How to make a callback class (non-MFC)

[FrozenKnight]

it took me a while to get this completely working but I eventually figured it out, this works with Visual C++ 2008 express I haven't tested it with other compilers.

First is the template I made to get this whole thing working.

Code

GeSHi (cpp):
template <class T, LRESULT (T::*Callback)(HWND,UINT,WPARAM,LPARAM)>
LRESULT CALLBACK ClassWindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
        T *wndClass = 0;
        switch (uMsg){
                case WM_NCCREATE:
                        wndClass = reinterpret_cast<T*>(reinterpret_cast<CREATESTRUCT*>(lParam)->lpCreateParams);
                        SetWindowLongPtr(hWnd, GWLP_USERDATA, reinterpret_cast<LONG_PTR>(wndClass));
                        break;
                default: 
                        wndClass = reinterpret_cast<T*>(GetWindowLongPtr(hWnd, GWLP_USERDATA));
        }
        return (wndClass->*Callback)(hWnd,uMsg,wParam,lParam);
}
Created by GeSHI 1.0.7.18

At first glance this may be a bit confusing, but trust me it isn't that hard to get working.
here is an example of how to use it.

Code

GeSHi (cpp):
wc.style                        = CS_HREDRAW | CS_VREDRAW | CS_OWNDC;.
wc.lpfnWndProc                  = ClassWindowProc<MyClass,&MyClass::MyCallBack>; //MyClass is the name of your class, &MyClass::MyCallBack is the CallBack function in your class.
wc.cbClsExtra                   = 0;
wc.cbWndExtra                   = 0;
wc.hInstance                    = hInstance;
wc.hIcon                        = LoadIcon(NULL, IDI_WINLOGO);
wc.hCursor                      = LoadCursor(NULL, IDC_ARROW);
wc.hbrBackground                = NULL;
wc.lpszMenuName                 = NULL;
wc.lpszClassName                = "MyWin";
 
RegisterClass(&wc)
 
hWnd=CreateWindowEx(dwExStyle,
                        "MyWin",
                        title,
                        dwStyle |
                        WS_CLIPSIBLINGS |
                        WS_CLIPCHILDREN,
                        0, 0,
                        WindowRect.right-WindowRect.left,
                        WindowRect.bottom-WindowRect.top,
                        NULL,	
                        NULL,	
                        hInstance,
                        reinterpret_cast<LPVOID>(this)); //pass a this pointer to WM_CREATE and WM_NCCREATE
 
Created by GeSHI 1.0.7.18

I placed comments at the important sections in the code, you must create the window inside a function in your class for this to work correctly, . Some of you will notice that it is possible to do outside the class but i won't discuss how to do that here.

now for an explanation of how this works.
First is the template,

Code

GeSHi (cpp):
ClassWindowProc<MyClass,&MyClass::MyCallBack>
Created by GeSHI 1.0.7.18

The first parameter is your class name the second is the address of your classes callback function. (It is possible to have two windows going using one class, as long as they use different callbacks, if you do, do so with caution, it is possible to mess up the class address and crash the program)
the template uses the GWLP_USERDATA of the desired window to store a pointer to the class (which you pass from the last parameter of CreateWindow or CreateWindowEx also known as lpParam) this pointer is then used to rout all calls to the member function you specify.

You must also specify the template ClassWindowProc as your callback function in WNDCLASS.lpfnWndProc when you register the WNDCLASS structure. Hope this makes things easier for all of you who wanted to use classes for your windows. 

[Joel]

Good idea, I use friend functions in the WinProc just to avoid the unnecessary of the usage of "static member" for it.