Object references, assignment and as function arguments

As of PHP5, objects are treated REFERENCE ! (same behaviour as C++ and Java)

1) assignment from an objects to another copies the reference, not the content !!

What is a reference ? it's a variabile/object that ONLY contains the pointer to the effective object in the memory. As I said in a previous post, the assigment instruction copy ONLY the value (pointer) of the object-pointer, it doesn't duplicate the content (members) of the class !

Let's demonstrate with an example, using the unset() function

class A { public $foo = 1; }

$a = new A;

$aRef = $a;

unset($aRef ); //delete ONLY the object-pointer !

print $a->foo; //print 1. This is the proof that $aRef is only a pointer

$aRef = $a;

unset($aRef ->foo); //through the object-pointer $c, it delete the real content !!

print $a->foo; //Notice: Undefined property !! This is the proof that $aRef is not a copy !!

2) objects are passed to functions by REFERENCE !

class A { public $foo = 1; }

$a = new A;

function foo($obj) {

$obj->foo = 2;

unset($obj); // delete local reference (that is a copy), no problem !

}

foo($a);

echo $a->foo; //2 ! this is the proof that the argument is passed as reference (the function doesn't return any value !) and not as a copy (the value outside the function is changed!)

Note 1: arrays are passed by copy, unless the function declare the argument as reference with "&"

function arrayDeleteFirst(&$ar) { unset($ar[0]); }

Note2: references work with variables as well

$a = 1;

$b = &$a;

$b = 2;

unset($b); //no problem. It deletes the reference, not the variabile pointed !

print $a; //2

PHP5: object references, object cloning and __clone()

Let's make a simple class with a value (default=1), set and get methods

class Obj
{

private $var=1;
public function setVal($var) { $this->var = $var; }
public function getVal() { return $this->var; }

}

Following, another class that includes the previous one (Or better, it includes a reference to the previous class, and the object will be instantiated by the constructor)

class A
{

private $obj; //reference to the previous class
private $i=1; //internal variabile
public function __construct() { $this->obj = new Obj(); }

//set the private properties and the val of the internal object !
public function setAll($v) { $this->obj->setVal($v); $this->i =$v; }
public function getObj() { return $this->obj->getVal(); }
public function getI() { return $this->i; }
function __clone()
{
$this->obj = clone $this->obj;
}

}

OBJECT ASSIGN (IT'S NOT A COPY)

Now we instantiate a class and assign its reference to other variables

$a = new A();
$aRef = &$a; // REFERENCE
$aCopy = $a; // REFERENCE as well, the same as before!!!

$a->setAll(2);

print $aRef-> getObj(); //2
print $aRef->getI();//2
print $aCopy-> getObj();//2
print $aCopy->getI();//2

CLONING

to copy all the elements of an object to another reference/variabile, you must clone it, using "clone" keyword.

$a = new A();
$aClone = clone $a; //CLONE
$a->setAll(2); //the cloned object ($aClone) won't be modified !

print $aClone->getI(); // "1"
print $aClone->getObj(); //"1"

thanks to the overriding __clone() method,

otherwise "2"