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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Can't export object of private type
Date: 1999/02/28
Message-ID: <m3lnhi18jn.fsf@mheaney.ni.net>#1/1
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References: <7b1k4h$13k6@news3.newsguy.com> <F7p7Dq.DE1@syd.csa.com.au>
NNTP-Posting-Date: Sun, 28 Feb 1999 14:10:04 PDT
Newsgroups: comp.lang.ada
Date: 1999-02-28T00:00:00+00:00
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nospam@thanks.com.au (Don Harrison) writes:

> Actually, I don't necessarily want to do that. What I'm really after 
> is polymorphic singletons - objects that
> 
>   1) Have only one instance
>   2) Are polymorphic
>   3) Are visible to clients
>   4) Can be called without a package prefix.
> 
> I can acheive two or three, but not all four. :(

Hmmm.  So you really do want a polymorphic singleton.  I did just this
in some of my early posts to the patterns archive.  See the stuff about
abstract factories and factory methods:

<http://www.acm.org/archives/patterns.html>

1) If you want one instance, make the type limited and indefinite.  I
explain all this in my posts about singletons, also in the archives.

2) If you want polymorphism, then declare the type as tagged, and refer
   to a class-wide object.

3) Export a function that returns a reference to the object (which is
   declared in the body of the package).

4) Don't use a state machine package, use a type.

package Singletons is

  type Root_Singleton_Type (<>) is abstract tagged limited private;

  <primitive ops for Root_Singleton_Type>

  type Singleton_Access is 
     access all Root_Singleton_Type'Class;  
     -- T'Class, to get dynamic dispatching

  function Ref return Singleton_Access;

private

  type Root_Singleton_Type is abstract tagged limited record...;

end Singletons;

package Singletons.Blue is
  
  type Blue_Singleton_Type is new Root_Singleton with private;

  <override primitive ops>

...
end;


with Singlestons.Blue;
package body Singetons is

  Singleton : aliased Blue_Singleton_Type;

  function Ref return Singleton_Access is
  begin
    return Singleton'Access;
  end;

end Singletons;


You could even choose the specific type of the singleton at run-time;
this is precisely what I did in my abstract factory and factory method
examples.