Object cloning
In response to "rune at zedeler dot dk"s comment about class contents being equal, I have a similar issue. I want to sort an array of objects using sort().
I know I can do it with usort(), but I'm used to C++ where you can define operators that allow comparison. I see in the zend source code that it calls a compare_objects function, but I don't see any way to implement that function for an object. Would it have to be an extension to provide that interface?
If so, I'd like to suggest that you allow equivalence and/or comparison operations to be defined in a class definition in PHP. Then, the sorts of things rune and I want to do would be much easier.
Comparing objects
In PHP 5, object comparison is more complicated than in PHP 4 and more in accordance to what one will expect from an Object Oriented Language (not that PHP 5 is such a language).
When using the comparison operator (==), object variables are compared in a simple manner, namely: Two object instances are equal if they have the same attributes and values, and are instances of the same class.
On the other hand, when using the identity operator (===), object variables are identical if and only if they refer to the same instance of the same class.
An example will clarify these rules.
Example #1 Example of object comparison in PHP 5
<?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
{
public $flag;
function Flag($flag = true) {
$this->flag = $flag;
}
}
class OtherFlag
{
public $flag;
function OtherFlag($flag = true) {
$this->flag = $flag;
}
}
$o = new Flag();
$p = new Flag();
$q = $o;
$r = new OtherFlag();
echo "Two instances of the same class\n";
compareObjects($o, $p);
echo "\nTwo references to the same instance\n";
compareObjects($o, $q);
echo "\nInstances of two different classes\n";
compareObjects($o, $r);
?>
The above example will output:
Two instances of the same class o1 == o2 : TRUE o1 != o2 : FALSE o1 === o2 : FALSE o1 !== o2 : TRUE Two references to the same instance o1 == o2 : TRUE o1 != o2 : FALSE o1 === o2 : TRUE o1 !== o2 : FALSE Instances of two different classes o1 == o2 : FALSE o1 != o2 : TRUE o1 === o2 : FALSE o1 !== o2 : TRUE
Note: Extensions can define own rules for their objects comparison.
add a note
User Contributed NotesComparing objects
cross+php at distal dot com
05-Mar-2008 08:50
05-Mar-2008 08:50
dionyziz at deviantart dot com
10-Mar-2007 07:20
10-Mar-2007 07:20
Note that classes deriving from the same parent aren't considered equal when comparing even using ==; they should also be objects of the same child class.
<?php
class Mom {
private $mAttribute;
public function Mom( $attribute ) {
$this->mAttribute = $attribute;
}
public function Attribute() {
return $this->mAttribute;
}
}
final class Sister extends Mom {
public function Sister( $attribute ) {
$this->Mom( $attribute );
}
}
final class Brother extends Mom {
public function Brother( $attribute ) {
$this->Mom( $attribute );
}
}
$sister = new Sister( 5 );
$brother = new Brother( 5 );
assert( $sister == $brother ); // will FAIL!
?>
This assertion will fail, because sister and brother are not of the same child class!
If you want to compare based on the parent class object type only, you might have to define a function for comparisons like these, and use it instead of the == operator:
<?php
function SiblingsEqual( $a, $b ) {
if ( !( $a instanceof Mom ) ) {
return false;
}
if ( !( $b instanceof Mom ) ) {
return false;
}
if ( $a->Attribute() != $b->Attribute() ) {
return false;
}
return true;
}
assert( SiblingsEqual( $sister, $brother ) ); // will succeed
?>
rune at zedeler dot dk
28-Feb-2007 08:34
28-Feb-2007 08:34
Whoops, apparently I hadn't checked the array-part of the below very well.
Forgot to test if the arrays had same length, and had some misaligned parenthesis.
This one should work better :+)
<?
function deepCompare($a,$b) {
if(is_object($a) && is_object($b)) {
if(get_class($a)!=get_class($b))
return false;
foreach($a as $key => $val) {
if(!deepCompare($val,$b->$key))
return false;
}
return true;
}
else if(is_array($a) && is_array($b)) {
while(!is_null(key($a)) && !is_null(key($b))) {
if (key($a)!==key($b) || !deepCompare(current($a),current($b)))
return false;
next($a); next($b);
}
return is_null(key($a)) && is_null(key($b));
}
else
return $a===$b;
}
?>
rune at zedeler dot dk
27-Feb-2007 08:27
27-Feb-2007 08:27
I haven't found a build-in function to check whether two obects are identical - that is, all their fields are identical.
In other words,
<?
class A {
var $x;
function __construct($x) { $this->x = $x; }
}
$identical1 = new A(42);
$identical2 = new A(42);
$different = new A('42');
?>
Comparing the objects with "==" will claim that all three of them are equal. Comparing with "===" will claim that all are un-equal.
I have found no build-in function to check that the two identicals are
identical, but not identical to the different.
The following function does that:
<?
function deepCompare($a,$b) {
if(is_object($a) && is_object($b)) {
if(get_class($a)!=get_class($b))
return false;
foreach($a as $key => $val) {
if(!deepCompare($val,$b->$key))
return false;
}
return true;
}
else if(is_array($a) && is_array($b)) {
while(!is_null(key($a) && !is_null(key($b)))) {
if (key($a)!==key($b) || !deepCompare(current($a),current($b)))
return false;
next($a); next($b);
}
return true;
}
else
return $a===$b;
}
?>
jazfresh at hotmail.com
08-Dec-2006 02:36
08-Dec-2006 02:36
Note that when comparing object attributes, the comparison is recursive (at least, it is with PHP 5.2). That is, if $a->x contains an object then that will be compared with $b->x in the same manner. Be aware that this can lead to recursion errors:
<?php
class Foo {
public $x;
}
$a = new Foo();
$b = new Foo();
$a->x = $b;
$b->x = $a;
print_r($a == $b);
?>
Results in:
PHP Fatal error: Nesting level too deep - recursive dependency? in test.php on line 11