I haven't actually happened upon this problem yet but I just want to know if I know how to deal with it for the future. My question is: how does the serialization work (or not work) if we add or remove indexes from the type? My suspicion is that one would have to do something like this: // version 1: BOOST_CLASS_VERSION(Thingy,1); typedef multi_index_container<thing,indexes> myset1_t; myset1_t m_MySet; ... template<typename Ar> void serialize(Ar & a, unsigned int) { ar & m_MySet; } // version 2 BOOST_CLASS_VERSION(Thingy,2); typedef multi_index_container<thing,indexes> myset1_t; typedef multi_index_container<thing,indexes2> myset2_t; myset2_t m_MySet; // using new version ... template<typename Ar> void serialize(Ar & a, unsigned int fv) { if(Ar::is_loading && fv == 1) { myset1_t set; ar & set; m_MySet.clear(); m_MySet.insert(m_MySet.begin(),set.begin(),set.end()); } else ar & m_MySet; } Would this be OK, errors in syntax aside? Thanks, Sohail