virtual析构函数（作用）

virtual析构函数的作用? .
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;34       class Base5       {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 Base19     {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。

原文：http://blog.csdn.net/han_348154920/article/details/5944351