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Newsgroups: comp.lang.ada
Subject: Re: private classes
References: <cjn3e9$moq$05$1@news.t-online.com>
 <cjn5p0$f6v$03$1@news.t-online.com>
From: Ludovic Brenta <ludovic.brenta@insalien.org>
Date: Sun, 03 Oct 2004 21:11:40 +0200
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Date: 2004-10-03T21:15:06+02:00
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"Rick Santa-Cruz" writes:
> Ok, I got it. The declaration of the procedure has to be before the "type 
> Derived_1 is new Base_1 with private;". Although I don't understand the 
> sense of this, but ok ;).

Dennis Lee Bieber already explained why in a previous post.  I think
his explanation is good and intuitive.  The technical term for this is
"freezing rules", defined formally in RM 13.14.  You might be
interested in reading that.  Note that few people actually bother to
really understand all the implications of freezing rules; they are of
interest primarily to compiler writers.

In your case, the declaration of type Derived_1 freezes type Base_1
(RM 13.14(7): "The declaration of a record extension causes freezing
of the parent subtype.").  After this point, no more primitive
subprograms ("methods") can be added to type Base_1.  This means that:

- you can declare procedure Proc (B : Base_1), but it is not a
  primitive subprogram, just a regular one.

- no dynamic dispatching can take place, since dynamic dispatching
  only involves primitive subprograms.

-- 
Ludovic Brenta.