A structure containing a weak reference to a Object.
A `GWeakRef` can either be empty (i.e. point to null), or point to an object for as long as at least one "strong" reference to that object exists. Before the object's dispose method is called, every WeakRef associated with becomes empty (i.e. points to null ).
Like Value, WeakRef can be statically allocated, stack- or heap-allocated, or embedded in larger structures.
Unlike weak_ref and add_weak_pointer, this weak reference is thread-safe: converting a weak pointer to a reference is atomic with respect to invalidation of weak pointers to destroyed objects.
If the object's dispose method results in additional references to the object being held (‘re-referencing’), any WeakRefs taken before it was disposed will continue to point to null. Any WeakRefs taken during disposal and after re-referencing, or after disposal has returned due to the re-referencing, will continue to point to the object until its refcount goes back to zero, at which point they too will be invalidated.
It is invalid to take a WeakRef on an object during dispose without first having or creating a strong reference to the object.