220 2702 <510E1906.3030604@wanadoo.fr> article Path: news.gmane.org!not-for-mail From: "Vicente J. Botet Escriba" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: A proposal to add swap traits to the standard library Date: Sun, 03 Feb 2013 09:00:06 +0100 Lines: 178 Approved: news@gmane.org Message-ID: <510E1906.3030604@wanadoo.fr> References: <1359827596.1381.2.camel@Nokia-N900> <41C25E3D-C254-43AE-BFEA-ED83B7DAF74E@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1359878408 4502 80.91.229.3 (3 Feb 2013 08:00:08 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 3 Feb 2013 08:00:08 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDH67CONY4PBBB5SXCEAKGQE6C6QLXI@isocpp.org Sun Feb 03 09:00:28 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lb0-f200.google.com ([209.85.217.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1U1uV0-0000lZ-1y for gclcip-std-proposals@m.gmane.org; Sun, 03 Feb 2013 09:00:26 +0100 Original-Received: by mail-lb0-f200.google.com with SMTP id k6sf6092672lbo.3 for ; Sun, 03 Feb 2013 00:00:07 -0800 (PST) X-Received: by 10.180.74.141 with SMTP id t13mr856902wiv.5.1359878407420; Sun, 03 Feb 2013 00:00:07 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.78.34 with SMTP id y2ls424635wiw.14.canary; Sun, 03 Feb 2013 00:00:07 -0800 (PST) X-Received: by 10.180.87.170 with SMTP id az10mr4754051wib.3.1359878406988; Sun, 03 Feb 2013 00:00:06 -0800 (PST) X-Received: by 10.180.87.170 with SMTP id az10mr4754049wib.3.1359878406968; Sun, 03 Feb 2013 00:00:06 -0800 (PST) Original-Received: from smtp.smtpout.orange.fr (smtp03.smtpout.orange.fr. [80.12.242.125]) by mx.google.com with ESMTP id r9si2289312wiv.76.2013.02.03.00.00.06; Sun, 03 Feb 2013 00:00:06 -0800 (PST) Received-SPF: neutral (google.com: 80.12.242.125 is neither permitted nor denied by best guess record for domain of vicente.botet@wanadoo.fr) client-ip=80.12.242.125; Original-Received: from iMac-de-Vicente-Botet-Escriba.local ([2.11.186.153]) by mwinf5d57 with ME id vk061k0043K0JVN03k06Vq; Sun, 03 Feb 2013 09:00:06 +0100 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:17.0) Gecko/20130107 Thunderbird/17.0.2 In-Reply-To: X-Original-Sender: vicente.botet@wanadoo.fr X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 80.12.242.125 is neither permitted nor denied by best guess record for domain of vicente.botet@wanadoo.fr) smtp.mail=vicente.botet@wanadoo.fr 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:2702 Archived-At: Le 03/02/13 06:52, Jeffrey Yasskin a =E9crit : > On Sat, Feb 2, 2013 at 10:00 AM, Howard Hinnant > wrote: >> On Feb 2, 2013, at 12:53 PM, Ville Voutilainen wrote: >> >>> ----- Original message ----- >>>> Hi all - >>>> >>>> I'd like to propose the addition of two new type relationship traits t= o >>>> the C++ standard library: >>>> >>>> template >>>> struct std::is_swappable; >>>> >>>> template >>>> struct std::is_nothrow_swappable; >>>> >>>> The purpose of these two new traits is primarily to make it easier to >>>> correctly query the noexcept status of swap when writing a noexcept >>>> expression. >>> Just quickly, what's the issue with >>> noexcept(swap(x, y)) ? >> Challenge: Make this code compile without a using directive or using de= claration: >> >> #include >> >> template >> void >> foo(T& x, T& y) noexcept(noexcept(swap(x, y))); >> >> struct A >> { >> A() =3D default; >> A(const A&); >> A& operator=3D(const A&); >> }; >> >> void swap(A&, A&) noexcept; >> >> int main() >> { >> int i; >> A a; >> static_assert(noexcept(foo(i, i)), ""); >> static_assert(noexcept(foo(a, a)), ""); >> } >> >> Howard > :) > > At one point, I think Dave suggested making ::std::begin(foo) > automatically delegate to ADL when that's possible. I forget exactly > how that worked, but if it's possible, we should consider doing it for > swap() as well. > > The following code based on the idea of boost::swap works with clang 3.2 "The template function boost::swap allows the values of two variables to=20 be swapped, using argument dependent lookup to select a specialized swap=20 function if available. If no specialized swap function is available,=20 std::swap is used." namespace nstd_swap_impl { using std::swap; template void swap_impl(T& x, T& y) noexcept(noexcept(swap(x, y))) { swap(x, y); } } namespace nstd { template void swap(T1& x, T2& y) noexcept(noexcept(::nstd_swap_impl::swap_impl(x,= y))) { ::nstd_swap_impl::swap_impl(x, y); } } template void foo(T& x, T& y) noexcept(noexcept(nstd::swap(x, y))); struct A { A() =3D default; A(const A&); A& operator=3D(const A&); }; void swap(A&, A&) noexcept; int main() { int i; A a; static_assert(noexcept(foo(i, i)), ""); static_assert(noexcept(foo(a, a)), ""); } I don't know if Dave suggestion was related to this technique, but at least= the nstd::swap is reusable. Vicente P.S. I needed to rename swap on for make working gcc namespace nstd { template void swap2(T1& x, T2& y) noexcept(noexcept(::nstd_swap_impl::swap_impl(x= , y))) { ::nstd_swap_impl::swap_impl(x, y); } } template void foo(T& x, T& y) noexcept(noexcept(nstd::swap2(x, y))); as otherwise .../example/swap_test.cpp:19:77: error: template instantiation depth exceeds= maximum of 128 (use -ftemplate-depth=3D to increase the maximum) substitut= ing =91template void nstd_swap_impl::swap_impl(T&, T&) [with T =3D= ]=92 void swap(T1& x, T2& y) noexcept(noexcept(::nstd_swap_impl::swap_impl(x= , y))) = ^ .../example/swap_test.cpp:19:77: required from =91void nstd::swap(T1&, T2&= ) [with T1 =3D int; T2 =3D int]=92 .../example/swap_test.cpp:27:50: required from =91void nstd_swap_impl::swa= p_impl(T&, T&) [with T =3D int]=92 .../example/swap_test.cpp:19:77: required from =91void nstd::swap(T1&, T2&= ) [with T1 =3D int; T2 =3D int]=92 .../example/swap_test.cpp:27:50: required from =91void nstd_swap_impl::swa= p_impl(T&, T&) [with T =3D int]=92 .../example/swap_test.cpp:19:77: required from =91void nstd::swap(T1&, T2&= ) [with T1 =3D int; T2 =3D int]=92 .../example/swap_test.cpp:27:50: [ skipping 118 instantiation contexts, us= e -ftemplate-backtrace-limit=3D0 to disable ] .../example/swap_test.cpp:27:50: required from =91void nstd_swap_impl::swa= p_impl(T&, T&) [with T =3D int]=92 .../example/swap_test.cpp:19:77: required from =91void nstd::swap(T1&, T2&= ) [with T1 =3D int; T2 =3D int]=92 .../example/swap_test.cpp:27:50: required from =91void nstd_swap_impl::swa= p_impl(T&, T&) [with T =3D int]=92 .../example/swap_test.cpp:19:77: required from =91void nstd::swap(T1&, T2&= ) [with T1 =3D int; T2 =3D int]=92 .../example/swap_test.cpp:27:50: required from =91void foo(T&, T&) [with T= =3D int]=92 .../example/swap_test.cpp:42:36: required from here --=20 ---=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. 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