c++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

1.全局函数做友元

#include <iostream>
#include <string>
using namespace std;
class Building
{
    //goodfriend是Building的好朋友  可以访问Building的私有成员变量
    friend void goodfriend(Building* building);
public:
    Building()
    {
        sittingroom = "客厅";
        bedroom="卧室";
    
    }
public:
    string sittingroom;
private:
    string bedroom;

};

void goodfriend(Building* building)
{
    cout << "好朋友全局函数，正在访问" <<building->sittingroom<< endl;
}

void text01()
{
    Building building;
    goodfriend(&building);
}
int main()
{
    text01();

}

2. 类做友元

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

class Building
{

    friend class Goodgay;
public:
    Building()
    {
    this->sittingroom= "客厅";
    this->bedroom= "卧室";

    };
public:
    string sittingroom;
private:
    string bedroom;
};
class Goodgay
{
    public:
        Goodgay()
        {
        //创建建筑对象
        building = new Building;
        };
        void visit();//参观函数 访问Building中的属性
    private:
        Building *building;


};


//类外写成员函数
/*Building::Building()
{
    this->sittingroom= "客厅";
    this->bedroom= "卧室";

}*/
/*Goodgay::Goodgay()
{
    //创建建筑对象
    building = new Building;
}*/
void Goodgay::visit()
{
    cout << "好基友类正在访问" << building->sittingroom << endl;
    cout << "好基友类正在访问" << building->bedroom<< endl;
}
void text01()
{
    Goodgay gg;
    gg.visit();
}
int main()
{
    void text01();
}

3.成员函数做友元

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


class Building;
class goodgay
{

public:
    goodgay();
    void visit();
    void visit1();

private:
    Building*building;

};
class Building
{
    friend  void goodgay::visit();
public:
    Building();
public:
    string sittingroom;
private:
    string bedroom;
};
goodgay::goodgay()
{
    building=new Building;
}
Building::Building()
{
  sittingroom="客厅";
  bedroom="卧室";

}
void goodgay::visit()
{
    cout << "好基友正在访问" << building->sittingroom << endl;
    cout << "好基友正在访问" << building->bedroom<< endl;
}

void goodgay::visit1()
{
    cout << "好基友正在访问" << building->sittingroom << endl;
    //cout << "好基友正在访问" << building->m_BedRoom << endl;
}

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

}

int main(){

    test01();


}