Difference between ArrayIterator, ArrayObject and Array in PHP

Question

Can somebody explain clearly the fundamental differences between ArrayIterator, ArrayObject and Array in PHP in terms of functionality and operation? Thanks!

Answer 1

Array is a native php type. You can create one using the php language construct array(), or as of php 5.4 onwards []

ArrayObject is an object that work exactly like arrays. These can be created using new keyword

ArrayIterator is like ArrayObject but it can iterate on itself. Also created using new

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Comparing Array vs (ArrayObject/ArrayIterator)

They both can be used using the php's array syntax, for eg.

$array[] = 'foo'; $object[] = 'foo'; // adds new element with the expected numeric key  $array['bar'] = 'foo'; $object['bar'] = 'foo'; // adds new element with the key "bar"  foreach($array as $value); foreach($object as $value); // iterating over the elements 

However, they are still objects vs arrays, so you would notice the differences in

is_array($array); // true is_array($object); // false  is_object($array); // false is_object($object); // true 

Most of the php array functions expect arrays, so using objects there would throw errors. There are many such functions. For eg.

sort($array); // works as expected sort($object); // Warning: sort() expects parameter 1 to be array, object given in ...... 

Finally, objects can do what you would expect from a stdClass object, i.e. accessing public properties using the object syntax

$object->foo = 'bar'; // works $array->foo = 'bar'; // Warning: Attempt to assign property of non-object in .... 

Arrays (being the native type) are much faster than objects. On the other side, the ArrayObject & ArrayIterator classes have certain methods defined that you can use, while there is no such thing for arrays

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Comparing ArrayObject vs ArrayIterator

The main difference between these 2 is in the methods the classes have.

The ArrayIterator implements Iterator interface which gives it methods related to iteration/looping over the elements. ArrayObject has a method called exchangeArray that swaps it's internal array with another one. Implementing a similar thing in ArrayIterator would mean either creating a new object or looping through the keys & unseting all of them one by one & then setting the elements from new array one-by-one.

Next, since the ArrayObject cannot be iterated, when you use it in foreach it creates an ArrayIterator object internally(same as arrays). This means php creates a copy of the original data & there are now 2 objects with same contents. This will prove to be inefficient for large arrays. However, you can specify which class to use for iterator, so you can have custom iterators in your code.

Answer 2

ArrayObject and array are somewhat alike. Merely a collection of objects (or native types). They have some different methods that you can call, but it mostly boils down to the same thing.

However, an Iterator is something else completely. The iterator design pattern is a way to secure your array (making it only readable). Lets take the next example:

You have a class that has an array. You can add items to that array by using addSomethingToMyArray. Note however, that we do something to item before we actually add it to the array. This could be anything, but lets for a moment act like it is very important that this method is fired for EVERY item that we want to add to the array.

class A {     private $myArray;     public function returnMyArray()     {         return $this->myArray;     }      public function addSomethingToMyArray( $item )     {         $this->doSomethingToItem( $item );          array_push( $item );     } } 

The problem with this, is that you pass the array by reference here. That means that classes that actually use returnMyArray get the real myArray object. That means that classes other than A can add things to that array, and therefor also change the array inside A without having to use the addSOmethingToMyArray. But we needed to doSOmethingToItem, remember? This is an example of a class not in control of its own inner status.

The solution to this is an iterator. Instead of passing the array, we pass the array to a new object, that can only READ things from the array. The most simple Iterator ever is something like this:

<?php  class MyIterator{      private $array;     private $index;      public function __construct( $array )     {         $this->array = $array;     }      public function hasNext()     {         return count( $this->array ) > $this->index;     }      public function next()     {         $item = $this->array[ $this->index ];         this->$index++;          return $item;     }        } 

?>

As you can see, i have no way of adding new items to the given array, but i do have posibilities to read the array like this:

while( $iterator->hasNext() )     $item = $iterator->next(); 

Now there is again, only one way to add items to myArray in A, namely via the addSomethingToArray method. So that is what an Iterator is, it is somewhat of a shell around arrays, to provide something called encapsulation.