目的：让一个函数或者类访问另一个类中的私有成员

全局函数做友元

#include <iostream>
#include <string>

using namespace std;

class Building
{
	friend void GoodGay(Building& build); //全局函数作为友元,可以访问私有成员
private:
	string m_Bedroom;
public:
	string m_sittingroom;

	Building()
	{
		m_Bedroom = "卧室";
		m_sittingroom = "客厅";
	}
};

void GoodGay(Building& build)
{
	cout << "好基友正在访问：" << build.m_sittingroom << endl;
	cout << "好基友正在访问：" << build.m_Bedroom << endl;

}

void test()
{
	Building build;
	GoodGay(build);
}

int main(void)
{
	test();
	system("pause");
	return 0;
}

类做友元

#include <iostream>
#include <string>

using namespace std;

class Building
{     
	friend class GoodGay;//类做友元，GoodGay可以访问本类的私有属性
private:
	string m_Bedroom;
public:
	string m_sittingroom;

	Building();
};

Building::Building()
{
	m_Bedroom = "卧室";
	m_sittingroom = "客厅";
}

class GoodGay
{
public:
	void visit(); //访问building中的所有属性

	Building* building;

	GoodGay();
};

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 test()
{
	GoodGay gg;
	gg.visit();
}

int main(void)
{
	test();
	system("pause");
	return 0;
}

类中的成员函数做友元

#include <iostream>
#include <string>

using namespace std;

class Building;
class GoodGay
{
public:
	void visit(); //访问building中的私有成员
	void visit2();//不可以访问building中的私有成员

	Building* building;

	GoodGay();
};

class Building
{     
	friend void GoodGay::visit(); //GoodGay下的visit可以访问本类的私有成员
private:
	string m_Bedroom;
public:
	string m_sittingroom;

	Building();
};

Building::Building()
{
	m_Bedroom = "卧室";
	m_sittingroom = "客厅";
}


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;
}

void test()
{
	GoodGay gg;
	gg.visit();
	gg.visit2();
}

int main(void)
{
	test();
	system("pause");
	return 0;
}

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