[C++知识库]C++ 友元函数

友元函数

全局函数作为友元函数

• 利用friend关键字让全局函数goodGay作为本类好朋友，可以访问私有成员
• friend void goodGay(Building * buliding);

#include <string>
class Building
{
	//利用friend关键字让全局函数,goodGay作为本类友元，可以访问类的私有成员
	friend void goodGay(Building * buliding);

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

//全局函数  可以访问Building的私有属性
void goodGay( Building * buliding)
{
	cout << "好基友正在访问：" << buliding->m_SittingRoom << endl;

	cout << "好基友正在访问：" << buliding->m_BedRoom << endl;
}

void test()
{
	Building buliding;
	goodGay(&buliding);
}

类作为友元类

让goodGay访问Building类的私有属性，需要将goodGay类作为Building的友元类，在类Building中添加friend class GoodGay

#include <string>

//类声明
class Building;
class GoodGay
{
public:

	GoodGay();

	void visit();

	Building* m_building;
};


//建筑
class Building
{
	//让goodGay类作为 Building的好朋友，可以访问私有成员
	friend class GoodGay;

public:
	Building();

	string m_SittingRoom;

private:
	string m_BedRoom;
};

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

//好基友类构造
GoodGay::GoodGay()
{
	this->m_building = new Building;
}

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

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

类中的成员函数作为友元函数

• 让GoodGay类中的 visit成员函数作为友元
• friend void GoodGay::visit();

//类声明
class Building;
class GoodGay
{
public:

	GoodGay();

	void visit();
	void visit2();
	Building* m_building;
};


//建筑
class Building
{
	//成员函数作为友元函数，只允许指定对象指定的函数访问私有属性
	friend void GoodGay::visit();

public:
	Building();

	string m_SittingRoom;

private:
	string m_BedRoom;
};

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

//好基友类构造
GoodGay::GoodGay()
{
	this->m_building = new Building;
}

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

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