From -6318677877046408807 X-Google-Thread: f78e5,be8dba9addd8a207 X-Google-NewGroupId: yes X-Google-Attributes: gid7894ca11fe,domainid0,public,usenet X-Google-Language: ENGLISH,ASCII Received: by 10.68.201.132 with SMTP id ka4mr191365pbc.8.1338315943235; Tue, 29 May 2012 11:25:43 -0700 (PDT) MIME-Version: 1.0 Path: pr3ni63148pbb.0!nntp.google.com!news2.google.com!goblin1!goblin.stu.neva.ru!news.albasani.net!.POSTED!not-for-mail From: =?ISO-8859-1?Q?Daniel_Kr=FCgler?= Newsgroups: comp.std.c++ Subject: Re: Smart pointer comparison - why a difference? Date: Tue, 29 May 2012 11:25:39 -0700 (PDT) Organization: A noiseless patient Spider Lines: 39 Sender: std-cpp-request@vandevoorde.com Approved: stephen.clamage@oracle.com Message-ID: References: NNTP-Posting-Host: X/RiL1VE8fbEFzZRmEtibTMPTsBtapTp2vPC4/Q1Zro= X-Trace: news.albasani.net 5JFq4Edh/2LyWyAcZJ1znHwqybul1iWP9IXVaosvxJmNdqsRSzeZfV1LdTZqF+k+CWdqziY3l5+ZNHbJYyFNAA== X-Complaints-To: abuse@albasani.net NNTP-Posting-Date: Tue, 29 May 2012 18:25:42 +0000 (UTC) X-Mailer: Perl5 Mail::Internet v2.05 X-Submission-Address: std-cpp-submit@vandevoorde.com Cancel-Lock: sha1:4oRx8DZat6sNunM51nbXNoVkpA0= X-Original-Date: Tue, 29 May 2012 09:29:14 +0200 Content-Type: text/plain; charset=ISO-8859-1; format=flowed On 2012-05-28 23:53, Bo Persson wrote: > In C++11, operator< for unique_ptr [unique.ptr.special] is defined as > > std::less()(x.get(), y.get()) > > where CT is the common_type of the unique_ptr's internal pointer typedefs. > > > On the other hand, operator< for shared_ptr [util.smartptr.shared.cmp] > is defined as > > std::less()(a.get(), b.get()) > > "where V is the composite pointer type (5.9) of T* and U*." > > > What exactly is the difference, and why? The wording difference exists, because std::unique_ptr can hold both built-in pointers and pointer-like types while std::shared_ptr can only hold pointers. The intention is that unique_ptr's wording has exactly the same effect when it holds a pointer. Currently these definitions are not the same because of a wording defect of std::common_type. An issue has already been raised for this but is not yet visible in the official list. In short words, that defect is because std::common_type::type evaluates to int*&& and not int*. This was an unintended side-effect because of change of the original definition of the conditional operator in the presence of xvalues. HTH& Greetings from Bremen, Daniel Kr�gler -- [ 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 ]