I'm am using boost::multi_index to hold a set of polymorphic entities.  Due to efficiency concerns, I have to be able sort the container based on actual type.  I was planning on sorting based on the return result of typeid.  When I try this I get errors in <functional>.  I am using VC++ 2005 Standard Edition SP1.  What am I doing wrong, or, what is a better way of accomplishing what I am trying to do?

#ifndef ENTITIES_ENTITYCONTAINER_H_

#define ENTITIES_ENTITYCONTAINER_H_

#include <boost/multi_index_container.hpp>

#include <boost/multi_index/ordered_index.hpp>

#include "Entity.h"

namespace Entities

{

    struct type{};

    struct extractType

    {

        typedef const type_info& result_type; 

        result_type operator()(const Entity * r)const // operator() must be const

        {

            return typeid(*r);

        }

    };

    typedef boost::multi_index::multi_index_container<

        Entity *,

        boost::multi_index::indexed_by<

            boost::multi_index::ordered_non_unique<boost::multi_index::tag<type>, extractType>

        >

    > EntityContainer;

};

#endif