The header <boost/core/use_default.hpp> provides the type boost::use_default which is used by other Boost libraries as a sentinel type in a templates to indicate defaults.

template<class Derived, class Base,
   class Value = boost::use_default,
   class CategoryOrTraversal = boost::use_default,
   class Reference = boost::use_default,
   class Difference = boost::use_default>
class iterator_adaptor;

template<class Value>
class node_iterator
    : public iterator_adaptor<node_iterator<Value>, Value*,
        boost::use_default, boost::forward_traversal_tag>;

namespace boost {
  struct use_default { };
}