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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: ANNOUNCE: data structures with limited items pattern
Date: 1999/03/01
Message-ID: <m3aexycbbo.fsf@mheaney.ni.net>#1/1
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Sender: matt@mheaney.ni.net
NNTP-Posting-Date: Sun, 28 Feb 1999 16:14:35 PDT
Newsgroups: comp.lang.ada
Date: 1999-03-01T00:00:00+00:00
List-Id: <comp.lang.ada>

I have submitted an article to the ACM patterns archive, on how to
implement a collection of items in which the item-type is limited.  A
description appears below.

<http://www.acm.org/archives/patterns.html>

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Data Structures Containing Limited Items

In this article I show how to implement a stack whose items are limited.

Most data structures you see are implemented this way:

generic
  type Item_Type is private;
package Stacks is

  type Stack_Type is limited private;

  procedure Push (Item : in     Item_Type;
                  On   : in out Stack_Type);
...
end Stacks;

The generic formal type Item_Type is non-limited, so it comes with
assignment.  This allows you to implement operation Push (for a bounded
stack) like this:

  procedure Push (Item : in     Item_Type;
                  On   : in out Stack_Type) is
  begin
     On.Top := On.Top + 1;
     On.Items (On.Top) := Item;
  end;

But suppose you need a stack of, say, files?  Type File_Type, in
Text_IO, is limited:

  type File_Type is limited private;

so you wouldn't be able to instantiate the stack package above.  But
even if you could, it's not clear how you'd implement Push, which
requires assignment.