Why should I declare a virtual destructor for an abstract class in C++?

Question

I know it is a good practice to declare virtual destructors for base classes in C++, but is it always important to declare virtual destructors even for abstract classes that function as interfaces? Please provide some reasons and examples why.

Answer 1

It's even more important for an interface. Any user of your class will probably hold a pointer to the interface, not a pointer to the concrete implementation. When they come to delete it, if the destructor is non-virtual, they will call the interface's destructor (or the compiler-provided default, if you didn't specify one), not the derived class's destructor. Instant memory leak.

For example

class Interface
{
   virtual void doSomething(void) = 0;
};

class Derived : public Interface
{
   Derived(void);
   ~Derived(void) 
   {
      // Do some important cleanup...
   }
};

void myFunc(void)
{
   Interface* p = new Derived();
   delete p; // calls Interface::~Interface, not Derived::~Derived
}

Answer 2

Yes it is always important. Derived classes may allocate memory or hold reference to other resources that will need to be cleaned up when the object is destroyed. If you do not give your interfaces/abstract classes virtual destructors, then every time you delete a derived class instance via a base class handle your derived class' destructor will not be called.

Hence, you're opening up the potential for memory leaks

class IFoo
{
  public:
    virtual void DoFoo() = 0;
};

class Bar : public IFoo
{
  char* dooby = NULL;
  public:
    virtual void DoFoo() { dooby = new char[10]; }
    void ~Bar() { delete [] dooby; }
};

IFoo* baz = new Bar();
baz->DoFoo();
delete baz; // memory leak - dooby isn't deleted

Answer 3

Kevin, first of all, please don't take my jumping on your comments in our previous discussion personally, I didn't intend to come off so harshly. Anyway, the main answer to the question.

It is not always required, but I find it to be good practice. What it does, is allows a derived object to be safely deleted through a pointer of a base type.

So for example: boost::shared_pointer<Base> p(new Derived); is illegal if Base doesn't have a virtual destructor, because it will attempt to delete the object as if it were a Base *.

Answer 4

It's not only good practice. It is rule #1 for any class hierarchy.

1. The base most class of a hierarchy in C++ must have a virtual destructor

Now for the Why. Take the typical animal hierarchy. Virtual destructors go through virtual dispatch just as any other method call. Take the following example.

Animal* pAnimal = GetAnimal();
delete pAnimal;

Assume that Animal is an abstract class. The only way that C++ knows the proper destructor to call is via virtual method dispatch. If the destructor is not virtual then it will simply call Animal's destructor and not destroy any objects in derived classes.

The reason for making the destructor virtual in the base class is that it simply removes the choice from derived classes. Their destructor becomes virtual by default.

Answer 5

The answer to your question is often, but now always. If your abstract class forbids clients to call delete on a pointer to it (or if it says so in its documentation), you are free to not declare a virtual destructor.

You can forbid clients to call delete on a pointer to it by making its destructor protected. Working like this, it is perfectly safe and reasonable to omit a virtual destructor.

You will eventually end up with no virtual method table, and end up signalling your clients your intention on making it non-deleteable through a pointer to it, so you have indeed reason not to declare it virtual in those cases.

[See item 4 in this article: http://www.gotw.ca/publications/mill18.htm]