I think I'll need your example to understand this question. Generally, my opinion is that I want the weakest dependency I can get away with. And a dependency to a template method of an event seems to me a weaker one than a dependency to a submachine class. Both ways because the submachine would also have to make some assumptions about the main machine's behaviour.
In the case of event parameter, I completely agree with you. But I'm not sure how to propagate the sub-machine's action result to main-machine without adding the interface to sub-machine.
I have two ideas. 1. Using the internal transition. (See 13_PropagateActionResult.cpp Event2) Can I call the member function "process_event" inside the action functor?? (See Line 69)
Yes. I didn't try your example out but it looks good.
More generally, does the msm provide interfaces that send events to their own state machine?
Sure. That's one of the main reasons to pass the fsm as parameter to the action.
2. Add the mutable variable to event classes, and modify the variable in action functor. I think it is ugly.
I agree :)
If there are no good ideas to do that, your approach solves only event parameter. I think it's too partial.
I hope 1 is OK. Or do you have any better ideas?
What you are doing should work without problems. As a side note, I like the Guard1 / Not_<Guard1> construct. It's the best way to ensure that guards are mutually exclusive. Regards, Christophe