【C++】友元

1. 友元的概念

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。

友元的三种实现：

• 全局函数做友元
• 类做友元
• 成员函数做友元

2. 友元的实现方式

2.1 全局函数做友元

#include 
using namespace std;
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;
}

int main()
{
    Building b;
    goodGay(&b);
    return 0;
}

2.2 类做友元

#include
using namespace std;
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;
}

int main()
{
    goodGay gg;
    gg.visit();
    return 0;
}

2.3 成员函数做友元

#include 
using namespace std;
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;
}

int main()
{
    goodGay gg;
    gg.visit();
    return 0;
}