I'm very new to codeigniter , I wanted to know what is the meaning of a constructor in a controller . I saw the following code in a codeigniter tutorial - 

class upload extends CI_Controller {

    function __construct() {
        parent::__construct();
        $this->load->helper(form);
    }

    // rest of the class...

My question is when is the constructor invoked - is it called every time the controller serves a request (e.g the controller class is instantiated for each request it receives?)

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