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

#include <string>

struct Peer
{
    std::string userid;
};
struct CPeer:public Peer
{
    int index;
};
struct userid{};

typedef multi_index_container<
   CPeer,
   indexed_by<
      hashed_unique<tag<userid>,member<CPeer,std::string,&CPeer::userid> >
   >
> CPeersType;

//how could i use userid as an index in  CPeer ,not in Peer,