220 2762 <5095aadf-58c1-458b-bc9f-6bbd7499aa59@isocpp.org> article Path: news.gmane.org!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: optional references -- take 2 Date: Wed, 6 Feb 2013 19:55:22 -0800 (PST) Lines: 125 Approved: news@gmane.org Message-ID: <5095aadf-58c1-458b-bc9f-6bbd7499aa59@isocpp.org> References: <8d57e9e9-e4cc-441c-abeb-e6526531fad5@isocpp.org> <3534ECE0-D67E-443E-9593-76B644FDDF6A@gmail.com> <3aca8f41-7a21-40d8-9bf9-9c27dfa778f3@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_1536_7806428.1360209322599" X-Trace: ger.gmane.org 1360209322 2912 80.91.229.3 (7 Feb 2013 03:55:22 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 7 Feb 2013 03:55:22 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCEKFTV6ZUMBBK6LZSEAKGQEJR2L67Q@isocpp.org Thu Feb 07 04:55:43 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vc0-f199.google.com ([209.85.220.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1U3IaM-0007Lg-Ph for gclcip-std-proposals@m.gmane.org; Thu, 07 Feb 2013 04:55:43 +0100 Original-Received: by mail-vc0-f199.google.com with SMTP id fo13sf2743760vcb.10 for ; Wed, 06 Feb 2013 19:55:23 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:x-beenthere:x-received:date:from:to:message-id :in-reply-to:references:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:x-google-group-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=QWdWuSLoMrnl47s+q18MgvBTwAzP+Eq9YlWmFvdFxCI=; b=nsurvMACMrTKhXooHE0l/1B5EHRHI1MCiCPc4NIMMAgGpLT5u0CfLhyD7GK6DeIlau /LVIJQodcrvBkn6NUIifhrobZy/cfXNh5OtBaV6eJoBldBV1uYqUaMa3j1p89k/wyTh9 XxUefXP3dqc7HXQKkJWXSoWTwaHIc8PvxWaGKKLWrjmlf2idmD1Ep0YA5jvFpa9W+dPJ Rj7i2veUz3LUAFZ5Uxf8xv9LPmW2YSsmS8LHNQoY3F+sVAVVFLPcl/wwprEec8BpDRIo 5rzZ04kixEjGnCgpP8KwLwvZD/4UuvkZtabHG5VE913hFlqdU/JCulBF0U4hAIFNQ5kF HdXQ== X-Received: by 10.236.147.234 with SMTP id t70mr14677445yhj.12.1360209323275; Wed, 06 Feb 2013 19:55:23 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.127.174 with SMTP id nh14ls630883qeb.28.gmail; Wed, 06 Feb 2013 19:55:22 -0800 (PST) X-Received: by 10.49.12.97 with SMTP id x1mr2703819qeb.25.1360209322903; Wed, 06 Feb 2013 19:55:22 -0800 (PST) In-Reply-To: X-Original-Sender: jmckesson@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:2762 Archived-At: ------=_Part_1536_7806428.1360209322599 Content-Type: text/plain; charset=ISO-8859-1 On Wednesday, February 6, 2013 7:25:09 PM UTC-8, Nevin ":-)" Liber wrote: > > On 6 February 2013 20:40, Nicol Bolas >wrote: > >> That's really the question I have about this whole `optional` thing: >> what exactly is the use case for this functionality that `T*` can't also >> cover? >> > > How about optional? > I would say that should be illegal. Consider: T&& t1 = GetT(); T t2(t1); //Calls copy constructor. T t3(std::move(t1)); //Explicit std::move required to cause move. optional ot1 = GetOptT(); T t2(*ot1); //Calls move constructor. No explicit std::move required. Much language was expended in the C++11 standard to ensure that you *must*use an explicit std::move for anything that isn't a temporary or the return from a variable that's going out of scope. optional breaks this, allowing movement without the explicit use of std::move. optionalis dangerous. -- --- 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/?hl=en. ------=_Part_1536_7806428.1360209322599 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable

On Wednesday, February 6, 2013 7:25:09 PM UTC-8, Nevin ":-)" Liber = wrote:
On 6 February 2013 20:40, Nicol Bolas <jmck..= ..@gmail.com> wrote:
That's really the question I have about this whole `optional<T&= >` thing: what exactly is the use case for this functionality that `T*` = can't also cover?

How about optional<T&&>?

I would say that should be illegal. Consider:

T&& t1 =3D GetT();
T t2(t1); //Calls copy co= nstructor.=
T t3(<= /span>std<= span style=3D"color: #660;" class=3D"styled-by-prettify">::move(t1)); //Explicit std::move required to cause move.

optional<T&&> ot1 =3D GetOptT();
T t2(*ot= 1);= //Calls move constructor. = No explicit std::move required.

Much language was ex= pended in the C++11 standard to ensure that you must use an explicit= std::move for anything that isn't a temporary or the return from a variabl= e that's going out of scope. optional<T&&> breaks this, allow= ing movement without the explicit use of std::move.
optional<T&= amp;&>is dangerous.

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