Re: getline() problem

From	James Kuyper <jameskuyper@alumni.caltech.edu>
Newsgroups	comp.lang.c++
Subject	Re: getline() problem
Date	2022-04-26 11:49 -0400
Organization	A noiseless patient Spider
Message-ID	<t494b6$a1v$1@dont-email.me> (permalink)
References	(5 earlier) <t45de2$1sln$2@gioia.aioe.org> <t46f86$or8$1@dont-email.me> <Kwz9K.23424$x9Ea.11744@fx45.iad> <t46np1$5uj$1@dont-email.me> <t480p6$1gdc$1@gioia.aioe.org>

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On 4/26/22 01:43, Juha Nieminen wrote:
> Andrey Tarasevich <andreytarasevich@hotmail.com> wrote:
>> It is the abstract language semantics implemented by the compiler. The 
>> compiler that actively uses UB as an optimization opportunity at a 
>> rather abstract level.
> 
> By the way, doesn't this break the rule that optimizations must not change
> the observable behavior of the program (other than by its execution time)?
> 
> (Granted, I haven't actually checked that the standard actually mandates
> that "optimizations must not change observable behavior". I'm not sure
> where I have got that idea from. I would assume that the standard does
> require this, in one form of another. ...

You're confusing two different kinds of optimization. The first kind are
the "as-if" optimizations. It's not true that such optimizations can't
change the observable behavior, but that's close to being the correct
description. For any given piece of code, because of the many things
that the standard leaves unspecified, there's often multiple (sometimes,
infinitely many) different sequences of observable behavior that are
allowed. When a program is well-formed, the implementation is required
generate one of those sequences. An optimization can change which of
those sequences occurs, but it cannot allow a sequence not on that list
to occur.

The other kinds of optimizations are enabled by the fact that certain
kinds of code can have undefined behavior under some circumstances. An
implementation is free to ignore the possibility that those
circumstances might occur, which sometimes enables optimizations for
code that doesn't have undefined behavior.. For example, `b+a-b` for
signed integer types can be optimized to a, because the overflow that
might occur when b+a is evaluated has undefined behavior, so the
compiler doesn't have to worry about that  possibility. If the behavior
were defined, the compiler might (depending upon the target platform)
have to complicate the code in order to ensure that the defined behavior
actually occurs.

> ... Or does UB "take precedence" over
> that requirement, making it void?)

Undefined behavior releases an implementation from ALL requirements
imposed by the standard, even than one.

Thread

getline() problem Jivanmukta <jivanmukta@poczta.onet.pl> - 2022-04-23 19:07 +0200
  Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-23 10:17 -0700
    Re: getline() problem Jivanmukta <jivanmukta@poczta.onet.pl> - 2022-04-23 19:40 +0200
      Re: getline() problem Jivanmukta <jivanmukta@poczta.onet.pl> - 2022-04-23 19:59 +0200
        Re: getline() problem Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-23 21:47 +0300
          Re: getline() problem Juha Nieminen <nospam@thanks.invalid> - 2022-04-25 06:00 +0000
            Re: getline() problem Öö Tiib <ootiib@hot.ee> - 2022-04-25 00:12 -0700
            Re: getline() problem Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-25 11:40 +0300
            Re: getline() problem James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-25 11:37 -0400
              Re: getline() problem Vir Campestris <vir.campestris@invalid.invalid> - 2022-04-25 16:54 +0100
                Re: getline() problem James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-25 12:29 -0400
                Re: getline() problem scott@slp53.sl.home (Scott Lurndal) - 2022-04-25 17:29 +0000
                Re: getline() problem Manfred <noname@add.invalid> - 2022-04-25 20:17 +0200
                Re: getline() problem Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-04-25 10:19 -0700
                Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-25 10:41 -0700
                Re: getline() problem Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-25 21:58 +0300
              Re: getline() problem scott@slp53.sl.home (Scott Lurndal) - 2022-04-25 15:56 +0000
                Re: getline() problem James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-25 13:12 -0400
                Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-25 11:03 -0700
                Re: getline() problem scott@slp53.sl.home (Scott Lurndal) - 2022-04-25 19:05 +0000
                Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-25 12:48 -0700
                Re: getline() problem Ben <ben.usenet@bsb.me.uk> - 2022-04-25 21:46 +0100
                Re: getline() problem Juha Nieminen <nospam@thanks.invalid> - 2022-04-26 05:43 +0000
                Re: getline() problem David Brown <david.brown@hesbynett.no> - 2022-04-26 09:36 +0200
                Re: getline() problem Juha Nieminen <nospam@thanks.invalid> - 2022-04-26 09:18 +0000
                Re: getline() problem Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-26 02:47 -0700
                Re: getline() problem Juha Nieminen <nospam@thanks.invalid> - 2022-04-26 11:06 +0000
                Re: getline() problem David Brown <david.brown@hesbynett.no> - 2022-04-26 15:52 +0200
                Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-26 07:04 -0700
                Re: getline() problem Manfred <noname@add.invalid> - 2022-04-26 16:49 +0200
                Re: getline() problem James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-26 12:01 -0400
                Re: getline() problem James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-04-26 11:49 -0400
                Re: getline() problem Manfred <noname@add.invalid> - 2022-04-25 20:23 +0200
            Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-25 10:33 -0700
              Re: getline() problem "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-04-25 10:52 -0700
                Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-25 11:08 -0700
                Re: getline() problem "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-04-25 21:18 -0700
                Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-25 22:04 -0700
              Re: getline() problem Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-25 22:06 +0300
                Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-25 13:02 -0700
        Re: getline() problem Barry Schwarz <schwarzb@delq.com> - 2022-04-23 12:54 -0700
        Re: getline() problem Montmorency <none@none.com> - 2022-04-23 14:45 -0700
          Re: getline() problem Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-23 16:49 -0700
            Re: getline() problem Ben <ben.usenet@bsb.me.uk> - 2022-04-24 01:04 +0100
              Re: getline() problem Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-04-23 18:24 -0700
                Re: getline() problem Ben <ben.usenet@bsb.me.uk> - 2022-04-24 03:16 +0100
                Re: getline() problem Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-24 22:51 -0700
                Re: getline() problem Ben <ben.usenet@bsb.me.uk> - 2022-04-25 11:07 +0100
            Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-23 17:15 -0700
              Re: getline() problem Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-23 18:23 -0700
          Re: getline() problem Jivanmukta <jivanmukta@poczta.onet.pl> - 2022-04-24 05:28 +0200
  Re: getline() problem Jivanmukta <jivanmukta@poczta.onet.pl> - 2022-04-25 12:41 +0200
    Re: getline() problem Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-25 14:33 +0300
    Re: getline() problem Jivanmukta <jivanmukta@poczta.onet.pl> - 2022-04-25 18:53 +0200
      Re: getline() problem Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-25 22:08 +0300
        Re: getline() problem Jivanmukta <jivanmukta@poczta.onet.pl> - 2022-04-26 09:01 +0200
          Re: getline() problem Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-26 11:15 +0300

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