220 33272 article Path: news.gmane.org!.POSTED!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Adding operator overloading case to structure binding Date: Fri, 21 Jul 2017 17:15:24 +0300 Lines: 98 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1500646529 12352 195.159.176.226 (21 Jul 2017 14:15:29 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 21 Jul 2017 14:15:29 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBC5JHI7A7ALRB7UYZDFQKGQEEQ5EOHY@isocpp.org Fri Jul 21 16:15:23 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qt0-f199.google.com ([209.85.216.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dYYi9-0002mH-Pq for gclcip-std-proposals@m.gmane.org; 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[2607:f8b0:400c:c08::22e]) by mx.google.com with ESMTPS id 91si387729uah.327.2017.07.21.07.15.25 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 21 Jul 2017 07:15:25 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400c:c08::22e as permitted sender) client-ip=2607:f8b0:400c:c08::22e; Original-Received: by mail-ua0-x22e.google.com with SMTP id f9so46555377uaf.4 for ; Fri, 21 Jul 2017 07:15:25 -0700 (PDT) X-Received: by 10.176.3.132 with SMTP id 4mr4864100uau.51.1500646525071; Fri, 21 Jul 2017 07:15:25 -0700 (PDT) Original-Received: by 10.103.103.65 with HTTP; Fri, 21 Jul 2017 07:15:24 -0700 (PDT) In-Reply-To: X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.b=VO1SGFLw; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400c:c08::22e 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:33272 Archived-At: On 21 July 2017 at 17:04, Micha=C5=82 Dominiak wrote: > There is currently a customization point (or, a set of customization > points...) for this feature in the language: std::tuple_size, > std::tuple_element, and get(). They aren't pretty to use, but I'm not > convinced your proposed way to customize the behavior is really better. Right, however, in general, types convertible to types that can be structured-bound aren't supported by structured bindings. If you have struct X : tuple {}; you can't do structured bindings on it. On the other hand, allowing conversions gets tricky; what if I have data members in X? Then it 'obviously' shouldn't do structured bindings via a conversion to tuple, so what we would get via the conversions is not necessarily better. It would certainly make the rules more complex. > > On Fri, Jul 21, 2017 at 3:42 PM Logan Song wrot= e: >> >> According to the C++ standard, structure binding has three cases right >> now: >> >> binding an array >> binding a tuple-like type >> binding to public data members >> >> For example: >> #include >> #include >> >> class tag_ptr { >> private: >> void* holder; >> >> public: >> explicit operator ::std::tuple() { >> return ::std::tuple(ptr(), tag()); >> } >> >> void* ptr() {...} >> ::std::byte tag() {...} >> }; >> >> tag_ptr tp; >> >> auto ptr =3D tp.ptr(); >> auto tag =3D tp.tag(); //Uhhhh >> >> //error: cannot decompose non-public member 'holder' of 'tag_ptr' >> //auto [ptr, tag] =3D tp; >> >> Assume we have a class tag_ptr. It could store some bits compressed with= a >> pointer, which we couldn't expose it directly. >> As we can see, it's very cumbrous to init variables one by one. >> >> But if we relax the restrictions of rule2 to allow binding type which ha= s >> tuple-like type cast operator. >> The code will be more clear and it could make interfaces more graceful. >> >> That's my proposal. :) >> >> -- >> You received this message because you are subscribed to the Google Group= s >> "ISO C++ Standard - Future Proposals" group. >> To unsubscribe from this group and stop receiving emails from it, send a= n >> 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/d992c1f5-72= f1-4127-9b53-3a4d360a58d6%40isocpp.org. > > -- > 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/CAPCFJdSxx8T= BEC%2BbdPr%2BMnZPBYvZKrFK350zOnwE7U7Ug0BmmQ%40mail.gmail.com. --=20 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 e= mail 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/CAFk2RUa%2B-_mw2QNh_pYR-QakwEh60OgoeCMaGsWW1yans= RrV%2Bw%40mail.gmail.com. .