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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Help - Constructors - ASAP.
Date: 1998/08/10
Message-ID: <EACHUS.98Aug10171006@spectre.mitre.org>#1/1
X-Deja-AN: 379831015
References: <6p75qi$rcj@news.latnet.lv> <m3lnpg7j5s.fsf@mheaney.ni.net>
 <6pi4jq$j73$1@nnrp1.dejanews.com> <EACHUS.98Jul30114523@spectre.mitre.org>
 <dewar.901945397@merv> <m3vhodvevl.fsf@mheaney.ni.net>
Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.ada
Date: 1998-08-10T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <m3vhodvevl.fsf@mheaney.ni.net> Matthew Heaney <matthew_heaney@acm.org> writes:

>   This is helpful when you have a type that comes into existence because
>   of a nested instantation.  For example...

>   with GQ;
>   pragma Elaborate_All (GQ);

>   package P is
>   ...
>      package Q is new GQ (...);

>      type T is new Q.T with null record;
       -- and this is the case where you want to inherit the construtor:
       function Create_T(...) return T;
       -- of course, if you forget to do this, the compiler will be
       -- sure to remind you.  ;-)
>  ...
>  end P;

   -- and also in the package body:
   function Create_T(...) return T is 
   begin return Q.T(Q.Create_T(...)); end Create_T;

   It is a bit of a pain when you need to do it right, but as you can
see, it amounts to a line in the spec and two in the body.

>  What you do NOT want to happen is to for a client to have to refer to
>  P.Q.T.  The client shouldn't have to care that T was brought into
>  existence because of an instantiation; it's really an implementation
>  detail.  By using transitivity of visibility, the client can just refer
>  to P.T, in effect "hiding" the fact that T comes from Q.

   Exactly, the user of the abstraction should never need to pay
attention to the man behind the curtin.
--

					Robert I. Eachus

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