php-doc-en/language/oop5/cloning.xml

<?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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