220 33173 <9434787.nHRs7m2U1a@tjmaciei-mobl1> article Path: news.gmane.org!.POSTED!not-for-mail From: Thiago Macieira Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Add a few missing things from the iterator library Date: Mon, 10 Jul 2017 23:14:03 -0700 Lines: 82 Approved: news@gmane.org Message-ID: <9434787.nHRs7m2U1a@tjmaciei-mobl1> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" X-Trace: blaine.gmane.org 1499753657 15246 195.159.176.226 (11 Jul 2017 06:14:17 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 11 Jul 2017 06:14:17 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRBLWZSHFQKGQESN5CPYI@isocpp.org Tue Jul 11 08:14:08 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f71.google.com ([209.85.215.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dUoQs-0003Bn-Ey for gclcip-std-proposals@m.gmane.org; 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[2a01:4f8:d13:f81:21c:14ff:fe01:12a3]) by mx.google.com with ESMTP id k4si8579350wmi.44.2017.07.10.23.14.06 for ; Mon, 10 Jul 2017 23:14:06 -0700 (PDT) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 2a01:4f8:d13:f81:21c:14ff:fe01:12a3 as permitted sender) client-ip=2a01:4f8:d13:f81:21c:14ff:fe01:12a3; Original-Received: from tjmaciei-mobl1.localnet (unknown [IPv6:2601:1c0:4501:5f9f:c87e:2c9:9bc3:4bac]) by gondolin.macieira.info (Postfix) with ESMTPSA id 75BB611A7A6 for ; Mon, 10 Jul 2017 23:14:05 -0700 (PDT) In-Reply-To: X-Original-Sender: thiago@macieira.org X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of thiago@macieira.org designates 2a01:4f8:d13:f81:21c:14ff:fe01:12a3 as permitted sender) smtp.mailfrom=thiago@macieira.org 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:33173 Archived-At: On segunda-feira, 10 de julho de 2017 22:21:10 PDT Giovanni Funchal wrote: > I agree with Daniel's point that the 1:1 relation between container and > iterator types is not currently mandated. However: > - I believe it only affects (3). Strictly speaking, yes. But (2) is also difficult to achieve. You're asking for a way to transform an iterator to const_iterator, but those types are often unrelated. For example, for libstdc++'s std::unordered_map, the two iterator classes are just aliases to std::__detail::_Node_const_iterator and std::__detail::_Node_iterator. A const iterator is not the same as a const_iterator. > - Due to std::iterator_traits, iterators know something about > their container (iterator_category) so there is already limited scope for > an iterator to be used in multiple containers. BTW, perhaps > std::iterator_traits::iterator_container would be a better place > for (3). First, the category is the category of the iterator, not of the container. Second, you cannot have a link back to the container, period. (3) is simply unachievable. Just think of this, what should this be std::iterator_traits::container choose: std::vector std::string std::basic_string really... std::array QVector QByteArray > - I'm not sure I understand your claim that existing code would break. I > envision it as a template specialized to std container's iterators and > other container libraries could add their own specializations similarly to > what is already done for example in std::hash. Right, if you're adding a new set of functions and new members in the template, existing generic code that isn't using those functions or those members cannot break. The problem is when people start using them, but their generic code works with only a few containers that have been updated. When someone else tries to use it with some non-updated container, it breaks. There needs to be a good reason to do what you're asking. > > I think 2 is possible though it may not necessarily have desirable > > properties. It can have a generic implementation that returns a > > std::const_iterator (where std::const_iterator is an adaptor > > template similar to std::move_iterator). It can be partially specialized > > on > > T* to return const T* and then container libraries can add custom > > overloads > > for their own iterator types that will be found via ADL, but only if the > > iterator type isn't T* or something like that. > > I like Brian's idea of std::const_iterator as an adaptor template. std::const_itererator is not std::const_iterator. That means that if you tried to use this in generic code that actually used the container's const_iterator, it would fail. Just try: void f(std::unordered_map::const_iterator); void g(std::unordered_map::iterator it) { f(std::make_const_iterator(it)); } -- Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org Software Architect - Intel Open Source Technology Center -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/9434787.nHRs7m2U1a%40tjmaciei-mobl1. .