220 33047 article Path: news.gmane.org!.POSTED!not-for-mail From: "Arthur O'Dwyer" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: std::optional - support for sentinel values Date: Fri, 30 Jun 2017 12:42:25 -0700 Lines: 114 Approved: news@gmane.org Message-ID: References: <6e23864f-827a-4bdb-89de-a60d8f5be993@isocpp.org> <2991b2b2-2f62-42e2-83d1-f42dae9c32c1@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="94eb2c0de474e2fb670553329d12" X-Trace: blaine.gmane.org 1498851752 11455 195.159.176.226 (30 Jun 2017 19:42:32 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 30 Jun 2017 19:42:32 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBDLZJYWNDQIKHU62ZICRUBCDGYKZQ@isocpp.org Fri Jun 30 21:42:28 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f71.google.com ([209.85.215.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dR1o7-0002Tf-Ef for gclcip-std-proposals@m.gmane.org; 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[2a00:1450:400c:c09::232]) by mx.google.com with ESMTPS id z130si10824061wmb.116.2017.06.30.12.42.26 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 30 Jun 2017 12:42:26 -0700 (PDT) Received-SPF: pass (google.com: domain of arthur.j.odwyer@gmail.com designates 2a00:1450:400c:c09::232 as permitted sender) client-ip=2a00:1450:400c:c09::232; Original-Received: by mail-wm0-x232.google.com with SMTP id i127so55772904wma.0 for ; Fri, 30 Jun 2017 12:42:26 -0700 (PDT) X-Received: by 10.80.146.79 with SMTP id j15mr6590309eda.17.1498851745919; Fri, 30 Jun 2017 12:42:25 -0700 (PDT) Original-Received: by 10.80.173.204 with HTTP; Fri, 30 Jun 2017 12:42:25 -0700 (PDT) In-Reply-To: <2991b2b2-2f62-42e2-83d1-f42dae9c32c1@isocpp.org> X-Original-Sender: arthur.j.odwyer@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.b=PgWBpSuL; spf=pass (google.com: domain of arthur.j.odwyer@gmail.com designates 2a00:1450:400c:c09::232 as permitted sender) smtp.mailfrom=arthur.j.odwyer@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:33047 Archived-At: --94eb2c0de474e2fb670553329d12 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, Jun 30, 2017 at 12:28 PM, Barry Revzin wrote: > >> This is optional>. >> As for why optional is a terrible, horrible, no-good, very bad idea, >> please see Matt Calabrese's talk >> from C++Now. >> > > That's not at all the impression I got from that talk. One of the things > he specifically mentions as a good idea is to support references for > optional/variant, but internally wrap the references so that they behave = as > regular types (that is, like pointers. so operator=3D would rebind > references, not assign through them). > I'll have to re-watch it, then. I would say that "an optional of a regular type that behaves like a pointer" is a good idea, but *that* one is spelled optional. Notice that part of behaving like a regular type is making sure that the semantics of operator=3D=3D match the semantics of your copy-constructor: a copy of a value should compare =3D=3D to the original value, and modifying the copy should not modify the original. I'm pretty sure Matt talks about *that* pretty extensively. The places where we put "foo" into the standard are either places where the rebinding question never comes up (e.g. std::promise) or places where the rebinding question *does* come up and the answer is "oh crap, maybe that was a bad idea" (e.g. std::tuple). I said as much during Matt's talk (hence why I haven't watched the video). IMO, if C++11 had had proper multiple-assignment, then it wouldn't have needed assign-throughable std::tie, and if it didn't have std::tie, then it wouldn't have had std::tuple. For the cases where we use std::tie and std::forward_as_tuple today, we could equally well be using a "std::forwarding_tuple" with perfect-forwarding read-through semantics and no operator=3D at all. But, hindsight is 20/20. =E2=80=93Arthur --=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/CADvuK0JexAJBZn1LsAPcZnabCJ4pz3J3HUahGeX_FEW8HBP= Ctw%40mail.gmail.com. --94eb2c0de474e2fb670553329d12 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Fri, Jun 30, 2017 at 12:28 PM, Barry Revzin <ba= rry.revzin@gmail.com> wrote:

= This is optional<reference_wrapper<T>>.
As for w= hy optional<T&> is a terrible, horrible, no-good, very bad idea, = please see Matt Calabrese's talk from C++Now.

That's not at all the impressi= on I got from that talk. One of the things he specifically mentions as a go= od idea is to support references for optional/variant, but internally wrap = the references so that they behave as regular types (that is, like pointers= .. so operator=3D would rebind references, not assign through them).

I'll have to re-watch it, then. I = would say that "an optional of a regular type that behaves like a poin= ter" is a good idea, but that one is spelled optional<T*>= ..=C2=A0 Notice that part of behaving like a regular type is making sure tha= t the semantics of operator=3D=3D match the semantics of your copy-construc= tor: a copy of a value should compare =3D=3D to the original value, and mod= ifying the copy should not modify the original.=C2=A0 I'm pretty sure M= att talks about that pretty extensively.

Th= e places where we put "foo<T&>" into the standard are e= ither places where the rebinding question never comes up (e.g. std::promise= <T&>) or places where the rebinding question does come up = and the answer is "oh crap, maybe that was a bad idea" (e.g. std:= :tuple<T&>).=C2=A0 I said as much during Matt's talk (hence w= hy I haven't watched the video).=C2=A0 IMO, if C++11 had had proper mul= tiple-assignment, then it wouldn't have needed assign-throughable std::= tie, and if it didn't have std::tie, then it wouldn't have had std:= :tuple<T&>.=C2=A0 For the cases where we use std::tie and std::fo= rward_as_tuple today, we could equally well be using a "std::forwardin= g_tuple<T>" with perfect-forwarding read-through semantics and n= o operator=3D at all.=C2=A0 But, hindsight is 20/20.

=E2=80=93Arthur

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