trait-constants.md

Trait Constants

Traits are intended to allow code re-use across classes, but currently do not allow the definition of constants, which limits their ability to provide some types of functionality in a "clean" manner.

Proposal

Allow traits to declare constants, in a similar fashion to classes and interfaces.

Access

Trait constants must be accessed via either the name of a class that uses the trait, or via the self keyword from within the trait, or via a class referencing keyword (i.e. self, static, parent) from within a class (or one of it's child classes) that uses the trait.

Trait constants cannot be accessed directly via the name of the trait. (See https://wiki.php.net/rfc/deprecations_php_8_1#accessing_static_members_on_traits)

Conflicts

As with properties of traits, the values of overlapping trait constants used in a class must be compatible, i.e. they must have the same value, otherwise they are considered conflicting, and a fatal error is produced. Conflict resolution (using the as or insteadof operators) is not supported.

Changing constant visibility

As with methods of traits, constant visibility within the class can be changed using the as operator.

Examples

trait Foo {
	
	public const FLAG_1 = 1;
	
	public const FLAG_2 = 2;

	public function doFoo(int $flags): void {
		if (($flags & self::FLAG_1) === self::FLAG_1) {
			echo 'Got flag 1';
		}
		if (($flags & self::FLAG_2) === self::FLAG_2) {
			echo 'Got flag 2';
		}
	}
}

trait Bar {
	protected const FLAG_1 = 'one';
	
	protected function doBar(string $flag): void {
		switch ($flag) {
			case self::FLAG_1:
				echo 'Got flag one';
			break;
		}
	}
}

trait Baz {
	use Foo;
	
	public const FLAG_2 = 2;
	
	...
}


// Legal:

class A {
	use Foo;
}

class B {
	use Bar {
		FLAG_1 as public;
	}
}

class C {
	use Foo, Baz;
	
	public function __construct() {
		var_dump(self::FLAG_2);
	}
}

class D {
	public static function testD() {
		var_dump(
			self::FLAG_2,
			parent::FLAG_2,
			static::FLAG_2
		);
	}
}


var_dump(C::FLAG_2);


// Fatal Error

class D {
	use Foo, Bar;
}

class E {
	use Foo;
	
	protected const FLAG_2 = 'two';
}

var_dump(Baz::FLAG_2);