C++中的友元

友元的功能是让一个函数或者类，能够访问另一个类中的私有成员。

实现方法：

1.全局函数作为友元：

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#include<iostream>
using namespace std;
#include<string>

class Building{//类
friend void goodfri(Building *building);//这句话就是让全局函数做Building类的友元
//这样，private的m_bedroom才可以被goodfri访问
    public:
        Buliding(){//构造函数
            this->m_sittingroom="keting";//在其中赋值
            this->m_bedroom="woshi";
        }
    public:
        string m_sittingroom;
    private:
        string m_bedroom;
};

void goodfri(Building *building){//全局函数
    cout<<"now is:"<<building->m_sittingroom<<endl;
    cout<<"now is:"<<building->m_bedroom<<endl;
}

void test01(){
    Building b;//创建一个Building型的b
    goodfri(&b);
}
int main(){
    test01();
    return 0;
}

2.类做友元：

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#include<iostream>
using namespace std;
#include<string>

class Building;//提前声明一下有Building这个类
class goodfri{
    public:
        goodfri();//构造函数
        void visit();//一个普通函数
    private:
        Building *building;//一个Building类型的指针
};

class Building{
    friend class goodfri;//这句话就实现了类做友元，告诉Building,goodfri类是你的朋友(友元)
    public:
        Building();//构造函数
    public:
        string m_sittingroom;
    private:
        string m_bedroom;
};

Building::Building(){ //在类外面写构造函数
    this->m_sittingroom="keting";
    this->m_bedroom="woshi";
}

goodfri::goodfri(){//在类外面写构造函数
    building=new Building;//创建了一个新的对象，new返回的是这个新的对象的地址，所以用指针来接收
    //building指针是在goodfri中的private类型
}

void goodfri::visit(){
    cout<<"now in:"<<building->m_bedroom<<endl;
    cout<<"now in:"<<building->m_sittingroom<<endl;
}

void test01(){
    goodfri hfri;//创建了一个goodfri类型的对象hfri
    hfri.visit();
}

int main(){
    test01();
    system("pause");
    return 0;
}

3.成员函数作为友元：

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#include<iostream>
using namespace std;
#include<string>
class Building;
class goodfri{
    public:
        goodfri();//构造函数
        void visit();
        void visit2();//只是用来和visit函数做对比的
    private:
        Building *building;
};

class Building{
    friend void goodfri::visit();//这句话就是让成员函数visit为友元

    public:
        Building();
    public:
        string m_sittingroom;
    private:
        string m_bedroom;
};

Building::Building(){
    this->m_bedroom="woshi";
    this->m_sittingroom="keting";
}

goodfri::goodfri(){
    building=new Building;
}
void goodfri::visit(){
    cout<<"now is:"<<building->m_sittingroom<<endl;
    cout<<"now is:"<<building->m_bedroom<<endl;
}
void goodfri::visit2(){
    cout<<"now is:"<<building->m_sittingroom<<endl;
    //cout<<"now is:"<<building->m_bedroom<<endl;这句话不能访问，
}
void test01(){
    goodfri hhh;
    hhh.visit();
}
int main(){
    test01();
    system("pause");
    return 0;
}