Union C++ standard

From	Hans-Peter Diettrich <DrDiettrich1@netscape.net>
Newsgroups	comp.compilers
Subject	Union C++ standard
Date	2021-11-25 11:11 +0100
Organization	Compilers Central
Message-ID	<21-11-004@comp.compilers> (permalink)

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Can somebody explain why the access to members of a union is "undefined"
except for the most recently written member?

What can be undefined in a union of data types of the same typesize end
alignment? Any member written will result in a unique bit/byte pattern
in memory, whose reading may not make sense in a different type but
undoubtedly is well defined.

DoDi
[I think it's undefined in a standards sense.  In any individual
implementation the result is predictable, but it's not portable. -John]

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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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