cast class member functions to unsigned long (to obtain their addresses in code)

memfn_cast.cpp

template<class T, class R, class... As>
constexpr unsigned long memfn_cast(R(T::*mfnp)(As...))
{
  union {
    unsigned long u_ul;
    R(T::*u_mfnp)(As...);
  } addr;
   
  addr.u_mfnp = mfnp;
  return addr.u_ul;
}