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From: dale@cs.rmit.edu.au (Dale Stanbrough)
Subject: Re: Private Children
Date: 1999/06/23
Message-ID: <dale-2306991359560001@dale.cs.rmit.edu.au>#1/1
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 03:59:55 GMT)
Organization: RMIT
NNTP-Posting-Date: 23 Jun 1999 03:59:55 GMT
Newsgroups: comp.lang.ada
Date: 1999-06-23T03:59:55+00:00
List-Id: <comp.lang.ada>

Matthew Heaney wrote:

" Every child (C1, C2) has access to the private part of its parent (P).
  There's no such thing as "hiding within a hierarchy.""

There is. Consider the following...


   pacakge P is
   end P;

   package P.A is
      type X is private;
   ...
   end A;


   with P.A;
   package P.B is
      type Y is private;
      -- operations on Y

   private
      type Y is record
            Item : X;
         end record;
   end P.B;


Clearly package P.B has no access to the private implementations
of package P.A (and they -are- in the same hierachy, it's just that
one is not a lineal (package) descendent of the other).

Consider the following...

The example above compiles fine. External users of type Y are happy.
Package P.B is prevented from knowing the internal implementation of
type X.

But let's also assume that we didn't want to give access to type X to
the rest of the world. What I would -like- to do is to make P.A private,
so that it can't be seen, but still retain type X's private status.

It would be nice if i could simply make P.A. a private package.
Unfortunately Ada confuses "compilation privacy" with "implementation
privacy". I am forced (simply because I want to hide some internal 
details) to sacrifice my privacy designs to the gods of compilation
efficiency (which is the opposite of what "private" means in the
rest of Ada).


" Child packages were designed not to hide visibility from within the
  hierarchy.  They were designed to /deliberately/ expose the private part
  to children, and to allow you to easily identify which packages have
  that private knowledge."


Yes I know that, but I'm not concerned about parent/child packages, but
about sibling (or rather non lineal descendant) packages/privacy issues.

I think that the current situation forces too much exposure of
implementation issues that will cause package hierachies to end up 
a large entagled mess, because we are continually forced to abandon
internal privacy goals to meet external ones.

Dale