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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Can't export object of private type
Date: 1999/03/01
Message-ID: <m31zjaat3o.fsf@mheaney.ni.net>#1/1
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References: <7b6nqe$75m$1@remarQ.com> <F7w4s3.F1p@syd.csa.com.au>
NNTP-Posting-Date: Sun, 28 Feb 1999 17:33:31 PDT
Newsgroups: comp.lang.ada
Date: 1999-03-01T00:00:00+00:00
List-Id: <comp.lang.ada>

nospam@thanks.com.au (Don Harrison) writes:

> That view seems to be widely held in Ada circles. However, I can't help 
> noticing that purity very neatly deals with the singleton issue (.. and a 
> number of other Ada problems).  :)

I think the "problem" is that the proper idioms for implementing
singleton abstractions in Ada aren't well-known.  The articles I've been
submitting to the ACM patterns archive are an attempt to overcome this.

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

As I explained in my previous posts in this thread, a polymorphic
singleton can be implemented in Ada by using a tagged, limited, and
indefinite type, and declaring an instance of that type in the package
body.

The singleton package exports a function that returns a pointer to that
object (if you need an object) or just returns the object (if you need a
value).

declare
  Singleton : Singleton_Type'Class renames Ref.all;
begin
  <call dispatching operations on Singleton>
end;

This is how you implement Eiffel's once-functions in Ada95.

Note that the languages have made different decisions here: Eiffel moved
once-functions directly into the language, while Ada95 gives you the
primitives for assembling them yourself.