C++解構函式加上virtual是為了防止記憶體洩漏。

用在C++實現多型的時候，其基類要加virtual。

原因跟動態繫結有關，大家都知道，多型是通過虛擬函式實現的，而虛擬函式又是通過動態繫結實現的。

先舉個例子：

class Base
{
	public:
	virtual void function()
	{
		cout<<"this is Base's function"<<endl;
	}		
	
	virtual ~Base()
	{
		cout<<"this is Base's destroy"<<endl;
	}
};

class Derived:public Base
{
	public:
	void function()
	{
		cout<<"this is Derived's function"<<endl;
	}
	
	~Derived()
	{
		cout<<"this is Derived's destroy"<<endl;
	}
};

int main()
{
	Base *ptr = new Derived();
	ptr->function();
	delete ptr;
	return 0;
}
 

程式碼輸出結果：

this is Derived's function

this is Derived's destroy

this is Base's destroy

你可以嘗試一下把基類的解構函式前的virtual刪掉（第9行），輸出結果將是：

this is Derived's function

this is Base's destroy