why do we still need parent constructor when controller class extends a parent controller?

Question

I'm a beginner in CodeIgniter and OOP. I was reading a page of CI tutorial here. I found something that made a question in my mind.
Look at this code:

<?php
class News extends CI_Controller {
    public function __construct()
    {
        parent::__construct();
        $this->load->model('news_model');
    }

I think if we made a class that extends CI_Controller, we assume it must have all methods and properties in its parent class (Although we can override them). So, why there is parent::__construct(); in the code?

Answer 1

__construct() is the constructor method of a class. It runs if you declare a new object instance from it. However, if a class implemented its own __construct(), PHP would only run the constructor of itself, not of its parent. For example:

<?php

class A {
  public function __construct() {
    echo "run A's constructor\n";
  }
}

class B extends A {
  public function __construct() {
    echo "run B's constructor\n";
  }
}

// only B's constructor is invoked
// show "run B's constructor\n" only
$obj = new B();

?>

In this case, if you need to run class A's constructor when $obj is declared, you'll need to use parent::__construct():

<?php

class A {
  public function __construct() {
    echo "run A's constructor\n";
  }
}

class B extends A {
  public function __construct() {
    parent::__construct();
    echo "run B's constructor\n";
  }
}

// both constructors of A and B are invoked
// 1. show "run A's constructor\n"
// 2. show "run B's constructor\n"
$obj = new B();

?>

In CodeIgniter's case, that line runs the constructor in CI_Controller. That constructor method should have helped your controller codes in some way. And you'd just want it to do everythings for you.

Answer 2

I believe the need of calling the parent constructor/method is a code smell, known as Call super. Besides the error-sensitivity (forgetting this call, you can get unexpected results), it's procedural instead of OOP. After all, the order of statements can lead to unexpected results too.

Read more here: https://martinfowler.com/bliki/CallSuper.html