220 33271 article Path: news.gmane.org!.POSTED!not-for-mail From: =?UTF-8?Q?Micha=C5=82_Dominiak?= Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Adding operator overloading case to structure binding Date: Fri, 21 Jul 2017 14:04:06 +0000 Lines: 255 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: multipart/alternative; boundary="94eb2c18828a3c56940554d457c5" X-Trace: blaine.gmane.org 1500645865 17963 195.159.176.226 (21 Jul 2017 14:04:25 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 21 Jul 2017 14:04:25 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBCA6D5ULVYBRBYMTZDFQKGQEWRC4KHA@isocpp.org Fri Jul 21 16:04:16 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-it0-f69.google.com ([209.85.214.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dYYXN-00047y-Ex for gclcip-std-proposals@m.gmane.org; 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[2607:f8b0:400e:c00::233]) by mx.google.com with ESMTPS id n78si3062708pfg.444.2017.07.21.07.04.17 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 21 Jul 2017 07:04:17 -0700 (PDT) Received-SPF: neutral (google.com: 2607:f8b0:400e:c00::233 is neither permitted nor denied by best guess record for domain of admin@griwes.info) client-ip=2607:f8b0:400e:c00::233; Original-Received: by mail-pf0-x233.google.com with SMTP id q85so24602896pfq.1 for ; Fri, 21 Jul 2017 07:04:17 -0700 (PDT) X-Received: by 10.84.210.141 with SMTP id a13mr8396501pli.199.1500645856918; Fri, 21 Jul 2017 07:04:16 -0700 (PDT) In-Reply-To: X-Original-Sender: griwes@griwes.info X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@griwes-info.20150623.gappssmtp.com header.b=nHFqm4oq; spf=neutral (google.com: 2607:f8b0:400e:c00::233 is neither permitted nor denied by best guess record for domain of admin@griwes.info) smtp.mailfrom=admin@griwes.info 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:33271 Archived-At: --94eb2c18828a3c56940554d457c5 Content-Type: text/plain; charset="UTF-8" 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. On Fri, Jul 21, 2017 at 3:42 PM Logan Song wrote: > According to the C++ standard, structure binding has three cases right now: > > 1. binding an array > 2. binding a tuple-like type > 3. 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 = tp.ptr(); > auto tag = tp.tag(); //Uhhhh > > //error: cannot decompose non-public member 'holder' of 'tag_ptr' > //auto [ptr, tag] = 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 has > 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 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/d992c1f5-72f1-4127-9b53-3a4d360a58d6%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/CAPCFJdSxx8TBEC%2BbdPr%2BMnZPBYvZKrFK350zOnwE7U7Ug0BmmQ%40mail.gmail.com. --94eb2c18828a3c56940554d457c5 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
There is currently a customization point (or, a set of cus= tomization points...) for this feature in the language: std::tuple_size, st= d::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.
On Fri, Jul 21, 2017 at = 3:42 PM Logan Song <ashenveil= song@gmail.com> wrote:
According to the C++ standard,=C2=A0structure binding has three= cases right now:
  1. binding an array
  2. binding a tuple-like= type
  3. binding to public data members
For exampl= e:
#include <c= stddef>
#include <tuple>= ;

class tag_ptr {=
private:
=C2=A0 =C2=A0 v= oid* holder;

publi= c:
=C2=A0 =C2=A0 explicit operator ::std::tupl= e<void*,= ::std::byte>() {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 <= /span>return ::std::tu= ple(ptr(), tag());=
=C2=A0 =C2=A0 }

=C2=A0 =C2=A0 v= oid* ptr() {...}
=C2=A0 =C2=A0 ::std::byte tag() {...}
};<= /span>

tag_ptr tp;

auto ptr =3D tp.ptr();<= /span>
auto tag =3D tp.tag(); //Uhhhh
<= br>//e= rror: cannot decompose non-public member 'holder' of 'tag_ptr&#= 39;
//auto [ptr, tag] =3D tp;

Assume we have a class tag_ptr. It could store=C2=A0= some bits compressed with a pointer, which we couldn't expose it direct= ly.
As we can see, it's very cumbrous to init variables one b= y one.

But if we relax the restrictions of rule2 t= o allow binding type which has tuple-like type cast operator.
The= code will be more clear and it could make interfaces more graceful.
<= div>
That's my proposal. :)

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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/d992c1f5-72f1-= 4127-9b53-3a4d360a58d6%40isocpp.org.

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To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CAPCFJdSxx8TB= EC%2BbdPr%2BMnZPBYvZKrFK350zOnwE7U7Ug0BmmQ%40mail.gmail.com.
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