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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 17-1. Example of object comparison in PHP 4
<?php function bool2str($bool) { if ($bool === false) { return 'FALSE'; } else { return 'TRUE'; } }
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); ?>
We will see:
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 17-2. 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); ?>
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