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Newsgroups: comp.lang.ada
Subject: Re: Ada Singleton Pattern
References: <%Fi1d.245967$OR2.11136154@news3.tin.it>
From: Matthew Heaney <matthewjheaney@earthlink.net>
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Date: Wed, 15 Sep 2004 05:43:58 GMT
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Date: 2004-09-15T05:43:58+00:00
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Luca Stasio <stasio2000@tin.it> writes:

> Hi, there is a way to implement the Singleton Pattern in Ada?  There
> are some examples out there?  Thanx.

I prefer to do it this way:

package P is
   pragma Elaborate_Body;

   type T (<>) is limited private;
   type T_Access is access all T;

   function Object return T_Access;

   procedure Op (O : access T);
   ...
private
   type T is limited record ... end record;
end;

package body P is
   State : aliased T;

   function Object return T_Access is
   begin
      return State'Access;
   end;
   ...
end P;

Technically, since there's only one object, then you don't really need
to declare the state as components of type T.  T can just be a dummy
type (type T is limited null record;), and operations (like Op) can just
ignore the object passed as a parameter, and manipulate package state
directly.

But all of this is just syntactic overhead.  You can always get rid of
the type declaration, and just let the operations manipulate the package
state.  (Booch calls this an "abstract state machine.")

Of course, if this is a type hierarchy, then you probably do need a type
(since packages aren't "first class citizens," to use Wegner's phrase).