220 31768 <0d3f5162-b2f9-4389-a0ec-4e3928492fa3@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Complexities caused by using unique_ptr and move semantics Date: Sun, 26 Mar 2017 18:12:05 -0700 (PDT) Lines: 103 Approved: news@gmane.org Message-ID: <0d3f5162-b2f9-4389-a0ec-4e3928492fa3@isocpp.org> References: <46a85060-631b-49e5-94f3-ab07429d8085@isocpp.org> <86aeb556-e0d7-4d68-b553-1b4163461473@isocpp.org> <136fc743-bc63-48c8-b1d5-dc4084dd13cf@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2792_1819910155.1490577125313" X-Trace: blaine.gmane.org 1490577129 25820 195.159.176.226 (27 Mar 2017 01:12:09 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 27 Mar 2017 01:12:09 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBZON4HDAKGQEOQP2DWY@isocpp.org Mon Mar 27 03:12:04 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vk0-f70.google.com ([209.85.213.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1csJCT-0005qG-6O for gclcip-std-proposals@m.gmane.org; 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Sun, 26 Mar 2017 18:12:06 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.30.168 with SMTP id n37ls11994478otn.25.gmail; Sun, 26 Mar 2017 18:12:05 -0700 (PDT) X-Received: by 10.157.82.72 with SMTP id q8mr1544524otg.4.1490577125812; Sun, 26 Mar 2017 18:12:05 -0700 (PDT) In-Reply-To: X-Original-Sender: jmckesson@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:31768 Archived-At: ------=_Part_2792_1819910155.1490577125313 Content-Type: multipart/alternative; boundary="----=_Part_2793_825116473.1490577125313" ------=_Part_2793_825116473.1490577125313 Content-Type: text/plain; charset=UTF-8 On Sunday, March 26, 2017 at 6:11:08 PM UTC-4, Sean Middleditch wrote: > > On Fri, Mar 24, 2017 at 3:25 PM, Nicol Bolas > wrote: > >> >>> Basically, you want to write something like: >>> >>> iterable foo::get_things() { return make_iterable(_things, >>> [](auto& p){ return *p; }); } >>> >>> >> That's a really bad tradeoff: type-erasing performance plus the cost of >> filtering, all just to gain some increased genericity. >> > > Not at all, because that'd be a template that gets fully inlined and > result in identical machine code (assuming a suitably capable optimizer). > How? `iterable` doesn't specify the type of iterator used to iterate over `_things`, nor does it specify the type of range that `_things` is. But it still has to store either the range or an iterator pair over that range. Therefore, the storage for those things must be type-erased in some fashion. And that requires some form of indirect call to move to the next item in the iterator, as well as to call the functor. Not to mention having to type-erase the functor itself. And type-erasure is the enemy of inlining. If that were an `iterable, function_type>`, then I could buy the inlining. But not so long as type erasure has to be used. -- 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/0d3f5162-b2f9-4389-a0ec-4e3928492fa3%40isocpp.org. ------=_Part_2793_825116473.1490577125313 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Sunday, March 26, 2017 at 6:11:08 PM UTC-4, Sea= n Middleditch wrote:
On Fri, Mar 24, 2017 at 3:25 PM, N= icol Bolas <jmck...@gmail.com> wrote:

Basically, you want to = write something like:

=C2=A0 iterable<T&>= ; foo::get_things() { return make_iterable(_things, [](auto& p){ return= *p; }); }


That's a really bad tradeoff: type-erasing performance plus the c= ost of filtering, all just to gain some increased genericity.

Not at all, because that'd be a tem= plate that gets fully inlined and result in identical machine code (assumin= g a suitably capable optimizer).
<= br>How? `iterable` doesn't specify the type of iterator used to iterate= over `_things`, nor does it specify the type of range that `_things` is. B= ut it still has to store either the range or an iterator pair over that ran= ge. Therefore, the storage for those things must be type-erased in some fas= hion. And that requires some form of indirect call to move to the next item= in the iterator, as well as to call the functor. Not to mention having to = type-erase the functor itself.

And type-erasure is the enemy of inli= ning.

If that were an `iterable<span<T>, function_type>`= , then I could buy the inlining. But not so long as type erasure has to be = used.

--
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