Source Reference:

• Component: bbPress :: bbp-topic

  File: /bbp-includes/bbp-topic-functions.php :: Trac Source Line: 2100

  	else
  		$topic_id = bbp_get_topic_id( $topic_id );
  
  	// Get replies of topic if not passed
  	if ( empty( $reply_count ) )
  		$reply_count = bbp_get_public_child_count( $topic_id, bbp_get_reply_post_type() );
  
  	update_post_meta( $topic_id, '_bbp_reply_count', (int) $reply_count );
  
  	return apply_filters( 'bbp_update_topic_reply_count', (int) $reply_count, $topic_id );
  }
  
  /**
   * Adjust the total hidden reply count of a topic (hidden includes trashed and spammed replies)
   *
   * @since bbPress (r2740)
   *
   * @param int $topic_id Optional. Topic id to update
   * @param int $reply_count Optional. Set the reply count manually
   * @uses bbp_is_reply() To check if the passed topic id is a reply