Can not create own Driver

Hi, 
I'm new on codeIgniter.

I've tryed to create my own driver - like it is described on CodeIgniter documentation

My Controller never finds the subclass.

If I try to run  

Code:

$this->driver_name->subclass_1->subclass_1();

CodeIgniter throws an Error - undefined property "subclass_1"
I have seen that in other tutorials the classes have to be extendet by CI_Driver (subclasses) and CI_Driver_Library (Driver Main Class)
AND: I need a constructor in the main driver:

Code:

public function __construct() {

       $this->valid_drivers = ['subclass_1'];

   }

I have debuged (xdebug) the codeIgniter and I think that is the right way.
So why is it not on the codeigniter documantation?
I'm doing something wrong?
When this is the right way - are there other parts in the documentation, where something like this is missing?

Or do I make a mistake?

cu

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