友元及其实现

 全局函数做友元

 类做友元

#include<iostream>
#include<string>
using namespace std;

class Building
{
	//表明GoodGay是本类的好朋友，允许GoodGay访问本类的私有属性；
	friend class GoodGay;
public:
	//用构造函数初始化；
	Building()
	{
		m_SittingRoom = "客厅";
		m_BedRoom = "卧室";
	}

public:
	string m_SittingRoom;
private:
	string m_BedRoom;
};

class GoodGay
{
private:
	//在类里面创建对象；
	Building* build = new Building;
	//(Building*)& building=build;   指针的引用，没有报错，但是无法运行；

public:
	void visit()
	{
		cout << "好基友正在访问：" << build->m_SittingRoom << endl;
		cout << "好基友正在访问：" << build->m_BedRoom << endl;
	}
};

int main() 
{
	GoodGay jj;
	jj.visit();

	system("pause"); 
	return 0;
}

成员函数作为友元；

//设置good gay中的visit（）函数允许访问本类的私密属性；
	friend void GoodGay::visit();

下面的代码比较玄学，所有的函数需要放到类外，且building需要提前声明，否则不会报错，但不能运行；

#include<iostream>
#include<string>
using namespace std;

class Building;

class GoodGay
{
private:
	//在类里面创建对象；
	Building* build;

public:
	//设置该成员函数可以访问
	GoodGay();
	void visit();
};

class Building
{
	//设置good gay允许访问本类的私密属性；
	friend void GoodGay::visit();
public:
	//用构造函数初始化；
	Building();
	string m_SittingRoom;

private:
	string m_BedRoom;
};

GoodGay::GoodGay()
{
	build = new Building;
}

Building::Building()
{
	this->m_SittingRoom = "客厅";
	this->m_BedRoom = "卧室";
}

void GoodGay::visit()
{
	cout << "好基友正在访问：" << build->m_SittingRoom<< endl;
	cout << "好基友正在访问：" << build->m_BedRoom << endl;
}

int main() 
{
	GoodGay jj;
	jj.visit();

	system("pause"); 
	return 0;
}