MSM 1.20 allowed writing a state machine with actions/guards which modify some external state.

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I no longer instantiate the state machine directly and cannot pass parameters to constructor and it seems that msm::back::state_machine will not do this for me (after a quick glance at the definition it seemed to me that msm::back::state_machine expects its template parameter to be default constructible). Is this wrong or am I missing something obvious? Is there some way I could still achieve the old functionality?

No you're not doing anything wrong, it's a use case I simply oversaw, so it's not your fault but mine ;-) So, I corrected the problem and : - added a templated constructor, with up to 5 arguments (can be changed with the BOOST_MSM_CONSTRUCTOR_ARG_SIZE define) - actualized the doc - thanked you for this in the acknowledgments You can download the change (in back/state machine.hpp) from the sandbox (http://svn.boost.org/svn/boost/sandbox/msm/). I tested your case and a few more so it should be ok now. Thanks a lot for the report. I'm happy you like the library. Regards, Christophe