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Weak References

A weak reference is a link to an object that does not keep the object alive. The object can still vanish while the weak reference points at it.

A weak reference reads a live object

The create method builds a weak reference to the note object. The get method hands the object back, and its text prints.

<?php
class Note {
  public string $text = "Draft ready";
}
$note = new Note();
$weak = WeakReference::create($note);
echo $weak->get()->text . "\n";

The object vanishes under the weak reference

The unset line drops the only strong name for the object. The collector runs, and the get method now hands back null.

<?php
class Note {
  public string $text = "Draft ready";
}
$note = new Note();
$weak = WeakReference::create($note);
unset($note);
gc_collect_cycles();
var_dump($weak->get());

A WeakMap forgets a vanished key

Two objects become keys in a WeakMap, and the count prints 2. One object is unset and the collector runs, so the count prints 1 and the other entry still reads.

<?php
class Item {
  public string $name;
  public function __construct(string $name) {
    $this->name = $name;
  }
}
$map = new WeakMap();
$a = new Item("pen");
$b = new Item("cup");
$map[$a] = "first";
$map[$b] = "second";
echo count($map) . "\n";
unset($a);
gc_collect_cycles();
echo count($map) . "\n";
echo $map[$b] . "\n";

Example

<?php
class Page {
  public string $title;
  public function __construct(string $title) {
    $this->title = $title;
  }
}
$home = new Page("Home");
$help = new Page("Help");
$cache = new WeakMap();
$cache[$home] = "cached home page";
$cache[$help] = "cached help page";
echo count($cache) . "\n";
unset($help);
gc_collect_cycles();
echo count($cache) . "\n";
echo $cache[$home] . "\n";

Key points

  • WeakReference::create builds a weak reference from an object.
  • The get method hands back the object while the object still lives.
  • The get method hands back null once the object is gone.
  • The gc_collect_cycles function collects objects that no strong name keeps alive.
  • A WeakMap is a map whose keys are objects.
  • A WeakMap entry vanishes when its key object vanishes.

In this example, two page objects become keys in a WeakMap that stands in for a cache. The count prints 2 while both pages live. The help page is unset and the collector runs, so the count prints 1 and the home entry still reads back.

Note

A weak reference lets a program watch an object without owning it.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

What does the get method of a weak reference hand back after its object is gone and the collector has run?