Re: Enumeration representation

[Robert Dewar <robert_dewar@my-deja.com>]

In article <7rava6$kht@hobbes.crc.com>,
  "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com> wrote:
>
> Alex <busato@newtech.it> wrote in message
> news:37D8E3BC.175DB72C@newtech.it...
> > I've written the following example program for testing
enumeration
> > represetantion clause:
> >
> > with Ada.Text_IO; use Ada.Text_IO;
> >
> >
> > procedure MainEnum is
> >
> >       type Status is (a,b,c,d,e);
> >       pragma Discard_Names(Status);
> >
> >       for Status use
> >        ( a => 0,
> >          b => 20,
> >          c => 400,
> >          d => 800,
> >          e => 1600);
> >
> >        S : Status;
> > begin
> >       S := b;
> >       Put(" => " & S'img);
> > end MainEnum;
> >
> > I've compiled it with Gnat (the latest version)
> > the output of program is 1 but I was expecting another value
: 20.
> > Does  Gnat support this clause?
> I'm not sure to which clause you refer.
>
> > Did I use this clause badly?
> >
>
> About the discard_names pragma the LRM says C.5 (7):
> If the pragma applies to an enumeration type, then the
semantics
> of the Wide_Image and Wide_Value attributes are implementation
> defined for that type; the semantics of Image and Value are
still
> defined in terms of Wide_Image and Wide_Value. In addition,
the
> semantics of Text_IO.Enumeration_IO are implementation
defined.
> If the pragma applies to a tagged type, then the semantics of
the
> Tags.Expanded_Name function are implementation defined for
that
> type. If the pragma applies to an exception, then the
semantics
> of the Exceptions.Exception_Name function are implementation
> defined for that exception.
>
> The GNAT Reference Manual is not entirely correct in what it
> states about this pragma, in that it says it is ignored.  What
> really happens, is that the Image attribute returns instead
> of the image of the name, the image of the 'pos attribute.'

Well my copy of the reference manual does not say that, as I
quoted in my previous message. It is possible that versions of
the reference manual for old versions of GNAT are out of date ..
>
> To do what you want to do, you need an instance of
> Ada.Unchecked_Conversion, as demonstrated in this modification
> of your program:
>
> with Ada.Text_Io;
> use Ada.Text_Io;
> with Ada.Unchecked_Conversion;
> procedure MainEnum is
>
>    type Status is (a,b,c,d,e);
>    pragma Discard_Names(Status);
>
>    for Status use
>       ( a => 0,
>         b => 20,
>         c => 400,
>         d => 800,
>         e => 1600);
>    type Status_Rep is mod 2 ** Status'Size;
>
>    function Rep is new Ada.Unchecked_Conversion
>      (Source => Status,
>       Target => Status_rep);
>
> --   S : Status;
> begin
>    for S in Status'Range loop
>       Put("pos => " & S'img);
>       Put_Line ("; rep =>" & Rep(S)'Img);
>    end loop;
> end MainEnum;
>
> Abitavo a Milano dal 6/1963 al 11/1965, e il mio
> Italiano non e' quanto fluente come era allora ;)
>

Or, as noted in the previous post, 'Enum_Rep


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