I can't create a subscriber for ar_track_alvar.

edit

I would like to use the code below to subscribe to the position of the AR marker obtained by　” ar_track_alvar”　. However, I am getting the error shown below.

Can you please tell me why I am getting the error? Also, please let me know the correct code.

The development environment is Ubuntu kinetic.

[code]
    #include <ros/ros.h>    
    #include <ar_track_alvar_msgs/AlvarMarkers.h>

    #include <iostream>

using namespace std;


void poseCallback(const ar_track_alvar_msgs::AlvarMarkers& pos){


    ROS_INFO("position (x,y,z) = (%f : %f : %f)  \n", pos->position.x, pos->position.y, pos->position.z);

}



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

  ros::init(argc, argv, "basic_simple_listener");
  ros::NodeHandle nh;


  ros::Subscriber sub = nh.subscribe<ar_track_alvar_msgs::AlvarMarkers>("ar_pose_marker", 100, poseCallback);

  ros::spin();

  return 0;
}

Here is the error

 error: ‘const AlvarMarkers {aka const struct ar_track_alvar_msgs::AlvarMarkers_<std::allocator<void> >}’ has no member named ‘position’
     ROS_INFO("position (x,y,z) = (%f : %f : %f)  \n", pos.position.x, pos->position.