Since ostream is a class, ostream& is a reference to an ostream class. Note that we’re also taking an ostream& as a parameter. ostream is typically passed by reference because we don’t want to make a copy of it as we pass it around.

int x = 11;

int& ref()
{
return x;
}

int main()
{
int *ptr = &ref(); //after the return by reference
//takes place, &ref() is &x

*ptr = 12;
cout < < x << endl;

return 0;
}

how is it possible?

this should work now

class Date
{
private:
    int m_nMonth;
    int m_nDay;
    int m_nYear;

    Date() { } // private default constructor

public:
    Date(int nMonth, int nDay, int nYear)
    {
        SetDate(nMonth, nDay, nYear);
    }

    void SetDate(int nMonth, int nDay, int nYear)
    {
        m_nMonth = nMonth;
        m_nDay = nDay;
        m_nYear = nYear;
    }

    friend Date& operator+ ( const Date& date1, const Date& date2);
    friend ostream& operator<< (ostream& out, const Date& date1);

    void PrintMonth()  {cout<<m_nMonth<<endl;}

    int GetMonth() const { return m_nMonth; }
    int GetDay() const { return m_nDay; }
    int GetYear() const { return m_nYear; }
};

Date& operator+ (const  Date& date1, const Date& date2)
{
	return Date(date1.m_nMonth+date2.m_nMonth, date1.m_nDay+date2.m_nDay, date1.m_nYear+date2.m_nYear);
}

ostream& operator<< (ostream& out, const Date& date1)
{
	out<<date1.m_nYear<<"/"<<date1.m_nMonth<<"/"<<date1.m_nDay;
	return out;
}

int _tmain(int argc, _TCHAR* argv[])
{
	Date date1(11,12,2009);
	Date date2(1,13,2009);
	//error,i wonder why, even the following format is alos wrong..cout<<(date1+date2)
	cout<<date1+date2;
	Date date3 = date1 + date2;
	//right
	cout<<endl<<date3;
	return 0;
}