virtual.cpp

#include <iostream>

using namespace std;

void
base_function (void *const this_obj)
{
  std::cout << "b" << std::endl;
}

void
derived_function (void *const this_obj)
{
  std::cout << "d" << std::endl;
}

struct base
{
  struct VFT
  {
    void (*virtual_function) (void *const this_obj) = nullptr;
  };

  VFT *vft;
};

struct derived : public base
{
  int r;
};

base::VFT * construct_base_vft ()
{
  static base::VFT * vtf = new base::VFT ();
  vtf->virtual_function = base_function;
  return vtf;
}

base::VFT * construct_derived_vft ()
{
  static base::VFT * vtf = new base::VFT ();
  vtf->virtual_function = derived_function;
  return vtf;
}

base *
construct_base ()
{
  base *obj = new base ();
  obj->vft = construct_base_vft ();
  return obj;
}

derived *
construct_derived ()
{
  derived *obj = new derived ();
  obj->vft = construct_derived_vft ();
  return obj;
}

void one_point(void* base_ptr) {
  reinterpret_cast<base*>(base_ptr)->vft->virtual_function(base_ptr);
}

int main ()
{
  derived* d = construct_derived();
  base* b = construct_base();
  
  one_point(b);
  one_point(d);
  
  delete d;
  delete b;

  delete construct_base_vft();
  delete construct_derived_vft();
  return 0;
}