The system maintains a WNDCLASSEX structure internally for each window class in the system. When an application registers a window class, it can direct the system to allocate and append a number of additional bytes of memory to the end of the WNDCLASSEX structure. This memory is called extra class memory and is shared by all windows belonging to the class. Use the extra class memory to store any information pertaining to the class.
Because extra memory is allocated from the system's local heap, an application should use extra class memory sparingly. The RegisterClassEx function fails if the amount of extra class 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 class memory.
The SetClassWord and SetClassLong functions copy a value to the extra class memory. To retrieve a value from the extra class memory, use the GetClassWord and GetClassLong functions. The cbClsExtra member of the WNDCLASSEX structure specifies the amount of extra class memory to allocate. An application that doesn't use extra class memory must initialize the cbClsExtra member to zero.