友元类，友元函数

友元是c++提供的一种破坏数据封装和数据隐藏特性的机制；

分为友元类和友元函数；

友元函数；

1、由关键字friend 修饰说明的非成员函数；

2、在函数体中可以通过类的对象访问私有成员和protect成员；

作用：增加灵活性，使程序员可以在封装和快速性方面做合理选择。

#include <iostream>
#include <cmath>
using namespace std; 
class Point    //Point类声明
{ 
        public:    //外部接口
        Point(int xx=0, int yy=0) {X=xx;Y=yy;}
        int GetX() {return X;}
        int GetY() {return Y;}
        friend float Distance(Point &a, Point &b); 
        private:    //私有数据成员
  　　　　 int X,Y;
};
 
double Distance( Point& a, Point& b)
{
  double dx=a.X-b.X;
  double dy=a.Y-b.Y;
  return sqrt(dx*dx+dy*dy);
}

int main()
{ Point p1(3.0, 5.0), p2(4.0, 6.0);
  double d=Distance(p1, p2);
  cout<<"The distance is "<<d<<endl;
  return 0;
}

友元类：

1、若一个类为另一个类的友元。则此类的所有成员都能访问对方类的私有成员（即谁是友元，它就可以访问对方类中的所有成员）

#include<iostream>
using namespace std;
class Date;
class Time;
class Date
{
    friend class Time;
    private:
    int year;
    int month;
    int day;
    public:
        void setdate(int year,int month,int day)
        {
            this->year = year;
            this->month = month;
            this->day = day;
        }
        void display();
};
void Date::display()
{
    cout<<year<<"."<<month<<"."<<day<<"\n";
}
class Time
{
    Date D2;
    int hour;
    int miniter;
    int second;
    public:
        Time(int hour,int miniter,int second);
        
void display()
{    D2.setdate(2016,12,13);
    D2.display();
    cout<<hour<<":"<<miniter<<":"<<second<<"\n";
}
};
        Time::Time(int hour,int miniter,int second)
        {
            this->hour = hour;
            this->miniter = miniter;
            this->second = second;
        }
int main()
{
Time T1(11,12,1);
T1.display();
}

将一个类中的成员函数声明为另一类的友元：

#include <iostream>
using namespace std;

class Date;

class Time
{
public:
    Time(int h,int m,int s):hour(h),minute(m),sec(s){};
    void display(Date &);
private:
    int hour;
    int minute;
    int sec;
    };

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

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

int main()
{
    Time t1(12,13,52);
    Date d1(2016,8,3);

    t1.display(d1);
    return 0;
    }