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From: Mark Lorenzen <mark.lorenzen@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: What would be the Ada solution?
Date: Fri, 4 Feb 2011 08:38:13 -0800 (PST)
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Date: 2011-02-04T08:38:13-08:00
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On 4 Feb., 17:33, KK6GM <mjsi...@scriptoriumdesigns.com> wrote:
> A colleague is working on some old code, written in C, that uses an
> "out of range" integer value to indicate no valid value. =A0Thus, a
> default value (in this case 0x7FFFFFFF) means no value has been
> entered. =A0All code that uses any values should check for this no-value
> and act accordingly, but of course not all the code actually does
> that, and there are odd cases where the no-value value gets processed
> as a valid value and then Bad Things Happen.
>
> I'm curious what the Ada approach to this issue would be, the issue
> being to differentiate between valid and invalid values, and to catch
> (compile time or run time) any manipulation of an invalid value as if
> it were a valid value.

I guess something like this:

type T is range ...;

type Opt_T (Is_Valid : Boolean :=3D False) is
   record
      case Is_Valid is
         when True =3D>
            Value : T;
         when False =3D>
            null;
      end case;
  end record;

- Mark L