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__vcall_offsetof_vfunc(D, f)

Where D is a class and f is a virtual function defined in D, or a base class of D.

The function returns the offset of the slot in the vtable that holds the pointer to the virtual function, f.

If __vcall_offsetof_vfunc(D, f) is used when f is not a virtual member function it returns an arbitrary value designed to cause an assembly error if used.