220 26328 <71f9f321-59e4-452e-84d1-74d2c476c2de@isocpp.org> article Path: news.gmane.org!not-for-mail From: inkwizytoryankes@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: statement folding on variadic templates parameters pack Date: Mon, 13 Jun 2016 16:18:24 -0700 (PDT) Lines: 368 Approved: news@gmane.org Message-ID: <71f9f321-59e4-452e-84d1-74d2c476c2de@isocpp.org> 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/mixed; boundary="----=_Part_1127_1727022655.1465859904296" X-Trace: ger.gmane.org 1465859909 531 80.91.229.3 (13 Jun 2016 23:18:29 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 13 Jun 2016 23:18:29 +0000 (UTC) Cc: inkwizytoryankes@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDDLTAGNTIBBBQP67S5AKGQEW3CL72A@isocpp.org Tue Jun 14 01:18:28 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f197.google.com ([209.85.220.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bCb7j-0008EX-MP for gclcip-std-proposals@m.gmane.org; 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Mon, 13 Jun 2016 16:18:26 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.245.2 with SMTP id k2ls1048994ith.39.canary; Mon, 13 Jun 2016 16:18:25 -0700 (PDT) X-Received: by 10.36.213.134 with SMTP id a128mr322539itg.8.1465859905637; Mon, 13 Jun 2016 16:18:25 -0700 (PDT) In-Reply-To: <1015518c-c0e4-4397-b946-df7834a2c3c0@isocpp.org> X-Original-Sender: inkwizytoryankes@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:26328 Archived-At: ------=_Part_1127_1727022655.1465859904296 Content-Type: multipart/alternative; boundary="----=_Part_1128_1958846099.1465859904297" ------=_Part_1128_1958846099.1465859904297 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Monday, June 13, 2016 at 7:56:48 PM UTC+2, 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=20 > =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= =20 >> 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= =20 > parameters with 'for constexpr' is to have type list outside of template? > =20 > >> This will allow usage outside of template function and simplify complex= =20 >> 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=20 > instead of iteraating over single element list you are iterating through = 4=20 > elements list since you are grabbing parameters of type mapped my templat= e=20 > typedef but not parameters of teemplate typedef itself. The simpliest way= =20 > to have list of types which is never can accidentally create some nonvoid= =20 > 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(); > } > > No, because type that is pass to `for` is `void` without any parameters. It= =20 will behave same if you pass `void_t` to template and look what= =20 template specialization is chosen. It will be `void`. =20 > My personal opinion on such feature: there should be separate syntax to= =20 > 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. In D it'= s=20 > possible to implement params_of in library code since it's possible to=20 > typedef parameters pack and then use it when needed. The following C++ co= de=20 > rewritten in D works as expected: > > template > struct test { > using params =3D L; > }; > > int main() { > for constexpr (typename T, test::params) { > std::cout << sizeof(T) << std::endl; > } > } > > From my personal point of view this way to have typelist outside of=20 > template is much better then grabbing parameters from some intermediate= =20 > template which is needed only to enlist types you need. This approach als= o=20 > allows to have better of your MetaFunc template without issues I've=20 > mentioned above. > My idea was that we have concrete template type with some parameters. Then= =20 `for constexpr` will extract parameters of this template. This is why=20 template alias are skipped. They can never by concrete. They more like type= =20 to type functions that templates.=20 --=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/71f9f321-59e4-452e-84d1-74d2c476c2de%40isocpp.or= g. ------=_Part_1128_1958846099.1465859904297 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Monday, June 13, 2016 at 7:56:48 PM UTC+2, Serg= ey 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 = 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= parameters from templates, some thing like that:
using type =3D std::tuple<int, long long, char= >;

int main()
{
=C2=A0 =C2=A0 for constexpr (typename T : type) //without `...`<= span style=3D"color:#000">
=C2=A0 =C2=A0 {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 f<T>();
= =C2=A0 =C2=A0 } //equal: `f<int>(); f<l= ong long>(); 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 function and simpli= fy complex operation:
template<typename T>
using MetaFunc =3D std::tuple<int, T, T, char>;


int main()
{
=C2=A0 =C2=A0 for constexpr (typename= T : MetaFunc<= /span><long>)
=C2=A0 =C2=A0 {
=C2=A0 =C2=A0 =C2=A0= =C2=A0 f<T>();
=C2=A0 =C2=A0 } //equal: `f<int>= ;(); f<long>(); f<long>(); f<char>();`
=C2=A0 =C2=A0 for constexpr<= /span> (typename T : = MetaFunc<ch= ar>)=
=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 <= /span>{
=C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0g&l= t;T,TT>();<= /span>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 }
=C2=A0 =C2= =A0 }}


=C2=A0I see one issue with this example. Your Met= aFunc is template typedef and instead of iteraating over single element lis= t you are iterating through=C2=A0 4 elements list since you are grabbing pa= rameters of type mapped my template 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<= span style=3D"color:#660"><typename... T>
using void_= t =3D <= /span>void;

how the fo= llowing loop should work then?

for constexpr= (typename T: void_t<= span style=3D"color:#660"><int8_t, int16_t, int32_t<= /span>, int64_= t>) = {
= =C2=A0 =C2=A0 do_something();
}


No, because type that is pass to `for` is `void` without any p= arameters. It will behave same if you pass `void_t<int, long>` to tem= plate and look what template specialization is chosen. It will be `void`.=C2=A0
My personal opinion on such feature: there should be separate syntax = to grab template parameters like

for<= span style=3D"color:#000"> constexpr (typename T= : params_of<void_t= <int= 8_t, in= t16_t, = int32_t, int64_t>>) {
=C2=A0 =C2=A0 do_something();
}

but it requ= ire some extra work on how this should actually work. In D it's possibl= e to implement params_of in library code since it's possible to typedef= parameters pack and then use it when needed. The following C++ code rewrit= ten in D works as expected:

template<= span style=3D"color:#660"><typename... L>
struct test= {
= =C2=A0 =C2=A0 using params =3D L;
};

int main() {
=C2=A0 =C2=A0 for constexpr (typename T, test<char, int>::params) {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 std::cout << sizeof(T) << std::endl;
=C2=A0 =C2=A0 }
}

From my personal point of= view this way to have typelist outside of template is much better then gra= bbing parameters from some intermediate template which is needed only to en= list types you need. This approach also allows to have better of your MetaF= unc template without issues I've mentioned above.

My idea was that we have concrete template type with some p= arameters. Then `for constexpr` will extract parameters of this template. T= his is why template alias are skipped. They can never by concrete. They mor= e like type to type functions that templates.

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/71f9f321-59e4-452e-84d1-74d2c476c2de= %40isocpp.org.
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