Best method for storing this pointer for use in WndProc

Question

I'm interested to know the best / common way of storing a this pointer for use in the WndProc. I know of several approaches, but each as I understand it have their own drawbacks. My questions are:

What different ways are there of producing this kind of code:

CWindow::WndProc(UINT msg, WPARAM wParam, LPARAM) {   this->DoSomething(); } 

I can think of Thunks, HashMaps, Thread Local Storage and the Window User Data struct.

What are the pros / cons of each of these approaches?

Points awarded for code examples and recommendations.

This is purely for curiosities sake. After using MFC I've just been wondering how that works and then got to thinking about ATL etc.

Edit: What is the earliest place I can validly use the HWND in the window proc? It is documented as WM_NCCREATE - but if you actually experiment, that's not the first message to be sent to a window.

Edit: ATL uses a thunk for accessing the this pointer. MFC uses a hashtable lookup of HWNDs.

Answer 1

While using the SetWindowLongPtr and GetWindowLongPtr to access the GWL_USERDATA might sound like a good idea, I would strongly recommend not using this approach.

This is the exactly the approached used by the Zeus editor and in recent years it has caused nothing but pain.

I think what happens is third party windows messages are sent to Zeus that also have their GWL_USERDATA value set. One application in particular was a Microsoft tool that provied an alternative way to enter Asian characters in any windows application (i.e. some sort of software keyboard utility).

The problem is Zeus always assumes the GWL_USERDATA data was set by it and tries to use the data as a this pointer, which then results in a crash.

If I was to do it all again with, what I know now, I would go for a cached hash lookup approach where the window handle is used as the key.

Answer 2

In your constructor, call CreateWindowEx with "this" as the lpParam argument.

Then, on WM_NCCREATE, call the following code:

SetWindowLongPtr(hwnd, GWLP_USERDATA, (LONG_PTR) ((CREATESTRUCT*)lParam)->lpCreateParams); SetWindowPos(hwnd, 0, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE | SWP_NOZORDER); 

Then, at the top of your window procedure you could do the following:

MyWindowClass *wndptr = (MyWindowClass*) GetWindowLongPtr(hwnd, GWL_USERDATA); 

Which allows you to do this:

wndptr->DoSomething(); 

Of course, you could use the same technique to call something like your function above:

wndptr->WndProc(msg, wparam, lparam); 

... which can then use its "this" pointer as expected.