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Subject: Re: [boost] Meta State Machine (MSM) review
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-12-07 11:05:12

Christophe Henry wrote:
> >I wonder if the library can provide a debugging mode that
> >would provide a helpful default implementation
> >of no_transition that could help identify the unknown
> >transition rather than requiring the user to write something?
> >Such a function might access internal details, perhaps only
> >provided in the said debugging mode,
> >that would be clearer about the source state and event for
> >which no transition was provided than would user-level code.
> Right now you only get an assert in debug mode. I also wonder if more
> would be possible. About the event, sure, it is already done, about
> the source state, I can't promise anything, but I will have a look at
> it.
> If I understand correctly, you would be satisfied with an assert or
> error message bringing a bit more debug information?

Yes. One person had a question about translating an integer back to user-level information. Another noted the need to write no_transition in order to discover some information about a missing transition rather than getting your assertion. It occurred to me that the library might do better at identifying the needed transition.

I presume doing so would entail storing typeid(T).name() strings mapped against state and event identifiers, if the type information is lost at the point where it is known that a transition is missing. I also presume that whatever you do is likely to be more costly in performance or memory usage, thus my "debugging mode" suggestion.

It boils down to the no_transition handling is likely to be to discover state machine problems in most cases, so the library should be in the best position do that. A more dynamic state machine might need to account for special cases and thus use no_transition as a fallback, so I'm not suggesting removing that facility.

BTW, it would be nice if the library provided a hook for producing the missing transition diagnostic. The library would generate a description and call the hook. The default would write to stderr or assert as now, but the user could override it to write a log file, etc.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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