最近在利用地铁通勤时间学习c++相关知识，在csdn看到一篇关于为什么析构函数前加virtual的文章，csdn原文：析构函数前加Virtual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：

#include<iostream>
using namespace std;
 
class Base
{
public:
    Base() {}; //Base的构造函数
    ~Base() //Base的析构函数
    {
        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的构造函数
    ~Derived() //Derived的析构函数
    {
    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类的指针
    pTest1->DoSomething();
    delete pTest1;
 
    cout << endl;
 
    Base *pTest2 = new Derived(); //Base类的指针
    pTest2->DoSomething();
    delete pTest2;
 
    return 0;
}

程序输出:

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。