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PHP : Language Reference : Classes and Objects (PHP 4) : References inside the constructor

References inside the constructor

Creating references within the constructor can lead to confusing results. This tutorial-like section helps you to avoid problems.

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<?php
class Foo {
   function Foo($name) {
       // create a reference inside the global array $globalref
       global $globalref;
       $globalref[] = &$this;
       // set name to passed value
       $this->setName($name);
       // and put it out
       $this->echoName();
   }

   function echoName() {
       echo "<br />", $this->name;
   }

   function setName($name) {
       $this->name = $name;
   }
}
?>

Let us check out if there is a difference between $bar1 which has been created using the copy = operator and $bar2 which has been created using the reference =& operator...

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<?php
$bar1 = new Foo('set in constructor');
$bar1->echoName();
$globalref[0]->echoName();

/* output:
set in constructor
set in constructor
set in constructor */

$bar2 =& new Foo('set in constructor');
$bar2->echoName();
$globalref[1]->echoName();

/* output:
set in constructor
set in constructor
set in constructor */
?>

Apparently there is no difference, but in fact there is a very significant one: $bar1 and $globalref[0] are _NOT_ referenced, they are NOT the same variable. This is because "new" does not return a reference by default, instead it returns a copy.

Note:

There is no performance loss (since PHP 4 and up use reference counting) returning copies instead of references. On the contrary it is most often better to simply work with copies instead of references, because creating references takes some time where creating copies virtually takes no time (unless none of them is a large array or object and one of them gets changed and the other(s) one(s) subsequently, then it would be wise to use references to change them all concurrently).

To prove what is written above let us watch the code below.

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<?php
// now we will change the name. what do you expect?
// you could expect that both $bar1 and $globalref[0] change their names...
$bar1->setName('set from outside');

// as mentioned before this is not the case.
$bar1->echoName();
$globalref[0]->echoName();

/* output:
set from outside
set in constructor */

// let us see what is different with $bar2 and $globalref[1]
$bar2->setName('set from outside');

// luckily they are not only equal, they are the same variable
// thus $bar2->name and $globalref[1]->name are the same too
$bar2->echoName();
$globalref[1]->echoName();

/* output:
set from outside
set from outside */
?>

Another final example, try to understand it.

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<?php
class A {
   function A($i) {
       $this->value = $i;
       // try to figure out why we do not need a reference here
       $this->b = new B($this);
   }

   function createRef() {
       $this->c = new B($this);
   }

   function echoValue() {
       echo "<br />","class ",get_class($this),': ',$this->value;
   }
}


class B {
   function B(&$a) {
       $this->a = &$a;
   }

   function echoValue() {
       echo "<br />","class ",get_class($this),': ',$this->a->value;
   }
}

// try to understand why using a simple copy here would yield
// in an undesired result in the *-marked line
$a =& new A(10);
$a->createRef();

$a->echoValue();
$a->b->echoValue();
$a->c->echoValue();

$a->value = 11;

$a->echoValue();
$a->b->echoValue(); // *
$a->c->echoValue();

?>

The above example will output:

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class A: 10
class B: 10
class B: 10
class A: 11
class B: 11
class B: 11

Code Examples / Notes » language.oop.newref

geronimo
To set properties of a class dynamically:
....
foreach($arr_properties as $key_prop=>$value_prop) {
$temp = &$this->$key_prop;//property reference
$temp ="new value";
}
....

jjh
This is another sample script to demonstrate the use of references on objects.
To test the script first replace all <b r> (note the space!) tags by normal
HTML-linke-brakes! Normal line brakes (what a shame!) cannot be shown inside a
php.net-note!!!
<?
// Class C contains instances of class A and class B.
//
// A should "know" B and B should "know" A.
// In other words: Object A should have a Reference on
// object B and vice versa.
class A {
var $a;
var $a_b; // this links to object B
function A() {
$this->a="set by constructor of A";
}

function getToKnowB(&$b) {
$this->a_b =& $b;
}
}
class B {
var $b;
var $b_a; // this links to object A
function B() {
$this->b="set by constructor of B";
}

function getToKnowA(&$a) {
$this->b_a =& $a;
}
}
class C {
var $myA;
var $myB;

function C() {
// Create Objects A and B
$this->myA = new A;
$this->myB = new B();

// "Introduce" A and B to each other
$this->myA->getToKnowB($this->myB);
$this->myB->getToKnowA($this->myA);

// Prove that A and B are using References (not copies!)
echo "Prove that myA->a equals myB->b_a->a
";
$this->myA->a = "one";
echo "this->myA->a is set to ".$this->myA->a."
";

$this->myB->b_a->a = "two";
echo "this->myB->b_a->a is set to ".$this->myB->b_a->a."
";

echo "this->myA->a: ".$this->myA->a."
";
echo "this->myB->b_a->a: ".$this->myB->b_a->a."
";
echo "
";

echo "Prove that myB->b equals myA->a_b->b
";
$this->myB->b = "one";
echo "this->myB->b is set to ".$this->myB->b."
";

$this->myA->a_b->b = "two";
echo "this->myA->a_b->b is set to ".$this->myA->a_b->b."
";

echo "this->myB->b: ".$this->myB->b."
";
echo "this->myA->a_b->b: ".$this->myA->a_b->b."
";
}
}
$myC = new C;
/*
evaluates to:
Prove that myA->a equals myB->b_a->a
this->myA->a is set to one
this->myB->b_a->a is set to two
this->myA->a: two
this->myB->b_a->a: two
Prove that myB->b equals myA->a_b->b
this->myB->b is set to one
this->myA->a_b->b is set to two
this->myB->b: two
this->myA->a_b->b: two
*/
?>

info
Regarding the use of assignment versus reference operators as they pertain to using classes which register shutdown functions.
Here's an example:
<?php
class testclass {
      var $value;
      function testclass() {
              register_shutdown_function(array(&$this, 'tcshutdown'));
              $this->value = 1;
              print 'Construction value=[' . $this->value ."]\n";
      }
      function setvalue($value) {
              $this->value = $value;
      }
      function tcshutdown() {
              print 'Shutdown value=[' . $this->value ."]\n";
      }
}
$testobj = new testclass();
$testobj->setvalue(2);
?>
Despite changing the value to 2 after the object is constructed, the above code will result in the following output:
Construction value=[1]
Shutdown value=[1]
The reason is new testclass() constructs the object and registers tcshutdown function from that copy of the object. Then assigning it to $testobj makes a copy of the object accessible as $testobj. Altering $testobj values confines those changes to the $testobj copy and doesn't affect the original ("new") copy of the object. When shutdown time comes around and the tcshutdown() function is called, it calls the original ("new") copy of tcshutdown() NOT $testobj's copy. Thus tcshutdown() sees only the original member value that was initialized by the constructor before the object was copied to $testobj.
The way to resolve this is to, again, convert from assignment to a reference with the following alteration:
<?php
$testobj =& new testclass();
?>
With that change, running the code again will result in what we expected:
Construction value=[1]
Shutdown value=[2]
This of course allows the tcshutdown() function to perform shutdown operations based on parameters which were altered subsequent to the construction of the object.

blueshadow822
Original : rick_php_net at roundeye dot net
Date : 01-Oct-2002 05:37
Modify : bitmore.co.kr
class StaticList {
var $list = 0;
function StaticList($name = '') {
global $master;
if (!isset($master)) {
print "creating new list $name<Br>\n";
//$master = $this;
$master =& $this;
$master->list = array();
}else {
print "creating new list $name... using global list
\n";
}
//$this->list =& $master->list;
//$this->list = $master->list;
return ($this);
}
function show() {
if (is_array($this->list)) {
if (count($this->list)) {
reset($this->list);
while (list($key, $value) = each($this->list)) {
print "list[$key] => $value
\n";
}
}else {
print "list empty.
\n";
}
}else {
print "list is not a list.
\n";
}
}
function append($val) {
if (!is_array($this->list)) {
print "forced to create list...
\n";
$this->list = array();
}else {
print "appending $val to list
\n";
$this->list[] = $val;
}
}
}
$a = new StaticList("a");
$a->append(1);
$a->append(2);
$b = new StaticList("b");
$b->append(3);
$b->append(4);
$a->append(5);
$b->append(6);
print "Contents of a:
\n";
$a->show();
print "Contents of a:
\n";
$b->show();
this Show Printing
creating new list a
appending 1 to list
appending 2 to list
creating new list b
appending 3 to list
appending 4 to list
appending 5 to list
appending 6 to list
Contents of a:
list[0] => 1
list[1] => 2
list[2] => 5
Contents of a:
list[0] => 3
list[1] => 4
list[2] => 6

aib
Note that the "new" operator no longer returns a copy of the constructed object, but the object itself. This behavior may be as of PHP 5.x.
This means that
$foo = new class();
and
$foo =& new class();
are equivalent now, and you don't have to worry about the problems stated on this page.
Consider the following example:
<?php
class foo
{
var $num;
function foo()
{
global $globalref;
$globalref[] = &$this;
}
}
$bar = new foo();
$bar->num = 1;
$globalref[0]->num = 2;
echo "\$bar->num = ".$bar->num."\n";
echo "\$globalref[0]->num = ".$globalref[0]->num."\n";
?>
On PHP 5.0.1, it will print:
$bar->num = 2
$globalref[0]->num = 2
Whereas on PHP 4.4.0, it would print:
$bar->num = 1
$globalref[0]->num = 2
unless you assigned $bar to a new foo using a reference.

rick_php_net
I was attempting to find a way to share data structures between objects and ideally wanted to be able to have a single shared object that would be returned as a reference from the constructor -- without having the caller use a different syntax.
I couldn't get the effect I wanted, but was able to share the internal data structures (which is where the bulk of the storage was in my application).  This is an example class I constructed (requires php >= 4.0.4) which illustrates the idea:
   class StaticList
   {
       var $list = 0;
       function StaticList($name = '')
       {
           global $master;
           if (!isset($master))
           {
               print "creating new list $name<Br>\n";
               $master = $this;
               $master->list = array();
           }
           else
           {
               print "creating new list $name... using global list
\n";
           }
           $this->list =& $master->list;
           return ($this);
       }
       function show()
       {
           if (is_array($this->list))
           {
               if (count($this->list))
               {
                   reset($this->list);
                   while (list($key, $value) = each($this->list))
                   {
                       print "list[$key] => $value
\n";
                   }
               }
               else
               {
                   print "list empty.
\n";
               }
           }
           else
           {
               print "list is not a list.
\n";
           }
       }
       function append($val)
       {
           if (!is_array($this->list))
           {
               print "forced to create list...
\n";
               $this->list = array();
           }
           else
           {
               print "appending $val to list
\n";
               $this->list[] = $val;
           }
       }
   }
   $a = new StaticList("a");
   $a->append(1);
   $a->append(2);
   $b = new StaticList("b");
   $b->append(3);
   $b->append(4);
   $a->append(5);
   $b->append(6);
   print "Contents of a:
\n";
   $a->show();
   print "Contents of a:
\n";
   $b->show();

kyong
Here's a reminder to those of you are going to use
PHP 5.0.0b3. You don't need to add & before $this any more
to get that object reference.

mentepollo phpero
Continuing aib's note (31-May-2006 11:20) on the new behavior of PHP 5.x:
Not only the "new" operator now returns a reference of the object (instead of a copy of the object as in PHP4), but more importantlly: now the assignment operator when acting on objects assigns by reference not by value.
This means operator =& is equal to operator = , but only when acting on objects. If you want to copy (clone) an object use: $B = clone $A;
Java creators got it right from the begining :)

denis
Construction Methods
In case of PHP versions earlier 5.x (or both) you have to consider, that an instance of a class is not fully allocated while the construction method(s) aren't finished.
For example, this solution could fail:
class Child
{
  var $c;
 
  function Child(&$parent)
  {
     $this->c = &$parent;
  }
}
class Test
{
  var $a;
  var $var;
 
  function Test ()
  {
     $this->var = 123;
     $this->a = &new Child($this);
  }
}
$b = &new Test();
$b->a->c->var = 456;
echo $b->var;
[outputs: 123]
In order of really allocated instance references, append referenced objects after (and not during) the construction.

php dot net
As a bit of a newbie to objects, I found the final example very hard to work out at first.  I'd like to try and explain more clearly:
It all centres around what the $this variable actually is.  The answer is, it's an object.  It's the object that the $this variable is currently inside.
So let's say we call the first instance of class A that is created by my script "Instance1".  If I say,
<?php
class A {
function test() {
$foo = $this;
}
}
$myclass = new A();
?>
... then $this is the object Instance1.  Note that it is *NOT* the same as the object $myclass, as $myclass is simply a *COPY* of the object Instance1.  This means that if you create an object using the copy (=) operator, there is an instance of the object (in this case, Instance1) hanging around unreferenced.
Whereas if we use the reference (=&) operator to create an object, the first instance (Instance1) is directly referenced by the variable specified, and no copy is created.  eg.
<?php
class A {
function test() {
$foo = $this;
}
}

$myclass =& new A();
?>
... this assigns the variable $myclass directly to Instance1, with no need for Instance1 to be copied.  There is only one instance of the class created, and it is assigned to $myclass.
In brief, if we use <--> to indicate a reference between a variable and an instance of a class:
Statement               | Results in...
$myclass = new A();     | Instance1 Instance2<-->$myclass      (Instance2 is a copy)
$myclass =& new A();    | Instance1<-->$myclass
All seems simple when you see the diagram, eh?  :-)  Of course, you may be asking the question, "Why doesn't the '=' operator just make $myclass reference first object Instance1?  Why would you ever need Instance1 to be copied, surely all you'd ever want or need is a direct reference to the newly created object Instance1?"  Well, that's the same question I and others are asking.  Hopefully this behaviour will change in a later version of PHP.
Hope that helps.

phpnotes
An earler note asked why the equal operator doesn't just make a reference, assuming that a direct reference would always be desired.
The answer is because a direct reference is *not* always desired.
When you instantiate a class, you are creating an object that has its own copy of all the class variables, access to the class methods, etc.
Now say you've created an instance of a class "MyClass" called "myObjectA" and you've done some processing to it, set some variables or whatever. Now say you want to create a duplicate of the object to do some changes, but you want to *keep the original.*
That is what the assignment (=) operator is for.
If you'd made a reference (&=), then any changes you make to the duplicate are also reflected in the original. This is undesireable.
So there are circumstances where the assignment (=) operator is required, and there are circumstances where the reference (&=) operator is required.
It just depends on what you're doing.
Hope this helps.

rpav
Ah HA! I just found an easy workaround with a bit of playing.  This works:
--- cut ---
$OBJECTS = array();
class Object {
   function &Object() {
       global $OBJECTS;
       $OBJECTS[] = &$this;
   }
   function _init() {
   }
}
class Derived {
   function &Derived() {
       Object::Object();
   }
}
function blah() {
   global $FOO;
   $x =& new Derived;
   $GLOBALS['FOO'] = &$x;
   $x->foo = 42;
   display("x", $x);
}
function display($text, &$o) {
   echo("($text) =>" . serialize($o));
}
blah();
display("GLOBALS[FOO]", $GLOBALS['FOO']);
display("OBJECTS", $OBJECTS);
--- cut ---
As you can see, simply adding & before the constructor to make it return a reference does the trick.  The bit about "returning a copy by default" lead me to try this trick, and it seems to work.  Have fun!
-rjp

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Scope Resolution Operator (::)
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Serializing objects - objects in sessions
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