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From: sjw <simon.j.wright@mac.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada-Singleton-Why does it work like this?
Date: Wed, 25 Mar 2009 15:29:25 -0700 (PDT)
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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;

Usually the application problem is to do something (eg print stuff)
and there's a side note that there's only one printer.

We could decide to apply the singleton-in-package pattern above to
ensure the limit is met.

In general, though, having only one printer isn't an integral part of
the application problem, and it'll be a real pain when it turns out
you need to add one. Better to be general about it.

My code generator allowed for singletons (no visible creation/deletion
operations, "instance" operations don't need a This parameter, ...)
because I thought it would be useful, but in fact it's been a feature
I deeply regret; it introduces inconsistency and irregularity. Now I
prefer to specify that there is a bounded number of instances and make
the bounds 0..1.