Codeigniter 2 difference between index and __construct and what to put in __construct

Question

When is __construct called and when is index called? And are there any other differences?

And what to put in __construct? Whats the best practice, should I put $this->load calls... ? what else?

class Site extends CI_Controller {

      public function __construct() {

          parent::__construct();
          echo 'Hello World2';

     }

     public function index() {

          echo 'Hello World1';

     }
}

Answer 1

__construct() is called first, then according to URL is called index() or other functions.

public function __construct() should contain:

1. allocating resources used in entire class ex. $this->load
2. check user authentication (if entire class requires it)

public function index() should contain:

1. allocating resources used only in this function
2. calling views or displaying anything

it is bad design if public function __construct() contain:

1. Displaying anything
2. Any code required only for one function.