生活中你的家有客廳(Public)，有你的臥室(Private)

客廳所有來的客人都可以進去，但是你的臥室是私有的，也就是說只有你能進去

但是呢，你也可以允許你的好閨蜜好基友進去。

在程式裡，有些私有屬性 也想讓類外特殊的一些函式或者類進行訪問，就需要用到友元的技術

友元的目的就是讓一個函式或者類 訪問另一個類中私有成員

友元的關鍵字為 ==friend==

友元的三種實現

* 全域性函式做友元
* 類做友元
* 成員函式做友元

1.全域性函式做友元

#include <iostream>
#include <string>
using namespace std;
 
class Building
{
    //goodfriend是Building的好朋友  可以訪問Building的私有成員變數
    friend void goodfriend(Building* building);
public:
    Building()
    {
        sittingroom = "客廳";
        bedroom="臥室";
    
    }
public:
    string sittingroom;
private:
    string bedroom;

};

void goodfriend(Building* building)
{
    cout 
 
<< "好朋友全域性函式，正在訪問" <<building->sittingroom<< endl;
}

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

}

2.類做友元

#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= "客廳";
    this->bedroom= "臥室";

    };
public:
    string sittingroom;
private:
    string bedroom;
};
class Goodgay
{
    public:
        Goodgay()
        {
        //建立建築物件
        building = new Building;
        };
        void visit();//參觀函式 訪問Building中的屬性
    private:
        Building *building;


};


//類外寫成員函式
/*Building::Building()
{
    this->sittingroom= "客廳";
    this->bedroom= "臥室";

}*/
/*Goodgay::Goodgay()
{
    //建立建築物件
    building = new Building;
}*/
void Goodgay::visit()
{
    cout << "好基友類正在訪問" << building->sittingroom << endl;
    cout << "好基友類正在訪問" << building->bedroom<< endl;
}
void text01()
{
    Goodgay gg;
    gg.visit();
}
int main()
{
    void text01();
}

3.成員函式做友元

#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="客廳";
  bedroom="臥室";

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

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

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

}

int main(){

    test01();


}