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Try:

#include <ros/ros.h>
#include <std_msgs/Float32.h>

void chatterCallback(const std_msgs::Float32::ConstPtr& msg)
{
  ROS_INFO("I heard: [%f]", msg->data);
}

int main(int argc, char **argv)
{
  ros::init(argc, argv, "listener");
  ros::NodeHandle n;
  ros::Subscriber sub = n.subscribe("chatter", 500, chatterCallback);
  ros::spin();

  return 0;
}

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