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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Runtime type selection
Date: Mon, 12 Oct 2009 17:59:53 -0700 (PDT)
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Date: 2009-10-12T17:59:53-07:00
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On Oct 11, 12:54=A0pm, Niklas Holsti <niklas.hol...@tidorum.invalid>
wrote:
> xorque wrote:
> > On Oct 11, 5:02 pm, Niklas Holsti <niklas.hol...@tidorum.invalid>
> > wrote:
> >> Keep a list or array of (accesses to) a factory function for each
> >> possible (derived) type.
>
> > 'Lo,
>
> > That does sound like the sort of thing I'd have done in C.
>
> Yes. Ada makes it easier since you can use elaboration-time statement
> blocks in package bodies to register stuff.
>
> > I still
> > have a nagging sensation that Ada actually provides other means to
> > do this based on such things as the 'Tag attribute.
>
> Did you read about the package Ada.Tags (RM 3.9)? There is a generic
> function Ada.Tags.Generic_Dispatching_Constructor that takes a tag as
> parameter and creates an object with that tag, but I don't hink there is
> any way to find the tags for all types in a given derivation class. So
> even with =A0Ada.Tags.Generic_Dispatching_Constructor your program still
> has to register the types, or their tags, or a prototype object of each
> type, to create the program-accessible set of types/tags that can be in
> the graph.
>
> Perhaps some expert can explain why this reflection capability (find
> tags for all derived types in a class) was not included in Ada.Tags?
> Perhaps because Ada run-time systems do not need such a function for
> their own purposes?

Maybe I don't understand the problem clearly enough, but would
Ada.Tags.Descendant_Tag help?  This takes an external tag (a string)
and the 'Tag of the ancestor type, and will find a type in the
ancestor type's class whose 'External_Tag is the given external tag
string (as long as it's not more deeply nested than the ancestor
type).

                                     -- Adam