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From: "Nick Roberts" <nickroberts@adaos.worldonline.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: List Container Straw Man
Date: Wed, 7 Nov 2001 23:58:31 -0000
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Date: 2001-11-07T23:58:31+00:00
List-Id: <comp.lang.ada>

I probably haven't explained it well, but I think you may be getting hold of
the wrong end of the stick a little.

In general, according my suggested design, you would need two instantiations
to start making use of a utility container. The first would instantiate the
package (called Iteration let us say) of abstract types:

   package Widget_Iteration is new Iteration(Aerial_Widget);

The second would instantiate the package declaring the utility container,
using the first package - a 'signature' package - as its generic parameter:

   package Widget_Storage is new
Linear_Linked_List_Storage(Widget_Iteration);

You might then want to use a 'use', or more likely a subtype declaration to
give the widget container type a convenient name:

   subtype Widget_Store is Widget_Storage.Store_Type;

and then you can start using it:

   Nuts, Bolts: Widget_Store; -- initialised empty

To recap, the great advantage of this design is that Widget_Store is now a
general-purpose iterator type (derived from
Widget_Iteration.Reproducable_Sequence probably), and so any unit that takes
a widget iterator type can have Widget_Stores passed into it with no further
ado. A generic procedure:

   generic
      package Specific_Iteration is new Iteration(<>);
   procedure Print_a_List (List: in out
Specific_Iteration.Terminating_Sequence'Class);

   procedure Print_a_List (List: in out
Specific_Iteration.Terminating_Sequence'Class) is
      Item: Specific_Iteration.Element_Type;
   begin
      while not End_of_Data(List) loop -- dispatching
         Read(List,Item); -- dispatching
         Put_Line(Item);
      end loop;
   end;

can be used to print out the items in any container of any type (that
conforms to my design):

   procedure Print_Widgets is new Print_a_List(Widget_Iteration);

   ...

   Print_Widgets(Nuts);
   Print_Widgets(Bolts);

Phew! I hope this clarifies things. It's really not too difficult in actual
usage, is it? In big software, it would save a huge amount of effort, and
Ada is an industrial language.

--
Best wishes,
Nick Roberts