codeigniter check for user session in every controller

I have this private session in one of my controllers that checks if a user is logged in:

function _is_logged_in() {

   $user = $this->session->userdata('user_data');

   if (!isset($user)) { 
      return false; 
   } 
   else { 
      return true;
   }

}

Problem is that I have more than one Controller. How can I use this function in those other controllers? Redefining the function in every Controller isn't very 'DRY'.

Any ideas?