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From: danmcleran@hotmail.com
Newsgroups: comp.lang.ada
Subject: Re: Possible Ada deficiency?
Date: 31 Dec 2004 17:52:49 -0800
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Date: 2004-12-31T17:52:49-08:00
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Jeffrey Carter wrote:
> danmcleran@hotmail.com wrote:
>
> > In C++ and Java, no private data can be seen by child classes. I
think
> > that Ada would benefit from extending its information hiding
> > capabilities by allowing a package writer to conceal type
information
> > from child packages.
>
> You're confusing C++ classes with Ada packages. In C++, classes serve
a
> multitude of purposes, including encapsulation, information hiding,
type
> extension, and dynamic dispatching. In Ada, encapsulation, name space

> control, and information hiding are handled by packages. Type
extension
> and dispatching are handled by tagged types.
>
> Private components in C++ prevent the components from being visible
to
> derived types. This is also possible in Ada:
>
> package P is
>     type T is tagged private;
>
>     procedure Op (A : in out T);
> private -- P
>     type T is tagged record
>        I : Integer := 0;
>     end record;
> end P;
>
> with P;
> package Q is
>     type T is new P.T with record
>        F : Float := 0.0;
>     end record;
>
>     procedure Op_2 (A : in out T);
> end Q;
>
> package body Q is
>     procedure Op_2 (A : in out T) is
>        -- null;
>     begin -- Op_2
>        A.I := 2; -- Illegal.
>     end Op_2;
> end Q;
>
> Child packages in Ada provide the equivalent of friends in C++; thus,

> the private part of a package is akin to protected in C++, with the
> package body corresponding to C++'s private.

There's one problem with this analogy. In C++, a class grants
friendship status to other classes and/or functions. In Ada, a child
package assumes friendship with the parent package whether the parent
likes it or not.