WordPress: the_post() in depth

the_post(), as a matter of fact, is the most used WordPress function in any theme but also almost unknown to the masses. We all know that this function initializes The Loop, but we actually don't know what kind of internal routines it uses. Let's see them together.

This function is defined within the core wp-includes/query.php file. This file, in turn, contains the definition of the famous WP_Query class, namely the main engine of the WordPress Loop.

the_post() is both defined as an external function and as a method of the aforementioned class. the_post() relies on the following code:

/**
  * Sets up the current post.
  *
  * Retrieves the next post, sets up the post, sets the 'in the loop'
  * property to true.
  *
  * @since 1.5.0
  * @access public
  * @uses $post
  * @uses do_action_ref_array() Calls 'loop_start' if loop has just started
  */
 function the_post() {
  global $post;
  $this->in_the_loop = true;

  if ( $this->current_post == -1 ) // loop has just started
   do_action_ref_array('loop_start', array(&$this));

  $post = $this->next_post();
  setup_postdata($post);
 }

WP_Query::the_post() sets a Boolean flag to notify other class members that we're in the Loop, Then it checks whether The Loop has started and sets the current post by moving each time to the next post in the queue, until the post queue ends.

the_post() itself, instead, is defined as follows:

/**
 * Iterate the post index in the loop.
 *
 * @see WP_Query::the_post()
 * @since 1.5.0
 * @uses $wp_query
 */
function the_post() {
 global $wp_query;

 $wp_query->the_post();
}

$wp_query is an instance of the WP_Query class. Bear in mind, however, that this class uses a backward-compatible object-oriented notation (as of PHP 4), not the latest OO features available in PHP 5.