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From: =?ISO-8859-2?Q?Pawe=B3_=27Nivertius=27_P=B3azie=F1ski?=
 <nivertius@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Access to function returning class-wide type
Date: Thu, 21 Aug 2008 21:30:28 +0200
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Date: 2008-08-21T21:30:28+02:00
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Dmitry A. Kazakov wrote:

> So you could replace it with a map condition->type tag and then proceed
> with Ada.Tags.Generic_Dispatching_Constructor, because the map
> tag->constructor is for free, here you do not need pointers any more, in
> Ada 2005. The compiler will force to override the "constructor" for each
> concrete derived type.
> -------------
> Having said that, there still is a difficult problem with construction of
> the map. I don't know if it is an issue for you, but it is for the
> persistency layer I designed. The problem is that if the map is
> open-ended, you need some mechanism of registration of each new type in
> the map. This is still unresolved in Ada. The pattern I am using is that
> each package declaring a new type is required to call some class-wide
> procedure registering the type upon package elaboration. In your case it
> is the condition associated with the type tag. That cannot be enforced,
> and it works only at the library level, i.e. for "immortal" types.

I think that the map condition->type tag is in my case same problem as map
condition->constructor pointer. I need the map registration anyway, because
it basically needs to be changed at run-time. Altrough, all the types I
need to register are library-level types, so the registration mechanism is
pretty simple.

> A possible solution in the future language revisions could be to introduce
> type constructors/destructors: user-defined procedures called upon
> derivation and leaving the scope of a type.

I can't wait to see that :-)

-- 
Pawe� P�azie�ski aka Nivertius
"In the end, there will be Ada, XML and gzip"