220 29822 article Path: news.gmane.org!.POSTED!not-for-mail From: =?UTF-8?Q?Jonathan_M=C3=BCller?= Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: New smart pointer: CopyConstructible and CopyAssignable flavor of unique_ptr Date: Fri, 16 Dec 2016 13:05:29 +0100 Lines: 92 Approved: news@gmane.org Message-ID: References: <13702f32-32c2-447f-9996-3624fd678675@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=001a1144c922dd50ff0543c56286 X-Trace: blaine.gmane.org 1481889931 32147 195.159.176.226 (16 Dec 2016 12:05:31 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 16 Dec 2016 12:05:31 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCUMHC4Z6QGBBCVRZ7BAKGQETLILK3A@isocpp.org Fri Dec 16 13:05:27 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb0-f200.google.com ([209.85.213.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cHrGQ-0007i4-Qq for gclcip-std-proposals@m.gmane.org; Fri, 16 Dec 2016 13:05:26 +0100 Original-Received: by mail-yb0-f200.google.com with SMTP id d128sf103565923ybh.6 for ; Fri, 16 Dec 2016 04:05:31 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :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=kw1SpdwijuhaBIOE9IkKu8aOAnLBBuCNCc3BioKDNiI=; b=mZ+m9L/GSGGEaJRTYacCZEHezPqQnsxPry3QgGUnvIseTyVccO4Pu80tTfW8HIDB9v 0q+qkqReJf6coSPBYI3So4m33yvqN8HFDmyoxlRA1blAm4pPhKnV72M0pFUXfO5OPNdv qqXZZISW72NUKSoC7GIljngAQJsNSyg/TMZqPHvJfh6Y/FtXnn/uKcYq2QgpZg4kcuZg VFr+5/UQU3frrZl0g57R0v5V7kFqkhZCYT5BpK0xfGLfmsyyZAxk07lUMUk9Q8bcI8Ir dibwcikC+g8sRyozRQ9qWAdtp4MPDLYI8S4Kh0UtyCYJUrE4rpKkWith/MBQxqvJsgx9 aNCQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to: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=kw1SpdwijuhaBIOE9IkKu8aOAnLBBuCNCc3BioKDNiI=; b=S6UCYjsuuwyKUL2eK92l30cbfrYtLEHjtFHc6/fT99kpvFMALQ2ZQDnq4455AKnzBF np9mZBAzsc8ATvDlJiDzlSrtWUm6PbboMDRTz3L6OQkFAKq6/m8uwr70AsVOga5lpr+S 1v56qVhRVX7GwFNVW7WtNnuz7Nmk3FQGjA1BSAnlwu7qXy28PKe5OCcIJw7k+z+IK5iu o6NQOjFblKbKGabVqwb029LZkIAsXo/JeD/11GEmF8bbaRcQ2/VxdrBAnTsPHO59Aok6 VgZX0XAm4a49WRfMteyGHwj1gKI3R8LHMX0W/d8G4CZl4nIuZO+JcFOlSer1NjLGspMY cKtA== X-Gm-Message-State: AKaTC03joXtgLfWSMCatxS9PjvyPNzRqdu86xoWfBEjt/NTRHQj+7Q+is94C57S0QCMFCA== X-Received: by 10.129.137.6 with SMTP id z6mr652592ywf.137.1481889930952; Fri, 16 Dec 2016 04:05:30 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.24.9 with SMTP id b9ls8891666ote.15.gmail; Fri, 16 Dec 2016 04:05:30 -0800 (PST) X-Received: by 10.55.23.213 with SMTP id 82mr2264898qkx.40.1481889930289; Fri, 16 Dec 2016 04:05:30 -0800 (PST) Original-Received: from mail-qk0-x22b.google.com (mail-qk0-x22b.google.com. [2607:f8b0:400d:c09::22b]) by mx.google.com with ESMTPS id x58si2981337qtx.289.2016.12.16.04.05.30 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 16 Dec 2016 04:05:30 -0800 (PST) Received-SPF: pass (google.com: domain of jonathanmueller.dev@gmail.com designates 2607:f8b0:400d:c09::22b as permitted sender) client-ip=2607:f8b0:400d:c09::22b; Original-Received: by mail-qk0-x22b.google.com with SMTP id n204so88231581qke.2 for ; Fri, 16 Dec 2016 04:05:30 -0800 (PST) X-Received: by 10.55.33.2 with SMTP id h2mr2272741qkh.230.1481889929841; Fri, 16 Dec 2016 04:05:29 -0800 (PST) Original-Received: by 10.55.74.202 with HTTP; Fri, 16 Dec 2016 04:05:29 -0800 (PST) Original-Received: by 10.55.74.202 with HTTP; Fri, 16 Dec 2016 04:05:29 -0800 (PST) In-Reply-To: X-Original-Sender: jonathanmueller.dev@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of jonathanmueller.dev@gmail.com designates 2607:f8b0:400d:c09::22b as permitted sender) smtp.mailfrom=jonathanmueller.dev@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:29822 Archived-At: --001a1144c922dd50ff0543c56286 Content-Type: text/plain; charset=UTF-8 On Dec 16, 2016 12:48, "Ville Voutilainen" wrote: > > Right, and I think My Little Pony is under-appreciated, but if I > really want to convey useful information to anyone, > I explain why. Those semantics happen to be superior to the alternatives. Take std::optional, when you move it the old optional will not become empty: move does basically the same as copy, just uses T's copy instead of move constructor. And this behavior makes sense: optional is supposed to behave like a T. std::unique_ptr on the other hand is only moveable and transfers ownership. This also makes sense. Now, std::optional does not work with polymorphic objects. So I see a point for a smart pointer with deep copying semantics. But then it should behave like the type so moving should also da a "move clone". Now consider a smart pointer with deep copying semantics but ownership transfer on move: What is it supposed to model? It does not behave like the type T as moving does something different. Consider using the pointer with a type that is only copyable. If you have code that takes a reference directly and (somehow) invokes a move operation, the move will copy, so it is safe to use moved-from object there. But if you use that with this smart pointer, it is not safe anymore, as the pointer will become null. I personally think that this kind of semantic difference is not a good idea. -- 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/CAEddoJb9BfQzhoWxLz4-QvD-PkEupxucguMvQEETfcUM7Ed%3DJA%40mail.gmail.com. --001a1144c922dd50ff0543c56286 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

On Dec 16, 2016 12:48, "Ville Voutilainen" <ville.voutilainen@gmail.com= > wrote:
>
> Right, and I think My Little Pony is under-appreciated, but if I
> really want to convey useful information to anyone,
> I explain why. Those semantics happen to be superior to the alternativ= es.

Take std::optional, when you move it the old optional will n= ot become empty: move does basically the same as copy, just uses T's co= py instead of move constructor.
And this behavior makes sense: optional<T> is supposed to behave like= a T.

std::unique_ptr on the other hand is only moveable and trans= fers ownership. This also makes sense.

Now, std::optional does not work with polymorphic objects. S= o I see a point for a smart pointer with deep copying semantics. But then i= t should behave like the type so moving should also da a "move clone&q= uot;.

Now consider a smart pointer with deep copying semantics but= ownership transfer on move: What is it supposed to model? It does not beha= ve like the type T as moving does something different.

Consider using the pointer with a type that is only copyable= .. If you have code that takes a reference directly and (somehow) invokes a = move operation, the move will copy, so it is safe to use moved-from object = there. But if you use that with this smart pointer, it is not safe anymore,= as the pointer will become null.

I personally think that this kind of semantic difference is = not a good idea.

--
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