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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: 'Size hack for enumerated types
Date: Sun, 06 Jul 2014 21:59:28 +0100
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Date: 2014-07-06T21:59:28+01:00
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Victor Porton <porton@narod.ru> writes:

> -- rdf-auxilary.ads
> with Interfaces.C;
>
> package RDF.Auxilary is
>
>    generic
>       type Enum_Type is range <>;
>    package Convert_Enum is
>       function To_C(Argument: Enum_Type) return Interfaces.C.int;
>       function From_C(Argument: Interfaces.C.int) return Enum_Type;
>    end;
>
> end RDF.Auxilary;

If you want Enum_Type to be an enumeration, you should use a formal
discrete type definition[1]:

   type Enum_Type is (<>);

But I don't see what you're trying to achieve that can't easily be done
using simple language facilities:

   with Ada.Text_IO; use Ada.Text_IO;
   procedure Enums is
      type Flag_Type is (Libxml_Error_Save,
                         Libxml_Structured_Error_Save,
                         URI_Interning,
                         WWW_Skip_Init_Finish) with Convention => C;
      for Flag_Type use (Libxml_Error_Save => 1,
                         Libxml_Structured_Error_Save => 2,
                         URI_Interning => 3,
                         WWW_Skip_Init_Finish => 4);
      function Increment (F : Flag_Type) return Flag_Type
      with Import, Convention => C, External_Name => "increment";
   begin
      Put_Line ("incremented "
                  & Libxml_Structured_Error_Save'Img
                  & " is "
                  & Increment (Libxml_Structured_Error_Save)'Img);
   end Enums;

and, in increment.c,

   #include <stdio.h>

   typedef enum {LIBXML_ERROR_SAVE = 1,
                 LIBXML_STRUCTURED_ERROR_SAVE,
                 URI_INTERNING,
                 WWW_SKIP_INIT_FINISH} flag_type;

   flag_type increment(flag_type f) {
     flag_type result = f;
     ++result;
     printf("increment received %d, returning %d\n", f, result);
     return result;
   }

with

   $ gcc -c increment.c
   $ gnatmake enums.adb -largs increment.o
   $ ./enums
   increment received 2, returning 3
   incremented LIBXML_STRUCTURED_ERROR_SAVE is URI_INTERNING

[1] http://www.ada-auth.org/standards/12rm/html/RM-12-5-2.html#p2