史上最详细友言函数示例

   友元函数：

                         在类里的声明一个普通的函数，加上关键字friend,就成了该类的友元函数，它可以访问该类的一切成员，其原型为：

              friend<类型><友元函数名>(<参数表>);

                 友元函数声明的位置可以在类的任何地方，public，protect，private;

class Point
{
public:
    Point(double xi, double yi){ X = xi, Y = yi; }
    friend double length(Point &a, Point &b);
private:
    double X, Y;
};

class Point
{
public:
    Point(double xi, double yi){ X = xi, Y = yi; }
private: 
    friend double length(Point &a, Point &b);
    double X, Y;
};

      友元函数不是类定义的函数，但它的特性是可以访问类的private成员；

double length(Point &a, Point &b)
{
	double dx = a.X - b.X;
	double dy = a.Y - b.Y;
	return dx + dy;
}

*******特别注意---友言函数的使用在main函数之前，这里务必注意函数名与friend的是否一致，编译器不会报错告诉你名字出错了！只会告诉你无法访问private的成员变量！

友元类

      除函数之外，一个类也可以被声明为另一个类的友元，该类称为友元类。

  友元类的声明格式为：     friend class <类名>

class student
{
public :
	friend class teacher;
	student(){};
private:
	int number, score;
};
class teacher
{
public :
	teacher(int i, int j);
	void display();
private:
	student a;
};

 teacher类是student类的友元类，teacher类所有的成员函数都可以访问类中student类的任意成员！

teacher 类的成员display（）引用了student类的两个私有成员number和score

teacher::teacher(int i, int j)
{
	a.number = i;
	a.score = j;
}
void teacher::display()
{
	cout << "No= " << a.number << " ";
	cout << "score =" << a.score << endl;
}

经典示例：

#include<iostream>
using namespace std;
class Ruler;
class Book
{
public:
	Book(){ weight = 0; }
	Book(double x)
	{
		weight = x;
	}
	friend double tatalweight( Book &a,  Ruler &b);
private:
	double weight;
};

class Ruler
{
public:
	//friend class Book;
	Ruler(){ weight = 0; }
	Ruler(double x)
	{
		weight = x;
	}
	friend double tatalweight( Book &a,  Ruler &b);
private:
	double weight;
};

double tatalweight(Book &a,  Ruler &b)
{
	return a.weight + b.weight;
}
int main()
{
	Book a(5);
	Ruler b(5);
	std::cout << "总重量为：" << tatalweight(a, b) << std::endl;
	system("pause");
	return 0;
}