#include <iostream>

struct A{
  typedef void (*callback_type)(void*, int);
  callback_type callback;
  void* user_data;
  void set_callback(callback_type cb, void* ud){
     callback = cb; user_data = ud;
  }
  void invoke(){ callback(user_data, 42); }
};

struct B{
  static void cb_foo(void* vself, int data){
    B* self = static_cast<B*>(vself);
    self->foo(data);
  }
  void foo(int data){ std::cout << data * 2 << "\n"; }
};

struct C{
  static void cb_bar(void* vself, int data){
    C* self = static_cast<C*>(vself);
    self->bar(data);
  }
  void bar(int data){ std::cout << data / 2 << "\n"; }
};

int main(){
  A a;
  B b;
  a.set_callback(&B::cb_foo, &b);
  a.invoke();
  C c;
  a.set_callback(&C::cb_bar, &c);
  a.invoke();
}