visual c++,mfc program to handle window messages..

                   /* TO HANDLE WINDOW MESSAGES*/
#include<afxwin.h>
class myframe:public CFrameWnd
{
public:
    myframe()
    {
        Create(0,"HELLO");
    }

    void OnLButtonDown()
    {
        MessageBox("HELLO","GOOD MORNING");
    }

    DECLARE_MESSAGE_MAP()
};

BEGIN_MESSAGE_MAP(myframe,CFrameWnd)
ON_WM_LBUTTONDOWN()
END_MESSAGE_MAP()

class myapp:public CWinApp
{
    virtual BOOL InitInstance()
    {
        m_pMainWnd=new myframe();
        m_pMainWnd->ShowWindow(1);
        m_pMainWnd->UpdateWindow();
        return TRUE;
    }
};
myapp app;

Default handlers for standard Windows messages (WM_) are predefined in class CWnd. The class library bases names for these handlers on the message name. For example, the handler for the WM_PAINT message is declared in CWnd as:

afx_msg void OnPaint();
The afx_msg keyword suggests the effect of the C++ virtual keyword by distinguishing the handlers from other CWnd member functions. Note, however, that these functions are not actually virtual; they are instead implemented through message maps. Message maps depend solely on standard preprocessor macros, not on any extensions to the C++ language. The afx_msg keyword resolves to white space after preprocessing.
To override a handler defined in a base class, simply define a function with the same prototype in your derived class and to make a message-map entry for the handler. Your handler "overrides" any handler of the same name in any of your class's base classes.
In some cases, your handler should call the overridden handler in the base class so the base class(es) and Windows can operate on the message. Where you call the base-class handler in your override depends on the circumstances. Sometimes you must call the base-class handler first and sometimes last. Sometimes you call the base-class handler conditionally, if you choose not to handle the message yourself. Sometimes you should call the base-class handler, then conditionally execute your own handler code, depending on the value or state returned by the base-class handler.