Generic autoloader for classes named in WordPress coding style

if ( ! class_exists( 'RT_WP_Autoload' ) ){
 /**
  * Generic autoloader for classes named in WordPress coding style.
  */
  class RT_WP_Autoload {
    /**
     * @var string Current directory absolute path
     */
    public $dir;

    /**
     * @param string $dir optional, default value is current
     */
    function __construct( $dir = '' ) {
      if ( ! empty( $dir ) ){
        $this->dir = trailingslashit( $dir );
      } else {
        /** __DIR__ is actually equivalent to 
         * realpath(dirname(__FILE__)) and Added in PHP 5.3.0
         * Bellow line will also work 
         * for php  5.2.4 (WordPress required php  5.2.4 or greater )
         */
        $this->dir = trailingslashit( ( @__DIR__ == '__DIR__' ) && define( '__DIR__', realpath( dirname( __FILE__ ) ) ) );
      }
      spl_autoload_register( 
          array( $this, 'spl_autoload_register' ) );
     }

    /**
     * @param string $class_name
     */
    function spl_autoload_register( $class_name ) {
      $class_path = $this->dir . strtolower( 
       str_replace( '_', '-', $class_name ) ) . '/class-' 
       . strtolower( str_replace( '_', '-', $class_name ) ) . '.php';
      
      if ( file_exists( $class_path ) ){
        include $class_path;
      }
    }
  }
}

/**
 * Initiate Loader class
 */

$obj_rt_wp_loader = new RT_WP_Autoload();