Problem using subscribed motor position topic

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Hi, I am new to ROS and I am trying to get access to my servo motor position to use in broadcasting a transform. I have done the listener tutorial and my listen program below runs and compiles fine, it even triggers the void function when the motor publishes a position topic. How can I get pull the position variable that I am subscribed to in order to use it in a program? Specifically what do I need to change "sg->data.c_str()" to in order for it to say the motor position integer in the message?

======================listener.cpp==============

#include "ros/ros.h"

#include "ax12_driver_core/MotorStateList.h"

void motorchatterCallback(const ax12_driver_core::MotorStateList::ConstPtr&)
{

 // ROS_INFO("I heard: [%s]", msg->data.c_str());

}

int main(int argc, char **argv)
{
   ros::init(argc, argv, "listener");

  ros::NodeHandle n;

  ros::Subscriber sub = n.subscribe("motor_states/ttyUSB0", 1000, motorchatterCallback);

ros::spin();

  return 0;

}