//5-6友元函数
#include<iostream>
#include<cmath>
using namespace std;
class Point{
public:Point(int x = 0, int y = 0):x(x), y(y){}int getX(){return x;}int getY(){return y;}friend float dist(Point &a, Point &b);
private:int x,y;
};
float dist(Point &a, Point &b){double x = a.x - b.x;double y = a.y - b.y;return static_cast<float>(sqrt(x*x+y*y));
}
int main(){Point p1(1,1), p2(3,4);cout << "The distance is:";cout << dist(p1,p2) << endl;return 0;
}

class A{friend class B;
public:void display(){cout << x << endl;}
private:int x;
}
class B{
public:void set(int i);void display();
private:A a;//A是B的组件
}
void B::set(int i){a.x = i;
}
void B::display(){a.display();
}

class A{
public:A(int i, int j){x = i; y = j;}
private:int x,y;
};
A const a(3,4);//a是常对象，不能被更新

//5-7 常成员函数举例
#include<iostream>
using namespace std;
class R{
public:R(int r1, int r2):r1(r1),r2(r2){}void print();void print() const;
private:int r1,r2;
};
void R::print(){cout << r1 << " : " << r2 << endl;
}
void R::print() const{//编译器会审查内部有没有改变数据cout << r1 << " : " << r2 << endl;
}
int main(){R a(5,4);a.print();//调用void print()const R b (20,39);b.print();//调用void print() constreturn 0;
}

//5-8 常数据成员举例
#include<iostream>
using namespace std;
class A{
public: A(int i);void print();
private:const int a;static const int b;//静态常数据成员
};
const int A::b = 10;
A::A(int i):a(i){}//常成员a只能在初始化列表中初始化，不能再函数体中赋值
void A::print(){cout << a << " : " << b << endl;
}
int main(){//建立对象a和b，并以100和0作为初值，分别调用构造函数，//通过构造函数的初始化例表给对象的场数据成员赋初值A a1(100), a2(0);a1.print();a2.print();return 0;
}

//5-9常引用作形参
#include<iostream>
#include<cmath>
using namespace std;
class Point{
public:Point(int x = 0, int y = 0):x(x),y(y){}int getX(){return x;}int getY(){return y;}friend float dist(const Point &p1, const Point &p2);
private:int x,y;
};
float dist(const Point &p1, const Point &p2){double x = p1.x - p2.x;double y = p1.y - p2.y;return static_cast<float>(sqrt(x*x+y*y));
}
int main(){const Point myp1(1,1),myp2(4,5);cout << "The distance is: ";cout << dist(myp1,myp2) << endl;return 0;
}