[开发工具]C++使用一个类的成员函数做另一个类的友元函数

写在一个文件中main.cpp

#include <iostream>
#include <cmath>

#include "Point.h"
#include "ManagerPoint.h"

using namespace std;
//
//class Point;	//前向声明
//
//class ManagerPoint
//{
//public:
//	double Distance(Point& a, Point& b);
//};
//
//
//class Point
//{
//public:
//	Point(double xx, double yy)
//	{
//		x = xx;
//		y = yy;
//	}
//	void Getxy();
//	friend double ManagerPoint::Distance(Point& a, Point& b);
//private:
//	double x, y;
//};
//
//void Point::Getxy()
//{
//	cout << "(" << x << "," << y << ")" << endl;
//}
//
//double ManagerPoint::Distance(Point& a, Point& b)
//{
//	double dx = a.x - b.x;
//	double dy = a.y - b.y;
//	return sqrt(dx * dx + dy * dy);
//}

int main()
{
	Point p1(3.0, 4.0), p2(6.0, 8.0);
	p1.Getxy();
	p2.Getxy();
	ManagerPoint mp;
	double d = mp.Distance(p1, p2);
	cout << "Distance is" << d << endl;

	return 0;
}

多文件写法
Point.h

#pragma once

#include "ManagerPoint.h"

class Point
{
public:
	Point(double xx, double yy);
	void Getxy();
	friend double ManagerPoint::Distance(Point& a, Point& b);
private:
	double x, y;
};

Point.cpp

#include "Point.h"
#include <iostream>

using namespace std;

Point::Point(double xx, double yy)
{
	x = xx;
	y = yy;
}

void Point::Getxy()
{
	cout << "(" << x << "," << y << ")" << endl;
}

ManagerPoint.h

#pragma once

//#include "Point.h"
class Point;

class ManagerPoint
{
public:
	double Distance(Point& a, Point& b);
//private:

};

ManagerPoint.cpp

#include "ManagerPoint.h"
#include <cmath>
#include "Point.h"	//注意不要忘记此处包含头文件

double ManagerPoint::Distance(Point& a, Point& b)
{
    double dx = a.x - b.x;
	double dy = a.y - b.y;
	return sqrt(dx * dx + dy * dy);
}