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在多数情况下,我们并不需要完全复制一个对象来获得其中属性。但有一个情况下确实需要:如果你有一个 GTK 窗口对象,该对象持有窗口相关的资源。你可能会想复制一个新的窗口,保持所有属性与原来的窗口相同,但必须是一个新的对象(因为如果不是新的对象,那么一个窗口中的改变就会影响到另一个窗口)。还有一种情况:如果对象 A 中保存着对象 B 的引用,当你复制对象 A 时,你想其中使用的对象不再是对象 B 而是 B 的一个副本,那么你必须得到对象 A 的一个副本。
对象复制可以通过 clone 关键字来完成(如果可能,这将调用对象的 __clone() 方法)。对象中的 __clone() 方法不能被直接调用。
$copy_of_object = clone $object;
当对象被复制后,PHP 5 会对对象的所有属性执行一个浅复制(shallow copy)。所有的引用属性 仍然会是一个指向原来的变量的引用。
当复制完成时,如果定义了 __clone() 方法,则新创建的对象(复制生成的对象)中的 __clone() 方法会被调用,可用于修改属性的值(如果有必要的话)。
Example #1 复制一个对象
<?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 ()
{
// 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 );
print( "Cloned Object:\n" );
print_r ( $obj2 );
?>
以上例程会输出:
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 ))
[#1] jason at jewelrysupply dot com [2015-09-24 21:44:39]
@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).
[#2] dennis [2015-07-08 22:45:16]
I had some trouble with cloning an array with objects in another object.
But I found a function on the internet that was very helpful:
function array_clone( array $array ) {
$result = array();
foreach( $array as $key => $val ) {
if( is_array( $val ) ) {
$result[$key] = arrayCopy( $val );
} elseif ( is_object( $val ) ) {
$result[$key] = clone $val;
} else {
$result[$key] = $val;
}
}
return $result;
}
//== my test case:
class Employee
{
public $sName = '';
}
class EmployeesContainer
{
private $arrObjects = array();
public function __clone()
{
//try to comment out the line below, you will see the out differs
$this->arrObjects = array_clone($this->arrObjects);
}
public function add(Employee $objEmployee)
{
$this->arrObjects[] = $objEmployee;
}
public function getInternalArray()
{
return $this->arrObjects;
}
}
$objContainerOriginal = new EmployeesContainer();
$objEmpl1 = new Employee();
$objEmpl1->sName = 'erny';
$objContainerOriginal->add($objEmpl1);
$objEmpl2 = new Employee();
$objEmpl2->sName = 'bert';
$objContainerOriginal->add($objEmpl2);
$objContainerClone = clone $objContainerOriginal;
if ($objContainerClone->getInternalArray()[1] === $objEmpl2)
echo 'SAME object: '.$objContainerClone->getInternalArray()[0]->sName.' <-> '.$objEmpl1->sName;
else
echo 'OTHER object: '.$objContainerClone->getInternalArray()[0]->sName.' <-> '.$objEmpl1->sName;
[#3] stanislav dot eckert at vizson dot de [2014-12-13 12:53:59]
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";
?>
[#4] obaidmaroof at gmail dot com [2014-04-29 14:01:17]
<?php
class A{
public $my_var;
function __construct() { $this->my_var = 'aaa'; }
}
$a = new A;
$b = clone $a;
$b->my_var = 'bbb';
echo($a->my_var); // 'aaa'
echo($b->my_var); // 'bbb'
$c = new A;
$d = $c;
$d->my_var = 'bbb';
echo($c->my_var); // 'bbb'
echo($d->my_var); // 'bbb'
?>
simple example to understand the basic concept behind clone.
[#5] seriously at something dot com [2012-06-06 16:58:08]
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.
[#6] walkman at walkman dot pk [2011-10-13 05:11:57]
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.
[#7] jojor at gmx dot net [2010-06-07 13:47:32]
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>
[#8] jeffrey at whinger dot nl [2010-04-15 05:41:08]
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
?>
[#9] henke at henke37 dot cjb dot net [2010-04-10 03:31:14]
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.
[#10] olivier dot pons at goo dot without dot oo dot mail dot com [2010-03-18 22:14:10]
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.
[#11] emile at webflow dot nl [2010-03-02 01:27:20]
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."
[#12] ben at last dot fm [2009-06-05 10:33:07]
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);
$ref =& $a->p; // note that this CONVERTS $a->p into a reference type!!
var_dump($a);
?>
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);
?>
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
?>
[#13] koyama [2009-03-03 07:28:18]
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 $foo;
// check if we reach this point
echo 'Finished cloning!';
?>
Replacing the __clone() method with the one shown in edit by danbrown at php dot net we run into an infinite loop, and we never get message 'Finished cloning!'.
[#14] cheetah at tanabi dot org [2008-11-18 05:15:27]
Want deep cloning without too much hassle?
<?php
function __clone() {
foreach($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]
[#15] wbcarts at juno dot com [2008-10-02 11:41:03]
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 $platoon[197] . '<br>';
?>
COMMAND ISSUED: Deploy Troops
Soldier[id=1, status=deploying]
Soldier[id=225, status=deploying]
Soldier[id=14, status=deploying]
COMMAND ISSUED: March Forward
Soldier[id=49, status=marching forward]
Soldier[id=165, status=marching forward]
COMMAND ISSUED: Fire!
Soldier[id=250, status=shot fired]
Soldier[id=70, status=shot fired]
COMMAND ISSUED: Retreat!
Soldier[id=28, status=course reversed]
Soldier[id=199, status=course reversed]
[#16] Jim Brown [2008-07-19 15:34:17]
Regarding the generic deep __clone() example provided by david ashe at metabin:
If your object has a variable that stores an array of objects, that particular __clone() example will NOT perform a deep copy on your array of objects.
[#17] alex dot offshore at gmail dot com [2008-05-19 03:23:39]
Remember that in PHP 5 ALL objects are assigned BY REFERENCE.
<?php
function foo($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
[#18] crrodriguez at suse dot de [2008-03-12 21:52:12]
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.
[#19] Hayley Watson [2007-12-17 15:51:49]
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);
[#20] Alexey [2007-02-08 07:18:38]
To implement __clone() method in complex classes I use this simple function:
function clone_($some)
{
return (is_object($some)) ? clone $some : $some;
}
In this way I don't need to care about type of my class properties.
[#21] MakariVerslund at gmail dot com [2007-01-21 16:30:49]
I ran into the same problem of an array of objects inside of an object that I wanted to clone all pointing to the same objects. However, I agreed that serializing the data was not the answer. It was relatively simple, really:
public function __clone() {
foreach ($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.
[#22] jorge dot villalobos at gmail dot com [2005-03-30 15:29:55]
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.