OrderedHashMap similar to Python OrderedDict implemented with Boost

OrderedHashMap.h

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/member.hpp>
#include <boost/multi_index/sequenced_index.hpp>
using namespace ::boost;
using namespace ::boost::multi_index;


template <typename T1,typename T2>
struct mutable_pair
{
  typedef T1 first_type;
  typedef T2 second_type;

  mutable_pair():first(T1()),second(T2()){}
  mutable_pair(const T1& f,const T2& s):first(f),second(s){}
  mutable_pair(const std::pair<T1,T2>& p):first(p.first),second(p.second){}

  T1         first;
  mutable T2 second;
};


struct by_seq{};

template <typename Key, typename T>
struct OrderedHashMap_set_indices:
indexed_by
<
   hashed_unique<member
   <
      mutable_pair<Key, T>,
      Key,
      &mutable_pair<Key, T>::first
   > >,
   sequenced<tag<by_seq> >
>
{};

template <typename Key, typename T>
class OrderedHashMap:
   public multi_index_container
   <
      mutable_pair<Key, T>,
      OrderedHashMap_set_indices<Key, T>
   >
{
//public:
//   multi_index_container<mutable_pair<Key, T> >::index<by_seq>::type::iterator begin()
//   {
//      return get<by_seq>().begin();
//   };
};