Question

Someone has suggested to e to use SplObjectStorage to keep track of a set of unique things. Great, except it doesn’t work with strings. An error says ” SplObjectStorage::attach() expects parameter 1 to be object, string given in fback.php on line 59″

Any ideas?

Answer 1

The SplObjectStorage is what its name says: a storage class for storing objects. In contrast to some other programming languages strings are not objects in PHP, they are, well, strings ;-). It therefore makes no sense to store strings in a SplObjectStorage – even if you wrap your strings in an object of class stdClass.

The best way to store a collection of unique strings si to use arrays (as hashtables) with the string as the key as well as the value (as suggested by Ian Selby).

$myStrings = array();
$myStrings['string1'] = 'string1';
$myStrings['string2'] = 'string2';
// ...

You could however wrap this functionality into a custom class:

class UniqueStringStorage // perhaps implement Iterator
{
    protected $_strings = array();

    public function add($string)
    {
        if (!array_key_exists($string, $this->_strings)) {
            $this->_strings[$string] = $string;
        } else {
            //.. handle error condition "adding same string twice", e.g. throw exception
        }
        return $this;
    }

    public function toArray()
    {
        return $this->_strings;
    }

    // ... 
}

By the way you san simulate the behavior of SplObjectStorage for PHP < 5.3.0 and to get a better understanding of what it does.

$ob1 = new stdClass();
$id1 = spl_object_hash($ob1);
$ob2 = new stdClass();
$id2 = spl_object_hash($ob2);
$objects = array(
    $id1 => $ob1,
    $id2 => $ob2
);

SplObjectStorage stores a unique hash for each instance (like spl_object_hash()) to
be able to identify object instances. As I said above: a string is not an object at all, it therefore does not have an instance hash. A string’s uniqueness can be checked by comparing the string values – two strings are equal when they contain the same set of bytes.