Attributes

General

Programmers can invent a new kind of declarative information, called an attribute. Attributes can be attached to various program entities, and information about those attribute can be retrieved at run-time via reflection (see library class Reflection, et al).

Consider the following example:

<<Help("http://www.MyOnlineDocs.com/Widget.html")>>
class Widget {
  …
}

$rc = new ReflectionClass('Widget');
$attrHelp = $rc->getAttribute('Help');

The method getAttribute returns an array containing the values corresponding to an attribute, in lexical order of their specification. As the Help attribute on class Widget has only one value, a string, array element 0 is the string containing that string, in this case, a URL at which the corresponding help information can be found.

A number of predefined attributes (§§) affect the way in which source code is compiled.

Attribute Specification

Syntax

attribute-specification:
  <<  attribute-list  >>
attribute-list:
  attribute
  attribute-list  ,  attribute
attribute:
  attribute-name  attribute-value-list_opt
attribute-name:
  name
attribute-value-list:
  (  attribute-values_opt  )
attribute-values
  attribute-value
  attribute-values  ,  attribute-value
attribute-value:
  expression

name is defined in §§; and expression is defined in §§.

Constraints

The names in an attribute-list must be distinct.

The expressions in an attribute-list must be constant-expressions (§§).

Semantics

A name in a user-defined attribute means whatever the programmer wants.

The names do not have any scope, per se. They appear only in the context of an attribute-specification, and do not hide nor conflict with the same names used in other contexts.

Omitting attribute-value-list is equivalent to specifying it without attribute-values.

Examples

<<A1(123), A2>>
class C {
  <<A3(true, 100)>> public function __construct() { … }
  <<A4>> public function doit(): void { … }
}

Predefined Attributes

General

The following attributes are recognized by a conforming implementation:

__ConsistentConstruct (§§)
__Memoize (§§)
__Override (§§)

Attribute `__ConsistentConstruct`

This attribute can be applied to classes; it has no attribute values.

When a method is overridden in a derived class, it must have exactly the same number, type, and order of parameters as that in the base class (§§). However, that is not the case for constructors (§§). Having a family of constructors with different signatures can cause a problem, however, especially when using new static (§§).

Consider the following example:

<<__ConsistentConstruct>>
class Base {
  public function __construct() { … }

  public static function make(): this {
    return new static();
  }
}

class Derived extends Base {
  public function __construct() {
    …
    parent::__construct();
  }
}

$v2 = Derived::make();

When make is called on a Derived object, new static results in Derived's constructor being called knowing only the parameter list of Base's constructor. As such, Derived's constructor must either have the exact same signature as Base's constructor, or the same plus an ellipses (§§) indicating a trailing variable-argument list.

Attribute `__Memoize`

This attribute can be applied to functions and static or instance methods; it has no attribute values.

The presence of this attribute causes the designated method to cache automatically each value it looks up and returns, so future requests for that same lookup can be retrieved more efficiently. The set of parameters is hashed into a single hash key, so changing the type, number, and/or order of the parameters can change that key.

Consider the following example:

class Item {
  <<__Memoize>>
  public static function getNameFromProductCode(int $productCode): string {
    return Item::getNameFromStorage($productCode);  // conditional call
  }
  private static function getNameFromStorage(int $productCode): string {
    // get name from alternate store
    return …;
  }
}

Item::getNameFromStorage will only be called if the given product code is not in the cache.

The types of the parameters are restricted to the following: null, bool, int, float, string, any object type that implements IMemoizeParaminterface, enum constants, tuples, shapes, and arrays/collections containing any supported element type.

The interface type IMemoizeParam (§§) assists with memorizing objects passed to async functions.

Attribute `__Override`

This attribute can be applied to static or instance methods; it has no attribute values.

The presence of this attribute indicates that the designated method is intended to override a method having the same name in a direct or indirect base class. If no such base-class method exists, a compile-time error occurs.

Consider the following example:

class Button {
  public function draw(): void { /* … */ }
}
class CustomButton extends Button {
  <<__Override>>
  public function draw(): void { /* … */ }
}

If a subsequent refactoring of class Button results in the removal of method draw, the presence of the attribute in class CustomButton will cause the dependence to be reported.

When __Override is applied to a method in a trait, the check for whether the overridden method exists takes place when a class uses that trait.

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