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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: 'Protected' abstract subprograms
Date: Sat, 11 Jan 2014 09:59:11 +0100
Organization: cbb software GmbH
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Date: 2014-01-11T09:59:11+01:00
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On Fri, 10 Jan 2014 14:00:31 -0800 (PST), sbelmont700@gmail.com wrote:

> I find it to be a fairly common occurrence and, depending on who you ask,
> a programming best practice to have a publically visible non-abstract
> dispatching operation re-dispatch to an abstract subprogram (i.e. the
> 'template pattern').

Fortunately, in Ada you don't need to re-dispatch because Ada directly
supports composing operations out of primitive operations. That is what
class-wide operations are for.

   type T is abstract ...;
   procedure Bar (X : T) is abstract; -- Primitive operation
   procedure Foo (X : T'Class); -- Class-wide operation

   procedure Foo (X : T'Class) is
   begin
       ...
       Bar (X); -- This dispatches
       ...
   end Foo;

> In any case, does anyone know of a workaround to achieve the same sort of
> behavior?

First, and foremost, never re-dispatch, that is semantically broken [*],
potentially unmaintainable (due to substitutability hell) and inefficient
too.

Secondly. There is another method of hiding operations. You pack it into a
set of implementation packages. The packages declare the operation public,
but the type itself is from some ancestor without the operation:

   type T is new Public_View with private;
private
   type T is new Implementation_View with ...;

The user of T does not know about Implementation_View, which itself is
public of course.

The drawback of this model is construction of new objects derived from T.
But since that is completely broken in Ada, anyway, I mean the mess of
limited aggregates, constructing functions etc, you will have fight that in
any case.

---------
* When you dispatch in a body of an operation that means that the body is
valid for more than one type. Many types = class. Operation for all class =
class-wide operation. It cannot be primitive, per definition. Re-dispatch
breaks this model and effectively makes your design un/weakly typed.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de