Using Extra Fields in Window Class Structure

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 3.1
• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5 and 3.51
      - Microsoft Windows 95 version 4.0
  

In order to generate several child windows of the same class, each having its own set of static variables and independent of the sets of the variables in the sibling windows, you need to use the cbWndExtra field in WNDCLASS, the window-class data structure, when registering a window; then, use SetWindowWord() (or Long) and GetWindowWord() (or Long). These functions will either get or set additional information about the window identified by hWnd.

Use positive offsets as indexes to access any additional bytes that were allocated when the window class structure was created, starting at zero for the first byte of the extra space. Similarly, if you want to refer to bytes already defined by Windows within the structure, use offsets defined with the GWW and GWL prefixes.