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From: "Nick Roberts" <Nick.Roberts@dial.pipex.com>
Subject: Re: Can't export object of private type
Date: 1999/02/27
Message-ID: <7b8cvc$ii5$3@plug.news.pipex.net>#1/1
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References: <7b4517$2bbr@news3.newsguy.com> <F7r3xr.3Fr@syd.csa.com.au>
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Newsgroups: comp.lang.ada
Date: 1999-02-27T00:00:00+00:00
List-Id: <comp.lang.ada>

I think possibly you guys are trying to use a sledgehammer to crack a nut
here.

One possible solution is: (a) provide a base (or common) package, which
defines a selector type; (b) implement each singleton as a (child) package;
(c) provide a 'gateway' package which provides dynamic selection of the
singletons.

E.g. (choosing the child package option):

   package Planetary is
      type Distance is digits 6 range 0.0 .. 1.0e14; -- metres
      type Time is ...;
      ...
      type Planet_Id is private;
      Planet_Error: exception;
   private
      type Planet_Id is new Positive range 1..99;
      ...
   end Planetary;

   package Planetary.Mercury is
      function Equatorial_Diameter return Distance;
      function Solar_Distance (T: in Time) return Distance;
      ...
      Id: constant Planet_Id;
   private
      ...
      Id: constant Planet_Id := 1;
   end Planetary.Mercury;

   ... etc. for the other planets, numbered 2, 3, etc.

   package Planetary.General_Planet is
      function Equatorial_Diameter (Planet: in Planet_Id) return Distance;
      function Solar_Distance (Planet: in Planet_Id; T: in Time) return
Distance;
      ...
   end Planetary.General_Planet;

The bodies of the functions in General_Planet contain a 'case' statement
which selects the appropriate function of the appropriate package to call:

      function Equatorial_Diameter (Planet: in Planet_Id) return Distance is
      begin
         case Planet is
            when Mercury.Id => return Mercury.Equatorial_Distance;
            ...
            when others => raise Planet_Error;
         end case;
      end Equatorial_Diameter;

In reality, the implementation may have to be more sophisticated than this.

To add a new planet, you only have to edit and (re)compile: (a) the new
planet package (spec & body); (b) the General_Planet package body.  Simple!

There will be many variations possible on this theme for each different
specific application, many of them neater than the above.

Another possible scheme that might suit you is: (a) declare a limited tagged
type which is derived from Finalization.Limited_Controlled; (b) implement
Initialize so that it increments a counter (which is itself initialized to
0), and if it goes above 1 (or some other number), raise an exception; (c)
implement Finalize so it decrements the counter.  This way you can
polymorphise the type as normal, but be sure no more than one object of the
type exists at any one time.

One needs to make a distinction between a design concept and the
implementational mechanism of that design concept, and not make the mistake
of thinking that a particular design is inextricably wedded to a particular
mechanism.  It all depends on what you're trying to do (or, rather, why
you're trying to do it).

In all cases: power to your arm (and brain).

-------------------------------------
Nick Roberts
-------------------------------------