From 9001552840203100101 X-Google-Thread: f78e5,e8381186f8304247 X-Google-NewGroupId: yes X-Google-Attributes: gid7894ca11fe,domainid0,public,usenet X-Google-Language: ENGLISH,UTF8 Path: g2news2.google.com!news2.google.com!Xl.tags.giganews.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local2.nntp.dca.giganews.com!news.giganews.com.POSTED!not-for-mail NNTP-Posting-Date: Sun, 09 Jan 2011 10:40:02 -0600 Return-Path: Sender: std-cpp-request@vandevoorde.com Approved: stephen.clamage@oracle.com Message-ID: Newsgroups: comp.std.c++ From: Joe Gottman Subject: Re: templates, lambda, and dependent names Organization: EasyNews, UseNet made Easy! References: <2621c508-cc6e-40f0-8298-3f6ef3fc9fec@c2g2000yqc.googlegroups.com> Content-Type: text/plain; charset=UTF-8; format=flowed X-Original-Date: Sun, 09 Jan 2011 09:18:56 -0500 X-Submission-Address: std-c++-submit@vandevoorde.com To: undisclosed-recipients:; Date: Sun, 9 Jan 2011 10:38:05 CST Lines: 76 X-Usenet-Provider: http://www.giganews.com X-Trace: sv3-BPNmSKW4NXoSaOlg3o6YFmv8v+nnWAHe2npS6stF3P5yj2tcAWvSdp6TaQzZVbbdticF4dLe+iWi0Yq!uzcGdVTYspvaS6vfVk19po33POQ5S5xivrxyH01DoCkzcL5o7vBIqfRP2Z/FjnoDguda+ajp3KPH!U620kbaP X-Complaints-To: abuse@giganews.com X-DMCA-Notifications: http://www.giganews.com/info/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 X-Original-Bytes: 4155 Xref: g2news2.google.com comp.std.c++:3107 On 1/8/2011 8:40 PM, Inconnu wrote: > > On 5 янв, 22:33, Noah Roberts wrote: >> I've run into something I did not expect when trying to create a lambda >> function within a templated function, namely dependent names are not >> behaving as I would expect. I'm trying to find out if I'm working with >> a bad implementation or if my intuition and experience fail in this >> case. >> >> template< typename T> >> function f() >> { >> return []() { typedef typename T::type type; }; >> >> } >> >> This one fails for two reasons: 1) the compiler insists that typename is >> invalid here and 2) it doesn't see the T symbol. Taking the typename >> out gets rid of the first error. >> >> template< typename T> >> struct get_type { typedef typename T::type type; }; >> >> template< typename T> >> function f() >> { >> return [](){ typedef typename get_type::type type; }; >> >> } >> >> This one only fails because the compiler insists that typename can't be >> used here. Getting rid of that and the program compiles and works >> correctly. >> >> My original code uses boost's function, but afaict that wouldn't make >> any difference. >> >> I've tried various other places before coming here. Everyone is >> guessing though. Best reply so far is this one:http://stackoverflow.com/questions/4589056/templates-typename-lambda- >> dependent-names-not-dependent/4593286#4593286 >> >> I'm looking for something definitive though. Is "icecrime's" answer >> correct? >> >> The compiler is MSVC2010 but I'm looking for what the standard specifies >> here. >> >> Thanks. >> > > Yes, "icecrime's" answer is correct, "Motti's" answer is wrong. The > standard though, as far as I understand, doesn't permit now an > implicit conversion from lambda expression to function<...> or any > other user-written class. And, as far as MSVC is concerned, it looks > very much like that it doesn't parse template function bodies at all, > it parses them during instantiation, substituting template parameters > with their values, that's why it isn't strange that 'typenames' only > hamper it to work. Actually, I think it does. std::function has the non-explicit constructor template function(F f) I'm pretty sure that if the parameter types and return types of the lambda function match those of the function template that implicit conversion will occur. Joe Gottman -- [ comp.std.c++ is moderated. To submit articles, try posting with your ] [ newsreader. If that fails, use mailto:std-cpp-submit@vandevoorde.com ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ]