友员函数：friend

#include<iostream>
using namespace std;
class Time;
class Data
{
	public:
	Data(int d):day(d){}
	friend void display(Data &,Time &);
	private:
	int day;
};
class Time
{
	public:
	Time(int h):hour(h){}
	friend void display(Data &,Time &);
	private:
	int hour;
};
void display(Data &d,Time &t)
{
	cout<<"data:"<<d.day<<"time:"<<t.hour<<endl;
}

int main()
{
	Time t(12);
	Data d(1);
	display(d,t);
	return 0;
}