1、全局函数做友元

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

//建筑物类
class Building {

	//goodGay全局函数是	Building好朋友 可以访问Building中私有成员
	friend  void goodGay(Building* building);

public:

	Building() {
		m_SittingRoom = "客厅";
		m_BedRoom = "卧室";
	}
	string m_SittingRoom;//客厅

private:
	string m_BedRoom;//卧室

};

//全局函数
void goodGay(Building* building) {
	cout << "好基友全局函数 正在访问：" << building->m_SittingRoom << endl;

	cout << "好基友全局函数 正在访问：" << building->m_BedRoom << endl;

}

void test01() {
	Building building;
	goodGay(&building);
}

 
int main() {
	test01();


	system("pause");
	return 0;
}

---

2、类做友元

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

//类做友元
class Building;

class GoodGay {
	
public:
	GoodGay();

	void visit();//参观函数 访问Building中的属性
	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();
	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 test01() {
	GoodGay gg;
	gg.visit();
}

int main() {

	test01();

	system("pause");
	return 0;
}

---

3、成员函数做友元

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

class Building;

class GoodGay {
	
public:
	GoodGay();

	void visit();// 访问Building中私有成员
	void visit2();// 不可以访问Building中私有成员

	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();
	
public:
	Building();
	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 << "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 << "visit2 函数正在访问：" << building->m_SittingRoom << endl;

	//cout << "visit 函数正在访问：" << building->m_BedRoom << endl;

}

void test01() {
	GoodGay gg;
	gg.visit();
	gg.visit2();
}

int main() {

	test01();

	system("pause");
	return 0;
}