【C++】友元

在程序里，有些私有属性想让类外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者类访问另一个类中私有成员。
关键字：friend

三种实现方式

1、全局函数做友元

语法：friend void 函数名();

class student {
	friend void goodGay(student* s);
public:
	string name;
	student() {
		name = "张三";
		id = 111111;
	}
private:
	int id;

};
//全局函数
void goodGay(student *s) {
	cout << "访问姓名"<<s->name << endl;
	cout << "访问ID:" << s->id << endl;
}
void test() {
	student s1;
	goodGay(&s1);
}

2、类做友元

语法：friend class 类名;

class student {
//类友元
	friend class goodGay;
public:
	string name;
	student();
private:
	int id;
};
//类外写成员函数
student::student() {
	name = "张三";
	id = 111111;
}
//全局函数
class goodGay {
public:
	goodGay();
	void visit();//访问student中的属性
private:
	student* s;
};
goodGay::goodGay() {
	s = new student;
}
void goodGay::visit() {
	cout << "goodGay正在访问" << s->name << endl;
	cout << "goodGay正在访问" << s->id << endl;
}
void test() {
	goodGay g;
	g.visit();
}

3、成员函数做友元

语法：friend void 类名::函数名();

class GoodGay;
class student;
class GoodGay {
public:
	GoodGay();
public:
	void visit();//可访问私有成员
	void visit1();//不可访问私有成员
private:
	student* s;
};
class student {
	friend void GoodGay::visit1();
public:
	string name;
	student();
private:
	int id;
};
//类外写成员函数
student::student() {
	name = "张三";
	id = 111111;
}

GoodGay::GoodGay() {
	s = new student;
}
void GoodGay::visit() {
	cout << "goodGayDevisit正在访问" << s->name << endl;
}
void GoodGay::visit1() {
	cout << "goodGayDevisit1正在访问" << s->name << endl;
	cout << "goodGayDevisit1正在访问" << s->id << endl;
}