220 26326 article Path: news.gmane.org!not-for-mail From: "Arthur O'Dwyer" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: statement folding on variadic templates parameters pack Date: Mon, 13 Jun 2016 11:58:55 -0700 Lines: 306 Approved: news@gmane.org Message-ID: References: <1e3daa7c-e10a-4589-8118-26f47bc1a0df@isocpp.org> <9c425e18-e592-4cfd-9e41-b37b212a6b9d@isocpp.org> <1015518c-c0e4-4397-b946-df7834a2c3c0@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=001a113a55d4ff8ecb05352d7a20 X-Trace: ger.gmane.org 1465844340 17675 80.91.229.3 (13 Jun 2016 18:59:00 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 13 Jun 2016 18:59:00 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBDLZJYWNDQIPDBH4XICRUBGFKQZOE@isocpp.org Mon Jun 13 20:58:59 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f69.google.com ([209.85.220.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bCX4d-0005IN-7q for gclcip-std-proposals@m.gmane.org; Mon, 13 Jun 2016 20:58:59 +0200 Original-Received: by mail-pa0-f69.google.com with SMTP id he1sf82964360pac.0 for ; Mon, 13 Jun 2016 11:58:58 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=dL9/UR7gkJd22qq9oh3fuK/ORw+RW50THZMPkTyDsKc=; b=DnKCAMjOFDbi/StmOcjCDOoLOK7jcp4GBKr2YjH8abA2R7DowPtB8FA0ivhZzKsyQs Jrpl2EqNPQFhcyxNasn2xrplTrLwcH5kwqhpQ4wX1S9XYhjNkLqu2s+N2X3j9Xg1Bfd5 QVtsKsLrkt/aJVAOh1u187O/cLiaSsz5PzPIJsV8IjCFBagn/6p2JSWC+yZfDAKKg3H+ FCxQvkJjkHWujXA7EJvpP4eE07Mazd2weuFFkUfsO/Vr/4Yn7Uildn1MIsjsswP/oEiS 1Ho1izgjEU/4pgI3DeqJ5lbNrkB3OS6LZSJ50IQMMTV4ha89+R48LSTRkEHSFiT8e0YN HXfg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=dL9/UR7gkJd22qq9oh3fuK/ORw+RW50THZMPkTyDsKc=; b=ND7z6iVQzj6joXiaCNbe2QnN2Gro4ncLxnUJL7pqrs+BLTFoX3WZ5UYBo1yQbwnDkv uiD72B0yWi0DJvayRFOfgZjABQd4xhK0JLE5fqxWAhu4hKqjpsl4TqXdbbv9oQBHsGQ+ SIGkvxYIDfArNKSZ3hqIev83BFMmmjbNhOnW1PT0NR6oJRzNOwYFQwNoD1lZfH5iosmY TPA/R0GUoTfkh9tombMlSifEVvvyzleqMsiaErAiTjVhdJOrXkbkiuRR8oYF+YDvQnBR 9QPfyrBQBO4FFuHvz9LGFT4FGonNP8ABq8ZEZa3YOcNh5TVENRzhgSrUybbR3DXJZd6O //rg== X-Gm-Message-State: ALyK8tIL16O319fY5ENO+Lpb4u/pSpBU7+QWCniorMpkkxYfy1IB03m3k+HO5s0mpA9JtQ== X-Received: by 10.98.29.15 with SMTP id d15mr13326304pfd.5.1465844338264; Mon, 13 Jun 2016 11:58:58 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.35.83 with SMTP id m77ls3501061qgm.24.gmail; Mon, 13 Jun 2016 11:58:57 -0700 (PDT) X-Received: by 10.55.11.6 with SMTP id 6mr16737335qkl.160.1465844337166; Mon, 13 Jun 2016 11:58:57 -0700 (PDT) Original-Received: from mail-qg0-x232.google.com (mail-qg0-x232.google.com. [2607:f8b0:400d:c04::232]) by mx.google.com with ESMTPS id w62si16015720qhw.12.2016.06.13.11.58.57 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 13 Jun 2016 11:58:57 -0700 (PDT) Received-SPF: pass (google.com: domain of arthur.j.odwyer@gmail.com designates 2607:f8b0:400d:c04::232 as permitted sender) client-ip=2607:f8b0:400d:c04::232; Original-Received: by mail-qg0-x232.google.com with SMTP id l44so72225653qgd.0 for ; Mon, 13 Jun 2016 11:58:57 -0700 (PDT) X-Received: by 10.140.96.199 with SMTP id k65mr15289040qge.27.1465844336920; Mon, 13 Jun 2016 11:58:56 -0700 (PDT) Original-Received: by 10.200.51.177 with HTTP; Mon, 13 Jun 2016 11:58:55 -0700 (PDT) In-Reply-To: <1015518c-c0e4-4397-b946-df7834a2c3c0@isocpp.org> X-Original-Sender: arthur.j.odwyer@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of arthur.j.odwyer@gmail.com designates 2607:f8b0:400d:c04::232 as permitted sender) smtp.mailfrom=arthur.j.odwyer@gmail.com; dmarc=pass (p=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-Spam-Checked-In-Group: std-proposals@isocpp.org 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:26326 Archived-At: --001a113a55d4ff8ecb05352d7a20 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Mon, Jun 13, 2016 at 10:56 AM, Sergey Vidyuk wrote: > =D1=81=D1=83=D0=B1=D0=B1=D0=BE=D1=82=D0=B0, 11 =D0=B8=D1=8E=D0=BD=D1=8F 2= 016 =D0=B3., 1:10:04 UTC+6 =D0=BF=D0=BE=D0=BB=D1=8C=D0=B7=D0=BE=D0=B2=D0=B0= =D1=82=D0=B5=D0=BB=D1=8C inkwizyt...@gmail.com > =D0=BD=D0=B0=D0=BF=D0=B8=D1=81=D0=B0=D0=BB: >> >> I think that `for constexpr` should be allowed to extracting parameters >> from templates, some thing like that: >> using type =3D std::tuple; >> >> int main() >> { >> for constexpr (typename T : type) //without `...` >> { >> f(); >> } //equal: `f(); f(); f();` >> } >> >> > Am I right tha the main and only reason you would like to grab template > parameters with 'for constexpr' is to have type list outside of template? > > >> This will allow usage outside of template function and simplify complex >> operation: >> template >> using MetaFunc =3D std::tuple; >> >> >> int main() >> { >> for constexpr (typename T : MetaFunc) >> { >> f(); >> } //equal: `f(); f(); f(); f();` >> for constexpr (typename T : MetaFunc) >> { >> for constexpr (typename TT : MetaFunc) >> { >> g(); >> } >> } >> } >> >> >> I see one issue with this example. Your MetaFunc is template typedef an= d > instead of iteraating over single element list you are iterating through = 4 > elements list since you are grabbing parameters of type mapped my templat= e > typedef but not parameters of teemplate typedef itself. The simpliest way > to have list of types which is never can accidentally create some nonvoid > variable is template typedef std::void_t which can be implemented as > > template > using void_t =3D void; > > how the following loop should work then? > > for constexpr (typename T: void_t) { > do_something(); > } > > My personal opinion on such feature: there should be separate syntax to > grab template parameters like > > for constexpr (typename T: params_of int64_t>>) { > do_something(); > } > > but it require some extra work on how this should actually work. > Sorry, that's impossible in C++. Intuitively, using result =3D void_t; ... some expression involving params_of ... is a non-starter for the same reason that auto sum =3D a + b + c + d; ... some expression involving addends_of ... is a non-starter. Alias templates are referentially transparent (at compile time) in the same way that ints are (at runtime); void_t *is* void as far as the compiler is concerned. That's an intentional feature; it allows type deduction to "see through" parameters whose types are template aliases whereas type deduction can't "see through" parameters whose types are member typedefs of template classes. HTH, Arthur --=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/CADvuK0JihA%3DK8PMhBMG3OKkq6JTRgqDtCkNM%2B9FWnuT= xvV-d1w%40mail.gmail.com. --001a113a55d4ff8ecb05352d7a20 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Mon, Jun 13, 2016 at 10:56 AM, Sergey Vidyuk <sir= ..vestnik@gmail.com> wrote:
=D1= =81=D1=83=D0=B1=D0=B1=D0=BE=D1=82=D0=B0, 11 =D0=B8=D1=8E=D0=BD=D1=8F 2016 = =D0=B3., 1:10:04 UTC+6 =D0=BF=D0=BE=D0=BB=D1=8C=D0=B7=D0=BE=D0=B2=D0=B0=D1= =82=D0=B5=D0=BB=D1=8C inkwizyt...@gmail.com =D0=BD=D0=B0=D0=BF=D0=B8=D1=81=D0=B0=D0=BB:=
= =C2=A0I think that `for constexpr` should be allowed to extracting paramete= rs from templates, some thing like that:
us= ing type =3D std::tuple<int, lon= g long<= /span>, char>;

int= main()=
{
=C2=A0 =C2=A0 for constexpr (typename T : type) //without `...`
=C2=A0 =C2=A0 {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 f<T>();
=C2=A0 =C2= =A0 } <= /span>//equal: `f<int>(); f<long long&g= t;(); f<char>();`
}


Am I right tha the main and only reason you would like to = grab template=20 parameters with 'for constexpr' is to have type list outside of=20 template?
=C2=A0
This will allow usage outside of template func= tion and simplify complex operation:
template<typena= me T>= ;
us= ing Met= aFunc = =3D std= ::tuple= <int= , T, T,= char>;

int ma= in(){
= =C2=A0 =C2=A0 for constexpr (typename T : MetaFunc<long>)
=C2=A0 =C2=A0 {
=C2= =A0 =C2=A0 =C2=A0 =C2=A0 f<T>();
=C2=A0 =C2=A0 } //eq= ual: `f<int>(); f<long>(); f<long>(); f<char>();`
=C2=A0 =C2=A0 for constexpr (typename T : MetaFunc<char>)
=C2=A0 =C2=A0 {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 for constexpr (typename TT : MetaFunc<T>)
=C2=A0 =C2=A0= =C2=A0 =C2=A0 {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0g<T,TT>();
=C2=A0 =C2=A0 =C2= =A0 =C2=A0 }
=C2=A0 =C2=A0 }
}


=C2=A0I see one issue w= ith this example. Your MetaFunc is template typedef and instead of iteraati= ng over single element list you are iterating through=C2=A0 4 elements list= since you are grabbing parameters of type mapped my template typedef but n= ot parameters of teemplate typedef itself. The simpliest way to have list o= f types which is never can accidentally create some nonvoid variable is tem= plate typedef std::void_t which can be implemented as

template<typename... T>
using void_t =3D void;
=

how the following loop should work then?

for constexpr (typename T: void_t<int8_t, int16_t, int32_t, int64_t>) {
=C2=A0 =C2=A0 do_something();
}
=

My personal opinion on such feature: there should be separate syn= tax to grab template parameters like

for constexpr<= /span> (typename T: params_o= f<vo= id_t<int8_t, int16_t, int32_t, int64_t>>) {
=C2=A0 =C2=A0 do_something= ();
= }

but it= require some extra work on how this should actually work.

Sorry, that's impossible in C++.=C2=A0 Intu= itively,

=C2= =A0 =C2=A0 using result =3D void_t<A,B,C,D>;
=C2=A0 =C2=A0 ... some expression involving pa= rams_of<result> ...

is a non-starter = for the same reason that

=C2=A0 =C2=A0 auto sum =3D a + b + c + d;
=C2=A0 =C2=A0 ... some expression involving = addends_of<sum> ...

is a non-starter. Alias templates are referentially transparent (at c= ompile time) in the same way that ints are (at runtime); void_t<A,B,C,D> is void as far as the compiler is concerned.= =C2=A0 That's an intentional feature; it allows type deduction to "= ;see through" parameters whose types are template aliases whereas type= deduction can't "see through" parameters whose types are mem= ber typedefs of template classes.

HTH,
Arthur

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