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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Use of abstract tagged types without access types -- again and
 complete
Date: Thu, 09 Jun 2011 22:19:19 +0200
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Date: 2011-06-09T22:19:19+02:00
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"Prof. Dr. Carl Weierstrass" writes on comp.lang.ada:
>    type Parent_Type is new Ada.Finalization.Controlled with record
>          Child : Child_Type'Class;
>    end record;
>
> This doesn't work, because:
> class-wide subtype with unknown discriminants in component declaration
>
> Of course I could use an access type to avoid the error but is there a
> way to do something like this without access types.

No.  The reason is that the size of Child is unknown, so you cannot
encapsulate a Child into a Parent_Type, which has a fixed size.  In
contrast, the size of an access value is fixed at comile time, so you
can encapsulate an access value in the Parent_Type.

What is it exactly that you want to achieve?  Why do you think you need
a record of a specific type to contain a record of a class-wide type?
The reason I ask is because the usefulness of Parent_Type escapes me,
since you can simply pass objects of type Child_Type'Class to
subprograms so they dispatch dynamically.

Also, why do you want to avoid access values in this case?  Would you be
happy with a reference-counted smart pointer instead?

-- 
Ludovic Brenta.