Problem with autoloader and namespaces

Question

I'm writing a simple plugin with oop and using namespaces and autoloading. Here is my main file:

add_action('plugins_loaded', array(My_Test_Plugin::get_instance(), 'plugin_setup'));

class My_Test_Plugin
{
    /**
     * Plugin instance.
     *
     * @see get_instance()
     * @type object
     */
    protected static $instance = NULL;
    /**
     * URL to this plugin's directory.
     *
     * @type string
     */
    public $plugin_url = '';
    /**
     * Path to this plugin's directory.
     *
     * @type string
     */
    public $plugin_path = '';

    /**
     * Access this plugin’s working instance
     *
     * @wp-hook plugins_loaded
     * @since   2012.09.13
     * @return  object of this class
     */
    public static function get_instance()
    {
        NULL === self::$instance and self::$instance = new self;
        return self::$instance;
    }

    /**
     * Used for regular plugin work.
     *
     * @wp-hook plugins_loaded
     * @return  void
     */
    public function plugin_setup()
    {
        $this->plugin_url = plugins_url('/', __FILE__);
        $this->plugin_path = plugin_dir_path(__FILE__);
        $this->load_language('vt-plugin');

        spl_autoload_register(array($this, 'autoload'));

    }
    /**
     * @param $class
     *
     */
    public function autoload($class)
    {
        $class = str_replace('\\', DIRECTORY_SEPARATOR, $class);

        if (!class_exists($class)) {
            $class_full_path = $this->plugin_path . 'includes/' . $class . '.php';
            if (file_exists($class_full_path)) {
                require $class_full_path;
            }
        }
    }
}

In includes/PostTypes directory there is a file called Book.php and it's containing this:

namespace Book;

class Book {
    public function vt_create_book_post_type() {
        $labels = array(
            'name'                  => _x( 'Books', 'vt-plugin' ),
            'singular_name'         => _x( 'Book', 'vt-plugin' ),
            'menu_name'             => _x( 'Books', 'vt-plugin' ),
            ),
        );

        register_post_type( 'book', $args );
    }
}

It just registers a simple post type. Now my question is Where I should use my init action to register post type?

I used this code in the plugin_setup function in my main file:

add_action( 'init', array('Book', 'vt_register_book_post_type') );

But it gave me this error:

Warning: call_user_func_array() expects parameter 1 to be a valid callback 

Update If I change my autoloader function to this:

public function autoload($class)
    {
        $class = str_replace('\\', DIRECTORY_SEPARATOR, $class);

        if (!class_exists($class)) {
            $class_full_path = $this->plugin_path . 'includes/' . $class . '.php';
            if (file_exists($class_full_path)) {
                require $class_full_path;
                echo $class_full_path; 
            }
        }
    }

And echo the full path inside the if statement, it does not echo anything and I think this means that files aren't included.

Answer 1

The callback [ 'Book', 'vt_register_book_post_type' ] is adding a hook to the static method vt_register_book_post_type in the Book class of the global namespace.

If you want to add a callback to a static class of the Book namespace, you need to use [ '\Book\Book\', 'vt_register_book_post_type' ] as a callback, but that will also cause a warning or error because your method is not static. You can declare your method static, but what I think you want to do is something like this:

namespace Book;
class book {
  public function vt_create_book_post_type() {
    $labels = [
      'name'                  => _x( 'Books', 'vt-plugin' ),
      'singular_name'         => _x( 'Book', 'vt-plugin' ),
      'menu_name'             => _x( 'Books', 'vt-plugin' ),
    ];
    register_post_type( 'book', $args );
  }
}
$book = new book();
\add_action( 'init', [ $book, 'vt_create_book_post_type' ] );

There's nothing wrong with your autoloader. Or at least that's not what's causing the error.