C++基础知识（友元）

1. friend关键字只出现在声明处
2. 其他类、类成员函数、全局函数都可声明为友元
3. 友元函数不是类的成员，不带this指针
4. 友元函数可访问对象任意成员属性，包括私有属性

全局函数做友元函数

         全局函数写到 类中做声明 并且前面加上关键字 friend

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

class Building
{
	//让全局的好基友函数  变为我的好朋友  友元函数
	friend void goodGay(Building * building);
	//客厅  卧室
public:
	Building()
	{
		this->m_SittingRoom = "客厅";
		this->m_BedRoom = "卧室";
	}
public:
	string m_SittingRoom;

private:
	string m_BedRoom;
};

//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 目的访问类中的私有成员属性
void test01()
{
	Building * building = new Building;
	goodGay(building);
}

int main(void)
{
	test01();
	system("pause");
	return 0;
}

整个类做友元函数

friend class 类名

   [友元类注意]

1. 友元关系不能被继承。
2. 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。

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

class Building
{
	//让好基友类  作为Building的好朋友
	friend class goodGay;
public:
	Building();
public:
	string m_SittingRoom; //客厅
private:
	string m_BedRoom;   //卧室
};
class goodGay
{
public:
	goodGay();
	void visit();
private:
	Building *building;
};

goodGay::goodGay()
{
	building = new Building;
}

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

Building::Building()
{
	this->m_SittingRoom = "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

friend void goodGay::visit();

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

//只让visit可以做Building的好朋友 visit2不可以作为友元
class Building;

class goodGay
{
public:
	goodGay();
	void visit();
	void visit2();
private:
	Building *building;
};

class Building
{
	//让成员函数visit  作为Building的友元
	friend void goodGay::visit();
public:
	Building();
public:
	string m_SittingRoom; //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_SittingRoom << 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_SittingRoom << endl;
	cout << "好基友正在访问" << this->building->m_BedRoom << endl;
}

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

void test01()
{
	goodGay gg;
	gg.visit();
	//gg.visit2();  只能访问到m_SittingRoom
}

int main()
{
	test01();
	system("pause");
	return 0;
}