友元
友元分为：友元函数和友元类
友元提供了一种突破封装的方式，有时提供了便利。但是友元会增加耦合度，破坏了封装，所以友元不宜多 用。
全局函数做友元函数
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全局函数写到类中做声明 并且最前面写关键字 friend
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友元函数可访问类的私有成员，但不是类的成员函数
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友元函数不能用const修饰 友元函数可以在类定义的任何地方声明，不受类访问限定符限制
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一个函数可以是多个类的友元函数
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友元函数的调用与普通函数的调用和原理相同
#include<iostream>#include<string>using namespace std;class Building{//让全局好基友函数变为我的好朋友 友元函数friend void goodGay(Building *building);public:Building(){this->m_SittingRoom = "客厅";this->m_BedRoom = "卧室";}//客厅 卧室public:string m_SittingRoom;//客厅private:string m_BedRoom;//卧室};//全局函数 好基友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;}//友元函数的目的：访问类中的私有成员属性void test01(){Building *building=new Building;goodGay(building);}int main(){test01();system("pause");return 0;}
类作为友元类
- 友元关系不能被继承
- 友元关系是单向的，类A是类B的朋友，但类B不一定是类A的朋友。
- 友元关系不具有传递性，类B是类A的朋友，类C是类B的朋友，但类C不一定是类A的朋友
#include
#include
using namespace std;
class Building;class goodGay
{public:goodGay();void visit();private:Building * building;
};class Building
{//让好基友类 作为Building的好朋友friend class goodGay;
public:Building();public:string m_SiteingRoom;//客厅
private:string m_BedRoom;//卧室
};goodGay::goodGay()
{building = new Building;
}void goodGay::visit()
{cout << "好基友正在访问：" << this->building->m_SiteingRoom << endl;cout << "好基友正在访问：" << this->building->m_BedRoom<< endl;
}Building::Building()
{this->m_SiteingRoom = "客厅";this->m_BedRoom = "卧室";}void test01()
{goodGay gg;gg.visit();
}int main()
{test01();system("pause");return 0;
}
成员函数作为友元函数
#include<iostream>#include<string>using namespace std;//只让visit可以作为Buildingde的好朋友，visit2不可以访问私有属性class Building;class goodGay{public:goodGay();void visit();void visit2();private:Building * building;};class Building{//让成员函数visit作为友元函数friend void goodGay:: visit();public:Building();public:string m_SiteingRoom;//客厅private:string m_BedRoom;//卧室};goodGay::goodGay(){building = new Building;}void goodGay::visit(){cout << "好基友正在访问：" << this->building->m_SiteingRoom << endl;cout << "好基友正在访问：" << this->building->m_BedRoom<< endl;}void goodGay::visit2(){cout << "好基友正在访问：" << this->building->m_SiteingRoom << endl;//cout << "好基友正在访问：" << this->building->m_BedRoom << endl;}Building::Building(){this->m_SiteingRoom = "客厅";this->m_BedRoom = "卧室";}void test01(){goodGay gg;gg.visit();gg.visit2();}int main(){test01();system("pause");return 0;}
)
