defaultdict in PHP

defaultdict.php

<?php

# http://scribu.net/blog/defaultdict-in-php.html

class Defaultdict implements ArrayAccess {

	private $container = array();
	private $default;

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

	public function offsetSet( $offset, $value ) {
		if ( is_null($offset) ) {
			trigger_error( sprintf( "Trying to use %s as a list.", __CLASS__ ), E_USER_WARNING );
			return;
		}

		$this->container[$offset] = $value;
	}

	public function offsetExists( $offset ) {
		return true;
	}

	public function offsetUnset( $offset ) {
		unset( $this->container[$offset] );
	}

	public function &offsetGet( $offset ) {
		if ( !isset( $this->container[$offset] ) ) {
			if ( is_callable($this->default) )
				$value = call_user_func( $this->default, $offset );
			else
				$value = $this->default;

			$this->container[$offset] = $value;
		}

		return $this->container[$offset];
	}
}

usage-advanced.php

<?php

$instances = new Defaultdict( function( $key ) {

	$value = new stdClass;
	$value->id = $key;

	return $value;
} );

print_r( $instances['bar'] );  // Result: stdClass Object ( [id] => bar )

usage-simple.php

<?php

$counts = new Defaultdict(1);

echo $counts['foo'];  // Result: 1

$counts['bar']++;

echo $counts['bar'];  // Result: 2