220 6845 article Path: news.gmane.org!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: auto with copy in expression Date: Tue, 1 Oct 2013 18:23:41 +0300 Lines: 105 Approved: news@gmane.org Message-ID: References: <65473e60-c6ee-48e4-ae03-fcbe251d7b9d@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=001a11c22634a5a7dc04e7af8951 X-Trace: ger.gmane.org 1380641022 21597 80.91.229.3 (1 Oct 2013 15:23:42 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 1 Oct 2013 15:23:42 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC5JHI7A7ALRB7WRVOJAKGQEAW5764A@isocpp.org Tue Oct 01 17:23:45 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-bk0-f69.google.com ([209.85.214.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VR1nd-0001oq-2f for gclcip-std-proposals@m.gmane.org; Tue, 01 Oct 2013 17:23:45 +0200 Original-Received: by mail-bk0-f69.google.com with SMTP id na10sf8761108bkb.0 for ; Tue, 01 Oct 2013 08:23:44 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=DPbaU1EgCSnmx9/1IebPpzC4stoHI/KZaReHKq20tfg=; b=Roy7y4eheLR2EmC9GdBYDIzJD7y61ND9bVNrDR+AE4pkr/XdYvWURJdbiO3P6Mkuu4 z1waD1llDn9PIPa7Ja34bVny9XdlMTpvFSsKLWt6De1WDcC2KIu36pMJ3r8X2y/4EzYY kX5I6TF+EqlXci8+6NoK67mJbin4EaqEE5JtP9zX9ec7SgiSNM295ykK8lvf1M1PgkLj bx73/25EYFcY9o40MuDULIiOFo+fOe+H7xYodI1QwhNuVP+8LvpppI8T3Ojxrq5A4cxg aaif2doRS+MWSHB6WiisIDR7EcJ75Q3/g8iKfXpdAwRKtwB5dVB0UcsdAq1D1Uufsoif rdaA== X-Received: by 10.180.21.137 with SMTP id v9mr10439315wie.7.1380641024103; Tue, 01 Oct 2013 08:23:44 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.84.71 with SMTP id w7ls878553wiy.28.canary; Tue, 01 Oct 2013 08:23:41 -0700 (PDT) X-Received: by 10.194.94.101 with SMTP id db5mr1620183wjb.67.1380641021557; Tue, 01 Oct 2013 08:23:41 -0700 (PDT) Original-Received: from mail-wg0-x22f.google.com (mail-wg0-x22f.google.com [2a00:1450:400c:c00::22f]) by mx.google.com with ESMTPS id bf10si2568889wjc.69.1969.12.31.16.00.00 (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 01 Oct 2013 08:23:41 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2a00:1450:400c:c00::22f as permitted sender) client-ip=2a00:1450:400c:c00::22f; Original-Received: by mail-wg0-f47.google.com with SMTP id f12so7332949wgh.2 for ; Tue, 01 Oct 2013 08:23:41 -0700 (PDT) X-Received: by 10.180.9.203 with SMTP id c11mr19072354wib.64.1380641021466; Tue, 01 Oct 2013 08:23:41 -0700 (PDT) Original-Received: by 10.194.42.70 with HTTP; Tue, 1 Oct 2013 08:23:41 -0700 (PDT) In-Reply-To: <65473e60-c6ee-48e4-ae03-fcbe251d7b9d@isocpp.org> X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2a00:1450:400c:c00::22f as permitted sender) smtp.mail=ville.voutilainen@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:6845 Archived-At: --001a11c22634a5a7dc04e7af8951 Content-Type: text/plain; charset=ISO-8859-1 On 24 September 2013 15:35, Michael AV wrote: > Hi, > > I would like to create a copy of the return value of a function. This > result shall be passed to another function. The two-line solution would be: > > auto result = foo(); > bar(result); > > bar expects a reference and foo returns a reference. However, in this case > it is crucial for bar to only receive a copy because (for this specific > result argument), bar changes what foo delivers. That does not mean, > however, that bar does always change it's argument. > > So, basically I just need a copy and would like to use it within an > expression. The most intuitive syntax for me would be: > > bar(auto(foo())); > > which does not work at the moment (as tested with the - clearly obsolete - > VS2010). > > I tried searching this group and thinking about why this syntax does not > make sense but did not come to any result. > > What do you think? > > > bar([](){auto x(foo()); return x;}()); You can't create temporaries with auto, as far as I understand the language rules. Also, in whichever way this temporary is created, bar() needs to accept an rvalue reference, since lvalue references can't bind to temporaries. -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. --001a11c22634a5a7dc04e7af8951 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable



On 24 September 2013 15:35, Michael AV <donnerdose@googlem= ail.com> wrote:
Hi,

I would like to create a copy of the return value of a function. Thi= s result shall be passed to another function. The two-line solution would b= e:

auto result =3D foo();
bar(result);

bar expects a reference and = foo returns a reference. However, in this case it is crucial for bar to onl= y receive a copy because (for this specific result argument), bar changes w= hat foo delivers. That does not mean, however, that bar does always change = it's argument.

So, basically I just need a copy and would like to use = it within an expression. The most intuitive syntax for me would be:

bar(auto(foo()));

which does not work at the moment (as tested with the - clearly obso= lete - VS2010).

I tried searching this group and t= hinking about why this syntax does not make sense but did not come to any r= esult.

What do you think?



bar([](){auto x(foo()); return x;}());

You can't create temporarie= s with auto, as far as I understand the language rules.
Also, in whichever way this temporary is c= reated, bar() needs to accept an rvalue
reference, since lvalue referenc= es can't bind to temporaries.

--
 
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