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NNTP-Posting-Date: Thu, 25 Aug 2011 21:00:08 -0500
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From: Bart van Ingen Schenau<bart@ingen.ddns.info>
Subject: Re: New strongly type enum proposal featuring inherited enums
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Date: Thu, 25 Aug 2011 20:51:07 CST
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Xref: x330-a1.tempe.blueboxinc.net comp.std.c++:257


Daniel Krügler<daniel.kruegler@googlemail.com>  Wrote:

>  Am 24.08.2011 23:06, schrieb Phil Bouchard:
>  >   On 8/18/2011 9:54 AM, Daniel Krügler wrote:
>  >>
>  >>>   2) Inheriting from another enum would be quite useful. For
>  example:
>  >>>   enum class Widget {button, combobox, listview};
>  >>>   enum class SuperWidget : Widget {iconview}
>  >>
>  >>   The current state would not disallow such an extension in the
>  future,
>  >>   so what is the problem?
>  >
>  >   I don't think it'll be a good idea to make the two coexist
>  together:
>  >   enum class SuperWidget : Widget {iconview} // derived
>  >   enum class SuperWidget : unsigned int {iconview} // subtyping
>
>  I'm missing the rationale for your dislike. Widget is still a type that
>  has an integral type as underlying type, so what is the drastic
>  difference, if you extend scoped enums to allow for extending
>  another enum? We have no enumeration values in integral types, so
>  we have we don't have an observable difference. If you want to
>  define that SuperWidget contains the enumeration values of
>  Widget, this is simply a definition problem.

The dislike probably comes from how an enum is typically viewed.
To my knowledge, an enum is typically viewed as an integer with a limited
set of possible values (that some/most of those values also have a symbolic
name is incidental). A scoped enum gives you the added value of being able
to specify the underlying storage.

With this view the interpretation of the declarations would be:
  enum class Widget : unsigned int { list_view, box, button }; /* RESTRICT a
storage cell of size unsigned int to store only the specified values */
  enum class SuperWidget : Widget { iconview }; /* EXTEND? the allowed values
to include iconview */

OTOH, you seem to view a (scoped) enum as its base-type with some of the
values given a symbolic name:

  enum class Widget : unsigned int { list_view, box, button }; /* Can store
all values of an unsigned int, with three of them given a specific,
symbolic, name */
  enum class SuperWidget : Widget { iconview }; /* Extend the list of
symbolic names to include iconview */

>
>  Greetings from Bremen,
>
>  - Daniel Krügler

Bart v Ingen Schenau


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