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From: Martin <martin.dowie@btopenworld.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada-Singleton-Why does it work like this?
Date: Tue, 24 Mar 2009 17:10:11 -0700 (PDT)
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Date: 2009-03-24T17:10:11-07:00
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On Mar 24, 8:47=A0pm, "Jeffrey R. Carter"
<spam.jrcarter....@spam.acm.org> wrote:
> patrick.gu...@googlemail.com wrote:
> > Hi all,
> > I=B4m currently working on implementing several design patterns in Ada,
> > and I found some code concerning the Singleton-pattern when searching
> > through the posts of this group. I used this code and it all worked as
> > expected. My problem is, that I don=B4t ave any idea, why it does...her=
e
> > is my code:
>
> > package Singleton is
>
> > =A0 =A0type Singleton_Type (<>) is tagged limited private;
> > =A0 =A0type Singleton_Access is access all Singleton_Type;
>
> > =A0 =A0function return_Single return Singleton_Access;
>
> > =A0 =A0procedure setValues(input : in out Singleton_Access; valueIN :
> > Integer; valueIN2 : Integer);
> > =A0 =A0procedure print_something(input : Singleton_Access);
>
> > =A0 =A0private
> > =A0 =A0type Singleton_Type is tagged limited record
> > =A0 =A0 =A0 =A0 =A0 =A0value : Integer;
> > =A0 =A0 =A0 =A0 =A0 =A0value2 : Integer;
> > =A0 =A0end record;
>
> > end Singleton;
>
> This is a very poor solution to the problem. A much simpler solution is
>
> package Singleton is
> =A0 =A0 procedure Set (Value_1 : in Integer; Value_2 : in Integer);
>
> =A0 =A0 procedure Print;
> end Singleton;

But you can not derive a class from this:

with Singletons; use Singletons;
package Print_Managers is
   type Print_Manager is new Singleton with private;
   -- There should only be 1 print manager
private
   -- implementation
end Print_Managers;

Now you could argue that there should just be a "package
Print_Manager" and no need for a Print_Manager type...

...and the idea that you can derive from a 'Singleton' type rather
defeats it as a 'pattern' but that's no bad thing IMHO! :-)

Cheers
-- Martin