220 29055 <20161023175710.4911185.4764.19015@gmail.com> article Path: news.gmane.org!.POSTED!not-for-mail From: Tony V E Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: [propagate_const] Implicit conversion to T Date: Sun, 23 Oct 2016 13:57:10 -0400 Lines: 201 Approved: news@gmane.org Message-ID: <20161023175710.4911185.4764.19015@gmail.com> References: <1f43ffe8-b267-4c8c-8834-f3e60035f18d@isocpp.org> <5c682d18-9cf2-4a80-9e58-6b8fae2cf7e8@isocpp.org> <20161023161002.4911185.19138.19005@gmail.com> <08231f25-9d2b-40be-a79e-92225a0b0336@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1477245460 18787 195.159.176.226 (23 Oct 2016 17:57:40 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 23 Oct 2016 17:57:40 +0000 (UTC) Cc: jbcoe@me.com, joseph.thomson@gmail.com, mihailnajdenov@gmail.com To: "joseph.thomson@gmail.com" , ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCUZ5QWKNQIPR45TYACRUBFIONDRW@isocpp.org Sun Oct 23 19:57:35 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb0-f197.google.com ([209.85.213.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1byN1D-0001Iw-1A for gclcip-std-proposals@m.gmane.org; Sun, 23 Oct 2016 19:57:11 +0200 Original-Received: by mail-yb0-f197.google.com with SMTP id g68sf29488643ybi.1 for ; Sun, 23 Oct 2016 10:57:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:content-transfer-encoding:message-id:date:subject:from :in-reply-to:references:to:cc: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=bpQhoFRV7v6N5TD6uSQ0RebsgX7MX/WAfMua1gzTrLM=; b=Q0lPCIFiwcJZwH/nkFu8MHmuI43OJKihSabu1C7juACmA2pxDfXNHfB/5mc8kquz1Q Aogb8nHL4Ei+O0AGmmeDoYMd8rM0AyHWmAvT4jTpyHkmSWYuPgWCI+lmNMLNaOsHxv5a p7DGxW7qJ3ANoCdz0zC9bHQfyR58oZiihI2/LoHFgpg1ZcUJqNH1jy0kyMiWPH17i0kI vPNlWWVKzgzNMeFfeoi7I2jF/2MdeiCqUGaNLToQVCbLq6U3qmAgBVD1yacR8E6+E5Ck yGXiohkZn0hRXWMTVQ9zN4HlemaeSGdhYYL3NOOS5KM1W2BStMpT56Urzz1jEUXJ8rEf K4MA X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:content-transfer-encoding :message-id:date:subject:from:in-reply-to:references:to:cc :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=bpQhoFRV7v6N5TD6uSQ0RebsgX7MX/WAfMua1gzTrLM=; b=cTw+M711LOiOlnc5+XFxVM+bZoRHJM2XD5oxW7n57wwNR52aKy6V48K8iEIbE9l+hB MUA+rSgtRTIsPhgOXanEdAzYkUzYNtiPmoZJd6V7oc2pcM5ySQQWuh8/wAW592FeCRi0 jOVIbxRwKbIPQJOSCToYYhpmcy0nUNhclBvO4P9M1sLMEHfbmkScnZd4YCB+EPlJF183 3mz2DHzZBrhZPGvGrXdYGolbgn8LmCB8UYpXfObicXDdTVAa2KELNqXaEdec7+pNGrFB ZPTcDv76b9eugV+8mh/HYb6tMuDcF9MywPgHOKTBwrSpUBvDHekH2f9g7aaFAXblgqZK X-Gm-Message-State: AA6/9Rmf/GDdOebq9M1JPI/73DBM8LAzZBFcizQOUtm7+2IhvNhwzlVjd9Ruf6GTkB4mkg== X-Received: by 10.129.165.14 with SMTP id c14mr7931580ywh.67.1477245433345; Sun, 23 Oct 2016 10:57:13 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.27.195 with SMTP id v3ls9331924otv.13.gmail; Sun, 23 Oct 2016 10:57:12 -0700 (PDT) X-Received: by 10.37.104.139 with SMTP id d133mr11923952ybc.160.1477245432565; Sun, 23 Oct 2016 10:57:12 -0700 (PDT) Original-Received: from mail-yw0-x243.google.com (mail-yw0-x243.google.com. [2607:f8b0:4002:c05::243]) by mx.google.com with ESMTPS id d13si4383728yba.258.2016.10.23.10.57.12 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 23 Oct 2016 10:57:12 -0700 (PDT) Received-SPF: pass (google.com: domain of tvaneerd@gmail.com designates 2607:f8b0:4002:c05::243 as permitted sender) client-ip=2607:f8b0:4002:c05::243; Original-Received: by mail-yw0-x243.google.com with SMTP id e5so4971670ywc.3 for ; Sun, 23 Oct 2016 10:57:12 -0700 (PDT) X-Received: by 10.107.157.14 with SMTP id g14mr12108781ioe.70.1477245432331; Sun, 23 Oct 2016 10:57:12 -0700 (PDT) Original-Received: from [127.0.0.1] ([142.114.191.80]) by smtp.gmail.com with ESMTPSA id h90sm5973572iod.18.2016.10.23.10.57.10 (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Sun, 23 Oct 2016 10:57:11 -0700 (PDT) X-Mailer: BlackBerry Email (10.3.2.2876) In-Reply-To: <08231f25-9d2b-40be-a79e-92225a0b0336@isocpp.org> X-Original-Sender: tvaneerd@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of tvaneerd@gmail.com designates 2607:f8b0:4002:c05::243 as permitted sender) smtp.mailfrom=tvaneerd@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:29055 Archived-At: =
Yes, value semantics is more accurate.
= =

= = =
Sent from my BlackBe= rry portable Babbage Device
= = = <= /tr>
From: joseph.thom= son@gmail.com
Sent: Sunday, October 23, 2016 1:52 PM
=
To: ISO C++ Standard - Future Proposals
Reply To: = std-proposals@isocpp.org
Cc: jbcoe@me.com; joseph.thom= son@gmail.com; mihailnajdenov@gmail.com
Subject: Re: [std-= proposals] Re: [propagate_const] Implicit conversion to T

On Monday, 24 October 201= 6 00:10:06 UTC+8, Tony V E wrote:
=
I would say propagate_const= *implies* ownership which implies deep-copy. (or no copy).

I would be suspect of situatio= ns that use propagate_const =E2=80=8Ebut don't have ownership semantics.
=
 
I'd say propagate_const implies value= .. const implies = that you can't change an object's value (even if it doesn't guarantee it in= practice). Lack of pro= pagate_const is like the mutable keyword: it is an indication that some part of an o= bject's representation does not affect its value, and can therefore be chan= ged even if the object is co= nst. propagate_const requir= es deep-copy or no copy simply because C++ copy operations take referen= ces to const. There is no logical reason why non-const objects using propagate_const should= n't be copied; it just isn't supported well by C++. Ownership is a separate= matter (though unique ownership by itself requires deep-copy or no copy).<= br> 
= = Sent from my BlackBerry portable Babb= age Device
= =
Sent: Sunday, October 23, 2016 12:00 PM
To: ISO C++ Standard - Future Proposals
Subject: Re: [std-pr= oposals] Re: [propagate_const] Implicit conversion to T


<= br>On Sunday, October 23, 2016 at 4:59:57 PM UTC+3, joseph....@gmail.com= wrote:
..= ..
Deep copying wouldn't be a problem. I'm just having a hard time figuri= ng out what exactly a deep-copying indirect type would do or how it would b= e useful. I guess I'll have to wait for the details of this std::indirect.

I meant, if the copy ctor and assign do a&nbs= p; deep copy propaga= te_const is all you need, no downsides - new data + new propaga= te_const instance.=

struct S
{
 S() : _p(std:= :make_unique<X>()) {}
 S(S&&) =3D default;
 S= & operator=3D(S&&) =3D default;
 S(const S& o) : _p= (std::make_unique<X>(*o._p)) {}
 S& operator=3D(const S&a= mp; o) { if(&o !=3D this) { _p =3D std::make_unique<X>(*o._p); } = return *this; };

 propagate_const<std::unique_ptr<X>> _p;
};

In this scenario propagate_const will behave completely correct= ..

In the shared_ptr scenario, copy-on-write is th= e only way to make the "casting-const" copying safe, Qt style.
<= br>
So, no-copy, or deep-copy are the only two ways to use propag= ate_const in composition in another object.
One might argue= , these are the only ways to do any const propagation in general, at= least without some magic language support. 

=

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