Re: What if pre-processor was moved to parser?

Path	csiph.com!news.swapon.de!eternal-september.org!feeder.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: What if pre-processor was moved to parser?
Date	Thu, 16 Feb 2017 14:02:31 -0800
Organization	None to speak of
Lines	37
Message-ID	<lnbmu1rdzc.fsf@kst-u.example.com> (permalink)
References	<11c906d6-9be0-44df-82b4-157fdba3b91c@googlegroups.com> <dd3ed896-e224-4263-ab5c-ea782b3c84f5@googlegroups.com> <o81egc$b6q$1@dont-email.me> <436a6e24-df32-45d9-b24e-c1e135c32c24@googlegroups.com> <8b49e198-86a5-45b4-ade7-a27ecb4f5e07@googlegroups.com> <d879efad-7e8b-44dd-aa5d-cab5950f7de2@googlegroups.com> <o82dcg$u62$1@dont-email.me> <d771fe14-7515-422b-9759-1d2b947a941f@googlegroups.com> <o843j2$i30$1@dont-email.me> <7cc856f7-997e-496d-8ebf-ba413cb89d75@googlegroups.com> <o84d1v$il5$1@dont-email.me> <7dc5b761-29bb-4244-aa99-ed20cbfa5fcd@googlegroups.com> <lnwpcqq8v7.fsf@kst-u.example.com> <ab4395b0-213c-4c70-beab-7a19ea1f020e@googlegroups.com> <o8525p$30h$1@dont-email.me> <lnk28prh30.fsf@kst-u.example.com> <o8566m$gra$1@dont-email.me>
Mime-Version	1.0
Content-Type	text/plain; charset=us-ascii
Injection-Info	mx02.eternal-september.org; posting-host="4389ae8ca80d0229613347d13905988d"; logging-data="27355"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+Nj7trGKChDOWyamU38TY5"
User-Agent	Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)
Cancel-Lock	sha1:5pPYPOMJHuU+TVZ6xbOX8DapGEA= sha1:rWvuQznBUDp69hGHvkFVC7+66Kc=
Xref	csiph.com comp.lang.c:104041

Show key headers only | View raw

David Brown <david.brown@hesbynett.no> writes:
> On 16/02/17 21:55, Keith Thompson wrote:
>> David Brown <david.brown@hesbynett.no> writes:
>> [...]
>>> However, the reason "type punning" via unions was standardised in C99 is
>>> that compilers already implemented it and people relied upon it.  I
>>> suspect you will find that any C++ compiler will support type punning
>>> via unions, at least when constructors and destructors are not involved
>>> - if you use type punning to gain access to an invalid object, that will
>>> be UB.  Certainly C++ does not /require/ that such type punning is UB.
>>
>> I'm not sure that last sentence makes sense.  Either it's undefined
>> behavior or it isn't.  If the C++ standard doesn't define the behavior,
>> then it's UB -- even if all implementations behave consistently.  UB
>> doesn't mean "it blows up".
>
> Supercat said that it "requires [that it] yields UB", which I 
> interpreted as him thinking the compiler /had/ to leave the behaviour 
> here without definition.  It is not defined by the C Standards, but 
> there is no requirement for an implementation to leave it without a 
> definition.

Supercat was either incorrect or stating something trivial, depending on
how you interpret it.  If a construct has undefined behavior, that means
only that the standard does not define its behavior.  It places no
requirements of any kind on implementations.  If a constructs behavior
is not defined by the standard and an implementation chooses to define
its behavior, then it still has "undefined behavior" as the standard
defines and uses that phrase.

I'm not sure what it means for something to "yield UB".

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-14 15:37 -0800
  Re: What if pre-processor was moved to parser? fir <profesor.fir@gmail.com> - 2017-02-15 00:05 -0800
    Re: What if pre-processor was moved to parser? fir <profesor.fir@gmail.com> - 2017-02-15 00:57 -0800
      Re: What if pre-processor was moved to parser? fir <profesor.fir@gmail.com> - 2017-02-15 07:21 -0800
        Re: What if pre-processor was moved to parser? fir <profesor.fir@gmail.com> - 2017-02-15 07:24 -0800
          Re: What if pre-processor was moved to parser? fir <profesor.fir@gmail.com> - 2017-02-15 08:10 -0800
  Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 03:05 -0800
    Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-15 11:38 +0000
      Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 04:28 -0800
        Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-15 13:22 +0000
          Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 05:47 -0800
        Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 10:06 -0800
      Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 05:24 -0800
        Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-15 13:57 +0000
          Re: What if pre-processor was moved to parser? Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-19 10:27 -0800
        Re: What if pre-processor was moved to parser? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-15 15:16 +0000
          Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 08:14 -0800
            Re: What if pre-processor was moved to parser? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-15 18:01 +0000
              Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-15 18:25 +0000
                Re: What if pre-processor was moved to parser? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-15 19:14 +0000
              Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-15 10:25 -0800
                Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-15 18:45 +0000
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-15 20:03 +0100
        Re: What if pre-processor was moved to parser? jameskuyper@verizon.net - 2017-02-15 10:12 -0800
          Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-15 10:51 -0800
          Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 12:14 -0800
            Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-15 20:25 +0000
              Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 12:50 -0800
                Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-16 11:50 +0000
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-16 04:44 -0800
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-16 05:28 -0800
                Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-16 14:32 +0000
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-16 09:13 -0800
                Re: What if pre-processor was moved to parser? Keith Thompson <kst-u@mib.org> - 2017-02-16 10:38 -0800
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-16 11:55 -0800
                Re: What if pre-processor was moved to parser? Keith Thompson <kst-u@mib.org> - 2017-02-16 12:24 -0800
                Re: What if pre-processor was moved to parser? David Brown <david.brown@hesbynett.no> - 2017-02-16 21:32 +0100
                Re: What if pre-processor was moved to parser? Keith Thompson <kst-u@mib.org> - 2017-02-16 12:55 -0800
                Re: What if pre-processor was moved to parser? David Brown <david.brown@hesbynett.no> - 2017-02-16 22:41 +0100
                Re: What if pre-processor was moved to parser? Keith Thompson <kst-u@mib.org> - 2017-02-16 14:02 -0800
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-16 15:08 -0800
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-16 13:32 -0800
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-17 21:19 +1300
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-17 12:01 +0100
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-17 09:16 -0800
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-17 18:55 +0100
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-18 08:07 +1300
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-17 20:12 +0100
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-18 11:04 +1300
                Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-17 22:38 +0000
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-18 11:47 +1300
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-17 14:55 -0800
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-18 12:11 +1300
                Re: What if pre-processor was moved to parser? "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-02-17 15:26 -0800
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-17 16:15 -0800
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-18 13:22 +1300
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-17 16:37 -0800
                Re: What if pre-processor was moved to parser? David Brown <david.brown@hesbynett.no> - 2017-02-18 11:57 +0100
                Re: What if pre-processor was moved to parser? Richard Heathfield <rjh@cpax.org.uk> - 2017-02-18 07:13 +0000
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-18 20:26 +1300
                Re: What if pre-processor was moved to parser? Richard Heathfield <rjh@cpax.org.uk> - 2017-02-18 09:17 +0000
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-18 01:18 +0100
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-18 13:23 +1300
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-18 01:45 +0100
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-18 16:18 +1300
                Re: What if pre-processor was moved to parser? Richard Heathfield <rjh@cpax.org.uk> - 2017-02-18 07:19 +0000
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-18 20:30 +1300
                Re: What if pre-processor was moved to parser? Richard Heathfield <rjh@cpax.org.uk> - 2017-02-18 09:18 +0000
                Re: What if pre-processor was moved to parser? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-02-18 07:15 -0800
                Re: What if pre-processor was moved to parser? Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-24 09:24 -0800
                Re: What if pre-processor was moved to parser? Richard Heathfield <rjh@cpax.org.uk> - 2017-02-24 17:51 +0000
                Re: What if pre-processor was moved to parser? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-24 10:08 -0700
                Re: What if pre-processor was moved to parser? Richard Heathfield <rjh@cpax.org.uk> - 2017-04-24 18:58 +0100
                Re: What if pre-processor was moved to parser? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2017-04-26 05:03 +0000
                Re: What if pre-processor was moved to parser? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 11:21 -0700
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-25 09:15 +1300
                Re: What if pre-processor was moved to parser? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-02-24 17:29 -0800
                Re: What if pre-processor was moved to parser? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-26 06:19 -0700
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-04-26 16:04 +0200
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 07:08 -0700
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-04-26 16:46 +0200
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 09:32 -0700
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-04-26 20:19 +0200
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 11:29 -0700
                Re: What if pre-processor was moved to parser? scott@slp53.sl.home (Scott Lurndal) - 2017-04-26 18:51 +0000
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-04-27 09:32 +1200
                Re: What if pre-processor was moved to parser? David Brown <david.brown@hesbynett.no> - 2017-04-27 11:01 +0200
                Re: What if pre-processor was moved to parser? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-04-27 02:23 -0700
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-04-27 21:31 +1200
                Re: What if pre-processor was moved to parser? Ike Naar <ike@iceland.freeshell.org> - 2017-04-27 16:15 +0000
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-04-27 09:35 -0700
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-04-28 15:12 +0200
                Re: What if pre-processor was moved to parser? Gareth Owen <gwowen@gmail.com> - 2017-04-27 19:13 +0100
                Re: What if pre-processor was moved to parser? David Brown <david.brown@hesbynett.no> - 2017-04-27 11:37 +0200
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-04-27 15:18 +0200
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-18 08:43 +0100
                Re: What if pre-processor was moved to parser? Richard Heathfield <rjh@cpax.org.uk> - 2017-02-18 09:22 +0000
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-18 09:44 -0800
                Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-18 11:33 +0000
                Re: What if pre-processor was moved to parser? David Brown <david.brown@hesbynett.no> - 2017-02-18 13:25 +0100
                Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-18 13:47 +0000
                Re: What if pre-processor was moved to parser? Jean-Marc Bourguet <jm@bourguet.org> - 2017-02-18 15:23 +0100
                Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-18 14:53 +0000
                Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-18 16:56 +0000
                Re: What if pre-processor was moved to parser? Robert Wessel <robertwessel2@yahoo.com> - 2017-02-18 22:50 -0600
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-19 09:45 +0100
                Re: What if pre-processor was moved to parser? David Brown <david.brown@hesbynett.no> - 2017-02-19 13:35 +0100
                Re: What if pre-processor was moved to parser? "James R. Kuyper" <jameskuyper@verizon.net> - 2017-02-18 11:24 -0500
                Re: What if pre-processor was moved to parser? Robert Wessel <robertwessel2@yahoo.com> - 2017-02-18 22:48 -0600
                Re: What if pre-processor was moved to parser? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-02-18 10:04 -0800
                Re: What if pre-processor was moved to parser? David Brown <david.brown@hesbynett.no> - 2017-02-19 17:12 +0100
                Re: What if pre-processor was moved to parser? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-02-19 11:11 -0800
                Re: What if pre-processor was moved to parser? David Brown <david.brown@hesbynett.no> - 2017-02-19 21:14 +0100
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-19 21:22 +0100
                Re: What if pre-processor was moved to parser? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-02-19 12:33 -0800
                Re: What if pre-processor was moved to parser? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-02-19 12:31 -0800
                Re: What if pre-processor was moved to parser? fir <profesor.fir@gmail.com> - 2017-02-19 13:14 -0800
                Re: What if pre-processor was moved to parser? fir <profesor.fir@gmail.com> - 2017-02-19 14:50 -0800
                Re: What if pre-processor was moved to parser? fir <profesor.fir@gmail.com> - 2017-02-19 15:04 -0800
                Re: What if pre-processor was moved to parser? David Brown <david.brown@hesbynett.no> - 2017-02-20 10:13 +0100
                Re: What if pre-processor was moved to parser? Gareth Owen <gwowen@gmail.com> - 2017-02-19 21:09 +0000
                Re: What if pre-processor was moved to parser? Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-23 15:32 -0800
                Re: What if pre-processor was moved to parser? Ian Collins <ian-news@hotmail.com> - 2017-02-24 12:44 +1300
                Re: What if pre-processor was moved to parser? Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-19 21:04 -0800
                Re: What if pre-processor was moved to parser? Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-19 21:14 -0800
                Re: What if pre-processor was moved to parser? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2017-02-16 14:48 -0800
                Re: What if pre-processor was moved to parser? Keith Thompson <kst-u@mib.org> - 2017-02-16 15:46 -0800
                Re: What if pre-processor was moved to parser? Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-23 16:16 -0800
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-20 04:18 -0800
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-20 06:47 -0800
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-20 09:50 -0800
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-21 04:33 -0800
                Re: What if pre-processor was moved to parser? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-21 15:41 +0100
                Re: What if pre-processor was moved to parser? supercat@casperkitty.com - 2017-02-21 09:39 -0800
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-21 09:59 -0800
                Re: What if pre-processor was moved to parser? TickleMe Elmo <tmelmosfire@gmail.com> - 2017-02-21 10:22 -0800
                Re: What if pre-processor was moved to parser? BartC <bc@freeuk.com> - 2017-02-20 15:49 +0000
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-20 09:43 -0800
            Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 12:31 -0800
              Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-16 03:33 -0800
            Re: What if pre-processor was moved to parser? jameskuyper@verizon.net - 2017-02-15 12:47 -0800
              Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 13:06 -0800
                Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-02-15 13:15 -0800
  Re: What if pre-processor was moved to parser? "Patrick.Schluter" <Patrick.Schluter@free.fr> - 2017-02-18 12:36 +0100
    Re: What if pre-processor was moved to parser? "Patrick.Schluter" <Patrick.Schluter@free.fr> - 2017-02-18 12:41 +0100
  Re: What if pre-processor was moved to parser? John Bode <jfbode1029@gmail.com> - 2017-04-26 09:42 -0700
    Re: What if pre-processor was moved to parser? Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 09:58 -0700

csiph-web