Re: redeclaration of enumerators?

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: redeclaration of enumerators?
Date	2021-09-29 20:14 +0200
Organization	A noiseless patient Spider
Message-ID	<sj2adj$emu$1@dont-email.me> (permalink)
References	(3 earlier) <sj053r$6c4$1@dont-email.me> <sj1q5a$a7i$1@dont-email.me> <sj1ro3$ng2$1@dont-email.me> <sj1u8e$bds$1@dont-email.me> <sj1vgt$lfe$1@dont-email.me>

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On 29/09/2021 17:08, Bart wrote:
> On 29/09/2021 15:46, David Brown wrote:
>> On 29/09/2021 16:03, Bart wrote:
> 
>>> Yeah, you're right. It would be tricky anyway, because it can't be
>>> sorted using normal name resolution rules, since it relies on type info
>>> which may not be dealt with until a subsequent stage.
>>>
>>> But also, you could have this:
>>>
>>>     enum E2 {A=1, B, C};
>>>     enum E2 A, B;
>>>
>>>         A = E2::C
>>>         B = A;
>>>
>>> Is the RHS the variable A (which contains 3), or is it intended to be
>>> E2::A, which is 1?
> 
>> I checked on the "using enum" in C++.  It was added in C++20, and lets
>> you write:
>>
>>     enum class colours { red, green, blue };
>>     colours my_favourite_colour;
>>
>>     my_favourite_colour = colours::red;
>>     using enum colours;
>>     my_favourite_colour = blue;
> 
> It took me a while to find something that supported C++20 (godbolt does)
> to try it out.
> 
> How it works is that if you use:
> 
>    using enum colours;
> 
> then you can't declare:
> 
>    colours blue;
> 
> So the ambiguity I pointed out can't occur.
> 
> It stops you using 'int blue;' as well. In fact, all those enum names
> become 'open', and you will get the same issues as you have in C with
> clashes with other identifiers and other open enums.
> 
> (Except that this feature allowes you to restrict those open enums to a
> particular scope.)

Of course.  Typically, you'll have the "using enum" clause in quite a
small scope - just the block of code where you need it.  It's not
something you'd have at file level.

> 
> It doesn't help with my other example either: you can't use both:
> 
>   using enum colours;
>   using enum lights;
> 
> as 'green' will clash; as well 'red', even though it they have the same
> value.

With strong enumerations, you shouldn't even be thinking of them as
having an integer value - it doesn't make sense to compare the "value"
of a "colours" enumeration element with one from a "lights" enumeration.
 That is a large part of the point of having better enumeration types
than were available in C.

Thread

redeclaration of enumerators? Thiago Adams <thiago.adams@gmail.com> - 2021-09-27 10:39 -0700
  Re: redeclaration of enumerators? John Bode <jfbode1029@gmail.com> - 2021-09-28 16:21 -0500
    Re: redeclaration of enumerators? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-28 14:57 -0700
      Re: redeclaration of enumerators? Thiago Adams <thiago.adams@gmail.com> - 2021-09-28 15:23 -0700
        Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-09-29 15:23 +0200
        Re: redeclaration of enumerators? Mark Bluemel <mark.bluemel@gmail.com> - 2021-09-30 01:15 -0700
      Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-09-28 23:31 +0100
        Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-09-29 15:36 +0200
          Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-09-29 15:03 +0100
            Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-09-29 16:46 +0200
              Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-09-29 16:08 +0100
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-09-29 20:14 +0200
                Re: redeclaration of enumerators? Thiago Adams <thiago.adams@gmail.com> - 2021-09-29 13:37 -0700
                Re: redeclaration of enumerators? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-29 18:25 -0700
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-09-30 12:45 +0200
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-09-30 07:36 -0700
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-01 09:20 +0200
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 11:24 +0100
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 04:49 -0700
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 13:06 +0100
                Re: redeclaration of enumerators? Tony Oliver <guinness.tony@gmail.com> - 2021-10-01 05:24 -0700
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 13:32 +0100
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 05:32 -0700
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 13:43 +0100
                Re: redeclaration of enumerators? Mark Bluemel <mark.bluemel@gmail.com> - 2021-10-01 06:13 -0700
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 06:29 -0700
                Re: redeclaration of enumerators? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-10-01 15:30 +0100
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 08:28 -0700
                Re: redeclaration of enumerators? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-10-01 17:08 +0100
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 15:12 -0700
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-01 15:37 +0200
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 15:30 +0100
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-01 19:12 +0200
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 19:06 +0100
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-01 15:32 +0200
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 15:38 +0100
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-01 17:40 +0100
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-02 12:16 +0100
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-02 15:34 +0200
                Re: redeclaration of enumerators? Bart <bc@freeuk.com> - 2021-10-02 15:16 +0100
                Re: redeclaration of enumerators? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-01 13:58 -0700
                Re: redeclaration of enumerators? Richard Damon <Richard@Damon-Family.org> - 2021-10-01 17:21 -0400
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-10-01 14:56 -0700
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-02 12:21 +0200
                Re: redeclaration of enumerators? scott@slp53.sl.home (Scott Lurndal) - 2021-09-30 14:47 +0000
                Re: redeclaration of enumerators? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-09-30 10:10 -0700
                Re: redeclaration of enumerators? David Brown <david.brown@hesbynett.no> - 2021-10-01 09:24 +0200
            Re: redeclaration of enumerators? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-09-29 12:43 -0700
    Re: redeclaration of enumerators? Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-29 03:40 +0000

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