Re: Union C++ standard

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From	Derek Jones <derek@NOSPAM-knosof.co.uk>
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Subject	Re: Union C++ standard
Date	Mon, 29 Nov 2021 00:09:39 +0000
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David,

>>> In C, type-punning via unions is allowed (i.e., fully defined behaviour
>>
>> That is not true.  Writing into one member and then reading from
>> another member is undefined behavior.
>
> No, it is correct.  It would be helpful if you looked at the full

You have misunderstood the C conformance model, which revolves around
the use of "shall" and "shall not", and the kind of section in which
they appear (e.g., Constraints).  See:
http://c0x.shape-of-code.com/4..html

For a longer discussion see: http://knosof.co.uk/cbook/

> """
> If the member used to read the contents of a union object is not the
> same as the member last used to store a value in the object, the
> appropriate part of the object representation of the value is
> reinterpreted as an object representation in the new type as described
> in 6.2.6 (a process sometimes called "type punning"). This might be a
> trap representation.
> """
>
> These quotations are from C18 (draft N2346), which is the current C
> standard (until C23 is finalised).  They have not changed since C99,

This footnote was added in response to this DR (so it must have come
after C99):
http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_283.htm

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Union C++ standard Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2021-11-25 11:11 +0100
  Re: Union C++ standard Kaz Kylheku <480-992-1380@kylheku.com> - 2021-11-26 18:06 +0000
  Re: Union C++ standard gah4 <gah4@u.washington.edu> - 2021-11-26 12:16 -0800
  Re: Union C++ standard David Brown <david.brown@hesbynett.no> - 2021-11-27 16:59 +0100
    Re: Union C++ standard Derek Jones <derek@NOSPAM-knosof.co.uk> - 2021-11-28 12:51 +0000
      Re: Union C++ standard David Brown <david.brown@hesbynett.no> - 2021-11-28 19:00 +0100
        Re: Union C++ standard Derek Jones <derek@NOSPAM-knosof.co.uk> - 2021-11-29 00:09 +0000
          Re: Union C++ standard David Brown <david.brown@hesbynett.no> - 2021-11-29 21:00 +0100
            Re: Union C++ standard Derek Jones <derek@NOSPAM-knosof.co.uk> - 2021-11-30 00:46 +0000
              Re: Union C++ standard George Neuner <gneuner2@comcast.net> - 2021-11-30 17:18 -0500
                Re: Union C++ standard terminology Derek Jones <derek@knosof.co.uk> - 2021-12-01 13:35 +0000
              Re: Union C++ standard David Brown <david.brown@hesbynett.no> - 2021-11-30 23:24 +0100
        Re: Union C++ standard Kaz Kylheku <480-992-1380@kylheku.com> - 2021-11-29 16:39 +0000
          Re: Union C++ standard Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-29 14:32 -0800

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