Boost statechart - compilation error when using state chart as template parameter - c++

Question

I'm trying to implement a simple state machine using boost statechart.
Since I have several variation of this state machine, I thought it might be a good idea to wrap it in a template and pass the state machine as template parameter.
However, I'm getting compilation errors.
Code:
#include <boost/statechart/state_machine.hpp>
#include <boost/statechart/simple_state.hpp>
#include <boost/statechart/transition.hpp>
namespace sc = boost::statechart;
class ComponentType
{
};
class FSM {
protected:
struct stInit ;
public:
struct Machine : sc::state_machine< Machine, stInit > {};
protected:
struct stInit : ComponentType, sc::simple_state< stInit, Machine > {};
};
template <class fsm>
void run() {
typename fsm::Machine m_fsm;
const ComponentType &t = m_fsm.state_cast<const ComponentType &>();
(void) t;
}
int main() {
run<FSM>();
}
The compilation errors:
fsmtest.cpp: In function ‘void run()’:
fsmtest.cpp:33:45: error: expected primary-expression before ‘const’
const ComponentType &t = m_fsm.state_cast<const ComponentType &>();
^
fsmtest.cpp:33:45: error: expected ‘,’ or ‘;’ before ‘const’
However, when using typedef instead of template:
typedef FSM fsm;
//template <class fsm>
and
run();
// run<FSM>();
Everything compiles without any errors.
What am I missing?
(compiler: g++ 4.8.4, OS: Ubuntu 14.04, boost: 1.54)

Answer 1

You have to let compiler know that you want to call state_cast template function, so it would parse the line correctly. Change:
const ComponentType &t = m_fsm.state_cast<const ComponentType &>();
to:
const ComponentType &t = m_fsm.template state_cast<const ComponentType &>();
Check Where and why do I have to put the "template" and "typename" keywords? for more info.