Re: [Boost-users] [range] Trying to make an "is_range" metafunction

17 Jun 2008

      John Moeller  writes:
...
Hello all,
I am trying to make an "is_range" metafunction that (essentially) returns
mpl::true_ when the template parameter is a range (as defined by the
boost::range library).
I can do this if I create a specialization for every "non-range" type currently
specialized by the library (such as built-in arrays, char strings, etc.). 
However, I'd like to have is_range work for a user-defined range without the
user *also* needing to specialize is_range.
I tried something like this:
//////////////////////////////////////////////////////////////////////
BOOST_MPL_HAS_XXX_TRAIT_DEF(type)
template < bool B > 
struct is_range_impl 
  : boost::mpl::false_ 
{};
template <>
struct is_range_impl< true >
  : boost::mpl::true_
{};
template < typename R >
struct is_range 
  : is_range_impl< has_type< boost::range_iterator< R > >::value >
{};
//////////////////////////////////////////////////////////////////////
Which does fine if R is a range (even when you use the non-intrinsic method of
defining a custom class as a range), but if R is *not* a range, the compiler
complains that "iterator" is not a member of the class that I'm passing as the
parameter, because it defaults to using the embedded "iterator" type in the
parameter.
Is there a better way to define is_range so that it will work?  I imagine that
I'd have to use SFINAE somehow, but I can't quite see how to put it together.
Many thanks ahead of time,
--
John Moeller
I figured out how to do it (and it's pretty simple):

template < typename R, typename V = void >
struct is_range_impl
  : boost::mpl::false_
{};

template < typename R >
struct is_range_impl< R, boost::range_iterator< R > >
  : boost::mpl::true_
{};

template < typename R > 
struct is_range 
  : is_range_impl< R >
{};

Feel free to pick this apart; I wouldn't mind if it were made better.

Thanks,

--

John Moeller