====== PHP RFC: Objects as hash keys ======
  * Version: 1.0
  * Date: 2014-10-26
  * Author: Stas Malyshev (stas@php.net), krakjoe
  * Status: Declined
  * First Published at: http://wiki.php.net/rfc/objkey

The purpose of this RFC is to introduce a magic method that will allow objects to serve as hash keys. 

===== Introduction =====

In PHP array keys can be only represented as numbers and strings. However, there already are several classes that represent different kinds of numbers (such as GMP objects) and some objects can also represent strings (e.g. Unicode strings) or string-like objects. It would be convenient to be able to use such objects as array keys to.

It could be achieved as conversion to string, but this is not ideal since object's string representation does not always match object's identity and there may be case where human-readable string may be different from value for the purposes of hashing. Also, some object may prefer to produce numeric index. 

===== Proposal =====
Create a new magic method, %%__hash()%% which is called when object is supplied as a hash key, and returns string or integer that is used as the hash key.

A number of languages implement the same facility, namely:
  * Java has hashCode() and toString()
  * Python has %%__str__, __repr__ and __hash__%%
  *  Ruby has object.hash

The method should produce a value which is acceptable as a key (not including objects), 
otherwise the engine will still produce an illegal offset type error. The objects not having this
method implemented would produce an illegal offset type error when used in hash key position as before.

<code php>
<?php
class Foo {
    public function __hash() {
        return "Foo";
    }
}

$foo = new Foo();
$test = [
    $foo => true
];
</code>

Inheritance shall work as any other magic method:

<code php>
<?php
class Foo {
    public function __hash() {
        return "Foo";
    }
}

class Bar extends Foo {
    /* shall use Foo::__hash unless Bar::__hash is implemented */
}

$bar = new Bar();
$test = [
    $bar => true
];
</code>

Returning a non-scalar shall fail as it did before:

<code php>
<?php
class Foo {
    public function __hash() {
        return [];
    }
}

$foo = new Foo();
$test = [
    $foo => true
];
</code>

Shall yield:

<code>
Warning: Illegal offset type in %s on line %d
</code>

The current behavior of %%__toString%% is unchanged.

==== Backward Incompatible Changes =====

Should not break anything as we don't allow this now.

===== Proposed PHP Version(s) =====
Targeted for PHP 7

===== RFC Impact =====
==== To SAPIs ====

No impact on SAPIs

==== To Existing Extensions ====

If somebody implements something like ArrayAccess they may want to update it to accommodate objects.

==== To Opcache ====

Since objects are run-time, should not have any effects on opcache.


===== Open Issues =====

  - Should SplFixedArray support object indexes?
  - Should SplObjectStorage support calling %%__hash%% for an object if it exists? 


===== Vote =====

New magic method requires 2/3 majority. The vote also includes choice for the name - %%__hash%% or %%__toKey%%. Vote for either is counted as the vote for the proposal. 

<doodle title="Should we support using objects as keys as described in this proposal?" auth="stas" voteType="single" closed="true">
   * Yes, via method __hash
   * Yes, via method __toKey
   * No
</doodle>

The vote runs from December 16, 2014 to the end of day (PDT) January 6, 2015. 

===== Implementation =====

https://github.com/php/php-src/pull/881

===== References =====
Discussed here:

http://marc.info/?t=141145969600001&r=1&w=2
===== Rejected Features =====

 - Using %%__toString%%  for the key conversion, for the reasons discussed above. 
 - Supporting the use of objects for string indexes