Object Cloning

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 keyword (which calls the object's method if possible). clone__clone()

$copy_of_object = clone $object;

When an object is cloned, PHP will perform a shallow copy of all of the object's properties. Any properties that are references to other variables will remain references.

void **__clone**

Once the cloning is complete, if a method is defined, then the newly created object's method will be called, to allow any necessary properties that need to be changed. __clone()__clone()

Voorbeeld: 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);

?>

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
        )

)

It is possible to access a member of a freshly cloned object in a single expression:

Voorbeeld: Access member of freshly cloned object

<?php
$dateTime = new DateTime();
echo (clone $dateTime)->format('Y');
?>

2016