C++友元friend

1普通函数声明友元函数

#include<iostream>
using namespace std;
class Timer
{
	public:
		Timer(int,int,int);
		friend void display(Timer& t);
	private:
		int h;
		int m;
		int s;
};
Timer::Timer(int a,int b,int c)
{
	h=a;
	m=b;
	s=c;
}
void display(Timer& t)
{
	cout<<t.h<<endl;
	cout<<t.m<<endl;
	cout<<t.s<<endl;
}
int main()
{
	Timer timer(12,34,56);
	display(timer);
	return 0;
}

2成员函数声明友元函数

#include<iostream>
using namespace std;
class Date;
class Time
{
	public:
		Time(int,int,int);
		void display(Date& d);
	private:
		int hour;
		int min;
		int sec;
	
};
class Date
{
	public:
		Date(int,int,int);
		friend void Time::display(Date& d);
	private:
		int year;
		int mon;
		int day;
};
Time::Time(int a,int b,int c)
{
	hour=a;
	min=b;
	sec=c;
}
Date::Date(int a,int b,int c)
{
	year=a;
	mon=b;
	day=c;
}
void Time::display(Date& d)
{
	cout<<d.year<<"  "<<d.mon<<d.day<<endl;
	cout<<hour<<min<<sec<<endl;	
}
int main()
{
	Time t1(11,22,33);
	Date d1(44,55,66);
	t1.display(d1);
	return 0;
}

3友元类

#include<iostream>
using namespace std;
class Date;
class Time
{
	public:
		Time(int,int,int);
		void display(Date& d);
	private:
		int hour;
		int min;
		int sec;
	
};
class Date
{
	public:
		Date(int,int,int);
		friend class Time;
	private:
		int year;
		int mon;
		int day;
};
Time::Time(int a,int b,int c)
{
	hour=a;
	min=b;
	sec=c;
}
Date::Date(int a,int b,int c)
{
	year=a;
	mon=b;
	day=c;
}
void Time::display(Date& d)
{
	cout<<d.year<<"  "<<d.mon<<d.day<<endl;
	cout<<hour<<min<<sec<<endl;	
}
int main()
{
	Time t1(11,22,33);
	Date d1(44,55,66);
	t1.display(d1);
	return 0;
}