Elegant solution for pointer-to-a virtual member function to function pointer

Hello,

I would like to see a cleaner solution than the one I have written in
the example below for passing a pointer-to-a-virtual member function of
an abstract class to a function pointer.

/*
  * Suppose this is the interface provided as API by an external library
  * written in C, containing the pointer to a function to be called by
  * the library itself.
  */
struct S {
     void *o;
     void (*callback)(S* p);
};

#include <iostream.h>

/*
  * This is my abstract class with the virtual member function.
  */
class A {
public:
     virtual void f() = 0;
     static void wrapper(S* p) {((A*)(p->o))->f();};
};

/*
  * These are derived classes.
  */
class B : public A {
     void f() {cout << "B::f() called\n";};
};
class C : public A {
     void f() {cout << "C::f() called\n";};
};

S s;
A* a;

int main(int argc, char**argv) {

     // dynamic linking
     if (argc > 1)
         a = new(B);
     else
         a = new(C);

     s.callback = A::wrapper;
     s.o = a;

     //...

     s.callback(&s); // dynamic behaviour in the library

     return 0;
}