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--- rfc:weak_maps [2019/11/04 18:46] – Add a use-case example nikic
+++ rfc:weak_maps [2020/01/03 09:56] (current) – Implemented nikic
@@ Line 3: / Line 3: @@
   * Author: Nikita Popov <nikic@php.net>
   * Proposed Version: PHP 8.0
-  * Status: Under Discussion
+  * Status: Implemented
   * Implementation: https://github.com/php/php-src/pull/4882
@@ Line 83: / Line 83: @@
 ===== Vote =====
-Add WeakMap class in PHP 8.0? Yes/No.
+Voting started 2019-12-18 and closes 2020-01-01.
+<doodle title="Add WeakMap class in PHP 8.0?" auth="nikic" voteType="single" closed="true">
+   * Yes
+   * No
+</doodle>
+===== 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:
+<PHP>
+// 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;
+</PHP>
+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.

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