220 25352 article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allowing uncopyable allocators Date: Sun, 27 Mar 2016 00:59:06 +0800 Lines: 179 Approved: news@gmane.org Message-ID: References: <657cb0a7-4854-41c6-963f-5a11cc677157@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 9.1 \(3096.5\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_BA5DEE2C-01DA-445B-B5C1-7F78B7413E42" X-Trace: ger.gmane.org 1459011570 7457 80.91.229.3 (26 Mar 2016 16:59:30 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 26 Mar 2016 16:59:30 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBZX73K3QKGQEDM5IFYY@isocpp.org Sat Mar 26 17:59:22 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qg0-f72.google.com ([209.85.192.72]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ajrYX-0005Wu-DT for gclcip-std-proposals@m.gmane.org; Sat, 26 Mar 2016 17:59:21 +0100 Original-Received: by mail-qg0-f72.google.com with SMTP id c67sf118895870qgc.0 for ; Sat, 26 Mar 2016 09:59:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=from:message-id:mime-version:subject:date:references:to:in-reply-to :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=NtXOM1pUPohPIGx74Uu6+PQs3WyAPbloNj5P9UMIhT8=; b=OWfbT+bDbn8gI5/uLjjs9SWMRF9F+iWWFhLKBiMzaY7wfDHAwipxhFGMCyt6HHTEFq dwnkfzmBBYCIUpUk4d9X/XX1W10taGvIyYrLXQ3+NdF/u1uIARZEw4IX06ADqZIdB33E SM4V8qckFWsal5a5wm9zLK9P/iwjOr96HRY1+tOtfsHLF+XbzqxN+9eAOrY7GcQq2Z5d 86PvFKUPmUWeHmt1DJysXGXj6N87Og9fx5X5US/l3Pw60amYk6CgCsWVryiYKgvIG/Xg 0XtHFrmQ5RxtXomRoMnHU7ZRR+eEzoA+0uj5apiarCY6Zu/9oKGtzFWvqWWWw6q4ovDp KteQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:message-id:mime-version:subject:date :references:to:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=NtXOM1pUPohPIGx74Uu6+PQs3WyAPbloNj5P9UMIhT8=; b=D8w3gHl7XcTktnONOWb5rjKUHLXwhWaNMFo4TGibzUNLjVPe4i3ow7TFPXUsUWYBzI NQvhZu6wKHiG9TTHb2VjLo97ScZ5TnzFAjrQI85VfTzjSXX+9ontehejPko+9mfnK0GS kqv/+We+SguATkjE91PeH0gw2PMsnEWdmvA7xUwJ4e7FTQUzSNm0yqWQl1VH9jg1/HNw Ff+ZzHj/RATL8XpqvtFFl/uftwy5nPWpGUt6D4XYfAeuO5gQUBBcrfb2wS7uUysbL/2+ 1T45eyMJUbVhCbblzcDs30GHpqoOF65tN0PZzrBhkOoBlecQO/n7TVUUeUD8WYGgjZ+L SuJw== X-Gm-Message-State: AD7BkJJO0zOtnu1m0u2l/MrhsHHkIKa1LluMgqKnmVTrxzgl5mHoiBurFXg8QuoggZyl3Q== X-Received: by 10.140.97.244 with SMTP id m107mr12375088qge.20.1459011559704; Sat, 26 Mar 2016 09:59:19 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.33.85 with SMTP id h82ls993818ioh.63.gmail; Sat, 26 Mar 2016 09:59:18 -0700 (PDT) X-Received: by 10.98.74.17 with SMTP id x17mr30049380pfa.14.1459011558522; Sat, 26 Mar 2016 09:59:18 -0700 (PDT) Original-Received: from mail-pf0-x243.google.com (mail-pf0-x243.google.com. [2607:f8b0:400e:c00::243]) by mx.google.com with ESMTPS id q75si2221571pfq.207.2016.03.26.09.59.18 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 26 Mar 2016 09:59:18 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c00::243 as permitted sender) client-ip=2607:f8b0:400e:c00::243; Original-Received: by mail-pf0-x243.google.com with SMTP id x3so15189347pfb.0 for ; Sat, 26 Mar 2016 09:59:18 -0700 (PDT) X-Received: by 10.98.15.142 with SMTP id 14mr29927044pfp.6.1459011558201; Sat, 26 Mar 2016 09:59:18 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.140]) by smtp.gmail.com with ESMTPSA id f8sm24253765pfj.49.2016.03.26.09.59.14 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Sat, 26 Mar 2016 09:59:17 -0700 (PDT) In-Reply-To: X-Mailer: Apple Mail (2.3096.5) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c00::243 as permitted sender) smtp.mailfrom=potswa@gmail.com; dmarc=pass (p=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-Spam-Checked-In-Group: std-proposals@isocpp.org 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:25352 Archived-At: --Apple-Mail=_BA5DEE2C-01DA-445B-B5C1-7F78B7413E42 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 > On 2016=E2=80=9301=E2=80=9317, at 7:38 AM, quicknir@gmail.com wrote: >=20 > But what if your allocators want to own their own resources? Such allocat= ors can never really allocate or deallocate each other's pointers, so they = can never compare equal. This sounds like a big deal, but if your allocator= is uncopyable, then it's not an issue; there's no real situation where you= would ever necessarily expect them to compare equal. It would be nice to see this avenue pursued. It would allow =E2=80=9Csmall= =E2=80=9D or =E2=80=9Clocal=E2=80=9D vectors to be specializations of std::= vector. Locally-optimized versions of all the standard containers would app= ear for minimal effort. > On 2016=E2=80=9301=E2=80=9317, at 12:12 PM, Zhihao Yuan w= rote: >=20 > There won't be any issue after >=20 > http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2015/p0135r0.html >=20 > being accepted. The interface for initializing and retrieving the internal allocator would = need to be revised, at least for the new local allocators. It would be nice= to even remove it, but querying the remaining capacity is too useful. (Regarding =E2=80=9Cnever equal,=E2=80=9D pedantically, if you can obtain a= reference to a local allocator, then it should compare equal to itself.) > On 2016=E2=80=9301=E2=80=9317, at 3:50 PM, quicknir@gmail.com wrote: >=20 > Not sure what polymorphic means in this context (polymorphic allocator?), Polymorphic allocators exist on the opposite end of the overall problem dom= ain of allocators. Every allocator object gets reduced to a polymorphic poi= nter, and every allocation and deallocation is dispatched as a virtual func= tion. This helps interoperability at the expense of performance. It=E2=80= =99s part of the Fundamentals TS; it may or may not be adopted by C++17. > I've literally seen examples of people rewriting std::function from scrat= ch so that they could change the size of the small function optimization be= cause heap allocation is a major issue for them. This seems to be fairly common. Unfortunately, the small function optimizat= ion takes place outside the allocator. You could perhaps hack a local alloc= ator into my function_container te= mplate. But you=E2=80=99d want to forbid converting that back to an ordinar= y function, or else do something clever like heap-allocate the local alloca= tor. The library doesn=E2=80=99t provide for that. Also, the ordinary small= -function optimization would be made redundant=E2=80=A6 it=E2=80=99s possib= le that this is an entirely different problem. > On 2016=E2=80=9303=E2=80=9326, at 11:44 PM, Yun Chen wrote: >=20 > We should extend it to move construction through select_on_container_move= _construction, as we need the ability of default constructing the allocator= too. This would be required to move construct an container in an arena fro= m stack. =20 =E2=80=A6 and lest it be overlooked, if these allocators aren=E2=80=99t cop= yable, they=E2=80=99re not movable either. --=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/C2CA1C21-42FC-448F-BCEE-F6B4F3EA8E55%40gmail.com= .. --Apple-Mail=_BA5DEE2C-01DA-445B-B5C1-7F78B7413E42 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
On = 2016=E2=80=9301=E2=80=9317, at 7:38 AM, quicknir@gmail.com wrote:

But what if your allocators want to own their own re= sources? Such allocators can never really allocate or deallocate each other= 's pointers, so they can never compare equal. This sounds like a big deal, = but if your allocator is uncopyable, then it's not an issue; there's no rea= l situation where you would ever necessarily expect them to compare equal.<= /span>

It would be nice to see this avenue pursued. It would all= ow =E2=80=9Csmall=E2=80=9D or =E2=80=9Clocal=E2=80=9D vectors to be special= izations of std::vector. Locally-o= ptimized versions of all the standard containers would appear for minimal e= ffort.


On 2016=E2=80=9301=E2=80=9317, at 12:12 PM, Zhihao Yuan = <zy@miator.net> wrote= :

There won't be any issue after

 http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2015/p013= 5r0.html

being accepted.

The interface fo= r initializing and retrieving the internal allocator would need to be revis= ed, at least for the new local allocators. It would be nice to even remove = it, but querying the remaining capacity is too useful.

(Regarding =E2=80=9Cnever equal,=E2= =80=9D pedantically, if you can obtain a reference to a local allocator, th= en it should compare equal to itself.)

=

On 2016=E2=80=9301=E2=80=9317, at 3:50 PM, quicknir@gmail.com wrote:


Polymorphic allocators exist on the opposite end of the overall probl= em domain of allocators. Every allocator object gets reduced to a polymorph= ic pointer, and every allocation and deallocation is dispatched as a virtua= l function. This helps interoperability at the expense of performance. It= =E2=80=99s part of the Fundamentals TS; it may or may not be adopted by C++= 17.

I've= literally seen examples of people rewriting std::function from scratch so = that they could change the size of the small function optimization because = heap allocation is a major issue for them.

This seems to be fairly common. Unfortunately= , the small function optimization takes place outside the allocator. You co= uld perhaps hack a local allocator into my functi= on_container template. But you=E2=80=99d want to forbid con= verting that back to an ordinary function= , or else do something clever like heap-allocate the local allocator= .. The library doesn=E2=80=99t provide for that. Also, the ordinary small-fu= nction optimization would be made redundant=E2=80=A6 it=E2=80=99s possible = that this is an entirely different problem.


On 2016=E2=80=9303=E2=80=9326, a= t 11:44 PM, Yun Chen <yun.go.chen@gmail.com> wrote:

We should extend it to move constr= uction through select_on_container_move_construction, as we need the abilit= y of default constructing the allocator too. This would be required to move= construct an container in an arena from stack.  

=E2=80=A6 and lest it be overlooke= d, if these allocators aren=E2=80=99t copyable, they=E2=80=99re not movable= either.

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