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
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全局函数做友元
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类做友元
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成员函数做友元
一、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;}二、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;}三、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;}
