Why does the CreateWindow() function want the WindowClass's name member and not a pointer to the class itself?

Question

I'm reading Luna's Introduction to 3D Game Programming with DirectX 11. Having always programmed for the Linux command line, I decided to start by reading Appendix A, a win32 programming primer, and I don't understand a certain behavior of the CreateWindow() function. Its first parameter is the name of the window class you want to create - so you first have to declare a window class, then "register" it (which I assume means adding the class to some class stack somewhere in the mysterious win32 API), and then you pass the window class's lpszClassName member to the function, like this:

WNDCLASS wc;
//set all the various members of wc
wc.lpszClassName = L"BasicWndClass";
RegisterClass(&wc);
ghMainWindow = CreateWindow(L"BasicWndClass", L"LOL", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, 0, 0, instanceHandle, 0);

I don't understand why the last line isn't something along the lines of

ghMainWindow = CreateWindow(&wc, L"LOL", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, 0, 0, instanceHandle, 0);

Is there some historical or practical reason I'm not aware of?

EDIT: Also, is it bad practice to do something like this?

ghMainWindow = CreateWindow(wc.lpszClassName, L"LOL", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, 0, 0, instanceHandle, 0);

Answer 1

Because the CreateWindow and the RegisterClass calls are generally not in the same module. CreateWindow is an application-level call, RegisterClass is a library-level call. The canonical examples are the "Edit" and "Listbox" classes, buried inside the OS. Using a string or an atom is a very simple way to avoid having to rely on a implementation dependent structure.

Compare WNDCLASS vs WNDCLASSEX to see why that's a good idea.

Answer 2

RegisterClass or RegisterClassEx return an ATOM that Window's uses internally. So what you are passing on is an internal reference to that ATOM in the name, also you can create multiple similar windows without having to re-do the RegisterClass part. Sometimes, people also do the RegisterClass and CreateWindow(Ex) calls in different parts of the code.

Your edited question about using wc.lpszClassName is technically fine, if you change the name in the WNDCLASS section, you won't have issues, though I use an std::string, and assign that to wc.lpszClassName = string.c_str(); then CreateWindow(string.c_str(), ... );

Hope this helps :)