c++ 友元

#include<iostream>using namespace std;class date;class time;class time{public:time(int ,int ,int );friend void display(date &d,time &t);private:int hour;int minute;int sec;};class date{public:date(int ,int ,int);friend void display(date &d,time &t);private:int month;int day;int year;};time::time(int h,int m,int s){hour=h;minute=m;sec=s;}date::date(int m ,int d ,int y){month=m;day=d;year=y;}void display(date &d,time &t){cout<<d.month<<"/"<<d.day<<"/"<<d.year<<endl;cout<<t.hour<<"/"<<t.minute<<"/"<<t.sec<<endl;}int main(){  date a(1,2,3);  time b(4,5,6);  display(a,b);return 0;}