220 32494 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 14:57:17 -0400 Lines: 62 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 1494961043 22974 195.159.176.226 (16 May 2017 18:57:23 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 16 May 2017 18:57:23 +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+bncBDBNXSHG6UDBBD4X5XEAKGQEUOUWEMA@isocpp.org Tue May 16 20:57:18 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pg0-f70.google.com ([74.125.83.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dAhek-0005mi-Jl for gclcip-std-proposals@m.gmane.org; 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[67.192.241.131]) by mx.google.com with ESMTPS id k13si55645oih.87.2017.05.16.11.57.18 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 May 2017 11:57:18 -0700 (PDT) Received-SPF: neutral (google.com: 67.192.241.131 is neither permitted nor denied by best guess record for domain of tom@honermann.net) client-ip=67.192.241.131; Original-Received: from smtp21.relay.dfw1a.emailsrvr.com (localhost [127.0.0.1]) by smtp21.relay.dfw1a.emailsrvr.com (SMTP Server) with ESMTP id 67C1E401EB for ; Tue, 16 May 2017 14:57:18 -0400 (EDT) X-Auth-ID: tom@honermann.net Original-Received: by smtp21.relay.dfw1a.emailsrvr.com (Authenticated sender: tom-AT-honermann.net) with ESMTPSA id 303B84018D for ; Tue, 16 May 2017 14:57:18 -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 14:57:18 -0400 In-Reply-To: X-Original-Sender: tom@honermann.net X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 67.192.241.131 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:32494 Archived-At: On 05/16/2017 01:35 PM, Ville Voutilainen wrote: > On 16 May 2017 at 20:01, Tom Honermann wrote: >>> 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 > Why on earth would it change from being a structured binding to something else? Nicol's response nicely explains this perspective; if one expects ' ' to declare a variable as it does, I think, everywhere else that syntax is currently allowed, then use of that syntax here to declare something other than a variable is surprising. An argument that this syntax, rather than declaring a structured binding, declare a variable initialized by the decomposed element, is sound. I'm not saying we need or should have that capability (to declare a variable instead of a binding), just that it is arguably the semantic behavior one might expect; exactly as Nicol described. > >> 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. > > Funny, such "type constraints" were something I always considered to > be an integral and fundamental > part of variable declarations. They certainly are; a variable declaration necessarily identifies a type (in C++). But a type constraint doesn't necessarily imply a variable declaration. > I don't write > > auto x = foo(); static_assert(is_same_v); > > but for some reason it's been suggested that I should do that for > structured bindings, conveniently forgetting > that then I will need to put that "type constraint" far far away from > my structured bindings, like into a for-loop body > when the structured binding is in the loop header. > I'm not following this. We're discussing: for (auto [Type x] : c) {} vs for (auto [x -> Type] : c) {} Unless you were responding to questions elsewhere regarding the motivation to be able to express a type constraint at all? I agree with you that some means to constrain the type in the structured binding declaration is desirable. 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/c0bf3a06-53e5-3411-0967-b167da5d0dcd%40honermann.net. .