220 29890 <20161217210804.GA10967@manuel-ThinkPad-L440.localdomain> article Path: news.gmane.org!.POSTED!not-for-mail From: Manuel Bergler Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: New smart pointer: CopyConstructible and CopyAssignable flavor of unique_ptr Date: Sat, 17 Dec 2016 22:08:04 +0100 Lines: 150 Approved: news@gmane.org Message-ID: <20161217210804.GA10967@manuel-ThinkPad-L440.localdomain> References: <3df7e8eb-01a0-7804-f476-504b5a25a9fe@gmail.com> <20161217170428.GA10214@manuel-ThinkPad-L440.localdomain> <20161217171633.4919375.58072.21574@gmail.com> <15884364-3238-41c7-bc5a-aa5595a64ade@isocpp.org> <20161217182130.GA4347@manuel-ThinkPad-L440.localdomain> <5eae5f16-e129-4c95-8001-fc73cc4f3d03@isocpp.org> <20161217192812.GA17421@manuel-ThinkPad-L440.localdomain> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="XsQoSWH+UP9D9v3l" X-Trace: blaine.gmane.org 1482008933 32412 195.159.176.226 (17 Dec 2016 21:08:53 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 17 Dec 2016 21:08:53 +0000 (UTC) User-Agent: Mutt/1.5.24 (2015-08-30) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDOZBAUFX4MRBZGS23BAKGQEL2OZ4EY@isocpp.org Sat Dec 17 22:08:49 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f69.google.com ([209.85.215.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cIMDo-0007eG-TC for gclcip-std-proposals@m.gmane.org; Sat, 17 Dec 2016 22:08:48 +0100 Original-Received: by mail-lf0-f69.google.com with SMTP id o20sf17575309lfg.2 for ; Sat, 17 Dec 2016 13:08:53 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent: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=55ub9n1//ZuuSwbIDR2SSxSjxQpR+RZKhXHdt/PNWhQ=; b=R8pLZgLNL8IaS46ge3HK9wneRKCEAIcsjnOWkSTnoIzNprogpqJ3JVQdtKE+T+M7mP g5vptBhzCZOQ+fy0xtbysq+1jZOy4lcdKnO7d2t8Z4lGdXSufocOTR/udUtcFrd1SOE5 mOXpx3pvonzXpilbggbfLkUC/XiSDi35UeWQwZHcAOtswWm7ORCGNLOa2CjxrRKKAzn9 Tq0hBZ7J/oFt7IN3jgaX04lq3CJtDI8KdP0Gcnn7Pb30VScJAOxMCRkFxibgTabTSv9G EzuxWdaxhnDyuHBnzxEvkMAPTPtKQiWq4sBN17DE2u5l7EBRDqshMseyLK96i+kBauUk /Z X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent :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=55ub9n1//ZuuSwbIDR2SSxSjxQpR+RZKhXHdt/PNWhQ=; b=Ew+MVuaBr9VNx/CW+0YAQufFH0dZPO61arUFy35Ix9nm8nK/Zy81EzBi8tuI1e3MJ8 4FF3iq8XomKWzXZ+w6o7NtMQrSX7dcvsMHkHpC/Q5I1bvnj0APxrq54gZtTDXMdhbeWG 94owK8p3GqMi/X+p082R+gsZN68S5frJC71shpcvxoVxQZBPKoyeeyI5KjKDE4KN8daZ An0rqT20GtW0PlE1GkKtyQ+BU/XEpYBNg8MdNo3v9pdu0P0CYIvguiynHJN60BE0vohb 3WO5Qp11UKdnKOxAx2gOzthH3WEhyxAFmVPlnQ+kfaXPi61EtCC7pWX1wbO738xaElZe X-Gm-Message-State: AKaTC01DUilDfX0UvFzuuDqwfYFYV3hegsnxAxg8BX6LEl65dt2KlgA/FEW0KlNLKLRqnw== X-Received: by 10.25.202.26 with SMTP id a26mr753903lfg.26.1482008933252; Sat, 17 Dec 2016 13:08:53 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.28.27.7 with SMTP id b7ls741309wmb.15.canary-gmail; Sat, 17 Dec 2016 13:08:52 -0800 (PST) X-Received: by 10.28.211.72 with SMTP id k69mr7780781wmg.138.1482008932007; Sat, 17 Dec 2016 13:08:52 -0800 (PST) Original-Received: from mail-wj0-x244.google.com (mail-wj0-x244.google.com. [2a00:1450:400c:c01::244]) by mx.google.com with ESMTPS id y124si9167394wme.83.2016.12.17.13.08.51 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 17 Dec 2016 13:08:51 -0800 (PST) Received-SPF: pass (google.com: domain of berglerma@gmail.com designates 2a00:1450:400c:c01::244 as permitted sender) client-ip=2a00:1450:400c:c01::244; Original-Received: by mail-wj0-x244.google.com with SMTP id he10so18845038wjc.2 for ; Sat, 17 Dec 2016 13:08:51 -0800 (PST) X-Received: by 10.194.146.131 with SMTP id tc3mr8102629wjb.129.1482008931396; Sat, 17 Dec 2016 13:08:51 -0800 (PST) Original-Received: from localhost (x5d856f73.dyn.telefonica.de. [93.133.111.115]) by smtp.gmail.com with ESMTPSA id e6sm12959764wjw.33.2016.12.17.13.08.50 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sat, 17 Dec 2016 13:08:50 -0800 (PST) Content-Disposition: inline In-Reply-To: X-Original-Sender: berglerma@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of berglerma@gmail.com designates 2a00:1450:400c:c01::244 as permitted sender) smtp.mailfrom=berglerma@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:29890 Archived-At: --XsQoSWH+UP9D9v3l Content-Type: text/plain; charset=UTF-8; format=flowed Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Sat, Dec 17, 2016 at 12:35:03PM -0800, Nicol Bolas wrote: > It is different. > > Consider for example something like > > =C2=A0 =C2=A0 my_obj =3D std::accumulate(input.begin(), input.end(), > std::move(my_obj), > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0= =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0some_function); > > Afterwards references to my_obj will still be valid and correctly poi= nt > to the > modified object. > > On the other hand, if you have > > =C2=A0 =C2=A0 auto f =3D [&someObj] (auto& acc, auto const& in) -> de= cltype(auto) { > =C2=A0 =C2=A0 =C2=A0 =C2=A0 acc->doSomethingModifyingThePointeeObj(in= ); > =C2=A0 =C2=A0 =C2=A0 =C2=A0 someObj.doSomething(acc) =C2=A0// note I = really want the cloning here > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0= =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 // so I need to pass in t= he clone_ptr > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0= =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 // and not the pointee > =C2=A0 =C2=A0 =C2=A0 =C2=A0 return std::move(acc); > =C2=A0 =C2=A0 }; > > =C2=A0 =C2=A0 my_clone_ptr =3D std::accumulate(input.begin(), input.e= nd(), > std::move(my_clone_ptr), f); > > now suddenly all references you got by calling `*my_clone_ptr` somewh= en > before > might or might not be invalid, > > Actually, the standard guarantees that it isn't valid. Your > `move(my_clone_ptr)` will move-construct the `init` parameter of > `accumulate`. However, the standard says: > > > Computes its result by initializing the accumulator acc with the init= ial > value init > > This means that it creates a new `T` called `acc`, which is initialized= by > the `init` value. Therefore, it will initialize `acc` by copy. `init` w= ill > hold the original pointer until the end of `accumulate`, when it will b= e > destroyed. What will be returned is `acc`, which is a copy of `init`. > =C2=A0 Apparently clang and gcc interpret this differently: They move it into plac= e due to the std::move (see http://coliru.stacked-crooked.com/a/cbe39ffef2e= bf716). So in case of clang/gcc in my example the call to std::accumulate wouldn't = invalidate existing references. But who knows, maybe some implementer of th= e standard library will interpret the requirements the same way you did and= actually perform a copy. But only forgetting to `return std::move(acc);` i= n the lambda used for `std::accumulate` and using just `return acc;` result= s in a copy. This is the entire point: When even very experienced people are not able to predict what will happen, how is this a good feature? It is just too hard t= o use correctly. > > even though I moved the original clone_ptr into > the algorithm to ensure that it doesn't get copied. > > Replace `clone_ptr` with `any`. Or `vector`. Many types have this probl= em > with `accumulate`; I don't see why adding one more will hurt anything. This is because these types don't act as pointers; you're not supposed to s= tore references to individual elements of a vector or a std::any, whereas t= he interface of this copy_ptr is designed to do exactly that, even though i= t is dangerous/unpredictable. The only way to get around that issue is to not have additional references = to the pointee, but then there is no need to have the smart pointer interfa= ce and you can just use `indirect`. > > -- > You received this message because you are subscribed to the Google Grou= ps > "ISO C++ Standard - Future Proposals" group. > To unsubscribe from this group and stop receiving emails from it, send = an > email to [1]std-proposals+unsubscribe@isocpp.org. > To post to this group, send email to [2]std-proposals@isocpp.org. > To view this discussion on the web visit > [3]https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/cb822c5= c-fc50-48e5-ad60-fdb14580329c%40isocpp.org. > >References > > Visible links > 1. mailto:std-proposals+unsubscribe@isocpp.org > 2. mailto:std-proposals@isocpp.org > 3. https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/cb822c5= c-fc50-48e5-ad60-fdb14580329c%40isocpp.org?utm_medium=3Demail&utm_source=3D= footer --=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/20161217210804.GA10967%40manuel-ThinkPad-L440.lo= caldomain. --XsQoSWH+UP9D9v3l Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEi3F6I6jTgMCHgGvofAJ3kFaTK3oFAlhVqTEACgkQfAJ3kFaT K3p1hQ/+JLqh4xIB0WtYR+TNENu1DcJXcXDpuhX3kpNTow+joCLuDbic3HSFXchz 3s7WdTGGPmLpcyMtbBqWodgfIdE5ob/N6awY1M2guEJWIBXbhEVDy+C8M7xlfzMw Iz5w7B41nEHww08V7xQg6JkcgSfGhmJ1ul8VbTmNNlAguTkCAM/ZeZFwtNz3ubjz t1I0f+OcUVazwAS74UyDjvQdPxGHutmTCTbJiYgnvtabe6oMTdSceXdT5f+LIGvC Cg8hT1Bl3NrVPIE1zjplLrdZW0DdjOY5rj9xHI1cymo73CRKRlVi/q82q3G0RXYx 8l1ljnpcVdklmGKgi/mQ+THBSkeZgtIF8r4JT6/Z28HAz4sMLr53lj87vOr/nU/Z HkNNwHGLRH1l3WW2cor93gg/thibMqVYvuZZjKSByPhsUtQVcsC4LWi7wLgupteo QaIw/8RJ5o0pThOK/RcFoLCVR14Yu+VoAH+X2+YhZoD6qcanlx3BNp8dVycie6PC iMPq7VkPzxPZLzNCdx3jBlqay9i3gC0KksKoy0szvtL2YfarKR+UWQHmR/zUoMMS cPW9ylESAi1yREIz4DuHCiC3gF3D5N4fskpoDx0WJWiy/lUgN2cWZI9iyrnQAdMr uJkJU45z8+hClULaq0f4onWz8tBRkHw28arM4lJnPilabVhDbnw= =KGYm -----END PGP SIGNATURE----- --XsQoSWH+UP9D9v3l-- .