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From: Jeffrey Carter <spam@spam.com>
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Newsgroups: comp.lang.ada
Subject: Re: Possible Ada deficiency?
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Date: Fri, 31 Dec 2004 19:12:14 GMT
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Date: 2004-12-31T19:12:14+00:00
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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.

-- 
Jeff Carter
"Ada has made you lazy and careless. You can write programs in C that
are just as safe by the simple application of super-human diligence."
E. Robert Tisdale
72