4.4 友元

生活中你的家有客厅(Public),有你的卧室(Private)

客厅所有来的客人都可以进去,但是你的卧室是私有的,也就是说只有你能进去

但是呢,你也可以允许你的好闺蜜好基友进去。

在程序里,有些私有属性 也想让类外特殊的一些函数或者类进行访问,就需要用到友元的技术

友元的目的就是让一个函数或者类 访问另一个类中私有成员

友元的关键字为 ==friend==

友元的三种实现

  • 全局函数做友元
  • 类做友元
  • 成员函数做友元

4.4.1 全局函数做友元

class Building
{
    //告诉编译器 goodGay全局函数 是 Building类的好朋友,可以访问类中的私有内容
    friend void goodGay(Building * building);

public:

    Building()
    {
        this->m_SittingRoom = "客厅";
        this->m_BedRoom = "卧室";
    }


public:
    string m_SittingRoom; //客厅

private:
    string m_BedRoom; //卧室
};


void goodGay(Building * building)
{
    cout << "好基友正在访问: " << building->m_SittingRoom << endl;
    cout << "好基友正在访问: " << building->m_BedRoom << endl;
}


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

int main(){

    test01();

    system("pause");
    return 0;
}

4.4.2 类做友元

class Building;
class goodGay
{
public:

    goodGay();
    void visit();

private:
    Building *building;
};


class Building
{
    //告诉编译器 goodGay类是Building类的好朋友,可以访问到Building类中私有内容
    friend class goodGay;

public:
    Building();

public:
    string m_SittingRoom; //客厅
private:
    string m_BedRoom;//卧室
};

Building::Building()
{
    this->m_SittingRoom = "客厅";
    this->m_BedRoom = "卧室";
}

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

void goodGay::visit()
{
    cout << "好基友正在访问" << building->m_SittingRoom << endl;
    cout << "好基友正在访问" << building->m_BedRoom << endl;
}

void test01()
{
    goodGay gg;
    gg.visit();

}

int main(){

    test01();

    system("pause");
    return 0;
}

4.4.3 成员函数做友元


class Building;
class goodGay
{
public:

    goodGay();
    void visit(); //只让visit函数作为Building的好朋友,可以发访问Building中私有内容
    void visit2(); 

private:
    Building *building;
};


class Building
{
    //告诉编译器  goodGay类中的visit成员函数 是Building好朋友,可以访问私有内容
    friend void goodGay::visit();

public:
    Building();

public:
    string m_SittingRoom; //客厅
private:
    string m_BedRoom;//卧室
};

Building::Building()
{
    this->m_SittingRoom = "客厅";
    this->m_BedRoom = "卧室";
}

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

void goodGay::visit()
{
    cout << "好基友正在访问" << building->m_SittingRoom << endl;
    cout << "好基友正在访问" << building->m_BedRoom << endl;
}

void goodGay::visit2()
{
    cout << "好基友正在访问" << building->m_SittingRoom << endl;
    //cout << "好基友正在访问" << building->m_BedRoom << endl;
}

void test01()
{
    goodGay  gg;
    gg.visit();

}

int main(){

    test01();

    system("pause");
    return 0;
}