C++友元函数

友元的三种实现
1.全局函数做友元
2.类做友元
3.成员函数做友元

1.全局函数做友元

#include<iostream>
#include<string>
using namespace std;
class Building
{
	//goodgay全局函数是类Building的好朋友，可以访问类Building中私有成员 
	friend void goodgay(Building *building); 
	public:
		Building()
		{
			m_SittingRoom="客厅";
			m_BedRoom="卧室";
		}
	public:	
		string m_SittingRoom;
	private:
		string m_BedRoom;
};
//全局函数
void goodgay(Building *building)
{
	cout<<"好基友的全局函数正在访问："<<building->m_SittingRoom<<endl;
	cout<<"好基友的全局函数正在访问："<<building->m_BedRoom<<endl;
} 
void test1()
{
	Building building;
	goodgay(&building);
}
int main()
{
	test1();
	return 0;
}

2.类做友元

#include<iostream>
#include<string>
using namespace std;
class Building;
class goodgay 
{
	public:
	    goodgay(); 
		void visit(); //参观函数访问Building中的属性
	private:	
		Building *building; 
};
class Building
{
	//goodgay类是本类的好朋友，可以访问本类中私有成员 
	friend class goodgay;
	public: 
		Building();
	public:
		string m_SittingRoom;
	private:
	    string m_BedRoom;	
};
//类外写成员函数
Building::Building() 
{
	m_SittingRoom="客厅";
	m_BedRoom="卧室";
} 
goodgay::goodgay()
{
	//创建建筑物对象
	building=new Building; 
}
void goodgay::visit()
{
	cout<<"好基友类正在访问："<<building->m_SittingRoom<<endl;
	cout<<"好基友类正在访问："<<building->m_BedRoom<<endl;
}
void test1()
{
	goodgay gg;
	gg.visit();
}
int main()
{
	test1();
	return 0;
}

3.成员函数做友元

#include<iostream>
#include<string>
using namespace std;
class Building;
class goodgay 
{
	public:
	    goodgay(); 
		void visit(); //visit函数可以访问Building中的私有成员
		void visit2();//visit函数不可以访问Building中的私有成员 
	private:	
		Building *building; 
};
class Building
{
	//告诉编译器goodgay类下的visit成员函数作为本类的好朋友，可以访问私有成员 
	friend void goodgay::visit();
	friend void goodgay::visit2();
	public: 
		Building();
	public:
		string m_SittingRoom;
	private:
	    string m_BedRoom;	
};
//类外实现成员函数
Building::Building() 
{
	m_SittingRoom="客厅";
	m_BedRoom="卧室";
} 
goodgay::goodgay()
{
	building=new Building; 
}
void goodgay::visit()
{
	cout<<"visit函数正在访问："<<building->m_SittingRoom<<endl;
	cout<<"visit函数正在访问："<<building->m_BedRoom<<endl;
}
void goodgay::visit2() 
{
    cout<<"visit函数正在访问："<<building->m_SittingRoom<<endl;
	cout<<"visit函数正在访问："<<building->m_BedRoom<<endl;
}
void test1()
{
	goodgay gg;
	gg.visit();
	gg.visit2();
}
int main()
{
	test1();
	return 0;
}