PHP: rfc:readonly_classes

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====== PHP RFC: Readonly classes ======
  * Date: 2021-08-04
  * Author: Máté Kocsis <kocsismate@php.net>
  * Status: Accepted
  * Target Version: PHP 8.2
  * Implementation: https://github.com/php/php-src/pull/7305

===== Introduction =====

PHP 8.1 added support for readonly properties via [[rfc:readonly_properties_v2|PHP RFC: Readonly properties 2.0]]. However, it's still not easy to declare (quasi-)immutable classes, especially if they contain many properties. Therefore, this RFC proposes to add support for ''readonly'' classes.

===== Proposal =====

The usage of the ''readonly'' modifier added by [[rfc:readonly_properties_v2|PHP RFC: Readonly properties 2.0]] is extended to classes:

<PHP>
readonly class Test {
    public string $prop;
}
</PHP>

Doing so will implicitly mark all instance properties of a class as readonly. Furthermore, it will prevent the creation of dynamic properties.

<PHP>
readonly class Foo
{
    public int $bar;

    public function __construct() {
        $this->bar = 1;
    }
}

$foo = new Foo();
$foo->bar = 2;
// Fatal Error: Uncaught Error: Cannot modify readonly property Foo::$bar

$foo->baz = 1;
// Fatal Error: Uncaught Error: Cannot create dynamic property Foo::$baz
</PHP>

[[rfc:deprecate_dynamic_properties|PHP RFC: Deprecate dynamic properties]] added support for the ''#[AllowDynamicProperties]'' attribute which makes it possible to create dynamic properties without triggering errors. In order not to violate the read-only constraint, marking readonly classes with the above attribute is a compile-time error:

<PHP>
#[AllowDynamicProperties]
readonly class Foo {
}

// Fatal error: Cannot apply #[AllowDynamicProperties] to readonly class Foo
</PHP>


==== Restrictions ====

As neither untyped, nor static properties are covered by the [[rfc:readonly_properties_v2#restrictions|Readonly properties RFC]],
readonly classes cannot declare them either:

<PHP>
readonly class Foo
{
    public $bar;
}

// Fatal error: Readonly property Foo::$bar must have type
</PHP>

<PHP>
readonly class Foo
{
    public static int $bar;
}

// Fatal error: Readonly class Foo cannot declare static properties
</PHP>

==== Inheritance ====

Similarly how overriding of readonly properties works, a readonly class can only extend a readonly parent:

<PHP>
readonly class A {}
readonly class B extends A {} // valid
</PHP>

But both of the following are illegal:

<PHP>
readonly class A {}
class B extends A {}
// Fatal error: Non-readonly class B cannot extend readonly class A
</PHP>

<PHP>
class A {}
readonly class B extends A {}
// Fatal error: Readonly class B cannot extend non-readonly class A
</PHP>

==== Reflection ====

A ''ReflectionClass::isReadOnly()'' method is added, which reports whether a class is declared as read-only. Additionally, ''ReflectionClass::getModifiers()'' will also include the ''ReflectionClass::IS_READONLY'' flag.

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

None.

===== Errata =====

  * [[https://github.com/php/php-src/issues/9285|https://github.com/php/php-src/issues/9285]]: It used to be possible to add non-readonly properties to readonly classes via traits. As on PHP 8.2 RC 1, traits cannot be used by readonly classes if they define any non-readonly property, otherwise a compilation error is emitted.

===== Vote =====

Voted started on 2022-04-27, ending on 2022-05-11

<doodle title="Add readonly classes as proposed?" auth="kocsismate" voteType="single" closed="true">
   * Yes
   * No
</doodle>
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