C++中关于纯虚函数的问题

paddyluo

#include <iostream>
using namespace std;

class Shape
{
public:
    virtual float area(void) const{return 0.0;}
    virtual float volume(void) const{return 0.0;}
    // virtual float area(void) const = 0;
    // virtual float volume(void) const = 0;
    virtual void shapeName(void) const = 0;   
};

class Point:public Shape
{
public:
    Point(float a = 0, float b = 0):x(a), y(b){}
    void setPoint(float a, float b){x = a; y = b;}
    float getX(void) const{return x;}
    float getY(void) const{return y;}
    virtual void shapeName(void) const{cout << "Point: ";}   

    friend ostream & operator<<(ostream &output, const Point &p)
    {
        output << "[" << "p.x" << ", " << p.y << "]";
        return output;
    }
   
protected:
    float x, y;
};

class Circle:public Point
{
public:
    Circle(float a, float b, float r):Point(a, b), radius(r){}
    void setRadius(float r){radius = r;}
    float getRadius(void) const{return radius;}
    virtual float area(void) const{return (3.14159 * radius * radius);}
    virtual void shapeName(void) const{cout << "Circle: ";}   

    friend ostream & operator<<(ostream &output, const Circle &cc)
    {
        output << "[" << "cc.x" << ", " << cc.y << "], r = " << cc.radius << ", area = " << cc.area();
        return output;
    }
   
protected:
    float radius;
};

class Cylinder:public Circle
{
public:
    Cylinder(float a = 0, float b = 0, float r = 0, float h = 0):Circle(a, b, r), height(h){}
    void setHeight(float h){height = h;}
    float getHeight(void) const{return height;}
    virtual float area(void) const{return (2 * Circle::area() + 2 * 3.14159 * radius * height);}
    virtual float volume(void) const{return (Circle::area() * height);}
    virtual void shapeName(void) const{cout << "Cylinder: ";}
   
    friend ostream & operator<<(ostream &output, const Cylinder &cy)
    {
        output << "[" << "cy.x" << ", " << cy.y << "], r = " << cy.radius << ", h = " << cy.height
               << ", area = " << cy.area() << ", volume = " << cy.volume();
        return output;
    }
   
protected:
    float height;
};

int main(void)
{
    Point p(3.2, 4.5);
    Circle cc(2.4, 1.2, 5.6);
    Cylinder cy(3.5, 6.4, 5.2, 10.5);
   
    p.shapeName();
    cout << p << endl;
    cc.shapeName();
    cout << cc << endl;
    cy.shapeName();
    cout << cy << "\n\n";
   
    Shape *pt;
    pt = &p;
    pt->shapeName();
    cout << "x = " << p.getX() << ", y = " << p.getY() << "\narea = " << pt->area()
         << "\nvolume = " << pt->volume() << "\n\n";
         
     pt = &cc;
     pt->shapeName();
    cout << "x = " << cc.getX() << ", y = " << cc.getY() << "\narea = " << pt->area()
         << "\nvolume = " << pt->volume() << "\n\n";     
         
     pt = &cy;
     pt->shapeName();
    cout << "x = " << cy.getX() << ", y = " << cy.getY() << "\narea = " << pt->area()
         << "\nvolume = " << pt->volume() << "\n\n";
   
    return 0;
}

在抽象基类Shape中如果把前两行换成注释中的两行即改为纯虚函数，则编译不通过，为什么呢？

小甲鱼

已经帮您修改了粗心的错误~

1. #include <iostream>
   
2. using namespace std;
   
3. 
   
4. class Shape
   
5. {
   
6. public:
   
7.     virtual float area(void) const{return 0.0;}
   
8.     virtual float volume(void) const{return 0.0;}
   
9.     // virtual float area(void) const = 0;
   
10.     // virtual float volume(void) const = 0;
    
11.     virtual void shapeName(void) const = 0;   
    
12. };
    
13. 
    
14. class Point:public Shape
    
15. {
    
16. public:
    
17.     Point(float a = 0, float b = 0):x(a), y(b){}
    
18.     void setPoint(float a, float b){x = a; y = b;}
    
19.     float getX(void) const{return x;}
    
20.     float getY(void) const{return y;}
    
21.     virtual void shapeName(void) const{cout << "oint: ";}   
    
22. 
    
23.     friend ostream & operator<<(ostream &output, const Point &p)
    
24.     {
    
25.         output << "[" << "p.x" << ", " << p.y << "]";
    
26.         return output;
    
27.     }
    
28.    
    
29. protected:
    
30.     float x, y;
    
31. };
    
32. 
    
33. class Circle:public Point
    
34. {
    
35. public:
    
36.       Circle(float a, float b, float r):Point(a, b), radius(r){}
    
37.     void setRadius(float r){radius = r;}
    
38.     float getRadius(void) const{return radius;}
    
39.     virtual float area(void) const{return (3.14159 * radius * radius);}
    
40.     virtual void shapeName(void) const{cout << "Circle: ";}   
    
41. 
    
42.     friend ostream & operator<<(ostream &output, const Circle &cc)
    
43.     {
    
44.         output << "[" << "cc.x" << ", " << cc.y << "], r = " << cc.radius << ", area = " << cc.area();
    
45.         return output;
    
46.     }
    
47.    
    
48. protected:
    
49.     float radius;
    
50. };
    
51. 
    
52. class Cylinder:public Circle
    
53. {
    
54. public:
    
55.     Cylinder(float a = 0, float b = 0, float r = 0, float h = 0):Circle(a, b, r), height(h){}
    
56.     void setHeight(float h){height = h;}
    
57.     float getHeight(void) const{return height;}
    
58.     virtual float area(void) const{return (2 * Circle::area() + 2 * 3.14159 * radius * height);}
    
59.     virtual float volume(void) const{return (Circle::area() * height);}
    
60.     virtual void shapeName(void) const{cout << "Cylinder: ";}
    
61.    
    
62.     friend ostream & operator<<(ostream &output, const Cylinder &cy)
    
63.     {
    
64.         output << "[" << "cy.x" << ", " << cy.y << "], r = " << cy.radius << ", h = " << cy.height
    
65.                << ", area = " << cy.area() << ", volume = " << cy.volume();
    
66.         return output;
    
67.     }
    
68.    
    
69. protected:
    
70.     float height;
    
71. };
    
72. 
    
73. int main(void)
    
74. {
    
75.     Point p(3.2, 4.5);
    
76.     Circle cc(2.4, 1.2, 5.6);
    
77.     Cylinder cy(3.5, 6.4, 5.2, 10.5);
    
78.    
    
79.     p.shapeName();
    
80.     cout << p << endl;
    
81.     cc.shapeName();
    
82.     cout << cc << endl;
    
83.     cy.shapeName();
    
84.     cout << cy << "\n\n";
    
85.    
    
86.     Shape *pt;
    
87.     pt = &p;
    
88.     pt->shapeName();
    
89.     cout << "x = " << p.getX() << ", y = " << p.getY() << "\narea = " << pt->area()
    
90.          << "\nvolume = " << pt->volume() << "\n\n";
    
91.          
    
92.      pt = &cc;
    
93.      pt->shapeName();
    
94.     cout << "x = " << cc.getX() << ", y = " << cc.getY() << "\narea = " << pt->area()
    
95.          << "\nvolume = " << pt->volume() << "\n\n";     
    
96.          
    
97.      pt = &cy;
    
98.      pt->shapeName();
    
99.     cout << "x = " << cy.getX() << ", y = " << cy.getY() << "\narea = " << pt->area()
    
100.          << "\nvolume = " << pt->volume() << "\n\n";
     
101.    
     
102.     return 0;
     
103. }

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朋友记得以后贴代码用代码格式哦~

仰望天上的光

Point类没有重写基类中的纯虚函数，因此Point类还是虚基类，不能产生对象，但LZ在main函数中企图产生Point类型的对象，所以错了。

paddyluo

仰望天上的光 发表于 2011-6-13 12:05

                               
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Point类没有重写基类中的纯虚函数，因此Point类还是虚基类，不能产生对象，但LZ在main函数中企图产生Point类 ...

嗯，谢谢大哥~