Hello, I've taken a look at how the version number is dealt with in Boost.Serialization collection serialization code: * The relevant code at saving time is: if(3 < ar.get_library_version()){ const unsigned int item_version = version< BOOST_DEDUCED_TYPENAME Container::value_type >::value; ar << BOOST_SERIALIZATION_NVP(item_version); } * At loading time, the equivalent code is: unsigned int item_version; ar >> BOOST_SERIALIZATION_NVP(count); if(3 < ar.get_library_version()) ar >> BOOST_SERIALIZATION_NVP(item_version); else item_version = 0; Can someone (aka Robert :-) ) shed some light on this bit? Why the version number has to be given such manual processing when this is not needed if, for instance, serializating values via the simple ar&value? I'm trying to do the equivalent thing in Boost.MultiIndex. Thank you, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo