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From: Gautier write-only <gautier_niouzes@hotmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Type of the target of an assignment shall not be abstract
Date: Tue, 16 Feb 2010 04:14:06 -0800 (PST)
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Date: 2010-02-16T04:14:06-08:00
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> Choosing between both tricks, I'd prefer Dmitry's.
> I'll give a clear name to the concrete-before-abstract type, like
> "Pre_Root", and try to have it private.
> Let's see how it turns out...

It turns out perfectly.

*** Spec, private part:
  -- We have a concrete type as hidden ancestor of the
Excel_Out_Stream root
  -- type. A variable of that type is initialized with default values
and
  -- can help re-initialize a Excel_Out_Stream when re-used several
times.
  -- See the Reset procedure in body.
  --
  type Excel_Out_Pre_Root_Type is tagged record
    [the 14 components]
  end record;

  type Excel_Out_Stream is abstract new Excel_Out_Pre_Root_Type with
null record;

*** Body:
  procedure Reset(
    xl        : in out Excel_Out_Stream'Class;
    format    :        Excel_type:= Default_Excel_type
  )
  is
    dummy_xl_with_defaults: Excel_Out_Pre_Root_Type;
  begin
    ...
    dummy_xl_with_defaults.format:= format;
    Excel_Out_Pre_Root_Type(xl):= dummy_xl_with_defaults;
  end Reset;

*** GNAT and ObjectAda are happy, I as well :-)
Philosophically, it seems reasonable that an abstract type should be
somewhat "limited" in its use (NB: I've put ""'s around the word
'limited' :-) )
______________________________________________________________
Gautier's Ada programming -- http://gautiersblog.blogspot.com/