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From: Lutz Donnerhacke <lutz@iks-jena.de>
Newsgroups: comp.lang.ada
Subject: Re: Upcasting
Date: Tue, 5 Nov 2002 12:49:25 +0000 (UTC)
Organization: IKS GmbH Jena
Message-ID: <slrnasffii.pj.lutz@taranis.iks-jena.de>
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Date: 2002-11-05T12:49:25+00:00
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* Matthew Baulch wrote:
> My question is: How would you do this 'upcasting' if varying
> types existed? Obviously the compiler could tell where each
> value must go based on their type if only one of their type
> was needed in the transition. How does this type of thing work?
> (or does the A95 compiler not allow it)

procedure t is
   type Base is tagged null record;
   type Derivated is new Base with null record;

   d  : Derivated;
   dc : Base'Class := d;
   b  : Base       := Base(d);
   bc : Base'Class := b;
begin
-- d := Base (b);       -- will not compile
   d := Derivated (dc);
   d := Derivated (bc); -- will raised CONSTRAINT_ERROR on run time
end t;