Subscriber node

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In the ROS tutorial, they give an example of publisher and subscriber nodes.

  34 void chatterCallback(const std_msgs::String::ConstPtr& msg)
  35 {
  36   ROS_INFO("I heard: [%s]", msg->data.c_str());
  37 }

I am trying to write a subscriber node for transform_broadcaster, so it will have to receive tfMessages instead of Strings. I will obviously change the body, but I am unsure of how to replace the parameter to accept tfMessages. How should I replace it?