$wp_query is returning nothing - wordpress

Question

I have the following code in my plugin file:
// SET UP REWITE RULES FOR LISTING PERMALINKS //
function my_rewrite_tags() {
add_rewrite_tag('%listingId%', '([^&]+)');
}
add_action('init', 'my_rewrite_tags', 10, 0);
function my_rewrite_rules() {
add_rewrite_rule('^listing/([^/]*)/?','index.php?pagename=listing&listingId=$matches[1]','top');
}
add_action('init', 'my_rewrite_rules', 10, 0);
This idea is that I have a page called "Listing" with the permalink "listing" and I want to be able to have the listing's ID number after it (i.e., /listing/12345/)
I then have a shortcode running on the "Listing" page
// SHORTCODE FOR SINGLE LISTING PAGE //
function my_single_listing(){
ob_start();
include('templates/single-listing.php');
return ob_get_clean();
}
add_shortcode('listing','my_single_listing');
...and the first thing it does is try to get that listing ID with the code:
$listingId = $wp_query->query_vars['listingId'];
I've done this with other plugins I've written in the past, but in this case it's decided to not work. In fact, if I enter the code:
print_r($wp_query);
I get absolutely nothing returned from it at all. (All other content on the page is displaying fine though.)
Any ideas?

Answer 1

Your issue with $wp_query being blank might be due to it not being accessed as a global variable. Prefacing it with a global declaration will allow it to access the global query:
global $wp_query;
print_r( $wp_query )
The issue with the listing ID not being picked up has to do with it not being declared as a possible custom query var. WordPress requires you to declare them before it loads them into the global wp_query for the page (presumably for security). $_GET was able to access them since that bypasses WordPress and just uses it with PHP.
function so_71685702_add_query_vars( $query_vars ) {
$query_vars[] = 'listingId';
return $qvars;
}
add_filter( 'query_vars', 'so_71685702_add_query_vars' );
Once you've got that, $wp_query->query_vars( 'listingid' ) should return a value.
Here's the query_vars hook information page, and the get_query_var hook information page which might be useful for further reading - might cover some things you'll run into based on the way you're setting up custom rewrites and query vars.