From: Luca Stasio <stasio2000@tin.it>
Subject: Re: Ada Singleton Pattern
Date: Tue, 14 Sep 2004 13:56:39 GMT
Date: 2004-09-14T13:56:39+00:00 [thread overview]
Message-ID: <rMC1d.251418$OR2.11336646@news3.tin.it> (raw)
In-Reply-To: <1826velwc7x9x$.6mjo8v6zq4c4$.dlg@40tude.net>
Dmitry A. Kazakov wrote:
> On Mon, 13 Sep 2004 17:01:01 GMT, Luca Stasio 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?
>
>
> "(<>) is limited" is a kind of such class built in the language. It can be
> specified as a generic formal parameter for example. So formally you do not
> need to create it, it is already here. However, if you want singleton types
> to share something common, apart from being just singletons, then you can
> create an abstract type:
>
> with Ada.Finalization;
> package Singletons is
> type Singleton (<>) is abstract tagged limited private;
> -- This is an extensible type. Interface follows, it should
> -- have only in-methods, because Singleton is viewed as
> -- a constant. Looks nasty, but technically it is no problem,
> -- because the implementation may use a function returning
> -- the object so that the methods could access the object
> -- directly.
> function Get_Name (X : Singleton) return String is abstract;
> ...
> private
> type Singleton is abstract
> new Ada.Finalization.Limited_Controlled with null record;
> end Singletons;
>
> Note that the base Limited_Controlled is hidden. It means that only the
> packages having a private view on Singleton can in effect extend it. Though
> others will be able to declare an extension type, they will fail to create
> any concrete object of that type.
>
>
>> I mean: (1) a Singleton class (2) a DatbaseAccessSingleton or a
>>NetworkAccessSingleton or... heach one with its own methods but sharing
>>the Singleton behaviour.
>
>
> package Singletons.DB is
> type DB_Singleton is new Singleton with private;
> -- Implementation of the interface, overriding all abstracts:
> function Get_Name (X : DB_Singleton) return String;
> ...
> -- The singleton object itself. Alas, it cannot be declared as a
> -- variable, because the object is unconstrained in its public
> -- view, but you can declare it as either a constant or a function.
> -- For clients it will make no difference:
> DB : constant DB_Singleton;
> -- Or else
> function DB return DB_Singleton;
> private
> DB : constant DB_Singleton; -- If declared as a constant
>
> type DB_Singleton is new Singleton with
> record
> ...
> end record;
> procedure Finalize (X : in out DB_Singleton);
> procedure Initialize (X : in out DB_Singleton);
> end Singletons.DB;
>
> When DB is a function then in the body:
>
> package Singletons.DB is
> procedure Finalize (X : in out DB_Singleton) is
> begin
> ...
> end Finalize;
> procedure Initialize (X : in out DB_Singleton) is
> begin
> ...
> end Initialize;
>
> My_DB : DB_Singleton;
>
> function DB return DB_Singleton is
> begin
> return My_DB;
> -- Non-local limited objects can be returned
> -- by reference
> end DB;
> ...
>
> As for singletons in the sense of objects exclusively used by a scheduled
> item, see what other posters already wrote. You can use a mutex protected
> object or a task implementing a monitor and mix in to the singleton object
> trough an access discriminant.
>
Really thank you all
next prev parent reply other threads:[~2004-09-14 13:56 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-09-13 15:04 Ada Singleton Pattern Luca Stasio
2004-09-13 15:33 ` Dmitry A. Kazakov
2004-09-13 16:18 ` Luca Stasio
2004-09-13 17:01 ` Luca Stasio
2004-09-13 18:43 ` Martin Dowie
2004-09-13 19:37 ` Martin Dowie
2004-09-14 2:29 ` Steve
2004-09-14 8:52 ` Martin Dowie
2004-09-14 12:46 ` Jim Rogers
2004-09-14 13:57 ` Luca Stasio
2004-09-13 20:38 ` Georg Bauhaus
2004-09-14 8:17 ` Dmitry A. Kazakov
2004-09-14 13:56 ` Luca Stasio [this message]
2004-09-14 14:21 ` Florian Weimer
2004-09-14 14:48 ` Dmitry A. Kazakov
2004-09-14 15:04 ` Florian Weimer
2004-09-15 7:33 ` Dmitry A. Kazakov
2004-09-16 6:48 ` Florian Weimer
2004-09-16 7:45 ` Dmitry A. Kazakov
2004-09-14 15:38 ` Luca Stasio
2004-09-14 16:32 ` Florian Weimer
2004-09-14 17:43 ` Luca Stasio
2004-09-15 7:27 ` Martin Dowie
2004-09-15 19:38 ` Luca Stasio
2004-09-15 5:43 ` Matthew Heaney
2004-09-15 19:38 ` Luca Stasio
2004-09-18 21:47 ` Pylinius
2004-09-19 4:19 ` Matthew Heaney
2004-09-20 3:03 ` Pylinius
2004-09-23 7:35 ` Luca Stasio
2004-09-27 5:22 ` Pylinius
2004-09-27 8:05 ` Luca Stasio
2004-10-05 17:55 ` Luca Stasio
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