PHP 5.6.24 is released

Clonazione di oggetti

Creare una copia di un oggetto con tutte le proprietà replicate non è sempre il comportamento desiderato. Un esempio della necessità di costruttori per la copia, è che quando si ha un oggetto che rappresenta una finestra GTK e l'oggetto detiene le risorse di questa finestra GTK, quando si crea un duplicato si può voler creare una nuova finestra con le stesse proprietà e fare in modo che il nuovo oggetto detenga le risorse della nuova finestra. Un ulteriore esempio è il caso in cui un oggetto ha un riferimento ad un altro oggetto che utilizza e quando si replica il l'oggetto padre si vuole creare una nuova istanza di questo altro oggetto in modo che la replica ne abbia una propria copia separata dall'originale.

La copia di un oggetto viene creata utilizzando la parola chiave clone (che chiama se possibile il metodo __clone() dell'oggetto). Il metodo __clone() di un oggetto non può essere invocato direttamente.

$copy_of_object = clone $object;

Quando un oggetto viene clonato, PHP 5 esegue una copia superficiale di tutte le proprietà dell'oggetto. Ogni proprietà che sia un riferimento ad altre variabili rimarrà un riferimento.

void __clone ( void )

Completata la clonazione, se esiste un metodo __clone(), viene chiamato il metodo __clone() dell'oggetto appena creato per consentire l'aggiornamento di tutte le proprietà che necessitano di essere modificate.

Example #1 Clonazione di un oggetto

<?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()
    {
        
// forza la copia di $this->object1, che altrimenti
        // punterebbe alla stessa istanza.
        
$this->object1 = clone $this->object1;
    }
}

$obj = new MyCloneable();

$obj->object1 = new SubObject();
$obj->object2 = new SubObject();

$obj2 = clone $obj;


print(
"Oggetto originale:\n");
print_r($obj);

print(
"Oggetto clonato:\n");
print_r($obj2);

?>

Il precedente esempio visualizzerà:

Oggetto orginale:
MyCloneable Object
(
    [object1] => SubObject Object
        (
            [instance] => 1
        )

    [object2] => SubObject Object
        (
            [instance] => 2
        )

)
Oggetto clonato:
MyCloneable Object
(
    [object1] => SubObject Object
        (
            [instance] => 3
        )

    [object2] => SubObject Object
        (
            [instance] => 2
        )

)
add a note add a note

User Contributed Notes 14 notes

up
18
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);
up
12
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>
up
1
yinzw at chuchujie dot com
20 days 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] =>
        )

)
up
2
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
1
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
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
0
fabio at naoimporta dot com
5 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
0
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
-1
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
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";
?>
up
-1
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
jason at jewelrysupply dot com
10 months 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).
up
-3
cheetah at tanabi dot org
7 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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