220 33168 article Path: news.gmane.org!.POSTED!not-for-mail From: Brian Bi Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Add a few missing things from the iterator library Date: Mon, 10 Jul 2017 11:49:20 -0700 Lines: 128 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="001a1141c70c76c2260553fb0a53" X-Trace: blaine.gmane.org 1499712563 26044 195.159.176.226 (10 Jul 2017 18:49:23 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 10 Jul 2017 18:49:23 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDS5ZX5VU4NBBMMYR7FQKGQESKPUP4A@isocpp.org Mon Jul 10 20:49:18 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pg0-f72.google.com ([74.125.83.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dUdkD-0006Sr-MB for gclcip-std-proposals@m.gmane.org; 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[2607:f8b0:4003:c06::22c]) by mx.google.com with ESMTPS id q185si8791980oig.68.2017.07.10.11.49.21 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 10 Jul 2017 11:49:21 -0700 (PDT) Received-SPF: pass (google.com: domain of bbi5291@gmail.com designates 2607:f8b0:4003:c06::22c as permitted sender) client-ip=2607:f8b0:4003:c06::22c; Original-Received: by mail-oi0-x22c.google.com with SMTP id 191so82834475oii.2 for ; Mon, 10 Jul 2017 11:49:21 -0700 (PDT) X-Received: by 10.202.227.130 with SMTP id a124mr10507937oih.52.1499712561003; Mon, 10 Jul 2017 11:49:21 -0700 (PDT) Original-Received: by 10.74.56.14 with HTTP; Mon, 10 Jul 2017 11:49:20 -0700 (PDT) In-Reply-To: X-Original-Sender: bbi5291@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.b=NrMxtuXS; spf=pass (google.com: domain of bbi5291@gmail.com designates 2607:f8b0:4003:c06::22c as permitted sender) smtp.mailfrom=bbi5291@gmail.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=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:33168 Archived-At: --001a1141c70c76c2260553fb0a53 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, Jul 10, 2017 at 11:21 AM, Daniel Kr=C3=BCgler wrote: > 2017-07-10 19:47 GMT+02:00 : > > Hi, > > > > I propose to add to : > > [..] > > > 2) std::make_const_iterator(iterator) > > Semantics: return iterator static_cast-ed to const_iterator > > Why: We have std::make_move_iterator so again I believe this is missing= .. > > Container requirements (23.2) specify that iterators must be convertibl= e > to > > const_iterator so I believe this is just a static_cast. > > > > 3) std::container_of_t > > Semantics: determine the container type of the iterator > > Why: Helpful to implement 2 above, also helpful in a variety of > situations > > where there's a need to check requirements on the container of an > iterator. > > These two proposals make assumptions that are neither met by current > implementations nor implied by the standard: There is no 1:1 relation > between container type and iterator type, which makes both (2) and (3) > impossible. It would also break non-Standard-Library implementations > of containers such as from Boost or other third-party libraries. > 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. --=20 *Brian Bi* --=20 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 e= mail 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/CAMmfjbPSGa_AiSOieY9jo9FPY5EaZn852RGTMO85kV%2Bh7= CRMig%40mail.gmail.com. --001a1141c70c76c2260553fb0a53 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Mon, Jul 10, 2017 at 11:21 AM, Daniel Kr=C3=BCgler <daniel.= kruegler@gmail.com> wrote:
= 2017-07-10 19:47 GMT+02:00=C2=A0 <gafunchal@gmail.com>:
> Hi,
>
> I propose to add to <iterator>:

[..]

> 2) std::make_const_iterator(iterator)
> Semantics: return iterator static_cast-ed to const_iterator
> Why: We have std::make_move_iterator so again I believe this is missin= g.
> Container requirements (23.2) specify that iterators must be convertib= le to
> const_iterator so I believe this is just a static_cast.
>
> 3) std::container_of_t<decltype(iterator)>
> Semantics: determine the container type of the iterator
> Why: Helpful to implement 2 above, also helpful in a variety of situat= ions
> where there's a need to check requirements on the container of an = iterator.

These two proposals make assumptions that are neither met by current=
implementations nor implied by the standard: There is no 1:1 relation
between container type and iterator type, which makes both (2) and (3)
impossible. It would also break non-Standard-Library implementations
of containers such as from Boost or other third-party libraries.

I think 2 is possible though it may not necessari= ly have desirable properties. It can have a generic implementation that ret= urns a std::const_iterator<Iterator> (where std::const_iterator is an= adaptor template similar to std::move_iterator). It can be partially speci= alized 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 onl= y if the iterator type isn't T* or something like that.
= =C2=A0
--
Brian Bi
=

--
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