In our project, we need a generic interface to refer to different state machine (back-end). For example, in the following codes, we want to use SMBase* to refer to any concrete state machine. SMBase should have similar interface as state machine. struct User { A(SMBase* s) : sm(s) (); SMBase* sm; ... // User can use sm->start(), sm->process_event() etc } struct SMBase { virtual void start() = 0; ... // how to design the rest ??? } // a wrapper implement SMBase template < typename SM > class SMWrapper : public SMBase{ SMWrapper(SM & s): sm(s){}; virtual void start() { sm.start(); } ... SM sm; } main() { typedef boost::msm::back::state_machine SM1; typedef boost::msm::back::state_machine SM2; User a1(new SMWrapper(SM1()) ); // Wrap the concrete state machine and pass it to User. User a2(new SMWrapper(SM2()) ); // Pass a different state machine. ... } The problem of designing SMBase is the function "process_event". We want it to to be virtual function so SMWrapper can implement it. But it also needs to be a template according to state machine interface. But we cannot have a virtual template function. How can we achieve/design this generic state machine interface? Many thanks. Jerry