Re: [Boost-users] [MSM] Unable to retrieve correct value of attribute of a state from within actions. (Possible Bug?)

25 Feb 2013

      Hi Christophe,
...
...
// The attribute:
BOOST_MSM_EUML_DECLARE_ATTRIBUTE(int, Number)
BOOST_MSM_EUML_ATTRIBUTES((attributes_ << Number), NumberAttr)
// An event containing the attribute:
BOOST_MSM_EUML_EVENT_WITH_ATTRIBUTES(event1, NumberAttr)
// Two states with attributes:
BOOST_MSM_EUML_STATE((no_action, no_action, attributes_ << Number),
state1)
BOOST_MSM_EUML_STATE((no_action, no_action, attributes_ << Number),
state2)
// Entry-action which initializes the attributes:
BOOST_MSM_EUML_ACTION(init_events_and_states)
{
  template 
  void operator()(Evt const&, Fsm&, State&)
  {
    event1.get_attribute(Number) = 100;
    state1.get_attribute(Number) = 1;
    state2.get_attribute(Number) = 2;
  };
};
Ouch! I admit I didn't think about this use case. This is what happens
when an author tries to hide implementation details ;-)
event1, state1 and state2 are really just "lies".
eUML is simply some syntactic sugar on top of the functor front-end.
These variables are only here because in the review, several people
complained that you had to write in the stt something like State1() +
Event1() == State2().
state1 simply is a dummy variable, all I want from it is its type. Of
course, one instance exists, but normally, one does not care.
[snip]
So, there is no copy but msm takes the type of state1 and create its own
instance (though you still have the possibility to copy from an already
existing instance).
Thank you, that clarifies a lot.
...
What you are allowed to use are the template parameters: source state,
target state, event, and fsm (on which, if you call fsm. template
get_state() will give you a reference
to the real state1).
You are missing a '&' or '*' behind the macro, but you are right, that 
does what I wanted in the first place.
...
...
fsm.process_event(event1); // this really is event1!
[snip]
As you process event1, you really are changing the dummy instance, so
you get the same as if you used the event in the action.
Normally, you shouldn't care because events are temporary variables. I
don't recommend using this trick ;-)
Just for final clarification three more questions:

1.
You mean, that from within the "operator()" of an action I am only 
allowed to use the parameters for accessing states, events, and the FSM 
internals? (Neither "event_", "source_", "target_", "fsm_", nor 
accessing the dummy-instances directly.)

This is now clear for the current event, current source- and target 
state as well as for all other states of the FSM (by accessing them with 
"get_state").

2.
However, how to access other events than the current one?

It seems to work as I did before (by accessing the "dummy"-instance of 
the event), but if you do not recommend it, maybe there is a better 
solution?
(I need to access all events directly to initialize their event-number 
attributes from within the FSM's entry-action.)

3.
And all the underscore-accessors "event_", "source_", "target_", "fsm_" 
are only allowed to be used from within the transition table?

Maybe you could emphasize the real usage in the documentation, so that 
nobody misuses it (like I did).

Thanks,
Deniz

Re: [Boost-users] [MSM] Unable to retrieve correct value of attribute of a state from within actions. (Possible Bug?)

Deniz Bahadir