②类做友元

③成员函数做友元

友元

class Building{
    //goodGay全局函数是Building好朋友，可以访问Building中私有成员
    friend void goodGay(Building *building );
public:
    Building(){
        m_SittingRoom = "客厅"；
        m_BedRoom = "卧室"；       
    }

public:
    string m_SittingRoom;
private:	
    string m_BedRoom;
}

//全局函数
void goodGay(Building *building ){
    cout << "好基友全局函数正在访问："<<building->m_SittingRoom <<endl;
    cout << "好基友全局函数正在访问："<<building->m_BedRoom <<endl;
}

void test01(){
    Building building;
    goodGay(&building);
}

友元类

class Building;
class GoodGay{
public:
    void visit();// 参观函数 访问Building中的属性
    Building *building;
};

class Building{
    //GoodGay类是本类的好朋友，可以访问本类中的私有成员
    friend class GoodGay;
public:
    Building();
public:
    string m_SittingRom;
private:
    string m_BedRoom;
};

//类外写成员函数
Building::Building(){
    m_SittingRoom = "客厅"；
    m_BedRoom = "卧室"；
}

GoodGay::GoodGay(){
    //创建建筑物对象
    building = new Building  
}

GoodGay::visit(){
    cout << "好基友类正在访问：" <<building->m_SittingRoom <<endl;
    cout << "好基友类正在访问：" <<building->m_BedRoom <<endl;
}

void test01(){
    GoodGay gg;
    gg.visit();
}

成员函数做友元

class Building;
class GoodGay{
public:
    GoodGay();
    void visit();//让visit函数可以访问Building中私有成员
    void visit2();//让visit2函数不可以访问Building中私有成员
    Building *building;
};

class Building{
    //告诉编译器，GoodGay类下的visit成员函数作为本类的好朋友，可以访问私有成员
    friend void GoodGay::visit();
public:
    Building();
public:
    string m_SittingRom;
private:
    string m_BedRoom;
};

//类外实现成员函数
Building::Building(){
    m_SittingRoom = "客厅";
    m_BedRoom = "卧室";
}

GoodGay::GoodGay(){
    building = new Building;
}

void GoodGay::visit(){
    cout <<"visit函数正在访问："<<building->m_SittingRoom <<endl;
    cout <<"visit函数正在访问："<<building->m_BedRoom <<endl;
}

void GOOdGay::visit2(){
    cout <<"visit2函数正在访问："<<building->m_SittingRoom <<endl;   
    cout <<"visit函数正在访问："<<building->m_BedRoom <<endl;    
}

void test01(){
    GoodGay gg;
    gg.visit();
}