How to define a tranform listener

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I am using this code in order to relate the point cloud with the pose information.

Also, the code found here looks like below:

laser_geometry::LaserProjection projector_;
tf::TransformListener listener_;

void scanCallback (const sensor_msgs::LaserScan::ConstPtr& scan_in)
{
  sensor_msgs::PointCloud cloud;
  projector_.transformLaserScanToPointCloud("base_link",*scan_in,cloud,listener_);

  // Do something with cloud.
}

How should I define the "listener_" variable in order to use it?