Hi,    I have a query regarding macro BOOST_CLASS_VERSION().    I am running following code :     class testRep     {         friend std::ostream & operator<<(std::ostream &os, const testRep &tr);             public :         template<class Archive> void serialize(Archive &ar, const unsigned int version)         {             if(version >= 2)                 ar & BOOST_SERIALIZATION_NVP(Site);         }         private :             string Site;     };        BOOST_CLASS_VERSION(testRep, 2)        std::ostream & operator<<(std::ostream &os, const testRep &tr)     {         return os << '\n' << tr.Site << endl;     }        void read_file (testRep &s, const char * filename)     {         std::ifstream ifs(filename);         assert (ifs.good());         boost::archive::xml_iarchive ia(ifs);         ia >> BOOST_SERIALIZATION_NVP(s);     }     int main (void)     {         testRep tr;         read_file (tr, "Test.xml");         cout << tr << endl;         return 0;     }    This code runs fine.    But, if I remove the statement :    BOOST_CLASS_VERSION(testRep, 2)        then I get following error :    terminate called after throwing an instance of 'boost::archive::archive_exception'      what():  class version    Aborted (core dumped)        So, want to ask if it is mandatory to declare a macro BOOST_CLASS_VERSION()? and if not then how do I prevent this runtime error from happening?   Regards, Girish