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目录

一、基本概念

二、全局函数做友元

三、类做友元

四、成员函数做友元

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一、基本概念

目的：让一个函数或者类，访问另一个类中的私有成员

关键字：friend

• 友元的三种实现

1. 全局函数做友元
2. 类做友元
3. 成员函数做友元

二、全局函数做友元

代码示例

#include<iostream>
using namespace std;class Building{// 告诉编译器，goodgay全局变量是类Building 的好朋友，可以访问私有内容friend void goodgay(Building * building);public:Building(){keting = "客厅";badroom = "卧室";}
public:string keting;private:string badroom;};void goodgay(Building * building){cout<<"您的好友正在访问您的"<< "  " <<building->keting<<endl;cout<<"您的好友正在访问您的"<< "  " <<building->badroom<<endl;
}void test(){Building b;goodgay(&b);
}int main(){test();
}

三、类做友元

代码示例 

#include<iostream>
using namespace std;class Building;
class goodgay{public:goodgay();void vis();	 // 参观函数，访问Building中的属性private:Building * building;
};class Building{//告诉编译器，类goodgay是类Building 的好朋友，可以访问私有内容friend class goodgay; 
public:Building();
public:string keting;
private:string badroom;
};Building::Building(){ // 类外写成员函数this->keting = "客厅";this->badroom = "卧室";
}goodgay::goodgay(){ // 类外写成员函数building = new Building; //创建建筑物对象
}void goodgay::vis(){cout<<"您的好友正在访问您的"<<building->keting <<endl;cout<<"您的好友正在访问您的"<<building->badroom <<endl;
}void test(){goodgay gg;gg.vis();}
int main(){test();
}

四、成员函数做友元

代码示例 

#include<iostream>
using namespace std;class Building;class goodgay{
public:goodgay();void vis();void vis2();
private:Building * building;
};class Building{//告诉编译器，类goodgay中的成员变量vis是类Building 的好朋友，可以访问私有内容friend void goodgay::vis(); public:Building();
public:string keting;
private:string badroom;
};//       类外实现成员函数
Building::Building(){this->keting = "客厅";this->badroom = "卧室";
}goodgay::goodgay(){building = new Building;
}
void goodgay::vis(){cout<<"您的好友vis正在访问您的"<<building->keting<<endl;cout<<"您的好友vis正在访问您的"<<building->badroom<<endl;
}void goodgay::vis2(){cout<<"您的好友vis2正在访问您的"<<building->keting<<endl;
//	cout<<"您的好友vis正在访问您的"<<building->badroom<<endl;
}void test(){goodgay gg;gg.vis();gg.vis2();
}
int main(){test();}