Re: [Boost-users] [mpl] what is not a placeholder expression

From: David Abrahams (dave_at_[hidden])
Date: 2007-07-27 20:53:06

on Fri Jul 27 2007, "JOAQUIN LOPEZ MU?Z" <joaquin-AT-tid.es> wrote:

> According to the MPL reference, a placeholder expression
> X<a1,..,an> must satisfy (among others) the following
> condition:
>
>   "All of X's template parameters, including the
>   default ones, are types."
>
> According to that the following should then compile
> without errors:
>
>   #include <boost/mpl/apply.hpp>
>   #include <boost/mpl/assert.hpp>
>   #include <boost/type_traits/is_same.hpp>
>
>   using namespace boost;
>   using namespace boost::mpl;
>
>   template<typename T,int=0>
>   struct foo 
>   {
>     template<typename Q>
>     struct apply{
>       typedef Q type;
>     };
>   };
>
>   typedef apply1<foo<_1>,int>::type t1;
>
>   BOOST_MPL_ASSERT((is_same<t1,int>));
>
>   int main(){}

I don't see how you can conclude the above will compile without error
just from (a part of) the definition of placeholder expression.

Are you saying that since foo<_1> is not a placeholder expression,
when applied to int it should be treated as a metafunction class?

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com
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