Extra Class Memory
Windows maintains a
WNDCLASSEX structure internally for each window class in the system. When an application
registers a window class, it can direct Windows 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.
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