Retrieves the widget’s allocation.
Note, when implementing a layout container: a widget’s allocation will be its “adjusted” allocation, that is, the widget’s parent typically calls size_allocate with an allocation, and that allocation is then adjusted (to handle margin and alignment for example) before assignment to the widget. get_allocation returns the adjusted allocation that was actually assigned to the widget. The adjusted allocation is guaranteed to be completely contained within the size_allocate allocation, however.
So a layout container is guaranteed that its children stay inside the assigned bounds, but not that they have exactly the bounds the container assigned.
a pointer to a Allocation to copy to