友元函数，成员函数，普通函数的区别

#include <iostream>

using namespace std;

class Time

{

public:

    Time(double xx,double yy):x(xx),y(yy){}

    double getx(){return x;}

    double gety(){return y;}

    void aaa1(Time &t);

    friend  void aaa2(Time &t1,Time &t2);

private:

    double x;

    double y;

};

void Time::aaa1(Time &t)

{

    cout<<"("<<(x+t.x)/2<<","<<(y+t.y)/2<<")"<<endl;

}

void aaa2(Time &t1,Time &t2)//友元函数不有“Time::”

{

    cout<<"("<<(t1.x+t2.x)/2<<","<<(t1.y+t2.y)/2<<")"<<endl;

}

void aaa3(Time &t1,Time &t2)

{

    cout<<"("<<(t1.getx()+t2.getx())/2<<","<<(t1.gety()+t2.gety())/2<<")"<<endl;

}

int main()

{

    Time b1(1.5,2.6),b2(5.2 ,6.4);

    b1.aaa1(b2);//成员函数直接访问私有数据

    aaa2(b1,b2);//友元函数也可以直接访问私有函数

    aaa3(b1,b2);//普通函数只能通过外部接口

    return 0;

}