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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