#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
#include <boost/flyweight.hpp>
#include <boost/flyweight/no_tracking.hpp>
#include <functional>
#include <fstream>
#include <set>
#include <iostream>

template<typename Entry,typename Value,const char* Filename>
class serialized_set_factory_class:
  public boost::flyweights::factory_marker
{
  typedef std::set<Entry,std::less<Value> >   container_type;
public:
  typedef typename container_type::iterator   handle_type;
  typedef typename container_type::value_type entry_type;

  serialized_set_factory_class()
  {
    std::ifstream ifs(Filename);
    if(ifs){
      boost::archive::text_iarchive ia(ifs);
      typename container_type::size_type s=0;
      ia>>s;
      for(typename container_type::size_type n=s;n--;){
        Value v;
        ia>>v;
        cont.insert(cont.end(),Entry(v));
      }
      std::clog<<"loaded "<<s<<" entries from "<<Filename<<"\n";
    }
  }
  
  ~serialized_set_factory_class()
  {
    std::ofstream                      ofs(Filename);
    boost::archive::text_oarchive      oa(ofs);
    typename container_type::size_type s=cont.size();
    oa<<s;
    for(typename container_type::iterator it=cont.begin(),it_end=cont.end();
        it!=it_end;++it){
      oa<<static_cast<const Value&>(*it);
    }
    std::clog<<"saved "<<s<<" entries to "<<Filename<<"\n";
  }

  handle_type insert(const entry_type& x)
  {
    return cont.insert(x).first;
  }

  void erase(handle_type h)
  {
    cont.erase(h);
  }

  const entry_type& entry(handle_type h){return *h;}

private:  
  container_type cont;
};

template<const char* Filename>
struct serialized_set_factory:boost::flyweights::factory_marker
{
  template<typename Entry,typename Value>
  struct apply
  {
    typedef serialized_set_factory_class<
      Entry,Value,Filename
    > type;
  };
};

using namespace boost::flyweights;

extern const char filename[]="fw.bd";
typedef flyweight<
  std::string,
  serialized_set_factory<filename>,
  no_tracking
> fw_t;

int main()
{
  fw_t fw1("Hello"),fw2("Boost"),fw3("Farewell"),fw4("Boost");
}