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Re: Function casting - UB?

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Jens Gustedt <jens.gustedt@loria.fr> writes:

> Am 28.06.2012 09:27, schrieb Johannes Bauer:
>> On 28.06.2012 09:23, Johannes Bauer wrote:
>>> On 27.06.2012 19:37, Tim Rentsch wrote:
>>>> The other example you give,
>>>> assigning to one member of a union and reading from another, is
>>>> actually defined behavior, _not_ undefined behavior.
>>>
>>> I also was under the impression that writing to member x and reading
>>> member y of a union is UB. Wikipedia says "This is not, however, a safe
>>> use of unions in general.", which is pretty vague (i.e. it's not clear
>>> which cases are safe and which are not).
>>>
>>> Could you elaborate on why you think this is well-defined?
>> 
>> Ah, I just read James' response further down. Interesting. Really
>> thought this was undefined. Is this a recent change?
>
> I think this was not considered as a change in contents but given as
> more precision on the intent. n1256.pdf has modification marks in this
> region so I suppose that these came with TC3. [snip]

Yes, if you read the Defect Report that prompted the change I
think you'll find that the intention was that the behavior
required was supposed to be the same all along (ie, since C90 and
presumably also before that), but changes in wording in other
places raised a concern that this (unchanged) requirement was not
evident enough without the footnote.

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Thread

Function casting - UB? nroberts <roberts.noah@gmail.com> - 2012-06-27 08:50 -0700
  Re: Function casting - UB? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-27 12:49 -0400
    Re: Function casting - UB? Heikki Kallasjoki <fis+usenet@zem.fi> - 2012-06-27 17:54 +0000
      Re: Function casting - UB? "io_x" <a@b.c.invalid> - 2012-06-28 18:58 +0200
  Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-27 10:37 -0700
    Re: Function casting - UB? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-28 09:23 +0200
      Re: Function casting - UB? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-28 09:27 +0200
        Re: Function casting - UB? Jens Gustedt <jens.gustedt@loria.fr> - 2012-06-28 12:03 +0200
          Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-29 11:10 -0700
      Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-29 11:03 -0700
  Re: Function casting - UB? Les Cargill <lcargill99@comcast.com> - 2012-06-27 12:58 -0500
  Re: Function casting - UB? Les Cargill <lcargill99@comcast.com> - 2012-06-27 12:59 -0500
    Re: Function casting - UB? James Kuyper <jameskuyper@verizon.net> - 2012-06-27 14:45 -0400
      Re: Function casting - UB? "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-06-27 14:55 -0700
        Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-27 17:44 -0700
          Re: Function casting - UB? Joshua Maurice <joshuamaurice@gmail.com> - 2012-07-07 16:46 -0700
            Re: Function casting - UB? Barry Schwarz <schwarzb@dqel.com> - 2012-07-08 09:19 -0700
              Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-07-08 11:13 -0700
                Re: Function casting - UB? Barry Schwarz <schwarzb@dqel.com> - 2012-07-08 15:24 -0700
                Re: Function casting - UB? Joshua Maurice <joshuamaurice@gmail.com> - 2012-07-08 18:12 -0700
                Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-07-10 10:40 -0700
            Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-07-08 11:06 -0700

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