From -2794511676994472821 X-Google-Thread: 7894ca11fe,bda36747929f8c7f X-Google-Attributes: gid7894ca11fe,public,usenet X-Google-NewGroupId: yes X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news1.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: Sat, 31 Oct 2009 22:10:15 -0500 Return-Path: To: (Usenet) From: "Johannes Schaub (litb)" Newsgroups: comp.std.c++ Subject: Re: How many parameters in T({x, y, z}) ? Followup-To: comp.std.c++ Organization: T-Online Sender: cppmods@cs.rpi.edu Approved: stephen.clamage@sun.com Message-ID: References: Content-Type: text/plain; charset="ISO-8859-1" X-Original-Date: Sat, 31 Oct 2009 17:58:35 +0100 X-Submission-Address: std-c++@netlab.cs.rpi.edu Date: Sat, 31 Oct 2009 22:06:31 CST Lines: 90 X-Usenet-Provider: http://www.giganews.com X-Trace: sv3-SxbfPwXX9LEJ0e1HwGQVlzk6G0GiRjPcS/aD50xMleSGt1P055cRDguiHtCrN9wQ6DpKb6XcrsoBRXk!YLjtbfBKePO90PDOSgRrWZmCHuFPUtI8d65JXrU1vAt+5rn+n4aJuz3TrMBu 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 Xref: g2news2.google.com comp.std.c++:1624 Scott Meyers wrote: > Consider the following attempt to construct a Widget object: > > Widget w( {5, 10, 15} ); > > How many arguments are being passed to the constructor? I say one, and > gcc 4.4 seems to agree with me: > > #include > #include > > class Widget { > public: > Widget(int, int, int) { std::cout << "3 ints\n"; } > private: > Widget(const Widget&) { std::cout << "copy\n"; } > }; > > int main() > { > Widget w ({ 10, 20, 30 }); // error! copy ctor is private > } > > Presumably the compiler is trying to create a temporary from the 3-int > constructor to pass to the copy constructor, which it thinks I'm trying to > call by passing only one constructor argument. > > If this is the case, adding a constructor taking an initializer_list > should make > no difference: I'd still be calling a constructor with a single argument. > The initializer_list constructor would be called to create the temporary, > but I'd > still have to have access to the copy constructor. But that's not what > gcc does: > > #include > #include > > class Widget { > public: > Widget(int, int, int) { std::cout << "3 ints\n"; } > Widget(std::initializer_list) { std::cout << "init list\n"; } > private: > Widget(const Widget&) { std::cout << "copy\n"; } > }; > > int main() > { > Widget w ({ 10, 20, 30 }); // compiles, runs, prints "init list" > } > > Is this a bug in my understanding or a bug in gcc's implementation? > There is a subtle difference that i think is worth noting so it's clear: Widget w{10, 20, 30}; Is different from the following Widget w({10, 20, 30}); The first is list-initialization, and will end up in 13.3.1.7 for finding candidates, and the second is direct initialization (because the initializer is a braced expression list), and ends up at 13.3.1.3. So in fact the argument in your case consists of 1 instead of 3 arguments - so only the copy constructor is viable. For this, 13.3.3.1.5 does the mapping to the three forms of implicit conversion sequences, and selects 13.3.3.1.5/5, and the reference binding will make the conversion sequence a user defined conversion sequence by going back to 13.3.3.1.5/3. Now, if you add the initializer list, there are two viable candidate functions, instead of only one: The initializer list constructor, and the copy constructor. 13.3.3.1.5 maps the conversion sequence for the initializer list constructor to an identity conversion by 13.3.3.1.5/2. In this case, the copy constructor is still viable, but reference binding is going back to 13.3.3.1.5/3 again (because it converts the initializer list to "Widget" - *not* to initializer_list which is merely a parameter of the constructor used in the user defind conversion sequence. Since using the initializer list ctor results in a better conversion sequence, it's used over the copy constructor by GCC. I think GCC does it all right here. -- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@netlab.cs.rpi.edu] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ]