How to disable the auto instantiate in Models or libraries

[eluser]Diego Pessoa[/eluser]
Hi, I have a problem:

Code:

class Foo extends Model {
public $bar;

public function __construct($bar) {
parent::__construct();
$this->bar = $bar;
}

}

A simple classs right?

Let's to load this:

Code:

$this->load->model('Foo');

The result:
Message: Missing argument 1 for Foo.

How I can do to disable this auto-instantiate?