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From: Nick Roberts <nick.roberts@acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Possible Ada deficiency?
Date: Fri, 31 Dec 2004 23:23:49 +0000
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Date: 2004-12-31T23:23:49+00:00
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danmcleran@hotmail.com wrote:

> I would like to know if anyone else thinks that the inability to hide
> private information from child packages is a deficiency in Ada95. I am
> discussing ways to do this on another thread, (See Private area and child
> packages), but it seems wrong that one has to go through a bunch of
> machinations to do something like this. Wouldn't a language construct for
> this be advantageous?

Perhaps the ability to declare a 'body type' within a private part would do
the trick:

   package Some_Package is
      type Fairly_Secret_Type is private;
   private
      body type Really_Secret_Stuff is
         record
            ...;
         end record;
      type Fairly_Secret_Type is
         record
            ...;
            Secret: Really_Secret_Stuff;
         end record;
   end Some_Package;

This would prevent the type Really_Secret_Stuff from being visible anywhere
but in the body of the package Some_Package (so that components of the
component Secret would be hidden from private children of Some_Package).

Unfortunately, it is now too late for any further changes to Ada 2005. This
might be something to be considered at the next revision.

Personally, I've often found it useful to use an incomplete type declaration
in the private part to completely isolate the actual contents of a visible
type from everything outside the package body.

   package P is
      type T is private;
      ...
   private
      type Innards_Bearer(<>);
      type Innards is access Innards_Bearer;
      type T is
         record
            Data: Innards;
         end record;
   end P;

   package body P is
      ...
      type Innards_Bearer is
         record
            ... -- actual contents here
         end record;
      ...
   end P;

Obviously this technique requires explicit allocation (and maybe
deallocation), and the inefficiency of a (further) level of dereferencing,
but these are usually not a significant problem (and are confined to the
body). The additional advantage is that the actual contents can be changed
within the body, so not clobbering the specification (or therefore, more
importantly, any of its dependants).

This technique will hide the actual contents from the private children of
the package. It comes close to achieving what you suggest, Dan. I haven't
read the whole of the other thread you mention, so perhaps this technique
has already been mentioned there.

-- 
Nick Roberts