Detect the the vtable offset of a specific virtual function (using Visual C++)

Question

Can the vtable offset of a specific virtual function be inspected?

Why? I'd like to be able to detect unintentional binary compatibility breaks (see http://techbase.kde.org/Policies/Binary_Compatibility_Issues_With_C%2B%2B for what I mean by binary compatibility).

I'm aware of the undocumented and unsupported technique of "/d1reportSingleClassLayout" (http://blogs.msdn.com/b/vcblog/archive/2007/05/17/diagnosing-hidden-odr-violations-in-visual-c-and-fixing-lnk2022.aspx), and I plan to use this technique, but I'd like to also use some simple compile time or run time checks if possible.

Answer 1

Inspired by Jerry's answer, I managed to write up this function that can do the same thing for any function signature:

#include <iostream>

struct A
{
    virtual void a() {}
    virtual void b() {}
    virtual void c() {}
};

template <class T>
int SeeBits(T func)
{
    union
    {
        T ptr;
        int i;
    };
    ptr = func;

    return i;
}

int main()
{
    int i = SeeBits(&A::a);
    int j = SeeBits(&A::b);
    int k = SeeBits(&A::c);

    std::cout << i << " " << j << " " << k << std::endl;

    return 0;
}