220 20705 <2711C197-0E67-4ADB-83A1-2D7B15E21693@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Null -conditional and -coalescing operators Date: Wed, 23 Sep 2015 17:31:54 +0800 Lines: 92 Approved: news@gmane.org Message-ID: <2711C197-0E67-4ADB-83A1-2D7B15E21693@gmail.com> References: <4c3e2ee7-2159-4e93-a439-7d5c94c38c9c@isocpp.org> <2081160.jZretSXBOl@tjmaciei-mobl4> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 8.2 \(2104\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_49C9D21C-13B2-4AC7-A590-9C0B299084F3" X-Trace: ger.gmane.org 1443000736 31943 80.91.229.3 (23 Sep 2015 09:32:16 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 23 Sep 2015 09:32:16 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBEHDRGYAKGQEYE2X2BQ@isocpp.org Wed Sep 23 11:32:03 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f200.google.com ([209.85.223.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZegPC-000379-HE for gclcip-std-proposals@m.gmane.org; Wed, 23 Sep 2015 11:32:02 +0200 Original-Received: by ioiz6 with SMTP id z6sf68304536ioi.2 for ; Wed, 23 Sep 2015 02:32:01 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:content-type:message-id:mime-version :subject:date:references:to:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=TMIxDGZbODtkrJ/EEmXQhZVfYHkdeZwwYyHp+ibaJgU=; b=Ce7oxLrJZfmkGFraAImVLZPc2wabnfRkgMz2woypkt+r2PtLRzrxHFSH/yuKuG4cSx CsJbVuCzyw5tAPLwSr2Ned6mjsgJiS0qg3u1ebTDOaWBgq/S//WEbVASM4r+EUQ90M3A liE+aeCcQYF4Pi1U47DAGs9dCq8dAxt8XDATC3GD396nZvMqO6Ej16yoUhaBrAXcNmVJ Rcfg5JL73SAq1Km3UE+X5MeRV2SdshUdz5OYSP93eKGRJ1j9lq9mtdz4nVHv3Ai4zrmv uttXeVVLFNpYtM5D2cMzKYTd4yVncv05MV7baejMmN3e6UIFb6j6zwNEair0+XGjECp2 /VFw== X-Gm-Message-State: ALoCoQnYNzOSm888izU7n23HcTuDdHFXyBQxcUk5Ti34X1T25A1KUnIs6e9wgg2EivxaDt1XgCtE X-Received: by 10.50.225.102 with SMTP id rj6mr7376483igc.9.1443000721439; Wed, 23 Sep 2015 02:32:01 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.79.201 with SMTP id l9ls395858igx.19.gmail; Wed, 23 Sep 2015 02:32:00 -0700 (PDT) X-Received: by 10.50.8.3 with SMTP id n3mr22603642iga.26.1443000720546; Wed, 23 Sep 2015 02:32:00 -0700 (PDT) Original-Received: from mail-pa0-x235.google.com (mail-pa0-x235.google.com. [2607:f8b0:400e:c03::235]) by mx.google.com with ESMTPS id hh18si16064088igb.55.2015.09.23.02.32.00 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 23 Sep 2015 02:32:00 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::235 as permitted sender) client-ip=2607:f8b0:400e:c03::235; Original-Received: by pacex6 with SMTP id ex6so35949285pac.0 for ; Wed, 23 Sep 2015 02:32:00 -0700 (PDT) X-Received: by 10.66.144.135 with SMTP id sm7mr36956413pab.106.1443000720006; Wed, 23 Sep 2015 02:32:00 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.183]) by smtp.gmail.com with ESMTPSA id zf5sm6847220pbc.36.2015.09.23.02.31.58 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 23 Sep 2015 02:31:59 -0700 (PDT) In-Reply-To: <2081160.jZretSXBOl@tjmaciei-mobl4> X-Mailer: Apple Mail (2.2104) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::235 as permitted sender) smtp.mailfrom=potswa@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-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:20705 Archived-At: --Apple-Mail=_49C9D21C-13B2-4AC7-A590-9C0B299084F3 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 All this confusion aside, it would be nice to have such a thing in the libr= ary. It would be on a similar level of abstraction as std::exchange. Not tested, but it might look something like this: template< typename alternative, typename value > alternative value_or( value && v, alternative && a ) { if ( v ) return std::forward< value >( v ); else return std::forward< alternative >( a ); } Given such a facility, experimental::optional shouldn=E2=80=99t need a memb= er value_or. This one does work a bit differently, but the goal is the same= .. It would be better if it returned by reference. This would be enabled by my= lifetime extension proposal (bit.ly/genlife ) and m= arking both parameters as export. Unlike the OP, the second argument gets evaluated even if it=E2=80=99s not = used. Hopefully at some point the language will support user-defined short-= circuit operators via lazy argument evaluation. In the big picture, the def= iciency is minor. --=20 ---=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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --Apple-Mail=_49C9D21C-13B2-4AC7-A590-9C0B299084F3 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8 All this confusion= aside, it would be nice to have such a thing in the library. It would be o= n a similar level of abstraction as std::= exchange.

Not te= sted, but it might look something like this:

template<&= nbsp;typename alter= native, typena= me value >
alternative value_or( value && v, alternative && a ) {
    if= ( v ) return std::forward< value >( v );
    else return std::forward&l= t; alternative >( a );
}

Given such a facility, experimental= ::optional shouldn=E2=80=99t need a member value_or. This one does work a bit differently, but the goa= l is the same.

It= would be better if it returned by reference. This would be enabled by my l= ifetime extension proposal (bi= t.ly/genlife) and marking both parameters as export.

Unlike the OP, the second argument gets evaluated even if it=E2= =80=99s not used. Hopefully at some point the language will support user-de= fined short-circuit operators via lazy argument evaluation. In the big pict= ure, the deficiency is minor.

--

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