Re: [Boost-users] Nullary metafunction

Subject: Re: [Boost-users] Nullary metafunction
From: Joaquin M Lopez Munoz (joaquin_at_[hidden])
Date: 2009-02-03 15:50:45

er <erwann.rogard <at> gmail.com> writes:

> 
> Hi All,
> 
> I'm looking for template<typename> class F such that
> 
> F<T>::type is T::type if it exists, T itself otherwise.
> 
> I was hoping apply<T>::type would work, by analogy with the well known 
> example apply<std::vector<_>,X>::type, but apparently not.
> 

Something like this could do:

#include <boost/mpl/if.hpp>

namespace nested_type_detail{
typedef char yes;
struct       no{char x[2];};
template<typename T>
yes helper(T*,typename T::type* =0);
no  helper(...);
}
template<typename T>
struct nested_type
{
  struct fallback{typedef T type;};
  typedef typename boost::mpl::if_c<
    sizeof(nested_type_detail::helper(static_cast<T*>(0)))==
    sizeof(nested_type_detail::yes),
    T,
    fallback
  >::type::type type;
};

/* testing */

#include <boost/mpl/assert.hpp>
#include <boost/type_traits/is_same.hpp>

struct test1{typedef char type;};
struct test2{typedef char tope;};
typedef char test3;

int main()
{
  BOOST_MPL_ASSERT((boost::is_same<nested_type<test1>::type,char>));
  BOOST_MPL_ASSERT((boost::is_same<nested_type<test2>::type,test2>));
  BOOST_MPL_ASSERT((boost::is_same<nested_type<test3>::type,test3>));
}

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo