The SetClassWord function replaces the 16-bit (word) value at the specified offset into the extra class memory for the window class to which the specified window belongs.
The GCW_ values supported by 16-bit Windows are obsolete. You must use the SetClassLong function to set the class values previously set by using the GCW_ values with the SetClassWord function.
WORD SetClassWord(
HWND hWnd, // handle of window
int nIndex, // index of value to change
WORD wNewWord // new value
);
If the function succeeds, the return value is the previous value of the specified 16-bit integer. If the value was not previously set, the return value is zero.
If the function fails, the return value is zero. To get extended error information, call GetLastError.
Reserve extra class memory by specifying a nonzero value in the cbClsExtra member of the WNDCLASS structure used with the RegisterClass function.
Window Classes Overview, Window Class Functions, GetClassWord, RegisterClass, SetClassLong, WNDCLASS