220 33019 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 13:30:28 +0300 Lines: 49 Approved: news@gmane.org Message-ID: References: <6e23864f-827a-4bdb-89de-a60d8f5be993@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 1498732233 31763 195.159.176.226 (29 Jun 2017 10:30:33 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 29 Jun 2017 10:30:33 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBC5JHI7A7ALRBRVN2PFAKGQEC5HBGUA@isocpp.org Thu Jun 29 12:30:27 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qt0-f198.google.com ([209.85.216.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dQWiQ-0007xs-Bt for gclcip-std-proposals@m.gmane.org; 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[2607:f8b0:4001:c0b::230]) by mx.google.com with ESMTPS id a128si734091ita.122.2017.06.29.03.30.29 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 29 Jun 2017 03:30:29 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:4001:c0b::230 as permitted sender) client-ip=2607:f8b0:4001:c0b::230; Original-Received: by mail-it0-x230.google.com with SMTP id m84so4520324ita.0 for ; Thu, 29 Jun 2017 03:30:29 -0700 (PDT) X-Received: by 10.36.33.202 with SMTP id e193mr12406254ita.92.1498732229307; Thu, 29 Jun 2017 03:30:29 -0700 (PDT) Original-Received: by 10.36.122.211 with HTTP; Thu, 29 Jun 2017 03:30:28 -0700 (PDT) In-Reply-To: <6e23864f-827a-4bdb-89de-a60d8f5be993@isocpp.org> X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.b=ITsuM34H; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:4001:c0b::230 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:33019 Archived-At: On 29 June 2017 at 12:51, Igor Baidiuk wrote: > At the moment, std::optional type, in the way it was accepted by committee, > always has storage block for its value and a flag field, which determines if > actual value is present. > Although, there are often safe wrapper types (like std::reference_wrapper, > or wrapper around OS handle, or a raw pointer) which have kind of > "impossible state" on their own. For such types, it's perfectly possible to > represent std::nullopt state as this "impossible" value: > > Allow to define optional sentinel value for a type. Current approach in STL > seems to be trait struct for the type and trait adaptor used in > std::optional to access values (similar to std::allocator + > std::allocator_traits). There might be a requirement that sentinel > value should be constexpr. > If type defines sentinel value, then std::optional's nullopt is represented > with that sentinel value, and there's no additional flag > > There are several benefits coming with this approach: > > Space efficiency for types with "invalid value" > Type safety. Sentinel value simply cannot be used the same way as normal > value. Roughly saying, no "null pointer dereference" issue > > The other extension, orthogonal to first one, is to allow value to be stored > using alternate representation. As an example, reference type can be stored > as a pointer and simply presented as reference. This will also allow to have > cheap and relatively easy support for storing references in std::optional, > without need of completely separate type specialization. > > > Any thoughts on this? That's a type different from optional. And optional-with-sentinel that considers a certain raw pointer value to be the sentinel value is not the same thing as an optional that adds a nullopt state. The latter can't be made to act like the former the way you suggest, because once a trait is present, it can't be removed, or we have an odr violation. You need a new type. -- 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. 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