Re: Ada Singleton Pattern

[Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>]

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