Why is wp_query causing an uncaught error to is_user_logged_in() not defined?

Question

I am building a plugin and have used the simple query:

function getTotal($postType){
  $total = 0;

  $args = array(
   'post_type' => $postType,
  );
  $metrics = new WP_Query( $args );

  foreach( $metrics->get_posts() as $post ) {
      $total++;
  }

  wp_reset_postdata();

  return $total;
}

But the moment I add this code to my plugin, the following error:

Fatal error: Uncaught Error: Call to undefined function is_user_logged_in() in C:\xampp\htdocs\sam\wp-includes\class-wp-query.php:2352 Stack trace: #0 C:\xampp\htdocs\sam\wp-includes\class-wp-query.php(3222): WP_Query->get_posts() #1 C:\xampp\htdocs\sam\wp-includes\class-wp-query.php(3328): WP_Query->query(Array) #2 C:\xampp\htdocs\sam\wp-content\plugins\sample_plugin\Models\dataModels.php(62): WP_Query->__construct(Array) #3 C:\xampp\htdocs\sam\wp-content\plugins\sample_plugin\Views\counselorTable.php(3): getCounselors() #4 C:\xampp\htdocs\sam\wp-content\plugins\sample_plugin\sample_plugin.php(20): require('C:\xampp\htdocs...') #5 C:\xampp\htdocs\sam\wp-settings.php(305): include_once('C:\xampp\htdocs...') #6 C:\xampp\htdocs\sam\wp-config.php(91): require_once('C:\xampp\htdocs...') #7 C:\xampp\htdocs\sam\wp-load.php(37): require_once('C:\xampp\htdocs...') #8 C:\xampp\htdocs\sam\wp-blog-header.php(13): require_once('C:\xampp\htdocs...') #9 C:\xampp\htdocs\sam\index.php(17): require('C:\xampp\htdocs. in C:\xampp\htdocs\sam\wp-includes\class-wp-query.php on line 2352

I've been looking for some time now, but can't find any direct explanation for the issue here. Guidance please?

Answer 1

One of the function call was written in the global scope, and firing before all the scripts could be properly loaded. Moving the function call out of the global space, the error went away. Functions that require or use WordPress functions should not be called before WordPress has successfully loaded it's scripts.