When an application creates a window, the system allocates a block of memory for storing information specific to the window, including the address of the window procedure that processes messages for the window. When the system needs to pass a message to the window, it searches the window-specific information for the address of the window procedure and passes the message to that procedure.
Subclassing is a technique that allows an application to intercept and process messages sent or posted to a particular window before the window has a chance to process them. By subclassing a window, an application can augment, modify, or monitor the behavior of the window. An application can subclass a window belonging to a system global class, such as an edit control or a list box. For example, an application could subclass an edit control to prevent the control from accepting certain characters. However, you cannot subclass a window or class that belongs to another application. All subclassing must be performed within the same process.
An application subclasses a window by replacing the address of the window's original window procedure with the address of a new window procedure, called the subclass procedure. Thereafter, the subclass procedure receives any messages sent or posted to the window.
The subclass procedure can take three actions upon receiving a message: it can pass the message to the original window procedure, modify the message and pass it to the original window procedure, or process the message and not pass it to the original window procedure. If the subclass procedure processes a message, it can do so before, after, or both before and after it passes the message to the original window procedure.
The system provides two types of subclassing: instance and global. In instance subclassing, an application replaces the window procedure address of a single instance of a window. An application must use instance subclassing to subclass an existing window. In global subclassing, an application replaces the address of the window procedure in the WNDCLASS structure of a window class. All subsequent windows created with the class have the address of the subclass procedure, but existing windows of the class are not affected.