PHP: Object cloning

Last updated: Sun, 25 Nov 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.

Example#1 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
        )

)

Comparing objects

Final Keyword

Last updated: Sun, 25 Nov 2007

add a note User Contributed Notes
Object cloning

Jim Brown
19-Jul-2008 03:34


Regarding the generic deep __clone() example provided by david ashe at metabin:



If your object has a variable that stores an array of objects, that particular __clone() example will NOT perform a deep copy on your array of objects.

alex dot offshore at gmail dot com
19-May-2008 03:23


Remember that in PHP 5 ALL objects are assigned BY REFERENCE.



<?php



  function foo($a) // notice that '&' near $a is missing

  {

    $a['bar'] = 10;

  }



  $x = array('bar' => 0); // built-in array() is not an object

  $y = new ArrayObject(array('bar' => 0));



  echo "\$x['bar'] == ${x['bar']};\n\$y['bar'] == ${y['bar']};\n\n";



  foo($x);

  foo($y);



  echo "\$x['bar'] == ${x['bar']};\n\$y['bar'] == ${y['bar']};\n";



?>



Output:

$x['bar'] == 0;

$y['bar'] == 0;



$x['bar'] == 0;

$y['bar'] == 10;



Hope this will be useful.



By the way, to determine whether the variable is compatible with ArrayAccess/ArrayObject see http://php.net/manual/en/function.is-array.php#48083

crrodriguez at suse dot de
12-Mar-2008 10:52


Keep in mind that since PHP 5.2.5, trying to clone a non-object correctly results in a fatal error, this differs from previous versions where only a Warning was thrown.

Anonymous
20-Feb-2008 11:55


I sorely apologize if this point seems to be obvious, but being slightly impatient, I do not care to know how many times I overlooked this.



For God's sake, notice the lack of the '$' in front of objects when referencing them with '$this'.  Not included, this allows class variables to be properly addressed.



eg, $this->$a, will NOT work properly with clone(), but this will: $this->a.

Hayley Watson
17-Dec-2007 03:51


It should go without saying that if you have circular references, where a property of object A refers to object B while a property of B refers to A (or more indirect loops than that), then you'll be glad that clone does NOT automatically make a deep copy!



<?php



class Foo

{

    var $that;



function __clone()

{

    $this->that = clone $this->that;

}



}



$a = new Foo;

$b = new Foo;

$a->that = $b;

$b->that = $a;



$c = clone $a;

echo 'What happened?';

var_dump($c);

david ashe at metabin
02-Dec-2007 10:18


Here is a function to clone all of the objects automatically 

(useful if you use a base class that has this method)



    function __clone(){

        foreach($this as $name => $value){

            if(gettype($value)=='object'){

                $this->$name= clone($this->$name);

            }

        }

    }

tomi at cumulo dot fi
13-Nov-2007 03:57


It should be noticed that __clone() does not allow you to return a value. Basically the idea is that you implement this magic method only when you want to execute operations inside the cloned object, immediately prior to the cloning. In this way __clone() is similar to the default destructor (__destruct()), in that it executes code right before the object is destroyed.

muratyaman at gmail dot com
08-Oct-2007 07:43


I think this is a bit awkward:



<?php

class A{

    public $aaa;

}



class B{

    public $a;

    public $bbb;

    

    function __clone(){

        $this->a = clone $this->a;//clone MANUALLY!!!

    }

}



$b1 = new B();

$b1->a = new A();

$b1->a->aaa = 111;

$b1->bbb = 1;



$b2 = clone $b1;

$b2->a->aaa = 222;//BEWARE!!

$b2->bbb = 2;//no problem on basic types



var_dump($b1); echo '<br />';

var_dump($b2);

/*

OUTPUT BEFORE implementing the function __clone()

object(B)#2 (3) { ["a"]=>  object(A)#3 (1) { ["aaa"]=>  int(222) } ["bbb"]=>  int(1)  }

object(B)#4 (3) { ["a"]=>  object(A)#3 (1) { ["aaa"]=>  int(222) } ["bbb"]=>  int(2)  }



OUTPUT AFTER implementing the function __clone()

object(B)#1 (3) { ["a"]=>  object(A)#2 (1) { ["aaa"]=>  int(111) } ["bbb"]=>  int(1)  }

object(B)#3 (3) { ["a"]=>  object(A)#4 (1) { ["aaa"]=>  int(222) } ["bbb"]=>  int(2)  }

*/

?>



Whenever we use another class inside, we must clone it manually. If you have 10s of classes related, this is rather tedious. I don't want to even think about classes dynamically populated with other objects. Be careful when designing your classes! You should look after your objects all the time! This major change on PHP5 vs PHP4 regarding "references" definitely has very good performance improvements but comes with very dangerous side effects as well..

Mr.KTO
13-Jun-2007 01:00


In PHP4 the clone keyword isn't defined, if you want a compatible code (using a copy of an object) - use this fix:

$c = (PHP_VERSION < 5) ? $b : clone($b);

It is possible cause php checks function existence just before its calling.

Alexey
08-Feb-2007 07:18


To implement __clone() method in complex classes I use this simple function:



function clone_($some)

{

   return (is_object($some)) ? clone $some : $some;

}



In this way I don't need to care about type of my class properties.

MakariVerslund at gmail dot com
21-Jan-2007 04:30


I ran into the same problem of an array of objects inside of an object that I wanted to clone all pointing to the same objects. However, I agreed that serializing the data was not the answer. It was relatively simple, really:



    public function __clone()

    {

        foreach ($this->varName as &$a)

    {

            foreach ($a as &$b)

        {

        $b = clone $b;

        }

    }

    }



Note, that I was working with a multi-dimensional array and I was not using the Key=>Value pair system, but basically, the point is that if you use foreach, you need to specify that the copied data is to be accessed by reference.

felix gilcher
22-Sep-2006 07:20


Copying objects with



<?php

public function copy()

{

   $serialized_contents = serialize($this);

   return unserialize($serialized_contents);

}

?>



is a pretty dangerous thing. What if your objects hold references to very large shared objects? They all get copied as well. Serializing objects may break database connections and references. What you probably want to do is implement a proper __clone() method for each of your classes so that each object in your tree gets to decide what subobjects need to be copied and which ones should better remain untouched.



regards



felix

paul at accessdesigns dot org dot uk
03-Aug-2006 01:47


I noticed when trying to clone a series of objects in a loop and and adding them to an array, the objects were still passed by reference, so all objects took the same value as the last object cloned and added to the array.



The solution was to use the copy function as below which truly clones objects.



Here is an example:

<?php

$temp = new $this->_doname;

foreach($rows as $id=>$row) {

        $do = clone $temp;

        $do->load($id);

        $this->_dataobjects[]= clone $do;

}

?>



This results in every object having the same value.

However using:



<?php

$this->_dataobjects[]= $this->copy($do);

?>

with 

<?php

public function copy()

{

   $serialized_contents = serialize($this);

   return unserialize($serialized_contents);

}

?>



Actually produces the expected result.



Paul Bain

olle dot remove_this dot suni at rdnsoftware dot com
16-May-2006 04:49


As jorge dot villalobos at gmail dot com pointed out, the "__clone is NOT an override". 



However, if one needs a true copy of an object (which has real copies of all subobjects too, not references), one can define a class method such as this to the object-to-be-cloned: 



public function copy()

{

    $serialized_contents = serialize($this);

    return unserialize($serialized_contents);

}



The method makes a string representation of the object's contents (including subobjects), which can then be unserialized back to an object. 



The method usage is simple:



$original_object = new MyCloneable(); //can have sub objects

$copied_object = $original_object->clone(); //only makes copies of the values, not references



With serialization, you surely don't have to worry about references, since serialized object is just simple text.

ove at junne dot dk
08-Feb-2006 08:02


Consider this:



function myfunc()

{

    $A = new myobject(); // Create another mysql connection

    $A->method1();         // Runs a query (works)

}



$A = new myobject(); // Create a mysql connection

myfunc();

$A->method1();  // This query fails! Possible because leaving the function has closed the connection.



Conclusion: Declaration of objects within and outside a function WITH THE SAME NAME, will affect each other. Do not relay on the rules of scope.



Take care when using recursive structures!!



It took me a long time to figure out.

jorge dot villalobos at gmail dot com
30-Mar-2005 03:29


I think it's relevant to note that __clone is NOT an override. As the example shows, the normal cloning process always occurs, and it's the responsibility of the __clone method to "mend" any "wrong" action performed by it.

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Comparing objects

Final Keyword

Last updated: Sun, 25 Nov 2007