为什么无法获得正确的面积和周长

Question

//Shape.h
#include<iostream>
using namespace std;
class Shape
{ public :
	virtual void GetArea()=0;
	virtual void GetPerimeter()=0;};
class Circle:public Shape
{ protected:
int m_r;

public: 
	Circle();
	Circle(int);
	virtual void GetArea();
    virtual void GetPerimeter();
  friend ostream &operator<<( ostream &output, 
                                       const Circle &C ) ;
  Circle&operator=( Circle&);
double area,perimeter;};
class Rectangle:public Shape
{ private:
 int m_w,m_h;
 
public:
	Rectangle();
	Rectangle(int,int);
	virtual void GetArea();
	virtual void GetPerimeter();
  friend ostream &operator<<( ostream &output, 
                                       const Rectangle &R)   ;
 Rectangle&operator=(Rectangle&);
double area,perimeter;}; 

//8.2.cpp
#include "stdafx.h"
#include"Shape.h"
#include<string>
#include <iostream>
using namespace std;

int main()
{   int r=3;
    int w=5,h=8;
	int m_r=r;
	int m_w=w;
	int m_h=h;
	Circle circle(r);
	Rectangle rectangle(w,h);
	cout<<"圆的面积为："<<circle.area<<endl
		<<"圆的周长为："<<circle.perimeter<<endl
		<<"矩形的面积为："<<rectangle.area<<endl
		<<"矩形的周长为："<<rectangle.perimeter<<endl;
	return 0;
}


//Shape.cpp
#include"stdafx.h"
#include"Shape.h"
using namespace std;

Circle::Circle( int r)
{ m_r=r;}
 void Circle::GetArea()
 { int PI=3;
	 double area=PI*m_r*m_r;}
 void Circle::GetPerimeter()
 {   int PI=3;
	 double perimeter=2*PI*m_r;}
 ostream &operator<<( ostream &output, 
                                       const Circle &C)
      { 
		  output << "面积为：" << C.area << "周长为：" << C.perimeter;
		  return output;     }
Circle&Circle:: operator=( Circle& C)
 { int r=0;
	 if(&C!=this)
 { C.m_r=r; }
	   return *this; }
 Rectangle::Rectangle(int w,int h)
 {m_w=w;
m_h=h;}
 void Rectangle::GetArea()
 {  double area=m_w*m_h;}
 void Rectangle::GetPerimeter()
 { double perimeter=2*m_w+2*m_h;}
ostream &operator<<( ostream &output, 
                                       const Rectangle&R)
      { 
		  output << "面积为：" << R.area << "周长为：" << R.perimeter;
		  return output;     }
Rectangle&Rectangle::operator=(Rectangle &R)
{int w=0,h=0;
if(&R!=this)
{R.m_h=h;
R.m_w=w;}
return *this;}
void ShapeArea(Shape &s)
{ s.GetArea();}
  void ShapePerimeter(Shape &s)
  { s.GetPerimeter();}

Answer 1

原因是，你在实现GetArea，GetPerimeter的时候，都重新在函数内新声明了一个变量double area = ...;double perimeter = ...;

你本意是修改类内public double area, perimeter的值，但是在这里因为double area = ...;，将计算的所得值赋给了一个函数内的局部变量，所以算半天还是初始值。

另外，记得在main()函数里面调用

    Circle circle = Circle(r);
    circle.GetArea();
    circle.GetPerimeter();
    Rectangle rectangle = Rectangle(w, h);
    rectangle.GetArea();
    rectangle.GetPerimeter();
以计算需要的数值。

 

我将你的代码进行了修改，在我本机上运行是没问题的。

代码我贴到这里了：

main文件：https://paste.ubuntu.com/p/T2T9QBS7P9/

.h头文件：https://paste.ubuntu.com/p/sQNJFR538H/

shape.cpp文件：https://paste.ubuntu.com/p/9z74jvxdXh/