220 14524 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: Thu, 13 Nov 2014 11:33:35 -0600 Lines: 177 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=001a113dd29c7f9ce60507c0e93d X-Trace: ger.gmane.org 1415900027 17147 80.91.229.3 (13 Nov 2014 17:33:47 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 13 Nov 2014 17:33:47 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCHLPXGUXMJBB4GWSORQKGQE7SKLHBQ@isocpp.org Thu Nov 13 18:33:40 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ig0-f200.google.com ([209.85.213.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1XoyH3-0001tt-N3 for gclcip-std-proposals@m.gmane.org; Thu, 13 Nov 2014 18:33:37 +0100 Original-Received: by mail-ig0-f200.google.com with SMTP id h15sf417860igd.3 for ; Thu, 13 Nov 2014 09:33:37 -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=LhgY1/oG4csbhXTabyjUOxsntA/xhIcPh9dAOzLl9FI=; b=COToh6h37pDDGfFRmuXETJedgzflZ2S+kCewRf7DioLe/YMR3RCYsP4lbVbHd/OaTL K8imzd00Tvb/zv6yM5urGcY+DR61D+VzwCurbCahu67k0Ocndve5ISHZW8YuIeBhxrIN ev3Oh99bi4PRlb6oLFDQjoDKNRvAIc1pmelT6pI19RVNyXpgYOgWBcBS3lErTGyDnKIo TY7GWf+ccCzCwekq/b+DPndiOubE99loKLsh/V8AUMGlrYnojlx980iSkx8gdW3eHEDX E77ZeNN6rItv2gR8fQow3piU/0K+ibVINdXEdaXXeMowQe1cogIGjVeoVPqPFBTiMxbN Jpeg== X-Gm-Message-State: ALoCoQnxGBjdx/fZP3Etd2P+PL3Be/5GgmgTfLWhwwhh5FnU6D0LYfsnRbgZf1FSDLDGI6zxHGSu X-Received: by 10.50.3.97 with SMTP id b1mr296115igb.4.1415900016962; Thu, 13 Nov 2014 09:33:36 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.233.198 with SMTP id ty6ls1257636obc.76.gmail; Thu, 13 Nov 2014 09:33:36 -0800 (PST) X-Received: by 10.202.98.195 with SMTP id w186mr234653oib.104.1415900016314; Thu, 13 Nov 2014 09:33:36 -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 l12si30277568oer.70.2014.11.13.09.33.36 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 13 Nov 2014 09:33:36 -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 nt9so12228908obb.29 for ; Thu, 13 Nov 2014 09:33:36 -0800 (PST) X-Received: by 10.202.67.85 with SMTP id q82mr2926071oia.35.1415900016121; Thu, 13 Nov 2014 09:33:36 -0800 (PST) Original-Received: by 10.202.232.77 with HTTP; Thu, 13 Nov 2014 09:33:35 -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:14524 Archived-At: --001a113dd29c7f9ce60507c0e93d Content-Type: text/plain; charset=UTF-8 I believe I was misinterpreting your intent. I had interpreted &begin() as a *begin() typo, not a &*begin() typo. If the intent is to return pointer-to-dereferenced-value, then that creates a separate issue. If *begin() returns say int (rather than int&) then that would return pointer-to-temporary-on-the-stack. >Brent, that's basically *what I wrote*, except using T* rather than optional. Certainly it's the same general idea. I agree. On Thu, Nov 13, 2014 at 10:23 AM, Matthew Woehlke < mw_triad@users.sourceforge.net> wrote: > On 2014-11-12 21:31, Brent Friedman wrote: > >> It doesn't assume that the iterators are pointers, if that's what you > >> mean. > > > > It assumes that it is possible to convert from nullptr_t to > > decltype(&range.begin()). > > Oops, yes, there is a '*' missing: 'decltype(&*range.begin())' (or > std::addressof as Benjamin suggested). This should be obvious > considering the RHS of the 'return'. > > I am confused; I am not returning the iterator. I am returning the > address of the iterator's value. While I would believe that might not be > possible in all cases, I don't see how it relates to the cases you are > concerned about. (Also, my C++11-fu is not that strong; there may be a > way to return the iterator value as a form of reference that would > bypass these issues also.) > > On 2014-11-12 21:52, Brent Friedman wrote: > > A function like first_or_null which returns an optional could > probably > > work alright for our purposes with only a bit of extra syntax over this > > range-if idea. > > Brent, that's basically *what I wrote*, except using T* rather than > optional. Certainly it's the same general idea. > > > It also assumes that this value constructed from nullptr necessarily > > represents an invalid value (converts to false) for that iterator type. > > Huh? The check is on begin == end. No other assumptions are made about > the behavior of begin except that operator==(end) will return true if > the range is not valid. Since AFAIUI this is how range-based for also > works, if this assumption is being violated I'd think you'd have other > problems... > > -- > 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/. --001a113dd29c7f9ce60507c0e93d Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
I believe I was misinterpreting your intent. I had interpr= eted &begin() as a *begin() typo, not a &*begin() typo. If the inte= nt is to return pointer-to-dereferenced-value, then that creates a separate= issue. If *begin() returns say int (rather than int&) then that would = return pointer-to-temporary-on-the-stack.=C2=A0

>Brent, that's b= asically *what I wrote*, except using T* rather than=C2=A0optional<T&>. C= ertainly it's the same general idea.
I agree.

On Thu, Nov 13, 2014 a= t 10:23 AM, Matthew Woehlke <mw_triad@users.sourceforge.net> wrote:
On = 2014-11-12 21:31, Brent Friedman wrote:
>> It doesn't assume that the iterators are pointers, if that'= ;s what you
>> mean.
>
> It assumes that it is possible to convert from nullptr_t to
> decltype(&range.begin()).

Oops, yes, there is a '*' missing: 'decltype(&*range= ..begin())' (or
std::addressof as Benjamin suggested). This should be obvious
considering the RHS of the 'return'.

I am confused; I am not returning the iterator. I am returning the
address of the iterator's value. While I would believe that might not b= e
possible in all cases, I don't see how it relates to the cases you are<= br> concerned about. (Also, my C++11-fu is not that strong; there may be a
way to return the iterator value as a form of reference that would
bypass these issues also.)

On 2014-11-12 21:52, Brent Friedman wrote:
> A function like first_or_null which returns an optional<T&> = could probably
> work alright for our purposes with only a bit of extra syntax over thi= s
> range-if idea.

Brent, that's basically *what I wrote*, except using T* rather t= han
optional<T&>. Certainly it's the same general idea.

> It also assumes that this value constructed from nullptr necessarily > represents an invalid value (converts to false) for that iterator type= ..

Huh? The check is on begin =3D=3D end. No other assumptions are made= about
the behavior of begin except that operator=3D=3D(end) will return true if the range is not valid. Since AFAIUI this is how range-based for also
works, if this assumption is being violated I'd think you'd have ot= her
problems...

--
Matthew

--

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