David Walker

David Walker

Contents

PHP RFC: list() Reference Assignment

Introduction

PHP has had list() assignment and reference assignment for a long time. However, it is not currently possible to use reference assignment with list(). This RFC proposes this new syntax to cover this. The original requests for this feature date back at least 16 years now [1][2]. We will be focusing on the formers syntax proposal for this RFC.

Proposal

Under this proposal, a new syntax is introduced:

$array = [1, 2];
list($a, &$b) = $array;

This would be equivalent to the following:

$array = [1, 2];
$a = $array[0];
$b =& $array[1];
/* Note; []= syntax works the same, so the following is functionally equivalent to the example */
[$a, &$b] = $array;

Of course, this works just like list() normally does, so you can use it with nested list() and skip values as well:

$array = [1, 2, 3, [3, 4]];
list(&$a, $b,, list(&$c, $d)) = $array;
var_dump($array);
/*
array(4) {
  [0]=>
  &int(1)
  [1]=>
  int(2)
  [2]=>
  int(3)
  [3]=>
  array(2) {
    [0]=>
    &int(3)
    [1]=>
    int(4)
  }
}
*/

It also works with foreach():

$array = [[1, 2], [3, 4]];
foreach ($array as list(&$a, $b)) {
    $a = 7;
}
var_dump($array)
/*
array(2) {
  [0]=>
  array(2) {
    [0]=>
    int(7)
    [1]=>
    int(2)
  }
  [1]=>
  array(2) {
    [0]=>
    &int(7)
    [1]=>
    int(4)
  }
}
*/

(notice here that the reference exists on $array[1][0] since $a is still in scope after the foreach())

The predominant advantage of adding support for this is that it allows you to use reference assignment for multiple variables at once, which is not currently possible. The syntax here is different from the traditional assignment syntax which places the & before the right-side value, not the left-side value; the advantage here is that you can reference assign some, but not all of the variables in list().

The RFC also takes into account the possibility of objects being the r-val of the assignment implementing ArrayAccess.

class RefArr implements ArrayAccess {
    private $s = [];
    function __construct(array $a) { $this->s = $a; }
    function offsetSet ($k, $v) { $this->s[$k] = $v; }
    function &offsetGet ($k) { return $this->s[$k]; }
    function offsetExists ($k) { return isset($this->s[$k]); }
    function offsetUnset ($k) { unset($this->s[$k]); }
}
$a = new RefArr(array(1, array(2, 3)));
list(&$one, list($two, &$three)) = $a;
$one++;
var_dump($a)
/*
object(RefArr)#1 (1) {
  ["s":"RefArr":private]=>
  array(2) {
    [0]=>
    &int(2)
    [1]=>
    array(2) {
      [0]=>
      int(2)
      [1]=>
      &int(3)
    }
  }
}
*/

Backward Incompatible Changes

This introduces no backwards incompatible changes.

Proposed PHP Version(s)

PHP 7.2

RFC Impact

To Opcache

Maybe, I don't know enough about opcache to say if changing current ops by adding an extended value would impact.

Patches and Tests

Implementation

References

Rejected Features

Change Log

  • v2.0 - Commandeered for revival for PHP 7 implementation.
  • v1.0(and prior) - Initial concept raised for PHP 5.x. Was withdrawn in 2014.

Votes

An option needs 2/3 votes to win

list() Reference Assignments (70.8% approved)
User Vote
ashnazg Yes
bishop Yes
bwoebi Yes
daverandom No
dm No
fmk Yes
galvao Yes
hywan No
joey No
kguest Yes
leigh Yes
levim No
lstrojny Yes
mariano Yes
ocramius No
peehaa No
pollita Yes
salathe Yes
sammyk Yes
stas Yes
svpernova09 Yes
tpunt Yes
trowski Yes
zeev Yes