220 11979 article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Controversy and debate: Uninitialized variables by default is a bug Date: Mon, 21 Jul 2014 12:21:24 +0800 Lines: 144 Approved: news@gmane.org Message-ID: References: <60caa4ef-b32d-40fe-97b6-8918fbb2bd51@isocpp.org> <20140620202026.GA29898@bacon.lysator.liu.se> <0329E32D-0EC2-4BE0-85D0-492A4CF22B0B@gmail.com> <8021febe-7c98-400e-8167-3d6dbb695a12@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_C2289781-1CE1-44E4-B74F-BAD09E53C120" X-Trace: ger.gmane.org 1405916503 21844 80.91.229.3 (21 Jul 2014 04:21:43 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 21 Jul 2014 04:21:43 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBTVKWKPAKGQEWO765IY@isocpp.org Mon Jul 21 06:21:36 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ie0-f200.google.com ([209.85.223.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X956V-00050Y-Sx for gclcip-std-proposals@m.gmane.org; Mon, 21 Jul 2014 06:21:36 +0200 Original-Received: by mail-ie0-f200.google.com with SMTP id at20sf56641896iec.3 for ; Sun, 20 Jul 2014 21:21:34 -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=tr2ZkQNwIyJmrqewTJ6eUC/QNXOgUZ4UXF2MmZ7ec70=; b=ja51MQxrKqmFyOV5a0N/guoZnxINB/UBVG6o83Z6vZs5j0klLRnrsf6KV8RYC5az0x HiARAXzwOxIupYLegEqfGbpSfvoBQIz3DfYg0mD6/b3/wahoWBtfXTGyotDGm7DV4VGA gYpXVEeM20q3Asos69eb0xT2cET7p/MVizdW6h56bnOIUux5qCZI5+h3REbk4TB2fFnD cQYxRJqjDecDJrnzZcw5MjUixvqSfBXYJeQP3Q5o1+h7u2IY05fnTXylcu8/IMUBKWPg 7pXCX/Cu4keGwBKTJv//jaXgrPm355rJdGDqZTtSrq1+8/A/7I0UUAZVaxUnfbIfjcZt 6AoA== X-Gm-Message-State: ALoCoQnY7DgLK/qPz67xWO55T6OwyJ5td0e/mWMQrFz1kAcib4/5F91b5irYxxTirqwRgT53Cgll X-Received: by 10.50.109.131 with SMTP id hs3mr202936igb.8.1405916494762; Sun, 20 Jul 2014 21:21:34 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.9.99 with SMTP id y3ls1334937iga.12.gmail; Sun, 20 Jul 2014 21:21:34 -0700 (PDT) X-Received: by 10.69.18.11 with SMTP id gi11mr11458979pbd.40.1405916493945; Sun, 20 Jul 2014 21:21:33 -0700 (PDT) Original-Received: from mail-pa0-x22c.google.com (mail-pa0-x22c.google.com [2607:f8b0:400e:c03::22c]) by mx.google.com with ESMTPS id fy14si6454141pdb.507.2014.07.20.21.21.33 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sun, 20 Jul 2014 21:21:33 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22c as permitted sender) client-ip=2607:f8b0:400e:c03::22c; Original-Received: by mail-pa0-f44.google.com with SMTP id eu11so9104027pac.3 for ; Sun, 20 Jul 2014 21:21:33 -0700 (PDT) X-Received: by 10.70.127.163 with SMTP id nh3mr811437pdb.139.1405916493577; Sun, 20 Jul 2014 21:21:33 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.63]) by mx.google.com with ESMTPSA id wp3sm12676725pbc.67.2014.07.20.21.21.31 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sun, 20 Jul 2014 21:21:32 -0700 (PDT) In-Reply-To: <8021febe-7c98-400e-8167-3d6dbb695a12@isocpp.org> X-Mailer: Apple Mail (2.1878.6) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22c 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:11979 Archived-At: --Apple-Mail=_C2289781-1CE1-44E4-B74F-BAD09E53C120 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-1 On 2014-07-21, at 12:00 PM, Matthew Fioravante wro= te: > Why not just a std::noinit wrapper? >=20 > std::vector>; //Does not initialize contents on construc= tion >=20 > Then the non-initialization is documented and enforced by the type system= .. Transparent wrappers are a can of worms. If wrapping isn't needed, then you can already use std::aligned_storage. I = mentioned this earlier in the thread. The nice thing about a singular value std::uninitialized is that it's also = guaranteed to propagate by the type system, but only to its own point of us= e. And, it can be used for overloading. For classes, it would generally con= vey the idea of "invalid until assignment or further initialization," and i= t would be implemented by propagating uninitialized initializers to members= .. A constructor overloaded on std::uninitialized_t, rather than the default= constructor, would be the canonical way to write an ugly partial-initializ= ation semantic. > Another option is a simple std::allocator implementation. That allocator could well be standardized. Here's an implementation I have = lying around, but I don't think it was ever tested. template< typename client > struct default_construct_allocator : std::allocator< client > { template< typename rebound, typename arg1, typename ... arg > void construct( rebound * p, arg1 && a1, arg && ... a ) { ::new ( static_cast< void * >( p ) ) rebound { std::forward< arg1 >( a1= ), std::forward< arg >( a ) ... }; } =09 template< typename rebound > void construct( rebound * p ) { ::new ( static_cast< void * >( p ) ) rebound; } =09 template< typename rebound > struct rebind { typedef default_construct_allocator< rebound > other; }; }; --=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=_C2289781-1CE1-44E4-B74F-BAD09E53C120 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1
On 2014&= ndash;07–21, at 12:00 PM, Matthew Fioravante <fmatthew5876@gmail.com> wrote:

Why not just a std::noinit<T> wrapper?
std::vector<std::noinit<int>>; //Does not initializ= e contents on construction

Then the non-initializa= tion is documented and enforced by the type system.

Transparent wrappers are a can of worms.
<= div>
If wrapping isn’t needed, then you can already use= std::aligned_storage. I mentioned this earli= er in the thread.

The nice thing about a singular = value std::uninitialized is that it= ’s also guaranteed to propagate by the type system, but only to its o= wn point of use. And, it can be used for overloading. For classes, it would= generally convey the idea of “invalid until assignment or further in= itialization,” and it would be implemented by propagating uninitialized initializers to members. A construct= or overloaded on std::uninitialized_t, rather= than the default constructor, would be the canonical way to write an ugly = partial-initialization semantic.

Another option is a simple std::allo= cator implementation.

Tha= t allocator could well be standardized. Here’s an implementation I ha= ve lying around, but I don’t think it was ever tested.

=
template< typename client >
struct default_construct_allocator : = std::allocator< client > {
template<= typename rebound, typename arg1, typename ... arg >
void construct( rebound * p, arg1 && a1, arg && = .... a )
{ ::new ( static_cast< void * >= ;( p ) ) rebound { std::forward< arg1 >( a1 ), std::forward< arg &= gt;( a ) ... }; }
template< typename rebound >
void c= onstruct( rebound * p )
{ ::new ( static_cas= t< void * >( p ) ) rebound; }
template< typename rebound >
<= font face=3D"Courier"> struct rebind { typedef default_construct_allocator< rebound= > other; };
};

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