Source

File: wp-includes/class-wp-query.php

public function the_post() {
		global $post;
		$this->in_the_loop = true;

		if ( -1 == $this->current_post ) { // Loop has just started.
			/**
			 * Fires once the loop is started.
			 *
			 * @since 2.0.0
			 *
			 * @param WP_Query $this The WP_Query instance (passed by reference).
			 */
			do_action_ref_array( 'loop_start', array( &$this ) );
		}

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