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<References inside the constructorClasses and Objects (PHP 5)>
Last updated: Thu, 19 May 2005

Comparing objects

In PHP 4, objects are compared in a very simple manner, namely: Two object instances are equal if they have the same attributes and values, and are instances of the same class. Similar rules are applied when comparing two objects using the identity operator (===).

If we were to execute the code in the example below:

Example 18-1. Example of object comparison in PHP 4

<?php
function bool2str($bool) {
   if ($bool === false) {
           return 'FALSE';
   } else {
           return 'TRUE';
   }
}

function compareObjects(&$o1, &$o2) {
   echo 'o1 == o2 : '.bool2str($o1 == $o2)."\n";
   echo 'o1 != o2 : '.bool2str($o1 != $o2)."\n";
   echo 'o1 === o2 : '.bool2str($o1 === $o2)."\n";
   echo 'o1 !== o2 : '.bool2str($o1 !== $o2)."\n";
}

class Flag {
   var $flag;

   function Flag($flag=true) {
           $this->flag = $flag;
   }
}

class SwitchableFlag extends Flag {

   function turnOn() {
       $this->flag = true;
   }

   function turnOff() {
       $this->flag = false;
   }
}

$o = new Flag();
$p = new Flag(false);
$q = new Flag();

$r = new SwitchableFlag();

echo "Compare instances created with the same parameters\n";
compareObjects($o, $q);

echo "\nCompare instances created with different parameters\n";
compareObjects($o, $p);

echo "\nCompare an instance of a parent class with one from a subclass\n";
compareObjects($o, $r);
?>

The above example will output:

Compare instances created with the same parameters
o1 == o2 : TRUE
o1 != o2 : FALSE
o1 === o2 : TRUE
o1 !== o2 : FALSE

Compare instances created with different parameters
o1 == o2 : FALSE
o1 != o2 : TRUE
o1 === o2 : FALSE
o1 !== o2 : TRUE

Compare an instance of a parent class with one from a subclass
o1 == o2 : FALSE
o1 != o2 : TRUE
o1 === o2 : FALSE
o1 !== o2 : TRUE
Which is the output we will expect to obtain given the comparison rules above. Only instances with the same values for their attributes and from the same class are considered equal and identical.

Even in the cases where we have object composition, the same comparison rules apply. In the example below we create a container class that stores an associative array of Flag objects.

Example 18-2. Compound object comparisons in PHP 4

<?php
class FlagSet {
   var $set;

   function FlagSet($flagArr = array()) {
       $this->set = $flagArr;
   }

   function addFlag($name, $flag) {
       $this->set[$name] = $flag;
   }

   function removeFlag($name) {
       if (array_key_exists($name, $this->set)) {
           unset($this->set[$name]);
       }
   }
}


$u = new FlagSet();
$u->addFlag('flag1', $o);
$u->addFlag('flag2', $p);
$v = new FlagSet(array('flag1'=>$q, 'flag2'=>$p));
$w = new FlagSet(array('flag1'=>$q));

echo "\nComposite objects u(o,p) and v(q,p)\n";
compareObjects($u, $v);

echo "\nu(o,p) and w(q)\n";
compareObjects($u, $w);
?>

The above example will output:

Composite objects u(o,p) and v(q,p)
o1 == o2 : TRUE
o1 != o2 : FALSE
o1 === o2 : TRUE
o1 !== o2 : FALSE

u(o,p) and w(q)
o1 == o2 : FALSE
o1 != o2 : TRUE
o1 === o2 : FALSE
o1 !== o2 : TRUE



User Contributed Notes
Comparing objects
pferlet at aston dot fr
29-Apr-2005 08:24
Using globals isn't really well... you can use pattern singleton to verify this.
jazfresh at hotmail dot com
09-Apr-2004 12:44
An addendum to the post below:

If you are comparing two objects and you know that they will be of the same type (you're just not sure if they refer to the same object or not), then there is an easier and faster way to do the comparison, and this also avoids the infinite recursion problem with circular references described in the post below.

In the constructor of your object, set a "serial" attribute from a global variable that is incremented whenever a new object is created. Then you just have to compare serial numbers to see if the objects are the same.

e.g.

<?php
$FooSerial = 0;
class Foo {
  var $serial;
  function Foo() {
   $this->serial = $GLOBALS['FooSerial']++;
   // Rest of constructor...
  }
  // Rest of class definition...
}

if($bar->serial == $baz->serial) {
  echo "Same";
}
?>
jazfresh at hotmail dot com
08-Apr-2004 12:18
PHP4 compares two objects (say, $a and $b) by comparing the type and all the attributes. If an attribute is an object, PHP4 will recursively call "if($a.attribute === $b.attribute)" to determine equality. However, this is a problem if a circular reference has been formed. The recursion will go on forever, and you will get the error message:

"Fatal error: Nesting level too deep - recursive dependency?"

Example, where the comparison will never terminate because PHP4 will forever recurse with comparisons of the attribute.
<?php
class Test {
       var $obj;
}
$foo =& new Test;
$bar =& new Test;
$foo->obj =& $bar; // Make circular reference
$bar->obj =& $foo;

if($foo === $bar) {
       baz();
}
?>
First PHP4 does ($foo === $bar), which expands into ($foo.obj === $bar.obj), which expands into ($bar.obj === $foo.obj), and so on and so on.

To avoid this situation, you must compare objects manually by comparing the two object's attributes and avoiding comparisons on attributes where a circular reference could arise.

This issue is easily avoided in PHP5, where objects can be compared via references rather than the object contents.

<References inside the constructorClasses and Objects (PHP 5)>
 Last updated: Thu, 19 May 2005
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Last updated: Thu May 19 18:35:34 2005 EDT