关于友元函数(成员友元函数, 普通友元函数)

#include<iostream>
#include<math.h>
using namespace std;
constdoublePI = 3.141593;
classCircle;
//classCalculate
//{
//public:
//doubledistance(Circle &p1, Circle &p2);
//};


classCircle
{
private:
doublex, y, r;
public:
//构造函数
Circle(double xx = 0, double yy = 0, double rr = 0)
{
x = xx;y = yy;r = rr;
}


//显示
voiddisplay()
{
cout << "\tCenter point: (" << x << ", " << y << ")" << endl;
cout << "\tRadius: " << r << endl;
cout << "\tArea: " << this->area() << endl;
cout << "\tCirculars: " << this->circular() << endl;
}


//面积
doublearea()
{
returnPI * r * r;
}


//周长
doublecircular()
{
return 2 * PI * r;
}

//普通友元函数
frienddoubledist(Circle &p1, Circle &p2);
//成员友元函数
//frienddoubleCalculate::distance(Circle &p1, Circle &p2);
};
//忘记了这个分号，结果报错：
//'Circle' followed by a double is illegal.(did you forgot a ';'?)
//doubleCalculate::distance(Circle &p1, Circle &p2)
//{
//returnsqrt((p1.x - p2.x) * (p1.x - p2.x) + (p1.y - p2.y) * (p1.y - p2.y));
//}
doubledist(Circle &p1, Circle &p2)
{
returnsqrt((p1.x - p2.x) * (p1.x - p2.x) + (p1.y - p2.y) * (p1.y - p2.y));
}


void main()
{
Circle c1(1, 1, 1), c2(3, 4, 5);
//Calculatecal;//必须声明一个Calculate 对象
cout << "Circle 1: " << endl;
c1.display();
cout << endl << endl;


cout << "Circle 2: " << endl;
c2.display();
//cout << "The distance is: " << cal.distance(c1, c2) << endl;
cout << "The distance is: " << dist(c1, c2) << endl;
}