220 33274 <1e3def09-89c9-49af-a126-722e35c8e34f@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Adding operator overloading case to structure binding Date: Fri, 21 Jul 2017 09:18:52 -0700 (PDT) Lines: 163 Approved: news@gmane.org Message-ID: <1e3def09-89c9-49af-a126-722e35c8e34f@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1506_2114818788.1500653932333" X-Trace: blaine.gmane.org 1500653934 9195 195.159.176.226 (21 Jul 2017 16:18:54 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 21 Jul 2017 16:18:54 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBB3OSZDFQKGQETH54AAA@isocpp.org Fri Jul 21 18:18:49 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pg0-f69.google.com ([74.125.83.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dYadc-00025m-Px for gclcip-std-proposals@m.gmane.org; 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Fri, 21 Jul 2017 09:18:54 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.143.88 with SMTP id r85ls8287437iod.5.gmail; Fri, 21 Jul 2017 09:18:53 -0700 (PDT) X-Received: by 10.31.164.205 with SMTP id n196mr30974vke.22.1500653932953; Fri, 21 Jul 2017 09:18:52 -0700 (PDT) In-Reply-To: X-Original-Sender: jmckesson@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:33274 Archived-At: ------=_Part_1506_2114818788.1500653932333 Content-Type: multipart/alternative; boundary="----=_Part_1507_585357188.1500653932334" ------=_Part_1507_585357188.1500653932334 Content-Type: text/plain; charset="UTF-8" On Friday, July 21, 2017 at 9:42:48 AM UTC-4, 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. > .... why? Why go through the intermediate `tuple`? If you have the ability to add `operator tuple` overloads, then you have the ability to add the `get/tuple_element/tuple_size` machinery to your type. Alternatively, why not simply have a `decompose` member function that returns said `tuple`? -- 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/1e3def09-89c9-49af-a126-722e35c8e34f%40isocpp.org. ------=_Part_1507_585357188.1500653932334 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Friday, July 21, 2017 at 9:42:48 AM UTC-4, Loga= n Song 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 example:
#includ= e <c= stddef>
#include <tuple>

class tag_ptr {
private:
=C2=A0 =C2=A0 void* holder;

public:
=C2=A0 =C2=A0 explicit <= span style=3D"color:#008">operator ::std<= span style=3D"color:#660">::tuple<= span style=3D"color:#660"><void= *, ::std::byte>() {
=C2=A0 =C2=A0 =C2=A0 =C2= =A0 return ::s= td::tup= le(ptr<= /span>(), tag<= /span>());
= =C2=A0 =C2=A0 }

=C2=A0 =C2=A0 void= * ptr() {...}
=C2=A0 =C2= =A0 ::s= td::byt= e tag()= {...}<= /span>
};

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. I= t could store=C2=A0some bits compressed with a pointer, which we couldn'= ;t expose it directly.
As we can see, it's very cumbrous to i= nit variables one by one.

But if we relax the rest= rictions of rule2 to allow binding type which has tuple-like type cast oper= ator.
The code will be more clear and it could make interfaces mo= re graceful.

... why? Why go through the i= ntermediate `tuple`? If you have the ability to add `operator tuple` overlo= ads, then you have the ability to add the `get/tuple_element/tuple_size` ma= chinery to your type.

Alternatively, why not simply have a `decomp= ose` member function that returns said `tuple`?

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/1e3def09-89c9-49af-a126-722e35c8e34f= %40isocpp.org.
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