How to get pose from april tag topic tag_detection?

edit

Hi, Currently, I have a bag file which publishes the /tag_detections coming from apriltag node. I'm having trouble in writing a callback function which could extract information like pose from the topic.

The details of the topic are:

rosmsg show apriltag_ros/AprilTagDetectionArray

std_msgs/Header header
  uint32 seq
  time stamp
  string frame_id
apriltag_ros/AprilTagDetection[] detections
  int32[] id
  float64[] size
  geometry_msgs/PoseWithCovarianceStamped pose
    std_msgs/Header header
      uint32 seq
      time stamp
      string frame_id
    geometry_msgs/PoseWithCovariance pose
      geometry_msgs/Pose pose
        geometry_msgs/Point position
          float64 x
          float64 y
          float64 z
        geometry_msgs/Quaternion orientation
          float64 x
          float64 y
          float64 z
          float64 w
      float64[36] covariance

The piece of code that I manage to write till now is:

#include "ros/ros.h"
#include "apriltag_ros/AprilTagDetectionArray.h"


void callback(const apriltag_ros::AprilTagDetectionArray::ConstPtr &msg)
{
    //What to do?
}

int main(int argc, char **argv)
{
    ros::init(argc, argv, "apriltag_location");
    ros::NodeHandle n;
    ros::Subscriber sub = n.subscribe("tag_detections",1000,callback);
    ros::spin();

}

Could someone point out on how to get the pose from here?