note 85769 added to language.oop5.object-comparison

This has already been mentioned (see jazfresh at hotmail.com's note), but here it is again in more detail because for objects the difference between == and === is significant. Loose equality (==) over objects is recursive: if the properties of the two objects being compared are themselves objects, then those properties will also be compared using ==. <?php class Link { public $link; function __construct($link) { $this->link = $link; } } class Leaf { public $leaf; function __construct($leaf) { $this->leaf = $leaf; } } $leaf1 = new Leaf(42); $leaf2 = new Leaf(42); $link1 = new Link($leaf1); $link2 = new Link($leaf2); echo "Comparing Leaf object equivalence: is \$leaf1==\$leaf2? ", ($leaf1 == $leaf2 ? "Yes" : "No"), "\n"; echo "Comparing Leaf object identity: is \$leaf1===\$leaf2? ", ($leaf1 === $leaf2 ? "Yes" : "No"), "\n"; echo "\n"; echo "Comparing Link object equivalence: is \$link1==\$link2? ",($link1 == $link2 ? "Yes" : "No"), "\n"; echo "Comparing Link object identity: is \$link1===\$link2? ", ($link1 === $link2 ? "Yes" : "No"), "\n"; ?> Even though $link1 and $link2 contain different Leaf objects, they are still equivalent because the Leaf objects are themselves equivalent. The practical upshot is that using "==" when "===" would be more appropriate can result in a severe performance penalty, especially if the objects are large and/or complex. In fact, if there are any circular relationships involved between the objects or (recursively) any of their properties, then a fatal error can result because of the implied infinite loop. <?php class Foo { public $foo; } $t = new Foo; $t->foo = $t; $g = new Foo; $g->foo = $g; echo "Strict identity: ", ($t===$g ? "True" : "False"),"\n"; echo "Loose equivalence: ", ($t==$g ? "True" : "False"), "\n"; ?> So preference should be given to comparing objects with "===" rather than "=="; if two distinct objects are to be compared for equivalence, try to do so by examining suitable individual properties. (Maybe PHP could get a magic "__equals" method that gets used to evaluate "=="? :) ) ---- Server IP: 69.147.83.197 Probable Submitter: 114.134.161.9 ---- Manual Page -- http://www.php.net/manual/en/language.oop5.object-comparison.php Edit -- https://master.php.net/note/edit/85769 Del: integrated -- https://master.php.net/note/delete/85769/integrated Del: useless -- https://master.php.net/note/delete/85769/useless Del: bad code -- https://master.php.net/note/delete/85769/bad+code Del: spam -- https://master.php.net/note/delete/85769/spam Del: non-english -- https://master.php.net/note/delete/85769/non-english Del: in docs -- https://master.php.net/note/delete/85769/in+docs Del: other reasons-- https://master.php.net/note/delete/85769 Reject -- https://master.php.net/note/reject/85769 Search -- https://master.php.net/manage/user-notes.php