220 8238 <f4ec2cca-5951-447f-a938-0b2e5e3e233f@isocpp.org> article
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From: Vincent Reverdy <vince.rev@gmail.com>
Newsgroups: gmane.comp.lang.c++.isocpp.proposals
Subject: A new keyword for either a type or a variable
 (typical use case for variadic template template)?
Date: Tue, 24 Dec 2013 15:32:57 -0800 (PST)
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Hi, 

Currently, it is impossible, as far as I know, to write a "
is_specialization_of" helper struct to detect whether a type is a 
specialization of a given template.
It can be illustrated with the following example:
----------------------------------------------------------------------
#include <iostream>
#include <type_traits>
#include <tuple>
#include <vector>
#include <array>

template <template <class...> class Template, class T>
struct is_specialization_of 
: std::false_type {};
 
template <template <class...> class Template, class... Args>
struct is_specialization_of<Template, Template<Args...>> 
: std::true_type {};

int main()
{
    std::cout<<is_specialization_of<std::tuple,  
std::tuple<double>>::value<<::std::endl;
    std::cout<<is_specialization_of<std::vector, 
std::tuple<double>>::value<<::std::endl;
    std::cout<<is_specialization_of<std::array,  
std::tuple<double>>::value<<::std::endl; // Error
    return 0;
}
----------------------------------------------------------------------
It works well for templates of the form template <class...> but it fails 
for mixed type/variable template like std::array.
I think that it would be great (particularly for template metaprogramming) 
to have a a new keyword (whose name has to be defined) to declare "a type 
or a variable".
With this new keyword, one could type:
----------------------------------------------------------------------
#include <iostream>
#include <type_traits>
#include <tuple>
#include <vector>
#include <array>

template <template <keyword...> class Template, class T>
struct is_specialization_of 
: std::false_type {};
 
template <template <keyword...> class Template, keyword... Args>
struct is_specialization_of<Template, Template<keyword...>> 
: std::true_type {};

int main()
{
    std::cout<<is_specialization_of<std::tuple,  
std::tuple<double>>::value<<::std::endl;
    std::cout<<is_specialization_of<std::vector, 
std::tuple<double>>::value<<::std::endl;
    std::cout<<is_specialization_of<std::array,  
std::tuple<double>>::value<<::std::endl; // Ok
    return 0;
}
----------------------------------------------------------------------

Has it been already discussed? 
If not, what is you opinion about such a thing?
If yes, what were the issues?
Do you have any suggestion for the name of this keyword?

Thank you!

Vincent R.

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<div dir=3D"ltr">Hi, <br><br>Currently, it is impossible, as far as I know,=
 to write a "<span style=3D"font-family: courier new,monospace;">is_special=
ization_of</span>" helper struct to detect whether a type is a specializati=
on of a given template.<br>It can be illustrated with the following example=
:<br><span style=3D"font-family: courier new,monospace;">------------------=
----------------------------------------------------</span><br><span style=
=3D"font-family: courier new,monospace;">#include &lt;iostream&gt;<br>#incl=
ude &lt;type_traits&gt;<br>#include &lt;tuple&gt;<br>#include &lt;vector&gt=
;<br>#include &lt;array&gt;<br><br>template &lt;template &lt;class...&gt; c=
lass Template, class T&gt;<br>struct is_specialization_of <br>: std::false_=
type {};<br>&nbsp;<br>template &lt;template &lt;class...&gt; class Template=
, class... Args&gt;<br>struct is_specialization_of&lt;Template, Template&lt=
;Args...&gt;&gt; <br>: std::true_type {};<br><br>int main()<br>{<br>&nbsp;&=
nbsp;&nbsp; std::cout&lt;&lt;is_specialization_of&lt;std::tuple,&nbsp; std:=
:tuple&lt;double&gt;&gt;::value&lt;&lt;::std::endl;<br>&nbsp;&nbsp;&nbsp; s=
td::cout&lt;&lt;is_specialization_of&lt;std::vector, std::tuple&lt;double&g=
t;&gt;::value&lt;&lt;::std::endl;<br>&nbsp;&nbsp;&nbsp; std::cout&lt;&lt;is=
_specialization_of&lt;std::array,&nbsp; std::tuple&lt;double&gt;&gt;::value=
&lt;&lt;::std::endl; // Error<br>&nbsp;&nbsp;&nbsp; return 0;<br>}</span><b=
r><span style=3D"font-family: courier new,monospace;">---------------------=
-------------------------------------------------</span><br>It works well f=
or templates of the form <span style=3D"font-family: courier new,monospace;=
">template &lt;class...&gt;</span> but it fails for mixed type/variable tem=
plate like <span style=3D"font-family: courier new,monospace;">std::array</=
span>.<br>I think that it would be great (particularly for template metapro=
gramming) to have a a new keyword (whose name has to be defined) to declare=
 "a type or a variable".<br>With this new keyword, one could type:<br><span=
 style=3D"font-family: courier new,monospace;">----------------------------=
------------------------------------------</span><br><span style=3D"font-fa=
mily: courier new,monospace;">#include &lt;iostream&gt;<br>#include &lt;typ=
e_traits&gt;<br>#include &lt;tuple&gt;<br>#include &lt;vector&gt;<br>#inclu=
de &lt;array&gt;<br><br>template &lt;template &lt;keyword...&gt; class Temp=
late, class T&gt;<br>struct is_specialization_of <br>: std::false_type {};<=
br>&nbsp;<br>template &lt;template &lt;keyword...&gt; class Template, keywo=
rd... Args&gt;<br>struct is_specialization_of&lt;Template, Template&lt;keyw=
ord...&gt;&gt; <br>: std::true_type {};<br><br>int main()<br>{<br>&nbsp;&nb=
sp;&nbsp; std::cout&lt;&lt;is_specialization_of&lt;std::tuple,&nbsp; std::t=
uple&lt;double&gt;&gt;::value&lt;&lt;::std::endl;<br>&nbsp;&nbsp;&nbsp; std=
::cout&lt;&lt;is_specialization_of&lt;std::vector, std::tuple&lt;double&gt;=
&gt;::value&lt;&lt;::std::endl;<br>&nbsp;&nbsp;&nbsp; std::cout&lt;&lt;is_s=
pecialization_of&lt;std::array,&nbsp; std::tuple&lt;double&gt;&gt;::value&l=
t;&lt;::std::endl; // Ok<br>&nbsp;&nbsp;&nbsp; return 0;<br>}</span><br><sp=
an style=3D"font-family: courier new,monospace;">--------------------------=
--------------------------------------------</span><br><br>Has it been alre=
ady discussed? <br>If not, what is you opinion about such a thing?<br>If ye=
s, what were the issues?<br>Do you have any suggestion for the name of this=
 keyword?<br><br>Thank you!<br><br>Vincent R.<br><br></div>

<p></p>

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