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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: how to use "in out" for the "self" parameter in a function in a
 protected object?
Date: Mon, 2 Oct 2017 19:08:34 -0500
Organization: JSA Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:48299
Date: 2017-10-02T19:08:34-05:00
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"Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message 
news:oplc73$jqn$1@dont-email.me...
> On 09/17/2017 12:40 AM, Frank Buss wrote:
>>
>> What's the rationale for this behavior and where can I find the 
>> definition, that it is not allowed? Doesn't look useful to me, because I 
>> can do this with a procedure. In fact now I have written a procedure with 
>> an "out" parameter and it works. But it is annoying to declare an extra 
>> temporary variable for it when I call it. Would be nice, if I could do 
>> this with a function.
>
> Modifying the state of a protected object requires exclusive access to it 
> (a lock of some sort). Simply reading the state does not require a lock, 
> and multiple reads may proceed in parallel. The definition of POs in 1995 
> used protected functions for non-exclusive, read-only access, and 
> protected procedures and entries for exclusive write access. One can 
> debate whether this was the best decision, but that's the way POs are 
> defined.

Note that the Ada 2012 Corrigendum added aspect "Exclusive_Functions", which 
changes the behavior of protected functions to support exclusive read-only 
access. (This allows a PO to be used as a concurrency wrapper around some 
non-concurrent data structure without having to change all of the functions 
of the wrapped data structure into procedures.)

Some of us complained about tying read-only access to the use of protected 
functions long ago, but we got nowhere then and probably wouldn't get 
anywhere now, either. 98% of the time you can use a protected procedure 
instead, but there are cases where a procedure isn't possible and you are 
just stuck.

                                      Randy.