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 the developer asks to create a new copy of an object, PHP 5 will check if a __clone() method has been defined or not. If not, it will call a default __clone() which will copy all of the object's properties. If a __clone() method is defined, then it will be responsible to set the necessary properties in the created object. For convenience, the engine will supply a function that imports all of the properties from the source object, so that they can start with a by-value replica of the source object, and only override properties that need to be changed.

例子 14-5. Cloning an object


<?php
class MyCloneable {
   static $id = 0;

   function MyCloneable() {
       $this->id = self::$id++;
   }

   function __clone() {
       $this->address = "New York";
       $this->id = self::$id++;
   }
}

$obj = new MyCloneable();

$obj->name = "Hello";
$obj->address = "Tel-Aviv";

print $obj->id . "\n";

$obj_cloned = clone $obj;

print $obj_cloned->id . "\n";
print $obj_cloned->name . "\n";
print $obj_cloned->address . "\n";
?>