On 5/27/2010 10:38 AM, Lorenzo Caminiti wrote:
Hello all,
How do I use Boost.ConceptCheck and/or Bosot.EnableIf to implement concept-based overloading (as discussed in N2081)?
For example, from N2081, the following functions are selected based on `Iter` matching one of the two specified concepts:
template<InputIterator Iter> // proposed concept syntax (not C++) void advance(Iter& x, Iter::distance_type n) { while (n> 0) { ++x; --n; } }
template<BidirectionalIterator Iter> // proposed concept syntax (not C++) void advance(Iter& x, Iter::distance_type n) { if (n> 0) while (n> 0) { ++x; --n; } else while (n< 0) { --x; ++n; } }
How do I program this in C++ using Boost libraries?
N2081 briefly mentions this can be done in C++ using SFINAE, etc. I think I could use `enable_if` if I had some sort of `is_input_iterator` metafunction... Please just point me to the documentation if this has already been addressed there.
Thank you very much.
Do you mean the iterator_category / iterator_traversal metafunctions in boost/iterator/iterator_traits.hpp ? http://www.boost.org/libs/iterator/doc/iterator_traits.html template< class Iterator > typename boost::disable_if< boost::is_convertible< typename boost::iterator_traversal< Iterator >::type, boost::bidirectional_traversal_tag >
::type advance(...) { ... } // for IncrementableIterator's
template< class Iterator > typename boost::enable_if< boost::is_convertible< typename boost::iterator_traversal< Iterator >::type, boost::bidirectional_traversal_tag >
::type advance(...) { ... } // for BidirectionalIterator's
I think the primary way to currently effect "concept-based" overloading is via tags or similar metafunction-based methods. There do exist introspection techniques, though, such as proto's can_be_called or is_incrementable in boost/detail/, so it is possible to overload based on (some) syntactic properties. Is that what you're looking for? - Jeff