Re: [Boost-users] Nullary metafunction

AMDG Joaquin M Lopez Munoz wrote:

er 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,X>::type, but apparently not.

Something like this could do:

#include

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(0)))== sizeof(nested_type_detail::yes), T, fallback

...

::type::type type; };

/* testing */

Wouldn't it be easier to use the MPL components that already exist? BOOST_MPL_HAS_XXX_TRAIT_DEF(type); template<class T> struct nested_type : boost::mpl::eval_if {}; In Christ, Steven Watanabe