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.

Example 19-29. Cloning an object


<?php

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_r($obj);



print("Cloned Object:\n");

print_r($obj2);



?>

The above example will output:


Original Object:

MyCloneable Object

(

    [object1] => SubObject Object

        (

            [instance] => 1

        )



    [object2] => SubObject Object

        (

            [instance] => 2

        )



)

Cloned Object:

MyCloneable Object

(

    [object1] => SubObject Object

        (

            [instance] => 3

        )



    [object2] => SubObject Object

        (

            [instance] => 2

        )



)