virtual析构函数的作用?

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大家知道,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
2       using namespace std;
3
4       class Base
5       {
6       public:
7                Base() {};       //Base的构造函数
8               ~Base()        //Base的析构函数
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() {};     //Derived的构造函数
22              ~Derived()      //Derived的析构函数
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。

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