You copied the Doc URL to your clipboard.

__mcall_offsetof_vbase(D, f)

Where D is a class type and f is a nonstatic member function defined in a virtual base class of D, in other words __mcall_is_in_vbase(D,f) returns {TRUE}.

__mcall_offsetof_vbase() 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).

The base offset is the this adjustment necessary when making a call to f with a pointer to a D.

Note

The offset returns a positive number that then has to be subtracted from the value of the vtable pointer.