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
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;
}
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()
{
return((length*width)*height);
}
int main()
{
rectangle r;
box b;
r.setlength();
r.getlength();
r.setwidth();
r.getwidth();
b.setheight();
b.getheight();
cout<<"the box's volume is:"<<b.volume()<<endl;
}

如果你非要用protected继承，那只能这样：

#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;
}

既然是继承，派生类就不要再定义这些成员了。

这个格式太惨了。

你的box型的b和rectangle型的r**实例化之后**之间没有关联。

你试试这样吧：

rectangle r;
box * p = (box*)&r;
r.setlength();
r.getlength();
r.setwidth();
r.getwidth();
p->setheight();
p->getheight();
cout<<"the box's volume is:"<<p->volume()<<endl;

可能是数据类型不对，但是程序总的来看是没错的

1. class box:public rectangle 这里改为public继承

rectangle * r;
box * p = new box;
r = p;
r->setlength();
r->getlength();
r->setwidth();
r->getwidth();
p->setheight();
p->getheight();
cout<<"the box's volume is:"<<p->volume()<<endl;
delete p;
p = NULL;