关于C++虚析构函数

以你之姓@ 2022-05-15 01:30 242阅读 0赞

在上一篇博文《[**关于C++对象的自杀** ][C_] 》末尾提到了基类的**虚析构函数** ，于是再添一篇。

什么时候析构函数需要加个virtual 来修饰呢？

**当要通过指向基类的指针删除子类对象时。**

Why?

**这是为了保证基类和子类的析构函数都得到调用。**

*个人理解* ：因为基类类型的指针指向的是子类对象中的基类部分，如果析构函数不为虚函数，则无法调用到子类的析构函数。

如下代码：

\#include <iostream> using namespace std; class ClsBase\{ public: ClsBase()\{ cout << "ClsBase constructor." << endl; \} ~ClsBase()\{ cout << "ClsBase destructor." << endl; \} \}; class ClsDerived : public ClsBase \{ public: ClsDerived()\{ cout << "ClsDerived constructor." << endl; \} ~ClsDerived()\{ cout << "ClsDerived destructor." << endl; \} \}; int main()\{ ClsBase \*p = new ClsDerived(); delete p; return 0; \}

输出为：

ClsBase constructor. ClsDerived constructor. ClsBase destructor.

**说明** 了基类析构函数不为虚函数时，delete指向基类的指针时无法调用到子类的析构函数。

将基类析构函数修改为虚函数，得到如下代码：

\#include <iostream> using namespace std; class ClsBase\{ public: ClsBase()\{ cout << "ClsBase constructor." << endl; \} virtual ~ClsBase()\{ cout << "ClsBase destructor." << endl; \} \}; class ClsDerived : public ClsBase \{ public: ClsDerived()\{ cout << "ClsDerived constructor." << endl; \} ~ClsDerived()\{ cout << "ClsDerived destructor." << endl; \} \}; int main()\{ ClsBase \*p = new ClsDerived(); delete p; return 0; \}

输出为：

ClsBase constructor. ClsDerived constructor. ClsDerived destructor. ClsBase destructor.

此时，子类的析构函数得到调用。

***个人理解*** ：

**这是因为基类中将析构函数声明为虚函数，则析构函数（的索引/指针）会被放在虚表vtbl中。**

**在继承过程中，vtbl属于基类部分，即基类类型的指针p可以指到的范围。**

**并且，析构函数的虚函数性质会在继承过程中得到保持（虽然函数名不同），于是子类的析构函数也位于虚表中，确保delete p时可以调用到子类的析构函数。**

**于是，析构函数自底向上得到调用了。**

最后引用C++ FAQ的一个段落：

***BTW, if you're interested, here are the mechanical details of why you need a virtual destructor when someone says delete using a Base pointer that's pointing at a Derived object. When you say***  ***delete p*** ***, and the class of p has a virtual destructor, the destructor that gets invoked is the one associated with the type of the object***  ***\*p*** ***, not necessarily the one associated with the type of the pointer. This is A Good Thing. In fact, violating that rule makes your program undefined. The technical term for that is, "Yuck."***

[C_]: http://blog.csdn.net/jasonblog/archive/2010/11/05/5989224.aspx