Final Keyword
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
?>
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
)
)
add a note
User Contributed Notes
Object Cloning - [22 notes]
ben at last dot fm ¶
4 years ago
jojor at gmx dot net ¶
3 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>
wbcarts at juno dot com ¶
4 years ago
CLONED ARMIES? USE STATIC DATA
When I think of cloning, I always think of Star Wars "Cloned Army"... where the number of clones are in the hundreds of thousands. So far, I have only seen examples of one or two clones with either shallow, deep, or recursive references. My fix is to use the static keyword. With static, you choose the properties your objects share... and makes scaling up the number of so-called "clones" much easier.
<?php
class Soldier {
public static $status; // this is the property I'm trying to clone
protected static $idCount = 0; // used to increment ID numbers
protected $id; // each Soldier will have a unique ID
public function __construct() {
$this->id = ++self::$idCount;
}
public function issueCommand($task) {
switch($task){
case 'Deploy Troops': self::$status = 'deploying'; break;
case 'March Forward': self::$status = 'marching forward'; break;
case 'Fire!': self::$status = 'shot fired'; break;
case 'Retreat!': self::$status = 'course reversed'; break;
default: self::$status = 'at ease'; break;
}
echo 'COMMAND ISSUED: ' . $task . '<br>';
}
public function __toString() {
return "Soldier[id=$this->id, status=" . self::$status . ']';
}
}
# create the General and the Cloned Army
$general = new Soldier();
$platoon = array();
for($i = 0; $i < 250; $i++) $platoon[] = new Soldier();
# issue commands, then check what soldiers are doing
$general->issueCommand('Deploy Troops');
echo $general . '<br>';
echo $platoon[223] . '<br>';
echo $platoon[12] . '<br>';
$general->issueCommand('March Forward');
echo $platoon[47] . '<br>';
echo $platoon[163] . '<br>';
$general->issueCommand('Fire!');
echo $platoon[248] . '<br>';
echo $platoon[68] . '<br>';
$general->issueCommand('Retreat!');
echo $platoon[26] . '<br>';
echo $platoon[197] . '<br>';
?>
COMMAND ISSUED: Deploy Troops
Soldier[id=1, status=deploying]
Soldier[id=225, status=deploying]
Soldier[id=14, status=deploying]
COMMAND ISSUED: March Forward
Soldier[id=49, status=marching forward]
Soldier[id=165, status=marching forward]
COMMAND ISSUED: Fire!
Soldier[id=250, status=shot fired]
Soldier[id=70, status=shot fired]
COMMAND ISSUED: Retreat!
Soldier[id=28, status=course reversed]
Soldier[id=199, status=course reversed]
Jim Brown ¶
4 years ago
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.
walkman at walkman dot pk ¶
1 year ago
If you want a property that gets the same value in every clone if changed, you can do this simple trick:
<?php
class A
{
public $name ;
public function __construct()
{
$this->name = & $this->name;
}
}
$a = new A;
$a->name = "George";
$b = clone $a;
$b->name = "Somebody else";
var_dump($a);
var_dump($b);
?>
this will output:
object(A)#1 (1) {
["name"]=>
&string(13) "Somebody else"
}
object(A)#2 (1) {
["name"]=>
&string(13) "Somebody else"
}
You can change any of the clones property and all of the others will change accordingly.
seriously at something dot com ¶
1 year ago
If you want a property that gets the same value in every clone if changed, you can do this simple trick:
<?php
class A
{
public static $name ;
}
$a = new A;
$a::$name = 'George';
$b = clone $a;
$b::$name = "Somebody else";
echo 'a: ' . $a::$name . "\n";
echo 'b: ' . $b::$name . "\n";
?>
this will output:
a: Somebody else
b: Somebody else
You can change any of the clones property and all of the others will change accordingly.
jeffrey at whinger dot nl ¶
3 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
?>
henke at henke37 dot cjb dot net ¶
3 years ago
Arrays are shallow cloned on assignment, so don't use the clone keyword on them, just assign it to a new variable. That would lead to an error instead.
koyama ¶
4 years ago
The __clone() method for deep cloning by cheetah at tanabi dot org also works when the object to be cloned contains references to itself. This is not the case for any variation of the __clone() method in edit by danbrown at php dot net.
We are taking advantage of the fact that one can serialize an object that references itself.
Example:
<?php
class Foo
{
function __construct()
{
$this->_myself = $this;
}
function __clone() {
foreach ($this as $key => $val) {
if (is_object($val) || (is_array($val))) {
$this->{$key} = unserialize(serialize($val));
}
}
}
}
// this object references itself
$foo = new Foo();
// create a deep clone
$bar = clone $foo;
// check if we reach this point
echo 'Finished cloning!';
?>
Replacing the __clone() method with the one shown in edit by danbrown at php dot net we run into an infinite loop, and we never get message 'Finished cloning!'.
cheetah at tanabi dot org ¶
4 years ago
Want deep cloning without too much hassle?
<?php
function __clone() {
foreach($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]
alex dot offshore at gmail dot com ¶
5 years ago
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 ¶
5 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.
Hayley Watson ¶
5 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);
tomi at cumulo dot fi ¶
5 years ago
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 ¶
5 years ago
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($b2);
/*
OUTPUT BEFORE implementing the function __clone()
object(B)#2 (3) { ["a"]=> object(A)#3 (1) { ["aaa"]=> int(222) } ["bbb"]=> int(1) }
object(B)#4 (3) { ["a"]=> object(A)#3 (1) { ["aaa"]=> int(222) } ["bbb"]=> int(2) }
OUTPUT AFTER implementing the function __clone()
object(B)#1 (3) { ["a"]=> object(A)#2 (1) { ["aaa"]=> int(111) } ["bbb"]=> int(1) }
object(B)#3 (3) { ["a"]=> object(A)#4 (1) { ["aaa"]=> int(222) } ["bbb"]=> int(2) }
*/
?>
Whenever we use another class inside, we must clone it manually. If you have 10s of classes related, this is rather tedious. I don't want to even think about classes dynamically populated with other objects. Be careful when designing your classes! You should look after your objects all the time! This major change on PHP5 vs PHP4 regarding "references" definitely has very good performance improvements but comes with very dangerous side effects as well..
Alexey ¶
6 years ago
To implement __clone() method in complex classes I use this simple function:
function clone_($some)
{
return (is_object($some)) ? clone $some : $some;
}
In this way I don't need to care about type of my class properties.
MakariVerslund at gmail dot com ¶
6 years ago
I ran into the same problem of an array of objects inside of an object that I wanted to clone all pointing to the same objects. However, I agreed that serializing the data was not the answer. It was relatively simple, really:
public function __clone() {
foreach ($this->varName as &$a) {
foreach ($a as &$b) {
$b = clone $b;
}
}
}
Note, that I was working with a multi-dimensional array and I was not using the Key=>Value pair system, but basically, the point is that if you use foreach, you need to specify that the copied data is to be accessed by reference.
jorge dot villalobos at gmail dot com ¶
8 years ago
I think it's relevant to note that __clone is NOT an override. As the example shows, the normal cloning process always occurs, and it's the responsibility of the __clone method to "mend" any "wrong" action performed by it.
olivier dot pons at goo dot without dot oo dot mail dot com ¶
3 years ago
If you think "clone" will create a new instance, thus calling "__constructor", you're wrong. clone seems to only allocate memory for the object cloned, and simply copies the variables memory from the original to the new one (imagine something alike memcpy() in C). Nothing more. Keep in mind you'll have to do all the rest by yourself.
emile at webflow dot nl ¶
3 years ago
Another gotcha I encountered: like __construct and __desctruct, you must call parent::__clone() yourself from inside a child's __clone() function. The manual kind of got me on the wrong foot here: "An object's __clone() method cannot be called directly."
gratcypalma at gmail dot com ¶
2 years ago
this is my simple method, use class_alias function
<?php
class Y {
function __construct($a) {
$this -> a = $a;
}
function __destruct() {
echo $this -> a;
}
}
$mam = new Y('Foo');
class_alias('Y', 'bar'); // clone class use class_alias function
$m = new bar('Bar');
?>
user at somethiong dot com ¶
2 years ago
Creating a Deep Copy
<?php
protected function deepCopy($object){
return unserialize(serialize($object));
}// End Function
?>