Why is child_frame_id not set for 'odom_msg' (and 'ground_truth_msg') in stageros.cpp?

[sayali-purdue]

nav_msgs::Odometry defintion states that twist data should be specified in the coordinate frame given by child_frame_id.

'odom_msg' (and 'ground_truth_msg') variable in stageros.cpp is of type nav_msgs::Odometry and contains twist data. Since, child_frame_id is not set, which co-ordinate frame does the twist data belong to? Also, I would like to know why is the ROS datatype definition not followed here.

Looking forward to your response.