The WeakMap class

Intro

A WeakMap is a map (or dictionary) that accepts objects as keys. However, unlike the otherwise similar SplObjectStorage, an object in a key of a 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

Examples

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)

Count Getiterator Offsetexists Offsetget Offsetset Offsetunset