220 33023 article Path: news.gmane.org!.POSTED!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: std::optional - support for sentinel values Date: Thu, 29 Jun 2017 14:13:07 +0300 Lines: 30 Approved: news@gmane.org Message-ID: References: <6e23864f-827a-4bdb-89de-a60d8f5be993@isocpp.org> <9f81f2b9-958c-4018-9f78-a6ac2e425899@isocpp.org> <730d0077-5a80-4dce-8250-19be62b1a23b@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" X-Trace: blaine.gmane.org 1498734790 19003 195.159.176.226 (29 Jun 2017 11:13:10 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 29 Jun 2017 11:13:10 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBC5JHI7A7ALRBRGB2PFAKGQEI7BEINI@isocpp.org Thu Jun 29 13:13:06 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f200.google.com ([209.85.223.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dQXNh-0004bE-GE for gclcip-std-proposals@m.gmane.org; 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[2607:f8b0:4001:c06::236]) by mx.google.com with ESMTPS id k195si4206810iok.285.2017.06.29.04.13.08 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 29 Jun 2017 04:13:08 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:4001:c06::236 as permitted sender) client-ip=2607:f8b0:4001:c06::236; Original-Received: by mail-io0-x236.google.com with SMTP id h134so4889443iof.2 for ; Thu, 29 Jun 2017 04:13:08 -0700 (PDT) X-Received: by 10.107.175.161 with SMTP id p33mr18320580ioo.20.1498734788161; Thu, 29 Jun 2017 04:13:08 -0700 (PDT) Original-Received: by 10.36.122.211 with HTTP; Thu, 29 Jun 2017 04:13:07 -0700 (PDT) In-Reply-To: <730d0077-5a80-4dce-8250-19be62b1a23b@isocpp.org> X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.b=QAsEiTLi; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:4001:c06::236 as permitted sender) smtp.mailfrom=ville.voutilainen@gmail.com; dmarc=pass (p=NONE sp=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:33023 Archived-At: On 29 June 2017 at 14:09, Igor Baidiuk wrote: >> It has nothing to do with includes. Altering the definition of >> optional to conditionally use >> a trait would be remotely feasible, but it would also be an abi-breaking >> change. >> > > Maybe I didn't express it well enough. I meant the case when someone > declares such "sentinel" locally for module, and then return that option to > unit where sentinel is not known. > Then I realized this is well-possible even if new optional would have trait > parameter. Right; multiple translation units must agree, which makes it an odr-problem, not an include-problem. The trait-approach might still work, it just requires that the trait gives the same result in every TU, but injecting such a trait-approach into the current optional is an abi break. Going for a separate type is safe from that abi break, which automatically removes some automatic opposition from the picture. -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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