Readonly Properties Cannot Have Default Values

[adrian-enspired]

Description

The following code:

<?php

class X {
    public readonly array $a = [];
}
var_dump(new X()->a);

class Y {
    public function __construct( public readonly array $a = [] ) {}
}

var_dump(new Y()->a);

The X example resulted in this output:

Fatal error: Readonly property X::$a cannot have default value ...

But I expected it to behave similarly / identically to the Y example:

array(0) {
}

Reading the documentation, I do understand that the result I expected initially - i.e., to behave like an argument default value - was misguided. However, I see no reason for a default to be disallowed simply because it would be (in the words of the manual) "essentially the same as a constant" and "not particularly useful." The behavior is inconsistent and unexpected.

Ideally (in my mind, at least) the default value would be assigned if the property was accessed before being initialized, or after construct if it hadn't yet been initialized. But I recognize this would be a larger change in behavior and likely much more complicated to implement.

PHP Version

PHP 8.4.10

Operating System

No response

[iluuu1994]

Hi there. See https://wiki.php.net/rfc/readonly_properties_v2#restrictions:

As the default value counts as an initializing assignment, a readonly property with a default value is essentially the same as a constant, and thus not particularly useful. The notion could become more useful in the future, if new expressions are allowed as property default values. At the same time, depending on how exactly property initialization would work in that case, having a default value on a readonly property could preclude userland serialization libraries from working, as they would not be able to replace the default-constructed object. Whether or not this is a concern depends on whether the property is initialized at time of object creation, or as an implicit part of the constructor (or similar). As these are open questions, the conservative choice is to forbid default values until these questions are resolved.

This behaves as specified and voted on, and needs an RFC to change. The open questions would also need to be resolved before relaxing this restriction. Please start a discussion on the internals mailing list ¹ if it's something you'd like to see, but given there's no real motivation the effort seems misplaced.

Footnotes

1. https://www.php.net/mailing-lists.php ↩

[adrian-enspired]

hi Ilija

Thanks, I'm aware that this is technically working as intended. I opened this at @sgolemon 's request after discussion on discord.

I mentioned my view on how it "should work," but did not expect that to be addressed here. If it sounds reasonable, though, I'd be happy to be part of a discussion and possible rfc.

[iluuu1994]

Thanks for the clarification.

If it sounds reasonable, though, I'd be happy to be part of a discussion and possible rfc.

That's good to hear. An initial e-mail as outlined by https://wiki.php.net/rfc/howto would be the first step. Thanks!