ID Number: Q76947
3.00
WINDOWS
Summary:
A Windows application can extend the behavior of a standard Windows
control by using the technique of superclassing. An application can
superclass a standard Windows control by retrieving its window class
information, modifying the fields of the WNDCLASS structure, and
registering a new class. For example, to associate status information
with each button control in an application, the buttons can be
superclassed to provide a number of window extra bytes.
This article describes a technique to access the WNDCLASS structure
associated with the standard "button" class.
More Information:
The following five steps are necessary to register a new class that
uses some information from the standard windows "button" class:
1. Call GetClassInfo() to fill the WNDCLASS structure.
2. Save the cbWndExtra value in a global variable.
3. Add the desired number of bytes to the existing cbWndExtra value.
4. Change the lpszClassName field.
5. Call RegisterClass() to register the new class.
The first step will fill the WNDCLASS structure with the data that was
used when the class was originally registered. In this example, the
second step is necessary so that when the "new" extra bytes are
accessed, the original extra bytes are not destroyed. Please note that
it is NOT safe to assume that the original cbWndExtra value was zero.
When accessing the "new" extra bytes, it is necessary to use the
original value of cbWndExtra as the base for any new data stored in
the extra bytes. The third step allocates the new extra bytes. The
fourth step specifies the new name of the class to be registered, and
the final step actually registers the new class.
Any new class created in this manner MUST have a unique class name.
Typically, this name would be similar but not identical to the
original class. For example, to superclass a button, an appropriate
class name might be "superbutton." There is no conflict with class
names used by other applications as long as the CS_GLOBALCLASS class
style is not specified. The standard Windows "button" class remains
unchanged and can still be used by the application as normal. In
addition, once a new class has been registered, any number of controls
can be created and destroyed with no extra coding effort. The
superclass is simply another class in the pool of classes that can be
used when creating a window.
The sample code below demonstrates this procedure:
BOOL DefineSuperButtonClass(void)
{
#define MYEXTRABYTES 8
HWND hButton;
WNDCLASS wc;
GetClassInfo(NULL, "button", (LPWNDCLASS)&wc);
iStdButtonWndExtra = wc.cbWndExtra; // Save this in a global
wc.cbWndExtra += MYEXTRABYTES;
lstrcpy((LPSTR)wc.lpszClassName, (LPSTR)"superbutton");
return(RegisterClass((LPWNDCLASS)&wc));
}
It is important to note that the lpszClassName, lpszMenuName, and
hInstance fields in the WNDCLASS structure are NOT returned by the
GetClassInfo() function. Please refer to page 4-153 of the "Microsoft
Windows Software Development Kit Reference Volume 1" for more
information. Also, each time a new class is registered, scarce system
resources are used. If it is necessary to alter many different
standard classes, the GetProp(), SetProp(), and RemoveProp() functions
should be used as an alternative approach to associating extra
information with standard Windows controls.