220 4771 article Path: news.gmane.org!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Implementability of std::optional Date: Fri, 31 May 2013 18:47:22 +0300 Lines: 118 Approved: news@gmane.org Message-ID: References: <00ac95c4-9e88-40eb-9564-41d8ed4d84ee@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=047d7bdca64ae290cc04de058755 X-Trace: ger.gmane.org 1370015246 27519 80.91.229.3 (31 May 2013 15:47:26 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 31 May 2013 15:47:26 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC5JHI7A7ALRBC4MUOGQKGQEVYXSHYA@isocpp.org Fri May 31 17:47:26 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qe0-f70.google.com ([209.85.128.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1UiRY5-0007Sx-3G for gclcip-std-proposals@m.gmane.org; Fri, 31 May 2013 17:47:25 +0200 Original-Received: by mail-qe0-f70.google.com with SMTP id 2sf2182364qea.9 for ; Fri, 31 May 2013 08:47:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-beenthere:mime-version:in-reply-to:references:date:message-id :subject:from:to:x-original-sender:x-original-authentication-results :reply-to:precedence:mailing-list:list-id:x-google-group-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=MpaksuHWcI5s2sPzb0wrhzUGo8MAKG+aPTwXkh1ke2Q=; b=OV/9uuTHDbRZ9fQ/nZj79Ukn+KGiL7DEkp5IGMy6YYXm+4sUQHv3x8VWeMX6ilQbze 5D/5KozBi3cw361witEvA8VBpHm6Ex/A5bEsNXhYhRJJLrW671sOMWiXSCfRBmPU/AIn sU7T0z41QG8ks5dLJEHHvLoPPUdbJppz/3dduzpAs8c4XHGMS09DristwhuceDzLmDZA Qe7y4h81Ut7O7qit2w12eWv8PsND9VPsTRnFY+6ZmAonoZh66CRDg+SD65Z7LDJnY+bj shmPSh05lr31+MkxADUXfEg030hNvu2XnW9pIOvADbbuJxY+YLi3SoCX+GCWYt3SBJPR p0pQ== X-Received: by 10.224.174.145 with SMTP id t17mr6941387qaz.4.1370015243984; Fri, 31 May 2013 08:47:23 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.4.1 with SMTP id g1ls955554qeg.64.gmail; Fri, 31 May 2013 08:47:22 -0700 (PDT) X-Received: by 10.224.7.195 with SMTP id e3mr10011664qae.5.1370015242952; Fri, 31 May 2013 08:47:22 -0700 (PDT) Original-Received: from mail-qe0-f46.google.com (mail-qe0-f46.google.com [209.85.128.46]) by mx.google.com with ESMTPS id c10si36395664qam.59.2013.05.31.08.47.22 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 31 May 2013 08:47:22 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 209.85.128.46 as permitted sender) client-ip=209.85.128.46; Original-Received: by mail-qe0-f46.google.com with SMTP id w7so190773qeb.5 for ; Fri, 31 May 2013 08:47:22 -0700 (PDT) X-Received: by 10.224.71.211 with SMTP id i19mr10106814qaj.27.1370015242816; Fri, 31 May 2013 08:47:22 -0700 (PDT) Original-Received: by 10.224.174.143 with HTTP; Fri, 31 May 2013 08:47:22 -0700 (PDT) In-Reply-To: <00ac95c4-9e88-40eb-9564-41d8ed4d84ee@isocpp.org> X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 209.85.128.46 as permitted sender) smtp.mail=ville.voutilainen@gmail.com; dkim=pass header.i=@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:4771 Archived-At: --047d7bdca64ae290cc04de058755 Content-Type: text/plain; charset=ISO-8859-1 On 31 May 2013 18:17, Nikolay Ivchenkov wrote: > On Friday, May 31, 2013 5:42:45 PM UTC+4, DeadMG wrote: >> >> A trivial example not involving optional would be, say, the last element >> of a std::vector. It's quite legal and normal for the user to overwrite >> reference locations by destructing the original object and reconstructing a >> new one in it's place. >> > > We can do that: > > #include > #include > #include > > struct A > { > > A(int &x) : ref(x) {} > int &ref; > }; > > int main() > { > std::vector v; > > int n1 = 0, n2 = 0; > > v.emplace_back(n1); > A *p = &v.back(); > > p->~A(); > A *q = new(p) A(n2); > > q->ref = 1; // q->ref shall refer to n2 > p->ref = 2; // p->ref may refer to n1 > > std::cout << n1 << " " << n2 << std::endl; > } > > > I suppose the vector internally has a pointer to the beginning of its buffer, which is practically a pointer to the first element, but since we reallocated a new element in its place, the internal pointer in the vector may or may not refer to the first element? -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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On 31 May 2013 18:17, Nikolay Ivchenkov <mk.ivchenkov@gmail.c= om> wrote:
On Friday, May 31, 2013 5:= 42:45 PM UTC+4, DeadMG wrote:
A trivial example not involving optional would be, say, the last eleme= nt of a std::vector. It's quite legal and normal for the user to overwr= ite reference locations by destructing the original object and reconstructi= ng a new one in it's place.

We can do that:

=A0=A0=A0 #include <iostream>
=A0=A0=A0 = #include <new>
=A0=A0=A0 #include <vector>

=A0=A0=A0 = struct A
=A0=A0=A0 {

=A0=A0=A0=A0=A0=A0=A0 A(int &x) : ref(x) {}
=A0=A0=A0=A0=A0=A0= =A0 int &ref;
=A0=A0=A0 };

=A0=A0=A0 int main()
=A0=A0=A0 = {
=A0=A0=A0=A0=A0=A0=A0 std::vector<A> v;
<= br>=A0=A0=A0=A0=A0=A0=A0 int n1 =3D 0, n2 =3D 0;

=A0=A0=A0=A0= =A0=A0=A0 v.emplace_back(n1);
=A0=A0=A0=A0=A0=A0=A0 A *p =3D &v.back();

=A0=A0=A0=A0=A0=A0=A0 = p->~A();
=A0=A0=A0=A0=A0=A0=A0 A *q =3D new(p) A(n2);

=A0=A0= =A0=A0=A0=A0=A0 q->ref =3D 1; // q->ref shall refer to n2
=A0=A0= =A0=A0=A0=A0=A0 p->ref =3D 2; // p->ref may refer to n1

=A0=A0=A0=A0=A0=A0=A0 std::cout << n1 << " " <<= n2 << std::endl;
=A0=A0=A0 }



I suppose the vector internally has a pointer to the= beginning of its buffer, which is practically
a pointer to the first element, but since we reallocated a new element in i= ts place, the internal
pointer in the vector may or may not r= efer to the first element?

--
 
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