本例对C++友元的使用从代码上作了一个简单的归纳，但不仅限于本问讲到的用法，友元还可以重载等。
**
/全局函数作为友元 /
**
include <iostream>
include <cmath>
using namespace std;
class Point{
public:
Point(doublex,doubley){ _x = x; _y = y;}voidgetFormatxy();frienddoubledistance(Point &a, Point &b);
private:
double_x, _y;
};
void Point::getFormatxy(){
cout<<"("<<_x<<","<<_y<<")"<
}
double distance(Point &a, Point &b){
doubledx = a._x - b._x;doubledy = a._y - b._y;returnsqrt(dx*dx + dy*dy);
}
int main()
{
Pointp1(3.0,4.0),p2(6.0,8.0);p1.getFormatxy();p2.getFormatxy();doubled = distance(p1, p2);cout<<"distance is "<
}
**
/一个类的成员函数作友元 /
**
include <iostream>
include <cmath>
using namespace std;
class Point; //前向声明,用于声明。
class ManagerPoint{
public:
double distance(Point&a, Point&b);
};
class Point{
public:
Point(doublex,doubley){ _x = x; _y = y;}voidgetFormatxy();frienddoubleManagerPoint::distance(Point &a, Point &b);
private:
double_x, _y;
};
void Point::getFormatxy(){
cout<<"("<<_x<<","<<_y<<")"<
}
double ManagerPoint::distance(Point &a, Point &b){
doubledx = a._x - b._x;doubledy = a._y - b._y;returnsqrt(dx*dx +dy*dy);
}
int main()
{
Pointp1(3.0,4.0),p2(6.0,8.0);p1.getFormatxy();p2.getFormatxy();ManagerPoint mp;floatd = mp.distance(p1, p2);cout<<"distance is"<
}
**
/友元类 /
**
include <iostream>
include <cmath>
using namespace std;
class Point{
public:
friendclassManagerPoint;Point(doublex,doubley){ _x = x; _y = y;}voidgetFormatxy();
private:
double_x, _y;
};
void Point::getFormatxy(){
cout<<"("<<_x<<","<<_y<<")"<
}
class ManagerPoint{
public:
double distance(Point&a, Point&b);
};
double ManagerPoint::distance(Point &a, Point &b){
doubledx = a._x - b._x;doubledy = a._y - b._y;returnsqrt(dx*dx + dy*dy);
}
int main()
{
Pointp1(3.0,4.0),p2(6.0,8.0);p1.getFormatxy();p2.getFormatxy();ManagerPoint mp;floatd = mp.distance(p1, p2);cout<<"distance is"<< d<
}
**
/友元小结 /
**
//友元声明以关键字friend开始,它只能出现在类定义中
//因为友元不是类授权的成员,所以它不受其所在区域的public private 和protected 的影响
//通常,把所有友元声明组织在一起并放在类头之后.
//友元利弊:
//友元不是类成员,但是它可以通过对象访问类中的私有成员,友元的作用在于提高程序的运行效率
//但是,它破坏了封装性,使得非成员函数可以访问类的私有成员
//注意: 友元关系不能被继承,友元关系不具有传递性
include <iostream>
using namespace std;
class Time{
public:
friendclassWatch;Time(inthour,intmin,intsec){ m_iHour =hour; m_iMinute = min; m_iSecond = sec;}
private:
intm_iHour;intm_iMinute;intm_iSecond;
};
class Watch{
public:
Watch(Time t): m_tTime(t){}voiddisplay(){cout<
public:
Timem_tTime;
};
int main()
{
Timet(6,30,20);Watchw(t);w.display();return0;
}
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