c++-----友元

不使用友元时 

#include<iostream>
#include<stdio.h>
#include<math.h>
using namespace std;
class Point {
public:
	Point(int x, int y)
	{
		this->x = x;
		this->y = y;
	}
	int getX()
	{
		return this->x;
	}
	int getY()
	{
		return this->y;
	}
private:
	int x;
	int y;

};

double PointDistance(Point &p1, Point &p2)
{
	double dis;
	int dd_x = p1.getX() - p2.getX();
	int dd_y = p1.getY() - p2.getY();
	dis = sqrt(dd_x*dd_x + dd_y * dd_y);
	return dis;
}
int main()
{
	Point p1(12, 31);
	Point p2(34, 45);
	cout << PointDistance(p1, p2) << endl;
	return 0;
}

 

友元的使用 

#include<iostream>
#include<stdio.h>
#include<math.h>
using namespace std;
class Point {
public:
	Point(int x, int y)
	{
		this->x = x;
		this->y = y;
	}
	//声明全局函数PointDistance是我类Point类的一个友元函数
	friend double PointDistance(Point &p1, Point &P2);
private:
	int x;
	int y;

};

double PointDistance(Point &p1, Point &p2)
{
	double dis;
	int dd_x = p1.x- p2.x;
	int dd_y = p1.y - p2.y;
	dis = sqrt(dd_x*dd_x + dd_y * dd_y);
	return dis;
}
int main()
{
	Point p1(12, 31);
	Point p2(34, 45);
	cout << PointDistance(p1, p2) << endl;
	return 0;
}