220 32486 article Path: news.gmane.org!.POSTED!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Allow Structured Binding Outside of Variable Declaration Date: Tue, 16 May 2017 17:37:09 +0300 Lines: 55 Approved: news@gmane.org Message-ID: References: <10b21241-b683-44d5-82ce-90681b2643a4@isocpp.org> <08ff73f1-6989-393e-ee4e-8158af2e7bed@honermann.net> 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 1494945430 31888 195.159.176.226 (16 May 2017 14:37:10 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 16 May 2017 14:37:10 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBC5JHI7A7ALRBFU55TEAKGQEO4ROBZA@isocpp.org Tue May 16 16:37:06 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f197.google.com ([209.85.223.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dAdb0-00088u-7A for gclcip-std-proposals@m.gmane.org; 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[2607:f8b0:4003:c06::235]) by mx.google.com with ESMTPS id v3si942335oib.199.2017.05.16.07.37.10 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 May 2017 07:37:10 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:4003:c06::235 as permitted sender) client-ip=2607:f8b0:4003:c06::235; Original-Received: by mail-oi0-x235.google.com with SMTP id b204so27663155oii.1 for ; Tue, 16 May 2017 07:37:10 -0700 (PDT) X-Received: by 10.157.56.140 with SMTP id p12mr5733813otc.206.1494945430121; Tue, 16 May 2017 07:37:10 -0700 (PDT) Original-Received: by 10.157.17.253 with HTTP; Tue, 16 May 2017 07:37:09 -0700 (PDT) In-Reply-To: <08ff73f1-6989-393e-ee4e-8158af2e7bed@honermann.net> X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:4003:c06::235 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:32486 Archived-At: On 16 May 2017 at 17:29, Tom Honermann wrote: >> On that note, I can cobble together something like >> >> std::pair parseKey(const char *); >> std::pair parseValue(const char *); >> >> std::pair parseKVPair(const char *in) >> { >> if (*in++ != '(') throw Failure(); >> auto [Key k, =in] = parseKey(in); >> if (*in++ != ',') throw Failure(); >> auto [Value v, =in] = parseValue(in); >> if (*in++ != ')') throw Failure(); >> return {KVPair(k, v), in}; >> } >> >> So two things there, allow specifying the type of a binding (my >> favorite thing that structured bindings >> don't let me do) and allow the binding to assign instead of initializing. >> > I like this, but still prefer the following syntax discussed in Issaquah to > specify a type constraint rather than something that looks like a variable > declaration with non-obvious conversion semantics: > > std::pair parseKey(const char *); > std::pair parseValue(const char *); > > std::pair parseKVPair(const char *in) > { > if (*in++ != '(') throw Failure(); > 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. :) -- 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/CAFk2RUaBhq7KVukgRMvqBMuBiBwZmCf%2BxZpD9jRtBberFoU2Rg%40mail.gmail.com. .