WM_DESTROY not called inside wrapped WndProc

Question

I have adopted the typical solution you find out there in order to use the WNDPROC as an object method, but it looks like the WM_DESTROY message is not sent to the object window's own WNDPROC and the program does not exit after closing the window.

My window class looks like this (irrelevant code removed):

class MyWindow : MyApp
{
public:
    MyWindow();
    void Create(void);
    LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
private:
    HWND _hWnd;
};

void MyWindow::Create()
{
    // Here I register my class and call CreateWindowEx
    // Everything works fine so far

    // Part of the code where I assign the static WNDPROC
    WNDCLASSEXW wcex;

    wcex.cbSize = sizeof(WNDCLASSEX);
    wcex.style = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc = MyApp::WndProc;
    wcex.cbClsExtra = 0;
    wcex.cbWndExtra = 0;
    wcex.hInstance = this->Instance;
    wcex.hIcon = LoadIcon(this->Instance, MAKEINTRESOURCE(32512));
    wcex.hCursor = LoadCursor(nullptr, IDC_ARROW);
    wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wcex.lpszMenuName = NULL;
    wcex.lpszClassName = "MyWindowClass";
    wcex.hIconSm = LoadIcon(this->Instance, MAKEINTRESOURCE(32512));

    RegisterClassExW(&wcex);

    this->_hWnd = CreateWindowExW(
        WS_EX_TOOLWINDOW | WS_EX_TOOLWINDOW,
        wcex.lpszClassName,
        "Window Title",
        WS_POPUP,
        10, 10,
        600, 400,
        nullptr, 
        nullptr, 
        this->Instance,
        nullptr
    );

    ShowWindow(this->_hWnd, SW_SHOW);
    UpdateWindow(this->_hWnd);
}

LRESULT CALLBACK MyWindow::WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch (msg)
    {
    case WM_CREATE:
    {
        // If I place a MessageBox here, it shows up
    }
    break;
    case WM_DESTROY:
        // It never gets to this point

        // Decrease windows count
        this->WindowsCount--;

        PostQuitMessage(0);
        break;
    break;
    default:
        return DefWindowProc(hWnd, msg, wParam, lParam);
    }
    return 0;
}

And now a class which holds the static WNDPROC, which is assigned at creation

class MyApp
{
public:
    static HINSTANCE Instance;
    static int WindowsCount;

    MyApp();
    static LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
};

and implementation

LRESULT CALLBACK MyApp::WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    // Window object
    MyWindow* myWindow = NULL;

    if (msg == WM_CREATE) {
        myWindow = reinterpret_cast<MyWindow *>(((LPCREATESTRUCT)lParam)->lpCreateParams);
        SetWindowLongPtr(hWnd, GWLP_USERDATA, (LONG_PTR)myWindow);
    }
    else {
        myWindow = reinterpret_cast<MyWindow *>(GetWindowLongPtr(hWnd, GWLP_USERDATA));
    }

    // If window not identified, identify now
    if (myWindow) {
        return myWindow->WndProc(hWnd, msg, wParam, lParam);
    }

    // Call window object's processor
    return DefWindowProc(hWnd, msg, wParam, lParam);
}

The WM_CLOSE message is not caught either. I really do not understand why these messages are not passed on

Answer 1

You are setting the lpParam parameter of CreateWindowEx() to nullptr, so myWindow is always nullptr in MyApp::WndProc(), thus MyWindow::WndProc() is never called. You need to pass this instead of nullptr.

You are also not doing any error checking to make sure RegisterClassExW() and CreateWindowEx() succeed before calling ShowWindow()/UpdateWindow().

Also, consider using SetWindowSubclass() instead of (Get|Set)WindowLongPtr(GWLP_USERDATA). See Subclassing Controls on MSDN, and Raymond Chen's blog article on Safer Subclassing.