确保模板功能的编译器错误

我正在尝试使用g ++ v.4.8.1编译以下代码：

#include <iostream>
#include <boost/msm/front/state_machine_def.hpp>
#include <boost/msm/front/euml/euml.hpp>
#include <boost/msm/front/functor_row.hpp>
#include <boost/msm/front/functor_row.hpp>
#include <boost/msm/back/state_machine.hpp>

typedef boost::msm::front::none none;

namespace events{
    class A{
        double member;

        template <typename T>
        struct field{
            typedef T field_type;
        };

    public:
        typedef field<double> A_double;
        typedef field<float> A_float;
        typedef field<int> A_int;

        A() : member(M_PI){}
        A(const double& d) : member(d){}

        double get_member() const{ return member; }

        template<typename T>
        typename T::field_type extract() const{
            typedef typename T::field_type ret;
            return static_cast<ret>(member);
        }
    };

    struct B{};
}

struct front : public boost::msm::front::state_machine_def<front>{
    // ----------------------- States ----------------------------
    struct S1 : public boost::msm::front::state<>{

        template <class Event, class Fsm> 
        void on_entry(Event const&, Fsm& fsm) { 
            std::cout << "executing S1::on_entry()" << std::endl;
        } 

        template <class Event, class Fsm> 
        void on_exit(Event const&, Fsm& fsm) { 
            std::cout << "executing S1::on_exit()" << std::endl; 
        }
    };

    struct S2 : public boost::msm::front::state<>{
        template <class Event, class Fsm> 
        void on_entry(Event const& evt, Fsm& fsm) {
            std::cout << "executing S2::on_entry()" << std::endl; 
            // Initialize a
            fsm.a = static_cast<events::A>(evt); 
        } 

        template <class Event, class Fsm> 
        void on_exit(Event const&, Fsm& fsm) { 
            std::cout << "executing S2::on_exit()" << std::endl; 
            std::cout << "a.get_member(): " << fsm.a.get_member() << std::endl; // OK
            int m = fsm.a.extract< events::A::A_int >();                        // ERROR
            std::cout << "a.extract<events::A::A_int>(): " << m << std::endl;
        } 
    };

    // --------------------------------- Initial state ------------------------------
    typedef S1 initial_state;

    // --------------------------------- Transition table ----------------------------------
    struct transition_table : boost::mpl::vector<
        //     Start    Event       Target  Action  Guard 
         _row< S1,  events::A,  S2          >,
         _row< S2,  events::A,  S2          >,
         _row< S2,  events::B,  S1          >
    > {};

    template <class Fsm, class Event>
        void no_transition(Event const& e, Fsm&, int state){ 
            std::cout << "No transition found" << std::endl; 
        }

    // Internal events::A member
    events::A a;
};

typedef boost::msm::back::state_machine<front> back; 

static char const* const state_names[] = { "S1", "S2" };
void pstate(back const& p){
        std::cout << " -> " << state_names[p.current_state()[0]] << std::endl;
}

int main(){
    back b;
    events::A tmp;
    std::cout << tmp.extract<events::A::A_double>() << std::endl;
    b.start();
    pstate(b); 
    std::cout << std::endl << "Firing event A()" << std::endl;
    b.process_event(events::A());
    pstate(b); 
    std::cout << std::endl << "Firing event A(153.13)" << std::endl;
    b.process_event(events::A(153.13));
    pstate(b); 
    std::cout << std::endl << "Firing event A(14)" << std::endl;
    b.process_event(events::A(14));
    pstate(b); 
    b.stop();
    return 0;
}

我收到以下错误：

main.cpp: In member function ‘void front::S2::on_exit(const Event&, Fsm&)’:
main.cpp:67:44: error: expected primary-expression before ‘>’ token
    int m = fsm.a.extract< events::A::A_int >();    // ERROR
                                            ^
main.cpp:67:46: error: expected primary-expression before ‘)’ token
    int m = fsm.a.extract< events::A::A_int >();    // ERROR
                                              ^

我尝试用以下内容替换该行：

int m = static_cast<events::A>(fsm.a).extract<events::A::A_int>();    // OK

现在，代码可以按照我的期望进行编译和工作。
有人可以帮助我了解原因吗？