repclass.cpp

#include <iostream>

class my_class
{
public:
  void setter(int a)
  {
    _some_member = a;
  }

  int getter() const
  {
    return _some_member;
  }

private:
  int _some_member = 42;
};

using setter_fn_t = void (void *_this, int a);
using getter_fn_t = int (const void *_this);

class replacement
{
public:
  void setter(int a)
  {
    _set(reinterpret_cast<void*>(this), a);
  }

  int getter() const
  {
    return _get(reinterpret_cast<const void*>(this));
  }

private:
  static constexpr setter_fn_t *_set = reinterpret_cast<setter_fn_t*>(&my_class::setter);
  static constexpr getter_fn_t *_get = reinterpret_cast<getter_fn_t*>(&my_class::getter);
};

int main()
{
  my_class instance;

  replacement *rep = reinterpret_cast<replacement*>(&instance);

  std::cout << rep->getter() << std::endl;
  rep->setter(1337);
  std::cout << rep->getter() << std::endl;
}