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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: A few questions
Date: Tue, 17 Nov 2015 15:33:39 -0600
Organization: JSA Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:28425
Date: 2015-11-17T15:33:39-06:00
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"Simon Wright" <simon@pushface.org> wrote in message 
news:ly1tbpipgd.fsf@pushface.org...
> "Randy Brukardt" <randy@rrsoftware.com> writes:
>
>> Since 'Unrestricted_Access is GNAT junk, it's doesn't have any value
>> for a sample implementation (if you really need 'Unrestricted_Access,
>> then the interface can't be implemented in Ada, and that would be a
>> major problem.)  So far as I know, you might need to use
>> 'Unchecked_Access, but that's necessary anytime one needs to get a
>> pointer to save the evaluated version of something.
>
> I can't of course speak for AdaCore, but I think that the problem is
> to do with obtaining a writable view of an in parameter; for example, in
> the Bounded Hashed Map (GCC 5) there is
>
>   function Iterate
>     (Container : Map) return 
> Map_Iterator_Interfaces.Forward_Iterator'Class
>   is
>      B  : Natural renames Container'Unrestricted_Access.all.Busy;
>   begin
>      return It : constant Iterator :=
>        (Limited_Controlled with
>           Container => Container'Unrestricted_Access)
>      do
>         B := B + 1;
>      end return;
>   end Iterate;
>
> I dare say something could be done with 'Address and Address To Access
> Conversions.

As Brad noted, you have to use the Rosen technique here. Not hacks like 
'Unrestricted_Access. (The Rosen technique is not a "trick", it's an 
intended part of the language in Ada 2012. I admit it originally was 
accidental, but it's too widely used to eliminate.)

                         Randy.