Destructuring assignment in php for objects / associative arrays-ThrowExceptions

Exception or error:

In CoffeeScript, Clojure, ES6 and many other languages we have destructuring of objects/maps/etc somewhat like this:

obj = {keyA: 'Hello from A', keyB: 'Hello from B'}
{keyA, keyB} = obj

I’ve found the list function in php which lets you destructure arrays like so:

$info = array('coffee', 'brown', 'caffeine');
list($drink, $color, $power) = $info;

Is there a way to destructure objects or associative arrays in PHP? If not in the core libs maybe someone wrote some smart helper function?

How to solve:

For PHP 7.0 and below that is beyond the functionality of list. The docs state:

list only works on numerical arrays and assumes the numerical indices start at 0.

One of the things that could suit your purpose would be the extract() function which imports variables from an array into the current symbol table. While with list you are able to define variable names explicitly, extract() does not give you this freedom.

Extracting an associative array

With extract you could do something like that:

<?php

$info = [ 'drink' => 'coffee', 'color' => 'brown', 'power' => 'caffeine' ];
extract($info);

var_dump($drink); // string(6) "coffee"
var_dump($color); // string(5) "brown"
var_dump($power); // string(8) "caffeine"

Extracting an Object

Extracting an object works almost the same. Since extract only takes an array as an argument we need to get the objects properties as an array. get_object_vars does that for you. It returns an associative array with all public properties as key and their values as value.

<?php

class User {

    public $name = 'Thomas';

}

$user = new User();
extract( get_object_vars($user) );

var_dump($name); // string(6) "Thomas"

Pitfalls

extract() is not the same as list since it does not allow you to explicitly define the variable names that get exported to the symbol table. The variable names correspond the array keys by default.

  • list is a language construct while extract() is a function
  • It might happen that you overwrite variables that you have defined beforehand unintentionally
  • Your array keys might be invalid as variable names

With the $flags parameter that you can pass as second argument to extract() you can influence the behavior in case of colliding or invalid variables. But still it’s important to know how extract() works and to use it with cauton.

Edit: As of PHP 7.1 this is possible:

http://php.net/manual/en/migration71.new-features.php#migration71.new-features.support-for-keys-in-list

You can now specify keys in list(), or its new shorthand [] syntax. This enables destructuring of arrays with non-integer or non-sequential keys.

https://php.net/manual/en/migration71.new-features.php#migration71.new-features.symmetric-array-destructuring

The shorthand array syntax ([]) may now be used to destructure arrays for assignments (including within foreach), as an alternative to the existing list() syntax, which is still supported.

For example this:

$test_arr = ['a' => 1, 'b' => 2];
list('a' => $a, 'b' => $b) = $test_arr;
var_dump($a);
var_dump($b);

Will output the following as of 7.1.0

int(1) 
int(2)

Answer:

I noticed the accepted answer missed out examples that use the short-hand notation, security issues with using extract, and IDE issues.

Numerical Array Destructing (PHP 7.1)

As of PHP 7.1 numerical array destructing (Symetric array destructuring) is supported like so:

<?php
$data = [55, 'John', 'UK'];
[$id, $name] = $data; // short-hand (recommended)
list($id, $name) = $data; // long-hand

Notice that you can miss items out if you don’t want them.

Associative Array Destructing (PHP 7.1)

You can also do array destructing on associative arrays (Support for keys in list) like so:

<?php
$data = ['id' => 55, 'firstName' => 'John', 'country' => 'UK']
['id' => $id, 'firstName' => $name] = $data; // short-hand (recommended)
list('id' => $id, 'firstName' => $name) = $data; // long-hand

Notice that you can miss items out if you don’t want them. Also the variable name can be different to the property name.

Object Destructing (PHP 7.1)

Unfortunately there is no object destructing. However you can convert an object to an associative array using get_object_vars, and then use associative array destructing.

<?php
class User {
    public $id;
    public $name;
    public $country;
}

$user = new User();
$user->id = 55;
$user->name = 'John';
$user->country = 'UK';

['id' => $id, 'firstName' => $name] = get_object_vars($user)

However, this can break some IDE features. These are some issues I noticed when using PHPStorm 2019.1:

  • IDE’s may no longer understand the type for the variables, so you would need to add some @var Type PHPDocs to maintain auto-complete functionality
  • Does not work well with refactoring tools. For example, if you rename one of the properties, the array destructing portion will not also automatically rename.

So I recommend just doing it the normal way:

$id = $user->id
$name = $user->firstName

Do NOT use extract

With extract, all variables are always set. There it is a really bad idea to use it because:

  • It can lead to security issues. Even if your careful, it can lead to non-obvious security holes in the future. If you do use it, don’t use it with user input (e.g. $_GET, $_POST), unless you want to make a malicious hacker’s day.
  • Can lead to hard to detect bugs
  • If the class or array changes in the future, by introducing new properties, it can break your code if it coincides with an already used variable, unless you use the EXTR_SKIP flag or similar

Answer:

Variable variables are one way you can do this. A foreach keeps you in-scope, so you can define your variables explicitly from the array like so:

$args = ['a' => 1, 'b' => 2, 'c' => 3];
foreach (['a', 'c'] as $v) $$v = $args[$v];
// $a is 1, $b is undefined, $c is 3

It’s really not pretty, and thankfully this has been addressed in 7.1 by https://wiki.php.net/rfc/short_list_syntax . This would let you say ['a' => $a, 'c' => $c] = $args; in the above example.

Since 7.1 includes a way to use a different name for your var than the assoc array key, I’ll mention that this is pretty straight-forward using my approach here too.

foreach (['a' => 'eh', 'b' => 'bee'] as $k => v) $$v = $args[$k];
// $eh is 1, $bee is 2

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