220 4620 <01190eaa-6223-4d3c-a9d3-b0d850aab98e@isocpp.org> article Path: news.gmane.org!not-for-mail From: potswa@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Changing lambda scope Date: Mon, 27 May 2013 04:52:17 -0700 (PDT) Lines: 262 Approved: news@gmane.org Message-ID: <01190eaa-6223-4d3c-a9d3-b0d850aab98e@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_2692_25349192.1369655538018" X-Trace: ger.gmane.org 1369655545 7398 80.91.229.3 (27 May 2013 11:52:25 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 27 May 2013 11:52:25 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRB5ERRWGQKGQEDEWAWVQ@isocpp.org Mon May 27 13:52:25 2013 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 1UgvyR-0001PU-ON for gclcip-std-proposals@m.gmane.org; Mon, 27 May 2013 13:52:24 +0200 Original-Received: by mail-ob0-f200.google.com with SMTP id ef5sf30166229obb.11 for ; Mon, 27 May 2013 04:52:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-beenthere:date:from:to:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:x-google-group-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=9HBSSPxfecNEpZpIiUkmtjVrRZ6bVNAJuUMyMO7EgCI=; b=FDMNQIzZcitxLFvfpq6bsZcBG2AJoz/WUlFmfZ6legNTLKUI9CPQe6L6fxsZdxvg/5 ItHJLZVlLE0dMDBwMAKlPgJSrzFWP+hk6fN8Lkp9kwdg33b+EKw4EguJ1LpVxE3V1ybp IGvFWvqyO2vrMUetxXVe4sG7Duj/6SbMv5695tRLmd+1YpGGQZCs+vKnzEn3N0CDtqvO HOcvMqztnx9hxlNmdl7BODoifQCKWdzlZUzVcsJclrUrBIcEyQzW4WCWhjlv29xc803m GEPiEWHvNL6AhTjaMuCUrmsS/vJMqk/51ack5yao+dQkAhggtp1eYHGboMrME3xZouUV H+DQ== X-Received: by 10.50.118.10 with SMTP id ki10mr8651365igb.7.1369655542292; Mon, 27 May 2013 04:52:22 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.124.69 with SMTP id mg5ls1885775igb.14.gmail; Mon, 27 May 2013 04:52:19 -0700 (PDT) X-Received: by 10.50.196.227 with SMTP id ip3mr936822igc.10.1369655539570; Mon, 27 May 2013 04:52:19 -0700 (PDT) In-Reply-To: X-Original-Sender: potswa@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:4620 Archived-At: ------=_Part_2692_25349192.1369655538018 Content-Type: text/plain; charset=ISO-8859-1 GCC 4.9 accepts your example already, but if it didn't, the fix would simply be a wrapper class template in the desired associated namespace. namespace X { template< typename t > // wrapper class struct adl_wrapper : t { using t::t; }; template< typename t > // wrapper factory adl_wrapper< t > adl_wrap( t o ) { return { o }; } template L operator+(L l, R r) { } auto f() { return adl_wrapper( [] {} ); } } It would be bad to make lambdas different from other local classes. Would applying the proposal so broadly break much? Before C++11 it was illegal to use a local class as a template argument and it we're only noticing this now because returning a local type is a C++14 feature. Here is a C++11 example of namespace association making a difference for a local type. It is correctly rejected by Clang but accepted by GCC. template< typename t, typename u > void f( t, u ) = delete; template< typename t > void c( t o ) { f( o, 5 ); } namespace n { template< typename t > void f( t, int ) {} void p() { c( []{} ); } } Why should local classes have associated namespaces? Because it's least surprising behavior. A local scope is just another means of encapsulation, and nobody expects functions to quarantine their contents this way. It just plays badly with templates and it's broken. -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/?hl=en. ------=_Part_2692_25349192.1369655538018 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable GCC 4.9 accepts your example already, but if it didn't, the fix would simpl= y be a wrapper class template in the desired associated namespace.

<= div class=3D"prettyprint" style=3D"background-color: rgb(250, 250, 250); bo= rder-color: rgb(187, 187, 187); border-style: solid; border-width: 1px; wor= d-wrap: break-word;">
namespace X {
template< typename t > // wrapper class
struct a= dl_wrapper : t { using t::t; };

template<= span style=3D"color: #660;" class=3D"styled-by-prettify">< typename t > // wrapper factory
adl_wrapper< t > adl_wrap( t o ) { return { o }; }=

template<typename L, typename R> L op= erator+(L l, R r) { }auto f() { return adl_wrapper( []<= /span> {} ); }
}

It would be bad to make lambdas different fr= om other local classes. Would applying the proposal so broadly break much? = Before C++11 it was illegal to use a local class as a template argument and= it we're only noticing this now because returning a local type is a C++14 = feature.

Here is a C++11 example of namespace association making a d= ifference for a local type. It is correctly rejected by Clang but accepted = by GCC.

template< typename t, typename u >
void f(<= /span> t, u ) =3D delete;
<= br>templat= e< typename t >
void c( t o ) { f( o, 5 <= /span>); }

namespace n {
template< type= name t >
void f( t, int ) {}

void p() { c( []{} ); }=
}

=

Why should local classes have associated namespaces?= Because it's least surprising behavior. A local scope is just another mean= s of encapsulation, and nobody expects functions to quarantine their conten= ts this way. It just plays badly with templates and it's broken.

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