Hello List After all of that I get caught with this Compiler error: xml.hpp:77: error: passing ‘const xml::entity’ as ‘this’ argument of ‘virtual void xml::entity::parse(boost::asio::buffers_iterator<boost::asio::const_buffers_1, char>, boost::asio::buffers_iterator<boost::asio::const_buffers_1, char>)’ discards qualifiers From within document class: typedef boost::asio::buffers_iterator< boost::asio::streambuf::const_buffers_type > iterator; void parse( iterator begin, iterator end ) { return state_cast<const entity&>().parse(begin,end); } So a short question is 'How do you pass data from "outside" the state machine into the state machine'. Is it worth it? Etienne