Storing a function pointer as a member within a class - c++

Question

I created this account to ask a question because I'm pretty lost right now and I couldn't find an answer. I'm not sure if what I'm trying to do is even possible, or perhaps I'm taking an incorrect approach.
I'm writing a windows socket wrapper class for asynchronous sockets, mostly so I never have to write this code again, and I'm trying to store a function pointer as a member variable inside my netcode class that points to a windows event handler that's called inside the application's normal win32 message pump that points elsewhere. Basically, I'm trying to create a programmer-defined callback function so that when I compile this netcode class into a .dll I never have to touch it again.
As a clarification, what I'm trying to do is, for example, write the event handler for WM_SOCKET (which is a windows message I have defined) somewhere other than inside the network code class (so it can be portable to different win32/C++ projects) and outside of the main win32 message pump (so my WndProc() doesn't become absolutely massive).
I don't even know what to search for, but all of the information I'm finding is about creating a pointer to a member function or a member of a template, which isn't what I want.
Here's the code I've already tried to write (only the relevant bits), which is what is giving me problems. I'm probably royally screwing something up here.
//// Sockets.h
class NetworkConnection
{
private:
void (*WMSocketFunction)(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam);
public:
void HandleEvents(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam);
void registerSocketFunction( void (*SocketFunction(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)));
};
//// Sockets.cpp
#include "Sockets.h"
void NetworkConnection::registerSocketFunction( void (*SocketFunction(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)))
{
WMSocketFunction = SocketFunction;
};
void NetworkConnection::HandleEvents(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
WMSocketFunction(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam);
}
//// Application.cpp
void SocketEventHandler(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
// Do socket stuff here
};
int SetupApp()
{
NetworkConnection NetConnection;
NetConnection.registerSocketFunction(&SocketEventHandler);
return 0;
}
LRESULT CALLBACK d3d::WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch( msg )
{
case WM_SOCKET:
{
NetConnection.HandleEvents(hwnd, msg, wParam, lParam);
break;
}
case WM_DESTROY:
{
ReleaseDevices();
break;
}
case WM_KEYDOWN:
{
#ifdef _DEBUG
if (lParam == VK_ESCAPE)
PostQuitMessage(0);
#endif
}
}
return ::DefWindowProc(hwnd, msg, wParam, lParam);
}
I'd be really grateful for some help, here. I totally have no clue why this isn't working.
The errors I receive are:
Error 1 error C2440: '=' : cannot convert from 'void *(__cdecl *)(HWND,UINT,WPARAM,LPARAM)' to 'void (__cdecl *)(HWND,UINT,WPARAM,LPARAM)' sockets.cpp (in function registerSocketFunction)
Error 2 error C2275: 'HWND' : illegal use of this type as an expression sockets.cpp (in function HandleEvents)
Error 3 error C2146: syntax error : missing ')' before identifier 'hwnd' sockets.cpp (in function HandleEvents)
Error 4 error C2059: syntax error : ')' sockets.cpp (in function HandleEvents)
Error 8 error C2664: 'NetworkConnection::registerSocketFunction' : cannot convert parameter 1 from 'void (__cdecl *)(HWND,UINT,WPARAM,LPARAM)' to 'void *(__cdecl *)(HWND,UINT,WPARAM,LPARAM)' application.cpp
If you need any better explanation, because I've likely explained this problem poorly, please don't hesitate to ask.
I'm using Visual Studio 2010, so I don't think I can use C++/11 features like std::function.

Answer 1

Exclude arguments names from function pointer declaration
Use typedef to make code cleaner
typedef void(*WMSocketFunction_Ptr)(HWND, UINT, WPARAM, LPARAM);
class NetworkConnection
{
private:
WMSocketFunction_Ptr WMSocketFunction;
public:
void HandleEvents(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam);
void registerSocketFunction(WMSocketFunction_Ptr SocketFunction);
};
void NetworkConnection::registerSocketFunction(WMSocketFunction_Ptr SocketFunction)
{
WMSocketFunction = SocketFunction;
};
Prefer to use STL facilities to naked pointers: std::function of boost::function
typedef std::function<void(HWND, UINT, WPARAM, LPARAM)> WMSocketFunction_Ptr;

Answer 2

You have to enclose the function pointer name in brackets like this: void (*SocketFunction)(...)
So instead of your method declaration:
void registerSocketFunction( void (*SocketFunction(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)));
You have to write:
void registerSocketFunction( void (*SocketFunction)(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam));
Also rewrite your method body to this:
void NetworkConnection::registerSocketFunction( void (*SocketFunction)(HWND, UINT, WPARAM, LPARAM))
{
WMSocketFunction = SocketFunction;
};
Next thing, i think your HandleEvents is unfinished yet, you have a syntax error there, i think you wanted to call the function behind the pointer.
void NetworkConnection::HandleEvents(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
WMSocketFunction(hwnd,msg,wParam,lParam);
}

Answer 3

Here is my suspicion. I think the way you've declared the function argument SocketFunction for registerSocketFunction is wrong.
void NetworkConnection::registerSocketFunction( void (*SocketFunction(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)))
should be,
void NetworkConnection::registerSocketFunction( void (*SocketFunction)(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam))
I ran a similar test on GCC, and it threw a similar error, so this might be your problem.