Virtual functions: To const, or not to const?

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From	Juha Nieminen <nospam@thanks.invalid>
Newsgroups	comp.lang.c++
Subject	Virtual functions: To const, or not to const?
Date	Fri, 7 Oct 2022 09:47:11 -0000 (UTC)
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I have encountered this small dilemma several times. It's a question of
const correctness and proper OO design in C++.

Should a virtual function in a base class be const or not? This especially
in cases where it's very possible that a derived implementation wants
to change its own state (and thus would want the function to not be
const).

The answer seems simple enough: Obviously it's better to not make it
const, to allow for this possibility.

Problem is, when it's not const, now it can't be called using a const
instance, or const reference or pointer to one. There may be situations
where the only thing you have is a const version of the object, and
there's no way around that.

(Ok, technically speaking you *can* call it, by using const_cast. But
that's a really ugly solution, bad design, and in some cases might
even potentially break something. (Const code might assume the object
doesn't change.))

"Well, in that case offer *both* versions of the virtual function in
the base class, duh! Problem solved!"

Except that sometimes, in some situations, there may not be any
reasonable way to implement a const version of a function, because
it *has to* change the object in order to function correctly.
Sometimes a const version just doesn't make any sense. (For example,
a const version of std::vector::push_back() wouldn't make much
sense. What would it be supposed to do? Nothing? That would make
it really odd design, and would have the potential to break
something.)

This isn't even a question of "const correctness". It's a question
of getting a compiler error if all you have is a const reference
(and no way around that) and the virtual function happens to be
non-const (even though the implementation doesn't actually change
anything).

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Virtual functions: To const, or not to const? Juha Nieminen <nospam@thanks.invalid> - 2022-10-07 09:47 +0000
  Re: Virtual functions: To const, or not to const? David Brown <david.brown@hesbynett.no> - 2022-10-07 12:50 +0200
    Re: Virtual functions: To const, or not to const? Juha Nieminen <nospam@thanks.invalid> - 2022-10-10 06:20 +0000
      Re: Virtual functions: To const, or not to const? Richard Damon <Richard@Damon-Family.org> - 2022-10-10 07:38 -0400
        Re: Virtual functions: To const, or not to const? Juha Nieminen <nospam@thanks.invalid> - 2022-10-10 12:08 +0000
          Re: Virtual functions: To const, or not to const? Richard Damon <Richard@Damon-Family.org> - 2022-10-10 20:47 -0400
      Re: Virtual functions: To const, or not to const? Paavo Helde <eesnimi@osa.pri.ee> - 2022-10-10 15:13 +0300
        Re: Virtual functions: To const, or not to const? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-10-10 09:43 -0700
      Re: Virtual functions: To const, or not to const? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-10-10 09:26 -0700
  Re: Virtual functions: To const, or not to const? Paavo Helde <eesnimi@osa.pri.ee> - 2022-10-07 14:00 +0300
  Re: Virtual functions: To const, or not to const? Richard Damon <Richard@Damon-Family.org> - 2022-10-07 07:29 -0400
  Re: Virtual functions: To const, or not to const? Öö Tiib <ootiib@hot.ee> - 2022-10-07 05:55 -0700
  Re: Virtual functions: To const, or not to const? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-10-08 14:56 +0200
  Re: Virtual functions: To const, or not to const? Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-08 16:35 +0200
    Re: Virtual functions: To const, or not to const? Michael S <already5chosen@yahoo.com> - 2022-10-08 11:03 -0700
      Re: Virtual functions: To const, or not to const? Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-08 20:36 +0200
      Re: Virtual functions: To const, or not to const? Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-08 20:19 +0100
        Re: Virtual functions: To const, or not to const? Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-10 17:58 +0200
      Re: Virtual functions: To const, or not to const? Muttley@dastardlyhq.com - 2022-10-09 09:19 +0000
  Re: Virtual functions: To const, or not to const? Manfred <noname@add.invalid> - 2022-10-08 21:42 +0200

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