220 32491 article Path: news.gmane.org!.POSTED!not-for-mail From: Tom Honermann Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Allow Structured Binding Outside of Variable Declaration Date: Tue, 16 May 2017 13:01:11 -0400 Lines: 66 Approved: news@gmane.org Message-ID: References: <10b21241-b683-44d5-82ce-90681b2643a4@isocpp.org> <08ff73f1-6989-393e-ee4e-8158af2e7bed@honermann.net> <20170516164245.GA16348@fukushima.lysator.liu.se> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8"; format=flowed X-Trace: blaine.gmane.org 1494954081 7168 195.159.176.226 (16 May 2017 17:01:21 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 16 May 2017 17:01:21 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDBNXSHG6UDBBWHA5TEAKGQENOJLXWY@isocpp.org Tue May 16 19:01:12 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 1dAfqO-0001VW-MG for gclcip-std-proposals@m.gmane.org; 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[108.166.43.67]) by mx.google.com with ESMTPS id n11si2528295ith.112.2017.05.16.10.01.12 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 May 2017 10:01:12 -0700 (PDT) Received-SPF: neutral (google.com: 108.166.43.67 is neither permitted nor denied by best guess record for domain of tom@honermann.net) client-ip=108.166.43.67; Original-Received: from smtp25.relay.ord1c.emailsrvr.com (localhost [127.0.0.1]) by smtp25.relay.ord1c.emailsrvr.com (SMTP Server) with ESMTP id 2BD282059B for ; Tue, 16 May 2017 13:01:12 -0400 (EDT) X-Auth-ID: tom@honermann.net Original-Received: by smtp25.relay.ord1c.emailsrvr.com (Authenticated sender: tom-AT-honermann.net) with ESMTPSA id 014B720597 for ; Tue, 16 May 2017 13:01:11 -0400 (EDT) X-Sender-Id: tom@honermann.net Original-Received: from [192.168.1.7] (pool-173-53-30-126.rcmdva.fios.verizon.net [173.53.30.126]) (using TLSv1.2 with cipher DHE-RSA-AES128-SHA) by 0.0.0.0:587 (trex/5.7.12); Tue, 16 May 2017 13:01:12 -0400 In-Reply-To: <20170516164245.GA16348@fukushima.lysator.liu.se> X-Original-Sender: tom@honermann.net X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 108.166.43.67 is neither permitted nor denied by best guess record for domain of tom@honermann.net) smtp.mailfrom=tom@honermann.net 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:32491 Archived-At: On 05/16/2017 12:42 PM, Magnus Fromreide wrote: > On Tue, May 16, 2017 at 12:36:52PM -0400, Tom Honermann wrote: >> On 05/16/2017 11:30 AM, Ville Voutilainen wrote: >>> On 16 May 2017 at 18:25, Tom Honermann wrote: >>>>>> auto [k -> Key, =in] = parseKey(in); >>>>>> if (*in++ != ',') throw Failure(); >>>>>> auto [v -> Value, =in] = parseValue(in); >>>>>> if (*in++ != ')') throw Failure(); >>>>>> return {KVPair(k, v), in}; >>>>>> } >>>>> Well, with the idea I cobbled together, I can write >>>>> >>>>> auto [x, =k->Key] = parseKey(); >>>>> >>>>> and it means something completely different, because -> is a member >>>>> access operator. So we can safely >>>>> assume that k is an entity that supports operator->, and my binding >>>>> there assigns to k->Key. It's such fun >>>>> to try to extend a language where all good syntaxes have been taken a >>>>> long time ago. :) >>>>> >>>> Oh, well, the solution to that is, of course, keyword spackling. >>>> >>>> auto [x, =k-> typename Key] = parseKey(); >>>> >>>> :) >>> What makes you think Key is a typename? :P >>> >> Because when I wrote that, I was intending to write a type constraint and >> not a member access. You can tell because I put 'typename' there :P >> >> Also because 'Key' starts with an uppercase letter but isn't all upper case >> letters. As we all know, that means it unambiguously designates a type. >> >> Actually, a type constraint doesn't makes much sense for an assignment >> binding, and a member access doesn't make much sense for a structured >> binding, so perhaps there is no ambiguity after all. >> >> auto [x -> SomeType, =k->Key] = parseKey(); // Constrains 'x' to 'SomeType', assigns 'k->Key' >> >> Yes, there is a certain amount of ick factor there. > But why invent a new way to declare/constrain types that is almost unheard > about? This actually isn't a new way; it comes from the Concepts TS. > > What is wrong with the good old way? > > auto [SomeType x, =k->Key] = parseKey(); There is the question of what happens if the decomposed type of the matching element is not reference compatible with type 'SomeType'. Ill-formed? Does 'x' change from being a structured binding to a variable of the declared type? If so, are conversions performed? What type is decltype(x) and does it depend on the type of the matching element? A syntax that indicates a type constraint (which is, I think, what is desired regardless of the syntax used) rather than a variable declaration avoids these questions. Tom. -- 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/ffcb4ca3-5fb8-41b4-8b38-cfd287148cd5%40honermann.net. .