rfc:weak_maps

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PHP RFC: WeakMap

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. This allows implementing caches and similar structures without causing memory leaks.

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.

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($map[$obj]); // int(42)
 
// The object is destroyed here, and the key is automatically removed from the weak map.
unset($obj);
var_dump(isset($map[$obj])); // bool(false)

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.

rfc/weak_maps.1572873322.txt.gz · Last modified: 2019/11/04 13:15 by nikic