Derived class with non-virtual destructor

Question

Are there any circumstances in which it is legitimate for a derived class to have a non-virtual destructor? A non-virtual destructor signifies that a class should not be used as a base-class. Will having a non-virtual destructor of a derived class act like a weak form of the Java final modifier?

I am especially interested in the case where the base class of the derived class has a virtual destructor.

Answer 1

Are there any circumstances in which it is legitimate for a derived class to have a non-virtual destructor?

Yes.

A non-virtual destructor signifies that a class should not be used as a base-class.

Not really; a non-virtual destructor signifies that deleting an instance of derived via a base pointer will not work. For example:

class Base {}; class Derived : public Base {};  Base* b = new Derived; delete b; // Does not call Derived's destructor! 

If you don't do delete in the above manner, then it will be fine. But if that's the case, then you would probably be using composition and not inheritance.

Will having a non-virtual destructor of a derived class act like a weak form of the Java final modifier?

No, because virtual-ness propagates to derived classes.

class Base { public:     virtual ~Base() {}     virtual void Foo() {}; };  class Derived : public Base { public:     ~Derived() {}  // Will also be virtual     void Foo() {}; // Will also be virtual }; 

There isn't a built-in language mechanism in C++03 or earlier to prevent subclasses(*). Which isn't much of an issue anyway since you should always prefer composition over inheritance. That is, use inheritance when a "is-a" relationship makes more sense than a true "has-a" relationship.

(*) 'final' modifier was introduced in C++11

Answer 2

It is perfectly valid to have an Base class with an non virtual destructor if you are never going to call delete on a Base class pointer pointing to an derived class object.

Follow Herb Sutter's Advice:

Guideline #: Only if derived classes need to invoke the base implementation of a virtual function, make the virtual function protected. For the special case of the destructor only:

Guideline #: A base class destructor should be either public and virtual, or protected and nonvirtual.

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Maybe your question actually is:
Does Destructor in Derived class needs to be virtual if Base class Destructor is virtual?

The answer is NO.
If Base class destructor is virtual then the Derived class destructor is implicitly virtual already, you don't need to specify it as virtual explicitly.