析构函数什么情况下要定义为虚函数？

 1 1.第一段代码  2 #include<iostream>

 3 using namespace std;

 4 class ClxBase{

 5 public:

 6     ClxBase() {};

 7     ~ClxBase() {cout << "Output from the destructor of class ClxBase!" << endl;};

 8     void DoSomething() { cout << "Do something in class ClxBase!" << endl; };

 9 };

10 class ClxDerived : public ClxBase{

11 public:

12     ClxDerived() {};

13     ~ClxDerived() { cout << "Output from the destructor of class ClxDerived!" << endl; };

14     void DoSomething() { cout << "Do something in class ClxDerived!" << endl; };

15 };

16   int   main(){  

17   ClxDerived *p =  new ClxDerived;

18   p->DoSomething();

19   delete p;

20   return 0;

21   }


22 运行结果：

23 Do something in class ClxDerived!            

24 Output from the destructor of class ClxDerived!

25 Output from the destructor of class ClxBase!  

26     这段代码中基类的析构函数不是虚函数,在main函数中用继承类的指针去操作继承类的成员,释放指针P的过程是:先释放继承类的资源,再释放基类资源. 

27 

 

28 2.第二段代码 29 #include<iostream>

30 using namespace std;

31 class ClxBase{

32 public:

33     ClxBase() {};

34     ~ClxBase() {cout << "Output from the destructor of class ClxBase!" << endl;};

35     void DoSomething() { cout << "Do something in class ClxBase!" << endl; };

36 };

37 class ClxDerived : public ClxBase{

38 public:

39     ClxDerived() {};

40     ~ClxDerived() { cout << "Output from the destructor of class ClxDerived!" << endl; };

41     void DoSomething() { cout << "Do something in class ClxDerived!" << endl; }

42 };

43   int   main(){  

44   ClxBase *p =  new ClxDerived;

45   p->DoSomething();

46   delete p;

47   return 0;

48   } 

49 输出结果：

50 Do something in class ClxBase!

51 Output from the destructor of class ClxBase!

52     这段代码中基类的析构函数同样不是虚函数,不同的是在main函数中用基类的指针去操作继承类的成员,释放指针P的过程是:只是释放了基类的资源,而没有调用继承类的析构函数.调用dosomething()函数执行的也是基类定义的函数.

53     一般情况下,这样的删除只能够删除基类对象,而不能删除子类对象,形成了删除一半形象,造成内存泄漏.

54     在公有继承中,基类对派生类及其对象的操作,只能影响到那些从基类继承下来的成员.如果想要用基类对非继承成员进行操作,则要把基类的这个函数定义为虚函数.

55     析构函数自然也应该如此:如果它想析构子类中的重新定义或新的成员及对象,当然也应该声明为虚的. 

56  

57 3.第三段代码： 58 #include<iostream>

59 using namespace std;

60 class ClxBase{

61 public:

62     ClxBase() {};

63     virtual ~ClxBase() {cout << "Output from the destructor of class ClxBase!" << endl;};

64     virtual void DoSomething() { cout << "Do something in class ClxBase!" << endl; };

65 };

66 class ClxDerived : public ClxBase{

67 public:

68     ClxDerived() {};

69     ~ClxDerived() { cout << "Output from the destructor of class ClxDerived!" << endl; };

70     void DoSomething() { cout << "Do something in class ClxDerived!" << endl; };

71 };

72   int   main(){  

73   ClxBase *p =  new ClxDerived;

74   p->DoSomething();

75   delete p;

76   return 0;

77   }  

78 运行结果：

79 Do something in class ClxDerived!

80 Output from the destructor of class ClxDerived!

81 Output from the destructor of class ClxBase!

82     这段代码中基类的析构函数被定义为虚函数,在main函数中用基类的指针去操作继承类的成员,释放指针P的过程是:只是释放了继承类的资源,再调用基类的析构函数.调用dosomething()函数执行的也是继承类定义的函数. 

83  

84     如果不需要基类对派生类及对象进行操作,则不能定义虚函数,因为这样会增加内存开销.当类里面有定义虚函数的时候,编译器会给类添加一个虚函数表,里面来存放虚函数指针,这样就会增加类的存储空间.所以,只有当一个类被用来作为基类的时候,才把析构函数写成虚函数.

 

转载自：http://blog.sina.com.cn/s/blog_7c773cc50100y9hz.html