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Newsgroups: comp.lang.ada
Subject: Re: Self pointer in limited record
From: Georg Bauhaus <rm.tsoh+bauhaus@maps.futureapps.de>
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Date: Thu, 06 Sep 2007 12:26:45 +0200
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Date: 2007-09-06T12:26:00+02:00
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On Thu, 2007-09-06 at 09:37 +0200, Dmitry A. Kazakov wrote:
> On Wed, 05 Sep 2007 23:38:34 +0200, Georg Bauhaus wrote:
> 
> > They can avoid surprises and nasty issues because they enforce body
> > visibility of data objects. Example:
> > 
> > <start of declarative part>
> > 
> >         package S is
> >                 type T is limited private;
> >                 procedure op(x: in out T);
> >         private
> >                 type T is limited record
> >                         null;
> >                 end record;
> >         end S;
> 
> This is IMO bad design, it should be:
> 
> package S is   -- Stateless
>    type T is limited private;
>    ...
> end S;

IIUC, this will still allow referencing S.T elsewhere.
And even when S has only subprograms, they are visible to
other parts of the program.
But S was intended to be used by some procedure,
and only by the procedure.

> package Data is -- Stateful
>    ... -- Operations having an instance of T as a hidden parameter
> end Data;
> 
> with S;
> package body Data is
>    Singleton : S.T;
>    ...
> end Data;
> 
> There is no need to "with" S for anybody else,

Do you think that "No need to with S" will convince programmers
to not touch it? When I need to make sure that S is not going
to be used elsewhere, it needs to be hidden. What are the options?
Either build some fairly complicated hierarchy with visibility in
private parts or simply declare S where it is needed: in the
declarative part of the enclosing subprogram.