220 19865 <3646B3DF-F7F0-465D-8EE1-536E4CFB226B@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: default arguments Date: Sun, 16 Aug 2015 08:19:49 +0800 Lines: 91 Approved: news@gmane.org Message-ID: <3646B3DF-F7F0-465D-8EE1-536E4CFB226B@gmail.com> References: <4671850d-b823-436d-b097-f514073cc92e@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 8.2 \(2098\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_74117612-8957-4073-BC71-32E748042A2B" X-Trace: ger.gmane.org 1439684406 9010 80.91.229.3 (16 Aug 2015 00:20:06 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 16 Aug 2015 00:20:06 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBLFOX6XAKGQES5IWBSY@isocpp.org Sun Aug 16 02:20:00 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f200.google.com ([209.85.214.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZQlg5-0000FR-Ql for gclcip-std-proposals@m.gmane.org; Sun, 16 Aug 2015 02:19:57 +0200 Original-Received: by obbop1 with SMTP id op1sf158185810obb.3 for ; Sat, 15 Aug 2015 17:19:56 -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=hGO3w0OlafYvixeCiW6DTmRN+rDvOLbNVyOoah9EkEA=; b=aI5e8F23d+L2uWk24OTC0MoQ6DAZF8PUKkWcX6UPmYy03mc+k3m4fyKlJTuSf58ZHL AphTyA/w0Mn1dZPFWIbFMPhAqWKrIFkfWCnvGYqpQzNqJnNSBgZmkCfQxLPLMLReOswH yCwh5g77LpdOdRN95KzOcPJVi7IHiOCw//8yFsD3npvotWKbRzVnYBzsfQRIoTUjTYl4 b28n7TAACjzLhxKJx2gzk8cJNNpBIznvDJ3NCt6LCgflnGDaQlz0Ghg5hEsaFdFjxElb yh0LE48NGH61TRe4WGwk8aPzZM5hu35IoGbv3Er+oqHnnXWzgHtiP2T8ToPMWmpRRTWp oWrg== X-Gm-Message-State: ALoCoQkpvbOgsHL6CLq097u080GbkbXwMytSqSBMN81+R8SWkqTAuxdMddqDUgA17+GCzIsa7IxP X-Received: by 10.182.85.197 with SMTP id j5mr46437005obz.23.1439684396577; Sat, 15 Aug 2015 17:19:56 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.57.18 with SMTP id e18ls580436igq.23.gmail; Sat, 15 Aug 2015 17:19:55 -0700 (PDT) X-Received: by 10.66.141.141 with SMTP id ro13mr81100004pab.68.1439684395865; Sat, 15 Aug 2015 17:19:55 -0700 (PDT) Original-Received: from mail-pd0-x22d.google.com (mail-pd0-x22d.google.com. [2607:f8b0:400e:c02::22d]) by mx.google.com with ESMTPS id b5si16870186pbu.237.2015.08.15.17.19.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 15 Aug 2015 17:19:55 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c02::22d as permitted sender) client-ip=2607:f8b0:400e:c02::22d; Original-Received: by pdrg1 with SMTP id g1so42838142pdr.2 for ; Sat, 15 Aug 2015 17:19:55 -0700 (PDT) X-Received: by 10.70.56.66 with SMTP id y2mr101005570pdp.168.1439684395721; Sat, 15 Aug 2015 17:19:55 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.44.88]) by smtp.gmail.com with ESMTPSA id hx7sm9868673pbc.84.2015.08.15.17.19.52 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sat, 15 Aug 2015 17:19:55 -0700 (PDT) In-Reply-To: <4671850d-b823-436d-b097-f514073cc92e@isocpp.org> X-Mailer: Apple Mail (2.2098) 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:c02::22d 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:19865 Archived-At: --Apple-Mail=_74117612-8957-4073-BC71-32E748042A2B Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 > On 2015=E2=80=9308=E2=80=9316, at 1:29 AM, Nicol Bolas wrote: >=20 > My only concern is that it could tie up more useful syntax. With Uniform = Initialization syntax, you can perform value initialization and constructio= n initialization. But there's no way to generically use default initializat= ion. So we might someday want `f(default)` to mean "default initialize the = parameter". C-style variadic arguments come close to implementing default-initialized (= uninitialized) parameters. The receiver needs to explicitly make a decision= whether to access the parameter and assert that it was actually passed. Default-initialized values, when distinct from value-initialized, are indet= erminate. Parameter passing uses copy-initialization, and you usually can= =E2=80=99t copy an indeterminate value. So f(default) would seem to be UB e= xcept when the argument is a temporary passed by reference. TL;DR: Default-initialized parameters, not very useful. --=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=_74117612-8957-4073-BC71-32E748042A2B Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
On 2015=E2=80=9308= =E2=80=9316, at 1:29 AM, Nicol Bolas <jmckesson@gmail.com> wrote:

My only concern is that it could tie= up more useful syntax. With Uniform Initialization syntax, you can perform= value initialization and construction initialization. But there's no way t= o generically use default initialization. So we might someday want `f(defau= lt)` to mean "default initialize the parameter".

C-sty= le variadic arguments come close to implementing default-initialized (unini= tialized) parameters. The receiver needs to explicitly make a decision whet= her to access the parameter and assert that it was actually passed.

Default-initialized valu= es, when distinct from value-initialized, are indeterminate. Parameter pass= ing uses copy-initialization, and you usually can=E2=80=99t copy an indeter= minate value. So f(default) would = seem to be UB except when the argument is a temporary passed by reference.<= /div>

TL;DR: Default-in= itialized parameters, not very useful.

=

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