C++ 习题 输出日期时间--友元类

/*Description设计一个日期类和时间类，编写display函数用于显示日期和时间。要求：将Time类声明为Date类的友元类，通过Time类中的display函数引用Date类对象的私有数据，输出年、月、日和时、分、秒。Input年、月、日和时、分、秒Output年、月、日和时、分、秒Sample Input2013 12 23 14 23 50Sample Output2013/12/23 14:23:50*/#include <iostream>using namespace std;class Date;class Time{private:    int hour,minute,second;public:    Time(int h,int mi,int se):hour(h),minute(mi),second(se) {}    void display(Date& D);};class Date{private:    int year,month,day;public:    Date(int y,int m,int d):year(y),month(m),day(d) {}    friend class Time ;};void Time::display(Date& D){    cout<<D.year<<"/"<<D.month<<"/"<<D.day<<" "<<hour<<":"<<minute<<":"<<second;}int main(){    int year,month,day;    cin>>year>>month>>day;    Date d1(year,month,day);    int hour,minute,second;    cin>>hour>>minute>>second;    Time t1(hour,minute,second);    t1.display(d1);    return 0;}