C++中虚构函数的作用

析构函数是为了在对象不被使用之后释放它的资源，虚函数是为了实现多态。那么把析构函数声明为vitual有什么作用呢？请看下面的代码：
1       #include <iostream>
2       using namespace std;
3
4       class Base
5       {
6       public:
7           Base() {};      
8           ~Base()       
9           {
10              cout << "Output from the destructor of class Base!" << endl;
11          };
12          virtual void DoSomething() 
13          {
14              cout << "Do something in class Base!" << endl;
15          };
16     };
17    
18     class Derived : public Base
19     {
20     public:
21              Derived() {};    
22              ~Derived()     
23              {
24                  cout << "Output from the destructor of class Derived!" << endl;
25              };
26              void DoSomething()
27              {
28                  cout << "Do something in class Derived!" << endl;
29              };
30     };
31    
32     int main()
33     {
34              Derived *pTest1 = new Derived();   //Derived类的指针
35              pTest1->DoSomething();
36              delete pTest1;
37    
38              cout << endl;
39    
40              Base *pTest2 = new Derived();      //Base类的指针
41              pTest2->DoSomething();
42              delete pTest2;
43    
44              return 0;
45     }
先看程序输出结果：
1       Do something in class Derived!
2       Output from the destructor of class Derived!
3       Output from the destructor of class Base!
4      
5       Do something in class Derived!
6       Output from the destructor of class Base!
代码第36行可以正常释放pTest1的资源，而代码第42行没有正常释放pTest2的资源，因为从结果看Derived类的析构函数并没有被调用。通常情况下类的析构函数里面都是释放内存资源，而析构函数不被调用的话就会造成内存泄漏。原因是指针pTest2是Base类型的指针，释放pTest2时 只进行Base类的析构函数。在代码第8行前面加上virtual关键字后的运行结果如下：
1       Do something in class Derived!
2       Output from the destructor of class Derived!
3       Output from the destructor of class Base!
4      
5       Do something in class Derived!
6       Output from the destructor of class Derived!
7       Output from the destructor of class Base!
此时释放指针pTest2时，由于Base的析构函数是virtual的，就会先找到并执行Derived类的析构函数，然后再执行Base类的析构函数，资源正常释放，避免了内存泄漏。
因此，只有当一个类被用来作为基类的时候，才会把析构函数写成虚函数。