220 11745 article Path: news.gmane.org!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: N4078: Rvalue reference overloads for value() method returns object by value Date: Wed, 9 Jul 2014 21:57:00 +0300 Lines: 44 Approved: news@gmane.org Message-ID: References: <764569c3-b26b-4938-970e-44a4681de132@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Trace: ger.gmane.org 1404932232 5172 80.91.229.3 (9 Jul 2014 18:57:12 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 9 Jul 2014 18:57:12 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC5JHI7A7ALRB7FA62OQKGQEVS6SNLY@isocpp.org Wed Jul 09 20:57:06 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f70.google.com ([209.85.220.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X4x38-0005Ue-E2 for gclcip-std-proposals@m.gmane.org; Wed, 09 Jul 2014 20:57:02 +0200 Original-Received: by mail-pa0-f70.google.com with SMTP id lj1sf53028952pab.9 for ; Wed, 09 Jul 2014 11:57:01 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state: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=YiwyNzFnRD8ND5wIrHW4cyb+7JJ8Z7kOJjJtKkyHoBI=; b=AajoMIy/tK97OoVTkWu0mgTiY47H2ZyKNLwlMAxd0enR4WStHiA4sX64Bn79i/cBZM cOFbKd/IcVB/upgdcbY7SnokIi4qdu3AXjgxa9JPCMdwXxXRq0AwIeTefGFJjr8yqiLo 6svWV3QfP9kBPoTbD+Z8A1NzkOV8Lkqd04eUCKOy+FBc+y9mVzMq5i0JNmqjS8fnQac9 oe4/MwY5aW5KRR2Cn5lSmrL1CbLZOD5p3eKP6gEBKt4QD2wvVpDSR9PZ9fU23jVf6uqt I7+0gXTselUGo92zQ034OKDdbmGb9LJcAjI5pPR2grYDJR5luTQq3Lu5VX9XurxCYkdA w+IA== X-Gm-Message-State: ALoCoQlGhEIVXYnErbwllMI0ieb3PTOYlwI1U5buj7G/H0d+jyGS4d8wSUBt9lXeJRx9OaI+xnZl X-Received: by 10.66.182.170 with SMTP id ef10mr9495208pac.0.1404932221364; Wed, 09 Jul 2014 11:57:01 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.81.106 with SMTP id e97ls2076220qgd.43.gmail; Wed, 09 Jul 2014 11:57:00 -0700 (PDT) X-Received: by 10.224.129.130 with SMTP id o2mr75276851qas.64.1404932220611; Wed, 09 Jul 2014 11:57:00 -0700 (PDT) Original-Received: from mail-qc0-x236.google.com (mail-qc0-x236.google.com [2607:f8b0:400d:c01::236]) by mx.google.com with ESMTPS id r6si22880908qcd.22.2014.07.09.11.57.00 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 09 Jul 2014 11:57:00 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400d:c01::236 as permitted sender) client-ip=2607:f8b0:400d:c01::236; Original-Received: by mail-qc0-f182.google.com with SMTP id m20so6909659qcx.27 for ; Wed, 09 Jul 2014 11:57:00 -0700 (PDT) X-Received: by 10.229.252.130 with SMTP id mw2mr70371486qcb.12.1404932220527; Wed, 09 Jul 2014 11:57:00 -0700 (PDT) Original-Received: by 10.224.31.137 with HTTP; Wed, 9 Jul 2014 11:57:00 -0700 (PDT) In-Reply-To: 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 2607:f8b0:400d:c01::236 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:11745 Archived-At: On 9 July 2014 21:34, wrote: > I want point out one other think that I run into, when I was creating a > handle_value_or_optional in > https://groups.google.com/a/isocpp.org/forum/?fromgroups#!topic/std-proposals/RTUK2xO0tzs. > I will provide here shorter version. Let assume we want to have the > following function handle_optional(opt, fv, fn) that when opt has value will > invoke fv(*opt) otherwis fn. > > I we have the r-value reference qualified operator* and value method > (instead of returning by value) I can write: > template //I removed the checking form > this function to make it simpler > delctype(auto) handle_optional(U&& u, FV&& fv, FN&& fn) > { > return u ? std::forward(fv)(*std::forward(u)) : > std::forward(fn); > } > Because *std::forward(u) will return a rvalue reference if such overload > exists. If the optional has no rvalue refence method > or the one returning value I must write to avoid making uncessary > temporaries: > template //I removed the checking form > this function to make it simpler > delctype(auto) handle_optional(U&& u, FV&& fv, FN&& fn) > { > return u ? std::forward(fv)(std::forward decay_t::value_type, U&&>>(*u)) : std::forward(fn); > } > //The same_cv_ref_as_t is needed to use std::forward if optional&& > is passed as first argument type. And actually this works because > u is lvalue here. Ah, I see. Forwarding a dereferenced named value will never create a temporary, *forward(u) might. -- --- 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/. .