c++ - Luabind - return userdata from script -

so after struggling bit, have decided give luabind try. i'm in process of figuring things out , currently, biggest problem returning userdata (class) created lua script. example i'm working with:

class testclass { public:    testclass(const std::string& s) : m_string(s) {}    void print_string() { std::cout << m_string << "\n"; }  private:    std::string m_string; }; 

this how register class in lua, using luabind:

module(l) [    class_<testclass>("testclass")      .def(constructor<const std::string &>())      .def("print_string", &testclass::print_string) ]; 

and content of lua script:

a = testclass('class created lua') return 

calling print_string() on 'a' in lua works fine. however, afterwards, i'm @ loss when trying retrieve 'a' lua stack , utilize in c++ program. i'm trying is:

testclass * tmp = (testclass*)lua_touserdata(lua_state, -1); tmp->print_string(); 

apparently, print_string() being called, because newline in output, however, i'd tmp->print_string()'s output "class created lua". how correctly retrieve userdata (the class) being returned script?