forked from Green-Sky/tomato
100 lines
4.4 KiB
Markdown
100 lines
4.4 KiB
Markdown
|
# 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.
|