Source Reference:

• Component: bbPress :: bbp-topic

  File: /bbp-includes/bbp-topic-template.php :: Trac Source Line: 1752

  	 * @uses get_post_meta() To get the topic reply count meta
  	 * @uses apply_filters() Calls 'bbp_get_topic_reply_count' with the
  	 *                        reply count and topic id
  	 * @return int Reply count
  	 */
  	function bbp_get_topic_reply_count( $topic_id = 0 ) {
  		$topic_id = bbp_get_topic_id( $topic_id );
  		$replies  = get_post_meta( $topic_id, '_bbp_reply_count', true );
  
  		return apply_filters( 'bbp_get_topic_reply_count', (int) $replies, $topic_id );
  	}
  
  /**
   * Output total post count of a topic
   *
   * @since bbPress (r2954)
   *
   * @param int $topic_id Optional. Topic id
   * @uses bbp_get_topic_post_count() To get the topic post count
   */