將例3.13中的Time類聲明爲Date類的友元類（第三章第十一題）

#include <iostream>
using namespace std;
class Time;
class Date
{public:
Date(int,int,int);
friend Time;
private:
int month;
int day;
int year;
};
Date::Date(int m,int d,int y):month(m),day(d),year(y){ }
class Time
{public:
Time(int,int,int);
void display(const Date &);
private:
int hour;
int minute;
int sec;
};
Time::Time(int h,int m,int s):hour(h),minute(m),sec(s){ }
void Time::display(const Date &d)
{
cout<<d.month<<"/"<<d.day<<"/"<<d.year<<endl;
cout<<hour<<":"<<minute<<":"<<sec<<endl;
}
int main()
{
Time t1(10,13,56);
Date d1(12,25,2004);
t1.display(d1);
return 0;
}