220 26324 <1015518c-c0e4-4397-b946-df7834a2c3c0@isocpp.org> article Path: news.gmane.org!not-for-mail From: Sergey Vidyuk Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: statement folding on variadic templates parameters pack Date: Mon, 13 Jun 2016 10:56:47 -0700 (PDT) Lines: 392 Approved: news@gmane.org Message-ID: <1015518c-c0e4-4397-b946-df7834a2c3c0@isocpp.org> References: <1e3daa7c-e10a-4589-8118-26f47bc1a0df@isocpp.org> <9c425e18-e592-4cfd-9e41-b37b212a6b9d@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_522_172920184.1465840607875" X-Trace: ger.gmane.org 1465840757 23657 80.91.229.3 (13 Jun 2016 17:59:17 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 13 Jun 2016 17:59:17 +0000 (UTC) Cc: inkwizytoryankes@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCB6HO5JRMARBYPH7O5AKGQERIBA65Q@isocpp.org Mon Jun 13 19:59:17 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f70.google.com ([209.85.220.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bCW6W-0002g8-12 for gclcip-std-proposals@m.gmane.org; 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Mon, 13 Jun 2016 10:56:50 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.36.135.197 with SMTP id f188ls115381ite.44.gmail; Mon, 13 Jun 2016 10:56:49 -0700 (PDT) X-Received: by 10.36.253.132 with SMTP id m126mr285898ith.10.1465840609111; Mon, 13 Jun 2016 10:56:49 -0700 (PDT) In-Reply-To: <9c425e18-e592-4cfd-9e41-b37b212a6b9d@isocpp.org> X-Original-Sender: sir.vestnik@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:26324 Archived-At: ------=_Part_522_172920184.1465840607875 Content-Type: multipart/alternative; boundary="----=_Part_523_718919558.1465840607876" ------=_Part_523_718919558.1465840607876 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable =D1=81=D1=83=D0=B1=D0=B1=D0=BE=D1=82=D0=B0, 11 =D0=B8=D1=8E=D0=BD=D1=8F 201= 6 =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 and= =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 template= =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(); } My personal opinion on such feature: there should be separate syntax to=20 grab template parameters like for constexpr (typename T: params_of>) { 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++ code= =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 also= =20 allows to have better of your MetaFunc template without issues I've=20 mentioned above. --=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/1015518c-c0e4-4397-b946-df7834a2c3c0%40isocpp.or= g. ------=_Part_523_718919558.1465840607876 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
=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 = (typena= me T :<= /span> type) //withou= t `...`
=C2=A0 =C2=A0 {
=C2=A0 =C2=A0 =C2= =A0 =C2=A0 f<T>();
=C2=A0 =C2=A0 } //equal: `f<int= >(); f<long long>(); f<char>();`
}

=

Am I right tha the main and only reason y= ou 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 sim= plify complex operation:
template<typename T>
using MetaFunc= =3D std::tuple<int, T, T, char>;


<= span style=3D"color:#008">int main= ()
<= span style=3D"color:#660">{
=C2=A0 =C2= =A0 for= constexpr (ty= pename T : Met= aFunc<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 fo= r const= expr (<= /span>typename= T : MetaFunc&= lt;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 }
}


<= /div>
=C2=A0I see one issue with this example. Your = MetaFunc is template typedef and instead of iteraating 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 not parameters of teempl= ate 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<typename... T>
<= /span>using void_t <= span style=3D"color: #660;" class=3D"styled-by-prettify">=3D void;

how the followi= ng loop should work then?

for constexpr (typename T: void_t<int8_t, int16_t, int32_t, i= nt64_t>= ) <= span style=3D"color: #660;" class=3D"styled-by-prettify">{
=C2=A0 =C2=A0 do_someth= ing();
}

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

for constexpr (type= name T: params_of<void_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 w= ork. In D it's possible 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 rewritten in D works as expected:

template<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 ha= ve typelist outside of template is much better then grabbing parameters fro= m some intermediate template which is needed only to enlist types you need.= This approach also allows to have better of your MetaFunc template without= issues I've mentioned above.

--
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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.
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/1015518c-c0e4-4397-b946-df7834a2c3c0= %40isocpp.org.
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