In entries where the term memberFxn is used, you must write your own member function for a derived CWnd class. You can give these functions any name you like. Other functions, such as OnActivate, are member functions of the CWnd base class that, if called, pass the message to the DefWindowProc Windows function. If you wish to process Windows notification messages, you must override the corresponding CWnd function in your derived class. Your function should call the overridden function in your base class so that the base class(es), and Windows, can operate on the message.
In all cases you must put the function prototype in the CWnd-derived class header, and you must code the message map entry as shown. See Chapter 14 of the Class Libraries User's Guide cookbook for message map examples.
The term id is any user-defined menu item ID (WM_COMMAND messages) or control ID (child window notification messages). The terms message and wNotifyCode are the Windows message IDs as defined in WINDOWS.H. The term nMessageVariable is the name of a variable that contains the return value from the RegisterWindowMessage Windows function. It must be declared NEAR.