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From: Jacob Sparre Andersen <sparre@nbi.dk>
Newsgroups: comp.lang.ada
Subject: Re: Handling invalid objects
Date: Sat, 18 Mar 2006 09:57:24 +0100
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Date: 2006-03-18T09:57:24+01:00
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Justin Gombos wrote:

> The 'Valid attribute exists to be able to handle abnormal objects.
>
> Here's a concrete example.  Suppose I have:
>
>   type clock_type is mod 12;
>
>   function hour_of_day return clock_type;
>
> If hour_of_day gets called and for whatever reason I cannot return
> an hour_of_day, the caller needs to know that.  Exceptions are a
> poor choice.  The quality and style guide advises against them for a
> good reason; exceptions are like gotos - and produce a questionable
> state.  It would be more graceful for me to return an invalid value
> (like -1), so my caller can simply do a 'Valid to discover whether
> the operation was successful.

No.  The way to do that is to have proper values for all the states
you want to handle, i.e.:

   type Hours is mod 12;

   type Clock_Type (Is_Set : Boolean := False) is new
      record
         case Is_Set is
            when True =>
               Time : Hours;
            when False =>
               null;
         end case;
      end record;

Greetings,

Jacob
-- 
ACRONYM: A Contrived Reduction Of Nomenclature Yielding Mnemonics