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

From: John Moeller (fishcorn_at_[hidden])
Date: 2008-06-17 13:08:22

John Moeller <fishcorn <at> gmail.com> 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