组合类的概念：一个类的成员是另一个类的对象，如下练习中Line类的成员a,b是Point类的对象。Point在类Line中创建对象a，b过程中，会调用构造函数对其进行初始化，而像类Line中Point对象的创建不含参数方式，也就需要Point类中含有不含参数的构造函数！

#include<iostream>
using namespace std;
class Point
{
public:
    Point() {};                          //不加这个会使得类Line中的声明变量不合法
    Point(float a,float b);
    void Display(void);
    float Distance(Point &point);
    float get_x() { return point_x; }
    float get_y() { return point_y; }
private: 
    float point_x, point_y;
    float distance;
};

Point::Point(float a, float b)
{
    point_x = a;
    point_y = b;
}

float Point::Distance(Point &point)
{
    float distance;
    distance = sqrt(pow(point_x - point.point_x,2) + pow(point_y - point.point_y,2));
    return distance;
}

void Point::Display(void)
{
    cout << "两点之间的距离：" << distance << endl;
}

class Line
{
public:
    Line(Line &line);                       //拷贝构造函数实现
    Line(Point &point1, Point &point2);
    double Display(Line &line);
private:
    Point a, b; 
    double distance;
};

Line::Line(Line &line)
{
    a = line.a;
    b = line.b;
}

Line::Line(Point &point1, Point &point2)
{
    a = point1;
    b = point2;
}

double Line::Display(Line &line)
{
    distance = sqrt(pow(line.a.get_x() - line.b.get_x(), 2) + pow(line.a.get_y() - line.b.get_y(), 2));
    return distance;
}

void main()
{
    Point a;
    Point b(10.2, 9.7), c(12.9, 15.6);
    a = c;
    cout << "两点之间的距离：" << a.Distance(b) << endl;
    Line s(a, b);
    Line s1(s);
    cout << s1.Display(s1) << endl;
}

本人菜鸟刚学c++，凡有不对的地方还请大神指正，谢谢~