virtual解構函式的作用? .
大家知道，解構函式是為了在物件不被使用之後釋放它的資源，虛擬函式是為了實現多型。那麼把解構函式宣告為vitual有什麼作用呢？請看下面的程式碼：

1         #include 
2       using namespace std;
3
4       class Base
5       {
6       public:
7                Base() {};       //Base的建構函式
8               ~Base()        //Base的解構函式
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的建構函式
22              ~Derived()      //Derived的解構函式
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類的指標
35              pTest1->DoSomething();
36              delete pTest1;
37    
38              cout << endl;
39    
40              Base *pTest2 = new Derived();      //Base類的指標
41              pTest2->DoSomething();
42              delete pTest2;
43    
44              return 0;
45     }
 

先看程式輸出結果：
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!
程式碼第36行可以正常釋放pTest1的資源，而程式碼第42行沒有正常釋放pTest2的資源，因為從結果看Derived類的解構函式並沒有被呼叫。通常情況下類的解構函式裡面都是釋放記憶體資源，而解構函式不被呼叫的話就會造成記憶體洩漏。原因是指標pTest2是Base型別的指標，釋放pTest2時只進行Base類的解構函式。在程式碼第8行前面加上virtual關鍵字後的執行結果如下：
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!

此時釋放指標pTest2時，由於Base的解構函式是virtual的，就會先找到並執行Derived類的解構函式，然後再執行Base類的解構函式，資源正常釋放，避免了記憶體洩漏。
因此，只有當一個類被用來作為基類的時候，才會把解構函式寫成虛擬函式。