PHP: rfc:class_name_literal_on_object

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====== PHP RFC: Allow ::class on objects  ======
  * Date: 2020-01-09
  * Author: Nikita Popov <nikic@php.net>
  * Target Version: PHP 8.0
  * Status: Implemented
  * Implementation: https://github.com/php/php-src/pull/5065

===== Introduction =====

The ''Foo\Bar::class'' syntax has been introduced in PHP 5.5 to allow fetching a class name as a string, in a way that respects context-dependent name resolution rules and is understood by IDEs. As it is syntactically similar to a class constant access, programmers intuitively expect the syntax ''$object::class'' to work as well and provide the same result as ''get_class($object)''. This RFC proposes to allow that syntax.

The original rationale for not allowing the ''$object::class'' syntax was that ''Foo\Bar::class'' is resolved at compile-time, which is not possible for ''$object::class''. However, this premise is already incorrect for two cases: First, ''static::class'' is always resolved at runtime, equivalent to ''get_called_class()''. Second, ''self::class'' and ''parent::class'' are also sometimes resolved at runtime, for example in closures.

Overall I think that ''$object::class'' has a well-defined meaning, and programmers familiar with the ''Foo\Bar::class'' syntax generally expect it to be available by symmetry with the general ''$object::CONST_NAME'' syntax. Not allowing the syntax is more surprising than allowing it.

===== Proposal =====

''$object::class'' is already permitted on the syntax level, but currently throws a compile error. This error will be removed when used in a normal expression context. However, ''$object::class'' will remain forbidden inside a constant expression context (as objects cannot be created there).

If ''$object'' is an object, then ''$object::class'' returns ''get_class($object)''. Otherwise it throws a ''TypeError'' exception.

<PHP>
$object = new stdClass;
var_dump($object::class); // "stdClass"

$object = null;
var_dump($object::class); // TypeError
</PHP>

This RFC does not permit ''$object'' to be a string and throws a ''TypeError'' in that case as well. It would be possible to allow strings to be consistent with the ''$className::CONST_NAME'' syntax. In that case, ''$str::class'' would return ''$str'' verbatim. As this doesn't seem practically useful, and likely somewhat surprising (esp. the fact that the class name is not validated at all, and can contain characters not allowed in class names), this functionality is omitted.

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

There are no backwards incompatible changes.

===== Vote =====

Voting started 2020-01-28 and ends 2020-02-11.

<doodle title="Allow ::class on objects?" auth="nikic" voteType="single" closed="true">
   * Yes
   * No
</doodle>
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