IteratorAggregate interface in PHP

Introduction

The IteratorAggregate interface helps you make your objects easy to loop over with foreach. It tells PHP how to get an iterator for your object.

When you want to allow users to loop through your custom object like an array.

When your object holds a collection of items and you want to control how they are accessed.

When you want to separate the logic of storing data from the logic of iterating over it.

Syntax

PHP

class ClassName implements IteratorAggregate {
    public function getIterator(): Traversable {
        // return an iterator object here
    }
}

The getIterator() method must return an object that implements Traversable, usually an ArrayIterator.

This interface makes your object compatible with foreach loops.

Examples

This example shows a class that holds fruits and allows looping through them with foreach.

PHP

<?php
class MyCollection implements IteratorAggregate {
    private array $items = [];

    public function __construct(array $items) {
        $this->items = $items;
    }

    public function getIterator(): Traversable {
        return new ArrayIterator($this->items);
    }
}

$collection = new MyCollection(['apple', 'banana', 'cherry']);
foreach ($collection as $item) {
    echo $item . "\n";
}

This example creates a range of numbers and lets you loop through them easily.

PHP

<?php
class NumberRange implements IteratorAggregate {
    private int $start;
    private int $end;

    public function __construct(int $start, int $end) {
        $this->start = $start;
        $this->end = $end;
    }

    public function getIterator(): Traversable {
        return new ArrayIterator(range($this->start, $this->end));
    }
}

$range = new NumberRange(1, 3);
foreach ($range as $number) {
    echo $number . "\n";
}

Sample Program

This program creates a shopping list object that can be looped over with foreach to print each item.

PHP

<?php
class ShoppingList implements IteratorAggregate {
    private array $items;

    public function __construct(array $items) {
        $this->items = $items;
    }

    public function getIterator(): Traversable {
        return new ArrayIterator($this->items);
    }
}

$list = new ShoppingList(['milk', 'bread', 'eggs']);
foreach ($list as $item) {
    echo "Buy: $item\n";
}

Important Notes

You can return any iterator from getIterator(), not just ArrayIterator.

Implementing IteratorAggregate is simpler than Iterator if you just want to provide an iterator.

Summary

The IteratorAggregate interface lets your object be used in foreach loops.

You must implement getIterator() to return an iterator.

This helps separate data storage from how you loop through data.

Practice

1.

What is the main purpose of the IteratorAggregate interface in PHP?

easy

A. To allow an object to be iterated with foreach by providing an external iterator.

B. To automatically sort arrays inside an object.

C. To enforce type checking on array elements.

D. To convert an object into a string representation.

Solution

1. Step 1: Understand the role of IteratorAggregate

   The interface requires implementing getIterator() which returns an iterator object.

2. Step 2: Connect to foreach usage

   This allows the object to be used in foreach loops by providing the iterator externally.

3. Final Answer:

   To allow an object to be iterated with foreach by providing an external iterator. -> Option A

4. Quick Check:

   IteratorAggregate enables foreach iteration [OK]

Hint: Remember: IteratorAggregate provides an iterator for foreach [OK]

Common Mistakes:

• Thinking it sorts data automatically
• Confusing it with string conversion
• Assuming it enforces type checks

2.

Which method must a class implement when it uses the IteratorAggregate interface?

easy

A. getItems()

B. iterate()

C. getIterator()

D. toArray()

Solution

1. Step 1: Recall interface requirements

   The IteratorAggregate interface requires the getIterator() method.

2. Step 2: Confirm method name

   This method must return an iterator object to enable iteration.

3. Final Answer:

   getIterator() -> Option C

4. Quick Check:

   IteratorAggregate requires getIterator() [OK]

Hint: Interface method is always getIterator() [OK]

Common Mistakes:

• Using iterate() instead of getIterator()
• Confusing with toArray() method
• Assuming getItems() is required

3.

What will be the output of this PHP code?

<?php
class MyCollection implements IteratorAggregate {
    private array $items = [1, 2, 3];
    public function getIterator(): Traversable {
        return new ArrayIterator($this->items);
    }
}

$collection = new MyCollection();
foreach ($collection as $item) {
    echo $item . ' ';
}

medium

A. ArrayIterator Object

B. 1 2 3

C. Error: getIterator must return array

D. No output

Solution

1. Step 1: Understand getIterator() return

   The method returns an ArrayIterator over the array [1, 2, 3].

2. Step 2: foreach loops over iterator

   The foreach loop iterates over the items 1, 2, and 3, echoing each with a space.

3. Final Answer:

   1 2 3 -> Option B

4. Quick Check:

   foreach prints each item from ArrayIterator [OK]

Hint: getIterator returns ArrayIterator to enable foreach [OK]

Common Mistakes:

• Expecting an error because getIterator returns ArrayIterator
• Thinking it prints object instead of values
• Missing the space after each number

4.

Identify the error in this PHP class implementing IteratorAggregate:

<?php
class Numbers implements IteratorAggregate {
    private array $nums = [10, 20, 30];
    public function getIterator() {
        return $this->nums;
    }
}

medium

A. getIterator() must return an object implementing Traversable, not an array.

B. Property $nums should be public, not private.

C. Class must implement Iterator, not IteratorAggregate.

D. getIterator() method must be static.

Solution

1. Step 1: Check getIterator() return type

   It returns an array, but getIterator() must return an object implementing Traversable.

2. Step 2: Understand interface contract

   Returning an array causes a type error because arrays do not implement Traversable.

3. Final Answer:

   getIterator() must return an object implementing Traversable, not an array. -> Option A

4. Quick Check:

   getIterator() returns Traversable object [OK]

Hint: getIterator must return Traversable object, not array [OK]

Common Mistakes:

• Returning array directly from getIterator()
• Thinking private properties cause error here
• Assuming getIterator() must be static

5.

You want to create a class FilteredCollection that implements IteratorAggregate and only iterates over items greater than 5 from an internal array. Which implementation of getIterator() is correct?

<?php
class FilteredCollection implements IteratorAggregate {
    private array $items;
    public function __construct(array $items) {
        $this->items = $items;
    }
    public function getIterator(): Traversable {
        // Which code below is correct?
    }
}

hard

A. return new ArrayIterator($this->items);

B. return new ArrayIterator(array_map(fn($v) => $v > 5, $this->items));

C. return $this->items;

D. return new ArrayIterator(array_filter($this->items, fn($v) => $v > 5));

Solution

1. Step 1: Filter items greater than 5

   Use array_filter with a callback to keep only values > 5.

2. Step 2: Return an ArrayIterator over filtered items

   Wrap filtered array in ArrayIterator to satisfy Traversable return type.

3. Final Answer:

   return new ArrayIterator(array_filter($this->items, fn($v) => $v > 5)); -> Option D

4. Quick Check:

   Filter then wrap in ArrayIterator [OK]

Hint: Filter array first, then return ArrayIterator [OK]

Common Mistakes:

• Returning unfiltered array directly
• Using array_map instead of array_filter
• Not wrapping result in an iterator object