rfc:weak_maps

PHP RFC: Weak maps

Introduction

Weak maps allow creating a map from objects to arbitrary values (similar to SplObjectStorage) without preventing the objects that are used as keys from being garbage collected. If an object key is garbage collected, it will simply be removed from the map.

In PHP 7.4 first-class support for weak references was already introduced. However, raw weak references are only of limited usefulness by themselves and weak maps are much more commonly used in practice. It is not possible to implement an efficient weak map on top of PHP weak references, because the ability to register a destruction callback is not provided.

The general use case for weak maps is to associate data with individual object instances, without forcing them to stay alive and thus effectively leak memory in long-running processes. For example, a weak map may be used to memoize a computation result:

class FooBar {
    private WeakMap $cache;
 
    public function getSomethingWithCaching(object $obj) {
        return $this->cache[$obj] ??= $this->computeSomethingExpensive($obj);
    }
 
    // ...
}

This will invoke the computeSomethingExpensive() method only once for each object. At the same time it will also drop the cached value from the map if the object is destroyed. Doing the same with a normal array (or rather SplObjectStorage) would result in a memory leak.

Proposal

Add a WeakMap class with the following prototype:

final class WeakMap implements ArrayAccess, Countable, Traversable {
    public function offsetGet($object);
    public function offsetSet($object, $value): void;
    public function offsetExists($object): bool;
    public function offsetUnset($object): void;
    public function count(): int;
}

Objects used as weak map keys are “weakly referenced”, which means that they are not prevented from being garbage collected. If an object that is used as a weak map key is garbage collected, the key is removed from the weak map. This is illustated in the following example:

$map = new WeakMap;
$obj = new stdClass;
$map[$obj] = 42;
var_dump($map);
// object(WeakMap)#1 (1) {
//   [0]=>
//   array(2) {
//     ["key"]=>
//     object(stdClass)#2 (0) {
//     }
//     ["value"]=>
//     int(42)
//   }
// }
 
// The object is destroyed here, and the key is automatically removed from the weak map.
unset($obj);
var_dump($map);
// object(WeakMap)#1 (0) {
// }

Some details of the WeakMap behavior are outlined in the following:

  • Just like WeakReference, WeakMap is not serializable and setting dynamic properties on it is forbidden.
  • Unlike WeakReference, WeakMap can be cloned. Cloning WeakReferences is not allowed because the objects are uniqued, which does not apply to WeakMaps.
  • Using a non-object key in $map[$key] or one of the offset*() methods results in a TypeError exception.
  • Appending to a weak map using $map[] results in an Error exception.
  • Reading a non-existent key results in an Error exception.
  • Overloaded operations on map entries are supported, i.e. $map[$obj][] = $x and similar work.
  • By-reference iteration of WeakMaps is supported.

Backward Incompatible Changes

No backwards incompatible changes apart from using the WeakMap class name.

Vote

Add WeakMap class in PHP 8.0? Yes/No.

Differences to spl_object_id() and WeakReference

Weak maps require first-class language support and cannot be implemented using existing functionality provided by PHP.

At first sight, it may seem that an array mapping from spl_object_id() to arbitrary values could serve the purpose of a weak map. This is not the case for multiple reasons:

  • spl_object_id() values are reused after the object is destroyed. Two different objects can have the same object ID – just not at the same time.
  • The object ID cannot be converted back into an object, so iteration over the map is not possible.
  • The value stored under the ID will not be released when the object is destroyed.

Using the WeakReference class introduced in PHP 7.4, it is possible to avoid the first two issues, by using the following construction:

// Insertion
$this->map[spl_object_id($object)] = [WeakReference::create($object), $data];
 
// Lookup
$id = spl_object_id($object);
if (isset($this->map[$id])) {
    [$weakRef, $data] = $this->map[$id];
    if ($weakRef->get() === $object) {
        return $data;
    }
    // This entry belongs to a destroyed object.
    unset($this->map[$id]);
}
return null;

This makes use of the WeakReference to determine whether the object ID has been reused. However, this does not solve the third problem: The data will not be released when the object is destroyed. It will only be released on the next access with an object that has the same reused ID, or if a garbage collection mechanism, which performs regular sweeps of the whole map, is implemented.

A native weak map implementation will instead remove the value from the weak map as soon as the object key is destroyed.

rfc/weak_maps.txt · Last modified: 2019/11/04 21:10 by nikic