How to call controller class from Rest controller class in Codigniter 3.0

I have applied Rest server successfully as per explain in link.

Now I am trying to apply REST code in my login module. I have created new controller for login to apply REST API. My PHP code is as follows:

require APPPATH.'/libraries/REST_Controller.php';
class Login_api extends REST_Controller {

    function __construct()
    {
        // Construct our parent class
        parent::__construct();


        // Configure limits on our controller methods. Ensure
        // you have created the 'limits' table and enabled 'limits'
        // within application/config/rest.php
        $this->methods['validate_user_get']['limit'] = 500; //500 requests per hour per user/key
        $this->methods['validate_user_post']['limit'] = 100; //100 requests per hour per user/key
    }

     function validate_user_get()
    {
        if(!$this->get('username') && !$this->get('password'))
        {
            $this->response(NULL, 400);
        }

        // $user = $this->some_model->getSomething( $this->get('id') );

        //$user = @$users[$this->get('id')];
        //$user =array($this->get('username'),$this->get('password'));

        if($user)
        {
            $this->response($user, 200); // 200 being the HTTP response code
        }

        else
        {
            $this->response(array('error' => 'User could not be found'), 404);
        }
    }

    function validate_user_post()
    {


        //$this->Login->login_user($this->input->post('username'),$this->input->post('password'));

        //$this->some_model->updateUser( $this->get('id') );
        //$message = array('id' => $this->get('id'), 'name' => $this->post('name'), 'email' => $this->post('email'), 'message' => 'ADDED!');

        $this->response($message, 200); // 200 being the HTTP response code
    }

It works fine when I print static data using get and post function. But I want to pass data in my CI_controller function from Rest controller. So what can I do for it ?

I have tried below code but not succeed :

I have write this code in Validate_user_post function :

$this->load->library('../controllers/Login');
$this->Login->login_user($this->input->post('username'),$this->input->post('password'));

Codeigniter 3.0 Session conflict with REST API