draw using mouse in visual studio 6

//save it as mouse.cpp in win32 c++
#include
#include "Mouse.h"

CMyApp myApp;

/////////////////////////////////////////////////////////////////////////
// CMyApp member functions

BOOL CMyApp::InitInstance ()
{
m_pMainWnd = new CMainWindow;
m_pMainWnd->ShowWindow (m_nCmdShow);
m_pMainWnd->UpdateWindow ();
return TRUE;
}

/////////////////////////////////////////////////////////////////////////
// CMainWindow message map and member functions

BEGIN_MESSAGE_MAP (CMainWindow, CFrameWnd)

ON_WM_LBUTTONDOWN()
ON_WM_MOUSEMOVE()
ON_WM_LBUTTONUP()

END_MESSAGE_MAP ()

CMainWindow::CMainWindow ()
{
Create (NULL, _T ("Testing Programs"));
}

void CMainWindow::OnLButtonDown(UINT flag,CPoint pt)
{
endpoint=startpoint=pt;
}

void CMainWindow::OnLButtonUp(UINT flag,CPoint pt)
{
CClientDC dc (this);
dc.SetROP2(R2_COPYPEN);
dc.MoveTo(startpoint);
dc.LineTo(endpoint);
}

void CMainWindow::OnMouseMove(UINT flag,CPoint pt)
{
CClientDC dc(this);
if(flag==MK_LBUTTON)
{
dc.SetROP2(R2_NOTXORPEN);
//erase line
dc.MoveTo(startpoint);
dc.LineTo(endpoint);

//draw line
dc.MoveTo(startpoint);
dc.LineTo(pt);
endpoint=pt;
}
}

//save it from as mouse.h in source filein visual studio 6
class CMyApp : public CWinApp
{
public:
virtual BOOL InitInstance ();
};

class CMainWindow : public CFrameWnd
{

private:
CPoint startpoint,endpoint;
public:
CMainWindow ();

protected:

afx_msg void OnLButtonDown (UINT,CPoint);
afx_msg void OnMouseMove(UINT,CPoint);
afx_msg void OnLButtonUp(UINT,CPoint);
DECLARE_MESSAGE_MAP ()
};