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From: "Matthew Heaney" <mheaney@on2.com>
Newsgroups: comp.lang.ada
Subject: Re: Pre-Elaboration clarification.
Date: Wed, 28 Nov 2001 15:13:34 -0500
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"Mark Lundquist" <up.yerz@nospam.com> wrote in message
news:fiaN7.89348$XJ4.48897553@news1.sttln1.wa.home.com...
> - If only the body needs the type, then put it in the body.

Or in the spec of a private child.

> - If code in other (non-child) packages depends on it, but only the
compiler
> (not the code) needs to know the representation, then put the full
> definition in the private part.
> - If the only things depending on the type are other types in the private
> part, there is no need for a private_type_declaration (that is, "is
private"
> in the public part).  Define the type in the private part but don't
mention
> it in the public part.
> - If the code in other (non-child) packages needs to see it, put it in the
> public part of the spec
>
> I wrote the cases in order from most- to least- encapsulated, because it's
> good to get in the habit of thinking that way.

Other techniques include:

o if only a pointer to the type is required, then you can defer the rep to
the body:

package P is
   type Handle is private;
private
   type Rep;
  type Handle is access Rep;
end P;

packge body P is
   type Rep is null record;  --or whatever
end P;


o sometimes it makes sense to declare an abstract interface, and then
provide a constructor for it:

package P is
   pragma Elaborate_Body;
   type T is abstract tagged null record;
   procedure Op (O : access T) is abstract;
   type T_Class_Access is access all T'Class;
end P;

--the "constructor"
--you could declare this in P, but then you won't be
--able to use a categorization pragma.
function P.New_T return T_Class_Access;

private package P.C is
   type Rep is new T with null record;  --or whatever
   procedure Op (O : access Rep);
end P.C;

with P.C;
function P.New_T return T_Class_Access is
begin
   return new P.C.Rep;
end;

In this way, you're able to hide all kinds of dependencies, because a client
only manipulates type P.T'Class.  This is essentially what Lakos described
as a "protocol class."

We could talk all day about this stuff, which is what I intend to do at next
years Ada-Europe conference in Vienna.