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Newsgroups: comp.lang.ada
Subject: Re: access private element of base-class
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From: Ludovic Brenta <ludovic.brenta@insalien.org>
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Date: Sat, 02 Oct 2004 13:32:07 +0200
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Date: 2004-10-02T13:35:27+02:00
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Brian May writes:
>>>>>> "Rick" == Rick Santa-Cruz writes:
>
>     >>> In C++ talk: Protection in Ada work on the "namespace" and not
>     >>> on the "class". This needs some getting use to but has the
>     >>> advantage that it works with classic (non OO) programming as
>     >>> well.
>     >>  It also eliminates the need for "friends", an ugly kludge.
>
>     Rick> Why this?
>
> In Ada you can put multiple "classes" in one package. Hence there is
> no need to use "friends".
>
> This isn't as flexible at C++ friends, where you can make any class a
> friend, but I have also heard complaints that C++ friends are too
> flexible, making code complicated.

You can also place things in child packages; these things can see the
private part of the parent package's spec.  By declaring a subprogram
in the private part of the spec, you get the effect of the C++
"protected", not only for child classes but for everything declared in
child packages.  "Things" may be types and subprograms. For example:

package P is
   type T is tagged private;
   procedure Foo (This : in T);
private
   type T is tagged record
      Member : Integer;
   end record;
   function Protected (This : in T) return Integer;
end P;

package P.Q is
   procedure Friend_Proc (That : in out T);
end P.Q;

package body P.Q is
   procedure Friend_Proc (That : in out T) is
   begin
      P.Foo (That);
      That.Member := P.Protected (That);
   end Friend_Proc;
end P.Q;

-- 
Ludovic Brenta.