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