Global Subclassing

To globally subclass a window class, the application must have a handle to a window of the class. The application also needs the handle to remove the subclass. To get the handle, an application typically creates a hidden window of the class to be subclassed. After obtaining the handle, the application calls the SetClassLong function, specifying the handle, the GCL_WNDPROC flag, and the address of the subclass procedure. SetClassLong returns the address of the original window procedure for the class.

The original window procedure address is used in global subclassing in the same way it is used in instance subclassing. The subclass procedure passes messages to the original window procedure by calling CallWindowProc. The application removes the subclass from the window class by calling SetClassLong again, specifying the address of the original window procedure, the GCL_WNDPROC flag, and the handle to a window of the class being subclassed. An application that globally subclasses a control class must remove the subclass when the application terminates; otherwise, an unrecoverable system error may occur.

Global subclassing has the same limitations as instance subclassing, plus some additional restrictions. An application should not use the extra bytes for either the class or the window instance without knowing exactly how the original window procedure uses them. If the application must associate data with a window, it should use window properties.

An application must not globally subclass a system global class. An unrecoverable system error may occur if more than one application globally subclasses a control class. If the application could benefit from globally subclassing a control class, use the technique called superclassing.

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