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From: Richard D Riehle <laoXhai@ix.netcom.com>
Subject: Re: Derivation + Access Discriminant = Headaches
Date: 1999/03/19
Message-ID: <7cu23r$miu@dfw-ixnews5.ix.netcom.com>#1/1
X-Deja-AN: 456572601
References: <m3r9qlvpuv.fsf@mheaney.ni.net>
Organization: Netcom
X-NETCOM-Date: Fri Mar 19 11:39:07 AM CST 1999
Newsgroups: comp.lang.ada
Date: 1999-03-19T11:39:07-06:00
List-Id: <comp.lang.ada>

In article <m3r9qlvpuv.fsf@mheaney.ni.net>,
	Matthew Heaney <matthew_heaney@acm.org> wrote:

>Below is a tagged type hierarchy.

  [ deleted because it is shown in Matthew's code example ]

>
>My expectation is that this should be legal, because NT is in T'Class,
>and should therefore be allowed to replace the discriminant that
>designates T'Class.

 On GNAT, an equivalent type hierarchy works fine.  For simplicity I
 did not separate this into separate packages or use private types.
 However, the same discriminant rules should apply.  

 package Discriminant_Derivations is

  type Tack is tagged limited null record;
  type Tack_Reference is access all Tack'Class;

  function Get return Tack_Reference;

  type Tack_N (Data :  access Tack'Class) is new Tack
              with null record;
  

  type Tack_N1(Data : access Tack'Class) is new Tack_N(Data)
              with null record;
  
  -- This derivation constrains with a function call
  type Tack_N1(Data : access Tack'Class) is new Tack_N(Get)
              with null record;

end Discriminant_Derivations; 


I wonder if this is OK on other Ada 95 compilers.  Have not tried
Aonix with it yet.   Do not have Rational Apex handy.  

Richard Riehle
richard@adaworks.com
http://www.adaworks.com
>
>--STX
>package P is
>
>   type T is abstract tagged limited private;
>
>   type NT (O : access T'Class) is
>     abstract new T with private;
>
>private
>
>   type T is abstract tagged limited null record;
>
>   type NT (O : access T'Class) is new T with null record;
>
>end P;
>
>
>with P; use P;
>package Q is
>
>   type NNT (O : access NT'Class) is
>     new NT with private;
>
>private
>
>   type NNT (O : access NT'Class) is
>     new NT (O) with null record;
>
>end Q;
>
>
>