__get-meta-field.php

<?php
  //Add meta fields
  update_post_meta( 1, 'hats', 'bats' );
  update_post_meta( 1, 'bread', array( 'rye', 'sourdough' ) );
  
  //get the WP_Post object
  $post = get_post( 1 );
  
  //use it's __get to get its meta fields.
  echo $post->hats; // "bats"
  print_r( $post->bread ); //the array
  var_dump( $post->not_a_real_key ); //empty string

wp-post-__get.php

<?php
//how it works in core
//Copied from post.php ~L718 in WordPress 4.3 (much GPL)

  /**
	 * Getter.
	 *
	 * @param string $key Key to get.
	 * @return mixed
	 */
	public function __get( $key ) {
		if ( 'page_template' == $key && $this->__isset( $key ) ) {
			return get_post_meta( $this->ID, '_wp_page_template', true );
		}

		if ( 'post_category' == $key ) {
			if ( is_object_in_taxonomy( $this->post_type, 'category' ) )
				$terms = get_the_terms( $this, 'category' );

			if ( empty( $terms ) )
				return array();

			return wp_list_pluck( $terms, 'term_id' );
		}

		if ( 'tags_input' == $key ) {
			if ( is_object_in_taxonomy( $this->post_type, 'post_tag' ) )
				$terms = get_the_terms( $this, 'post_tag' );

			if ( empty( $terms ) )
				return array();

			return wp_list_pluck( $terms, 'name' );
		}

		// Rest of the values need filtering.
		if ( 'ancestors' == $key )
			$value = get_post_ancestors( $this );
		else
			$value = get_post_meta( $this->ID, $key, true );

		if ( $this->filter )
			$value = sanitize_post_field( $key, $value, $this->ID, $this->filter );

		return $value;
	}