Codeigniter cannot extend controller

I have created an admin controller that extends the blog controller (both are located in application/controller). However, when I tried to access the controller, it gives me an error that the blog controller was not found. If I put require_once(APPPATH.'controllers/blog.php'); inside the admin.php file it works. But I'm just wondering if there's another possible way to extends the blog controller without having to use require_onc开发者_StackOverflow社区e inside the admin.php

---

From CI user guide

If you are extending the Controller core class, then be sure to extend your new class in your application controller's constructors.

class Welcome extends MY_Controller {

    function __construct()
    {
        parent::__construct();
    }

    function index()
    {
        $this->load->view('welcome_message');
    } 

}

That means your Blog controller must extends CI_Controller

Example:

class MY_Blog extends CI_Controller {

    function hello() {
        $data = 'something';
    }
}

class Admin extends MY_Blog {

    function do_something() {}
}

Userguide