[Boost-users] Another C++ inheritence/polymorphism problem

Hi, here is another problem I met: $ g++ main.cpp main.cpp: In member function `void schedule_vertex_writer<Graph>::operator()(std::ostream&, const Vertex&) const [with Vertex = unsigned int, Graph = boost::adjacency_list]': /usr/local/include/boost/graph/graphviz.hpp:264: instantiated from `void boost ::write_graphviz(std::ostream&, const Graph&, VertexPropertiesWriter, EdgeProper tiesWriter, GraphPropertiesWriter) [with Graph = boost::adjacency_list, VertexPropertiesWriter = schedule_vertex_writer >, EdgePropertiesWrite r = boost::default_writer, GraphPropertiesWriter = boost::default_writer]' /usr/local/include/boost/graph/graphviz.hpp:292: instantiated from `void boost ::write_graphviz(std::ostream&, const Graph&, VertexWriter) [with Graph = Graph, VertexWriter = schedule_vertex_writer >]' main.cpp:93: instantiated from here main.cpp:52: error: passing `const BasicOperator' as `this' argument of ` virtual OperatorType* BasicOperator::getOperatorType()' discards qualifiers The source code is in the attachment. Thanks!