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From: Jacob Sparre Andersen <sparre@nbi.dk>
Newsgroups: comp.lang.ada
Subject: Re: Ada-Singleton-Why does it work like this?
Date: Sun, 29 Mar 2009 09:29:02 +0200
Organization: Jacob Sparre Andersen
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Date: 2009-03-29T09:29:02+02:00
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"patrick.gunia@googlemail.com" <patrick.gunia@googlemail.com> wrote:

> I used a tagged type because I wanted to use the Singleton
> implementation as bsae class for other classes with more
> functionality though sharing the singleton behaviour.

That's a very complicated way of creating singletons, unless of course
that you actually have some operations which should work on your whole
class of singletons.

In general, it is easier to write:

   package Single_Car is
      procedure Start;
      procedur Stop;
   end Single_Car;

   package Single_Plane is
      procedure Start;
      procedur Stop;
   end Single_Plane;

than it is to write:

   package Singleton is
      type Instance (<>) is tagged limited private;
      ...
   private
      ...
   end Singleton;

   package Single_Car is
      type Instance is new Singleton.Instance with private;
      procedure Start (Item : in out Instance);
      procedure Stop (Item : in out Instance);
   private
      ...
   end Single_Car;

   package Single_Plane is
      type Instance is new Singleton.Instance with private;
      procedure Start (Item : in out Instance);
      procedure Stop (Item : in out Instance);
   private
      ...
   end Single_Plane;

Greetings,

Jacob
-- 
"Very small. Go to sleep" -- monster (not drooling)