When to use virtual destructors

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I understand the majority of OOP theory, but virtual destructors are particularly perplexing.

I assumed that the destructor was called for every object in the chain, no matter what.

When and why should you make them virtual?
Jun 2 in C++ by Nicholas
• 5,020 points
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1 answer to this question.

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When you want to delete an instance of a derived class using a pointer to the base class, virtual destructors come in handy:

class Base 
{
    // some virtual methods
};

class Derived : public Base
{
    ~Derived()
    {
        // Do some important cleanup
    }
};

You'll notice that I didn't make Base's destructor virtual in this case. 

Take a look at the following snippet now:

Base *b = new Derived();
// use b
delete b; // Here's the problem!

Delete b has undefined behaviour because Base's destructor is not virtual and b is a Base* pointing to a Derived object:

If the object to be deleted's static type is different from its dynamic type, the static type must be a base class of the object's dynamic type, and the static type must have a virtual destructor, or the behaviour is undefined.

In most implementations, the call to the destructor is resolved like any non-virtual code, which means that the base class's destructor is called but not the derived class's, resulting in a resource leak.

To summarise, whenever base classes are meant to be manipulated polymorphically, make their destructors virtual.

If you want to prevent an instance from being deleted through a base class pointer, make the base class destructor protected and nonvirtual; the compiler will not allow you to call delete on a base class pointer if you do so.

answered Jun 7 by Damon
• 4,960 points

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