Notes on reference:
A reference is not a pointer. However, an object handle IS a pointer. Example:
<?php
class Foo {
private static $used;
private $id;
public function __construct() {
$id = $used++;
}
public function __clone() {
$id = $used++;
}
}
$a = new Foo; // $a is a pointer pointing to Foo object 0
$b = $a; // $b is a pointer pointing to Foo object 0, however, $b is a copy of $a
$c = &$a; // $c and $a are now references of a pointer pointing to Foo object 0
$a = new Foo; // $a and $c are now references of a pointer pointing to Foo object 1, $b is still a pointer pointing to Foo object 0
unset($a); // A reference with reference count 1 is automatically converted back to a value. Now $c is a pointer to Foo object 1
$a = &$b; // $a and $b are now references of a pointer pointing to Foo object 0
$a = NULL; // $a and $b now become a reference to NULL. Foo object 0 can be garbage collected now
unset($b); // $b no longer exists and $a is now NULL
$a = clone $c; // $a is now a pointer to Foo object 2, $c remains a pointer to Foo object 1
unset($c); // Foo object 1 can be garbage collected now.
$c = $a; // $c and $a are pointers pointing to Foo object 2
unset($a); // Foo object 2 is still pointed by $c
$a = &$c; // Foo object 2 has 1 pointers pointing to it only, that pointer has 2 references: $a and $c;
const ABC = TRUE;
if(ABC) {
$a = NULL; // Foo object 2 can be garbage collected now because $a and $c are now a reference to the same NULL value
} else {
unset($a); // Foo object 2 is still pointed to $c
}
Objects and references
One of the key-points of PHP 5 OOP that is often mentioned is that "objects are passed by references by default". This is not completely true. This section rectifies that general thought using some examples.
A PHP reference is an alias, which allows two different variables to write to the same value. As of PHP 5, an object variable doesn't contain the object itself as value anymore. It only contains an object identifier which allows object accessors to find the actual object. When an object is sent by argument, returned or assigned to another variable, the different variables are not aliases: they hold a copy of the identifier, which points to the same object.
Example #1 References and Objects
<?php
class A {
public $foo = 1;
}
$a = new A;
$b = $a; // $a and $b are copies of the same identifier
// ($a) = ($b) = <id>
$b->foo = 2;
echo $a->foo."\n";
$c = new A;
$d = &$c; // $c and $d are references
// ($c,$d) = <id>
$d->foo = 2;
echo $c->foo."\n";
$e = new A;
function foo($obj) {
// ($obj) = ($e) = <id>
$obj->foo = 2;
}
foo($e);
echo $e->foo."\n";
?>
The above example will output:
2 2 2
add a note
User Contributed Notes 13 notes
miklcct at gmail dot com ¶
3 years ago
mjung at poczta dot onet dot pl ¶
3 years ago
Ultimate explanation to object references:
NOTE: wording 'points to' could be easily replaced with 'refers ' and is used loosly.
<?php
$a1 = new A(1); // $a1 == handle1-1 to A(1)
$a2 = $a1; // $a2 == handle1-2 to A(1) - assigned by value (copy)
$a3 = &$a1; // $a3 points to $a1 (handle1-1)
$a3 = null; // makes $a1==null, $a3 (still) points to $a1, $a2 == handle1-2 (same object instance A(1))
$a2 = null; // makes $a2 == null
$a1 = new A(2); //makes $a1 == handle2-1 to new object and $a3 (still) points to $a1 => handle2-1 (new object), so value of $a1 and $a3 is the new object and $a2 == null
//By reference:
$a4 = &new A(4); //$a4 points to handle4-1 to A(4)
$a5 = $a4; // $a5 == handle4-2 to A(4) (copy)
$a6 = &$a4; //$a6 points to (handle4-1), not to $a4 (reference to reference references the referenced object handle4-1 not the reference itself)
$a4 = &new A(40); // $a4 points to handle40-1, $a5 == handle4-2 and $a6 still points to handle4-1 to A(4)
$a6 = null; // sets handle4-1 to null; $a5 == handle4-2 = A(4); $a4 points to handle40-1; $a6 points to null
$a6 =&$a4; // $a6 points to handle40-1
$a7 = &$a6; //$a7 points to handle40-1
$a8 = &$a7; //$a8 points to handle40-1
$a5 = $a7; //$a5 == handle40-2 (copy)
$a6 = null; //makes handle40-1 null, all variables pointing to (hanlde40-1 ==null) are null, except ($a5 == handle40-2 = A(40))
?>
Hope this helps.
Rob Marscher ¶
2 years ago
Here's an example I created that helped me understand the difference between passing objects by reference and by value in php 5.
<?php
class A {
public $foo = 'empty';
}
class B {
public $foo = 'empty';
public $bar = 'hello';
}
function normalAssignment($obj) {
$obj->foo = 'changed';
$obj = new B;
}
function referenceAssignment(&$obj) {
$obj->foo = 'changed';
$obj = new B;
}
$a = new A;
normalAssignment($a);
echo get_class($a), "\n";
echo "foo = {$a->foo}\n";
referenceAssignment($a);
echo get_class($a), "\n";
echo "foo = {$a->foo}\n";
echo "bar = {$a->bar}\n";
/*
prints:
A
foo = changed
B
foo = empty
bar = hello
*/
?>
Aaron Bond ¶
3 years ago
I've bumped into a behavior that helped clarify the difference between objects and identifiers for me.
When we hand off an object variable, we get an identifier to that object's value. This means that if I were to mutate the object from a passed variable, ALL variables originating from that instance of the object will change.
HOWEVER, if I set that object variable to new instance, it replaces the identifier itself with a new identifier and leaves the old instance in tact.
Take the following example:
<?php
class A {
public $foo = 1;
}
class B {
public function foo(A $bar)
{
$bar->foo = 42;
}
public function bar(A $bar)
{
$bar = new A;
}
}
$f = new A;
$g = new B;
echo $f->foo . "\n";
$g->foo($f);
echo $f->foo . "\n";
$g->bar($f);
echo $f->foo . "\n";
?>
If object variables were always references, we'd expect the following output:
1
42
1
However, we get:
1
42
42
The reason for this is simple. In the bar function of the B class, we replace the identifier you passed in, which identified the same instance of the A class as your $f variable, with a brand new A class identifier. Creating a new instance of A doesn't mutate $f because $f wasn't passed as a reference.
To get the reference behavior, one would have to enter the following for class B:
<?php
class B {
public function foo(A $bar)
{
$bar->foo = 42;
}
public function bar(A &$bar)
{
$bar = new A;
}
}
?>
The foo function doesn't require a reference, because it is MUTATING an object instance that $bar identifies. But bar will be REPLACING the object instance. If only an identifier is passed, the variable identifier will be overwritten but the object instance will be left in place.
Ivan Bertona ¶
4 years ago
A point that in my opinion is not stressed enough in the manual page is that in PHP5, passing an object as an argument of a function call with no use of the & operator means passing BY VALUE an unique identifier for that object (intended as instance of a class), which will be stored in another variable that has function scope.
This behaviour is the same used in Java, where indeed there is no notion of passing arguments by reference. On the other hand, in PHP you can pass a value by reference (in PHP we refer to references as "aliases"), and this poses a threat if you are not aware of what you are really doing. Please consider these two classes:
<?php
class A
{
function __toString() {
return "Class A";
}
}
class B
{
function __toString() {
return "Class B";
}
}
?>
In the first test case we make two objects out of the classes A and B, then swap the variables using a temp one and the normal assignment operator (=).
<?php
$a = new A();
$b = new B();
$temp = $a;
$a = $b;
$b = $temp;
print('$a: ' . $a . "\n");
print('$b: ' . $b . "\n");
?>
As expected the script will output:
$a: Class B
$b: Class A
Now consider the following snippet. It is similar to the former but the assignment $a = &$b makes $a an ALIAS of $b.
<?php
$a = new A();
$b = new B();
$temp = $a;
$a = &$b;
$b = $temp;
print('$a: ' . $a . "\n");
print('$b: ' . $b . "\n");
?>
This script will output:
$a: Class A
$b: Class A
That is, modifying $b reflects the same assignment on $a... The two variables end pointing to the same object, and the other one is lost. To sum up is a good practice NOT using aliasing when handling PHP5 objects, unless your are really really sure of what you are doing.
kristof at viewranger dot com ¶
11 months ago
I hope this clarifies references a bit more:
<?php
class A {
public $foo = 1;
}
$a = new A;
$b = $a;
$a->foo = 2;
$a = NULL;
echo $b->foo."\n"; // 2
$c = new A;
$d = &$c;
$c->foo = 2;
$c = NULL;
echo $d->foo."\n"; // Notice: Trying to get property of non-object...
?>
Anonymous ¶
2 years ago
There seems to be some confusion here. The distinction between pointers and references is not particularly helpful.
The behavior in some of the "comprehensive" examples already posted can be explained in simpler unifying terms. Hayley's code, for example, is doing EXACTLY what you should expect it should. (Using >= 5.3)
First principle:
A pointer stores a memory address to access an object. Any time an object is assigned, a pointer is generated. (I haven't delved TOO deeply into the Zend engine yet, but as far as I can see, this applies)
2nd principle, and source of the most confusion:
Passing a variable to a function is done by default as a value pass, ie, you are working with a copy. "But objects are passed by reference!" A common misconception both here and in the Java world. I never said a copy OF WHAT. The default passing is done by value. Always. WHAT is being copied and passed, however, is the pointer. When using the "->", you will of course be accessing the same internals as the original variable in the caller function. Just using "=" will only play with copies.
3rd principle:
"&" automatically and permanently sets another variable name/pointer to the same memory address as something else until you decouple them. It is correct to use the term "alias" here. Think of it as joining two pointers at the hip until forcibly separated with "unset()". This functionality exists both in the same scope and when an argument is passed to a function. Often the passed argument is called a "reference," due to certain distinctions between "passing by value" and "passing by reference" that were clearer in C and C++.
Just remember: pointers to objects, not objects themselves, are passed to functions. These pointers are COPIES of the original unless you use "&" in your parameter list to actually pass the originals. Only when you dig into the internals of an object will the originals change.
Example:
<?php
//The two are meant to be the same
$a = "Clark Kent"; //a==Clark Kent
$b = &$a; //The two will now share the same fate.
$b="Superman"; // $a=="Superman" too.
echo $a;
echo $a="Clark Kent"; // $b=="Clark Kent" too.
unset($b); // $b divorced from $a
$b="Bizarro";
echo $a; // $a=="Clark Kent" still, since $b is a free agent pointer now.
//The two are NOT meant to be the same.
$c="King";
$d="Pretender to the Throne";
echo $c."\n"; // $c=="King"
echo $d."\n"; // $d=="Pretender to the Throne"
swapByValue($c, $d);
echo $c."\n"; // $c=="King"
echo $d."\n"; // $d=="Pretender to the Throne"
swapByRef($c, $d);
echo $c."\n"; // $c=="Pretender to the Throne"
echo $d."\n"; // $d=="King"
function swapByValue($x, $y){
$temp=$x;
$x=$y;
$y=$temp;
//All this beautiful work will disappear
//because it was done on COPIES of pointers.
//The originals pointers still point as they did.
}
function swapByRef(&$x, &$y){
$temp=$x;
$x=$y;
$y=$temp;
//Note the parameter list: now we switched 'em REAL good.
}
?>
Jon Whitener ¶
9 months ago
The use of clone may get you the behavior you expect when passing an object to a function, as shown below using DateTime objects as examples.
<?php
date_default_timezone_set( "America/Detroit" );
$a = new DateTime;
echo "a = " . $a->format('Y-m-j') . "\n";
// This might not give what you expect...
$b = upDate( $a ); // a and b both updated
echo "a = " . $a->format('Y-m-j') . ", b = " . $b->format('Y-m-j') . "\n";
$a->modify( "+ 1 day" ); // a and b both modified
echo "a = " . $a->format('Y-m-j') . ", b = " . $b->format('Y-m-j') . "\n";
// This might be what you want...
$c = upDateClone( $a ); // only c updated, a left alone
echo "a = " . $a->format('Y-m-j') . ", c = " . $c->format('Y-m-j') . "\n";
function upDate( $datetime ) {
$datetime->modify( "+ 1 day" );
return $datetime;
}
function upDateClone( $datetime ) {
$dt = clone $datetime;
$dt->modify( "+ 1 day" );
return $dt;
}
?>
The above would output something like:
a = 2012-08-15
a = 2012-08-16, b = 2012-08-16
a = 2012-08-17, b = 2012-08-17
a = 2012-08-17, c = 2012-08-18
Hayley Watson ¶
3 years ago
Using &$this can result in some weird and counter-intuitive behaviour - it starts lying to you.
<?php
class Bar
{
public $prop = 42;
}
class Foo
{
public $prop = 17;
function boom()
{
$bar = &$this;
echo "\$bar is an alias of \$this, a Foo.\n";
echo '$this is a ', get_class($this), '; $bar is a ', get_class($bar), "\n";
echo "Are they the same object? ", ($bar === $this ? "Yes\n" : "No\n");
echo "Are they equal? ", ($bar === $this ? "Yes\n" : "No\n");
echo '$this says its prop value is ';
echo $this->prop;
echo ' and $bar says it is ';
echo $bar->prop;
echo "\n";
echo "\n";
$bar = new Bar;
echo "\$bar has been made into a new Bar.\n";
echo '$this is a ', get_class($this), '; $bar is a ', get_class($bar), "\n";
echo "Are they the same object? ", ($bar === $this ? "Yes\n" : "No\n");
echo "Are they equal? ", ($bar === $this ? "Yes\n" : "No\n");
echo '$this says its prop value is ';
echo $this->prop;
echo ' and $bar says it is ';
echo $bar->prop;
echo "\n";
}
}
$t = new Foo;
$t->boom();
?>
In the above $this claims to be a Bar (in fact it claims to be the very same object that $bar is), while still having all the properties and methods of a Foo.
Fortunately it doesn't persist beyond the method where you committed the faux pas.
<?php
echo get_class($t), "\t", $t->prop;
?>
Anonymous ¶
1 year ago
this example could help:
<?php
class A {
public $testA = 1;
}
class B {
public $testB = "class B";
}
$a = new A;
$b = $a;
$b->testA = 2;
$c = new B;
$a = $c;
$a->testB = "Changed Class B";
echo "<br/> object a: "; var_dump($a);
echo "<br/> object b: "; var_dump($b);
echo "<br/> object c: "; var_dump($c);
// by reference
$aa = new A;
$bb = &$aa;
$bb->testA = 2;
$cc = new B;
$aa = $cc;
$aa->testB = "Changed Class B";
echo "<br/> object aa: "; var_dump($aa);
echo "<br/> object bb: "; var_dump($bb);
echo "<br/> object cc: "; var_dump($cc);
?>
lazybones_senior ¶
4 years ago
WHOA... KEEP IT SIMPLE!
In regards to secure_admin's note: You've used OOP to simplify PHP's ability to create and use object references. Now use PHP's static keyword to simplify your OOP.
<?php
class DataModelControl {
protected static $data = 256; // default value;
protected $name;
public function __construct($dmcName) {
$this->name = $dmcName;
}
public static function setData($dmcData) {
if(is_numeric($dmcData)) {
self::$data = $dmcData;
}
}
public function __toString() {
return "DataModelControl [name=$this->name, data=" . self::$data . "]";
}
}
# create several instances of DataModelControl...
$dmc1 = new DataModelControl('dmc1');
$dmc2 = new DataModelControl('dmc2');
$dmc3 = new DataModelControl('dmc3');
echo $dmc1 . '<br>';
echo $dmc2 . '<br>';
echo $dmc3 . '<br><br>';
# To change data, use any DataModelControl object...
$dmc2->setData(512);
# Or, call setData() directly from the class...
DataModelControl::setData(1024);
echo $dmc1 . '<br>';
echo $dmc2 . '<br>';
echo $dmc3 . '<br><br>';
?>
DataModelControl [name=dmc1, data=256]
DataModelControl [name=dmc2, data=256]
DataModelControl [name=dmc3, data=256]
DataModelControl [name=dmc1, data=1024]
DataModelControl [name=dmc2, data=1024]
DataModelControl [name=dmc3, data=1024]
... even better! Now, PHP creates one copy of $data, that is shared amongst all DataModelControl objects.
wbcarts at juno dot com ¶
4 years ago
A BIT DILUTED... but it's alright!
In the PHP example above, the function foo($obj), will actually create a $foo property to "any object" passed to it - which brings some confusion to me:
$obj = new stdClass();
foo($obj); // tags on a $foo property to the object
// why is this method here?
Furthermore, in OOP, it is not a good idea for "global functions" to operate on an object's properties... and it is not a good idea for your class objects to let them. To illustrate the point, the example should be:
<?php
class A {
protected $foo = 1;
public function getFoo() {
return $this->foo;
}
public function setFoo($val) {
if($val > 0 && $val < 10) {
$this->foo = $val;
}
}
public function __toString() {
return "A [foo=$this->foo]";
}
}
$a = new A();
$b = $a; // $a and $b are copies of the same identifier
// ($a) = ($b) = <id>
$b->setFoo(2);
echo $a->getFoo() . '<br>';
$c = new A();
$d = &$c; // $c and $d are references
// ($c,$d) = <id>
$d->setFoo(2);
echo $c . '<br>';
$e = new A();
$e->setFoo(16); // will be ignored
echo $e;
?>
- - -
2
A [foo=2]
A [foo=1]
- - -
Because the global function foo() has been deleted, class A is more defined, robust and will handle all foo operations... and only for objects of type A. I can now take it for granted and see clearly that your are talking about "A" objects and their references. But it still reminds me too much of cloning and object comparisons, which to me borders on machine-like programming and not object-oriented programming, which is a totally different way to think.
secure_admin ¶
4 years ago
USE OOP to ACCESS OBJECT REFERENCES
The PHP language itself offers a slew of nifty operators that can copy, clone, and alias objects and references in many ways. But that kind of syntax looks rather fearsome. Here, I use OOP to get the same results, but with cleaner and more practical code. Below, one DataModel object is instantiated so that many instances of DataControl can use and alter it. Regardless of how PHP works, the OOP styled setup keeps all DataControl instances "on the same page" because they are all looking at the "same model" - which this code clearly shows.
<?php
class DataModel {
protected $name, $data;
public function __construct($dmName, $dmData) {
$this->name = $dmName;
$this->setData($dmData);
}
public function setData($dmData) {
if(is_numeric($dmData)) {
$this->data = $dmData;
}
}
public function __toString() {
return "DataModel [name=$this->name, data=$this->data]";
}
}
class DataControl {
protected $name, $model;
public function __construct($dcName, $dcModel) {
$this->name = $dcName;
$this->model = $dcModel;
}
public function setData($dmData) {
$this->model->setData($dmData);
}
public function __toString() {
return "DataController [name=$this->name, model=" . $this->model->__toString() . "]";
}
}
# create one instance of DataModel...
$model = new DataModel('dm1', 128);
echo $model . '<br><br>';
# create several instances of DataControl, passing $model to each one...
$dc1 = new DataControl('dc1', $model);
$dc2 = new DataControl('dc2', $model);
$dc3 = new DataControl('dc3', $model);
echo $dc1 . '<br>';
echo $dc2 . '<br>';
echo $dc3 . '<br><br>';
# To change data, use any $dataControl->setData()...
$dc3->setData(512);
echo $dc1 . '<br>';
echo $dc2 . '<br>';
echo $dc3 . '<br><br>';
?>
* * * output * * *
DataModel [name=dm1, data=128]
DataController [name=dc1, model=DataModel [name=dm1, data=128]]
DataController [name=dc2, model=DataModel [name=dm1, data=128]]
DataController [name=dc3, model=DataModel [name=dm1, data=128]]
DataController [name=dc1, model=DataModel [name=dm1, data=512]]
DataController [name=dc2, model=DataModel [name=dm1, data=512]]
DataController [name=dc3, model=DataModel [name=dm1, data=512]]