Win32 C++ Create a Window and Procedure Within a Class

Question

Pre-Text/ Question

I am trying to make a fairly simple tool to help debug variable values. For it to be completely self contained within the class i

Answer 1

To use a non-static class method as a window procedure requires a dynamically-allocated thunk, which is an advanced technique that I will not get into it here.

The alternative is to declare the class method as static, then it will work as a window procedure. Of course, being a static method, it can no longer access non-static class members without an instance pointer. To get that pointer, you can have the class pass its this pointer to the lpParam parameter of CreateWindow/Ex(), then the window procedure can extract that pointer from the WM_NCCREATE message and store it in the window using SetWindowLong/Ptr(GWL_USERDATA). After that, subsequent messages can retrieve that pointer using GetWindowLong/Ptr(GWL_USERDATA) and thus be able to access non-static members of that object. For example:

class viewvars
{
private:
    HWND hWindow;
    LRESULT WindowProc(UINT message, WPARAM wParam, LPARAM lParam);

    static LRESULT CALLBACK ThisWindowProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam);
public:
    int CreateTestWindow(HINSTANCE hInst);
};

int viewvars::CreateTestWindow(HINSTANCE hInst)
{ 
    WNDCLASS wincl;

    if (!GetClassInfo(hInst, thisClassName, &wincl))
    {
        ...
        wincl.hInstance = hInst;
        wincl.lpszClassName = thisClassName;
        wincl.lpfnWndProc = &ThisWindowProc;

        if (RegisterClass(&wincl) == 0)
            return -1;
    }

    hWindow = CreateWindow(..., hInst, this);
    if (hWindow == NULL)
        return -1;

    ...

    MSG msg;
    while (GetMessage(&msg, hWindow, 0, 0))
    {
        TranslateMessage(&msg);  
        DispatchMessage(&msg); 
    }

    DestroyWindow(hWindow);
    hWindow = NULL;

    return msg.wParam;
}

LRESULT CALLBACK viewvars::ThisWindowProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    viewvars *view;

    if (message == WM_NCCREATE)
    {
        CREATESTRUCT *cs = (CREATESTRUCT*) lParam; 
        view = (viewvars*) cs->lpCreateParams;

        SetLastError(0);
        if (SetWindowLongPtr(hwnd, GWL_USERDATA, (LONG_PTR) view) == 0)
        {
            if (GetLastError() != 0)
                return FALSE;
        }
    }
    else
    {
        view = (viewvars*) GetWindowLongPtr(hwnd, GWL_USERDATA);
    }

    if (view)
        return view->WindowProc(message, wParam, lParam);

    return DefWindowProc(hwnd, message, wParam, lParam);
}

LRESULT viewvars::WindowProc(UINT message, WPARAM wParam, LPARAM lParam)
{
    // you can access non-static members in here...

    switch (message)
    {
        case WM_PAINT: 
            return 0;

        case WM_DESTROY:
            PostQuitMessage(0);       
            return 0;

        default:
            return DefWindowProc(hWindow, message, wParam, lParam);
    }
}