Extra Window Memory

Windows maintains an internal data structure for each window. When registering a window class, an application can specify a number of additional bytes of memory, called extra window memory. When creating a window of the class, Windows allocates and appends the specified amount of extra window memory to the end of the window's structure. An application can use this memory to store window-specific data.

Because extra memory is allocated from the system's local heap, an application should use extra window memory sparingly. With operating system version 4.0 or later, the RegisterClassEx function fails if the amount of extra window memory requested is greater than 40 bytes. If an application requires more than 40 bytes, it should allocate its own memory and store a pointer to the memory in the extra window memory.

The SetWindowWord and SetWindowLong functions copy a value to the extra memory. The GetWindowWord and GetWindowLong functions retrieve a value from the extra memory. The cbWndExtra member of the WNDCLASSEX structure specifies the amount of extra window memory to allocate. An application that doesn't use the memory must initialize cbWndExtra to zero.

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