Using Window Extra Bytes in Custom Controls

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

SUMMARY

In the Microsoft Windows environment, custom controls are often designed to store data about their state, color, or position. This data can be stored directly in window extra bytes associated with the custom control window or in a local or a global memory object.

If a control uses window extra bytes directly, Windows allocates memory from the user heap, which is a scarce system resource shared by all applications. In contrast, the LocalAlloc function allocates memory from the heap of the application or the dynamic-link library (DLL) in which the control is implemented.

If six or fewer bytes of data are required for each instance of a custom control, the demand placed on the system user heap is not large. In contrast, each window in Windows version 3.0 requires at least 62 bytes of user heap; therefore, using a small number of window extra bytes is easily justified.

If the control requires more than six bytes of data, the control class should set the number of window extra bytes to the size of one HANDLE data type. When the control is created, call the LocalAlloc function to allocate enough memory to hold the data associated with the control. Store the handle that LocalAlloc returns in the window extra bytes.

MORE INFORMATION

Depending on the amount of storage required, storing a handle to a local memory block in the window extra bytes is usually advantageous. This method requires calling the LocalAlloc function once at initialization time and calling the LocalLock and LocalUnlock functions each time the control processes a message that refers to or modifies any status information. Calling LocalLock and LocalUnlock is much faster than calling GetWindowWord to access each item of data.

Storing a memory handle in the window extra bytes also provides the control the option of using a global memory block if a very large amount of data must be stored.

Using an allocated memory also allows the application to vary the amount of data stored for each custom control by storing the size of the structure or an index that describes the structure at the beginning of the block of memory.