//@ The difference to a reference is that `Box` implies full ownership: Once you drop
//@ the box (i.e., when the entire `Callbacks` instance is dropped), the content it
//@ points to on the heap will be deleted.
//@ The difference to a reference is that `Box` implies full ownership: Once you drop
//@ the box (i.e., when the entire `Callbacks` instance is dropped), the content it
//@ points to on the heap will be deleted.