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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Universal type in Ada
Newsgroups: comp.lang.ada
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Date: Wed, 15 Jun 2005 15:55:15 +0200
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Date: 2005-06-15T15:55:15+02:00
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On 15 Jun 2005 06:07:12 -0700, zw wrote:

> Hi, I am trying to create a Universal type in Ada such as "Object" in
> Java so that I could define a function that returns a value in this
> Universal type, then I could lower-cast value in this type to its
> specific type such as String, Float or any user-defined types. Because
> in Java if a method returns an Object, it could be casted to any other
> types that are subtypes of Object. Is there a Universal type in Ada?

Yes. What you describe is a contravariant result. In Ada it is achieved by
returning a class-wide result. A function of this sort is also called
"abstract factory".

> Could anyone tell me how to create such types in Ada, please?

type Object is abstract tagged null record;
function Factory (...) return Object'Class;

type My_Special_Object is new Object with ...;

declare
   X : My_Special_Object;
begin
   ...
   X := My_Special_Object Factory (...));

Here the result of Factory is explicitly converted to My_Special_Object. If
that is impossible (because the result is not a My_Special_Object)
Constraint_Error exception is propagated at run-time.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de