From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,2a5cdfc27475c02f
X-Google-Attributes: gid103376,public
X-Google-ArrivalTime: 2003-06-04 08:56:40 PST
Path: archiver1.google.com!postnews1.google.com!not-for-mail
From: maa@liacc.up.pt (=?ISO-8859-1?Q?M=E1rio_Amado_Alves?=)
Newsgroups: comp.lang.ada
Subject: Re: How to model unknown values in a vector?
Date: 4 Jun 2003 08:56:39 -0700
Organization: http://groups.google.com/
Message-ID: <4a4de33a.0306040756.4bac474@posting.google.com>
References: <BAFE7E15.3898%harald.schmidt@anobject.net>
 <bbjgh7$acuc1$1@ID-25716.news.dfncis.de>
NNTP-Posting-Host: 195.23.231.53
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
X-Trace: posting.google.com 1054742200 21690 127.0.0.1 (4 Jun 2003 15:56:40
 GMT)
X-Complaints-To: groups-abuse@google.com
NNTP-Posting-Date: 4 Jun 2003 15:56:40 GMT
Xref: archiver1.google.com comp.lang.ada:38612
Date: 2003-06-04T15:56:40+00:00
List-Id: <comp.lang.ada>

Ah, the joys of special values :-)

  type Valid_Value is new Value
    range First_Valid_Value .. Last_Valid_Value;

  Unknown : Value := Value'Pred (First_Valid_Value); -- say

Or, the OO way

  type Value is tagged with null record;

  type Valid_Value is new Value with
    record
      The_Value : ...
    end record;

(But then you'll probably have to use pointers, and in that case there
is always null to signal unknown...)

Both are Ada ways, no implementation dependencies.