Hi all I have tried out the new feature of msm : using constructor arguments for substate/machines : namely : player p( back::states_ << Playing(back::states_ << Song1(some_Song1_data)) , boost::ref(data),3); see doc : http://svn.boost.org/svn/boost/trunk/libs/msm/doc/HTML/ch03s05.html#d0e2320 I have some problem with it :(. this case state Playing and Song1 will be constructed first with default constructor in back end constructor than the internally constructed Playing and Song1 will be assigned to the newly construed Playing / Song1 copy which got copied by value. The only reason to use this feature is to able to use reference members in the substates but for that reference members must be initialised in the member initialisation list of the constructor of the substates in order to provide the references constructor arguments must be used. Since subsates must provide default constructors it is not possible to always initialise reference members from outside. Furthermore substate must be copyable and must implement assignment which brings more harm than good :(. Is there a way to resolve these problems ? PS: Looking to the test: http://svn.boost.org/svn/boost/trunk/libs/msm/test/Constructor.cpp I think there is a bug : at with line : fsm.context_ = 20; according my understanding the compiler will create a SomeExternalContext object on the stack of the context of on_entry(Event const&,FSM& fsm) function and will replace the fsm.context_ reference to the reference of the newly constructed object one the stack so when the on_entry function exits the SomeExternalContext object will be removed from the stack so the fsm.context_ reference becomes invalid so code p.context_.bla == 20 actually should crash because it uses invalid reference. I think it is just working by chance ? p.start(); // after this p.context_ will be invalid BOOST_CHECK_MESSAGE(p.context_.bla == 20,"error with fsm entry behavior"); // accessing invalid reference Br. Richie