C++ class member function pointer to function pointer

Solution 1:

No. A member function is not a free function. The type is entirely different, and a pointer to a member function (PTMF) is a completely different, incompatible object from a function pointer. (A PTMF is usually much bigger, for example.) Most importantly a pointer-to-member must always be used together with an instance pointer to the object whose member you want to call, so you cannot even use a PTMF the same way you use a function pointer.

The easiest solution for interacting with C code is to write a global wrapper function that dispatches your call, or to make your member function static (in which case it becomes essentially a free function):

// global!

Engine * myEngine;
int theCallback(lua_State * L)
{
  return myEngine->pcall_log(L);
}

Engine::Run()
{
  /* ... */
  myEngine = this;
  luabind::set_pcall_callback(&theCallback);
  /* ... */
}

The conceptual problem here is that you have an engine class, although you will practically only have one single instance of it. For a genuine class with many objects, a PTMF wouldn't make sense because you'd have to specify which object to use for the call, whereas your engine class perhaps is essentially a singleton class which could be entirely static (i.e. a glorified namespace).

Solution 2:

Not suitable for your LUA problem, but maybe on other libraries: If a function requests a a function pointer like func(void* param, ...) and you can ensure that the lifetime of your object is greater than the stored function pointer, then you could technically also use a method pointer (looks the same on the stack), but C++ prevents direct casting of method pointers to function pointers.

But with a little trick, you can also cast method pointers to function pointers:

template<typename M> inline void* GetMethodPointer(M ptr)
{
    return *reinterpret_cast<void**>(&ptr);
}

Using that, you can use method pointers for example with libmicrohttpd:

this->m_pDaemon = MHD_start_daemon(MHD_USE_THREAD_PER_CONNECTION, this->m_wPort, NULL, NULL, reinterpret_cast<MHD_AccessHandlerCallback>(GetMethodPointer(&CMyWebServer::AccessHandlerCallback)), this, MHD_OPTION_END);

But be aware of it. You must take care of the lifetime of that object. Also the calling conventions must match.