220 26329 <0e2ea7f0-ee5e-44be-bade-413381bbb12b@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 22:02:03 -0700 (PDT) Lines: 266 Approved: news@gmane.org Message-ID: <0e2ea7f0-ee5e-44be-bade-413381bbb12b@isocpp.org> References: <1e3daa7c-e10a-4589-8118-26f47bc1a0df@isocpp.org> <9c425e18-e592-4cfd-9e41-b37b212a6b9d@isocpp.org> <1015518c-c0e4-4397-b946-df7834a2c3c0@isocpp.org> <71f9f321-59e4-452e-84d1-74d2c476c2de@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2311_1309869163.1465880523561" X-Trace: ger.gmane.org 1465880528 19081 80.91.229.3 (14 Jun 2016 05:02:08 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 14 Jun 2016 05:02:08 +0000 (UTC) Cc: inkwizytoryankes@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCB6HO5JRMARBTM7725AKGQEZF3RCQY@isocpp.org Tue Jun 14 07:02:08 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f200.google.com ([209.85.223.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bCgUJ-0003Sh-38 for gclcip-std-proposals@m.gmane.org; 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Mon, 13 Jun 2016 22:02:06 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.148.19 with SMTP id w19ls1990126iod.2.gmail; Mon, 13 Jun 2016 22:02:04 -0700 (PDT) X-Received: by 10.36.87.213 with SMTP id u204mr79178ita.10.1465880524855; Mon, 13 Jun 2016 22:02:04 -0700 (PDT) In-Reply-To: <71f9f321-59e4-452e-84d1-74d2c476c2de@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:26329 Archived-At: ------=_Part_2311_1309869163.1465880523561 Content-Type: multipart/alternative; boundary="----=_Part_2312_420184387.1465880523564" ------=_Part_2312_420184387.1465880523564 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable =D0=B2=D1=82=D0=BE=D1=80=D0=BD=D0=B8=D0=BA, 14 =D0=B8=D1=8E=D0=BD=D1=8F 201= 6 =D0=B3., 5:18:24 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 > > > My idea was that we have concrete template type with some parameters. The= n=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 ty= pe=20 > to type functions that templates.=20 > Ok. Got the point on alias templates. But it actually means that this=20 feature is only usable when it's strong guarantie that template you are=20 using not an alias to nontrivial implementation. For example there might be= =20 some problems with your examples if std::tuple is defined in such way: template using tuple =3D detail::tuple_impl; grbbing template by for constexpr seems only be usable together with some= =20 template class designed to be used for grabbing parameters. Something like: template struct types_list {}; which has strong guarantie that no extra types or nontype argument will=20 apear when it's used in "for constexpr". I think that D-like code I've=20 posted above solves the same issue in much better way: > template > struct test { > using params =3D L; > }; > > int main() { > for constexpr (typename T, test::params) { > std::cout << sizeof(T) << std::endl; > } > } > > =20 Thre are two different language constructs designed for their own purpuses= =20 which allows to have type list to iterate over in nontemplate function when= =20 those features are used together. From my personal point of view this way= =20 is much better than grabbing template parameters unless there are some use= =20 cases which are not just "some way to have iterable typelist in=20 non-template code". Do you have some example usecases for the feature? -- Sergey --=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/0e2ea7f0-ee5e-44be-bade-413381bbb12b%40isocpp.or= g. ------=_Part_2312_420184387.1465880523564 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


=D0=B2=D1=82=D0=BE=D1=80=D0=BD=D0=B8=D0=BA, 14 =D0= =B8=D1=8E=D0=BD=D1=8F 2016 =D0=B3., 5:18:24 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

My idea was that we have concrete templa= te type with some parameters. Then `for constexpr` will extract parameters = of this template. This is why template alias are skipped. They can never by= concrete. They more like type to type functions that templates.

Ok. Got the point on alias templates. But it actual= ly means that this feature is only usable when it's strong guarantie th= at template you are using not an alias to nontrivial implementation. For ex= ample there might be some problems with your examples if std::tuple is defi= ned in such way:

<= div class=3D"subprettyprint">template<typename... L>
using tuple =3D detail::tuple_impl<sizeof...(L), L...>;=
=

grbbing template by for constexpr seems only be usable tog= ether with some template class designed to be used for grabbing parameters.= Something like:

<= div class=3D"subprettyprint">template<typename... L>
struct types_list {};
which has strong guarantie that no extra types or nontype argument will ap= ear when it's used in "for constexpr". I think that D-like co= de I've posted above solves the same issue in much better way:

<= blockquote style=3D"margin: 0px 0px 0px 0.8ex; border-left: 1px solid rgb(2= 04, 204, 204); padding-left: 1ex;" class=3D"gmail_quote">
t= emplate<typename<= /span>... L>
struct test {
=C2=A0 = =C2=A0 using = params =3D= L;
};

int main() {
=C2=A0 =C2= =A0 for constexpr (typename T,= test<char, <= span style=3D"color:#008">int>::params) {=
=C2=A0 =C2=A0 =C2=A0 =C2=A0 std::cout << sizeof(T) << std::endl;<= /span>
=C2=A0 =C2=A0 = }
}

=C2=A0
Thre a= re two different language constructs designed for their own purpuses which = allows to have type list to iterate over in nontemplate function when those= features are used together. From my personal point of view this way is muc= h better than grabbing template parameters unless there are some use cases = which are not just "some way to have iterable typelist in non-template= code". Do you have some example usecases for the feature?

-- S= ergey

--
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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/0e2ea7f0-ee5e-44be-bade-413381bbb12b= %40isocpp.org.
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