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. 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.

例 19-31. 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);

?>

上例将输出：


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
        )

)

add a note User Contributed Notes

Mr.KTO
13-Jun-2007 08:00


In PHP4 the clone keyword isn't defined, if you want a compatible code (using a copy of an object) - use this fix:

$c = (PHP_VERSION < 5) ? $b : clone($b);

It is possible cause php checks function existence just before its calling.

Alexey
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


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.

felix gilcher
22-Sep-2006 02:20


Copying objects with



<?php

public function copy()

{

   $serialized_contents = serialize($this);

   return unserialize($serialized_contents);

}

?>



is a pretty dangerous thing. What if your objects hold references to very large shared objects? They all get copied as well. Serializing objects may break database connections and references. What you probably want to do is implement a proper __clone() method for each of your classes so that each object in your tree gets to decide what subobjects need to be copied and which ones should better remain untouched.



regards



felix

paul at accessdesigns dot org dot uk
03-Aug-2006 08:47


I noticed when trying to clone a series of objects in a loop and and adding them to an array, the objects were still passed by reference, so all objects took the same value as the last object cloned and added to the array.



The solution was to use the copy function as below which truly clones objects.



Here is an example:

<?php

$temp = new $this->_doname;

foreach($rows as $id=>$row) {

        $do = clone $temp;

        $do->load($id);

        $this->_dataobjects[]= clone $do;

}

?>



This results in every object having the same value.

However using:



<?php

$this->_dataobjects[]= $this->copy($do);

?>

with 

<?php

public function copy()

{

   $serialized_contents = serialize($this);

   return unserialize($serialized_contents);

}

?>



Actually produces the expected result.



Paul Bain

olle dot remove_this dot suni at rdnsoftware dot com
16-May-2006 11:49


As jorge dot villalobos at gmail dot com pointed out, the "__clone is NOT an override". 



However, if one needs a true copy of an object (which has real copies of all subobjects too, not references), one can define a class method such as this to the object-to-be-cloned: 



public function copy()

{

    $serialized_contents = serialize($this);

    return unserialize($serialized_contents);

}



The method makes a string representation of the object's contents (including subobjects), which can then be unserialized back to an object. 



The method usage is simple:



$original_object = new MyCloneable(); //can have sub objects

$copied_object = $original_object->clone(); //only makes copies of the values, not references



With serialization, you surely don't have to worry about references, since serialized object is just simple text.

ove at junne dot dk
08-Feb-2006 04:02


Consider this:



function myfunc()

{

    $A = new myobject(); // Create another mysql connection

    $A->method1();         // Runs a query (works)

}



$A = new myobject(); // Create a mysql connection

myfunc();

$A->method1();  // This query fails! Possible because leaving the function has closed the connection.



Conclusion: Declaration of objects within and outside a function WITH THE SAME NAME, will affect each other. Do not relay on the rules of scope.



Take care when using recursive structures!!



It took me a long time to figure out.

markus dot amsler at oribi dot org
20-Jun-2005 01:14


For php4 you can use the clone method from the PEAR PHP_Compat package (http://pear.php.net/package/PHP_Compat).

jorge dot villalobos at gmail dot com
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.

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