Is it necessary to use wp_reset_query() in a WP_Query call?

Hi @janoChen:

Simple answer: no.

What follows is what the PHP code for the function wp_reset_query() from /wp-includes/query.php in WordPRess v3.0.4 as well as the functions subsequently called. You can see that it’s primarily about in modifying global variables.

When you use new WP_Query($args) you will be assigning the return value from values to a local variable so, unless you are doing something so complex that you’d already know the answer to this question for then no, you don’t need to call wp_reset_query():

function wp_reset_query() {
  unset($GLOBALS['wp_query']);
  $GLOBALS['wp_query'] =& $GLOBALS['wp_the_query'];
  wp_reset_postdata();
}

function wp_reset_postdata() {
  global $wp_query;
  if ( !empty($wp_query->post) ) {
    $GLOBALS['post'] = $wp_query->post;
    setup_postdata($wp_query->post);
  }
}

function setup_postdata($post) {
  global $id, $authordata, $day, $currentmonth, $page, $pages, $multipage, $more, $numpages;

  $id = (int) $post->ID;

  $authordata = get_userdata($post->post_author);

  $day = mysql2date('d.m.y', $post->post_date, false);
  $currentmonth = mysql2date('m', $post->post_date, false);
  $numpages = 1;
  $page = get_query_var('page');
  if ( !$page )
    $page = 1;
  if ( is_single() || is_page() || is_feed() )
    $more = 1;
  $content = $post->post_content;
  if ( strpos( $content, '<!--nextpage-->' ) ) {
    if ( $page > 1 )
      $more = 1;
    $multipage = 1;
    $content = str_replace("\n<!--nextpage-->\n", '<!--nextpage-->', $content);
    $content = str_replace("\n<!--nextpage-->", '<!--nextpage-->', $content);
    $content = str_replace("<!--nextpage-->\n", '<!--nextpage-->', $content);
    $pages = explode('<!--nextpage-->', $content);
    $numpages = count($pages);
  } else {
    $pages = array( $post->post_content );
    $multipage = 0;
  }

  do_action_ref_array('the_post', array(&$post));

  return true;
}

-Mike