220 33352 <4DFE708E-6589-4A08-8A65-766B6CC4C19E@hsr.ch> article Path: news.gmane.org!.POSTED!not-for-mail From: Peter Sommerlad Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Trait for arithmetic types similar to std::chrono::treat_as_floating_point for the use in other parts of the library Date: Wed, 26 Jul 2017 20:49:00 +0200 Lines: 206 Approved: news@gmane.org Message-ID: <4DFE708E-6589-4A08-8A65-766B6CC4C19E@hsr.ch> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 (1.0) Content-Type: multipart/alternative; boundary="Apple-Mail-A9F30C1C-52A9-4A51-B2AE-EAA33E06DA54" Content-Transfer-Encoding: 7bit X-Trace: blaine.gmane.org 1501094950 27499 195.159.176.226 (26 Jul 2017 18:49:10 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 26 Jul 2017 18:49:10 +0000 (UTC) Cc: Peter Sommerlad To: Original-X-From: std-proposals+bncBD3OHQEN4AJBBIWI4PFQKGQEQQK2UXA@isocpp.org Wed Jul 26 20:49:05 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 1daRMk-0006jo-DV for gclcip-std-proposals@m.gmane.org; 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[2001:620:130:a036::31]) by mx.google.com with ESMTPS id s23si10066098edc.539.2017.07.26.11.49.06 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 26 Jul 2017 11:49:06 -0700 (PDT) Received-SPF: pass (google.com: domain of peter.sommerlad@hsr.ch designates 2001:620:130:a036::31 as permitted sender) client-ip=2001:620:130:a036::31; Original-Received: from localhost (localhost [127.0.0.1]) by mx1.hsr.ch (Postfix) with ESMTP id 5D6E423B61E8 for ; Wed, 26 Jul 2017 20:49:05 +0200 (CEST) Original-Received: from mx1.hsr.ch ([127.0.0.1]) by localhost (mx1.hsr.ch [127.0.0.1]) (amavisd-new, port 10024) with LMTP id E0Y2hW9iBTrK for ; Wed, 26 Jul 2017 20:49:02 +0200 (CEST) Original-Received: from webmail.hsr.ch (unknown [152.96.21.232]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-SHA256 (128/128 bits)) (No client certificate requested) by mx1.hsr.ch (Postfix) with ESMTPS id C79D123B61E7 for ; Wed, 26 Jul 2017 20:49:02 +0200 (CEST) Original-Received: from [192.168.1.19] (152.96.21.199) by sidv0232.hsr.ch (152.96.21.231) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.1034.26; Wed, 26 Jul 2017 20:49:02 +0200 X-Mailer: iPad Mail (14F89) In-Reply-To: X-Originating-IP: [152.96.21.199] X-ClientProxiedBy: sidv0231.hsr.ch (152.96.21.229) To sidv0232.hsr.ch (152.96.21.231) X-Original-Sender: peter.sommerlad@hsr.ch X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of peter.sommerlad@hsr.ch designates 2001:620:130:a036::31 as permitted sender) smtp.mailfrom=peter.sommerlad@hsr.ch 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:33352 Archived-At: --Apple-Mail-A9F30C1C-52A9-4A51-B2AE-EAA33E06DA54 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable I believe that discussion must also be done considering a more complete uni= ts library, beyond just duration. But the problems to address are essential= ly the same, so what works for duration might be a good starting point for = generalization. Thank you both for bringing this up. Regards Peter Sent from Peter Sommerlad's iPad +41 79 432 23 32 > On 26 Jul 2017, at 20:03, John McFarlane wrote: >=20 >> On Wed, Jul 26, 2017 at 9:14 AM Manuel Bergler wro= te: >> Hi everyone, > Hi Manual=20 >=20 >> However, allowing only float, double or long double is not a restriction= of the implementation (at least in the libstdc++ one I tested). As a matte= r of fact, just specializing std::is_floating_point for std::chrono::durati= on - even though it is not allowed and thus undefined behaviour - makes=20 >> =20 >> ```` >> using milliseconds_d =3D std::chrono::duration; >> std::normal_distribution distribution{milliseconds_d= {0}, milliseconds_d{1}}; >> ```` >>=20 >> compile and the distribution yields the exact same values as it does for= pure doubles. >=20 > I'm sure that you're aware that it's a very big leap from not observing a= ny UB in one implementation to writing code that is safe. But you might be= surprised by what implementors do with the assumptions you are breaking! >=20 >> So I was wondering where else in the library these restrictions are kind= of arbitrary and if it does make sense to use something like std::chrono::= treat_as_floating point in other places of the library. >=20 > I would not say that they are arbitrary. It's easy to start with tight r= estrictions that allow basic functionality and relax them in future revisio= ns. But it's much harder to shut the stable door once the horse has bolted= .. > =20 >> But before searching in all of the STL for components that are unnecessa= rily restrictive I first wanted to ask for feedback. >>=20 >> In light of the work on the Numerics TS as outlined in [P0101r0], which = will introduce more arithmetic types to the standard library I think it def= initely makes sense to make all kinds of STL components aware of user-defin= ed arithmetic types, but what is your opinion? >=20 > You might be interested in P0437. It proposes moving the functionality o= f `numeric_limits` into individual traits -- like those in --= but continuing to allow specialization for user-defined types. The specif= ic case of a test for floating-point is deferred but this seems to be the c= urrent approach to achieve what you're after. >=20 > There are some tough problems to address here though. Firstly, while `du= ration` or the types from P0101 can behave like floating-point types= , that does not mean that either of them are, themselves, floating-point ty= pes. They are essentially classes. So some other property may be what we'= re really after here: perhaps the ability to convert between floating-point= types or the ability to approximate real numbers. =20 >=20 > It's difficult to say what these properties are until more types exhibit = them but I hope that there is a way forward here that allows seamless inter= operability between all numeric types where that makes sense. > =20 > John > --=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= 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/isoc= pp.org/d/msgid/std-proposals/CABPJVnTsGJmiV3mePzgum4j_BLdcReBDXGjp4kCXCRnFD= U8%2BbA%40mail.gmail.com. --=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/4DFE708E-6589-4A08-8A65-766B6CC4C19E%40hsr.ch. --Apple-Mail-A9F30C1C-52A9-4A51-B2AE-EAA33E06DA54 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
I believe that discussion must als= o be done considering a more complete units library, beyond just duration. = But the problems to address are essentially the same, so what works for dur= ation might be a good starting point for generalization.

Thank you both f= or bringing this up.

Regards
Peter
Sent from Peter Sommerlad's iPad
+41 79 432 23 32=

On 26 Jul 2017, at 20:03, John McFarlane <john@mcfarlane.name> wrote:
=
On Wed, Jul 26, 2017 at 9:14 AM Manuel Bergler <= ;berglerma@gmail.com> wrote:<= br>
Hi everyone,
=
Hi Manual

However, allowing only float, double or long double is not a= restriction of the implementation (at least in the libstdc++ one I tested)= .. As a matter of fact, just specializing std::is_floating_point for std::ch= rono::duration - even though it is not allowed and thus undefined behaviour= - makes 
   
    ````
  &nb= sp; using milliseconds_d =3D std::chrono::duration<double, std::milli>= ;;
    std::normal_distribution<milliseconds_d> d= istribution{milliseconds_d{0}, milliseconds_d{1}};
    = ````

compile and the distribution yields the exact= same values as it does for pure doubles.

=
I'm sure that you're aware that it's a very big leap from not ob= serving any UB in one implementation to writing code that is safe.  Bu= t you might be surprised by what implementors do with the assumptions you a= re breaking!

<= div>So I was wondering where else in the library these restrictions are kin= d of arbitrary and if it does make sense to use something like std::chrono:= :treat_as_floating point in other places of the library.

I would not say that they are arbitrary.  It's easy to = start with tight restrictions that allow basic functionality and relax them= in future revisions.  But it's much harder to shut the stable door on= ce the horse has bolted.
 
But before searching in all of the STL for com= ponents that are unnecessarily restrictive I first wanted to ask for feedba= ck.

In light of the work on the Numerics TS as out= lined in [P0101r0], which will introduce more arithmetic types to the stand= ard library I think it definitely makes sense to make all kinds of STL comp= onents aware of user-defined arithmetic types, but what is your opinion?

You might be interested in P0437.=   It proposes moving the functionality of `numeric_limits` into indivi= dual traits -- like those in <type_traits> -- but continuing to allow= specialization for user-defined types.  The specific case of a test f= or floating-point is deferred but this seems to be the current approach to = achieve what you're after.

There are some tough problems = to address here though.  Firstly, while `duration<float>` or the= types from P0101 can behave like floating-point types, that does not mean = that either of them are, themselves, floating-point types.  They are e= ssentially classes.  So some other property may be what we're really a= fter here: perhaps the ability to convert between floating-point types or t= he ability to approximate real numbers. 

It's difficult to say= what these properties are until more types exhibit them but I hope that th= ere is a way forward here that allows seamless interoperability between all= numeric types where that makes sense.
 
J= ohn

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