220 16161 article Path: news.gmane.org!not-for-mail From: Arthur O'Dwyer Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: deprecate catch exception by value, at least in some instances? Date: Mon, 9 Feb 2015 15:29:05 -0800 (PST) Lines: 109 Approved: news@gmane.org Message-ID: References: <8be36e16-97f4-4d8a-8500-c045195b2f6b@isocpp.org> <56A0729B-434D-4300-B4FF-5D744D81506C@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_29_803476715.1423524545068" X-Trace: ger.gmane.org 1423524551 31999 80.91.229.3 (9 Feb 2015 23:29:11 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 9 Feb 2015 23:29:11 +0000 (UTC) Cc: gmisocpp@gmail.com To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDLZJYWNDQIMFBPFUYCRUBFA4HJ7A@isocpp.org Tue Feb 10 00:29:11 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ig0-f198.google.com ([209.85.213.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YKxlL-0007DI-Ua for gclcip-std-proposals@m.gmane.org; Tue, 10 Feb 2015 00:29:08 +0100 Original-Received: by mail-ig0-f198.google.com with SMTP id hl2sf123075777igb.1 for ; Mon, 09 Feb 2015 15:29:06 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:content-type:x-original-sender:reply-to:precedence :mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=Q1D91TcVk4SOOThan0KcqLlZTXSP9y50p8hnhQ7vAYk=; b=c2jNrMWU5jiIuxx60zRvtmORq4p6M3MVFpu+lAlnBT6aYqqmfBDEWmKn7Pq41a5AwV VC1PSuVWghXmz6tx854F8OfAunMqI2JynbdPHPpHkPfMvGr4OS9EcoaxKTMoEFj5Iufy XLkXtA0HDNluGCEsi3yIf25dHo633EHceoiz2ZfoAaggXPMYG+IT1OHUE0um0JJJo9JP n6QFuWx0+c6JNJ8NPDzBmDBELbpqXX8XZG3XUMQiwCA7y1QLgvEkHmtQ9HDJ7QzWTI+g dYv8rhAnpPzIZzyg+p9vmEN+MSHg83RI3DGq4jqEyn6aHUo2Zb0fUHIyXeuqT0gUFYSh 8K+g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:cc:message-id:in-reply-to :references:subject:mime-version:content-type:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=Q1D91TcVk4SOOThan0KcqLlZTXSP9y50p8hnhQ7vAYk=; b=EC6YkIP5VDdnn95xHvxSCxWxkajJlqpC1FJ+k0WXexDYbwkt+Zex841GBNJicWvtHj v09VcDeydYMqYcy31Y23y1hRfBNoQKWq9baNU2TpT/UeYLVkuwZWIlxgwR3HtifTr/lO qTZw6F505+guoR9bxPN7odU0iCg2rZTJVEnVt74jjFf9wjxmZ2hSpt1c1RBHjO8OgxOq zzwZrfZNOU0NVOpEPRP/w1hb0ePLW6E1rVaz9cNIMqIonyUnTAhTg+1POQ7tXyk/bhlY rPdGw3NS8M9ozyLKxSS8WBpkTniXUnquJQwWK7u8v+4NO8IrpctkzZyhswrlvF2cuiCJ E8PA== X-Gm-Message-State: ALoCoQkBoe5j2/HxuDfRKaNaKU31H67VMMrtxthK90Tn3b3QJkWXAiW0HkRjEuc51lcJ/wasoHhQ X-Received: by 10.50.87.97 with SMTP id w1mr16167939igz.1.1423524546694; Mon, 09 Feb 2015 15:29:06 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.150.202 with SMTP id y193ls1516452iod.78.gmail; Mon, 09 Feb 2015 15:29:06 -0800 (PST) X-Received: by 10.50.25.194 with SMTP id e2mr276817igg.16.1423524546038; Mon, 09 Feb 2015 15:29:06 -0800 (PST) In-Reply-To: X-Original-Sender: arthur.j.odwyer@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:16161 Archived-At: ------=_Part_29_803476715.1423524545068 Content-Type: multipart/alternative; boundary="----=_Part_30_655976669.1423524545068" ------=_Part_30_655976669.1423524545068 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Sunday, February 8, 2015 at 5:36:48 PM UTC-8, gmis...@gmail.com wrote: > > On Monday, February 9, 2015 at 1:28:33 PM UTC+13, David Krauss wrote: >> >> >> Perhaps it=E2=80=99s hard to remember because it doesn=E2=80=99t make mu= ch of a=20 >> difference either way. There=E2=80=99s no such thing as throwing by refe= rence, at=20 >> all. You can get a little extra efficiency by using std::move when=20 >> throwing, but there=E2=80=99s not really much point. [...] >> > > Hey that all strangely leads me off to a different idea: > When we throw x, why can't we treat that as a move in some instances, lik= e=20 > return does!? > There are two things going on here: - "return x;" and "throw x;", where x is a local variable, will both treat= =20 x as an rvalue rather than an lvalue. (This is specified in 12.8 /32 of=20 draft standard N4296.) - Copy/move elision. "return x;" and "throw x;" are both permitted to elide= =20 the copy/move construction. (These are specified in 12.8 /31.1 and /31.2=20 respectively.) In other words, "throw x;" will neither copy nor move in the usual case;=20 but if you somehow manage to disable copy elision in your compiler, it will= =20 still move. It is not *permitted* to make a copy, unless there are=20 extenuating circumstances (such as the move constructor having been=20 deleted). Writing "throw std::move(x);" will inhibit move elision, so don't do that. =E2=80=93Arthur --=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/. ------=_Part_30_655976669.1423524545068 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Sunday, February 8, 2015 at 5:36:48 PM UTC-8, gmis...@g= mail.com wrote:
On Monday, February 9, 2015 at 1:28:33 PM UTC+13, David Krauss wrote:

Perhaps it=E2=80=99s hard to remember because it= doesn=E2=80=99t make much of a difference either way. There=E2=80=99s no s= uch thing as throwing by reference, at all. You can get a little extra effi= ciency by using std::move when throwing, but = there=E2=80=99s not really much point. [...]
<= br>
Hey that all strangely leads me off to a different idea:
When we throw x, why can't we treat that as a move in some instances,= like return does!?

There are t= wo things going on here:
- "return x;" and "throw x;", where x is= a local variable, will both treat x as an rvalue rather than an lvalue. (T= his is specified in 12.8 /32 of draft standard N4296.)
- Copy/mov= e elision. "return x;" and "throw x;" are both permitted to elide the copy/= move construction. (These are specified in 12.8 /31.1 and /31.2 respectivel= y.)

In other words, "throw x;" will neither copy n= or move in the usual case; but if you somehow manage to disable copy elisio= n in your compiler, it will still move. It is not *permitted* to make a cop= y, unless there are extenuating circumstances (such as the move constructor= having been deleted).

Writing "throw std::move(x)= ;" will inhibit move elision, so don't do that.

= =E2=80=93Arthur

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