220 33041 article Path: news.gmane.org!.POSTED!not-for-mail From: inkwizytoryankes@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: std::optional - support for sentinel values Date: Fri, 30 Jun 2017 10:24:04 -0700 (PDT) Lines: 145 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/mixed; boundary="----=_Part_468_1176146734.1498843444502" X-Trace: blaine.gmane.org 1498843451 13678 195.159.176.226 (30 Jun 2017 17:24:11 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 30 Jun 2017 17:24:11 +0000 (UTC) Cc: target.san@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDDLTAGNTIBBBNMS3LFAKGQEIC2QLIA@isocpp.org Fri Jun 30 19:24:05 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f197.google.com ([209.85.192.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dQzeD-00031z-PE for gclcip-std-proposals@m.gmane.org; Fri, 30 Jun 2017 19:24:01 +0200 Original-Received: by mail-pf0-f197.google.com with SMTP id v26sf69494411pfa.0 for ; Fri, 30 Jun 2017 10:24:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc: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=Tpq0VbjEQbqMPORaeo3JhsQ6g7pho+bUgQeYEgkmszQ=; b=Se0rLtyu49dJNEXarGLFC/Hnq7TquBZzuvkXVoZf0tHxxSmbnS6PNSmyaY49miuFop Plh4k+oQkuW8JF0In65xiROl0vud7qGX/x0h3xNYjfVZD3s3zrwbKe714yp4+PRxTJH3 1CtX2N/22Td7txkW1gAlNNAE8wac71L5kxDfZSvvzETbexN/RS0KNBlKE6D03X15lGMj Qe/vbT9OB4+GPCnbUYkMfQj6cX0Y1u1BeGOBki5f06neJTJZcmm3rJ494I/zsjKNVmwM yyzrKF4v51D3NjdUIOwdEXssaEWw7Q8MXBc1aB8ezCp11rmy3tqZsvgVZQvpXGv61oS7 Xt0g== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc: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=Tpq0VbjEQbqMPORaeo3JhsQ6g7pho+bUgQeYEgkmszQ=; b=KNw8M0dLrgQSVrcxinRwtU3aqQzkY+K9TV+Pab3qV1g2OlYXUugEWLV3NX3ecGPWb0 mrhi4bRwH7Dl5eYywtpQGP4gx8nS5v2BleAhMItPsBQi/ltByJjLmyOpBGXpSFm2tLTU kxx2xEfsb/K82FU48xbMxzxom/4AqZ74wj9HNUEM4NweV11hCq6sE5oYAQ81zHAlrQQ0 N17mO+6kmUYxQiH7ZYYW9DtAKFdPjT+MKs4i+vgq9FTKYE9pRut0p5Ry0qQQPxDeAaC1 L2DbDyV1X2H1MBjcRAlQgH8dQ/ZC3mlH6BFj/KpXr5Jt0bAvEUu7FrGKMw3KMaKZPotH 9WAQ== 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:cc: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=Tpq0VbjEQbqMPORaeo3JhsQ6g7pho+bUgQeYEgkmszQ=; b=rsQLluJmnw+qg2300/73/qFMkgH9x2ZOSDLZNHQAYZrSnfwG/VF0g6E7fz42VHE/P4 /Xj5o4Hs4hAx3DWHuRH9AVBxlT0GKzzQAsjBGAGb45YqFuZwHzjgkpY21FKdFTmrWpmo +BiYeNZUhHvdBjcsEQElqZTBkxuwb/FVQJYCGLOXBd3PqOx4q2nHpgU96BATWi/jj7Ph 5IHzvvRZZ4ZOa5qzudJwIAhBIIjDFdixSh6Hf9MsgGUxPJghX27JwOa+w3+iANqSzQOS zsnbsFTHcFpqX6K6SE7Tj88ce8hd1cK38OehNqimeE1iQAcf0QxsJ9G43muYitgh7Qzn PgKw== X-Gm-Message-State: AKS2vOxwBkKfuLsKST6XjvhzM/HkPAb4JVrbliYmqOikJsDeDCUWf7jy /085tX2GReNxhBv8 X-Received: by 10.99.56.25 with SMTP id f25mr13673490pga.135.1498843446453; Fri, 30 Jun 2017 10:24:06 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.172.109 with SMTP id m45ls2708032iti.11.canary-gmail; Fri, 30 Jun 2017 10:24:05 -0700 (PDT) X-Received: by 10.36.34.10 with SMTP id o10mr369176ito.10.1498843445074; Fri, 30 Jun 2017 10:24:05 -0700 (PDT) In-Reply-To: <0dd0a5ae-5fc2-020f-73a9-f5c7959fed0f@gmail.com> X-Original-Sender: inkwizytoryankes@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:33041 Archived-At: ------=_Part_468_1176146734.1498843444502 Content-Type: multipart/alternative; boundary="----=_Part_469_1199741472.1498843444502" ------=_Part_469_1199741472.1498843444502 Content-Type: text/plain; charset="UTF-8" On Friday, June 30, 2017 at 5:44:57 PM UTC+2, Matthew Woehlke wrote: > > On 2017-06-30 10:21, Igor Baidiuk wrote: > > On Friday, June 30, 2017 at 2:05:35 AM UTC+3, Arthur O'Dwyer wrote: > >> - In fact, if you can enumerate the available bit-patterns, you can > store > >> a *lot* of extra information in the unused bits of a T. My STL uses > this > >> fact to make sizeof(optional>>) == 1. Your > >> approach would be completely unable to optimize this; you'd have to use > at > >> least 4 bytes. > > > > Let's imagine someone tries to take reference to doubly-optional bool > from > > your triple-optional bool. And assign it some value. > > I can imagine ways this can still work. (The complexity will depend on > what happens when you assign `true` to the inner-most `bool`, i.e. if it > stores 1 (0x1) or -1 (0xff).) > > > Also note that > > Some(Some(Some(false))), Some(Some(None)), Some(None) are three > different > > state of your triply-optional bool. Even with space compression, you'll > > need at least 2 bytes. > > Er... why? > > An `optional` needs `bitsof(T)+1` bits of storage. A `bool` has two > legal states, so `bitsof(bool) == 1`. Thus, an `optional` needs 2 > bits of storage. > > Applying this recursively: > > bitsof(optional) == 2 > bitsof(optional>) == 3 > bitsof(optional>>) == 4 > > Last I checked, there are (probably / at least) 8 bits in a byte. I > hardly need *16* bits to store optional^3. In fact, I could > (theoretically) store up to optional^7 in *one* byte. > > -- > Matthew > 16 because you need full byte object in optional otherwise you will get `std::vector` mk2. -- 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/cebea1ab-9c29-4c7d-acea-9d3ff6b4062c%40isocpp.org. ------=_Part_469_1199741472.1498843444502 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Friday, June 30, 2017 at 5:44:57 PM UTC+2, Matt= hew Woehlke wrote:
On 2017-06-3= 0 10:21, Igor Baidiuk wrote:
> On Friday, June 30, 2017 at 2:05:35 AM UTC+3, Arthur O'Dwyer w= rote:
>> - In fact, if you can enumerate the available bit-patterns, yo= u can store=20
>> a *lot* of extra information in the unused bits of a T. My STL= uses this=20
>> fact to make sizeof(optional<optional<optional<b= ool>>>) =3D=3D 1. Your=20
>> approach would be completely unable to optimize this; you'= d have to use at=20
>> least 4 bytes.
>
> Let's imagine someone tries to take reference to doubly-option= al bool from=20
> your triple-optional bool. And assign it some value.

I can imagine ways this can still work. (The complexity will depend on
what happens when you assign `true` to the inner-most `bool`, i.e. if i= t
stores 1 (0x1) or -1 (0xff).)

> Also note that=20
> Some(Some(Some(false))), Some(Some(None)), Some(None) are three di= fferent=20
> state of your triply-optional bool. Even with space compression, y= ou'll=20
> need at least 2 bytes.

Er... why?

An `optional<T>` needs `bitsof(T)+1` bits of storage. A `bool` ha= s two
legal states, so `bitsof(bool) =3D=3D 1`. Thus, an `optional<bool>= ;` needs 2
bits of storage.

Applying this recursively:

=C2=A0 bitsof(optional<bool>) =3D=3D 2
=C2=A0 bitsof(optional<optional<bool>>) =3D=3D 3
=C2=A0 bitsof(optional<optional<optional<bool>>>= ) =3D=3D 4

Last I checked, there are (probably / at least) 8 bits in a byte. I
hardly need *16* bits to store optional^3<bool>. In fact, I could
(theoretically) store up to optional^7<bool> in *one* byte.

--=20
Matthew

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

--
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+unsubscribe@isocpp.org.
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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/cebea1ab-9c29-4c7d-acea-9d3ff6b4062c= %40isocpp.org.
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