PHP 7.1.0 Released

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.

void __clone ( void )

Once the cloning is complete, 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
        )

)

PHP 7.0.0 introduced the possibility to access a member of a freshly cloned object in a single expression:

Example #2 Access member of freshly cloned object

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

The above example will output something similar to:

2016
add a note add a note

User Contributed Notes 14 notes

up
19
MakariVerslund at gmail dot com
9 years ago
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.
up
12
Hayley Watson
8 years ago
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);
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11
jojor at gmx dot net
6 years ago
Here is test script i wrote to test the behaviour of clone when i have arrays with primitive values in my class - as an additonal test of the note below by jeffrey at whinger dot nl

<pre>
<?php

class MyClass {

    private
$myArray = array();
    function
pushSomethingToArray($var) {
       
array_push($this->myArray, $var);
    }
    function
getArray() {
        return
$this->myArray;
    }

}

//push some values to the myArray of Mainclass
$myObj = new MyClass();
$myObj->pushSomethingToArray('blue');
$myObj->pushSomethingToArray('orange');
$myObjClone = clone $myObj;
$myObj->pushSomethingToArray('pink');

//testing
print_r($myObj->getArray());     //Array([0] => blue,[1] => orange,[2] => pink)
print_r($myObjClone->getArray());//Array([0] => blue,[1] => orange)
//so array  cloned

?>
</pre>
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4
jorge dot villalobos at gmail dot com
11 years ago
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.
up
2
stanislav dot eckert at vizson dot de
1 year ago
This base class automatically clones attributes of type object or array values of type object recursively. Just inherit your own classes from this base class.

<?php
   
class clone_base
   
{
        public function
__clone()
        {
           
$object_vars = get_object_vars($this);

            foreach (
$object_vars as $attr_name => $attr_value)
            {
                if (
is_object($this->$attr_name))
                {
                   
$this->$attr_name = clone $this->$attr_name;
                }
                else if (
is_array($this->$attr_name))
                {
                   
// Note: This copies only one dimension arrays
                   
foreach ($this->$attr_name as &$attr_array_value)
                    {
                        if (
is_object($attr_array_value))
                        {
                           
$attr_array_value = clone $attr_array_value;
                        }
                        unset(
$attr_array_value);
                    }
                }
            }
        }
    }
?>

Example:
<?php
   
class foo extends clone_base
   
{
        public
$attr = "Hello";
        public
$b = null;
        public
$attr2 = array();

        public function
__construct()
        {
           
$this->b = new bar("World");
           
$this->attr2[] = new bar("What's");
           
$this->attr2[] = new bar("up?");
        }
    }

    class
bar extends clone_base
   
{
        public
$attr;

        public function
__construct($attr_value)
        {
           
$this->attr = $attr_value;
        }
    }

    echo
"<pre>";

   
$f1 = new foo();
   
$f2 = clone $f1;
   
$f2->attr = "James";
   
$f2->b->attr = "Bond";
   
$f2->attr2[0]->attr = "Agent";
   
$f2->attr2[1]->attr = "007";

    echo
"f1.attr = " . $f1->attr . "\n";
    echo
"f1.b.attr = " . $f1->b->attr . "\n";
    echo
"f1.attr2[0] = " . $f1->attr2[0]->attr . "\n";
    echo
"f1.attr2[1] = " . $f1->attr2[1]->attr . "\n";
    echo
"\n";
    echo
"f2.attr = " . $f2->attr . "\n";
    echo
"f2.b.attr = " . $f2->b->attr . "\n";
    echo
"f2.attr2[0] = " . $f2->attr2[0]->attr . "\n";
    echo
"f2.attr2[1] = " . $f2->attr2[1]->attr . "\n";
?>
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1
yinzw at chuchujie dot com
4 months ago
It's clearly depicted in the manual, about the mechanism of clone process:
- First, shallow copy: properties of references will keep references (refer to the same target/variable)
- Then, change content/property as requested (calling __clone method which is defined by user).

To illustrate this process, the following example codes seems better, comparing the the original version:

class SubObject
{
    static $num_cons = 0;
    static $num_clone = 0;

    public $construct_value;
    public $clone_value;

    public function __construct() {
        $this->construct_value = ++self::$num_cons;
    }

    public function __clone() {
        $this->clone_value = ++self::$num_clone;
    }
}

class MyCloneable
{
    public $object1;
    public $object2;

    function __clone()
    {
        // 强制复制一份this->object, 否则仍然指向同一个对象
        $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($obj);
echo '<br>';
print("Cloned Object:\n");
print_r($obj2);

==================

the output is as below

Original Object:
MyCloneable Object
(
    [object1] => SubObject Object
        (
            [construct_value] => 1
            [clone_value] =>
        )

    [object2] => SubObject Object
        (
            [construct_value] => 2
            [clone_value] =>
        )

)
<br>Cloned Object:
MyCloneable Object
(
    [object1] => SubObject Object
        (
            [construct_value] => 1
            [clone_value] => 1
        )

    [object2] => SubObject Object
        (
            [construct_value] => 2
            [clone_value] =>
        )

)
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2
jeffrey at whinger dot nl
6 years ago
For me it wasn't very clear to how this cloning of objects really worked so I made this little bit of code:

<?php
class foo
{
    public
$test;
   
    public function
test()
    {
        echo
'give us a '.$this->test."<br>\n";
    }
}

class
bar
{
    public
$foo;
   
    public function
insertFoo($foo)
    {
       
$this->foo = $foo;
    }
}

$foo = new foo();

$foo->test = 'foo';

$bar = new bar();

$bar->insertFoo($foo);

$foo->test();

$bar->foo->test();

$foo->test = 'bar';

$foo->test();

$bar->foo->test();

$bar->foo = clone $foo;

$bar->foo->test = 'woop woop';

$foo->test();

$bar->foo->test();

// result:
// give us a foo
// give us a foo
// give us a bar
// give us a bar
// give us a bar
// give us a woop woop
?>
up
0
jason at jewelrysupply dot com
1 year ago
@DPB

I believe the two functions are not quite the same. The serialize followed by deserialize method is the way I've done deep cloning in other languages (bypasses any weird clone function behavior and ensures you have a no-strings-attached copy of the object).
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0
Alexey
9 years ago
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.
up
-1
crrodriguez at suse dot de
8 years ago
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.
up
-2
ben at last dot fm
7 years ago
Here are some cloning and reference gotchas we came up against at Last.fm.

1. PHP treats variables as either 'values types' or 'reference types', where the difference is supposed to be transparent. Object cloning is one of the few times when it can make a big difference. I know of no programmatic way to tell if a variable is intrinsically a value or reference type. There IS however a non-programmatic ways to tell if an object property is value or reference type:

<?php

class A { var $p; }

$a = new A;
$a->p = 'Hello'; // $a->p is a value type
var_dump($a);

/*
object(A)#1 (1) {
  ["p"]=>
  string(5) "Hello" // <-- no &
}
*/

$ref =& $a->p; // note that this CONVERTS $a->p into a reference type!!
var_dump($a);

/*
object(A)#1 (1) {
  ["p"]=>
  &string(5) "Hello" // <-- note the &, this indicates it's a reference.
}
*/

?>

2. unsetting all-but-one of the references will convert the remaining reference back into a value. Continuing from the previous example:

<?php

unset($ref);
var_dump($a);

/*
object(A)#1 (1) {
  ["p"]=>
  string(5) "Hello"
}
*/

?>

I interpret this as the reference-count jumping from 2 straight to 0. However...

2. It IS possible to create a reference with a reference count of 1 - i.e. to convert an property from value type to reference type, without any extra references. All you have to do is declare that it refers to itself. This is HIGHLY idiosyncratic, but nevertheless it works. This leads to the observation that although the manual states that 'Any properties that are references to other variables, will remain references,' this is not strictly true. Any variables that are references, even to *themselves* (not necessarily to other variables), will be copied by reference rather than by value.

Here's an example to demonstrate:

<?php

class ByVal
{
    var
$prop;
}

class
ByRef
{
    var
$prop;
    function
__construct() { $this->prop =& $this->prop; }
}

$a = new ByVal;
$a->prop = 1;
$b = clone $a;
$b->prop = 2; // $a->prop remains at 1

$a = new ByRef;
$a->prop = 1;
$b = clone $a;
$b->prop = 2; // $a->prop is now 2

?>
up
-2
emile at webflow dot nl
6 years ago
Another gotcha I encountered: like __construct and __desctruct, you must call parent::__clone() yourself from inside a child's __clone() function. The manual kind of got me on the wrong foot here: "An object's __clone() method cannot be called directly."
up
-1
fabio at naoimporta dot com
9 months ago
It's possible to know how many clones have been created of a  object. I'm think that is correct:

<?php

class Classe {

    public static
$howManyClones = 0;

    public function
__clone() {
        ++static::
$howManyClones;
    }

    public static function
howManyClones() {
        return static::
$howManyClones;
    }

    public function
__destruct() {
        --static::
$howManyClones;
    }
}

$a = new Classe;

$b = clone $a;
$c = clone $b;
$d = clone $c;

echo
'Clones:' . Classe::howManyClones() . PHP_EOL;

unset(
$d);

echo
'Clones:' . Classe::howManyClones() . PHP_EOL;
up
-4
cheetah at tanabi dot org
8 years ago
Want deep cloning without too much hassle?

<?php
function __clone() {
    foreach(
$this as $key => $val) {
        if(
is_object($val)||(is_array($val))){
           
$this->{$key} = unserialize(serialize($val));
        }
    }
}
?>

That will insure any object, or array that may potentially contain objects, will get cloned without using recursion or other support methods.



[EDIT BY danbrown AT php DOT net: An almost exact function was contributed on 02-DEC-2008-10:18 by (david ashe AT metabin):

<?php
   
function __clone(){
        foreach(
$this as $name => $value){
            if(
gettype($value)=='object'){
               
$this->$name= clone($this->$name);
            }
        }
    }
?>

Giving credit where it's due.  ~DPB]
[EDIT BY cmb AT php DOT net: the latter function fails to make deep copies of object arrays, and might end up with infinite recursion.]
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