【C++】 --- 纯虚函数和抽象类

1. /**
2. 纯虚函数的定义： virtual 返回类型 函数名(形参) = 0
3. 含有纯虚函数的类叫抽象类
4. 抽象类的特点：不能用抽象类去实例化对象
5. 抽象类的作用：提供一个统一的接口，让子类去实现，子类实现之后才可以实例化对象
6. */
7. 
8. class figure     //抽象类
9. {
10. public:
11.   virtual void getS() = 0;    //纯虚函数
12. 
13. protected:
14. private:
15. 
16. };
17. 
18. class Circle : public figure
19. {
20. public:
21.   Circle(int a)
22.   {
23.     this->a = a;
24.   }
25.   virtual void getS()
26.   {
27.     cout << "the area of the circle is " << 3.14*a*a << endl;
28.   }
29. 
30. protected:
31. private:
32.   int a;
33. };
34. 
35. class Triangle : public figure
36. {
37. public:
38.   Triangle(int a, int b)
39.   {
40.     this->a = a;
41.     this->b = b;
42.   }
43.   virtual void getS()
44.   {
45.     cout << "the area of the triangel is " << a*b/2 << endl;
46.   }
47. 
48. protected:
49. private:
50.   int a;
51.   int b;
52. };
53. 
54. 
55. void objplay(figure * base)
56. {
57.   base->getS();   //会发生多态
58. }
59. 
60. int main(void)
61. {
62.   //figure F; //抽象类不能实例化对象
63.   //figure *base = NULL;
64. 
65.   Circle c1(10);
66.   Triangle t1(10,20);
67. 
68.   objplay(&c1);
69.   objplay(&t1);
70. 
71.   system("pause");
72.   return 0;
73. }