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=-0.3 required=5.0 tests=BAYES_00,
	REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4
X-Google-Thread: 103376,eca91508b02e7e97
X-Google-Attributes: gid103376,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news4.google.com!news.germany.com!newsfeed.freenet.de!news.osn.de!diablo1.news.osn.de!noris.net!newsfeed.arcor.de!newsspool3.arcor-online.net!news.arcor.de.POSTED!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Amount of copying on returned constant objects
Newsgroups: comp.lang.ada
User-Agent: 40tude_Dialog/2.0.15.1
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Reply-To: mailbox@dmitry-kazakov.de
Organization: cbb software GmbH
References: <5dfsfnF1qav8bU1@mid.individual.net>
Date: Sat, 16 Jun 2007 08:48:04 +0200
Message-ID: <15bf7x8niwgcz.horig9zyjn4p.dlg@40tude.net>
NNTP-Posting-Date: 16 Jun 2007 08:47:58 CEST
NNTP-Posting-Host: d9e5db67.newsspool3.arcor-online.net
X-Trace: 
 DXC=G?Z[;D6GjoYaoembcbF;DQMcF=Q^Z^V3X4Fo<]lROoRQFl8W>\BH3YR\TG?<^IdHjSDNcfSJ;bb[UIRnRBaCd<M^3l^_ho`nTST<UF7gY@oShS
X-Complaints-To: usenet-abuse@arcor.de
Xref: g2news1.google.com comp.lang.ada:16206
Date: 2007-06-16T08:47:58+02:00
List-Id: <comp.lang.ada>

On Fri, 15 Jun 2007 19:19:00 +0300, Alex R. Mosteo wrote:

> I know that some "in" arguments may be passed as copies or as references, at
> compiler discretion (this is one of these things that "programmers
> shouldn't care about", many times quoted).
> 
> I wonder however about results of functions, that are not modified. Look for
> example at the Element function of the new Ada.Containers. They return the
> stored item, that may well be a quite large controlled tagged type, for
> example.
> 
> Now, many times I want to query an element just for read-only purposes. I'm
> faced with two options:
> 
> 1) Just call Element on the container Key/Index, and be done with it.
> 
> 2) Do a Find+Query_Element, which requires defining an extra procedure and
> somewhat breaks the flow of control, but ensures no copying.
> 
> I tend to go with 1) because of laziness and the "no premature optimization"
> rule. In C++ I could use constant references. Now, I wonder if 
> 
> a) is there something in the ARM that prevents an equivalent transparent
> optimization in the Ada side (returning the reference when it is detected
> that the returned object is not modified)?

No [*], but a way to help the compiler to know what you wanted is:

   declare
      Item : Element renames Get (Collection, Key);
                 -- Please, don't make new objects, if you can
   begin
      ... -- Use Item

> Thanks in advance, have a nice week-end.

Same to you.

---------------
* In Ada 95 there were limited types returned strictly by reference, but
this semantics was crapped in Ada 2005.

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