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From: Florian Weimer <fw@deneb.enyo.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada Singleton Pattern
Date: Tue, 14 Sep 2004 16:21:16 +0200
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Date: 2004-09-14T16:21:16+02:00
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* Dmitry A. Kazakov:

> On Mon, 13 Sep 2004 15:04:27 GMT, Luca Stasio wrote:
>
>> Hi, there is a way to implement the Singleton Pattern in Ada?
>> There are some examples out there?
>> Thanx.
>
> package Foo is
>    type Singleton (<>) is limited private;
>    ... -- Public interface subroutines
>
>    The_Only_One : constant Singleton; -- The value
> private
>    type Singleton is new Integer; -- Any implementation you want
>    The_Only_One : constant Singleton := 5;
>
> The public view of Singleton is unconstrained and limited, which prevents
> it from either being copied or declaring new objects of this type.

This is only a partial solution, unfortunately.

Ada *requires* that the package initialization code runs before
The_Only_One is accessed.  This is different from many other languages
(where the initialization order of global objects is mostly
unspecified and not related to actual usage patterns) and solves a big
chunk of the singleton problem.

However, you still have to deal with termination on your own: you must
ensure that no client is still using the singleton object before you
destroy it.  There is no direct support in the language for this part
of the problem, and you have to use one of the traditional approaches
if this matters to your application (or assignment 8-).

(I tried to avoid Ada-specific terminology above because Luca probably
isn't familiar with the Ada language.)