为什么这段代码中对象rectangle的各个成员函数输出的值是对的，而box的却都是错的

#include <iostream>
#include <math.h>
using namespace std;
class rectangle
{
protected:
  double length,width,l,w;
public:
  void setlength();
  void getlength();
  void setwidth();
  void getwidth();
  double area();
  double perimeter();
  double diagmonal();
};
void rectangle::setlength()
{
  cout<<"please enter the length of rectangle(box):";
  cin>>l;
}
void rectangle::getlength()
{
  length=l;
}
void rectangle::setwidth()
{
  cout<<"please enter the width of rectangle(box):";
  cin>>w;
}
void rectangle::getwidth()
{
  width=w;
}
double rectangle::area()
{
  return(length*width);
}
double rectangle::perimeter()
{
  return(2*(length+width));
}
double rectangle::diagmonal()
{
  double x=length*length+width*width;
  return(sqrt(x));
}
class box:protected rectangle
{
protected:
  double height,h;
public:
  void setheight();
  void getheight();
  double area();
  double perimeter();
  double diagmonal();
  double volume();
};
void box::setheight()
{
  cout<<"please enter the height for box:";
  cin>>h;
}
void box::getheight()
{
  height=h;
  cout << height << endl;
}
double box::area()
{
  return(2*(length*width+length*height+width*height));
}
double box::perimeter()
{
  return(4*(length+width+height));
}
double box::diagmonal()
{
  return(sqrt(length*length+width*width+height*height));
}
double box::volume()
{
  setlength();
  getlength();
  setwidth();
  getwidth();
  return((length*width)*height);
}
int main()
{
  box * p = new box;
  p->setheight();
  p->getheight();
  cout<<"the box's volume is:"<<p->volume()<<endl;
  delete p;
  p = NULL;
}

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