Multiple deferred events in Boost Statechart

Question

How do multiple deferred events behave in Boost Statechart?

Let's say I have 4 states and 3 events. The non-deferred transitions looks as follows:

S1--(E1)-->S2--(E2)-->S3--(E3)-->S4

The state machine starts in S1. Now events are posted in the following order: E2, E3, E1.

I want the state machine to reach S4 as a result of this sequence of events. What do I have to do in order to achieve this?

• I obviously have to defer E2 and E3 in S1.
• I probably also have to defer E3 in S2? I guess otherwise it will be discarded when S2 is reached.

I assume this part of the documentation: http://www.boost.org/doc/libs/1_55_0/libs/statechart/doc/reference.html#process_event contains the answer, but it is very technical (rather a description of the implementation than of the semantics -- the tutorial is not clearer either), and contains lots of references, so it is hard to know for sure.

Answer 1

You need to use sc::deferral to achieve the same. What you want to do can be achieved via following code.

// States
struct S1;
struct S2;
struct S3;
struct S4;


// Events
struct E1 : sc::event<E1> {};
struct E2 : sc::event<E2> {};
struct E3 : sc::event<E3> {};

struct statemachine : sc::state_machine<statemachine, S1>{};

struct S1 : sc::simple_state<S1, statemachine> {
    typedef mpl::list<
        sc::transition< E1, S2 >,
        sc::deferral< E2 >,
        sc::deferral< E3 >
    > reactions;

};

struct S2 : sc::simple_state<S2, statemachine> {
    typedef sc::transition<E2, S3> reactions;
};

struct S3 : sc::simple_state<S3, statemachine> {
    typedef sc::transition<E3,S4> reactions;
};

struct S4 : sc::simple_state<S4, statemachine> {
};

int main() {
    statemachine sm;
    sm.initiate();
// Event Sequence E2 -> E3 -> E1
    sm.process_event(E2());
    sm.process_event(E3());
    sm.process_event(E1());
    return 0;
}

In STATE "S1" , we deferred handling of Events "E2" and "E3" which is subsequently handled in STATE "S2" and "S3"

Hope its clear..

Note: you need to add a header file #include <boost/statechart/deferral.hpp> for using sc::deferral