PHP: rfc:class_name_scalars

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====== Request for Comments: Class Name Resolution As Scalar Via "class" Keyword ======
  * Version: 1.0
  * Date: 2012-04-17
  * Author: Ralph Schindler <ralph.at.ralphschindler.com>
  * Status: Under Discussion
  * First Published at: http://wiki.php.net/rfc/class_name_scalars

===== Introduction =====

This RFC proposes to add a new ''ClassName::class'' syntax, which provides the fully qualified class name as a string.

===== Use case =====

Class names in strings have to be fully qualified in PHP. It is not possible to utilize aliases registered through ''use'' statements:

<code php>
use A\Namespaced\ClassName;

// this will try to create a mock of the global class ClassName, not
// of the aliased class A\Namespaced\ClassName
$mock = $this->getMock('ClassName');
</code>

''ClassName::class'' allows the programmer to easily obtain the fully qualified class name from an aliased name:

<code php>
use A\Namespaced\ClassName;

// ClassName::class resolves to 'A\Namespaced\ClassName'
$mock = $this->getMock(ClassName::class);
</code>

==== Real World Usage Scenarios ====

1. PHPUnit's Mocking

<code php>

use MyVendor\SomeComponent\TargetSubNs;

// inside a test case
$this->getMock(TargetSubNs\Foo::class);
// as opposed to
$this->getMock('MyVendor\SomeComponent\TargetSubNs\Foo');
</code>

2. Doctrine

<code php>

use MyVendor\SomeComponent\TargetEntityNs as Entity;

// inside a test case
$entityManager->find(Entity\User::class, 5);
// as opposed to
$entityManager->find('MyVendor\SomeComponent\TargetEntityNs\User', 5);

</code>

3. Any Real (auto-wiring || auto-instantiating) Dependency Injection Container

<code php>

use MyVendor\SomeComponent\TargetNs as T;

// inside a test case
$dic->newInstance(T\Foo::class);
// as opposed to
$dic->newInstance('MyVendor\SomeComponent\TargetEntityNs\Foo');

</code>

4. General PHP Usage:

<code php>

use MyVendor\SomeComponent\TargetNs as T;

// reflection
$r = new ReflectionClass(T\Foo::class); // instead of new ReflectionClass('MyVendor\SomeComponent\TargetNs\Foo');

// class_exists, also applies to any php function that takes a class name (is_subclass_of, get_class_methods, etc)
if (class_exists(T\Foo::class, true)) {} // instead of class_exists('MyVendor\SomeComponent\TargetNs\Foo')

</code>
===== Choice of syntax =====

The ''ClassName::class'' syntax was chosen because it can not clash with existing constants (as ''class'' is a keyword). The feature addition thus is fully backwards compatible.

Furthermore the syntax resembles the semantically equivalent Java syntax ''ClassName.class''.

Note: since this feature reuses T_CLASS/class keyword, it is case-insensitive.  Therefore, Foo::class is semantically equivalent to Foo::Class, Foo::CLASS and the like.

===== More examples =====

From the test for the feature:

<code php>

--TEST--
class name as scalar from ::class keyword
--FILE--
<?php

namespace Foo\Bar {
    class One {
        // compile time constants
        const A = self::class;
        const B = Two::class;
    }
    class Two extends One {
        public static function run() {
            var_dump(self::class); // self compile time lookup
            var_dump(static::class); // runtime lookup
            var_dump(parent::class); // runtime lookup
            var_dump(Baz::class); // default compile time lookup
        }
    }
    class Three extends Two {
        // compile time static lookups
        public static function checkCompileTime(
            $one = self::class,
            $two = Baz::class,
            $three = One::A,
            $four = self::B
        ) {
            var_dump($one, $two, $three, $four);
        }
    }
    echo "In NS\n";
    var_dump(Moo::CLASS); // resolve in namespace
}

namespace {
    use Bee\Bop as Moo,
        Foo\Bar\One;
    echo "Top\n";
    var_dump(One::class); // resolve from use
    var_dump(Boo::class); // resolve in global namespace
    var_dump(Moo::CLASS); // resolve from use as
    var_dump(\Moo::Class); // resolve fully qualified
    $class = One::class; // assign class as scalar to var
    $x = new $class; // create new class from original scalar assignment
    var_dump($x);
    Foo\Bar\Two::run(); // resolve runtime lookups
    echo "Parent\n";
    Foo\Bar\Three::run(); // resolve runtime lookups with inheritance
    echo "Compile Check\n";
    Foo\Bar\Three::checkCompileTime();
}

?>
--EXPECTF--
In NS
string(11) "Foo\Bar\Moo"
Top
string(11) "Foo\Bar\One"
string(3) "Boo"
string(7) "Bee\Bop"
string(3) "Moo"
object(Foo\Bar\One)#1 (0) {
}
string(11) "Foo\Bar\Two"
string(11) "Foo\Bar\Two"
string(11) "Foo\Bar\One"
string(11) "Foo\Bar\Baz"
Parent
string(11) "Foo\Bar\Two"
string(13) "Foo\Bar\Three"
string(11) "Foo\Bar\One"
string(11) "Foo\Bar\Baz"
Compile Check
string(13) "Foo\Bar\Three"
string(11) "Foo\Bar\Baz"
string(11) "Foo\Bar\One"
string(11) "Foo\Bar\Two"
</code>

===== Considerations =====

One situation in need of a solution is what to do with ''self::class'', ''static::class'', and ''parent::class''

Do we:

  * <del>Throw a compile error?</del>
  * <del>Resolve as best as possible, meaning error in non-class context, do a runtime lookup in a class context</del>

**Note:** (as of 1/18/13)

In the current patch, the following resolutions take place:
  * self::class resolves the same as %%__CLASS__%% would
  * static::class resolves the same as get_called_class() would
  * parent::class resolves the same as get_parent_class() would, (in fact, would return false if not inherited.)
  * static::class & parent::class when used in compile-time only places (like method signatures), will throw an exception


===== Patch =====

* Pull Request located here: https://github.com/php/php-src/pull/187/files

===== Vote =====


<doodle title="Should the ::class feature be merged to master?" auth="ralphschindler" voteType="single" closed="true">
   * Yes
   * No
</doodle>

Note: This Vote has been closed as the RFC will be moved back to discussion at the time being while the RFC and associated patch become more mature.
===== Changelog =====

  * 2012-04-17 Initially created by Ralph Schindler
  * 2012-09-08 Updated RFC and added link to Pull Request (which addresses considerations and on-list questions)
  * 2013-01-18 Updated RFC to reflect design decisions, updated the test/example
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