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

Here is way too much information about member function pointers. There's some stuff about virtual functions under "The Well-Behaved Compilers", although IIRC when I read the article I was skimming that part, since the article is actually about implementing delegates in C++.

http://www.codeproject.com/KB/cpp/FastDelegate.aspx

The short answer is that it depends on the compiler, but one possibility is that the member function pointer is implemented as a struct containing a pointer to a "thunk" function which makes the virtual call.