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: 103376,54aae3da1cf935cd
X-Google-Attributes: gid103376,public
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news2.google.com!proxad.net!news.cs.univ-paris8.fr!informatik.uni-bremen.de!cs.tu-berlin.de!uni-duisburg.de!not-for-mail
From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada Singleton Pattern
Date: Mon, 13 Sep 2004 20:38:04 +0000 (UTC)
Organization: GMUGHDU
Message-ID: <ci50fd$kk9$1@a1-hrz.uni-duisburg.de>
References: <%Fi1d.245967$OR2.11136154@news3.tin.it>
 <14p6ezf3vze8j$.j05arkr066wi.dlg@40tude.net>
 <hnk1d.246585$OR2.11156430@news3.tin.it>
NNTP-Posting-Host: l1-hrz.uni-duisburg.de
X-Trace: a1-hrz.uni-duisburg.de 1095107884 21129 134.91.1.34 (13 Sep 2004
 20:38:05 GMT)
X-Complaints-To: usenet@news.uni-duisburg.de
NNTP-Posting-Date: Mon, 13 Sep 2004 20:38:04 +0000 (UTC)
User-Agent: tin/1.5.8-20010221 ("Blue Water") (UNIX) (HP-UX/B.11.00
 (9000/800))
Xref: g2news1.google.com comp.lang.ada:3696
Date: 2004-09-13T20:38:04+00:00
List-Id: <comp.lang.ada>

Luca Stasio <stasio2000@tin.it> wrote:
: Dmitry A. Kazakov ha scritto:
:> 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.
:> 
: Thanx for your answer.
: Know, sorry... but, there is a way to create a Singleton Class from wich 
: derive and create concrete singleton classes?
:  I mean: (1) a Singleton class (2) a DatbaseAccessSingleton or a 
: NetworkAccessSingleton or... heach one with its own methods but sharing 
: the Singleton behaviour.
: 

The following outlines a solution; in Ada 200Y it will be possible
to have initialised constants of a limited type, which might help,
the experts might tell you.
In the following, the constant is replaced with a function returning
the very same singleton.


with DB;  --  database stuff

package Single is

   pragma Elaborate_Body;

   type Singleton(<>) is tagged limited private;

    -- (this should be moved to another package: )
   type The_One_And_Only_DB is new Singleton with private;

   function DB17 return The_One_And_Only_DB;
    --  the database singleton

private
   type Singleton is tagged limited null record;

   type The_One_And_Only_DB is new Singleton with record
      Comp: DB.Stuff := DB.init;
      -- note that you can pass The_One_And_Only'Access to DB.init
      -- ...
   end record;

end Single;


package body Single is

   It: The_One_And_Only_DB;
    --  with default initialisation, values provided by DB.init

   function DB17 return The_One_And_Only_DB is
   begin
      return It;
   end DB17;

end Single;


-- Georg