PHP: オブジェクトのクローン作成

Last updated: Fri, 29 Aug 2008

オブジェクトのクローン作成

オブジェクトのコピーを作成する際、そのプロパティも全て二重化することが、常に望ましい動作であるわけではありません。コピーコンストラクタが必要となる例として、 GTKウインドウを表すオブジェクトを有しており、そのオブジェクトがGTKウインドウのリソースを保持している際、コピーを作成する時に、同じプロパティを有するウインドウを作成し、その新しいオブジェクトがその新しいウインドウのリソースを保持するようにしたい場合が考えられます。他の例としては、オブジェクトがそのオブジェクトが使用する他のオブジェクトへのリファレンスを保持しており、親オブジェクトをコピーする際に、そのコピーが独立したオブジェクトのコピーを有するように、そのオブジェクトのインスタンスを新たに作成したい場合が考えられます。

オブジェクトのコピーは、clone キーワード (これは、そのオブジェクトの __clone()メソッドがある場合にこれをコールします)により作成されます。オブジェクトの__clone()メソッドを直接コールすることはできません。

$copy_of_object = clone $object;

オブジェクトのクローンが作成される際、PHP 5 は、そのオブジェクトのプロパティを全てシャローコピーします。他の変数へのリファレンスを保持する全てのプロパティは、リファレンスのままとなります。 __clone() メソッドが定義された場合、新規の作成されたオブジェクトの __clone() メソッドがコールされるため、この中で、プロパティに必要な変更を行うことができます。

例1 オブジェクトのクローン作成


<?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()
    {
        // this->object のコピーを作成します。こうしないと、
        // 同じオブジェクトを指すことになってしまいます。
        $this->object1 = clone $this->object1;
    }
}

$obj = new MyCloneable();

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

$obj2 = clone $obj;


print("元のオブジェクト\n");
print_r($obj);

print("クローンオブジェクト\n");
print_r($obj2);

?>

上の例の出力は以下となります。

元のオブジェクト
MyCloneable Object
(
    [object1] => SubObject Object
        (
            [instance] => 1
        )

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

)
クローンオブジェクト
MyCloneable Object
(
    [object1] => SubObject Object
        (
            [instance] => 3
        )

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

)

オブジェクトの比較

finalキーワード

Last updated: Fri, 29 Aug 2008

add a note User Contributed Notes
オブジェクトのクローン作成

Jim Brown
20-Jul-2008 12:34


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 12:23


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 06:52


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
18-Dec-2007 12:51


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);

david ashe at metabin
02-Dec-2007 07:18


Here is a function to clone all of the objects automatically 

(useful if you use a base class that has this method)



    function __clone(){

        foreach($this as $name => $value){

            if(gettype($value)=='object'){

                $this->$name= clone($this->$name);

            }

        }

    }

tomi at cumulo dot fi
13-Nov-2007 12:57


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 04:43


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 04:18


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 01:30


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
31-Mar-2005 01:29


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.

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オブジェクトの比較

finalキーワード

Last updated: Fri, 29 Aug 2008