Move difference_type from the Forward Traversal Iterator concept to

the Single Pass Traversal concept, so that input iterators are still
input iterators.  It does make sense to measure the distance between
Single Pass iterators, provided you're willing to throw away the data.


[SVN r33870]

include/boost/iterator/iterator_archetypes.hpp

@@ -196,6 +196,8 @@ namespace detail
           explicit archetype(ctor_arg arg)
             : traversal_archetype_<Derived, Value, incrementable_traversal_tag>(arg)
           {}
+          
+          typedef std::ptrdiff_t difference_type;
       };
   };
 
@@ -219,7 +221,6 @@ namespace detail
           archetype() 
             : traversal_archetype_<Derived, Value, single_pass_traversal_tag>(ctor_arg())
           {}
-          typedef std::ptrdiff_t difference_type;
       };
   };