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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 2016-09-01 12:30 +0100
Organization Datemas.de http://www.news.datemas.de
Message-ID <20160901123037.3255928f@bother.homenet> (permalink)
References (2 earlier) <nq6p82$ijc$1@dont-email.me> <nq6rp7$3ip$1@gioia.aioe.org> <nq746l$r3b$1@dont-email.me> <20160831215212.63b20ea7@bother.homenet> <nq8k3g$51b$1@dont-email.me>

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On Thu, 01 Sep 2016 09:07:27 +0200
David Brown <david.brown@hesbynett.no> wrote:
> On 31/08/16 22:52, Chris Vine wrote:
> > On Wed, 31 Aug 2016 19:29:57 +0200
> > David Brown <david.brown@hesbynett.no> wrote:
> > [snip]  
> >> Apparently, you are not allowed to access the different parts of
> >> the union as I had thought.  My knowledge of such details in C++
> >> is a good deal less than my knowledge of C, and my knowledge of C
> >> has holes too!  
> > 
> > It has undefined behaviour in C89, and C99 as originally printed.
> > It has defined behaviour in C99 with Technical Corrigendum 3
> > applied and in C11, except that if there are padding bytes they can
> > have a trap representation (§6.2.6.1/6 and /7 of C11 read with
> > footnote 95 of §6.5.2.3/3 if you want the reference).  
> 
> Yes, that's how I remember the standards.  C90 standards did not allow
> this sort of access to different parts of the union, but most
> implementations did - and programmers relied on that as a way of
> changing types without changing the underlying representation.  So the
> C99 (after the TC's) standard put it in.
> 
> But I guess C++ gets most of its C stuff from C90.
> 
> >  The latter (trap
> > representations) can be ignored on any architecture running C++.  
> 
> Is that guaranteed by the C++ standards, or is it just that no one has
> made a C++ compiler for one of the weird architectures that has trap
> representations?
> 
> (In this particular case, the types in question were uint32_t and
> uint64_t - these cannot have trap representations in C.  But pointers,
> signed integers, and floating point types could have trap
> representations, in theory.)

A trap representation is (according to C) "an object representation
that need not represent a value of the object type".  §6.2.6.1/5 goes
on to say that "Certain object representations need not represent a
value of the object type.  If the stored value of an object has such a
representation and is read by an lvalue expression that does not have
character type, the behavior is undefined.  If such a representation is
produced by a side effect that modifies all or any part of the object
by an lvalue expression that does not have character type, the behavior
is undefined.  Such a representation is called a trap representation".

The resulting undefined behaviour may include the triggering of hardware
traps.

C++ has not yet imported the "trap representation" from C: C++ instead
refers to uninitialised values of trivial types having "an
indeterminate value".  In particular it does not so far permit access
to indeterminate values via the character types, but by the same token
I do not think that any architecture running C++ has hardware-level
traps for invalid signed arithmetic or floating points or for copying
invalid pointer values (but see below on pointers).

§8.5/12 of the draft C++17 standard sort-of introduces something similar
to the rules on trap representations by stating (in a convoluted
fashion) that if you evaluate an indeterminate value except via a
narrow unsigned character type you have undefined behaviour.

Since "evaluate" includes copy, this seems inconsistent with the
welcome licence given by §3.7.4.2/4 of C++14/17 which makes copying a
pointer value which is neither the null pointer nor the address of a
valid object implementation-defined behaviour instead of undefined
behaviour (C++14 tips it hat to C hardware traps by indicating that
that implementation-defined behaviour may include having "a
system-generated runtime fault").  No one yet seems to have spotted
this inconsistency.

Chris

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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

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