Returns the negative offset from the value of the vtable pointer of the vtable slot that holds the base offset (from the beginning of a
D object to the start of the base that
f is defined in).
D is a class type and
f is a nonstatic member
function defined in a virtual base class of
D, in other words
The base offset is the
this adjustment necessary when making a call to
f with a pointer to a
The offset returns a positive number that then has to be subtracted from the value of the vtable pointer.