为什么析构函数常声明为虚函数

       昨天看了火狐的源码，看到里面很多，应该说几乎所有的析构函数都申明为虚函数，一时想不起来为啥，就上网查了一下，才知道这样做的作用是：

       当你动态申请一个对象时，并且把这个对象的指针赋值给基类，这时当你用这个基类指针释放内存时，就有用了，因为这样可以用多态性原理调用对象实际的析构函数来析构内存。举例代码如下

#include "stdafx.h"#include <iostream>using namespace std;class CPen{public:CPen(int weight) : m_weight(weight){cout << "construct Cpen object.\n";}virtual ~CPen(){cout << "destruct CPen object\n";}//~CPen()//{//cout << "destruct CPen object\n";//}void PrintWeight() const{cout << "weight = " << m_weight;}private:int m_weight;};class CShape{public:int m_length;int m_thickness;int m_colour;};class CPencil : public CPen{public:CPencil(int weight, int length, int thickness, int colour) : CPen(weight){m_shapePtr = new CShape;m_shapePtr->m_length = length;m_shapePtr->m_thickness = thickness;m_shapePtr->m_colour = colour;//m_weight = weight; errorcout << "construct CPencil object.\n";}~CPencil(){cout << "destruct CPencil\n";delete m_shapePtr;}void ShowInfo() const{cout << "I am a pencil: ";PrintWeight();cout << " length = " << m_shapePtr->m_length << " thickness = " << m_shapePtr->m_thickness<< " colour = " << m_shapePtr->m_colour << endl;}private:CShape *m_shapePtr;};int _tmain(int argc, _TCHAR* argv[]){CPen *penPtr = new CPencil(10, 11, 12, 13);((CPencil *)penPtr)->ShowInfo();delete penPtr;penPtr = NULL;getchar();return 0;}

代码中注释了的析构函数是没有声明为虚函数的情况，大家如果也想测试的话，只要把注释去掉，在把虚析构函数注释起来，就可以对比出他们两的差别了。