220 11589 <29988A30-548A-47F3-A275-9B6099329877@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Teachability problems with std::move Date: Sat, 28 Jun 2014 06:30:17 +0800 Lines: 171 Approved: news@gmane.org Message-ID: <29988A30-548A-47F3-A275-9B6099329877@gmail.com> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.2\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_86B416BA-23C9-4A36-A216-1EA192BC5B5E" X-Trace: ger.gmane.org 1403908219 31395 80.91.229.3 (27 Jun 2014 22:30:19 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 27 Jun 2014 22:30:19 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRB4XAW6OQKGQENKSBZAQ@isocpp.org Sat Jun 28 00:30:13 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pd0-f198.google.com ([209.85.192.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X0eeq-0005JR-1Q for gclcip-std-proposals@m.gmane.org; Sat, 28 Jun 2014 00:30:12 +0200 Original-Received: by mail-pd0-f198.google.com with SMTP id y10sf20527063pdj.1 for ; Fri, 27 Jun 2014 15:30:10 -0700 (PDT) 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:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=fPkieLv5V28BqQOMMPkqaY2UJFoBmu7ASQNX1XX1E9M=; b=PLl7/1bvjeL3jOusxtDVHXrLldkL00gfjh+mAqbNKRpn8qJ27DH7ECSlil/LxrSl6l KvoZNFKWYxIodIsZ3X76uQ4BiYO5HBqdTVgJOqxd76ckqNf+dn6eNBHJV8A3mmE1oxiv 5DEkl/KA6U1BUvOpQzwBo/YQMZZZrggbCF1Hb34fpigNNA/NON2CtyFBcfj7rSCBOZuE LgGNfsPjRoXhAzTPUXxPMO8o8ZKYy0nqfHd6pmggEYV0kG2Zw7Kf/bb5g+G8dJtM31ja xv0nwPlxGUIkE6KtbyxX14T/fyFuEDgKFYIyYr6ZRn+CBao+xFP8PeWhk8XC/HRXzsuV Stxg== X-Gm-Message-State: ALoCoQk8K2YLiCgDy8mPamHN3IQSsaVoFP23mjvarR8Rd6LVRERV7CpfsMBlrVIv8jn2Ksgtfkn6 X-Received: by 10.66.236.136 with SMTP id uu8mr12727195pac.13.1403908210577; Fri, 27 Jun 2014 15:30:10 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.66.227 with SMTP id i3ls642983igt.32.canary; Fri, 27 Jun 2014 15:30:09 -0700 (PDT) X-Received: by 10.50.122.73 with SMTP id lq9mr16017727igb.13.1403908208938; Fri, 27 Jun 2014 15:30:08 -0700 (PDT) Original-Received: from mail-ig0-x22f.google.com (mail-ig0-x22f.google.com [2607:f8b0:4001:c05::22f]) by mx.google.com with ESMTPS id u4si468800igh.60.2014.06.27.15.30.08 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 27 Jun 2014 15:30:08 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:4001:c05::22f as permitted sender) client-ip=2607:f8b0:4001:c05::22f; Original-Received: by mail-ig0-f175.google.com with SMTP id h3so2503475igd.8 for ; Fri, 27 Jun 2014 15:30:08 -0700 (PDT) X-Received: by 10.42.72.198 with SMTP id p6mr23177857icj.52.1403908208704; Fri, 27 Jun 2014 15:30:08 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.57]) by mx.google.com with ESMTPSA id t1sm1021062igh.9.2014.06.27.15.30.06 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 27 Jun 2014 15:30:08 -0700 (PDT) In-Reply-To: X-Mailer: Apple Mail (2.1878.2) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:4001:c05::22f as permitted sender) smtp.mail=potswa@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:11589 Archived-At: --Apple-Mail=_86B416BA-23C9-4A36-A216-1EA192BC5B5E Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-1 On 2014-06-28, at 3:43 AM, 'Geoffrey Romer' via ISO C++ Standard - Future P= roposals wrote: >=20 > On Thu, Jun 26, 2014 at 6:17 PM, David Krauss wrote: >=20 > That doesn't sound like something that should be taught. Students shouldn= 't have the impression that they should avoid move, or omit it just because= they can. >=20 > Really? You wouldn't even teach students to omit move in an expression li= ke this? >=20 > Sink(std::move(Source())); That's going a bit far. Students should know that move casts an object-sema= ntic lvalue into a value-semantic rvalue, and application to an rvalue is a= lways redundant. But that's not avoiding it, or opportunistic omission, it = follows from the underlying theory. > When fixing this code, it would be a good idea to add a comment that the = result is expected to be constructed in-place, hence neither moved nor copi= ed. >=20 > By the way, this code is still giving you exactly what you asked for. Ask= ing for a move when you actually want copy elision is, though reasonable, a= confusion of two completely different things. >=20 > Can you give an example of when a programmer would not want a copy elisio= n? It's an optimization, not a choice. Teaching its interaction with moving is= asks the student to think about two theories at once, which almost by defi= nition relegates it to a more advanced course.=20 > Say move() when you want a move. >=20 > That's just begging the question; I'm trying to teach people how to know = when they should want a move I mean, say move() when you want an ownership transfer. > Debug the rest, and be aware that moves still carry a cost which may be a= s much as a copy.=20 >=20 > Would you say, then, that one should never apply std::move to a copyable = type? Absolutely not. For one thing, that often happens in generic programming. I= simply mean that while chasing performance bugs, after the fact of writing= a first draft, code with move semantics should not be dismissed out of han= d as "already fast." --=20 ---=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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --Apple-Mail=_86B416BA-23C9-4A36-A216-1EA192BC5B5E Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1
On 2014&= ndash;06–28, at 3:43 AM, 'Geoffrey Romer' via ISO C++ Standard - Futu= re Proposals <std-proposals@= isocpp.org> wrote:


On Thu, Jun 26, 2014 at 6:17 PM, David Krauss <potswa@gmail.com> wrote:

=
That doesn’t sound like something that should be= taught. Students shouldn’t have the impression that they should avoi= d move, or omit it just because they can.

Really? You wouldn't even teach students to omit mov= e in an expression like this?

Sink(std::move(Sourc= e()));

That’s= going a bit far. Students should know that move casts an object-semantic l= value into a value-semantic rvalue, and application to an rvalue is always = redundant. But that’s not avoiding it, or opportunistic omission, it = follows from the underlying theory.

When fixing this code, it would be a good idea to add a comment tha= t the result is expected to be constructed in-place, hence neither moved no= r copied.

By the way, this code is still giving yo= u exactly what you asked for. Asking for a move when you actually want copy= elision is, though reasonable, a confusion of two completely different thi= ngs.

Can you give an example of= when a programmer would not want a copy elision?

It’s an optimization, not a choice. Te= aching its interaction with moving is asks the student to think about two t= heories at once, which almost by definition relegates it to a more advanced= course. 

Say&= nbsp;move() when you want a move.
=
That's just begging the question; I'm trying to teach people= how to know when they should want a move

I mean, say move() when you want an ownership transf= er.

=
Debug the rest, and be aware = that moves still carry a cost which may be as much as a copy. 

Would you say, then, that one should never a= pply std::move to a copyable type?

=
Absolutely not. For one thing, that often happens in generic pro= gramming. I simply mean that while chasing performance bugs, after the fact= of writing a first draft, code with move semantics should not be dismissed= out of hand as “already fast.”

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