Source Reference:

• Component: bbPress:: bbp-topic

  File: /bbp-includes/bbp-topic-functions.php :: Trac Source Line: 2811

   *                        topic
   * @uses wp_untrash_post() To untrash the reply
   */
  function bbp_untrash_topic( $topic_id = 0 ) {
  	$topic_id = bbp_get_topic_id( $topic_id );
  
  	if ( empty( $topic_id ) || !bbp_is_topic( $topic_id ) )
  		return false;
  
  	do_action( 'bbp_untrash_topic', $topic_id );
  
  	// Get the replies that were not previously trashed
  	$pre_trashed_replies = get_post_meta( $topic_id, '_bbp_pre_trashed_replies', true );
  
  	// There are replies to untrash
  	if ( !empty( $pre_trashed_replies ) ) {
  
  		// Maybe reverse the trashed replies array
  		if ( is_array( $pre_trashed_replies ) )
  			$pre_trashed_replies = array_reverse( $pre_trashed_replies );