RSS (general)There have been activity recently about the different ways to uniquely identify objects in PHP. In fact, a function have been sitting in SPL unnoticed for quite some time and while people came across it, some got frustrated. I'm of course talking about spl_object_hash(). To summarize it: in PHP, you basically need two things to safely identify an object: a object index, the handle, and the class handlers which is how the object will react internally. This set of handlers is actually a pointer, and since disclosing valid pointers is not something that should be done, spl_object_hash is simply providing a MD5 hash of those two values concatenated. Now two problems comes from this MD5 hash:
- It's quite slow
- It may generate collisions
One of the usages of this hash that comes to mind is an object dictionary(or map): attach information to instances, for example:
<?php
$dict = array();
$dict[spl_object_hash($obj1)] = $info1;
$dict[spl_object_hash($obj2)] = $info2;
// and so on.
?>
Sadly, since PHP arrays are themselves hashtables, that means the hash will get hashed one more time, this is a waste of time.
Another example could be to mark nodes in a graph traversal algorithm, using a set of visited nodes.
SPL thankfully provides a class (as of PHP5.3) that can implement quite easily both examples without the problems stated above: SplObjectStorage.
Since an example is better than thousand words, here is a demonstration:
<?php
// Map
$dict = new SplObjectStorage;
$dict[$obj1] = $info1;
$dict[$obj2] = $info2;
var_dump($dict[$obj1]); // $info1
// Set
$set = new SplObjectStorage;
$set->attach($obj1);
var_dump($set->contains($obj1)); // True
?>
SplObjectStorage directly uses the unique identifier without pre-hashing it, so you spare time and you will be safe against collisions!
Your code is nice and all but what practical use does the SplObjectStorage class have? Can you give me a real world example of when you would put something into $set and need to check if it is there later?