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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada 95 and Class attribute for none-tagged incomplete type
Date: Fri, 14 Aug 2009 15:01:15 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2009-08-14T15:01:15-04:00
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"Randy Brukardt" <randy@rrsoftware.com> writes:

>...All this special rule eliminates is the need to declare 
> an incomplete type in this one unusual case (the private type in cases like 
> this is best declared as tagged, so this shouldn't happen very often). Seems 
> like a pile of work for an implementation and for the language just to 
> eliminate one line in a private part.

The problem is that declaring that incomplete type is illegal.
Eliminating the need for one _illegal_ line isn't such a bad thing.  ;-)

package P is
   type Priv is private;
private
   type Priv is tagged; -- Illegal!
   type Acc is access all Priv'Class;
   type Priv is tagged
      record
         Next : Acc;
      end record;
end P;

We could have made it legal, I suppose.  But having three views of a
type might make more trouble for the compiler than the existing
rules.

On the third hand, compilers have to deal with:

package P is
   type Priv is private;
private
   task type Priv is ...;
end P;

package body P is
   task body Priv is separate;
end P;

separate (P)
task body Priv is ...;

where there are four things called Priv, which are all really the same
thing.

- Bob