Encapsulate Win32 in a class - pass class pointer in CreateWindow - windows

Question

I viewed many tutorials and posts on this subject, and they all say I pass the class instance pointer (this) in my CreateWindowEx() function, and then store it in the window procedure function, when the WM_NCCREATE message is sent. I guess this is because WM_NCCREATE is supposedly the first message that gets sent to the window procedure since a window is created.
A few questions/notes:
From debugging I came to know that actually WM_GETMINMAXINFO is the first sent message, before WM_NCCREATE (at least on my machine). Does this mean I should listen for this message instead of WM_NCCREATE?
According to this popular article, the reason everyone calls SetWindowLongPtr() after the message WM_NCCREATE is received is because
If the value does not exist by the time WM_NCCREATE is called then,
by some mysterious Windows behavior that I still don't understand, the
value never gets inserted.
I tried to do exactly that (that is, call SetWindowLongPtr() after CreateWindowEx()). It turns out to be just fine. The application runs okay. Below is my code, please tell me if there's something wrong with this approach.
void GLWin32::CreateWindow(...)
{
...
_hwnd = CreateWindowEx(NULL, _wndclassex.lpszClassName, title.c_str(), WS_OVERLAPPEDWINDOW, x, y, width, height, NULL, NULL, _hinstance, NULL);
SetWindowLongPtr(_hwnd, GWL_USERDATA, reinterpret_cast<LONG_PTR>(this));
...
}
//static window procedure for all instances of this class
LRESULT CALLBACK GLWin32::_msgRouter(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
LONG l = GetWindowLongPtr(hwnd, GWLP_USERDATA);
GLWin32* thisPtr = reinterpret_cast<GLWin32*>(l);
if (thisPtr)
return thisPtr->_winProc(msg, wparam, lparam);
else
return DefWindowProc(hwnd, msg, wparam, lparam);
}
LRESULT GLWin32::_winProc(UINT msg, WPARAM wparam, LPARAM lparam)
{
switch (msg)
{
case WM_CLOSE:
{
PostQuitMessage(0);
return 0;
}
}
return DefWindowProc(_hwnd, msg, wparam, lparam);
}
Why is my approach not used instead of the popular approach?

Answer 1

The problem with this approach is that if you want to use the instance when processing any of the window creation messages (including "ordinary" messages that are sent as part of the creation process), you won't have access to it.
Suppose you want to create a button when processing WM_CREATE (typical scenario) and you want to set the button text to some instance member value. You want something like:
LRESULT GLWin32::_winProc(UINT msg, WPARAM wparam, LPARAM lparam)
{
switch (msg)
{
case WM_CREATE:
{
CreateWindow("BUTTON", this->buttonText, WS_VISIBLE | WS_CHILD,
10, 10, 50, 30, this->hwnd, NULL, this->hInstance, NULL);
return 0;
}
}
return DefWindowProc(_hwnd, msg, wparam, lparam);
}
Problem is, when WM_CREATE is processed (before CreateWindowEx returns), SetWindowLongPtr wasn't called yet and the instance pointer is missing, so _winProc isn't being called at all.