220 33172 article Path: news.gmane.org!.POSTED!not-for-mail From: Giovanni Funchal Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Add a few missing things from the iterator library Date: Tue, 11 Jul 2017 06:21:10 +0100 Lines: 166 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="94eb2c12542a6c5121055403e038" X-Trace: blaine.gmane.org 1499750513 32300 195.159.176.226 (11 Jul 2017 05:21:53 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 11 Jul 2017 05:21:53 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDLLHL6CZMFBB3WASHFQKGQELG6GWMI@isocpp.org Tue Jul 11 07:21:48 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-it0-f69.google.com ([209.85.214.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dUncJ-00084y-9c for gclcip-std-proposals@m.gmane.org; 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[2607:f8b0:400d:c0d::232]) by mx.google.com with ESMTPS id v24si12656196qth.390.2017.07.10.22.21.50 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 10 Jul 2017 22:21:50 -0700 (PDT) Received-SPF: pass (google.com: domain of gafunchal@gmail.com designates 2607:f8b0:400d:c0d::232 as permitted sender) client-ip=2607:f8b0:400d:c0d::232; Original-Received: by mail-qt0-x232.google.com with SMTP id 32so90180958qtv.1 for ; Mon, 10 Jul 2017 22:21:50 -0700 (PDT) X-Received: by 10.237.41.225 with SMTP id o88mr9018995qtd.27.1499750510381; Mon, 10 Jul 2017 22:21:50 -0700 (PDT) Original-Received: by 10.237.33.227 with HTTP; Mon, 10 Jul 2017 22:21:10 -0700 (PDT) In-Reply-To: X-Original-Sender: gafunchal@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.b=QH63vd9G; spf=pass (google.com: domain of gafunchal@gmail.com designates 2607:f8b0:400d:c0d::232 as permitted sender) smtp.mailfrom=gafunchal@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:33172 Archived-At: --94eb2c12542a6c5121055403e038 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, Jul 10, 2017 at 7:49 PM, Brian Bi wrote: > 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 : >> > 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 missin= g. >> > Container requirements (23.2) specify that iterators must be >> convertible 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 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). - 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). - 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. > 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 overloa= ds > 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. Thanks, -- Giovanni --=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/CAKDaRuKMi7w3umi3JZ%3D44Vypc3C_%2BCFiVpE9dHC4Ys-= uf0UDFg%40mail.gmail.com. --94eb2c12542a6c5121055403e038 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
= On Mon, Jul 10, 2017 at 7:49 PM, Brian Bi <bbi5291@gmail.com> wrote:
On Mon, Jul 10, 2017 at 11:21 AM, D= aniel 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 agree wi= th Daniel's point that the 1:1 relation between container and iterator = types is not currently mandated. However:
- I believe it only aff= ects (3).
- Due to std::iterator_traits<iterator>, iterator= s know something about their container (iterator_category) so there is alre= ady limited scope for an iterator to be used in multiple containers. BTW, p= erhaps std::iterator_traits<iterator>::iterator_container would be a = better place for (3).
- I'm not sure I understand your cl= aim that existing code would break. I envision it as a template specialized= to std container's iterators and other container libraries could add t= heir own specializations similarly to what is already done for example in s= td::hash.
=C2=A0
I think 2 is possibl= e though it may not necessarily have desirable properties. It can have a ge= neric implementation that returns a std::const_iterator<Iterator> (wh= ere std::const_iterator is an adaptor template similar to std::move_iterato= r). It can be partially specialized on T* to return const T* and then conta= iner libraries can add custom overloads for their own iterator types that w= ill be found via ADL, but only if the iterator type isn't T* or somethi= ng like that.

<= /div>
I like Brian's idea of std::const_iterator as an adaptor temp= late.

Thanks,
-- Giovanni

--
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