The parameter arrangement of a simple done callback function for an action client

edit

What is really the difference between this:

void doneCallback(const ardrone_as::ArdroneResultConstPtr &result,
                      const actionlib::SimpleClientGoalState &state) {
      ROS_INFO("[State Result]: %s", state.toString().c_str());
      ROS_INFO("Done Callback called");
      ros::shutdown();
    }

and this?

 void doneCallback(const actionlib::SimpleClientGoalState &state,
                      const ardrone_as::ArdroneResultConstPtr &result) {
      ROS_INFO("[State Result]: %s", state.toString().c_str());
      ROS_INFO("The Action has been completed");
      ros::shutdown();
    }

I do notice the fact that the arrangement of the parameter for my doneCallback is different, but for the first doneCallback function, I am receiving this compiler error:

 error: invalid initialization of reference of type 'const boost::shared_ptr<const ardrone_as::ArdroneResult_<std::allocator<void> > >&' from expression of type 'const actionlib::SimpleClientGoalState'
  117 |           BOOST_FUNCTION_RETURN(f(BOOST_FUNCTION_ARGS));

while the second one compiles just fine. I do not really understand the entire error message as it is a bit (maybe not a bit) too complicated for me to understand but what I do know is that I have issues with my const actionlib::SimpleClientGoalState. Can anyone explain to me why the arrangement matters?

Here is my whole code if you need it:

#include <actionlib/client/simple_action_client.h>
#include <ardrone_as/ArdroneAction.h>
#include <ros/ros.h>

// Compiler error
// void doneCallback(const ardrone_as::ArdroneResultConstPtr &result,
//                   const actionlib::SimpleClientGoalState &state) {
//   ROS_INFO("[State Result]: %s", state.toString().c_str());
//   ROS_INFO("Done Callback called");
//   ros::shutdown();
// }

int nImage = 0;

void doneCallback(const actionlib::SimpleClientGoalState &state,
                  const ardrone_as::ArdroneResultConstPtr &result) {
  ROS_INFO("[State Result]: %s", state.toString().c_str());
  ROS_INFO("The Action has been completed");
  ros::shutdown();
}

void activeCallback() { ROS_INFO("Active callback called"); }

void feedbackCallback(const ardrone_as::ArdroneFeedback::ConstPtr &feedback) {
  ROS_INFO("Feedback callback has been called");
}

int main(int argc, char **argv) {

  ros::init(argc, argv, "move_drone_node");

  actionlib::SimpleActionClient<ardrone_as::ArdroneAction> client(
      "ardrone_action_server", true);
  client.waitForServer();

  ardrone_as::ArdroneGoal goal;
  goal.nseconds = 10;

  client.sendGoal(goal, &doneCallback, &activeCallback, &feedbackCallback);

  ros::Rate loop_rate(2);
  actionlib::SimpleClientGoalState state_result = client.getState();

  while (state_result == actionlib::SimpleClientGoalState::ACTIVE ||
         state_result == actionlib::SimpleClientGoalState::PENDING) {
    ROS_INFO("Doing something");
    loop_rate.sleep();
    state_result = client.getState();
  }

  return 0;
}

This is the error if I use the first callback function:

T0, T1>::invoke(boost::detail::function::function_buffer&, T0, T1) [with FunctionPtr = void (*)(const boost::shared_ptr<const ardrone_as::ArdroneResult_<std::allocator<void> > >&, const actionlib::SimpleClientGoalState&); R = void; T0 = const actionlib::SimpleClientGoalState&; T1 = const boost::shared_ptr<const ardrone_as::ArdroneResult_<std::allocator<void> > >&]':
/usr/include/boost/function/function_template.hpp:931:38:   required from 'void boost::function2<R, T1, T2>::assign_to(Functor) [with Functor = void (*)(const boost::shared_ptr<const ardrone_as::ArdroneResult_<std::allocator<void> > >&, const actionlib::SimpleClientGoalState&); R = void; T0 =const actionlib::SimpleClientGoalState&; T1 = const boost::shared_ptr<const ardrone_as::ArdroneResult_<std::allocator<void> > >&]'
/usr/include/boost/function/function_template.hpp:720:7:   required from 'boost::function2<R, T1, T2>::function2(Functor, typename boost::enable_if_<(! boost::is_integral<T>::value), int>::type) [with Functor = void (*)(const boost::shared_ptr<const ardrone_as::ArdroneResult_<std::allocator<void> > >&, const actionlib::SimpleClientGoalState&); R = void; T0 = const actionlib::SimpleClientGoalState&; T1 = const boost::shared_ptr<const ardrone_as::ArdroneResult_<std::allocator<void> > >&; typename boost::enable_if_<(! boost::is_integral<T>::value), int>::type = int]'
/usr/include/boost/function/function_template.hpp:1068:16:   required from 'boost::function<R(T0, T1)>::function(Functor, typename boost::enable_if_<(! boost::is_integral<T>::value), int> ...