get function ptr to member function of instanced class?

Question

class gfx { 
    void resize(int x, int y);
}

gfx g;    

can i cast g.resize to a 'void (*)(int, int)' somehow?

Answer 1

No. gfx::resize is of type void(gfx::*)(int, int). You can't meaningfully convert it to type void(*)(int, int) because you can only call it as a member function on an object of type gfx.

A member function can only be called on an instance of the class, so given your gfx g;, you can call g.resize(), but you can't just call resize() like it were an ordinary function. An ordinary function pointer can't point to a member function because it doesn't have any way to bind the function call to an instance of the class.

Conceptually, a member function takes an additional parameter, the this parameter, that points to the instance of the class on which it was called.

If you want to be able to call a member function via an ordinary function pointer, you can create a nonmember function (or a static member function) wrapper with a parameter to which you can pass the instance of the object on which to call the member function. For example, you could have:

void resize(gfx* obj, int x, int y) {
    return obj->resize(x, y);
}

This nonmember resize function has type void(*)(gfx*, int, int) and can be called as a nonmember function. The generalized form of this approach is the std::function and std::bind facilities found in C++0x (you can also find them in Boost and in C++ TR1).

Answer 2

Yes, you can do it:

void easy_resize(int x, int y) { g.resize(x, y); }