第四课 类和对象-友元

4.4 友元
在程序中，有些私有属性也想让类外特殊的一些函数或者类进行访问，就需要用到友元的技术
友元的目的就是让一个函数或类访问另一个类中的私有成员
友元的关键字 friend
4.4.1 全局函数做友元

class Building
{
	friend void goodGay(Building *building);
public:
	Building()
	{
		m_SittingRoom = "客厅";
		m_BadRoom = "卧室";
	}

	string m_SittingRoom; // 客厅

private:
	string m_BadRoom; // 卧室

};

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_BadRoom << endl;
}

int main()
{
	Building building;
	goodGay(&building);

	system("pause");
	return 0;
}

4.4.2 类做友元

// 类做友元
class Building
{
	friend class GoodGay;
public:
	Building();

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

class GoodGay
{
public:

	GoodGay(); // 函数声明
	void visit(); // 参观函数，访问Building中的属性
	Building* building;
};
GoodGay::GoodGay()
{
	// 创建Building对象
	building = new Building;
}
void GoodGay::visit()
{
	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()
{
	GoodGay gg;
	gg.visit();
}

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

4.4.3 成员函数做友元

class Building;
class GoodGay
{
public:
	GoodGay();
	void visit();
	Building* building;
};

class Building
{
	friend void GoodGay::visit();
public:
	Building();
	string m_SettingRoom;
private:
	string m_BedRoom;
};

// 定义好类之后才能在类外定义成员函数

GoodGay::GoodGay()
{
	// 重新开辟一块空间
	building = new Building;
}

void GoodGay::visit()
{
	// 调用本类的 building 属性
	cout << "visit 访问：" << building->m_SettingRoom << endl;
	cout << "visit 访问：" << building->m_BedRoom << endl;

}

Building::Building()
{
	m_SettingRoom = "客厅";
	m_BedRoom = "卧室";
}

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

int main()
{
	test();

	system("pause");
	return 0;
}