Well, I found out that this is not a problem related to inheritence nor polymorphism, it is a problem with the BGL 1. Even if I make BasicOperator not inherit from Operator, the problem is the same. Here is the modified BasicOperator: class BasicOperator /* : public Operator */{ public: BasicOperator() {}; BasicOperator(OperatorType* type) : _type(type) {toString = _type->getMnemonic();}; BasicOperator(OperatorType* type, string cust) : _type(type), _cust(cust) {toString = _type->getMnemonic() + "&" + _cust;}; OperatorType* getOperatorType() { return _type; } string toString() { return toString; } string toString; private: OperatorType* _type; string _cust; }; And the vertex property writer is: void operator()(std::ostream& out, const Vertex& v) const { out << "[label=\"" << "(" << g[v].toString() << ")" << "\"]"; } The error is: main.cpp:52: error: passing `const BasicOperator' as `this' argument of ` std::string BasicOperator::toString()' discards qualifiers So it looks like that the problem is related to the "const" BasicOperator I passed into std::string BasicOperator::toString(). How can I fix this? Which means if the property of a vertex is a Class, how can I call its member function in the vertex writer? 2. If I changes the vertex property writer as: void operator()(std::ostream& out, const Vertex& v) const { out << "[label=\"" << "(" << g[v].toString << ")" << "\"]"; } Everything is OK. Which mean in the vertex property writer, I can access the data member without any problem, but I cannot access the member function. -----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf Of Welson Sun Sent: Thursday, February 10, 2005 4:42 PM To: boost-users@lists.boost.org Subject: [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<boost::vecS, boost::vecS, boost::bidirectionalS, BasicOperator, boost::no_property, boost::no_property, boost::listS>]': /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<boost::ve cS, boost::vecS, boost::bidirectionalS, BasicOperator, boost::no_property, boost ::no_property, boost::listS>, VertexPropertiesWriter = schedule_vertex_writer<bo ost::adjacency_list<boost::vecS, boost::vecS, boost::bidirectionalS, BasicOperat or, boost::no_property, boost::no_property, boost::listS> >, 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<boost::adjacency_list<boost::vecS, boost: :vecS, boost::bidirectionalS, BasicOperator, boost::no_property, boost::no_prope rty, boost::listS> >]' 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!