Chapter 9. Child Window Controls
Recall from Chapter 7 the programs in the CHECKER series. These programs display a grid of rectangles. When you click the mouse in a rectangle, the program draws an X. When you click again, the X disappears. Although the CHECKER1 and CHECKER2 versions of this program use only one main window, the CHECKER3 version uses a child window for each rectangle. The rectangles are maintained by a separate window procedure named ChildProc.
If we wanted to, we could add a facility to ChildProc to send a message to its parent window procedure (WndProc) whenever a rectangle is checked or unchecked. Here's how: The child window procedure can determine the window handle of its parent by calling GetParent,
hwndParent = GetParent ...