220 34185 article Path: news.gmane.org!.POSTED!not-for-mail From: Curious Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Structured bindings with polymorphic lambdas Date: Mon, 28 Aug 2017 12:20:29 -0700 (PDT) Lines: 275 Approved: news@gmane.org Message-ID: References: <8497b31b-7c9d-4334-ac22-0809c958cb93@isocpp.org> <15a70d94-e039-4f82-ba01-bde9d3fbacbd@isocpp.org> <6b50933d-291b-4b47-8c0c-82c97d9cb406@isocpp.org> <348b59eb-c288-482e-a7f4-473d8f1fbfea@isocpp.org> <029d244a-cc53-40de-9c2f-59b1bc776c1b@isocpp.org> <481b4d44-74e0-4cd3-8141-62d35dea8350@isocpp.org> <3509394f-e995-41d1-8fe0-3e7d8ecfe63e@isocpp.org> <27932a56-329e-4854-8d6f-e8a4812adadb@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_7146_1600220776.1503948029651" X-Trace: blaine.gmane.org 1503948032 19053 195.159.176.226 (28 Aug 2017 19:20:32 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 28 Aug 2017 19:20:32 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCNP5RVE2QIBB7WZSHGQKGQESS7D47I@isocpp.org Mon Aug 28 21:20:27 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f199.google.com ([209.85.217.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dmPaE-0004WH-HS for gclcip-std-proposals@m.gmane.org; Mon, 28 Aug 2017 21:20:26 +0200 Original-Received: by mail-ua0-f199.google.com with SMTP id k14sf1304954uad.17 for ; Mon, 28 Aug 2017 12:20:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=YUyAx6swaz+7Wga2ZnzCJzEA4Pntuekrv0FcDCvUioY=; b=gs3IRjg8ZVDGMfzgiKYBUgLYOi6yCVFHG4KSfh1YZPdiT1FTG86sV4lhgVecpO/CpD TsmWyNJUdiCAtlzslVwyhO6D1YBp4BddsDLrnR0YIFWnjwH6aO8dL1JjuZQUzP3CvGVY RSLL86UQRem1TbIc05AAAzNj9ZRhpKne9jucQ5Pf9kmhl7m9z7Xq5Vr/z+hgpsnzeCF7 A2SqFXAlY7m8EaQYv4KzUODUXYbHDelZNAfDi1cFvhB15/hAhESZmgXYWZjOgwmain4S 7teivvFuLu1R18syo4C6s/NUkpgP/4Pcym8hwZykj7KrEghc6e9wT5LJ3P/CDBIqBKwH swIA== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=YUyAx6swaz+7Wga2ZnzCJzEA4Pntuekrv0FcDCvUioY=; b=RZE/TbiOtW72wnD9AcOloiqEJJgLdMWoyClhi4wPyaNl9ly2RXwj5X9LhtcZBvS8WJ GG0SHvft2OWe3OwLWxNQu5HtfUs12r35ajbpf5WPEgutiYuZJUsrNHO0jdXGViqtxoLh 0AroGLoY1H1LdBuWBTH485KNM4iGM1bimwNIkzgsmCVdgwdv6TzUnJTZl9wLObUGgfCA 7LjGNzEieKHBSp+JnDXE2KdJ/ugLZxG3pgTnVX4CkUvgOj2MHMb338Kvn03qg8jF487J nmL9wsthCaSYtxqCVQmrvBehzHqHWc8GAb2N2DnXrhvwc5vFcaiRB6uOcHAiRi/WOK9v lMqA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=YUyAx6swaz+7Wga2ZnzCJzEA4Pntuekrv0FcDCvUioY=; b=PX4ctZ0rKJnJOKlrh2saHSyuk33xpWe17vr2DP6PcegYFyrFQkKFrllToK8l9Qj+Qj JWlAeeXzkOnwD4FO9DlDtQnVq5X6duuztWflUc75tz0VMv0NAnrahIha2dX2aTambO4u 6Bzfa8N8xfShtY6CJ0+Vm38GM8QceKx3VESLpyCwf2GyiYFvowXy0yI2OyV65OhWHOM3 GF7O88YlUsTgPnV/AZ+H3MqiVwvq/Iv1qjm1bv1fNteiZLj0KN4R9BmRAs4CTfZyCRYC aN0gKVX0vLEAMQ6GW7FYewkM6Zyw4iYA+rxca7jUhGiXGSmWLofg0BfSJFBfu0rVeFUj k58Q== X-Gm-Message-State: AHYfb5iEn8FEOd8MIkTPk5G0ecuPCnA7u4L6QQ1QWqfLTO2vcZqX9/gE edrCJnKMwQKe5g0t X-Received: by 10.159.36.133 with SMTP id 5mr1020685uar.42.1503948031323; Mon, 28 Aug 2017 12:20:31 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.65.217 with SMTP id b86ls915178itd.18.canary-gmail; Mon, 28 Aug 2017 12:20:30 -0700 (PDT) X-Received: by 10.31.3.106 with SMTP id 103mr19010vkd.11.1503948030040; Mon, 28 Aug 2017 12:20:30 -0700 (PDT) In-Reply-To: <27932a56-329e-4854-8d6f-e8a4812adadb@isocpp.org> X-Original-Sender: rmn100@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:34185 Archived-At: ------=_Part_7146_1600220776.1503948029651 Content-Type: multipart/alternative; boundary="----=_Part_7147_688418426.1503948029651" ------=_Part_7147_688418426.1503948029651 Content-Type: text/plain; charset="UTF-8" I think I wasn't clear. It is prone to ODR violations not neccesarily the same way as is_decomposable would be but like the following class Something { public: using iterator_category = int; using difference_type = int; using pointer = int*; using value_type = int; using reference = int&; }; void foo(Something) { auto iter_traits = std::iterator_traits{}; static_cast(iter_traits); } And then in another cpp file, which has the definition of Something visible namespace std { template <> class iterator_traits { public: using iterator_category = int; using difference_type = std::size_t; using pointer = int*; using value_type = int; using reference = int&; }; } // namespace std void bar(Something) { auto iter_traits = std::iterator_traits{}; static_cast(iter_traits); } Would it be best to just exclude is_decomposable from the proposal? Is there any other way to achieve the ability it provides to the instantiations of the lambda? On Monday, 28 August 2017 11:19:29 UTC-7, T. C. wrote: > > > On Monday, August 28, 2017 at 2:14:03 PM UTC-4, Curious wrote: >> >> @T. C, The is_decomposable trait has the same potential ODR issues as >> std::iterator_traits. >> > > It doesn't. Instantiating iterator_traits will always give you the same > definition if T is complete. And instantiating standard library templates > with incomplete types is generally prohibited. > > is_decomposable, OTOH, does not have the luxury of ignoring incomplete > tuple_sizes. > > > -- 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/e148a17c-acc0-44a3-854b-531b13c0520d%40isocpp.org. ------=_Part_7147_688418426.1503948029651 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
I think I wasn't clear. =C2=A0It is prone to ODR viola= tions not neccesarily the same way as is_decomposable would be but like the= following

<= span style=3D"color: #008;" class=3D"styled-by-prettify">class Something {
public:
=C2=A0 =C2=A0 using iterator_category =3D int;
=C2=A0 =C2= =A0 using<= /span> difference= _type =3D<= /span> int;
=C2=A0 =C2=A0 using pointer =3D int*;
=C2=A0 =C2=A0 using value_type =3D i= nt;=
=C2=A0 =C2=A0= using reference =3D int&;
};


void foo(Something) {
=C2=A0 =C2=A0 auto iter_traits <= span style=3D"color: #660;" class=3D"styled-by-prettify">=3D std::iterator_traits<Something>{};
=C2=A0 =C2=A0 static_cast<void>(iter_traits);
}

And then in another cpp = file, which has the definition of Som= ething visible

namespace std {
template <>
class iterator_traits<Something> {
<= span style=3D"color: #008;" class=3D"styled-by-prettify">public:
=C2=A0 =C2=A0 using iterator_category =3D int;
=C2=A0 =C2=A0 using difference_type =3D std::size_t;=
=C2=A0 =C2=A0 using pointer =3D int*;
=C2=A0 =C2=A0 using value_type =3D int;
=C2=A0 =C2=A0 using reference =3D int&;
};
} // namespace std


void bar(Something) {=C2=A0 =C2=A0 auto= iter_traits =3D std= ::<= span style=3D"color: #000;" class=3D"styled-by-prettify">iterator_traits<Something>{};
=C2=A0 =C2=A0 static_cast<void>(iter_traits);
}

Wou= ld it be best to just exclude is_deco= mposable from the proposal? =C2=A0Is there any other way to achieve = the ability it provides to the instantiations of the lambda?

On Mond= ay, 28 August 2017 11:19:29 UTC-7, T. C. wrote:

On Monday, August 28, 2017 at 2:14:0= 3 PM UTC-4, Curious wrote:
@T. C, The is_decomposable trait has the same potential ODR issues as std::iterator_traits.

It doesn't. Instantiating i= terator_traits<T> will always give you the same definition if T is co= mplete. And instantiating standard library templates with incomplete types = is generally prohibited.=C2=A0

is_decomposable, OT= OH, does not have the luxury of ignoring incomplete tuple_size<X>s.


--
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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/e148a17c-acc0-44a3-854b-531b13c0520d= %40isocpp.org.
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