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

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

Patrick Gunia wrote on comp.lang.ada:
> Dmitry A. Kazakov wrote:
> > Sorry, but a singleton with more than one instance is not a singleton.
>
> This is arguable. Gamma et al. tell explictily in their work, that it
> ´s one advantage of the Singleton-pattern that the number of creatable
> instances can be fixed later on without major code changes. Thus I
> think of the pattern more like a possibility to gain control over the
> number of instantiations of a concrete class, not necessarily
> restricted to one, though limited.

Then do not call this a Singleton; call it a
Bounded_Pool_Of_Instances. And in Ada, you still do not need tagged
types or public access types to implement this pattern:

package Pool_Of_Instances is
   type Handle (<>) is private;
   Null_Handle : constant Handle;
   function Next_Available return Handle;
   procedure Operate (On : in Handle);
   procedure Return (H : in out Handle);
   Pool_Exhausted : exception;
private
   type T;
   type Handle is access all T; -- note: the access type is private
   Null_Handle : constant Handle := null;
end Pool_Of_Instances;

package body Pool_Of_Instances is
   type T is record ... end record;
   N : constant := 10;
  Instances : array (1 .. N) of aliased T;
  Available : array (1 .. N) of Boolean := (others => True);

  function Next_Available return Handle is
  begin
      for J in Available'Range loop
         if Available (J) then
            Available (J) := False;
            return Instances (J)'Access;
         end if;
      end loop;
      raise Pool_Exhausted;
  end Next_Available;

   procedure Return (H : in out Handle) is
   begin
      H := Null_Handle;
   end Return;

end Pool_Of_Instances;

The above pattern can be extended to do reference counting and
deallocate unused instances, thereby making some slots available
again. For that you'd need to make Handle controlled:

package Pool_Of_Instances is
   -- as above
private
   type T;
   type Access_T is access all T;
   type Handle is new Ada.Finalization.Controlled with record
      Instance : Access_T;
   end record;
   overriding procedure Initialize (H : in out Handle);
   overriding procedure Adjust (H : in out Handle);
   overriding procedure Finalize (H : in out Handle);
end Pool_Of_Instances;

The Instance would contain the reference count; Initialize, Adjust and
Finalize would manage it. See http://www.adaic.com/learn/tech/safe_ptr.html
for inspiration.

--
Ludovic Brenta.