220 33129 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: Wed, 5 Jul 2017 14:47:25 -0700 Lines: 148 Approved: news@gmane.org Message-ID: References: <6e23864f-827a-4bdb-89de-a60d8f5be993@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="089e08221614266ea9055398f228" X-Trace: blaine.gmane.org 1499291254 29874 195.159.176.226 (5 Jul 2017 21:47:34 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 5 Jul 2017 21:47:34 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBDLZJYWNDQIO7PHVZICRUBCQR3YN6@isocpp.org Wed Jul 05 23:47:28 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f72.google.com ([209.85.215.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dSs8p-0007E6-0Q for gclcip-std-proposals@m.gmane.org; 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[2a00:1450:400c:c0c::233]) by mx.google.com with ESMTPS id y130si17452831wmd.197.2017.07.05.14.47.26 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 05 Jul 2017 14:47:26 -0700 (PDT) Received-SPF: pass (google.com: domain of arthur.j.odwyer@gmail.com designates 2a00:1450:400c:c0c::233 as permitted sender) client-ip=2a00:1450:400c:c0c::233; Original-Received: by mail-wr0-x233.google.com with SMTP id c11so2626479wrc.3 for ; Wed, 05 Jul 2017 14:47:26 -0700 (PDT) X-Received: by 10.80.224.9 with SMTP id e9mr13637928edl.21.1499291246290; Wed, 05 Jul 2017 14:47:26 -0700 (PDT) Original-Received: by 10.80.173.235 with HTTP; Wed, 5 Jul 2017 14:47:25 -0700 (PDT) In-Reply-To: X-Original-Sender: arthur.j.odwyer@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.b=SjzrWtrV; spf=pass (google.com: domain of arthur.j.odwyer@gmail.com designates 2a00:1450:400c:c0c::233 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:33129 Archived-At: --089e08221614266ea9055398f228 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, Jul 5, 2017 at 7:24 AM, Matt Calabrese wrote: > On Thu, Jun 29, 2017 at 7:05 PM, Arthur O'Dwyer > wrote: > >> The other extension, orthogonal to first one, is to allow value to be >>> stored using alternate representation. As an example, reference type ca= n be >>> stored as a pointer and simply presented as reference. This will also a= llow >>> to have cheap and relatively easy support for storing references in >>> std::optional, without need of completely separate type specialization. >>> >> 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. >> > > Erm, that's not what I said in my talk. In fact, quite the opposite. I > advocate support for optional > Huh! Well, you fooled me. What I took away from your talk was mostly the many examples of why value semantics and regular types are a good thing, and the subtlety of what it really means to be a "regular" type (for example, that copying ought to produce an identical, substitutable duplicate of the original, which implies that you can't be "regular" without supporting operator=3D=3D). I also remember the examples of how to mess things up by injecting "assign-through" types into generic value-semantic contexts, e.g. https://wandbox.org/permlink/gmmqCF4d1eSvutZC int main() { int i=3D3, j=3D2, k=3D1; using assign_through =3D std::tuple; std::vector v =3D { {i}, {j}, {k} }; std::sort(v.begin(), v.end()); printf("%d %d %d\n", i, j, k); // "3 3 3" } I thought you were holding up assign_through as a "bad", "irregular" type, not as a model. I also recall (more hazily) discussion of the sins of vector, although that might have been a different talk. Anyway, it sounds like I've misrepresented Matt's opinion. Let me backtrack and just say that *I personally* wouldn't support the addition of a type to the library which was (A) irregular and (B) spelled as if it were a specialization of an existing type that *is* regular. =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/CADvuK0LWcmzL5ofmFEB8-eYdwbhvOjNizjoEMFqGGCH0FzS= ztg%40mail.gmail.com. --089e08221614266ea9055398f228 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Wed, Jul 5, 2017 at 7:24 AM, Matt Calabrese=C2=A0wrote:
On Thu, Jun 29, 2017 at 7:05 PM, Arthur O'Dwyer <arthur.j.odwyer@gmail.com> wrote:

The other exte= nsion, orthogonal to first one, is to allow value to be stored using altern= ate representation. As an example, reference type can be stored as a pointe= r and simply presented as reference. This will also allow to have cheap and= relatively easy support for storing references in std::optional, without n= eed of completely separate type specialization.

This is optional<reference_wrapper<T>= ;>.
As for why optional<T&> is a terrible, horr= ible, no-good, very bad idea, please see Matt Calabrese's talk fro= m C++Now.

Erm, that'= s not what I said in my talk. In fact, quite the opposite. I advocate suppo= rt for optional<T&>=C2=A0

Huh!=C2=A0 Well, you fooled me.=C2=A0 What I took away fro= m your talk was mostly the many examples of why value semantics and regular= types are a good thing, and the subtlety of what it really means to be a &= quot;regular" type (for example, that copying ought to produce an iden= tical, substitutable duplicate of the original, which implies that you can&= #39;t be "regular" without supporting operator=3D=3D).=C2=A0 I al= so remember the examples of how to mess things up by injecting "assign= -through" types into generic value-semantic contexts, e.g.
<= br>

int main() {
=C2=A0 =C2=A0 int i=3D3, j=3D2, k=3D1;
=C2=A0 =C2=A0 using assig= n_through =3D std::tuple<int&>;
=C2=A0 =C2=A0 std::vector<assign_through> v =3D {= {i}, {j}, {k} };
=C2= =A0 =C2=A0 std::sort(v.begin(), v.end());
=C2=A0 =C2=A0 printf("%d %d %d\n", i, j, k); = =C2=A0// "3 3 3"
}

I thought you were holding up= =C2=A0assign_through=C2=A0as a &= quot;bad", "irregular" type, not as a model.

<= /div>
I also recall (more hazily) discussion of the sins of vector<bool>, although that might ha= ve been a different talk.

Anyway, it sounds like I= 've misrepresented Matt's opinion. Let me backtrack and just say th= at I personally wouldn't support the addition of a type to the l= ibrary which was (A) irregular and (B) spelled as if it were a specializati= on of an existing type that is=C2=A0regular.

=E2=80=93Arthur

--
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To view this discussion on the web visit htt= ps://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CADvuK0LWcmzL5ofm= FEB8-eYdwbhvOjNizjoEMFqGGCH0FzSztg%40mail.gmail.com.
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