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From: Mark Biggar <mark.a.biggar@attbi.com>
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Subject: Re: Refactoring and Ada
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Date: 2002-02-20T05:20:00+00:00
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Richard Riehle wrote:
> 
> Larry Kilgallen wrote:
> 
> > > What happens when you read junk off the interface?
> >
> > Doesn't 'Valid detect that ?
> 
> It turns out that 'Valid is not as useful as many of us thought it was. For
> example,  the result of an erroneous result from unchecked conversion,
> which we originally thought was harmless if we checked it ourselves
> with 'Valid before using it is a problem.   Consider the following,
> 
>                type X is ... ;
>                type Y is ... ;
> 
>                function Convert is new Ada.Unchecked_Conversion
>                                          (Source => Y, Target => X);
> 
>                 X := Convert (Y);     -- suppose X is erroneous or not valid
>                 if X'Valid then ...      -- We used to think this was OK.
>                                                  -- Some compilers fail on this because of an
> interpretation
>                                                  -- of the ALRM rules.
> 
>  At present, one must wonder about the usefulness of 'Valid.

There is an AI with a binding interpretation that was included in the
TC that says that at least for scalar types the above code is
not errorious and must work as expected.

SO any compiler that fail on the above are non-compliant.

--
Mark Biggar
mark.a.biggar@attbi.com