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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Implicit dereferences rational question
Date: Tue, 20 May 2014 16:26:08 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:19943
Date: 2014-05-20T16:26:08-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:1pkkrdwu7lh6y.7cjyncdfw33e.dlg@40tude.net...
> On Mon, 19 May 2014 16:51:56 -0500, Randy Brukardt wrote:
>
>> "Victor Porton" <porton@narod.ru> wrote in message
>> news:llb1aa$gth$1@speranza.aioe.org...
>>> Robert A Duff wrote:
>>>> Victor Porton <porton@narod.ru> writes:
>>>>
>>>>> Why implicit dereferences use only access discriminants, not arbitrary
>>>>> access fields?
>>>>
>>>> It helps prevent dangling pointers.
>>>
>>> In which way limiting to discriminants may prevent dangling pointers?
>>
>> Discriminants have a lengthy set of accessibility rules, which turn out 
>> to
>> be exactly what's needed for limiting dangling pointers. In particular, 
>> it's
>> almost never the case that one can make a copy of an access discriminant,
>> while that's easy for a "arbitrary access field".
>
> Except that dereference, be it implicit or explicit, does not create
> pointers. Obviously, it eliminates a pointer, as the name suggests.

I suppose, but you have to have a pointer or something very similar to a 
pointer if you want to be able to write to it. And that's the key feature 
here: that one can use these as variables as well as constants. It didn't 
make much sense to invent a new, almost pointer construct just for this 
purpose when we already had one with the correct semantics.

                                Randy.