References and Aliases are Different Mechanisms

Author: zkarakaya

Date 14/03/2001

Aliasing and Referencing are completely different mechanisms in PHP.

If you are Java or C++ programmer, you must be careful when using

Objects created on run-time.

Lets see an example;

class MyClass{
var $myData;
var $outManager;
cfunction MyClass($p){
$this->myData=$p;
$this->outManager = new MyOutManager($this);
}
cfunction display(){
$this->outManager->display();
}
}
class MyOutManager{
var $refObj;
cfunction MyOutManager(&$obj){
$this->refObj = &$obj;
}
cfunction display(){
echo $this->refObj->myData;
}
}
$myvar = new MyClass(10);
$myvar->myData = 20;
$myvar->display();
?>

What value be the output of this program code. Many programmer will

say "20", but this is not correct. Output is 10. Why? Because we have

created an instance of MyClass type on the right hand side of assignment

operator, and gave an initial value of 10. In the constructor of MyClass,

we have send the memory location of that newly created instance to another

object of type MyOutManager, and hold this value in $refObj. Now the

reference count for this object is 1, which is $refObf property of

outManager instance. Lets continue the execution. Constructor has finished

its job and returned to assignment operator. PHP4 now creates a new

reference named $myvar to newly created object. Now the reference count

of that object is 2. Be careful that $myvar is not an alies. So that when

you execute the next statement, which assigns a value 20 to its property

named $myData, PHP4 creates a new instance of MyClass type, copies

the contents of old one which is also referenced by its member outManager.

And then changes the contents of myData to 20.

>From now on you will have two different instance of type MyClass.

Our intend was not this. So to correct this problem, use alias

on the object creation statement, that is use;

$myvar = &new MyClass(10);

This will solve the problem. So if you are C++ and Java programmer,

you must be careful in writing PHP codes.

This description does not conflict with the information given in the

PHP 4: Reference

Counting and Aliasing

written by Andi

Gutmans.

Ziya Karakaya