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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<sect1 xml:id="language.oop5.cloning" xmlns="http://docbook.org/ns/docbook">
<title>Object Cloning</title>
<para>
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.
</para>
<para>
An object copy is created by using the <literal>clone</literal> keyword
(which calls the object's <link linkend="object.clone">__clone()</link>
method if possible).
</para>
<informalexample>
<programlisting>
<![CDATA[
$copy_of_object = clone $object;
]]>
</programlisting>
</informalexample>
<para>
When an object is cloned, PHP will perform a shallow copy of all of the
object's properties. Any properties that are references to other variables
will remain references.
</para>
<methodsynopsis xml:id="object.clone">
<type>void</type><methodname>__clone</methodname>
<void/>
</methodsynopsis>
<para>
Once the cloning is complete, if a <link linkend="object.clone">__clone()</link> method is defined, then
the newly created object's <link linkend="object.clone">__clone()</link> method will be called, to allow any
necessary properties that need to be changed.
</para>
<example>
<title>Cloning an object</title>
<programlisting role="php">
<![CDATA[
<?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);
?>
]]>
</programlisting>
&example.outputs;
<screen role="php">
<![CDATA[
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
)
)
]]>
</screen>
</example>
<para>
It is possible to access a member of a freshly cloned
object in a single expression:
</para>
<example>
<title>Access member of freshly cloned object</title>
<programlisting role="php">
<![CDATA[
<?php
$dateTime = new DateTime();
echo (clone $dateTime)->format('Y');
?>
]]>
</programlisting>
&example.outputs.similar;
<screen>
<![CDATA[
2016
]]>
</screen>
</example>
</sect1>
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-->
|
