友元函数

友元函数不是成员函数，但可以访问类的私有成员，它的作用在于提高程序的运行效率，但却破坏了类的封装性和隐蔽性。

#include <iostream>#include <math.h>using namespace std;class Point{public:Point(float i=0.0f, float j=0.0f){x=i;y=j;}friend float Distance(Point &left, Point &right);private:float x,y;};float Distance(Point &left, Point &right){float dx=right.x-left.x;float dy=right.x-left.x;return sqrt(dx*dx+dy*dy); }void main(){Point a(1.0, 3.5), b(1.5, 5.5);cout<<Distance(a, b)<<endl;}