类的组合

类名::类名(形参表)：内嵌对象1(形参表),内嵌对象2(形参表)...{
   //初始化函数体
}

class Point{
private:
    int x,y;
public:
    Point(int newX,int newY):x(newX),y(newY){}
};
class Circle{
private:
    double radius;
    point center;//圆的圆心点
public:

};

Circle(double r,point p):radius(r),center(p){
    //构造函数初始化代码
}

组合类的复制构造函数，默认状态下同样由编译器自行生成——它会自动调用各个内嵌对象的复制构造函数，为它们初始化。如果我们想要为组合类编写复制构造函数的话，那么我们需要为内嵌对象成员的复制构造函数传递参数。比如说，对于circle类，它的复制构造函数应该这样定义：

#include <iostream>
#include <cmath>
using std::cin;
using std::cout;
using std::endl;
class Point{
public:
    Point(int newX = 0,int newY = 0){
      x = newX;
      y = newY;
    }
    Point(Point &p){
      x = p.x;
      y = p.y;
    }
    int getX() {
          return x;
    }
    int getY() {
          return y;    
    }
private:
    int x,y;
};

class Line{
public:
    Line(Point new_p1, Point new_p2):p1(new_p1),p2(new_p2){
        double x = static_cast<double>(p1.getX() - p2.getX());
        double y = static_cast<double>(p1.getY() - p2.getY());
        len = sqrt(x*x + y*y);
    }
    Line(Line &l):p1(l.p1),p2(l.p2){
        len = l.len;
    }
    double getLen(){
        return len;
    }
private:
    Point p1, p2;
    double len;
};

int main(){
    Point pa(1,2);
    Point pb(3,5);
    Line L1(pa, pb);
    Line L2(L1);
    cout<<"The length of L1 is "<< L1.getLen()<<endl;
    cout<<"The length of L2 is "<< L2.getLen()<<endl;   
    return 0;
}