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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.

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$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 10.31. Cloning an object

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<?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:

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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
       )

) ?>



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