Hi, I want no_transition() to be called whenever no row matches, even if there is a row that does not match only because the guard returns false. How can I achieve this? I want to be strict about the input events. I can probably workaround this by putting guardless rows at the top whose action is to call no_transition(), but I'm hoping to find a cleaner solution. If I get this behavior in msm, then the following code should output "no transition..." as its last line. Right now, with boost 1.51, it doesn't. It just prints "entering State1". Josh #include <iostream> #include <boost/msm/back/state_machine.hpp> #include <boost/msm/front/functor_row.hpp> #include <boost/msm/front/state_machine_def.hpp> using namespace std; namespace mpl = boost::mpl; namespace msm = boost::msm; using namespace msm::front; struct Event { Event(bool data) : data(data) { } bool data; }; struct state_ : public msm::front::state_machine_def<state_> { struct MyGuard { template <class EVT, class FSM, class SourceState, class TargetState> bool operator()(EVT const& e, FSM&, SourceState&, TargetState&) { return e.data; } }; struct State1 : public msm::front::state<> { // optional entry/exit methods template <class Event,class FSM> void on_entry(Event const&, FSM& f) { std::cout << "entering: State1" << std::endl; } template <class Event,class FSM> void on_exit(Event const&,FSM& ) { std::cout << "leaving: State1" << std::endl; } }; struct State2 : public msm::front::state<> { // optional entry/exit methods template <class Event,class FSM> void on_entry(Event const&, FSM& f) { std::cout << "entering: State2" << std::endl; } template <class Event,class FSM> void on_exit(Event const&,FSM& ) { std::cout << "leaving: State2" << std::endl; } }; typedef State1 initial_state; struct transition_table : mpl::vector< Row< State1, Event , State2, none, MyGuard > > {}; template <class FSM, class Event> void no_transition(Event const& e, FSM&, int state) { std::cout << "no transition from state " << state << " on event " << typeid(e).name() << std::endl; } }; typedef msm::back::state_machine<state_> State; int main() { State state; state.start(); state.process_event(Event(false)); return 0; }