public static function url( $file = null, $key = null ) 
	{
		// get the storage key
		if ( is_null( $key ) ) 
		{
			$key = static::$default;
		}

		// check if path exists
		if ( !isset( static::$urls[$key] ) ) 
		{
			throw new CCException( 'CCStorage - use of undefined public url '.$key.'.' );
		}

		return CCUrl::to( static::$urls[$key].$file );
	}