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
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;
}
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;
}
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;
}
)
