Question

C++计算几何图形时计算结果总是是0

#ifndef SHAPE_H
#define SHAPE_H
#include <iostream>
using namespace std;
class Shape
{
public:
double circum() const { return 0; }
double volume() const { return 0; }
double area() const { return 0; }
};
#endif
---------------------------------------------------------------------------------

#ifndef CIRCLE_H
#define CIRCLE_H
#define PI 3.1415
#include <iostream>
using namespace std;
#include "shape.h"
class Circle:public Shape
{
public:
Circle(double r)
{
radius = r;
}
virtual double area() const
{
return radius * radius * PI;
}
virtual double circum() const
{
return radius * 2 * PI;
}
private:
double radius;
};
#endif
----------------------------------------------------------------------------------------------
#include <iostream>
#include "shape.h"
#include "circle.h"
using namespace std;
int main()
{
cout << "请输入圆的半径:";
cin >> R;
Circle circle(R);
Shape *aShape = &circle;
cout << "圆的周长为:" << aShape->circum() << endl;
cout << "圆的面积为:" << aShape->area() << endl;
system("pause");
system("cls");

}

计算结果总是0，麻烦大家帮忙看下哪里错了。。。

Answer 1

#include <stdio.h>
#include <iostream>
using namespace std;
class Shape
{
public:
    Shape(){}
    virtual double circum()const = 0;  // 定义成纯虚函数
    virtual double area()const = 0;
    virtual double volume()const = 0;
};

#define PI 3.1415
class Circle : public Shape
{
public:
    Circle(double r)
    {
        radius = r;
    }
    virtual double area() const
    {
        return radius * radius * PI;
    }
    virtual double circum() const
    {
        return radius * 2 * PI;
    }
    virtual double volume() const
    {
        return 0;
    }
private:
    double radius;
};

int main()
{
    double R;
    cout << "请输入圆的半径:";
    cin >> R;
    Circle circle(R);
    Shape *aShape = &circle;
    cout << "圆的周长为:" << aShape->circum() << endl;
    cout << "圆的面积为:" << aShape->area() << endl;
    system("pause");
    system("cls");
}