Saturday, March 24, 2007

Object cloning

Creating a copy of an object with fully replicated properties is not always the wanted behavior. A good example of the need for copy constructors, is if you have an object which represents a GTK window and the object holds the resource of this GTK window, when you create a duplicate you might want to create a new window with the same properties and have the new object hold the resource of the new window. Another example is if your object holds a reference to another object which it uses and when you replicate the parent object you want to create a new instance of this other object so that the replica has its own separate copy.
An object copy is created by using the clone keyword (which calls the object's __clone() method if possible). An object's __clone() method cannot be called directly.
$copy_of_object = clone $object;
When an object is cloned, PHP 5 will perform a shallow copy of all of the object's properties. Any properties that are references to other variables, will remain references. If a __clone() method is defined, then the newly created object's __clone() method will be called, to allow any necessary properties that need to be changed.

class SubObject
static $instances = 0;
public $instance;

public function __construct() {
$this->instance = ++self::$instances;

public function __clone() {
$this->instance = ++self::$instances;

class MyCloneable
public $object1;
public $object2;

function __clone()
// Force a copy of this->object, otherwise
// it will point to same object.
$this->object1 = clone($this->object1);

$obj = new MyCloneable();

$obj->object1 = new SubObject();
$obj->object2 = new SubObject();

$obj2 = clone $obj;

print("Original Object:\n");

print("Cloned Object:\n");


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