C友元初步

jiezi

5 年前

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 /

using namespace std;

class Point{
public:

Point(double x, double y){
    _x = x;
    _y = y;
}
void getFormatxy();
friend double distance(Point &a, Point &b);

private:

double _x, _y;

};

void Point::getFormatxy(){

cout<<"("<<_x<<","<<_y<<")"<<endl;

}
double 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.getFormatxy();
p2.getFormatxy();
double d = distance(p1, p2);
cout<<"distance is"<<d<<endl;
return 0;

}

/ 一个类的成员函数作友元 /

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(double x, double y){
    _x = x;
    _y = y;
}
void getFormatxy();
friend double ManagerPoint::distance(Point &a, Point &b);

private:

double _x, _y;

};

void Point::getFormatxy(){

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.getFormatxy();
p2.getFormatxy();
ManagerPoint mp;
float d = mp.distance(p1, p2);
cout<<"distance is"<<d<<endl;
return 0;

}

/ 友元类 /

using namespace std;

class Point{
public:

friend class ManagerPoint;
Point(double x, double y){
    _x = x;
    _y = y;
}
void getFormatxy();

private:

double _x, _y;

};

void Point::getFormatxy(){

cout<<"("<<_x<<","<<_y<<")"<<endl;

}

class ManagerPoint{
public:

double distance(Point &a, Point &b);

};

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.getFormatxy();
p2.getFormatxy();
ManagerPoint mp;
float d = mp.distance(p1, p2);
cout<<"distance is"<< d<<endl;
return 0;

}

/ 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

using namespace std;

class Time{
public:

friend class Watch;
Time(int hour, int min, int sec){
    m_iHour = hour;
    m_iMinute = min;
    m_iSecond = sec;
}

private:

int m_iHour;
int m_iMinute;
int m_iSecond;

};

class Watch{
public:

Watch(Time t): m_tTime(t){}
void display(){
    cout<<m_tTime.m_iHour<<endl;
    cout<<m_tTime.m_iMinute<<endl;
    cout<<m_tTime.m_iSecond<<endl;
}

public:

Time m_tTime;

};

int main()
{

Time t(6, 30, 20);
Watch w(t);
w.display();
return 0;

}

C友元初步

include <iostream>

include <cmath>

include <iostream>

include <cmath>

include <iostream>

include <cmath>

include <iostream>