haphpiness

WeakMap — Object Keys That Don't Leak Memory

PHP 8.0 introduced WeakMap, a map where the keys are objects but holding a key doesn't prevent that object from being garbage collected. It's the perfect structure for per-object caches that would otherwise cause memory leaks.

$cache = new WeakMap();

class QueryBuilder
{
    public function build(): string { /* ... */ return 'SELECT ...'; }
}

$qb = new QueryBuilder();

// Associate computed data with the object
$cache[$qb] = $qb->build();

echo $cache[$qb];  // "SELECT ..."
echo count($cache); // 1

// When the object goes out of scope, the WeakMap entry is automatically removed
unset($qb);

echo count($cache); // 0 — cleaned up automatically, no memory leak

Compare this to a plain SplObjectStorage or a regular array keyed by spl_object_id(): both keep the object alive as long as the cache exists. With WeakMap, the cache is truly a side-channel — it holds data about objects without claiming ownership of them.

// Real-world: memoize expensive per-object computations
class MetadataRegistry
{
    private WeakMap $cache;

    public function __construct()
    {
        $this->cache = new WeakMap();
    }

    public function getMetadata(object $obj): array
    {
        if (!isset($this->cache[$obj])) {
            $this->cache[$obj] = $this->computeExpensiveMetadata($obj);
        }
        return $this->cache[$obj];
    }
}