急，看不懂的常见提示（关于多继承），望高手指点

编译通过，建立时出错。错误提示如下：--------------------Configuration: 01 - Win32 Debug--------------------
Linking...
msvcrtd.lib(crtexe.obj) : error LNK2001: unresolved external symbol _main
Debug/01.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.01.exe - 2 error(s), 0 warning(s)
不知何意
源程序如下
#include <afxwin.h>class CHelloAppDlg : public CDialog, public CWinApp
{ 
public:
    CHelloAppDlg()
        { }    void* operator new(size_t nSize)
        { return CFrameWnd::operator new(nSize); }
    void operator delete(void* p)
        { CFrameWnd::operator delete(p); }    virtual BOOL InitInstance();    virtual BOOL OnInitDialog();
    afx_msg void OnPaint();
    DECLARE_MESSAGE_MAP()
};
BEGIN_MESSAGE_MAP(CHelloAppDlg, CDialog)
    ON_WM_PAINT()
END_MESSAGE_MAP()BOOL  CHelloAppDlg::OnInitDialog()
{
OnInitDialog();
return true;
}
void CHelloAppDlg::OnPaint()
{
    CPaintDC dc(this);
    CRect rect;
    GetClientRect(rect);
    CString s = "Hello, Windows!";
    dc.TextOut(rect.right / 2, rect.bottom / 2, s);
}BOOL CHelloAppDlg::InitInstance()
{
    DoModal();
    return TRUE;
}CHelloAppDlg theHelloAppAndDlg;