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Date: Fri, 21 Jun 2013 12:14:12 +0200
From: "G.B." <rm-dash-bau-haus@dash.futureapps.de>
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Subject: Re: Ada202X: Easy to use "UML private"-like components
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Date: 2013-06-21T12:14:12+02:00
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On 21.06.13 11:33, Martin wrote:
> On Friday, June 21, 2013 10:23:51 AM UTC+1, Dmitry A. Kazakov wrote:
>> On Fri, 21 Jun 2013 01:43:05 -0700 (PDT), Martin wrote: > Secondly, I have to explain that using Ada "private" gives you "UML protected": Ada's private gives both protected for children of the package and private for non-children. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de
>
> But no easy means of keeping member data private from children.

But no pointers needed; I hope the following example
is a reasonably accurate paraphrase of a hint given
by Tucker Taft some years ago, memory is vague. Not
sure if UML has a concept for capturing this, but confident
that Grady Booch knows Ada's visibility rules well.
(Or should one find a sentence that has "possibilities"
instead of "rules"?)

package Opaque is

    type Exposed is tagged private;

private

    package Invisible is
       type T is tagged private;
    private
       type Scalr is range 1 .. 2;
       type T is tagged record
          Comp_1 : Scalr;
       end record;
    end Invisible;

    type Exposed is new Invisible.T with null record;
end Opaque;

private package Opaque.Child
is
    A : Invisible.T;
    Y : Invisible.Scalr := A.Comp_1;
end Opaque.Child;


Compiling: opaque-child.ads (source file time stamp: 2013-06-21 10:06:44)

      4.    Y : Invisible.Scalr := A.Comp_1;
                         1          2
         >>> "Scalr" is not a visible entity of "Invisible"
         >>> no selector "Comp_1" for private type "T" defined at 
opaque.ads:8

  5 lines: 2 errors

Thus, even a private child cannot see anything behind
doubled fences. I imagine that explaining this needs
some spin doctoring.