220 30412 article Path: news.gmane.org!.POSTED!not-for-mail From: Chris Hallock Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: P0298: A byte type definition: with undefined pointer arithmetic? Date: Sat, 7 Jan 2017 17:13:39 -0800 (PST) Lines: 128 Approved: news@gmane.org Message-ID: References: <117168cf-8ab4-4048-aed6-38b68cb3ebca@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2349_1282421112.1483838019427" X-Trace: blaine.gmane.org 1483838024 12795 195.159.176.226 (8 Jan 2017 01:13:44 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 8 Jan 2017 01:13:44 +0000 (UTC) Cc: neilmac@microsoft.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCHINCHEQEFBBRFEY3BQKGQEALED6FY@isocpp.org Sun Jan 08 02:13:39 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f199.google.com ([209.85.223.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cQ23F-0002SE-8o for gclcip-std-proposals@m.gmane.org; Sun, 08 Jan 2017 02:13:37 +0100 Original-Received: by mail-io0-f199.google.com with SMTP id v96sf6243364ioi.5 for ; Sat, 07 Jan 2017 17:13:41 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=zx2Qrq7TLt9BWlt9t1oPYuAcFxgEZTDAFC3DJCsPhkc=; b=RevVEri7mQoW5WSBq2Wpzc7ATYRFjDIWPz8mdB+S2NwBXP2MuyciX9XtHm6cBBTaNr y+nUNX6FYYeTxWkwIcC8ULHvi/GT/1ujA8MwQBVpO8KreyTbpHlKARNAcPQMD+Oplswe YaDswfKderGLsyljp+drlCFBl/9EqKAx5y6JgnZ9DQwT6FgtEhegozqSPP+wgQZ+IFTr yTk/0LEI8xNUPhwDbQPxBXOsOy506mEzcvybRDhsr/EBzFZp8dG+TSlWCULdycAK91X3 83P18ccgBtjIB4jBPAOVVk1gVLOFSOntg3sz7QFDjLMBx4tVJQ+j/4duioMwjR2FrLUF q1lQ== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=zx2Qrq7TLt9BWlt9t1oPYuAcFxgEZTDAFC3DJCsPhkc=; b=Lq+bfDZTKTdPszrxWi10Uy5/IcLe+MFihcV28yCyKG7lOdodS2q9PkEV+G3qtLBpOQ uCLNr1AsRLUcjEy82cbdNHhMVTMEIp8NdcEzpWlWsBTUdaD4OBkjjWRBXSSKGbcBvZeh XWw1sBdoPKomi2dxzS21KiV2lB4luHr9EIGouFZkAJknPodq8zwQxyTUJ8LFiu282R7C RJzta0goskGJKjVGrDZ2T6/diM2k4FuDO6pJ/Ld41KxcE3iyLGRkxrveOmnFB9SQ6QFe Anakixx+3WQ7t53QO5pl8MhPQv2YiwfG4lKJB4SmjHBOgrH1UKkyyjgz7OuB2gWenHDb JVQw== 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:cc:message-id:in-reply-to :references:subject:mime-version:x-original-sender:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=zx2Qrq7TLt9BWlt9t1oPYuAcFxgEZTDAFC3DJCsPhkc=; b=VVZIyFGqrqL5IiSa70LaP/swHLW/8IRSmRJum2UlfIQOyJt1TAD3ykUBecOb9Mwf5e 2dffWKp7D8hfemEG8I1QQygb+TaUjKoqwhXV8LoCMnu5YgGVFTrSX5AMD2OU25Nb5dpa 8OmhB8jtj3SdtOyj5zlvec/8XK/pr2eoaolE4isN0Vi/kd2kjIriooDUl9I+f6aCmQmb WoLlhpZy/sBI2aprtwyw7rtMCa9ikVktAKLrnH/yC3BLoN64gcLWEsV6wAUTDaKLwZul woTGoAcU9D6r1JcQCzQosNMvyAtjVxGKHTyukhnqg/LLhtonTE67ejMUP4Rx5hHCVil/ rhGw== X-Gm-Message-State: AIkVDXLIaSPGp8cS70vINkho1xfk8bMjZKyZxx8H55FgiPw6d6NqT8zIvvCiSFu7QTvqAg== X-Received: by 10.107.156.78 with SMTP id f75mr3527820ioe.86.1483838020926; Sat, 07 Jan 2017 17:13:40 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.17.12 with SMTP id g12ls4349886ote.26.gmail; Sat, 07 Jan 2017 17:13:40 -0800 (PST) X-Received: by 10.157.8.134 with SMTP id 6mr638868otf.17.1483838020125; Sat, 07 Jan 2017 17:13:40 -0800 (PST) In-Reply-To: <117168cf-8ab4-4048-aed6-38b68cb3ebca@isocpp.org> X-Original-Sender: christopherhallock@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:30412 Archived-At: ------=_Part_2349_1282421112.1483838019427 Content-Type: multipart/alternative; boundary="----=_Part_2350_1731349889.1483838019427" ------=_Part_2350_1731349889.1483838019427 Content-Type: text/plain; charset=UTF-8 On Saturday, January 7, 2017 at 3:07:23 PM UTC-8, Ryou wrote: > > [...] > So I think this is well-formed. > > unsigned char storage[ sizeof(int) ] ; > int * p1 = new(storage) int(42) ; > unsigned char * p2 = reinterpret_cast(p1) ; > p2 + 1 ; // OK > Agreed, except that, since the int object is not pointer-interconvertable to storage[0], you also need to std::launder the result of the cast under C++17. This is also well-formed. > > union { int i ; unsigned char buf[sizeof(int)] ; } u ; > u.i = 42 ; > unsigned char * p3 = reinterpret_cast< unsigned char *>( &u ) ; > p3 + 1 ; // OK > This example also needs std::launder, but even with that, p3 would only point to the first byte in the object representation of u (or, equivalently, u.i), not u.buf[0]. u.buf[0] doesn't exist (as a live object). It doesn't exist because u.buf itself doesn't exist, and that's because it was never made the active member. There's only u.i. Since there is no live array object, the last line is strictly undefined under the current (but presumably defective) wording. But this one is undefined behaviour? > > int i = 42 ; > unsigned char * p4 = reinterpret_cast< unsigned char *>( &i ) ; > p4 + 1 ; // Undefined behaviour? > > Because the storage for object i is not associated with array of unsigned > char. > Yes, under the current (but presumably defective) wording. (Also, this needs std::launder.) -- 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/d5d59214-e783-4d5c-bc34-1598d3eb34e9%40isocpp.org. ------=_Part_2350_1731349889.1483838019427 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Saturday, January 7, 2017 at 3:07:23 PM UTC-8, Ryou wro= te:
[...]
<= div>So I think this is well-formed.

=C2=A0 = =C2=A0 unsigned char storage[ sizeof(int) ] ;
=C2=A0 =C2=A0 int *= p1 =3D new(storage) int(42) ;
=C2=A0 =C2=A0 unsigned char * p2 = =3D reinterpret_cast<unsigned char *>(p1) ;
=C2=A0 =C2=A0 p= 2 + 1 ; // OK

Agreed, except that, s= ince the int obj= ect is not pointer-inte= rconvertable to sto= rage[0], you also need to std::launder the result of the cast under C++17.


This is also = well-formed.

=C2=A0 =C2=A0 union { int i ; un= signed char buf[sizeof(int)] ; } u ;
=C2=A0 =C2=A0 u.i =3D 42 ;
=C2=A0 =C2=A0 unsigned char * p3 =3D reinterpret_cast< unsigned= char *>( &u ) ;
=C2=A0 =C2=A0 p3 + 1 ; // OK
<= /div>

This example also needs std::launder, but even with that, p3 would only point to= the first byte in the object representation of u (or, equivalently, u.i), not u.buf[0]. u.buf[0] doesn't exist (as a live object). It = doesn't exist because u.buf itself doesn't exist, and that's because it was nev= er made the active member. There's only u.i. Since there is no live array object, the l= ast line is strictly undefined under the current (but presumably defective)= wording.


But this one is undefined behaviour?

=C2=A0 =C2=A0 int i =3D 42 ;
=C2=A0 =C2=A0 = unsigned char * p4 =3D reinterpret_cast< unsigned char *>( &i ) ;=
=C2=A0 =C2=A0 p4 + 1 ; // Undefined behaviour?
<= br>
Because the storage for object i is not associated with array= of unsigned char.=C2=A0

Yes, under the cu= rrent (but presumably defective) wording. (Also, this needs std::launder.)

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