gh a pointer to its base class,
the library needs to determine whether or not the bas is abstract
(i.e. has at least one virtual function).  The library uses the
type trait macro <code style="white-space: normal">BOOST_IS_ABSTRACT(T)</code>
to do this.  Not all compilers support this type trait and corresponding
macro.  To address this, the macro <code style="white-space: normal">
BOOST_SERIALIZATION_ASSUME_ABSTRACT(T)</code> has been
implemented to permit one to explicitly indicate that a specified
type is