The WeakMap class

(PHP 8)

Introduction

A WeakMap is map (or dictionary) that accepts objects as keys. However, unlike the otherwise similar SplObjectStorage, an object in a key of WeakMap does not contribute toward the object's reference count. That is, if at any point the only remaining reference to an object is the key of a WeakMap, the object will be garbage collected and removed from the WeakMap. Its primary use case is for building caches of data derived from an object that do not need to live longer than the object.

WeakMap implements ArrayAccess, Traversable (via IteratorAggregate), and Countable, so in most cases it can be used in the same fashion as an associative array.

Class synopsis

final class WeakMap implements ArrayAccess, Countable, IteratorAggregate {

/* Methods */

public count(): int

public getIterator(): Iterator

public offsetExists(object $object): bool

public offsetGet(object $object): mixed

public offsetSet(object $object, mixed $value): void

public offsetUnset(object $object): void

}

Examples

Example #1 Weakmap usage example

<?php
$wm = new WeakMap();

$o = new stdClass;

class A {
    public function __destruct() {
        echo "Dead!\n";
    }
}

$wm[$o] = new A;

var_dump(count($wm));
echo "Unsetting...\n";
unset($o);
echo "Done\n";
var_dump(count($wm));

The above example will output:

int(1)
Unsetting...
Dead!
Done
int(0)

WeakMap::count — Counts the number of live entries in the map
WeakMap::getIterator — Retrieve an external iterator
WeakMap::offsetExists — Checks whether a certain object is in the map
WeakMap::offsetGet — Returns the value pointed to by a certain object
WeakMap::offsetSet — Updates the map with a new key-value pair
WeakMap::offsetUnset — Removes an entry from the map

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User Contributed Notes 2 notes

down

Samu ¶

1 year ago

PHP's implementation of WeakMap allows for iterating over the contents of the weak map, hence it's important to understand why it is sometimes dangerous and requires careful thought.

If the objects of the WeakMap are "managed" by other services such as Doctrine's EntityManager, it is never safe to assume that if the object still exists in the weak map, it is still managed by Doctrine and therefore safe to consume. 

Doctrine might have already thrown that entity away but some unrelated piece of code might still hold a reference to it, hence it still existing in the map as well.

If you are placing managed objects into the WeakMap and later iterating over the WeakMap (e.g. after Doctrine flush), then for each such object you must verify that it is still valid in the context of the source of the object.

For example assigning a detached Doctrine entity to another entity's property would result in errors about non-persisted / non-managed entities being found in the hierarchy.