Questions & Answers, tips

Where should I define a state machine?: The tutorials are implemented in a simple cpp source file for simplicity. I want to model dynamic behavior of a class as a state machine, how should I define the state machine?

Answer: Usually you’ll want to implement the state machine as an attribute of the class. Unfortunately, a concrete state machine is a typedef, which cannot be forward-declared. This leaves you with two possibilities:

  • Provide the state machine definition inside the header class and contain an instance as attribute. Simple, but with several drawbacks: using namespace directives are not advised, and compile-time cost for all modules including the header.

  • Keep the state machine as (shared) pointer to void inside the class definition, and implement the state machine in the cpp file. Minimum compile-time, using directives are okay, but the state machine is now located inside the heap.

Question: on_entry gets as argument the sent event. What event do I get when the state becomes default-activated (because it is an initial state)?

Answer: To allow you to know that the state was default-activated, MSM generates a boost::msm::InitEvent default event.

Question: Why do I see no call to no_transition in my submachine?

Answer: Because of the priority rule defined by UML. It says that in case of transition conflict, the most inner state has a higher priority. So after asking the inner state, the containing composite has to be also asked to handle the transition and could find a possible transition.

Question: Why do I get a compile error saying the compiler cannot convert to a function …​Fsm::*(some_event)?

Answer: You probably defined a transition triggered by the event some_event, but used a guard/action method taking another event.

Question: Why do I get a compile error saying something like too few'' or too many'' template arguments?

Answer: You probably defined a transition in form of an a_row or g_row where you wanted just a _row or the other way around. With Row, it could mean that you forgot a "none".

Question: Why do I get a very long compile error when I define more than 20 rows in the transition table?

Answer: MSM uses Boost.MPL under the hood and this is the default maximum size. Please define the following 3 macros before including any MSM headers:

#define BOOST_MPL_CFG_NO_PREPROCESSED_HEADERS
#define BOOST_MPL_LIMIT_VECTOR_SIZE 30 // or whatever you need
#define BOOST_MPL_LIMIT_MAP_SIZE 30 // or whatever you need

Question: Why do I get this error: ``error C2977: 'boost::mpl::vector' : too many template arguments''?

Answer: The first possibility is that you defined a transition table as, say, vector17 and have 18 entries. The second is that you have 17 entries and have a composite state. Under the hood, MSM adds a row for every event in the composite transition table. The third one is that you used a mpl::vector without the number of entries but are close to the MPL default of 50 and have a composite, thus pushing you above 50. Then you need mpl/vector60/70….hpp and a mpl/map60/70….hpp.

Question: Why do I get a very long compile error when I define more than 10 states in a state machine?

Answer: MSM uses Boost.Fusion under the hood and this is the default maximum size. Please define the following macro before including any MSM headers:

#define FUSION_MAX_VECTOR_SIZE 20 // or whatever you need
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