Mine

                class LexError : std::exception
                {
                        std::string myErrorMessage;
                        public:
                                LexError(std::string errorMessage) :
                                        myErrorMessage(errorMessage) {};
                                virtual const char* what() const throw()
                                {
                                        return myErrorMessage.c_str();
                                }
                                ~LexError() throw() {};
                };