C++ FAQ for me

1. 为什么需要虚析构函数,什么时候需要?

看下面的代码:

class ClxBase
{
public:
    ClxBase() {};
    virtual ~ClxBase() {cout << "Output from the destructor of class ClxBase!" << endl;};

    virtual void DoSomething() { cout << "Do something in class ClxBase!" << endl; };
};

class ClxDerived : public ClxBase
{
public:
    ClxDerived() {};
    ~ClxDerived() { cout << "Output from the destructor of class ClxDerived!" << endl; }; 

    void DoSomething() { cout << "Do something in class ClxDerived!" << endl; };
};

代码:

ClxBase *pTest = new ClxDerived;
pTest->DoSomething();
delete pTest;

的输出结果是

Do something in class ClxDerived!
Output from the destructor of class ClxDerived!

Output from the destructor of class ClxBase!
如果把类ClxBase析构函数前的virtual去掉，那输出结果就是下面的样子了：
Do something in class ClxDerived!

Output from the destructor of class ClxBase!

为什么ClxDerived的析构函数没有调用呢? pTest的动态类型是ClxDerived,但是静态类型是ClxBase. 如果ClxBase的析构函数不加virtual,当调用 delete pTest时, 系统会只调用ClxBase的析构函数,而ClxDerived的析构函数不会被调用。

当然了如果delete后面的指针的静态类型如果是ClxDerived就不会有这个问题了。