220 35044 article Path: news.gmane.org!.POSTED!not-for-mail From: Hyman Rosen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: =?UTF-8?Q?Re=3A_=5Bstd=2Dproposals=5D_Re=3A_std=3A=3Asink=3CT=3E_=E2=80=93_a_better_so?= =?UTF-8?Q?lution_to_the_=22nightmare_of_move_semantics=22_=28a=2Ek=2Ea_overload_?= =?UTF-8?Q?proliferation=29?= Date: Fri, 20 Oct 2017 11:34:47 -0400 Lines: 101 Approved: news@gmane.org Message-ID: References: <271bf2c1-854f-4f21-9593-f405f7cd3e8a@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="001a1140a828ba42a5055bfc375c" X-Trace: blaine.gmane.org 1508513708 12432 195.159.176.226 (20 Oct 2017 15:35:08 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 20 Oct 2017 15:35:08 +0000 (UTC) Cc: joseph.thomson@gmail.com To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDKJJPVBQMKRBLNPVDHQKGQEDPHBVAQ@isocpp.org Fri Oct 20 17:35:03 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f70.google.com ([74.125.82.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1e5ZKB-0002hp-20 for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id o8sor367676wmd.4.2017.10.20.08.35.09 for (Google Transport Security); Fri, 20 Oct 2017 08:35:09 -0700 (PDT) Received-SPF: pass (google.com: domain of hyman.rosen@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.28.159.72 with SMTP id i69mr1992821wme.53.1508513708391; Fri, 20 Oct 2017 08:35:08 -0700 (PDT) Original-Received: by 10.28.101.8 with HTTP; Fri, 20 Oct 2017 08:34:47 -0700 (PDT) In-Reply-To: X-Original-Sender: hyman.rosen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=O2tdR12J; spf=pass (google.com: domain of hyman.rosen@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=hyman.rosen@gmail.com; dmarc=pass (p=NONE sp=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:35044 Archived-At: --001a1140a828ba42a5055bfc375c Content-Type: text/plain; charset="UTF-8" On Fri, Oct 20, 2017 at 11:18 AM, Nicol Bolas wrote: > > Which will happen either way if you call any functions after that one > which can throw exceptions. Or if the function throws exceptions for > reasons other than the construction of its parameters. > .... > Moving state and the strong exception guarantee are somewhat mutually > exclusive. > I would argue that the initialization of function parameters should not be part of the function's contract at all. The function should say what it does with its parameters as given, but failure to initialize the parameters means that the function is never even called; how can it be expected to guarantee anything? Unless the function you're calling with std::move(lvalue) as an argument tells you otherwise, you should assume that the call expression puts the lvalue in a moved-from state. By "tells you otherwise," I mean code something like this: bool try_foo(Thing &&t); // return true if t is moved from bool try_bar(Thing &&t); // return true if t is moved from void baz() { Thing me; if (try_foo(std::move(me))) { /* ... */ } else if (try_bar(std::move(me))) { /* ... */ } else { /* me retains its original contents */ } } -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CAHSYqdadJQEwbHvKDV7m9qtgmMkV%2BAsw%3DNa0eEo9C%3DfjRFHYSA%40mail.gmail.com. --001a1140a828ba42a5055bfc375c Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On F= ri, Oct 20, 2017 at 11:18 AM, Nicol Bolas <jmckesson@gmail.com> wrote:
Which will happen either way if you call any functions after that = one which can throw exceptions. Or if the function throws exceptions for re= asons other than the construction of its parameters.=C2=A0
=
...=C2=A0
Moving state and the strong exception g= uarantee are somewhat mutually exclusive.

= I would argue that the initialization of function parameters should not be = part
of the function's contract at all.=C2=A0 The function should sa= y what it does with its
parameters as given, but failure to initialize t= he parameters means that the
function is never even called; how can it b= e expected to guarantee anything?

Unless the function you're cal= ling with std::move(lvalue) as an argument tells
you otherwise, you shou= ld assume that the call expression puts the lvalue in
a moved-from state= ..

By "tells you otherwise," I mean code something like thi= s:
bool try_foo(Thing &&t);= =C2=A0 // return true if t is moved from
bool try_bar(Thing &&t);=C2=A0=C2=A0// return true if t is moved fro= m
void baz() {
=C2=A0 =C2=A0 Thing me;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0if (try_foo(std::mo= ve(me))) { /* ... */ }
=C2=A0 =C2=A0 else if (try_bar(std::move(me))) { = /* ... */ }
=C2=A0 =C2=A0 else=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0{ /* me retai= ns its original contents */ }
}

--
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--001a1140a828ba42a5055bfc375c-- .