贺老师的教学链接  本课讲解


普通的友元函数

#include <iostream>
using namespace std;
class Time
{
public:
    Time(int,int,int);
    friend void display(Time &);
private:
    int hour;
    int minute;
    int sec;
};


Time::Time(int h,int m,int s)
{
    hour=h;
    minute=m;
    sec=s;
}


void display(Time& t)
{
    cout<<t.hour<<":"<<t.minute<<":"<<t.sec<<endl;
}


int main( )
{
    Time t1(10,13,56);
    display(t1);
    return 0;
}


友元成员函数——另一类中的成员函数作友元 
#include <iostream>
using namespace std;
class Date; //对Date类的提前引用声明
class Time
{
public:
    Time(int,int,int);
    void display(Date &);
private:
    int hour;
    int minute;
    int sec;
};


class Date
{
public:
    Date(int,int,int);
    friend void Time::display(Date &);
private:
    int month;
    int day;
    int year;
};


Time::Time(int h,int m,int s)
{
    hour=h;
    minute=m;
    sec=s;
}
void Time::display(Date &d)
{
    cout<<d.month<<"/"<<d.day<<"/"<<d.year<<endl;
    cout<<hour<<":"<<minute<<":"<<sec<<endl;
}


Date::Date(int m,int d,int y)
{
    month=m;
    day=d;
    year=y;
}


int main( )
{
    Time t1(10,13,56);
    Date d1(12,25,2004);
    t1.display(d1);
    return 0;
}


友元类 
class Date;  ///对Date类的提前引用声明
class Time
{
public:
    Time(int,int,int);
    void add(Date &);
    void display(Date &);
private:
    int hour;
    ……
};
void Time::add(Date &d)
{
    if(d.day>30) …
        if(d.month>12) …
        }
class Date
{
public:
    Date(int,int,int);
    friend class Time;
private:
    int month;
    ……
};
int main( )
{
    Time t1(23,59,32);
    Date d1(12,25,2004);
    t1.add(d1);
    t1.display(d1);
    return 0;
}