final Anahtar Sözcüğü
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
?>
Nesne Kopyalama
Bir nesnenin tüm özelliklerinin aynen kopyalanarak çoğaltılması her zaman istenilen durum değildir. Kurucuların kopyalanması iyi bir örnektir, bir GTK penceresini temsil eden bir nesneniz varsa ve bu nesne GTK penceresine ait özkaynağı tutuyorsa, bu nesnenin kopyasını oluşturduğunuzda, yeni nesnenin önceki pencere ile aynı özelliklere sahip yeni bir pencereye sahip olmasını ve önceki nesne ile aynı şekilde pencereye ait özkaynağı tutuyor olmasını isteyebilirsiniz. Bir diğer örnek, nesneniz kullandığı başka bir nesneye ait bir gönderim tutuyorsa, ebeveyn nesne kopyalandığında diğer nesnenin de yeni bir örneğinin oluşturulmasını isteyebilirsiniz, yani ebeveynin kopyasının kendine ait ayrı bir kopyası olur.
Bir nesnenin kopyası (mümkünse nesnenin __clone() yöntemini
çağıran) clone anahtar sözcüğü kullanılarak oluşturulur. Bir
nesnenin __clone() yöntemi doğrudan doğruya çağrılamaz.
$nesnenin_kopyası = clone $object;
Bir nesne kopyalandığında, PHP 5 nesnenin tüm özelliklerinin yüzeysel bir kopyasını çıkartacaktır. Diğer değişkenlere birer gönderim olan tüm özellikler gönderim olarak kalacaktır.
Kopyalama tamamlanır tamamlanmaz, bir __clone() yöntemi
tanımlanmışsa, değişmesi gereken lüzumlu tüm özelliklere izin vermek için
yeni oluşturulan nesnenin __clone() yöntemi çağrılacaktır.
Örnek 1 - Bir nesnenin kopyalanması
<?php
class AltNesne
{
static $örnek_sayısı = 0;
public $örnek;
public function __construct() {
$this->örnek = ++self::$örnek_sayısı;
}
public function __clone() {
$this->örnek = ++self::$örnek_sayısı;
}
}
class KopyalanabilirNesnem
{
public $nesne1;
public $nesne2;
function __clone()
{
// this->nesne1'in bir kopyasını oluşturmak için
// zorlayalım, yoksa aynı nesneyi gösterecek.
$this->nesne1 = clone $this->nesne1;
}
}
$nes = new KopyalanabilirNesnem();
$nes->nesne1 = new AltNesne();
$nes->nesne2 = new AltNesne();
$nes2 = clone $nes;
print("Özgün Nesne:\n");
print_r($nes);
print("Kopya Nesne:\n");
print_r($nes2);
?>
Yukarıdaki örneğin çıktısı:
Özgün Nesne:
KopyalanabilirNesnem Object
(
[nesne1] => AltNesne Object
(
[örnek] => 1
)
[nesne2] => AltNesne Object
(
[örnek] => 2
)
)
Kopya Nesne:
KopyalanabilirNesnem Object
(
[nesne1] => AltNesne Object
(
[örnek] => 3
)
[nesne2] => AltNesne Object
(
[örnek] => 2
)
)
add a note
User Contributed NotesNesne Kopyalama
ben at last dot fm
05-Jun-2009 05:33
05-Jun-2009 05:33
koyama
03-Mar-2009 03:28
03-Mar-2009 03:28
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 $foo;
// check if we reach this point
echo 'Finished cloning!';
?>
Replacing the __clone() method with the one shown in edit by danbrown at php dot net we run into an infinite loop, and we never get message 'Finished cloning!'.
cheetah at tanabi dot org
18-Nov-2008 01:15
18-Nov-2008 01:15
Want deep cloning without too much hassle?
<?php
function __clone() {
foreach($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]
wbcarts at juno dot com
02-Oct-2008 06:41
02-Oct-2008 06:41
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 $platoon[197] . '<br>';
?>
COMMAND ISSUED: Deploy Troops
Soldier[id=1, status=deploying]
Soldier[id=225, status=deploying]
Soldier[id=14, status=deploying]
COMMAND ISSUED: March Forward
Soldier[id=49, status=marching forward]
Soldier[id=165, status=marching forward]
COMMAND ISSUED: Fire!
Soldier[id=250, status=shot fired]
Soldier[id=70, status=shot fired]
COMMAND ISSUED: Retreat!
Soldier[id=28, status=course reversed]
Soldier[id=199, status=course reversed]
Jim Brown
19-Jul-2008 10:34
19-Jul-2008 10:34
Regarding the generic deep __clone() example provided by david ashe at metabin:
If your object has a variable that stores an array of objects, that particular __clone() example will NOT perform a deep copy on your array of objects.
alex dot offshore at gmail dot com
19-May-2008 10:23
19-May-2008 10:23
Remember that in PHP 5 ALL objects are assigned BY REFERENCE.
<?php
function foo($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
13-Mar-2008 05:52
13-Mar-2008 05:52
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.
Hayley Watson
17-Dec-2007 11:51
17-Dec-2007 11:51
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);
tomi at cumulo dot fi
13-Nov-2007 11:57
13-Nov-2007 11:57
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 02:43
08-Oct-2007 02:43
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($b2);
/*
OUTPUT BEFORE implementing the function __clone()
object(B)#2 (3) { ["a"]=> object(A)#3 (1) { ["aaa"]=> int(222) } ["bbb"]=> int(1) }
object(B)#4 (3) { ["a"]=> object(A)#3 (1) { ["aaa"]=> int(222) } ["bbb"]=> int(2) }
OUTPUT AFTER implementing the function __clone()
object(B)#1 (3) { ["a"]=> object(A)#2 (1) { ["aaa"]=> int(111) } ["bbb"]=> int(1) }
object(B)#3 (3) { ["a"]=> object(A)#4 (1) { ["aaa"]=> int(222) } ["bbb"]=> int(2) }
*/
?>
Whenever we use another class inside, we must clone it manually. If you have 10s of classes related, this is rather tedious. I don't want to even think about classes dynamically populated with other objects. Be careful when designing your classes! You should look after your objects all the time! This major change on PHP5 vs PHP4 regarding "references" definitely has very good performance improvements but comes with very dangerous side effects as well..
Alexey
08-Feb-2007 03:18
08-Feb-2007 03:18
To implement __clone() method in complex classes I use this simple function:
function clone_($some)
{
return (is_object($some)) ? clone $some : $some;
}
In this way I don't need to care about type of my class properties.
MakariVerslund at gmail dot com
22-Jan-2007 12:30
22-Jan-2007 12:30
I ran into the same problem of an array of objects inside of an object that I wanted to clone all pointing to the same objects. However, I agreed that serializing the data was not the answer. It was relatively simple, really:
public function __clone() {
foreach ($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.
jorge dot villalobos at gmail dot com
30-Mar-2005 11:29
30-Mar-2005 11:29
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.