c++ - Pointers to virtual member functions. How does it work?

Question

Consider the following C++ code:

class A
{
public:
      virtual void f()=0;
};


int main()
{
     void (A::*f)()=&A::f;
}

If I'd have to guess, I'd say that &A::f in this context would mean "the address of A's implementation of f()", since there is no explicit seperation between pointers to regular member functions and virtual member functions. And since A doesn't implement f(), that would be a compile error. However, it isn't.

And not only that. The following code:

void (A::*f)()=&A::f;
A *a=new B;            // B is a subclass of A, which implements f()
(a->*f)();

will actually call B::f.

How does it happen?

Answer 1

It works because the Standard says that's how it should happen. I did some tests with GCC, and it turns out for virtual functions, GCC stores the virtual table offset of the function in question, in bytes.

struct A { virtual void f() { } virtual void g() { } }; 
int main() { 
  union insp { 
    void (A::*pf)();
    ptrdiff_t pd[2]; 
  }; 
  insp p[] = { { &A::f }, { &A::g } }; 
  std::cout << p[0].pd[0] << " "
            << p[1].pd[0] << std::endl;
}

That program outputs 1 5 - the byte offsets of the virtual table entries of those two functions. It follows the Itanium C++ ABI, which specifies that.