220 36093 article Path: news.gmane.org!.POSTED!not-for-mail From: Eyal Rozenberg Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Sub-objects and their copy ellision Date: Tue, 19 Dec 2017 23:09:25 +0100 Lines: 76 Approved: news@gmane.org Message-ID: References: <1c6eff77-d71f-4dd7-8c13-f609460a03f0@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1513721257 14329 195.159.176.226 (19 Dec 2017 22:07:37 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 19 Dec 2017 22:07:37 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:57.0) Gecko/20100101 Thunderbird/57.0 To: std-proposals@isocpp.org, Nicol Bolas Original-X-From: std-proposals+bncBDHKZN7LR4ARBHE443IQKGQEWKVX5AA@isocpp.org Tue Dec 19 23:07:33 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f71.google.com ([74.125.82.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eRQ2s-0003Jx-VS for gclcip-std-proposals@m.gmane.org; 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[192.16.191.27]) by mx.google.com with ESMTPS id 93si453015ede.482.2017.12.19.14.09.32 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 19 Dec 2017 14:09:32 -0800 (PST) Received-SPF: pass (google.com: domain of srs0=kaojp=dp=technion.ac.il=eyalroz@cwi.nl designates 192.16.191.27 as permitted sender) client-ip=192.16.191.27; Original-Received: from fester.cwi.nl (fester.cwi.nl [192.16.191.27]) by fester.cwi.nl (8.14.4/8.12.3) with ESMTP id vBJM9V2a018192; Tue, 19 Dec 2017 23:09:31 +0100 Original-Received: from [192.168.2.1] (ip565d4370.direct-adsl.nl [86.93.67.112] (may be forged)) (authenticated bits=0) by fester.cwi.nl (8.14.4/8.12.3) with ESMTP id vBJM9UUj018184 (version=TLSv1/SSLv3 cipher=AES128-SHA bits=128 verify=NO); Tue, 19 Dec 2017 23:09:31 +0100 Openpgp: preference=signencrypt In-Reply-To: <1c6eff77-d71f-4dd7-8c13-f609460a03f0@isocpp.org> Content-Language: en-US X-Original-Sender: eyalroz@technion.ac.il X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of srs0=kaojp=dp=technion.ac.il=eyalroz@cwi.nl designates 192.16.191.27 as permitted sender) smtp.mailfrom=SRS0=KAOJP=DP=technion.ac.il=eyalroz@cwi.nl; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=technion.ac.il Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:36093 Archived-At: On 12/19/17 9:10 PM, Nicol Bolas wrote: > look at what [class.copy.elision]/1.1 says: >=20 >> when the expression is the name of a non-volatile automatic object > (other than a function parameter or a variable introduced by the > exception-declaration of a handler (18.3)) >=20 > The expression `v.first` is not "the name of a non-volatile automatic > object". `first` is not an automatic object; it is a member subobject of > `v`. And `v.first` is not its name; `first` is its name. `v.first` is an > expression that accesses a subobject from an object. Ah. Indeed, it seems only variables can have "automatic storage duration", even though their sub-objects have the same storage behavior as them. Ok. > Now, consider this: >=20 > | > stringfoo() > { > =C2=A0 pairpr; > =C2=A0 pr.first =3D...; > =C2=A0 returnpr.first; > } > | >=20 > The layout of `pair` is fixed. But when optimizing, the compiler is not committed to using that layout. In fact, in the example you've given it can completely drop pr, and just construct the returned string at the area where a string is supposed to end up. It will be "as if" the pr was used and the first element was copied= .. > But the/caller/ doesn't know that. The caller is the one who provides > memory for the return value object. And as far as the caller can tell, > all it needs to provide is `sizeof(string)` bytes. .... which is good enough with your specific example. Now, it's true that there could be code in foo() which depends on the layout being as it is that a compiler's optimizer will not be able to clear away; but there could also not be such code. And since, like you pointed out at the beginning of your reply, elision is just an option - why not give that option? > Not to mention, this creates a problem for the entire object model. To > make this work, `pr.first` would have to be able to outlive the object > it is stored within. But the object model doesn't allow that; if you > destroy a complete object, you destroy its subobjects too. That's require= d. Under certain assumptions, which std::pair meets, that should not be a problem. I guess a defaulted destructor would be one of those. > No, this is not gonna happen. Not unless someone comes up with a really > novel way of allowing it. Would you not say there's some leeway for what I describe (i.e. break up objects that are not necessary) rather than what you ruled out (negotiations over who destroys an object)? Under the "as if" rule? Eyal --=20 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 e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/c14fe13b-a658-e295-40a6-1465638d18db%40technion.= ac.il. .