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：

#include<iostream>
using namespace std;

class Base
{
public:
	Base() {}; //Base的建構函式
	~Base() //Base的解構函式
	{
		cout << "Output from the destructor of class Base!" << endl;
	};
	virtual void DoSomething()
	{
		cout << "Do something in class Base!" << endl;
	};
};

class Derived : public Base
{
public:
	Derived() {}; //Derived的建構函式
	~Derived() //Derived的解構函式
	{
		cout << "Output from the destructor of class Derived!" << endl;
	};
	void DoSomething()
	{
		cout << "Do something in class Derived!" << endl;
	};
};

int main()
{
	Derived *pTest1 = new Derived(); //Derived類的指標
	pTest1->DoSomething();
	delete pTest1;

	cout << endl;

	Base *pTest2 = new Derived(); //Base類的指標
	pTest2->DoSomething();
	delete pTest2;

	return 0;
}
 

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。