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Check a few things that could be wrong:

• Is your callback actually being called?
  • add a print statement in the callback to make sure
  • check rqt_graph to see if your node is actually subscribed to the topic
• Does your "Obstacle" object go out of scope?
• Does the subscriber object (sub) go out-of-scope?
• Did you remember to call ros::spin()?

Here is some example code that works for me. See below for usage.

#include <ros/ros.h>
#include <nav_msgs/MapMetaData.h>

class Obstacle
{
public:
  ros::Subscriber sub;
  unsigned int width, height;
  float resolution;

  Obstacle() : width(0), height(0), resolution(0)
  {
    ros::NodeHandle node;
    sub = node.subscribe("map_metadata", 1, &Obstacle::metaDataCallback, this);
  }

  void metaDataCallback(const nav_msgs::MapMetaData::ConstPtr &msg) {
    this->width = msg->width;
    this->height = msg->height;
    this->resolution = msg->resolution;
  }
};

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

  ros::init(argc, argv, "subTest");

  Obstacle obs;

  while (ros::ok())
  {
    ROS_INFO("width: %d, height: %d, resolution: %g",
             obs.width, obs.height, obs.resolution);
    ros::spinOnce();
    sleep(2);
  }

  return 0;
}

Build this node and run it. It should start printing out the current member-variable states (all-zeros). Then, publish your message. It should start printing the new values, once the messages are received. For testing, I manually published a message:

$ rostopic pub "map_metadata" nav_msgs/MapMetaData "{width: 1, height: 2, resolution: 3.14159}"