Source

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

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

		if ( ! ( $post instanceof WP_Post ) ) {
			$post = get_post( $post );
		}

		if ( ! $post ) {
			return;
		}

		$elements = $this->generate_postdata( $post );
		if ( false === $elements ) {
			return;
		}

		$id           = $elements['id'];
		$authordata   = $elements['authordata'];
		$currentday   = $elements['currentday'];
		$currentmonth = $elements['currentmonth'];
		$page         = $elements['page'];
		$pages        = $elements['pages'];
		$multipage    = $elements['multipage'];
		$more         = $elements['more'];
		$numpages     = $elements['numpages'];

		/**
		 * Fires once the post data has been set up.
		 *
		 * @since 2.8.0
		 * @since 4.1.0 Introduced `$this` parameter.
		 *
		 * @param WP_Post  $post The Post object (passed by reference).
		 * @param WP_Query $this The current Query object (passed by reference).
		 */
		do_action_ref_array( 'the_post', array( &$post, &$this ) );

		return true;
	}