220 14494 article Path: news.gmane.org!not-for-mail From: Brent Friedman Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Range-based if Date: Tue, 11 Nov 2014 20:39:32 -0600 Lines: 212 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=001a1130c512468fb00507a04e91 X-Trace: ger.gmane.org 1415759983 29159 80.91.229.3 (12 Nov 2014 02:39:43 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 12 Nov 2014 02:39:43 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCHLPXGUXMJBBZMQRORQKGQEX42C3LA@isocpp.org Wed Nov 12 03:39:36 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f69.google.com ([209.85.220.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1XoNqI-0004AA-PQ for gclcip-std-proposals@m.gmane.org; Wed, 12 Nov 2014 03:39:35 +0100 Original-Received: by mail-pa0-f69.google.com with SMTP id eu11sf69945731pac.4 for ; Tue, 11 Nov 2014 18:39:34 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:date :message-id:subject:from:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=nOYdCeelEX84X4K+5VSjBsDNx8VnMDPz1zdimcWrn+k=; b=OU3t7U7iZinS3PHn7fLnVA1x3kbzVkKUq/LDzu5LiyNLuuNxGi0vKnXTWOwOQZeGNg snX47y99gg298QqsU8nhXRu3JnGzbdt4Zq25CZYjx0hM+NCT6G3hovGusenNBMEgrgZ4 +nlxOMT5aQrjQzPEhDjdqN0uMZOTHVqWV2sMFxkt8OpDb7ZDnF3qGlr+Q1jSnJsWSF0H fuiL9LjdJTlWepx5+vR4C7pEh0BxxUGE0l0IUtLhoXV8n8gLb3eyhTpOzshIiG79i008 FsuS7J1K/EkNbufgoHdlfRQ80zrQHP9CHMsSEU9ownnPwCjcZcxpZzAZEAFeaqNblzZo zVjA== X-Gm-Message-State: ALoCoQmbHRDV784Y3o5YFpUKJNVwavPlXxrMf99lZti3GPu3Chx/KtL34JIcn5iZlneQ9pJEQp3I X-Received: by 10.66.250.165 with SMTP id zd5mr33838517pac.27.1415759974024; Tue, 11 Nov 2014 18:39:34 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.245.131 with SMTP id xo3ls956153obc.50.gmail; Tue, 11 Nov 2014 18:39:33 -0800 (PST) X-Received: by 10.182.125.3 with SMTP id mm3mr35168927obb.7.1415759973171; Tue, 11 Nov 2014 18:39:33 -0800 (PST) Original-Received: from mail-ob0-x22a.google.com (mail-ob0-x22a.google.com. [2607:f8b0:4003:c01::22a]) by mx.google.com with ESMTPS id cr8si25511311oec.91.2014.11.11.18.39.33 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 11 Nov 2014 18:39:33 -0800 (PST) Received-SPF: pass (google.com: domain of fourthgeek@gmail.com designates 2607:f8b0:4003:c01::22a as permitted sender) client-ip=2607:f8b0:4003:c01::22a; Original-Received: by mail-ob0-f170.google.com with SMTP id nt9so9169623obb.15 for ; Tue, 11 Nov 2014 18:39:33 -0800 (PST) X-Received: by 10.60.245.99 with SMTP id xn3mr36467640oec.15.1415759972960; Tue, 11 Nov 2014 18:39:32 -0800 (PST) Original-Received: by 10.202.232.77 with HTTP; Tue, 11 Nov 2014 18:39:32 -0800 (PST) In-Reply-To: X-Original-Sender: fourthgeek@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of fourthgeek@gmail.com designates 2607:f8b0:4003:c01::22a as permitted sender) smtp.mail=fourthgeek@gmail.com; dkim=pass header.i=@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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:14494 Archived-At: --001a1130c512468fb00507a04e91 Content-Type: text/plain; charset=UTF-8 > auto first_or_null(auto range) -> decltype(&range.begin()) This assumes pointers, which while addressing some use cases it isn't sufficient for the general case. > if (!range.empty()) > DoSomething(*range.begin()); This is more or less what we're doing currently. If you need to do more than one something, then it becomes if (!range.empty()) { auto&& front = *range.begin(); DoSomething( front ); //etc } 'range' may of course be an rvalue, perhaps the result of a function call. We do this a lot. Currently, we'll have to create a variable to store 'range'. So introducing an additional scope would be wise to contain 'range'. { auto& range = GetTheRange(); if (!range.empty()) { auto& front = *range.begin(); DoSomething( front ); //etc } } In the face of this worst-case alternative, I think it's only reasonable that someone would prefer the range-for-with-a-break. Anecdotally, in our codebase of a million or so LOC, this pattern of using a looping structure to get the first element in a range constituted about 70% of our unreachable code instances. On Tue, Nov 11, 2014 at 3:34 PM, Matthew Woehlke < mw_triad@users.sourceforge.net> wrote: > On 2014-11-11 16:12, Brent Friedman wrote: > > I've noted in our codebase that we lean heavily on the convenience of > > range-based for even when it's not perhaps the best way to express an > idea. > > > > //do something to the first item > > for (auto& Item : range) > > { > > DoSomething(Item); > > break; > > } > > auto first_or_null(auto range) -> decltype(&range.begin()) > { > auto begin = range.begin(); > auto end = range.end(); > return (begin == end ? nullptr : &(*begin)); > } > > if (auto item = first_or_null(range)) > > Basically you want that, right? :-) > > I'm not sure if that actually works, and it requires an additional > dereference when you go to use 'item', but OTOH it wouldn't require a > language change... > > That said, how often can you not write: > > if (!range.empty()) > DoSomething(*range.begin()); > > ...? > > -- > Matthew > > -- > > --- > 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. > Visit this group at > http://groups.google.com/a/isocpp.org/group/std-proposals/. > -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. --001a1130c512468fb00507a04e91 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
>=C2=A0auto first_or_null(auto range) -> decltype(&range.begin(= ))

This assumes pointers, which while addressing = some use cases it isn't sufficient for the general case.

=
>= =C2=A0 =C2=A0 if (!range.empty())
> =C2=A0 =C2=A0 =C2=A0 =C2=A0DoSomething(= *range.begin());

This is more or less what we'= ;re doing currently. If you need to do more than one something, then it bec= omes

if (!range.empty())
{
=C2= =A0 =C2=A0 auto&& front =3D *range.begin();
=C2=A0 =C2=A0= DoSomething( front );
=C2=A0 =C2=A0 //etc
}
=
'range' may of course be an rvalue, perhaps the resu= lt of a function call. We do this a lot. Currently, we'll have to creat= e a variable to store 'range'. So introducing an additional scope w= ould be wise to contain 'range'.

{
=C2=A0 auto& range =3D GetTheRange();
=C2=A0 if (!range.em= pty())
=C2=A0 {
=C2=A0 =C2=A0 =C2=A0 auto& front = =3D *range.begin();
=C2=A0 =C2=A0 =C2=A0 DoSomething( front );
=C2=A0 =C2=A0 =C2=A0 //etc
=C2=A0 }
}

In the face of this worst-case alternative, I think it'= s only reasonable that someone would prefer the range-for-with-a-break.

Anecdotally, in our codebase of a million or so LOC, = this pattern of using a looping structure to get the first element in a ran= ge constituted about 70% of our unreachable code instances.
=C2= =A0

On= Tue, Nov 11, 2014 at 3:34 PM, Matthew Woehlke <mw_triad@user= s.sourceforge.net> wrote:
<= span class=3D"">On 2014-11-11 16:12, Brent Friedman wrote:
> I've noted in our codebase that we lean heavily on the convenience= of
> range-based for even when it's not perhaps the best way to express= an idea.
>
> //do something to the first item
> for (auto& Item : range)
> {
>=C2=A0 =C2=A0 =C2=A0DoSomething(Item);
>=C2=A0 =C2=A0 =C2=A0break;
> }

=C2=A0 =C2=A0 auto first_or_null(auto range) -> decltype(&ran= ge.begin())
=C2=A0 =C2=A0 {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 auto begin =3D range.begin();
=C2=A0 =C2=A0 =C2=A0 =C2=A0 auto end =3D range.end();
=C2=A0 =C2=A0 =C2=A0 =C2=A0 return (begin =3D=3D end ? nullptr : &(*beg= in));
=C2=A0 =C2=A0 }

=C2=A0 =C2=A0 if (auto item =3D first_or_null(range))

Basically you want that, right? :-)

I'm not sure if that actually works, and it requires an additional
dereference when you go to use 'item', but OTOH it wouldn't req= uire a
language change...

That said, how often can you not write:

=C2=A0 =C2=A0 if (!range.empty())
=C2=A0 =C2=A0 =C2=A0 =C2=A0 DoSomething(*range.begin());

....?

--
Matthew

--

---
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-propo= sals+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
Visit this group at http://groups.google.com/a/isocpp.org/gro= up/std-proposals/.

--

---
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/<= /a>.
--001a1130c512468fb00507a04e91-- .