C 友元（friend

友元的作用在让另一个函数或类，可以访问私有成员，有三种形式

• 全局函数作友元
• 类作友元
• 成员函数作友元

全局函数作友元

#include <iostream>
#include <string>
using namespace std;
class Room{
    friend void toSleep();//声明全局函数toSleep可以返回Room私有函数及变量
    //私有函数
    private:void bedRoom(){
        cout << "in bedroom" << endl;
    }
};
void toSleep(){
    Room room;
    room.bedRoom();//可以正常调用
}
int main()
{
    toSleep();
    return 0;
}

类作友元

#include <iostream>
#include <string>
using namespace std;
class Room{
    friend class People;//声明People为友元
    //私有函数
    private:void bedRoom(){
        cout << "in bedroom" << endl;
    }
};
class People{
    public:void sleep(){
        Room r;
        r.bedRoom();
    }
};

类成员函数作友元

#include <iostream>
#include <string>
using namespace std;
class People;
class Room;
class People{
    public:void sleep();
};
class Room{
    friend void People::sleep();
    void bedRoom();
};
void People::sleep(){
    Room room;
    room.bedRoom();
}
void Room::bedRoom(){
    cout << "in bedroom" << endl;
}
int main()
{
    People p;
    p.sleep();
    return 0;
}