Re: Ada-Singleton-Why does it work like this?

[Martin <martin.dowie@btopenworld.com>]

On Mar 24, 8:47 pm, "Jeffrey R. Carter"
<spam.jrcarter....@spam.acm.org> wrote:
> patrick.gu...@googlemail.com wrote:
> > Hi all,
> > I´m 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´t 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;
>
> This is a very poor solution to the problem. A much simpler solution is
>
> package Singleton is
>     procedure Set (Value_1 : in Integer; Value_2 : in Integer);
>
>     procedure Print;
> end Singleton;

But you can not derive a class from this:

with Singletons; use Singletons;
package Print_Managers is
   type Print_Manager is new Singleton with private;
   -- There should only be 1 print manager
private
   -- implementation
end Print_Managers;

Now you could argue that there should just be a "package
Print_Manager" and no need for a Print_Manager type...

...and the idea that you can derive from a 'Singleton' type rather
defeats it as a 'pattern' but that's no bad thing IMHO! :-)

Cheers
-- Martin