From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!feeder.eternal-september.org!aioe.org!.POSTED!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Logical constants
Date: Fri, 29 Sep 2017 22:50:09 +0200
Organization: Aioe.org NNTP Server
Message-ID: <oqmbm4$chf$1@gioia.aioe.org>
References: <oqm2f1$1sdc$1@gioia.aioe.org>
NNTP-Posting-Host: MajGvm9MbNtGBKE7r8NgYA.user.gioia.aioe.org
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
X-Complaints-To: abuse@aioe.org
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101
 Thunderbird/52.3.0
Content-Language: en-US
X-Notice: Filtered by postfilter v. 0.8.2
Xref: news.eternal-september.org comp.lang.ada:48259
Date: 2017-09-29T22:50:09+02:00
List-Id: <comp.lang.ada>

On 2017-09-29 20:12, Victor Porton wrote:
> I pass a private type T (whose full type is a record) containing a Handle to
> a subprogram.
> 
> (In fact, a handle is a pointer to a C struct.)
> 
> All such subprograms can be written as subprograms with "in" mode for T,
> because they do not modify the handle.
> 
> But some of these programs logically modify the corresponding C object
> (without modifying the handle itself, they may modify the struct the handle
> points to).
> 
> Should I pass T arguments to such subprograms in "in out" mode? To use "in"
> only is allowed by the language rules, but logically them are "in out".
> 
> What are the arguments for using either "in" or "in out" in this case?

It is undecided.

1. A "transitive" approach is that the mode of a proxy object must 
reflect the mode of the target.

2. A "non-transitive" one is that modes of two objects are separate.

E.g. Ada's access types take the second approach. A pointer can be 
constant and target mutable and conversely.

Arguably the first one should be chosen when the proxy hides/represents 
the target and the second when both types are independently visible.

But Ada.Text_IO still takes the second approach. Ada.Text_IO.Create has 
in out File_Type. Ada.Text_IO.Put has in File_Type, though the target is 
clearly invisible.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de