Lua5.4.md

Lua 5.4

Lua 5.4 allows you to annotate local variables to either make them const, or const and closing. Closing variables when they go out of scope will call their __close metamethod.

The binding now handles __close in addition to __gc, by resetting the shared pointer, effectively releasing that refcount for the object. This allows you to have more deterministic releasing of resources, without having to force garbage collections.

Example

Say you have a binding for a texture class which looks after allocated memory on the GPU in addition to normal ram. You want this to be cleaned up ASAP after use.

function setCaption(dialogbox, text)
 local texture <close> = generateTextureFromText(text)
 dialogbox.setCaptionTexture(texture)
end

Once this function finishes - normally without the <close> annotation, the texture variable becomes garbage and eventually it's __gc metamethod called which will destroy the shared pointer - reducing the ref count.

However with the <close> annotation, at the end of the function as 'texture' goes out of scope, it's __close metamethod will be called. This calls std::shared_ptr<T>.reset() which will reduce the ref count. This happens as soon as the function finishes. Later, the __gc metamethod will destroy an empty (pointing at nullptr) shared pointer.