Why am I getting Fatal error when calling a parent's constructor?

Question

I am extending one of the SPL (Standard PHP Library) classes and I am unable to call the parent's constructor. Here is the error I am getting:

Fatal error: Cannot call constructor

Here is a link to the SplQueue's documentation: http://www.php.net/manual/en/class.splqueue.php

Here is my code:

$queue = new Queue();  class Queue extends SplQueue {      public function __construct() {         echo 'before';         parent::__construct();         echo 'I have made it after the parent constructor call';     }  }  exit; 

What could prevent me from calling the parent's constructor?

Answer 1

SplQueue inherits from SplDoublyLinkedList. Neither of these classes defines a constructor of its own. Therefore there's no explicit parent constructor to call, and you get such an error. The documentation is a little misleading on this one (as it is for many SPL classes).

To solve the error, don't call the parent constructor.

---

Now, in most object-oriented languages, you'll expect the default constructor to be called if there isn't an explicit constructor declared in a class. But here's the catch: PHP classes don't have default constructors! A class has a constructor if and only if one is defined.

In fact, using reflection to analyze the stdClass class, we see even that lacks a constructor:

$c = new ReflectionClass('stdClass'); var_dump($c->getConstructor()); // NULL 

Attempting to reflect the constructors of SplQueue and SplDoublyLinkedList both yield NULL as well.

My guess is that when you tell PHP to instantiate a class, it performs all the internal memory allocation it needs for the new object, then looks for a constructor definition and calls it only if a definition of __construct() or <class name>() is found. I went to take a look at the source code, and it seems that PHP just freaks out and dies when it can't find a constructor to call because you told it explicitly to in a subclass (see zend_vm_def.h).

Answer 2

This error gets thrown, usually, when the parent class being referenced in parent::__construct() actually has no __construct() function.