Use variable by reference in arrow function

Question

PHP 7.4 introduced Arrow functions. And it also introduced implicit by-value scope binding which eliminate the need for use keyword.

Now if we want to use a variable out of a closure's scope by reference with a regular anonymous function we would do this:

$num = 10;
call_user_func(function() use (&$num) {
$num += 5;
});
echo $num; // Output: 15

But using an arrow function it seems impossible

$num = 10;
call_user_func(fn() => $num += 5);

echo $num; // Output: 10

So how to use $num variable by reference?

I think you'll have to pass it as an argument, otherwise its not visible in the functions scope. Though I'm not sure how that'll work in practice, so it might not be possible. — Qirel
– Qirel, Commented Nov 28, 2019 at 6:56
Does this answer your question? Rewriting an anonymous function in php 7.4 — Dharman
– Dharman ♦, Commented Nov 28, 2019 at 7:27

Community · Accepted Answer · 2020-06-20 09:12:55Z

28

Reading the documentation about it, it says...

By-value variable binding
As already mentioned, arrow functions use by-value variable binding. This is roughly equivalent to performing a use($x) for every variable $x used inside the arrow function. A by-value binding means that it is not possible to modify any values from the outer scope:
$x = 1; 
$fn = fn() => $x++; // Has no effect 
$fn(); 
var_dump($x); // int(1)

So it is not possible ATM.

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered Nov 28, 2019 at 7:01

Nigel Ren

57.3k11 gold badges49 silver badges59 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

damask · Accepted Answer · 2024-08-05 06:01:03Z

5

You can use an object to do this.

$state = new stdClass; 
$state->index = 1;

$fn = fn() => $state->index++; // Now, has effect.
$fn();

var_dump($state->index); // int(2)

It works because objects always pass by reference. I don't encourage you to use this but in some places it's useful.

edited Aug 5, 2024 at 6:01

damask

5655 silver badges19 bronze badges

answered Jul 8, 2022 at 7:03

Mahdi Aslami Khavari

1,99119 silver badges27 bronze badges

1 Comment

Yes Barry Over a year ago

Clever workaround

JSowa · Accepted Answer · 2022-07-08 15:39:28Z

0

It's not possible with the argument-less arrow function, but you can always define an argument inside as a reference. There is no other way using it with arrow functions.

$num = 10;
$fn = fn(&$num) => $num += 5;
$fn($num);

echo $num; // Output: 15

answered Jul 8, 2022 at 15:39

JSowa

10.8k7 gold badges66 silver badges70 bronze badges

Collectives™ on Stack Overflow

Use variable by reference in arrow function

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related