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From: "patrick.gunia@googlemail.com" <patrick.gunia@googlemail.com>
Newsgroups: comp.lang.ada
Subject: Ada-Singleton-Why does it work like this?
Date: Tue, 24 Mar 2009 12:01:00 -0700 (PDT)
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Date: 2009-03-24T12:01:00-07:00
List-Id: <comp.lang.ada>

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...here
is my code:

package Singleton is

	type Singleton_Type (<>) is tagged limited private;
	type Singleton_Access is access all Singleton_Type;

	function return_Single return Singleton_Access;

	procedure setValues(input : in out Singleton_Access; valueIN :
Integer; valueIN2 : Integer);
	procedure print_something(input : Singleton_Access);


	private
	type Singleton_Type is tagged limited record
		value : Integer;
		value2 : Integer;
	end record;

end Singleton;

Here is the implementation part:

package body Singleton is
	It: aliased Singleton_Type;
	procedure setValues(input : in out Singleton_Access; valueIN :
Integer; valueIN2 : Integer) is
		begin
			input.value :=3D valueIN;
			input.value2 :=3D valueIN2;

		end setValues;

	function return_Single return Singleton_Access is
		begin
			return It'Access;
		end return_Single;

	procedure print_something(input : Singleton_Access) is
		begin
			put(input.value);
			put(input.value2);
		end print_something;

end Singleton;

As far as I get it, is the declaration "type Singleton_Type (<>) is
tagged limited private;" the key-feature to realize the Singleton
pattern, but how, and why does this work? I looked through my Ada-
book, searched the web but didn=B4t find an explanation for this
construct! Could you please tell me, what exactly happens when I use
(<>) within a type-declaration and why it is necessary for the
Singleton-Implementation?

Thanks a lot!
Patrick