Re: Union C++ standard

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.compilers
Subject	Re: Union C++ standard
Date	2021-11-27 16:59 +0100
Organization	A noiseless patient Spider
Message-ID	<21-11-008@comp.compilers> (permalink)
References	<21-11-004@comp.compilers>

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On 25/11/2021 11:11, Hans-Peter Diettrich wrote:
> 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]
>

In C++, objects of a class typically have some kind of invariant which
is established by the constructor, and kept consistent when accessed via
its public methods.  Messing with the underlying data representation
directly is going to risk losing that - it means you are accessing data
without going through the proper defined interface (the public or
protected methods and members).

In C, type-punning via unions is allowed (i.e., fully defined behaviour
in the standards), but not in C++ where the language is expected to
enforce higher-level aspects of the data.

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Thread

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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