A WeakMap polyfill for PHP 7.4
This polyfill aims to be 100% compatible with
WeakMap in PHP 8.
PHP 7.4 introduced
WeakReference, but didn't include a
WeakMap implementation. This has been implemented since, but is only available in PHP 8.
The RFC author, Nikita Popov, highlights why a userland
WeakMap is suboptimal:
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 (…). 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.
This is the trade-off this library offers: a 100% compatible implementation, but:
- whose values are not removed as soon as the object key is destroyed, but when you use the
WeakMapagain; note that this affects when object destructors are called as well
Here is how it works:
- calls to
count()will always garbage collect dangling references immediately
- using array-like features: set, get,
unset()will perform garbage collection at least every 100 operations (less often for large WeakMaps).
This offers a reasonable trade-off between performance and memory usage.
This library is installable via Composer:
composer require benmorel/weakmap-polyfill
This library requires PHP 7.4 or later.
$weakMap = new WeakMap(); $a = new stdClass(); $b = new stdClass(); $weakMap[$a] = 123; var_export(isset($weakMap[$a])); // true var_export(isset($weakMap[$b])); // false echo $weakMap[$a]; // 123 echo $weakMap[$b]; // Error echo count($weakMap); // 1 // removing the last reference to the object will remove it from the WeakMap unset($a); echo count($weakMap); // 0
weakreference_bc PECL backports a native polyfill for WeakReference and WeakMap to PHP 7.0-7.4.
The PECL has the following advantages:
weakreference_bcsupports PHP 7.0+
- Like the real WeakMap/WeakReference, values are removed as soon as the object key is destroyed.
- Native implementations are faster and use less memory.
WeakMap::count()is fast because it does not need to perform garbage collection.
This has the following drawbacks:
- Installing a PECL has more steps than installing a composer package.
Iteratorinstead of the more accurate
IteratorAggregateas of 0.4.1.