220 2392 article Path: news.gmane.org!not-for-mail From: Daniel James Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: optional, one more value or two states Date: Mon, 28 Jan 2013 09:07:23 +0000 Lines: 63 Approved: news@gmane.org Message-ID: References:

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Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 X-Trace: ger.gmane.org 1359364045 18090 80.91.229.3 (28 Jan 2013 09:07:25 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 28 Jan 2013 09:07:25 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCGPB3VVUAGBBTP7TCEAKGQENAYLKSA@isocpp.org Mon Jan 28 10:07:44 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wg0-f69.google.com ([74.125.82.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Tzkgp-0000zm-5u for gclcip-std-proposals@m.gmane.org; Mon, 28 Jan 2013 10:07:43 +0100 Original-Received: by mail-wg0-f69.google.com with SMTP id fn15sf2601007wgb.4 for ; Mon, 28 Jan 2013 01:07:25 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:x-beenthere:x-received:x-received:received-spf :mime-version:x-received: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=ARYpvfSan3/Fiykrfj9D4ZpXm3ftoUnqcKQwuqH8B5M=; b=rjjYGlJFVWIsVdR9L01RUWLNGS5k7pacsCej0BSMeLpzHGif9zrcAN37vpUk9VbHML MQh9Oeyb/CpKN/A8xK2zfYDyjY7WYFt1dqIu+0fnQzsfLF9X48ajOPcekZkkTia7k1um 0g/1kDxqPx9Tifsbn2GD5S3ABfVLUAdYYCBwDlkRcJ1zOFk17MC/hKim3B2NIM2Jdu09 q8qfih6wfuXoLV1Uee89S3+e8VQcMq5Ta7qSQaFpPag15oU3r9XkRRf/SzwwF2Thq5dL ARq+tXNL/xsUh4rTrITG7N1hkJZUK1PVt9znwLk81rRhanm1l X-Received: by 10.180.100.74 with SMTP id ew10mr1452837wib.7.1359364045251; Mon, 28 Jan 2013 01:07:25 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.14.105 with SMTP id o9ls860529wic.3.gmail; Mon, 28 Jan 2013 01:07:23 -0800 (PST) X-Received: by 10.180.101.99 with SMTP id ff3mr8333520wib.21.1359364043858; Mon, 28 Jan 2013 01:07:23 -0800 (PST) X-Received: by 10.180.101.99 with SMTP id ff3mr8333518wib.21.1359364043836; Mon, 28 Jan 2013 01:07:23 -0800 (PST) Original-Received: from mail-we0-x235.google.com ([2a00:1450:400c:c03::235]) by mx.google.com with ESMTPS id fy9si3130273wjb.199.2013.01.28.01.07.23 (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 28 Jan 2013 01:07:23 -0800 (PST) Received-SPF: pass (google.com: domain of dnljms@gmail.com designates 2a00:1450:400c:c03::235 as permitted sender) client-ip=2a00:1450:400c:c03::235; Original-Received: by mail-we0-f181.google.com with SMTP id t44so1334099wey.12 for ; Mon, 28 Jan 2013 01:07:23 -0800 (PST) X-Received: by 10.194.78.236 with SMTP id e12mr19659022wjx.32.1359364043677; Mon, 28 Jan 2013 01:07:23 -0800 (PST) Original-Received: by 10.194.46.7 with HTTP; Mon, 28 Jan 2013 01:07:23 -0800 (PST) In-Reply-To: X-Original-Sender: dnljms@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of dnljms@gmail.com designates 2a00:1450:400c:c03::235 as permitted sender) smtp.mail=dnljms@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:2392 Archived-At: On 27 January 2013 21:39, Olaf van der Spek wrote: > On Sun, Jan 27, 2013 at 9:27 PM, Daniel James wrote: >> But it could easily be changed to meet them. 'Optional' is a container >> (in the wider sense) with a maximum of 1 element. It is not in any >> sense a pointer. In fact, std::optional probably should meet the basic >> container requirements, as it would allow it to be used with generic >> functions. > > It's a bit like static_vector with max_size 1, true. Though I'm not > sure what supporting a container interface really buys you. It depends on how well the range algorithms works out. They might provide a more elegant way to manipulate optionals than if statements, similar to how a range algorithm can be more elegant than a for loop. This has worked out well in other languages, for example in Scala: http://www.scala-lang.org/api/current/index.html#scala.Option It also allows for use in generic code, the classic example is 'flatten', which flattens a single layer of a nested container, e.g. from vector> to vector. If optional works as a range then 'flatten' will also work for vector> resulting in a vector that only contains the elements with values. This is particularly useful when combined with 'transform', allowing you to use a function which returns many values or optional values and then flatten them into a single list. In functional languages this is called 'flatMap' and is considered a fundamental building blocks. In C++ such a function would have to have a different name (not sure what, 'flat_transform' feels a bit off), and perhaps not be quite so useful, but the point is that seeing optionals as ranges has a proven track record. >>> I think using operator* is natural. >> >> But it does currently imply certain semantics. The current situation >> is that if you write: >> >> auto x = y; >> *x = 2; >> >> Then for any standard type this is equivalent to assigning '2' to >> whatever y points to. By using 'operator*' in std::optional, the >> semantics of 'operator*' will be changed. That shouldn't be done >> lightly, especially since with templates and 'auto' a type is defined >> by the functions and operators that it supports.. > > Isn't that more of a problem with the constructor than with operator*? The structural type implies certain behaviour. When a class has 'operator*' it should be expected to behave like a pointer, or if you prefer, a proxy. Which implies that the constructor should also behave like a pointer/proxy. -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To post to this group, send email to std-proposals@isocpp.org. To unsubscribe from this group, send email to std-proposals+unsubscribe@isocpp.org. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/?hl=en. .