Re: [Boost-users] Boost.MSM -> Ignore events that were not explicitly defined for a state

Subject: Re: [Boost-users] Boost.MSM -> Ignore events that were not explicitly defined for a state
From: Christophe Henry (christophe.j.henry_at_[hidden])
Date: 2012-12-18 16:46:56

Hi,

<snip>

>                struct transition_table:mpl::vector<
>                    //          Start    Event     Next     Action
> Guard
>                    msmf::Row < State1, Event1,    State2, msmf::none,
> msmf::none >,
>                    msmf::Row < State2, Event2,    State3, msmf::none,
> msmf::none >
>                > {};
>    };
>    void test()
>    {
>        Sm1 sm1;
>        sm1.start();
>        std::cout << "> Send Event2" << std::endl;
>        sm1.process_event(Event2());
>        std::cout << "> Send Event1" << std::endl;
>        sm1.process_event(Event1());
>    }
> }

<snip>

> If I compile and execute the code above the software crashes due to the
> fact, that *Event2* is received in the state *State1*.
>
> My understanding of a state machine and boost.msm was, that all events 
> which
> are not defined for a particular state, will be just ignored and the state
> receiving this unknown event (here *Event2*) will keept.
>
> Is there a possibility in boost.msm ( maybe by adopting to the transition
> table) to stay in the current state if an event was received which was not
> assigned to this state?
>
> Thank you in advance!
>
> Best regards,
>
> RaRi

Strictly speaking, it is considered an error when an event is sent to a 
state machine which is not in a state where it can process it.
MSM's default is an assert, so you will get this only in debug mode.
You have 2 possibilities:
- the easy one, overwrite the default handler in the front-end:

        // Replaces the default no-transition response.
        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;
        }

- handle the event to show that you expected this case to arise. A simple 
internal transition in State1 will do (see 
http://svn.boost.org/svn/boost/trunk/libs/msm/doc/HTML/ch03s03.html#d0e1338). I 
personally add an action to this transition to log this but it's a matter of 
taste. Internal<Event2> would actually do.

HTH,
Christophe