How to use WndProc as a class function [duplicate]

Answer 1

Unfortunately you cannot use a class function as a wndproc because as the compiler tries to tell you the calling convention differs, even though the two functions have the same signature, a class function expects the this pointer to be passed to it. On 64 bit builds it will expect it to be in the RCX/ECX registry while on 32 bit builds it will expect the this pointer to be the last argument pushed on the stack. The window code won't do that when calling your WndProc essentially turning this into a function call on a garbage pointer.

What you can do is make a static method that does something like the following:

LRESULT Client::CreateMen(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
    // The OS makes sure GWLP_USERDATA is always 0 before being initialized by the application
    Client* client = (Client*)GetWindowLongPtr(hwnd, GWLP_USERDATA);

    if(msg == WM_INIT)
    {
        client = new Client();
        SetWindowLongPtr(hwnd, GWLP_USERDATA, client);
    }

    if(msg == WM_DESTROY)
    {
        client = (Client*)GetWindowLongPtr(hwnd, GWLP_USERDATA, client);
        SetWindowLongPtr(hwnd, GWLP_USERDATA, NULL);
        delete client;
        client = NULL;
    }

    if(client)
    {
        // Do stuff with the client instance
    }

    return DefWindowProc(hwnd, msg, wparam, lparam);
}

I haven't tested this, so it might have some bugs, but let me know if you have any problems with it and I'll refine it if need be.