May a destructor be final?

Question

Does the C++ standard allow a destructor to be declared as final? Like this:

 class Derived: public Base  {       ...       virtual ~Derived() final;  } 

And if so, does that prevent the declaration of a derived class:

 class FurtherDerived: public Derived {// allowed?  } 

If it is allowed, is a compiler likely to issue a warning? Is declaring a destructor to be final a workable idiom for indicating that a class is not intended to be used as a base class?

(There is no point in doing this in a ultimate base class, only a derived class.)

Answer 1

May a C++ destructor be declared as final?

Yes.

And if so, does that prevent declaration of a derived class:

Yes, because the derived class would have to declare a destructor (either explicitly by you or implicitly by the compiler), and that destructor would be overriding a function declared final, which is ill-formed.

The rule is [class.virtual]/4:

If a virtual function f in some class B is marked with the virt-specifier final and in a class D derived from B a function D​::​f overrides B​::​f, the program is ill-formed.

It's the derivation itself that is ill-formed, it doesn't have to be used.

Is declaring a destructor to be final a workable idiom for indicating that a class is not intended to be used as a base class?

Effectively, but you should just mark the class final. It's quite a bit more explicit.