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From: "Grein, Christoph" <christoph.grein@eurocopter.com>
Newsgroups: comp.lang.ada
Subject: Re: How do I create an object instance from a tag?
Date: Wed, 19 Feb 2003 10:39:56 +0100 (MET)
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Date: 2003-02-19T10:39:56+01:00

Problem with this proposal is abstract types. There must not be objects of an 
abstract type.

So if myTag denotes an abstract type, the proposal is illegal. It should at 
least propagate Program_Error.

So there are good reasons why there is not yet a solution for this problem.

(A solution, together with some trivial other amendments, could be used to find 
the nearest common ancestor of two tagged objects. Currently there is no way to 
find it.)

> IMO, the following is more consistent:
> 
>    myTag : Tags.Tag := ...;
>    value   : data_type'class (myTag);
> 
> and for dispatching:
> 
>    myTag : Tags.Tag := ...;
>    value   : data_type'class (myTag) := Create_Value;
> 
> or fully dynamically (qualified expression):
> 
>    myTag : Tags.Tag := ...;
>    value   : data_type'class :=
>        data_type'class (myTag)'(Create_Value);