From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,b88383a5d9c51aa0
X-Google-Attributes: gid103376,domainid0,public,usenet
X-Google-Language: ENGLISH,ASCII
Path: 
 g2news2.google.com!news4.google.com!feeder.news-service.com!news2.euro.net!xlned.com!feeder1.xlned.com!npeer.de.kpn-eurorings.net!npeer-ng1.de.kpn-eurorings.net!newsfeed.arcor.de!newsspool3.arcor-online.net!news.arcor.de.POSTED!not-for-mail
Date: Wed, 25 Mar 2009 12:26:40 +0100
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
User-Agent: Thunderbird 2.0.0.21 (Macintosh/20090302)
MIME-Version: 1.0
Newsgroups: comp.lang.ada
Subject: Re: Ada-Singleton-Why does it work like this?
References: <5a7a870c-40e2-4803-8753-0f9cfd2b800f@k2g2000yql.googlegroups.com>
 	<9a5fb100-c38d-45f6-a482-1c67b26c5866@z15g2000yqm.googlegroups.com>
 <19a1c6cf-4e4d-402c-902d-3ea2055b4779@c11g2000yqj.googlegroups.com>
In-Reply-To: 
 <19a1c6cf-4e4d-402c-902d-3ea2055b4779@c11g2000yqj.googlegroups.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
Message-ID: <49ca14f1$0$30231$9b4e6d93@newsspool1.arcor-online.net>
Organization: Arcor
NNTP-Posting-Date: 25 Mar 2009 12:26:41 CET
NNTP-Posting-Host: ebf778aa.newsspool1.arcor-online.net
X-Trace: 
 DXC=[RE@UbE5m1F0YVY]kmLTlDic==]BZ:afN4Fo<]lROoRA^YC2XCjHcbICW9^O>US0XI;9OJDO8_SKFNSZ1n^B98iJicFK?R8kU1F
X-Complaints-To: usenet-abuse@arcor.de
Xref: g2news2.google.com comp.lang.ada:5260
Date: 2009-03-25T12:26:41+01:00
List-Id: <comp.lang.ada>

patrick.gunia@googlemail.com schrieb:

> Still I don�t get how the unconstrained type limits the
> number of instances to 1.

It does, but outside package Singleton only.

The (<>) forces initialization.
The "limited" part prevents assignment (thus copying).
The type is private.

Any object to be created in whichever way must be
initialized because of (<>). Initialization is not
possible outside the package because Singleton_Type
has discriminants unknown outside the package.

Also, any object of the public Singleton_Type cannot be
assigned a new value (because it is limited).

The only place where objects of type Singleton_Type
can be created is where the full definition
of type Singleton_Type is visible.

The only way to get a reference to the object is
through function return_Single. This function
always returns the same access value, hence clients
of package Singleton always use the same (invisible)
object.