_type &emplace(const entity_type entt, Args &&...args) {
+        if constexpr(std::is_aggregate_v<value_type>) {
+            const auto it = emplace_element(entt, false, Type{std::forward<Args>(args)...});
+            return element_at(static_cast<size_type>(it.index()));
+        } else {
+            const auto it = emplace_element(entt, false, std::forward<Args>(args)...);
+            return element_at(static_cast<size_type>(it.index()));
+        }
+    }
+
+    /**
+     * @brief Updates the instance assigned to a given entity in-place.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entt A valid identifier.
+     * @param func Valid function objects.
+     * @return A reference to the updated instance.
+     */
+    template<typename... Func>
+    value_type &patch(const entity_type entt, Func &&...func) {
+        const auto idx = base_type::index(entt);
+        auto &elem = element_at(idx);
+        (std::forward<Func>(func)(elem), ...);
+        return elem;
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given instance.
+     *
+     * @warning
+     * Attempting to assign an entity that already belongs to the storage
+     * results in undefined behavior.
+     *
+     * @tparam It Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param value An instance of the object to construct.
+     */
+    template<typename It>
+    void insert(It first, It last, const value_type &value = {}) {
+        for(; first != last; ++first) {
+            emplace_element(*first, true, value);
+        }
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given range.
+     *
+     * @sa construct
+     *
+     * @tparam EIt Type of input iterator.
+     * @tparam CIt Type of input iterator.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param from An iterator to the first element of the range of objects.
+     */
+    template<typename EIt, typename CIt, typename = std::enable_if_t<std::is_same_v<typename std::iterator_traits<CIt>::value_type, value_type>>>
+    void insert(EIt first, EIt last, CIt from) {
+        for(; first != last; ++first, ++from) {
+            emplace_element(*first, true, *from);
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ a storage.
+     *
+     * The iterable object returns a tuple that contains the current entity and
+     * a reference to its component.
+     *
+     * @return An iterable object to use to _visit_ the storage.
+     */
+    [[nodiscard]] iterable each() noexcept {
+        return {internal::extended_storage_iterator{base_type::begin(), begin()}, internal::extended_storage_iterator{base_type::end(), end()}};
+    }
+
+    /*! @copydoc each */
+    [[nodiscard]] const_iterable each() const noexcept {
+        return {internal::extended_storage_iterator{base_type::cbegin(), cbegin()}, internal::extended_storage_iterator{base_type::cend(), cend()}};
+    }
+
+private:
+    compressed_pair<container_type, allocator_type> packed;
+};
+
+/*! @copydoc basic_storage */
+template<typename Type, typename Entity, typename Allocator>
+class basic_storage<Type, Entity, Allocator, std::enable_if_t<ignore_as_empty_v<Type>>>
+    : public basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>> {
+    using alloc_traits = std::allocator_traits<Allocator>;
+    static_assert(std::is_same_v<typename alloc_traits::value_type, Type>, "Invalid value type");
+    using underlying_type = basic_sparse_set<Entity, typename alloc_traits::template rebind_alloc<Entity>>;
+    using comp_traits = component_traits<Type>;
+
+public:
+    /*! @brief Base type. */
+    using base_type = underlying_type;
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Type of the objects assigned to entities. */
+    using value_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Extended iterable storage proxy. */
+    using iterable = iterable_adaptor<internal::extended_storage_iterator<typename base_type::iterator>>;
+    /*! @brief Constant extended iterable storage proxy. */
+    using const_iterable = iterable_adaptor<internal::extended_storage_iterator<typename base_type::const_iterator>>;
+
+    /*! @brief Default constructor. */
+    basic_storage()
+        : basic_storage{allocator_type{}} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_storage(const allocator_type &allocator)
+        : base_type{type_id<value_type>(), deletion_policy{comp_traits::in_place_delete}, allocator} {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_storage(basic_storage &&other) noexcept = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_storage(basic_storage &&other, const allocator_type &allocator) noexcept
+        : base_type{std::move(other), allocator} {}
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This storage.
+     */
+    basic_storage &operator=(basic_storage &&other) noexcept = default;
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return allocator_type{base_type::get_allocator()};
+    }
+
+    /**
+     * @brief Returns the object assigned to an entity, that is `void`.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.
+     *
+     * @param entt A valid identifier.
+     */
+    void get([[maybe_unused]] const entity_type entt) const noexcept {
+        ENTT_ASSERT(base_type::contains(entt), "Storage does not contain entity");
+    }
+
+    /**
+     * @brief Returns an empty tuple.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the storage results in
+     * undefined behavior.
+     *
+     * @param entt A valid identifier.
+     * @return Returns an empty tuple.
+     */
+    [[nodiscard]] std::tuple<> get_as_tuple([[maybe_unused]] const entity_type entt) const noexcept {
+        ENTT_ASSERT(base_type::contains(entt), "Storage does not contain entity");
+        return std::tuple{};
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     *
+     * @warning
+     * Attempting to use an entity that already belongs to the storage results
+     * in undefined behavior.
+     *
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid identifier.
+     */
+    template<typename... Args>
+    void emplace(const entity_type entt, Args &&...) {
+        base_type::try_emplace(entt, false);
+    }
+
+    /**
+     * @brief Updates the instance assigned to a given entity in-place.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entt A valid identifier.
+     * @param func Valid function objects.
+     */
+    template<typename... Func>
+    void patch([[maybe_unused]] const entity_type entt, Func &&...func) {
+        ENTT_ASSERT(base_type::contains(entt), "Storage does not contain entity");
+        (std::forward<Func>(func)(), ...);
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam It Type of input iterator.
+     * @tparam Args Types of optional arguments.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     */
+    template<typename It, typename... Args>
+    void insert(It first, It last, Args &&...) {
+        for(; first != last; ++first) {
+            base_type::try_emplace(*first, true);
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ a storage.
+     *
+     * The iterable object returns a tuple that contains the current entity.
+     *
+     * @return An iterable object to use to _visit_ the storage.
+     */
+    [[nodiscard]] iterable each() noexcept {
+        return {internal::extended_storage_iterator{base_type::begin()}, internal::extended_storage_iterator{base_type::end()}};
+    }
+
+    /*! @copydoc each */
+    [[nodiscard]] const_iterable each() const noexcept {
+        return {internal::extended_storage_iterator{base_type::cbegin()}, internal::extended_storage_iterator{base_type::cend()}};
+    }
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/storage_mixin.hpp

@@ -0,0 +1,236 @@
+#ifndef ENTT_ENTITY_SIGH_STORAGE_MIXIN_HPP
+#define ENTT_ENTITY_SIGH_STORAGE_MIXIN_HPP
+
+#include <utility>
+#include "../config/config.h"
+#include "../core/any.hpp"
+#include "../signal/sigh.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Mixin type used to add signal support to storage types.
+ *
+ * The function type of a listener is equivalent to:
+ *
+ * @code{.cpp}
+ * void(basic_registry<entity_type> &, entity_type);
+ * @endcode
+ *
+ * This applies to all signals made available.
+ *
+ * @tparam Type The type of the underlying storage.
+ */
+template<typename Type>
+class sigh_storage_mixin final: public Type {
+    using basic_registry_type = basic_registry<typename Type::entity_type, typename Type::base_type::allocator_type>;
+    using sigh_type = sigh<void(basic_registry_type &, const typename Type::entity_type), typename Type::allocator_type>;
+    using basic_iterator = typename Type::basic_iterator;
+
+    void pop(basic_iterator first, basic_iterator last) override {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+
+        for(; first != last; ++first) {
+            const auto entt = *first;
+            destruction.publish(*owner, entt);
+            const auto it = Type::find(entt);
+            Type::pop(it, it + 1u);
+        }
+    }
+
+    basic_iterator try_emplace(const typename basic_registry_type::entity_type entt, const bool force_back, const void *value) final {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        Type::try_emplace(entt, force_back, value);
+        construction.publish(*owner, entt);
+        return Type::find(entt);
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = typename Type::allocator_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename Type::entity_type;
+    /*! @brief Expected registry type. */
+    using registry_type = basic_registry_type;
+
+    /*! @brief Default constructor. */
+    sigh_storage_mixin()
+        : sigh_storage_mixin{allocator_type{}} {}
+
+    /**
+     * @brief Constructs an empty storage with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit sigh_storage_mixin(const allocator_type &allocator)
+        : Type{allocator},
+          owner{},
+          construction{allocator},
+          destruction{allocator},
+          update{allocator} {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    sigh_storage_mixin(sigh_storage_mixin &&other) noexcept
+        : Type{std::move(other)},
+          owner{other.owner},
+          construction{std::move(other.construction)},
+          destruction{std::move(other.destruction)},
+          update{std::move(other.update)} {}
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    sigh_storage_mixin(sigh_storage_mixin &&other, const allocator_type &allocator) noexcept
+        : Type{std::move(other), allocator},
+          owner{other.owner},
+          construction{std::move(other.construction), allocator},
+          destruction{std::move(other.destruction), allocator},
+          update{std::move(other.update), allocator} {}
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This storage.
+     */
+    sigh_storage_mixin &operator=(sigh_storage_mixin &&other) noexcept {
+        Type::operator=(std::move(other));
+        owner = other.owner;
+        construction = std::move(other.construction);
+        destruction = std::move(other.destruction);
+        update = std::move(other.update);
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given storage.
+     * @param other Storage to exchange the content with.
+     */
+    void swap(sigh_storage_mixin &other) {
+        using std::swap;
+        Type::swap(other);
+        swap(owner, other.owner);
+        swap(construction, other.construction);
+        swap(destruction, other.destruction);
+        swap(update, other.update);
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever a new instance is created and assigned to an entity.<br/>
+     * Listeners are invoked after the object has been assigned to the entity.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_construct() noexcept {
+        return sink{construction};
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance is explicitly updated.<br/>
+     * Listeners are invoked after the object has been updated.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_update() noexcept {
+        return sink{update};
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance is removed from an entity and thus destroyed.<br/>
+     * Listeners are invoked before the object has been removed from the entity.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_destroy() noexcept {
+        return sink{destruction};
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam Args Types of arguments to use to construct the object.
+     * @param entt A valid identifier.
+     * @param args Parameters to use to initialize the object.
+     * @return A reference to the newly created object.
+     */
+    template<typename... Args>
+    decltype(auto) emplace(const entity_type entt, Args &&...args) {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        Type::emplace(entt, std::forward<Args>(args)...);
+        construction.publish(*owner, entt);
+        return this->get(entt);
+    }
+
+    /**
+     * @brief Patches the given instance for an entity.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entt A valid identifier.
+     * @param func Valid function objects.
+     * @return A reference to the patched instance.
+     */
+    template<typename... Func>
+    decltype(auto) patch(const entity_type entt, Func &&...func) {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        Type::patch(entt, std::forward<Func>(func)...);
+        update.publish(*owner, entt);
+        return this->get(entt);
+    }
+
+    /**
+     * @brief Assigns entities to a storage.
+     * @tparam It Type of input iterator.
+     * @tparam Args Types of arguments to use to construct the objects assigned
+     * to the entities.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param args Parameters to use to initialize the objects assigned to the
+     * entities.
+     */
+    template<typename It, typename... Args>
+    void insert(It first, It last, Args &&...args) {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        Type::insert(first, last, std::forward<Args>(args)...);
+
+        for(auto it = construction.empty() ? last : first; it != last; ++it) {
+            construction.publish(*owner, *it);
+        }
+    }
+
+    /**
+     * @brief Forwards variables to derived classes, if any.
+     * @param value A variable wrapped in an opaque container.
+     */
+    void bind(any value) noexcept final {
+        auto *reg = any_cast<basic_registry_type>(&value);
+        owner = reg ? reg : owner;
+        Type::bind(std::move(value));
+    }
+
+private:
+    basic_registry_type *owner;
+    sigh_type construction;
+    sigh_type destruction;
+    sigh_type update;
+};
+
+} // namespace entt
+
+#endif

src/entt/entity/view.hpp

@@ -0,0 +1,861 @@
+#ifndef ENTT_ENTITY_VIEW_HPP
+#define ENTT_ENTITY_VIEW_HPP
+
+#include <array>
+#include <iterator>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include "../config/config.h"
+#include "../core/iterator.hpp"
+#include "../core/type_traits.hpp"
+#include "component.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+#include "sparse_set.hpp"
+#include "storage.hpp"
+
+namespace entt {
+
+/**
+ * @cond TURN_OFF_DOXYGEN
+ * Internal details not to be documented.
+ */
+
+namespace internal {
+
+template<typename Type, std::size_t Get, std::size_t Exclude>
+class view_iterator final {
+    using iterator_type = typename Type::const_iterator;
+
+    [[nodiscard]] bool valid() const noexcept {
+        return ((Get != 0u) || (*it != tombstone))
+               && std::apply([entt = *it](const auto *...curr) { return (curr->contains(entt) && ...); }, pools)
+               && std::apply([entt = *it](const auto *...curr) { return (!curr->contains(entt) && ...); }, filter);
+    }
+
+public:
+    using value_type = typename iterator_type::value_type;
+    using pointer = typename iterator_type::pointer;
+    using reference = typename iterator_type::reference;
+    using difference_type = typename iterator_type::difference_type;
+    using iterator_category = std::forward_iterator_tag;
+
+    constexpr view_iterator() noexcept
+        : it{},
+          last{},
+          pools{},
+          filter{} {}
+
+    view_iterator(iterator_type curr, iterator_type to, std::array<const Type *, Get> all_of, std::array<const Type *, Exclude> none_of) noexcept
+        : it{curr},
+          last{to},
+          pools{all_of},
+          filter{none_of} {
+        while(it != last && !valid()) {
+            ++it;
+        }
+    }
+
+    view_iterator &operator++() noexcept {
+        while(++it != last && !valid()) {}
+        return *this;
+    }
+
+    view_iterator operator++(int) noexcept {
+        view_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] pointer operator->() const noexcept {
+        return &*it;
+    }
+
+    [[nodiscard]] reference operator*() const noexcept {
+        return *operator->();
+    }
+
+    template<typename LhsType, auto... LhsArgs, typename RhsType, auto... RhsArgs>
+    friend constexpr bool operator==(const view_iterator<LhsType, LhsArgs...> &, const view_iterator<RhsType, RhsArgs...> &) noexcept;
+
+private:
+    iterator_type it;
+    iterator_type last;
+    std::array<const Type *, Get> pools;
+    std::array<const Type *, Exclude> filter;
+};
+
+template<typename LhsType, auto... LhsArgs, typename RhsType, auto... RhsArgs>
+[[nodiscard]] constexpr bool operator==(const view_iterator<LhsType, LhsArgs...> &lhs, const view_iterator<RhsType, RhsArgs...> &rhs) noexcept {
+    return lhs.it == rhs.it;
+}
+
+template<typename LhsType, auto... LhsArgs, typename RhsType, auto... RhsArgs>
+[[nodiscard]] constexpr bool operator!=(const view_iterator<LhsType, LhsArgs...> &lhs, const view_iterator<RhsType, RhsArgs...> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+template<typename It, typename... Type>
+struct extended_view_iterator final {
+    using difference_type = std::ptrdiff_t;
+    using value_type = decltype(std::tuple_cat(std::make_tuple(*std::declval<It>()), std::declval<Type>().get_as_tuple({})...));
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using iterator_category = std::input_iterator_tag;
+
+    constexpr extended_view_iterator()
+        : it{},
+          pools{} {}
+
+    extended_view_iterator(It from, std::tuple<Type *...> storage)
+        : it{from},
+          pools{storage} {}
+
+    extended_view_iterator &operator++() noexcept {
+        return ++it, *this;
+    }
+
+    extended_view_iterator operator++(int) noexcept {
+        extended_view_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] reference operator*() const noexcept {
+        return std::apply([entt = *it](auto *...curr) { return std::tuple_cat(std::make_tuple(entt), curr->get_as_tuple(entt)...); }, pools);
+    }
+
+    [[nodiscard]] pointer operator->() const noexcept {
+        return operator*();
+    }
+
+    template<typename... Lhs, typename... Rhs>
+    friend bool constexpr operator==(const extended_view_iterator<Lhs...> &, const extended_view_iterator<Rhs...> &) noexcept;
+
+private:
+    It it;
+    std::tuple<Type *...> pools;
+};
+
+template<typename... Lhs, typename... Rhs>
+[[nodiscard]] constexpr bool operator==(const extended_view_iterator<Lhs...> &lhs, const extended_view_iterator<Rhs...> &rhs) noexcept {
+    return lhs.it == rhs.it;
+}
+
+template<typename... Lhs, typename... Rhs>
+[[nodiscard]] constexpr bool operator!=(const extended_view_iterator<Lhs...> &lhs, const extended_view_iterator<Rhs...> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
+} // namespace internal
+
+/**
+ * Internal details not to be documented.
+ * @endcond
+ */
+
+/**
+ * @brief View implementation.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error, but for a few reasonable cases.
+ */
+template<typename, typename, typename>
+class basic_view;
+
+/**
+ * @brief Multi component view.
+ *
+ * Multi component views iterate over those entities that are at least in the
+ * given storage. During initialization, a multi component view looks at the
+ * number of entities available for each component and uses the smallest set in
+ * order to get a performance boost when iterating.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New elements are added to the storage.
+ * * The entity currently pointed is modified (for example, components are added
+ *   or removed from it).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the pools iterated by the view in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @tparam Get Types of storage iterated by the view.
+ * @tparam Exclude Types of storage used to filter the view.
+ */
+template<typename... Get, typename... Exclude>
+class basic_view<get_t<Get...>, exclude_t<Exclude...>> {
+    using underlying_type = std::common_type_t<typename Get::entity_type..., typename Exclude::entity_type...>;
+    using basic_common_type = std::common_type_t<typename Get::base_type..., typename Exclude::base_type...>;
+
+    template<typename, typename, typename>
+    friend class basic_view;
+
+    template<typename Type>
+    static constexpr std::size_t index_of = type_list_index_v<std::remove_const_t<Type>, type_list<typename Get::value_type...>>;
+
+    [[nodiscard]] auto opaque_check_set() const noexcept {
+        std::array<const base_type *, sizeof...(Get) - 1u> other{};
+        std::apply([&other, pos = 0u, view = view](const auto *...curr) mutable { ((curr == view ? void() : void(other[pos++] = curr)), ...); }, pools);
+        return other;
+    }
+
+    [[nodiscard]] auto filter_as_array() const noexcept {
+        return std::apply([](const auto *...curr) { return std::array<const base_type *, sizeof...(Exclude)>{curr...}; }, filter);
+    }
+
+    template<std::size_t Curr, std::size_t Other, typename... Args>
+    [[nodiscard]] auto dispatch_get(const std::tuple<underlying_type, Args...> &curr) const {
+        if constexpr(Curr == Other) {
+            return std::forward_as_tuple(std::get<Args>(curr)...);
+        } else {
+            return storage<Other>().get_as_tuple(std::get<0>(curr));
+        }
+    }
+
+    [[nodiscard]] auto reject(const underlying_type entt) const noexcept {
+        return std::apply([entt](const auto *...curr) { return (curr->contains(entt) || ...); }, filter);
+    }
+
+    template<std::size_t Curr, typename Func, std::size_t... Index>
+    void each(Func &func, std::index_sequence<Index...>) const {
+        for(const auto curr: storage<Curr>().each()) {
+            if(const auto entt = std::get<0>(curr); ((sizeof...(Get) != 1u) || (entt != tombstone)) && ((Curr == Index || storage<Index>().contains(entt)) && ...) && !reject(entt)) {
+                if constexpr(is_applicable_v<Func, decltype(std::tuple_cat(std::tuple<entity_type>{}, std::declval<basic_view>().get({})))>) {
+                    std::apply(func, std::tuple_cat(std::make_tuple(entt), dispatch_get<Curr, Index>(curr)...));
+                } else {
+                    std::apply(func, std::tuple_cat(dispatch_get<Curr, Index>(curr)...));
+                }
+            }
+        }
+    }
+
+    template<typename Func, std::size_t... Index>
+    void pick_and_each(Func &func, std::index_sequence<Index...> seq) const {
+        ((&storage<Index>() == view ? each<Index>(func, seq) : void()), ...);
+    }
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = underlying_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Common type among all storage types. */
+    using base_type = basic_common_type;
+    /*! @brief Bidirectional iterator type. */
+    using iterator = internal::view_iterator<base_type, sizeof...(Get) - 1u, sizeof...(Exclude)>;
+    /*! @brief Iterable view type. */
+    using iterable = iterable_adaptor<internal::extended_view_iterator<iterator, Get...>>;
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_view() noexcept
+        : pools{},
+          filter{},
+          view{} {}
+
+    /**
+     * @brief Constructs a multi-type view from a set of storage classes.
+     * @param value The storage for the types to iterate.
+     * @param exclude The storage for the types used to filter the view.
+     */
+    basic_view(Get &...value, Exclude &...exclude) noexcept
+        : pools{&value...},
+          filter{&exclude...},
+          view{[](const base_type *first, const auto *...other) { ((first = other->size() < first->size() ? other : first), ...); return first; }(&value...)} {}
+
+    /**
+     * @brief Constructs a multi-type view from a set of storage classes.
+     * @param value The storage for the types to iterate.
+     * @param excl The storage for the types used to filter the view.
+     */
+    basic_view(std::tuple<Get &...> value, std::tuple<Exclude &...> excl = {}) noexcept
+        : pools{std::apply([](auto &...curr) { return std::make_tuple(&curr...); }, value)},
+          filter{std::apply([](auto &...curr) { return std::make_tuple(&curr...); }, excl)},
+          view{std::apply([](const base_type *first, const auto *...other) { ((first = other->size() < first->size() ? other : first), ...); return first; }, pools)} {}
+
+    /**
+     * @brief Creates a new view driven by a given component in its iterations.
+     * @tparam Type Type of component used to drive the iteration.
+     * @return A new view driven by the given component in its iterations.
+     */
+    template<typename Type>
+    [[nodiscard]] basic_view use() const noexcept {
+        return use<index_of<Type>>();
+    }
+
+    /**
+     * @brief Creates a new view driven by a given component in its iterations.
+     * @tparam Index Index of the component used to drive the iteration.
+     * @return A new view driven by the given component in its iterations.
+     */
+    template<std::size_t Index>
+    [[nodiscard]] basic_view use() const noexcept {
+        basic_view other{*this};
+        other.view = &storage<Index>();
+        return other;
+    }
+
+    /**
+     * @brief Updates the internal leading view if required.
+     * @return A newly created and internally optimized view.
+     */
+    [[nodiscard]] basic_view refresh() const noexcept {
+        return std::apply([](auto *...elem) { return basic_view{*elem...}; }, std::tuple_cat(pools, filter));
+    }
+
+    /**
+     * @brief Returns the leading storage of a view.
+     * @return The leading storage of the view.
+     */
+    [[nodiscard]] const base_type &handle() const noexcept {
+        return *view;
+    }
+
+    /**
+     * @brief Returns the storage for a given component type.
+     * @tparam Comp Type of component of which to return the storage.
+     * @return The storage for the given component type.
+     */
+    template<typename Type>
+    [[nodiscard]] decltype(auto) storage() const noexcept {
+        return storage<index_of<Type>>();
+    }
+
+    /**
+     * @brief Returns the storage for a given index.
+     * @tparam Index Index of the storage to return.
+     * @return The storage for the given index.
+     */
+    template<std::size_t Index>
+    [[nodiscard]] decltype(auto) storage() const noexcept {
+        return *std::get<Index>(pools);
+    }
+
+    /**
+     * @brief Estimates the number of entities iterated by the view.
+     * @return Estimated number of entities iterated by the view.
+     */
+    [[nodiscard]] size_type size_hint() const noexcept {
+        return view->size();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the view.
+     *
+     * The returned iterator points to the first entity of the view. If the view
+     * is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the view.
+     */
+    [[nodiscard]] iterator begin() const noexcept {
+        return iterator{view->begin(), view->end(), opaque_check_set(), filter_as_array()};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the view. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the view.
+     */
+    [[nodiscard]] iterator end() const noexcept {
+        return iterator{view->end(), view->end(), opaque_check_set(), filter_as_array()};
+    }
+
+    /**
+     * @brief Returns the first entity of the view, if any.
+     * @return The first entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const noexcept {
+        const auto it = begin();
+        return it != end() ? *it : null;
+    }
+
+    /**
+     * @brief Returns the last entity of the view, if any.
+     * @return The last entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const noexcept {
+        auto it = view->rbegin();
+        for(const auto last = view->rend(); it != last && !contains(*it); ++it) {}
+        return it == view->rend() ? null : *it;
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const noexcept {
+        return contains(entt) ? iterator{view->find(entt), view->end(), opaque_check_set(), filter_as_array()} : end();
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     * @param entt A valid identifier.
+     * @return The components assigned to the given entity.
+     */
+    [[nodiscard]] decltype(auto) operator[](const entity_type entt) const {
+        return get(entt);
+    }
+
+    /**
+     * @brief Checks if a view is properly initialized.
+     * @return True if the view is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return view != nullptr;
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entt A valid identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const noexcept {
+        return std::apply([entt](const auto *...curr) { return (curr->contains(entt) && ...); }, pools)
+               && std::apply([entt](const auto *...curr) { return (!curr->contains(entt) && ...); }, filter);
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the view results in
+     * undefined behavior.
+     *
+     * @tparam Type Types of components to get.
+     * @param entt A valid identifier.
+     * @return The components assigned to the entity.
+     */
+    template<typename... Type>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        if constexpr(sizeof...(Type) == 0) {
+            return std::apply([entt](auto *...curr) { return std::tuple_cat(curr->get_as_tuple(entt)...); }, pools);
+        } else if constexpr(sizeof...(Type) == 1) {
+            return (storage<index_of<Type>>().get(entt), ...);
+        } else {
+            return std::tuple_cat(storage<index_of<Type>>().get_as_tuple(entt)...);
+        }
+    }
+
+    /**
+     * @brief Returns the components assigned to the given entity.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the view results in
+     * undefined behavior.
+     *
+     * @tparam First Index of a component to get.
+     * @tparam Other Indexes of other components to get.
+     * @param entt A valid identifier.
+     * @return The components assigned to the entity.
+     */
+    template<std::size_t First, std::size_t... Other>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        if constexpr(sizeof...(Other) == 0) {
+            return storage<First>().get(entt);
+        } else {
+            return std::tuple_cat(storage<First>().get_as_tuple(entt), storage<Other>().get_as_tuple(entt)...);
+        }
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a set of references to non-empty components. The
+     * _constness_ of the components is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &...);
+     * void(Type &...);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        pick_and_each(func, std::index_sequence_for<Get...>{});
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ a view.
+     *
+     * The iterable object returns a tuple that contains the current entity and
+     * a set of references to its non-empty components. The _constness_ of the
+     * components is as requested.
+     *
+     * @return An iterable object to use to _visit_ the view.
+     */
+    [[nodiscard]] iterable each() const noexcept {
+        return {internal::extended_view_iterator{begin(), pools}, internal::extended_view_iterator{end(), pools}};
+    }
+
+    /**
+     * @brief Combines two views in a _more specific_ one (friend function).
+     * @tparam OGet Component list of the view to combine with.
+     * @tparam OExclude Filter list of the view to combine with.
+     * @param other The view to combine with.
+     * @return A more specific view.
+     */
+    template<typename... OGet, typename... OExclude>
+    [[nodiscard]] auto operator|(const basic_view<get_t<OGet...>, exclude_t<OExclude...>> &other) const noexcept {
+        return std::make_from_tuple<basic_view<get_t<Get..., OGet...>, exclude_t<Exclude..., OExclude...>>>(
+            std::apply([](auto *...curr) { return std::forward_as_tuple(*curr...); }, std::tuple_cat(pools, other.pools, filter, other.filter)));
+    }
+
+private:
+    std::tuple<Get *...> pools;
+    std::tuple<Exclude *...> filter;
+    const base_type *view;
+};
+
+/**
+ * @brief Single component view specialization.
+ *
+ * Single component views are specialized in order to get a boost in terms of
+ * performance. This kind of views can access the underlying data structure
+ * directly and avoid superfluous checks.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New elements are added to the storage.
+ * * The entity currently pointed is modified (for example, components are added
+ *   or removed from it).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the pool iterated by the view in any way
+ * invalidates all the iterators and using them results in undefined behavior.
+ *
+ * @tparam Get Type of storage iterated by the view.
+ */
+template<typename Get>
+class basic_view<get_t<Get>, exclude_t<>, std::void_t<std::enable_if_t<!component_traits<typename Get::value_type>::in_place_delete>>> {
+    template<typename, typename, typename>
+    friend class basic_view;
+
+public:
+    /*! @brief Underlying entity identifier. */
+    using entity_type = typename Get::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = std::size_t;
+    /*! @brief Common type among all storage types. */
+    using base_type = typename Get::base_type;
+    /*! @brief Random access iterator type. */
+    using iterator = typename base_type::iterator;
+    /*! @brief Reversed iterator type. */
+    using reverse_iterator = typename base_type::reverse_iterator;
+    /*! @brief Iterable view type. */
+    using iterable = decltype(std::declval<Get>().each());
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_view() noexcept
+        : pools{},
+          filter{} {}
+
+    /**
+     * @brief Constructs a single-type view from a storage class.
+     * @param ref The storage for the type to iterate.
+     */
+    basic_view(Get &ref) noexcept
+        : pools{&ref},
+          filter{} {}
+
+    /**
+     * @brief Constructs a single-type view from a storage class.
+     * @param ref The storage for the type to iterate.
+     */
+    basic_view(std::tuple<Get &> ref, std::tuple<> = {}) noexcept
+        : pools{&std::get<0>(ref)},
+          filter{} {}
+
+    /**
+     * @brief Returns the leading storage of a view.
+     * @return The leading storage of the view.
+     */
+    [[nodiscard]] const base_type &handle() const noexcept {
+        return storage();
+    }
+
+    /**
+     * @brief Returns the storage for a given component type.
+     * @tparam Type Type of component of which to return the storage.
+     * @return The storage for the given component type.
+     */
+    template<typename Type = typename Get::value_type>
+    [[nodiscard]] decltype(auto) storage() const noexcept {
+        static_assert(std::is_same_v<std::remove_const_t<Type>, typename Get::value_type>, "Invalid component type");
+        return storage<0>();
+    }
+
+    /**
+     * @brief Returns the storage for a given index.
+     * @tparam Index Index of the storage to return.
+     * @return The storage for the given index.
+     */
+    template<std::size_t Index>
+    [[nodiscard]] decltype(auto) storage() const noexcept {
+        return *std::get<Index>(pools);
+    }
+
+    /**
+     * @brief Returns the number of entities that have the given component.
+     * @return Number of entities that have the given component.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return handle().size();
+    }
+
+    /**
+     * @brief Checks whether a view is empty.
+     * @return True if the view is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return handle().empty();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the view.
+     *
+     * The returned iterator points to the first entity of the view. If the view
+     * is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity of the view.
+     */
+    [[nodiscard]] iterator begin() const noexcept {
+        return handle().begin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the view. Attempting to dereference the returned iterator results in
+     * undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the view.
+     */
+    [[nodiscard]] iterator end() const noexcept {
+        return handle().end();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity of the reversed view.
+     *
+     * The returned iterator points to the first entity of the reversed view. If
+     * the view is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first entity of the reversed view.
+     */
+    [[nodiscard]] reverse_iterator rbegin() const noexcept {
+        return handle().rbegin();
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity of the reversed
+     * view.
+     *
+     * The returned iterator points to the entity following the last entity of
+     * the reversed view. Attempting to dereference the returned iterator
+     * results in undefined behavior.
+     *
+     * @return An iterator to the entity following the last entity of the
+     * reversed view.
+     */
+    [[nodiscard]] reverse_iterator rend() const noexcept {
+        return handle().rend();
+    }
+
+    /**
+     * @brief Returns the first entity of the view, if any.
+     * @return The first entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type front() const noexcept {
+        return empty() ? null : *begin();
+    }
+
+    /**
+     * @brief Returns the last entity of the view, if any.
+     * @return The last entity of the view if one exists, the null entity
+     * otherwise.
+     */
+    [[nodiscard]] entity_type back() const noexcept {
+        return empty() ? null : *rbegin();
+    }
+
+    /**
+     * @brief Finds an entity.
+     * @param entt A valid identifier.
+     * @return An iterator to the given entity if it's found, past the end
+     * iterator otherwise.
+     */
+    [[nodiscard]] iterator find(const entity_type entt) const noexcept {
+        return contains(entt) ? handle().find(entt) : end();
+    }
+
+    /**
+     * @brief Returns the identifier that occupies the given position.
+     * @param pos Position of the element to return.
+     * @return The identifier that occupies the given position.
+     */
+    [[nodiscard]] entity_type operator[](const size_type pos) const {
+        return begin()[pos];
+    }
+
+    /**
+     * @brief Returns the component assigned to the given entity.
+     * @param entt A valid identifier.
+     * @return The component assigned to the given entity.
+     */
+    [[nodiscard]] decltype(auto) operator[](const entity_type entt) const {
+        return storage().get(entt);
+    }
+
+    /**
+     * @brief Checks if a view is properly initialized.
+     * @return True if the view is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return std::get<0>(pools) != nullptr;
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entt A valid identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const noexcept {
+        return handle().contains(entt);
+    }
+
+    /**
+     * @brief Returns the component assigned to the given entity.
+     *
+     * @warning
+     * Attempting to use an entity that doesn't belong to the view results in
+     * undefined behavior.
+     *
+     * @tparam Type Type or index of the component to get.
+     * @param entt A valid identifier.
+     * @return The component assigned to the entity.
+     */
+    template<typename... Type>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        if constexpr(sizeof...(Type) == 0) {
+            return storage().get_as_tuple(entt);
+        } else {
+            static_assert((std::is_same_v<std::remove_const_t<Type>, typename Get::value_type> && ...), "Invalid component type");
+            return storage().get(entt);
+        }
+    }
+
+    /*! @copydoc get */
+    template<std::size_t Index>
+    [[nodiscard]] decltype(auto) get(const entity_type entt) const {
+        return storage().get(entt);
+    }
+
+    /**
+     * @brief Iterates entities and components and applies the given function
+     * object to them.
+     *
+     * The function object is invoked for each entity. It is provided with the
+     * entity itself and a reference to the component if it's a non-empty one.
+     * The _constness_ of the component is as requested.<br/>
+     * The signature of the function must be equivalent to one of the following
+     * forms:
+     *
+     * @code{.cpp}
+     * void(const entity_type, Type &);
+     * void(typename Type &);
+     * @endcode
+     *
+     * @note
+     * Empty types aren't explicitly instantiated and therefore they are never
+     * returned during iterations.
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        if constexpr(is_applicable_v<Func, decltype(*each().begin())>) {
+            for(const auto pack: each()) {
+                std::apply(func, pack);
+            }
+        } else if constexpr(ignore_as_empty_v<typename Get::value_type>) {
+            for(size_type pos{}, last = size(); pos < last; ++pos) {
+                func();
+            }
+        } else {
+            for(auto &&component: storage()) {
+                func(component);
+            }
+        }
+    }
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ a view.
+     *
+     * The iterable object returns a tuple that contains the current entity and
+     * a reference to its component if it's a non-empty one. The _constness_ of
+     * the component is as requested.
+     *
+     * @return An iterable object to use to _visit_ the view.
+     */
+    [[nodiscard]] iterable each() const noexcept {
+        return storage().each();
+    }
+
+    /**
+     * @brief Combines two views in a _more specific_ one (friend function).
+     * @tparam OGet Component list of the view to combine with.
+     * @tparam OExclude Filter list of the view to combine with.
+     * @param other The view to combine with.
+     * @return A more specific view.
+     */
+    template<typename... OGet, typename... OExclude>
+    [[nodiscard]] auto operator|(const basic_view<get_t<OGet...>, exclude_t<OExclude...>> &other) const noexcept {
+        return std::make_from_tuple<basic_view<get_t<Get, OGet...>, exclude_t<OExclude...>>>(
+            std::apply([](auto *...curr) { return std::forward_as_tuple(*curr...); }, std::tuple_cat(pools, other.pools, other.filter)));
+    }
+
+private:
+    std::tuple<Get *> pools;
+    std::tuple<> filter;
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Type Type of storage classes used to create the view.
+ * @param storage The storage for the types to iterate.
+ */
+template<typename... Type>
+basic_view(Type &...storage) -> basic_view<get_t<Type...>, exclude_t<>>;
+
+/**
+ * @brief Deduction guide.
+ * @tparam Get Types of components iterated by the view.
+ * @tparam Exclude Types of components used to filter the view.
+ */
+template<typename... Get, typename... Exclude>
+basic_view(std::tuple<Get &...>, std::tuple<Exclude &...> = {}) -> basic_view<get_t<Get...>, exclude_t<Exclude...>>;
+
+} // namespace entt
+
+#endif