220 32485 <08ff73f1-6989-393e-ee4e-8158af2e7bed@honermann.net> 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 10:29:03 -0400 Lines: 74 Approved: news@gmane.org Message-ID: <08ff73f1-6989-393e-ee4e-8158af2e7bed@honermann.net> References: <10b21241-b683-44d5-82ce-90681b2643a4@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"; format=flowed X-Trace: blaine.gmane.org 1494944946 19541 195.159.176.226 (16 May 2017 14:29:06 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 16 May 2017 14:29:06 +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+bncBDBNXSHG6UDBBMUZ5TEAKGQE5ZGMVNI@isocpp.org Tue May 16 16:29:02 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oi0-f70.google.com ([209.85.218.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dAdTB-0004w5-P1 for gclcip-std-proposals@m.gmane.org; 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[67.192.241.139]) by mx.google.com with ESMTPS id v187si962853oia.196.2017.05.16.07.29.06 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 May 2017 07:29:06 -0700 (PDT) Received-SPF: neutral (google.com: 67.192.241.139 is neither permitted nor denied by best guess record for domain of tom@honermann.net) client-ip=67.192.241.139; Original-Received: from smtp22.relay.dfw1a.emailsrvr.com (localhost [127.0.0.1]) by smtp22.relay.dfw1a.emailsrvr.com (SMTP Server) with ESMTP id AB8CD203E7 for ; Tue, 16 May 2017 10:29:05 -0400 (EDT) X-Auth-ID: tom@honermann.net Original-Received: by smtp22.relay.dfw1a.emailsrvr.com (Authenticated sender: tom-AT-honermann.net) with ESMTPSA id 6A404203BC for ; Tue, 16 May 2017 10:29:05 -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 10:29:05 -0400 In-Reply-To: X-Original-Sender: tom@honermann.net X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 67.192.241.139 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:32485 Archived-At: On 05/16/2017 01:52 AM, Ville Voutilainen wrote: > On 16 May 2017 at 08:37, Ville Voutilainen wrote: >> On 16 May 2017 at 08:13, Arthur O'Dwyer wrote: >>>> [a,text] = tuple(3, "Hello, world"); >>> >>> No, that's just std::tie; we have that. >>> What we don't have in any sane form, but which IMO every parser-programmer >>> is waiting for, is >>> >>> std::pair parseKey(const char *); >>> std::pair parseValue(const char *); >>> >>> std::pair parseKVPair(const char *in) >>> { >>> if (*in++ != '(') throw Failure(); >>> Key k [,in] = parseKey(in); >>> if (*in++ != ',') throw Failure(); >>> Value v [,in] = parseValue(in); >>> if (*in++ != ')') throw Failure(); >>> return {KVPair(k, v), in}; >>> } >>> >>> Now, this is terrible syntax, and it also doesn't really work if you were to >>> put the "const char *" first in the return type instead of last. So I'm by >>> no means proposing this syntax. I'm just using it to illustrate the semantic >> So your point is that std::tie does "assignment bindings", structured >> bindings do >> "initializing bindings", but neither of them can do a mixture of both. > > 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}; } Tom. -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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