设计一个矩形类,分别用继承和组合的方式设计一个长方体类,计算其表面积，这个怎么设计啊

我晕  没人会吗？？我自己调出来了
#include<iostream>
using namespace std;
 
class Rectangle
{
  public:
      void Init(float xx=0,float yy=0)
      {
          length=xx;
          width=yy;
      }

      void Change(float xx,float yy)
      {
          length=xx;
          width=yy;
      }

      float Getlength() {  return length; }
      float Getwidth()  {  return width; }
  
  private:
      float length,width;
};

class Rectangular_block: public Rectangle
{
     public:
         void Init_b(float xx ,float yy, float zz=0)
         {
             Init(xx,yy);
             height=zz;
         }
         
         float Getheight()
         {
             return height;
         }
 
         float Surface_area(float length,float width,float height)
         {
             return (length*width+width*height+length*height)*2;
         }        

     private:
         float  height;
};

void main()
{
    Rectangular_block A;
    float Length,Width,Height;   
    cout<<"请输入长方体的长、宽和高："<<endl;
    cin>>Length;
    cin>>Width;
    cin>>Height;
    A.Init_b(Length,Width, Height);
    cout<<"长方体的表面积为："<<A.Surface_area(Length,Width,Height)<<endl;
}