RE: How will you call a constructor in CodeIgniter?
Anyway, yes, the magic method __construct() is called (automatically) upon each instantiation of the class,
Usually, in CI is not necessary to call a constructor, unless you actually want one. You can alternatively put the library/helper/model you want to have alywas loaded in the respective autoload array in config/autoload.php (check “autoloading” in CI’s manual)
Once you define a constructor in your child Controller you’re compelled to call the parent constructor (of the mail CI_Controller class), because there is where the main CI object is created and all the classes are loaded, and you need those in your child controller too; if fail to do so your child class will construct separately and won’t inherit.