 operators. We'll
          have to implement our own filter factory instead. Filter factories derive
          from the <code class="computeroutput"><a class="link" href="../../boost/log/filter_factory.html" title="Struct template filter_factory">filter_factory</a></code>
          interface. This base class declares a number of virtual functions that
          will be called in order to create filters, according to the filter expression.
          If some functions are not overridden by the factory, the c