If the message loop is the heart of the program, the window procedure is the brain. This is where all the messages that are sent to our window get processed.
LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARA M lParam)
return DefWindowProc(hwnd, msg, wParam, lParam);
The window procedure is called for each message, the HWND parameter is the handle of your window, the one that the message applies to. This is important since you might have two or more windows of the same class and they will use the same window procedure (WndProc()). The difference is that the parameter hwnd will be different depending on
which window it is. For example when we get the WM_CLOSE message we destroy the window. Since we use the window handle that we received as the first paramter, any other windows will not be affected, only the one that the message was intended for.
WM_CLOSE is sent when the user presses the Close Button or types Alt-F4. This will cause the window to be destroyed by default, but I like to handle it explicitly, since this is the perfect spot to do cleanup checks, or ask the user to save files etc. before exiting the program. When we call DestroyWindow() the system sends the WM_DESTROY message to the window getting destroyed, in this case it's our window, and then destroys any remaining child windows before finally removing our window from the system. Since this is the only window in our program, we are all done and we want the program to exit, so we call
PostQuitMessage(). This posts the WM_QUIT message to the message loop. We never receive this message, because it causes GetMessage() to return FALSE, and as you'll see in our message loop code, when that happens we stop processing messages and return the final result code, the wParam of WM_QUIT which happens to be the value we passed into PostQuitMessage(). The return value is only really useful if your program is designed to be called by another program and you want to return a specific value.