Using wndproc as a member function of a class

Question

I've been following this thread for how to use the wndproc function as a member function but I keep getting read access violations (it won't tell me the exact line in my program though, but it's at line 120 of xmemory0 const uintptr_t _Ptr_container = _Ptr_user[-1]; and it says Unhandled exception thrown: read access violation. _Ptr_user was 0x7. I'm probably doing something funky that I shouldn't be doing. Does anyone see anything evident?

Code:

LRESULT CALLBACK wndproc::WndProc(HWND hwnd, UINT msg,
    WPARAM wParam, LPARAM lParam) {
    wndproc* pThis; //wndproc is the class name
    bool checked = true;

    HWND text, button, selection1, selection2;
    //hwnd is parent window

    switch (msg) {

    case WM_CREATE: {
        pThis = static_cast<wndproc*>(reinterpret_cast<CREATESTRUCT*>(lParam)->lpCreateParams);
        SetWindowLongPtrW(hwnd, GWLP_USERDATA, reinterpret_cast<::LONG_PTR>(pThis));
        text = CreateWindow("STATIC", "Please select options from below:", WS_VISIBLE | WS_CHILD, 20, 20, 300, 25, hwnd, NULL, NULL, NULL);
        button = CreateWindow("BUTTON", "Submit", WS_VISIBLE | WS_CHILD | WS_BORDER, 500, 500, 80, 25, hwnd, (HMENU)0, NULL, NULL);
        selection1 = CreateWindow("button", "Scan?", WS_VISIBLE | WS_CHILD | BS_CHECKBOX | WS_OVERLAPPED, 500, 460, 350, 20, hwnd, (HMENU)1001, NULL, NULL);
        CheckDlgButton(hwnd, 1001, BST_CHECKED);
        LPCREATESTRUCT lpcs = reinterpret_cast<LPCREATESTRUCT>(lParam);
        vector<string> *strings = reinterpret_cast<vector<string>*>(lpcs->lpCreateParams);
        int j = 40;

        pThis->checkVectorSize = strings->size();
        for (int i = 1; i != strings->size() + 1; i++)
        {
            CreateWindowA("button", (*strings)[i - 1].c_str(),
                WS_VISIBLE | WS_CHILD | BS_CHECKBOX | WS_OVERLAPPED,
                20, j, 185, 35, hwnd, (HMENU)i,
                NULL, NULL);
            CheckDlgButton(hwnd, 0, BST_UNCHECKED);

            j = j + 30;
        }
        //SetWindowLongPtrW(hwnd, GWLP_USERDATA, reinterpret_cast<::LONG_PTR>(strings));

        break;
    }
    case WM_COMMAND: {
        pThis = reinterpret_cast<wndproc*>(GetWindowLongPtrW(hwnd, GWLP_USERDATA));
        int i = wParam;
        if (i == 0) //LOWORD(wParam)
        {
            for (int j = 0; j != pThis->checkVectorSize; j++)
            {
                if (IsDlgButtonChecked(hwnd, j + 1) == true)
                {
                    pThis->check.push_back(j);
                }
            }
            if (IsDlgButtonChecked(hwnd, 1001) == true)
            {
                pThis->scan = true;
            }
            else
            {
                pThis->scan = false;
            }
            PostMessage(hwnd, WM_CLOSE, 0, 0);
            break;
        }
        checked = IsDlgButtonChecked(hwnd, i);
        if (checked) {

            CheckDlgButton(hwnd, i, BST_UNCHECKED);
        }
        else {
            CheckDlgButton(hwnd, i, BST_CHECKED);
        }
        break;
    }
    case WM_DESTROY: {

        PostQuitMessage(0);
        break;
    }
    }
    return DefWindowProcW(hwnd, msg, wParam, lParam);
}

EDIT: Wrapper function:

void wndproc::select(vector<string>& return)
{
    HINSTANCE hInstance = GetModuleHandle(NULL);
    WNDCLASSW wc = { 0 };
    MSG  msg;

    wc.lpszClassName = L"Selection1";
    wc.hInstance = hInstance;
    wc.hbrBackground = CreateSolidBrush(RGB(0, 0, 255));
    wc.lpfnWndProc = WndProc;
    wc.hCursor = LoadCursor(0, IDC_ARROW);
    wc.style = CS_DBLCLKS;

    RegisterClassW(&wc);
    CreateWindowW(wc.lpszClassName, L"Selection",
        WS_OVERLAPPEDWINDOW | WS_VISIBLE | WS_SYSMENU,
        CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, 0, 0, hInstance, &return);

    while (GetMessage(&msg, NULL, 0, 0)) {

        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
}

Answer 1

Non-static member functions are not the same as a non-member function. Non-static member function needs an object to be called on, which non-member functions doesn't need.

If you don't use any other (non-static) member functions or member variables, you could make the function static and it should work.

---

Another possible solution, considering that you're passing "this" along to the function in other ways, is partially the same as the solution mentioned above, together with a second non-static member function.

Something like

class wndproc
{
    ...

    static LRESULT WndProcWrapper(...)
    {
        wndproc *pThis;

        // Set pThis...

        // Call *real* window procedure
        return pThis->WndProc(...);
    }

private:
    LRESULT WndProc(...)
    {
        ...
    }
};

Then make wndproc::WndProcWrapper the window procedure.