C++ 友元

友元

目的：就是让一个函数或者类访问一个类中的私有成员

友元的三种实现

• 全局函数做友元：声明全局函数是本类的朋友（友元）， 全局函数便可以访问类的私有成员
• 类做友元：声明别的类是本类的朋友（友元），可以访问本类的私有成员
• 成员函数做友元：声明别的类的函数为本类的朋友（友元），可以访问本类的私有成员

全局函数做友元

#include <iostream>
#include<string>

using namespace std;

class MyHouse {

    //声明全局函数是本类的友元， 全局函数便可以访问类的私有成员
    friend void goodFiendVisit(MyHouse& house);
    friend void goodFiendVisit2(MyHouse* house);
public:
    MyHouse() {
        settingRoom = "-客厅";
        bedRoom = "-卧室";
    }

public:
    string settingRoom;
private:
    string bedRoom;
};


//全局函数

void goodFiendVisit(MyHouse& house) {
    
    cout << "好朋友正在访问：" << house.settingRoom << endl;
    cout << "好朋友正在访问：" << house.bedRoom << endl;
}

void goodFiendVisit2(MyHouse* house) {

    cout << "好朋友正在访问：" << house->settingRoom << endl;
    cout << "好朋友正在访问：" << house->bedRoom << endl;
}


int main() {

    MyHouse house;
    goodFiendVisit(house);

    //MyHouse& h = house;
    //goodFiendVisit(h);
    


    MyHouse* hp = new MyHouse();
    goodFiendVisit2(hp);

    return 0;
}

类做友元

#include <iostream>
#include<string>

using namespace std;

class MyHouse;

class GoodFriend
{
public:
    MyHouse* house;
    GoodFriend();
    void visit();


    ~GoodFriend() {
        if (house != NULL) {
            delete house;
            house = NULL;
            cout << "GoodFriend的析构函数被调用啦" << endl;
        }
    }
};

class MyHouse {

    //GoodFriend是本类的朋友（友元），可以访问私有成员
    friend GoodFriend;
public:
    MyHouse();

public:
    string settingRoom;
private:
    string bedRoom;
};

GoodFriend::GoodFriend() {
    house = new MyHouse();
}


void GoodFriend:: visit() {
    cout<< "好朋友正在访问："<<house->settingRoom<<endl;
    cout<< "好朋友正在访问："<<house->bedRoom<<endl;
}



MyHouse::MyHouse() {
    settingRoom = "客厅";
    bedRoom = "卧室";
}

void test() {
    GoodFriend goodF;
    goodF.visit();

    GoodFriend* goodFP = new GoodFriend();
    goodFP->visit();
    delete goodFP;
}

int main()
{
    test();

    return 0;
}

成员函数做友元

#include <iostream>
#include<string>

using namespace std;

class MyHouse;

class GoodFriend
{
public:
    MyHouse* house;
    GoodFriend();
    void visit();
    void visit2();


    ~GoodFriend() {
        if (house != NULL) {
            delete house;
            house = NULL;
            cout << "GoodFriend的析构函数被调用啦" << endl;
        }
    }
};


class MyHouse {
    //声明别的类的函数为本类的友元，可以访问本类的私有成员
    friend void GoodFriend::visit();
public:
    MyHouse() {
        settingRoom = "-客厅";
        bedRoom = "-卧室";
    }

public:
    string settingRoom;
private:
    string bedRoom;
};

GoodFriend::GoodFriend() {
    house = new MyHouse();
}

void GoodFriend::visit() {
    cout << "正在访问：" << house->settingRoom << endl;
    cout << "正在访问：" << house->bedRoom << endl;
}

void GoodFriend::visit2() {
    cout << "正在访问：" << house->settingRoom << endl;
    //cout << "正在访问：" << house->bedRoom << endl;
}

int main() {

    
    GoodFriend gf;
    gf.visit();
    gf.visit2();

    return 0;
}