How can I use the subscribed message?

edit

Hi,

I've written a very simple subscriber and now my task is to use the subscribed message as a "position value" for my servo. But how to do this ? Let's image that I would have a function called motor_position(uint16_t position) which I have to call and give it the new position value, when and where do I have to call the function?

So my question is, what is the simplest way to use the received message.

Thank you in advance

    #include "ros/ros.h"
    #include "message/message.h" //is a uint16_t var

    uint16_t a;

    void chatterCallback(const message::message::ConstPtr &msg)
    {
      a = msg->position;
      ROS_INFO("I heard: [%d]", a);


    }

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

    ros::init(argc, argv, "subscriber");
    ros::NodeHandle n;

    ros::Subscriber sub = n.subscribe("topicname", 1000, chatterCallback);
    ros::spinOnce();

    // HOW CAN I CALL MY FUNCTION ?
    // motor_position(a); THIS is wrong for sure.  

    return 0;
    }

////////////////////////////EDIT//////////////////////////////// Does not compile ?!

#include "ros/ros.h"
#include "message/message.h"


void chatterCallback(const message::message::ConstPtr &msg, int arg)
{
  //do something with arg
  uint16_t a = msg->position;
  ROS_INFO("I heard: [%d]", a);
}

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

int my_arg= 0;

ros::init(argc, argv, "subscriber");
ros::NodeHandle n;

ros::Subscriber sub = n.subscribe("topicname", 1000, boost::bind(chatterCallback, _1, my_arg ));
ros::spin();

return 0;
}