Every window has an associated data structure. The window data structure contains all the information specified for the window at the time it was created and any additional information supplied for the window since that time. Although the exact size and meaning of the information in the window data structure are private to the system, an application can access any of the following data items via system-provided functions:

• Pointer to window-instance data structure
  
• Pointer to window procedure
  
• Parent-window handle
  
• Owner-window handle
  
• Handle of first child window
  
• Handle of next sibling window
  
• Window size and position (expressed as a rectangle)
  
• Window style
  
• Window identifier
  
• Update-region handle
  
• Message-queue handle

An application can examine and modify this data by using functions such as WinQueryWindowUShort and WinSetWindowUShort. These functions let an application access data that is stored as 16-bit integers. Other functions let an application access data containing 32-bit integers and pointers. Several functions indirectly affect the data items in the window data structure. For example, the WinSubclassWindow function replaces the window-procedure pointer, and the WinSetWindowPos function changes the size and position of the window.

An application can extend the number of available data items in the window data structure by specifying a count of extra bytes when it registers the corresponding window class. Then, the window procedure can use these bytes to store information about the window. The WinQueryWindowUShort and WinSetWindowUShort functions give direct access to the extra bytes.

It generally is not a good idea to use direct storage in the window data. It is better to allocate a data structure dynamically and set a pointer to that data structure in the window words. This provides two advantages:

• Most importantly, it is a symbolic way of referencing the data structure. It is very easy to make mistakes and provide the wrong offsets to WinQueryWindowUShort and so forth.

• You now can add and remove fields without cross dependencies because you now use symbolic references; whereas, when you use the technique of putting window words directly in the window data structure, you have to account for changed offsets.