EnTT: Push EnTT across boundaries

EnTT 3.16.0

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Push EnTT across boundaries

# Table of Contents

- Working across boundaries
  - Smooth until proven otherwise
  - Meta context
  - Memory management

# 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.
Fortunately, nowadays EnTT works smoothly across boundaries.

## Smooth until proven otherwise

Many classes in EnTT make extensive use of type erasure for their purposes. This raises the need to identify objects whose type has been erased.
The type_hash class template is how identifiers are generated and thus made available to the rest of the library. In general, this class arouses little interest. The only exception is when a conflict between identifiers occurs (definitely uncommon though) or when the default solution proposed by EnTT is not suitable for the user's purposes.
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 are there to import or export symbols, so as to make everything work nicely across boundaries.
On the other hand, everything should run smoothly when working with plugins or shared libraries that do not export any symbols.

For those who need more details, the test suite contains many examples covering the most common cases (see the lib directory for all details).
It goes without saying that it is 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.
Since it is linked already to a static context to which the elements are attached and different contexts do not 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:

auto handle = entt::locator<entt::meta_ctx>::handle();
entt::locator::handle
static node_type handle() noexcept
Returns a handle to the underlying service.
Definition locator.hpp:124

Then, it is passed to the receiving space that sets it as its default context, thus discarding or storing aside the local one:

entt::locator<entt::meta_ctx>::reset(handle);
entt::locator::reset
static void reset(const node_type &other={}) noexcept
Resets or replaces a service.
Definition locator.hpp:134

From now on, both spaces refer to the same context and to it are all new meta types attached, no matter where they are created.
Note that replacing the main context does not also propagate changes across boundaries. In other words, replacing 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.
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.
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 is 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.
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.