220 33043 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 10:32:01 -0700 Lines: 103 Approved: news@gmane.org Message-ID: References: <6e23864f-827a-4bdb-89de-a60d8f5be993@isocpp.org> <0dd0a5ae-5fc2-020f-73a9-f5c7959fed0f@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="94eb2c1cca7887ffe8055330cb00" X-Trace: blaine.gmane.org 1498843927 2959 195.159.176.226 (30 Jun 2017 17:32:07 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 30 Jun 2017 17:32:07 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBDLZJYWNDQIJFFW2ZICRUBHBAUKUU@isocpp.org Fri Jun 30 19:31:59 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f70.google.com ([74.125.82.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dQzlt-0000MM-TS for gclcip-std-proposals@m.gmane.org; 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[2a00:1450:400c:c09::230]) by mx.google.com with ESMTPS id m79si6629404wmg.165.2017.06.30.10.32.02 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 30 Jun 2017 10:32:02 -0700 (PDT) Received-SPF: pass (google.com: domain of arthur.j.odwyer@gmail.com designates 2a00:1450:400c:c09::230 as permitted sender) client-ip=2a00:1450:400c:c09::230; Original-Received: by mail-wm0-x230.google.com with SMTP id i127so53029742wma.0 for ; Fri, 30 Jun 2017 10:32:02 -0700 (PDT) X-Received: by 10.80.195.21 with SMTP id a21mr6046683edb.144.1498843921773; Fri, 30 Jun 2017 10:32:01 -0700 (PDT) Original-Received: by 10.80.173.204 with HTTP; Fri, 30 Jun 2017 10:32:01 -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=IdWOg22G; spf=pass (google.com: domain of arthur.j.odwyer@gmail.com designates 2a00:1450:400c:c09::230 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:33043 Archived-At: --94eb2c1cca7887ffe8055330cb00 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, Jun 30, 2017 at 10:24 AM, wrote: > > 16 because you need full byte object in optional otherwise you will get > `std::vector` mk2. > I hate vector as much as you do, I promise. :) The analogy doesn't work here. vector is terrible because it *purports* to expose accessors returning bool& but its in-memory representation doesn't contain a full-byte bool anywhere, so it has to cheat and use proxy iterators and just generally mess things up. In the case of std::optional>, it purports to expose accessors returning bool& *if and only if* both the outer and inner optionals are engaged. When either optional is disengaged, the bool member does not exist and therefore there is no need to keep it around in memory (in fact, we are required by the Standard to destroy it[1]). Which means that we have a whole unused byte to play around with. We just need to make sure that when the optionals *are* engaged, the bool really truly exists = =E2=80=94 and I do make sure of that. Notice that I left out the case where "both optionals are disengaged." That's because no such state exists. When the outer optional is disengaged, *there is no inner optional* =E2=80=94 the inner optional is not in any state at a= ll =E2=80=94 it has been destroyed, and will not reappear until you engage the outer optional again. *An optional which is disengaged does not hold a T.* =E2=80=93Arthur [1] modulo the as-if rule --=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/CADvuK0JfnhrRMS1VM1MGz73kgrYRHoRdG_%3DT6N9Z%2Boh= BSoD5%3Dw%40mail.gmail.com. --94eb2c1cca7887ffe8055330cb00 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Fri, Jun 30, 2017 at 10:24 AM, <<= a href=3D"mailto:inkwizytoryankes@gmail.com" target=3D"_blank">inkwizytorya= nkes@gmail.com> wrote:

16 because you need full byte object in= optional otherwise you will get `std::vector<bool>` mk2.

I hate vector<bool> as much as y= ou do, I promise. :) =C2=A0The analogy doesn't work here. vector<boo= l> is terrible because it purports to expose accessors returning = bool& but its in-memory representation doesn't contain a full-byte = bool anywhere, so it has to cheat and use proxy iterators and just generall= y mess things up.

In the case of std::optional<= std::optional<bool>>, it purports to expose accessors returning bo= ol& if and only if both the outer and inner optionals are= engaged. When either optional is disengaged, the bool member does not exis= t and therefore there is no need to keep it around in memory (in fact, we a= re required by the Standard to destroy it[1]). Which means that we have a w= hole unused byte to play around with. We just need to make sure that when t= he optionals are engaged, the bool really truly exists =E2=80=94 and= I do make sure of that.

Notice that I left out th= e case where "both optionals are disengaged." That's because = no such state exists. When the outer optional is disengaged, there is no= inner optional =E2=80=94 the inner optional is not in any state at all= =E2=80=94 it has been destroyed, and will not reappear until you engage th= e outer optional again.

An optional<T> wh= ich is disengaged does not hold a T.

=E2=80=93= Arthur

[1] modulo the as-if rule

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