#ifndef _utl_MapKeyIterator_h_
#define _utl_MapKeyIterator_h_

#include <boost/iterator/transform_iterator.hpp>


namespace utl {

  template<class Pair>
  struct ExtractFirst {
    typedef typename Pair::first_type FirstType;
    FirstType& operator()(Pair& pair) const { return pair.first; }
    const FirstType& operator()(const Pair& pair) const { return pair.first; }
  };


  template<class Map>
  struct MapKeyIterator :
    public boost::transform_iterator<
      ExtractFirst<typename Map::value_type>,
      typename Map::const_iterator,
      typename ExtractFirst<typename Map::value_type>::FirstType&
    > {

    typedef ExtractFirst<typename Map::value_type> UnaryFunction;
    typedef typename Map::const_iterator Iterator;
    typedef typename ExtractFirst<typename Map::value_type>::FirstType& Reference;
    typedef boost::transform_iterator<UnaryFunction, Iterator, Reference> Super;

    MapKeyIterator(const Iterator& it) : Super(it) { }

  };

}


#endif