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.

例子 19-29. 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

olle dot remove_this dot suni at rdnsoftware dot com
16-May-2006 07:49


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
09-Feb-2006 12:02


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 09:14


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
31-Mar-2005 07:29


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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