Re: This will work or not?

From	Chris Vine <chris@cvine--nospam--.freeserve.co.uk>
Newsgroups	comp.lang.c++
Subject	Re: This will work or not?
Date	2016-09-02 10:42 +0100
Organization	Datemas.de http://www.news.datemas.de
Message-ID	<20160902104241.67323299@bother.homenet> (permalink)
References	(4 earlier) <nq746l$r3b$1@dont-email.me> <20160831215212.63b20ea7@bother.homenet> <kfn37ljpq2h.fsf@x-alumni2.alumni.caltech.edu> <20160901181638.34f18d48@bother.homenet> <kfnpoonnkt0.fsf@x-alumni2.alumni.caltech.edu>

Show all headers | View raw

On Thu, 01 Sep 2016 18:37:47 -0700
Tim Rentsch <txr@alumni.caltech.edu> wrote:
> Chris Vine <chris@cvine--nospam--.freeserve.co.uk> writes:
> > You cannot get a trap
> > representation from arithmetic operations on valid values of
> > unsigned integers, but that seems to me to be a different matter.
> > Integer values may have padding bits or parity bits.  
> 
> That is true for integer types generally, but the fixed-width
> types like uint32_t have additional guarantees that preclude the
> possibility of padding bits or trap representations.  You might
> want to look that up in the section for <stdint.h>.  The lack of
> trap representations for uint32_t forms the crux of my argument.
> (Probably I should have mentioned the more stringent guarantees
> for these fixed-width types before, but 20-20 hindsight and all
> that...)

OK that's good to know.  The thesis in my original post on this was
that if you compile the type-punning code as C99 or C11 and link to it
in your C++ code it would work, because no architectures in fact running
C++ have trap representations on integers in C.  You are, like Judge
Dredd, saying that it will work because it _is_ the law, for fixed size
unsigned integers at least.

Apropos of which, and moving on from indeterminate values arising from
padding to indeterminate values arising from uninitialized built-in
types, C appears to do this much better than the proposal in what is
now §8.6/12 of N4606 for C++.  C seems to provide that an uninitialised
variable may hold a trap representation, but it may equally well have a
valid (unknown) value in it, which is in fact what will happen on 99%
or architectures and all those running C++.  For the latter, these can
be evaluated, say by copying.  §8.6/12 of N4606 on the other hand
provides for blanket undefined behaviour for any copying or other
evaluation of an uninitialized object of built-in type which is not an
unsigned narrow character (unsigned char, and char where char is
unsigned).

I don't like the way C++ sprays around undefined behaviour at the
slightest provocation.  Unions are another example.

Chris

Thread

This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-08-31 14:59 +0200
  Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-08-31 15:20 +0200
    Re: This will work or not? mark <mark@invalid.invalid> - 2016-08-31 16:05 +0200
      Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-08-31 16:19 +0200
    Re: This will work or not? scott@slp53.sl.home (Scott Lurndal) - 2016-08-31 14:20 +0000
    Re: This will work or not? Bo Persson <bop@gmb.dk> - 2016-08-31 18:53 +0200
      Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-01 09:03 -0700
        Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-01 22:45 +0200
          Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-01 18:08 -0700
            Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 09:24 +0200
              Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-04 06:31 -0700
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-04 20:19 +0200
  Re: This will work or not? Manfred <noname@invalid.add> - 2016-08-31 15:45 +0200
    Re: This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-08-31 15:59 +0200
    Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-08-31 16:22 +0200
      Re: This will work or not? Manfred <noname@invalid.add> - 2016-08-31 17:06 +0200
        Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-08-31 19:29 +0200
          Re: This will work or not? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2016-08-31 21:52 +0100
            Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-01 09:07 +0200
              Re: This will work or not? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2016-09-01 12:30 +0100
            Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-01 09:01 -0700
              Re: This will work or not? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2016-09-01 18:16 +0100
                Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-01 18:37 -0700
                Re: This will work or not? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2016-09-02 10:42 +0100
                Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-04 06:59 -0700
          Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-08-31 23:17 -0400
            Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-01 09:35 +0200
              Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-01 15:34 -0400
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-01 13:50 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-01 21:01 -0400
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-01 19:02 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-01 22:17 -0400
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-02 08:19 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 12:39 -0400
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-02 12:26 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 16:38 -0400
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-02 13:51 -0700
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-02 14:16 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 19:58 -0400
                Re: This will work or not? Paavo Helde <myfirstname@osa.pri.ee> - 2016-09-02 23:56 +0300
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-03 17:58 +0200
                Re: This will work or not? Manfred <noname@invalid.add> - 2016-09-02 11:41 +0200
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 12:27 +0200
                Re: This will work or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 12:37 +0100
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 17:39 +0200
                Re: This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-09-02 17:40 +0200
                Re: This will work or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 16:54 +0100
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 20:22 +0200
                Re: This will work or not? Richard Damon <Richard@Damon-Family.org> - 2016-09-02 08:04 -0400
                Re: This will work or not? BartC <bc@freeuk.com> - 2016-09-02 13:51 +0100
                Re: This will work or not? Richard Damon <Richard@Damon-Family.org> - 2016-09-02 21:48 -0400
                Re: This will work or not? Manfred <noname@invalid.add> - 2016-09-02 15:17 +0200
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 17:46 +0200
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-01 23:14 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-01 21:02 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 09:35 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 08:50 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 17:59 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 12:49 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 20:36 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 16:32 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-03 18:08 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 18:25 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-03 23:51 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 21:48 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 03:48 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 23:05 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 05:02 +0100
                Re: This will work or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-04 10:57 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 09:40 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 16:39 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 12:46 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 17:51 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 13:07 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 18:37 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 17:16 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 22:23 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 18:54 -0400
                Re: This will work or not? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-09-04 09:58 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 13:08 -0400
                Re: This will work or not? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-09-04 10:21 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 17:18 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 22:24 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 18:56 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-05 00:27 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 22:53 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-05 18:18 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-05 15:27 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-05 20:32 +0100
                Re: This will work or not? woodbrian77@gmail.com - 2016-09-05 14:07 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-05 18:15 -0400
                Re: This will work or not? Ike Naar <ike@iceland.freeshell.org> - 2016-09-05 20:12 +0000
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-05 22:05 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-05 18:16 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-06 08:47 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-06 09:12 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-06 16:31 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-06 11:20 -0400
                Re: This will work or not? bighe.trole@gmail.com - 2016-09-04 18:55 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 22:55 -0400
                Re: This will work or not? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-09-04 17:17 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 22:57 -0400
                Re: This will work or not? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-09-05 11:20 -0700
                Re: This will work or not? Luuk <luuk@invalid.lan> - 2016-09-04 18:54 +0200
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 17:59 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 13:08 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-04 20:25 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 17:20 -0400
                Re: This will work or not? Öö Tiib <ootiib@hot.ee> - 2016-09-04 02:36 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 09:41 -0400
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 21:49 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 03:49 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 23:06 -0400
                Re: This will work or not? woodbrian77@gmail.com - 2016-09-03 21:16 -0700
  Re: This will work or not? Vir Campestris <vir.campestris@invalid.invalid> - 2016-08-31 21:08 +0100
  Re: This will work or not? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2016-09-01 02:28 +0200
    Re: This will work or not? Öö Tiib <ootiib@hot.ee> - 2016-09-01 01:28 -0700
    Re: This will work or not? "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-09-02 12:06 -0700
  Re: This will work or not? Öö Tiib <ootiib@hot.ee> - 2016-09-05 02:21 -0700
    Re: This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-09-05 13:38 +0200
  Re: This will work or not? Juha Nieminen <nospam@thanks.invalid> - 2016-09-05 11:34 +0000
    Re: This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-09-05 13:36 +0200
      Re: This will work or not? Vir Campestris <vir.campestris@invalid.invalid> - 2016-09-05 21:25 +0100

csiph-web