When an MDI application registers a window class, it may reserve extra space in the window structure for application data specific to this particular class of windows. To store and retrieve data in this extra space, the application uses the GetWindowWord, SetWindowWord, GetWindowLong, and SetWindowLong functions.
To maintain a large amount of data for a child window, an application can allocate memory for a data structure and then store the handle of the memory containing the structure in the extra space associated with the child window.