MyC++-learning-友元函数和友元类

c++ 之 友元函数和友元类

友元（friend）其实就是好朋友的意思，我是你的好朋友，我就能访问你的所有对象的私有或公有成员。

但是友元会破坏数据的封装性，所以使用要慎重。

一、友元函数

如果友元是普通函数，则称友元函数。它不是当前类的成员函数。
位置可以放在任何部分（私有部分或共有部分）。
例1：一个类的友元函数

#include<iostream>
#include<cmath>
using namespace std;
class Point{
	private:
		double x,y;
	public:
		Point(double xx = 0.0, double yy = 0.0);
		void disp();
		friend double dis(const Point &p1,const Point &p2);  //友元函数
}; 
Point::Point(double xx, double yy){
	x = xx;
	y = yy;
}
void Point::disp(){
	cout<<x<<","<<y<<endl;
}
double dis(const Point &p1,const Point &p2){       //不用Point::，因为它不是成员函数
	return sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y));
}
int main()
{
	Point p1(1,1),p2(2,2);
	p1.disp();
	p2.disp();
	cout<<dis(p1,p2)<<endl;
	return 0;
}

其中最好函数名不要用distance，原因链接：c++编译错误
例2：多个类的友元函数

#include<iostream>
#include<cmath>
using namespace std;
class Line;
class Point
{
private:
	double x, y;                  //x,y 坐标
	public: 
	Point(double x=0.0, double y=0.0);
	void disp( );
	friend double dist(const Point& p,const Line& l); //声明为Point的友元
};
Point::Point (double x, double y){
	this->x = x;  
	this->y = y; 
} 		
void  Point::disp( ){
	cout<<"点（"<<x<<","<<y<<"）";  
}
class Line
{
private:
	double a, b, c;             //直线为ax+by+c=0的系数
public: 
	Line(double a=0.0, double b=0.0, double c=0.0);
	void disp( );			
	friend double dist(const Point& p,const Line& l); //声明为Line的友元
};
Line::Line(double a, double b, double c){
	this->a = a;  
	this->b = b;  
	this->c = c; 
} 		
void  Line::disp( ){  
	cout<<"线（"<<a<<"x+"<<b<<"y+"<<c<<"=0）";
}
double dist(const Point& p,const Line& l){
	return fabs(l.a*p.x+l.b*p.y+l.c)/sqrt(l.a*l.a+l.b*l.b);
}	
int main(){
	Point p1(1.0,2.0);
	Line l1(3.0,4.0,5.0);
	p1.disp();
	l1.disp();
	cout<<"距离为："<<dist(p1,l1)<<endl;
	return 0;
}

二、友元成员函数

#include<iostream>
#include<cmath>
using namespace std;
class Line;
class Point
{
private:
	double x, y;                  //x,y 坐标
public: 
	Point(double x=0.0, double y=0.0);
	void disp( );
	double dist(Line l);  //声明为Point的成员
};
Point::Point (double x, double y){
	this->x = x;  
	this->y = y; 
} 		
void  Point::disp( ){  
	cout<<"点（"<<x<<","<<y<<"）";  
}
class Line
{
private:
	double a, b, c;             //直线为ax+by+c=0的系数
public: 
	Line(double a=0.0, double b=0.0, double c=0.0);
	void disp( );
	friend double Point::dist(Line l); //声明为Line的友元
};
Line::Line(double a, double b, double c){
	this->a = a;  this->b = b;  this->c = c;    
} 		
void  Line::disp( ){ 
	cout<<"线（"<<a<<"x+"<<b<<"y+"<<c<<"=0）";
	}
double Point::dist(Line l){
	return fabs(l.a*x+l.b*y+l.c)/sqrt(l.a*l.a+l.b*l.b);
}
int main()
{
	Point p1(1.0,2.0);
	Line l1(3.0,4.0,5.0);
	p1.disp();
	l1.disp();
	cout<<"距离为："<<p1.dist(l1)<<endl;
	return 0;
}

三、友元类

#include<iostream>
#include<cmath>
using namespace std;
class Point
{
private:
	double x, y;                  //x,y 坐标
public: 
	Point(double x=0.0, double y=0.0);
	void disp( );
	friend class ComputeTools; //声明为Point的友元类
};
Point::Point (double x, double y){
	this->x = x;  
	this->y = y; 
} 		
void  Point::disp( ){  
	cout<<"点（"<<x<<","<<y<<"）";  
}
class Line
{
private:
	double a, b, c;             //直线为ax+by+c=0的系数
public: 
	Line(double a=0.0, double b=0.0, double c=0.0);
	void disp( );		// 输出私有变量的成员函数
	friend class ComputeTools; //声明为Line的友元类
};
Line::Line(double a, double b, double c){
	this->a = a;  
	this->b = b;  
	this->c = c;  
} 		
void  Line::disp( ){  
	cout<<"线（"<<a<<"x+"<<b<<"y+"<<c<<"=0）";
}
class ComputeTools  // Point and Line 的友元类
{
public:
	static double distance(Point p1,Point p2); //重载点与点距离
	static double distance(Point p,Line l); //重载点与直线距离
};
double ComputeTools::distance(Point p1,Point p2){
	return sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y)); 
}
double ComputeTools::distance(Point p,Line l){
	return fabs(l.a*p.x+l.b*p.y+l.c)/sqrt(l.a*l.a+l.b*l.b);
}
int main(){
	Point p1(1.0,2.0),p2(3.0,4.0); 	
	Line l1(3.0,4.0,5.0);
	p1.disp(); 	p2.disp();
	cout<<"距离为："<<ComputeTools::distance(p1,p2)<<endl;
	p1.disp(); 	l1.disp();
	cout<<"距离为："<<ComputeTools::distance(p1,l1)<<endl;
	return 0;
}