How to use hooks? Already read user guide

[eluser]Emkay[/eluser]
I want the site to check if login cookies have been set on every page load, and subsequently log the user in if they haven't already. I am trying to use hooks for this purpose.

My hooks config file

Code:

$hook['post_controller_constructor'] = array(
    'class'    => 'Login',
    'function' => 'cookie_login',
    'filename' => 'login.php',
    'filepath' => 'controllers',
);

My Login class

Code:

class Login extends CI_Controller {

        function cookie_login() {
            if($this->input->cookie('email') && $this->input->cookie('password') && $this->session->userdata('active') != 1) {
                $password = $this->db->escape($this->input->cookie('password'));
                $email = $this->db->escape($this->input->cookie('email'));
                $sql = "SELECT users.user_id, username, email, user_type, email_verified, location_id
                                            FROM users WHERE (email = $email AND password = $password)";
                $query = $this->db->query($sql);
                if ($query->num_rows == 1 ) {
                    $user = $query->row();
                    $this->session->set_userdata('active','1');
                    $this->session->set_userdata($user);
                    $this->standard->update_location($user->location_id,'both');
                }
            }
        }

etc etc..

But I keep getting these errors:

A PHP Error was encountered

Severity: Notice
Message: Undefined property: Login::$session
Filename: controllers/login.php
Line Number: 17

Fatal error: Call to a member function userdata() on a non-object in C:\wamp\www\site\controllers\login.php on line 17

Help?

I have already autoloaded session and database libraries btw