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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: <m3d82u17lb.fsf@mheaney.ni.net>#1/1
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Sender: matt@mheaney.ni.net
References: <7b1k4h$13k6@news3.newsguy.com> <F7p7Dq.DE1@syd.csa.com.au>
 <7b4517$2bbr@news3.newsguy.com>
NNTP-Posting-Date: Sun, 28 Feb 1999 14:30:40 PDT
Newsgroups: comp.lang.ada
Date: 1999-02-28T00:00:00+00:00
List-Id: <comp.lang.ada>

Samuel Mize <smize@imagin.net> writes:

> By "singleton" and (1) you mean that there can exist only one object
> of the type.  But a visible type makes that impossible to ensure, so
> the approach you were taking is fatally flawed.  Oh, the humanity.

This is incorrect.  Implement the type as tagged, limited, and
indefinite, which prevents anyone from declaring instances of the type.

The 4 goals listed are all achievable, indeed very simply.

 
> I would represent each singleton with a package, which gives 1 and 3,
> and use programming by extension to get points 2 and 4.  Specifically,
> we start with:

I like the idea of a state machine package, but he wants dynamic
dispatching.  

The way to implement this abstraction is like this:

package P is

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

  <primitive ops>

  type T_Access is access all T'Class;

  function Ref return T_Access;

private

  <junk>

end P;


package P.Q is

  type NT is new T with private;

  ...
end;


with P.Q;
package body P is

  O : aliased Q.NT;

  function Ref return T_Access is
  begin
     return O'Access;
  end;

...
end P;



The function Ref, which returns a pointer to an object declared in the
body, is analogous to Eiffel's once functions.