Request for Comments: Weak References

• Version: 1.0
• Date: 2011-07-15
• Author: Etienne Kneuss colder@php.net
• Status: Under Discussion

This RFC discuss the introduction of weak references in PHP.

A weak reference is a reference to an object that does not prevent its collection from the garbage collector (GC). Weak references are trying to solve the problem of caches and secondary references in complex data-structures. In such scenarios, the cache prevents the garbage collector from collecting an object, even though it's explicit usage has stopped.

Weak references are useful to allow associating data to existing objects, without having to manually clean this information before the actual object gets collected.

<?php

class MyPlop {
   private $_store = array();
   
   public function getByID($id) {
     if (isset($this->_store[$id]) && $this->_store[$id]->valid()) {
       return $this->_store[$id]->get();
     } else {
       // compute $obj
       $this->_store[$id] = new SplWeakRef($obj);
       return $obj;
     }
   }
}

$plop = new MyPlop();

$a = $plop->getByID(42);

unset($a); // destroys object

Proposal is a SplWeakRef class, which implements some magic to allow GC collection. SplWeakRef would have the following prototype:

void   SplWeakRef::__construct(object ref)
object SplWeakRef::get()
bool   SplWeakRef::valid()

Patch is available here: http://patches.colder.ch/php-src/weakref-trunk.patch?markup