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

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

On 25 Mar, 09:30, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> On Tue, 24 Mar 2009 17:10:11 -0700 (PDT), Martin wrote:
> > On Mar 24, 8:47 pm, "Jeffrey R. Carter"
> >> 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:
>
> Firstly you can. There are child packages in Ada, they are equivalent to
> derivation.
>
> Secondly, you may not because it is a singleton. A derived variant would be
> meaningless without making an instance of, which would be *another*
> instance of now non-singleton.

Unless you use the "tagged type" version...and that would be ok, as a
type is not an object. So you would have something like:

package Singletons is
   type Singleton ...;
   function Get_Instance return Singleton;
end Singletons;

package Singletons.Thread_Safe is
   type Thread_Safe_Singleton is new Singleton ...;
   function Get_Instance return Thread_Safe_Singleton;
end Singletons.Thread_Safe;

But only one object of either of these types could be instantiated.

Cheers
-- Martin