后期静态绑定

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自 PHP 5.3.0 起，PHP 增加了一个叫做后期静态绑定的功能，用于在继承范围内引用静态调用的类。

准确说，后期静态绑定工作原理是存储了在上一个“非转发调用”（non-forwarding call）的类名。当进行静态方法调用时，该类名即为明确指定的那个（通常在 :: 运算符左侧部分）；当进行非静态方法调用时，即为该对象所属的类。所谓的“转发调用”（forwarding call）指的是通过以下几种方式进行的静态调用：self::，parent::，static:: 以及 forward_static_call() 。可用 get_called_class() 函数来得到被调用的方法所在的类名，static:: 则指出了其范围。

该功能从语言内部角度考虑被命名为“后期静态绑定”。“后期绑定”的意思是说，static:: 不再被解析为定义当前方法所在的类，而是在实际运行时计算的。也可以称之为“静态绑定”，因为它可以用于（但不限于）静态方法的调用。

self:: 的限制

使用 self:: 或者 __CLASS__ 对当前类的静态引用，取决于定义当前方法所在的类：

Example #1 self:: 用法

  <?php
 class  A  {
    public static function  who () {
        echo  __CLASS__ ;
    }
    public static function  test () {
         self :: who ();
    }
}

class  B  extends  A  {
    public static function  who () {
        echo  __CLASS__ ;
    }
}

 B :: test ();
 ?>

以上例程会输出：

后期静态绑定的用法

后期静态绑定本想通过引入一个新的关键字表示运行时最初调用的类来绕过限制。简单地说，这个关键字能够让你在上述例子中调用 test() 时引用的类是 B 而不是 A。最终决定不引入新的关键字，而是使用已经预留的 static 关键字。

Example #2 static:: 简单用法

  <?php
 class  A  {
    public static function  who () {
        echo  __CLASS__ ;
    }
    public static function  test () {
        static:: who ();  // 后期静态绑定从这里开始
     }
}

class  B  extends  A  {
    public static function  who () {
        echo  __CLASS__ ;
    }
}

 B :: test ();
 ?>

以上例程会输出：

Note:

在非静态环境下，所调用的类即为该对象实例所属的类。由于 $this-> 会在同一作用范围内尝试调用私有方法，而 static:: 则可能给出不同结果。另一个区别是 static:: 只能用于静态属性。

Example #3 非静态环境下使用 static::

  <?php
 class  A  {
    private function  foo () {
        echo  "success!\n" ;
    }
    public function  test () {
         $this -> foo ();
        static:: foo ();
    }
}

class  B  extends  A  {
    
 }

class  C  extends  A  {
    private function  foo () {
         
     }
}

 $b  = new  B ();
 $b -> test ();
 $c  = new  C ();
 $c -> test ();    //fails
 ?>

以上例程会输出：

success!
success!
success!Fatal error:  Call to private method C::foo() from context 'A' in /tmp/test.php on line 9

Note:

后期静态绑定的解析会一直到取得一个完全解析了的静态调用为止。另一方面，如果静态调用使用 parent:: 或者 self:: 将转发调用信息。
Example #4 转发和非转发调用

<?php class A { public static function foo () { static:: who (); } public static function who () { echo __CLASS__ . "\n" ; } } class B extends A { public static function test () { A :: foo (); parent :: foo (); self :: foo (); } public static function who () { echo __CLASS__ . "\n" ; } } class C extends B { public static function who () { echo __CLASS__ . "\n" ; } } C :: test (); ?>

以上例程会输出：
A
C
C

用户评论:

[#1] sskaje at gmail dot com [2015-08-31 04:01:56]

static::class and self::class can be used to get current class name,
work under 5.5 and 5.6
failed in 5.3.

<?php
class a{
function d() {
echo "=== self::class ===\n";
var_dump(self::class);
echo "=== static::class ===\n";
var_dump(static::class);
}
}
class b extends a{}
class c extends b{}

a::d();
b::d();
c::d();

[#2] tamilps2 at gmail dot com [2014-03-20 05:42:26]

I have implemented enum using late static binding.

<?php interface IEnum { public static function who(); } abstract class Enum { public $value; public function __construct($value) { $this->value = $value; } public static function Factory($type) { if (empty($type)) { return false; } // use of late static binding to get the class. $class = static::who(); if (array_key_exists($type, static::$_enums)) { return new $class($type); } return false; } public function getValue() { return $this->value; } public static function getValues() { return array_keys(static::$_enums); } public function getString() { return static::$_enums[$this->value]; } public function __toString() { return static::$_enums[$this->value]; } } class Fruits extends Enum implements IEnum { public static $_enums = array( 1 => 'Apple' 2 => 'Orange' 3 => 'Banana' ) public static function who() { return __CLASS__; } } // Usage // user input from dropdown menu of fruits list $input = 3; $fruit = Fruits::Factory($input); $fruit->getValue(); // 3 $fruit->getString(); // Banana ?>

[#3] mhh1422 at hotmail dot com [2013-12-29 11:58:15]

For abstract classes with static factory method, you can use the static keyword instead of self like the following:
<?php abstract class A{ static function create(){ //return new self(); //Fatal error: Cannot instantiate abstract class A return new static(); //this is the correct way } } class B extends A{ } $obj=B::create(); var_dump($obj); ?>

[#4] Taai [2012-08-21 10:36:54]

I discovered an interesting thing. The class name string must be accessed directly from "flat" variable. Late static binding code that get's it's variable from array that is passed by class instance, throws an syntax error. Bug?

<?php class A { public $metadata = array('class' => 'A'); public static function numbers() { return 123; } } $instance = new A(); // This throws an error // Parse error: syntax error, unexpected '::' (T_PAAMAYIM_NEKUDOTAYIM) var_dump( $instance->metadata['class']::numbers() ); // Get the class name and store it in "flat" variable and now it's ok $class_name = $instance->metadata['class']; var_dump( $class_name::numbers() ); // Other tests ------------------------------------------- $arr = array('class' => 'A'); // This works too. var_dump( $arr['class']::numbers() ); ?>

[#5] jrfish dot x at gmail dot com [2010-12-09 04:05:04]

also works the same way with static variables and constants

[#6] jrfish dot x at gmail dot com [2010-12-09 04:01:59]

consider this:

<?php class A { // some stuff.... public static function getInstance() { return new self(); } } class B extends A { //stuff... } $obj = B::getInstance(); //versus class A { // some stuff.... public static function getInstance() { return new static(); } } class B extends A { //stuff... } $obj = B::getInstance(); ?>

also works the same way with static variables and constants

[#7] Anonymous [2010-11-24 09:22:08]

THIS WORKED GREAT FOR ME:

<?php abstract class ParentClass { static function parent_method() { $child_class_str = self::get_child_class(); eval("\$r = ".$child_class_str."::abstract_static();"); return $r; }// CHILD MUST OVERRIDE TO PUT ITSELF INTO TRACE protected abstract static function abstract_static(); private static function get_child_class() { $backtrace = debug_backtrace(); $num = count($backtrace); for($i = 0; $i < $num; $i++) { if($backtrace[$i]["class"] !== __CLASS__) return $backtrace[$i]["class"]; } return null; } } class ChildClass extends ParentClass { static function parent_method(){ return parent::parent_method(); } protected static function abstract_static() { return __METHOD__."()"; }// From ParentClass } print "The call was: ". ChildClass::parent_method(); ?>

[#8] jakub dot lopuszanski at nasza-klasa dot pl [2010-11-08 10:12:48]

Suprisingly consts are also lazy bound even though you use self instead of static:
<?php class A{ const X=1; const Y=self::X; } class B extends A{ const X=1.0; } var_dump(B::Y); // float(1.0) ?>

[#9] tfn dot yldrm at hotmail dot com [2010-10-26 12:30:45]

example for static binding on 5.2 and real enumaration
<?php abstract class EnumBase { protected $num = 0; public $toString; public $toInt; public function __construct($enumKeyOrVal) { unset($this->toString,$this->toInt); $enums = $this->enums(); if( empty($enumKeyOrVal) || !(isset($enums[$this->num = $enumKeyOrVal]) || ($this->num = array_search($enumKeyOrVal,$enums)) !== false) ) $this->num = 0; } #5.3 e ge?ilirse gerek kalmaz. public function vars() { return get_class_vars(get_class($this)); } public function enums() { $vars = $this->vars(); return $vars['enums']; } public function __get($property) { if(method_exists($this,'__'.$property)) return $this->{'__'.$property}(); else return $this->__toString(); } public function __toInt() { return $this->num; } public function __toString() { $enums = $this->enums(); if(isset($enums[$this->num])) { return $enums[$this->num]; } else { return $enums[0]; } } } class Positions extends EnumBase { public static $enums = array( 0 => 'Bilinmiyor', 1 => 'Kale', 2 => 'Defans', 3 => 'Orta Saha', 4 => 'Forvet' ); } $a = new Positions('Orta Saha'); $b = new Positions(4); $c = (string)$a; // Orta Saha $d = (string)$b; // Forvet ?>

[#10] steven dot karas+nospam at gmail dot com [2010-09-27 14:38:48]

This function can be used as a workaround for late static binding in PHP >= 5.1.0. There was another similar version of this function elsewhere, but used eval.

<?php function & static_var($class, $name) { if (is_object($class)) { $class = get_class($class); } elseif ( ! is_string($class)) { throw new Exception('Must be given an object or a class name', NULL); } $class = new ReflectionClass($class); return $class->getStaticPropertyValue($name); } ?>

[#11] adam dot prall at thinkingman dot com [2010-06-11 09:51:23]

Just a quick reminder to always check your syntax. While I love LSB, I thought it wasn't working:

static::$sKey = not set

??until I realized that I??d completely forgotten to make it a variable variable:

$sKey = 'testStaticClassVarNameThatExistsInThisClassesScope';

static::$$sKey = is set

??of course this applies anywhere in PHP, but because of the (current) newness late static bindings, I??ve seen lots of code with this particular snafu in it from others.

[#12] joost dot t dot hart at planet dot nl [2009-08-21 06:34:40]

PHP5.3 unavailable, yet in the need for 'static', I did the following.

Any objections? Personally I hate using the the eval() statement...

<?php class mother { function setStatic( $prop, $val ) { // After this, self:: refers to mother, yet next $class refers to... // $class = get_class( $this ); eval( "$class::\$$prop = \$$val;" ); } } class child extends mother { protected static $sProp; function writer( $value ) { parent::setStatic( 'sProp', $value ); } function reader() { return self::$sProp; } } $c = new child(); $c->writer( 3 ); echo $c->reader(); // 3 ?>

[#13] tom [2009-07-03 06:28:57]

Something you may find useful for passive code-testing:

<?php class BaseClass { function __get($id) { throw new Exception("Trying to access undefined property '$id'."); } function __set($id) { throw new Exception("Trying to access undefined property '$id'."); } } class MyClass extends BaseClass { // my implementation } ?>

Using these magic function as described above will help you to find classes that try to access an undefined (and undocumented) class-member. In most cases: this is an error based on misspelled member names.

[#14] kenneth at kennethjorgensen dot com [2009-03-30 02:19:00]

Simple basic class which uses to get_called_class() to create singleton instances. A previous post by php at mikebird dot co dot uk explain how to do this, but the extended static variables require you to define them in child classes before they work.

<?php abstract class Singleton { private static $instances = array(); public function __construct() { $class = get_called_class(); if (array_key_exists($class, self::$instances)) trigger_error("Tried to construct a second instance of class \"$class\"", E_USER_WARNING); } public static function getInstance() { $class = get_called_class(); if (array_key_exists($class, self::$instances) === false) self::$instances[$class] = new $class(); return self::$instances[$class]; } } class A extends Singleton { } class B extends Singleton { } $a1 = A::getInstance(); $a2 = A::getInstance(); $b1 = B::getInstance(); $b2 = B::getInstance(); if (get_class($a1) == "A" && get_class($a2) == "A" && get_class($b1) == "B" && get_class($b2) == "B" && $a1 === $a2 && $b1 === $b2) echo "All good\n"; else echo "FAIL!\n"; ?>

You probably noticed the use of self:: rather than static::, this is because we want the static variable to be private, and using static:: will not allow us to do that.

[#15] gern_ at hotmail dot com [2008-09-18 07:51:29]

get_called_class for PHP < 5.3

<?php function get_called_class ($i_level = 1) { $a_debug = debug_backtrace(); $a_called = array(); $a_called_function = $a_debug[$i_level]['function']; for ($i = 1, $n = sizeof($a_debug); $i < $n; $i++) { if (in_array($a_debug[$i]['function'], array('eval')) || strpos($a_debug[$i]['function'], 'eval()') !== false) continue; if (in_array($a_debug[$i]['function'], array('__call', '__callStatic'))) $a_called_function = $a_debug[$i]['args'][0]; if ($a_debug[$i]['function'] == $a_called_function) $a_called = $a_debug[$i]; } if (isset($a_called['object']) && isset($a_called['class'])) return (string)$a_called['class']; $i_line = (int)$a_called['line'] - 1; $a_lines = explode("\n", file_get_contents($a_called['file'])); preg_match("#([a-zA-Z0-9_]+){$a_called['type']} {$a_called['function']}( )*\(#", $a_lines[$i_line], $a_match); unset($a_debug, $a_called, $a_called_function, $i_line, $a_lines); if (sizeof($a_match) > 0) $s_class = (string)trim($a_match[1]); else $s_class = (string)$a_called['class']; if ($s_class == 'self') return get_called_class($i_level + 2); return $s_class; } ?>

[#16] kx [2008-09-14 09:39:32]

At least as of PHP 5.3.0a2 there's a function get_called_class(), which returns the class on which the static method is called.

<?php class a { static public function test() { print get_called_class(); } } class b extends a { } a::test(); // "a" b::test(); // "b" ?>

[#17] iamscrumpyjack [2008-09-07 09:01:41]

I have been dying to see this issue resolved. I'm very much looking forward to the production release of PHP 5.3...

In my case I have been trying to do the following:

class A {
  function __construct() {
    echo "I was called by " . static::__CLASS__;
  }
}

class B extends A {
  function Foo() {
    echo "I am class " . __CLASS__;
  }
}

$b = new B; // Should echo "I was called by B"
$b->Foo(); // Should echo "I am class B"

At the moment I do the following workaround:

class A {
  function __construct($child) {
    echo "I was called by " . $child;
  }
}

class B extends A {
  function __construct() {
    parent::__construct(__CLASS__);
  }

  function Foo() {
    echo "I am class " . __CLASS__;
  }
}

$b = new B; // Echos "I was called by B"
$b->Foo(); // Echo "I am class B"

As you can see, my current workaround has some overhead and is not as water-tight as the late static binding method.

[#18] sebastien at info-conseil dot fr [2008-07-17 07:26:29]

Here is a small workaround I made for the static inheritance issue. It's not perfect, but it works.

<?php // BaseClass class will be extended by any class needing static inheritance workaroud class BaseClass { // Temporarily stores class name for Entry::getStatic() and Entry::setNextStatic() protected static $nextStatic = false; // Returns the real name of the class calling the method, not the one in which it was declared. protected static function getStatic() { // If already stored if (self::$nextStatic) { // Clean and return $class = self::$nextStatic; self::$nextStatic = false; return $class; } // Init $backTrace = debug_backtrace(); $class = false; // Walk through for ($i=0; $i<count($backTrace); $i++) { // If a class is defined if (isset($backTrace[$i]['class'])) { // Check if it is not a basic class if (!in_array($backTrace[$i]['class'], array('BaseClass', 'GenericClass'))) { return $backTrace[$i]['class']; } else { $class = $backTrace[$i]['class']; } } else { // Returns last known class return $class; } } // Default return $class; } // If a static method is called within global env, the previous method won't work, so we need to tell BaseClass which public static function setNextStatic($class) { // Save value self::$nextStatic = $class; } } // Generic class declaring various static methods class GenericClass extends BaseClass { public static $name = 'Generic'; public function getName() { $static = get_class_vars(get_class($this)); return $static['name']; } public static function basicClassName() { return self::$name; } public static function staticClassName() { // Get real name $staticName = self::getStatic(); // Return final class name $static = get_class_vars($staticName); return $static['name']; } } // Final class class SomeClass extends GenericClass { public static $name = 'Some'; public static function returnClassNameWith($string) { return $string.' : '.self::staticClassName(); } } // Instance call // Will print 'Some' $a = new SomeClass(); echo 'Name of $a : '.$a->getName().'<br />'; // Static calls // Will print 'Generic' echo 'Basic call to SomeClass::$name : '.SomeClass::basicClassName().'<br />'; // Will print 'Generic' echo 'Global call to SomeClass::$name : '.SomeClass::staticClassName().'<br />'; // Will print 'Some' BaseClass::setNextStatic('SomeClass'); echo 'Global call to SomeClass::$name with pre-set : '.SomeClass::staticClassName().'<br />'; // Will print 'Some' echo 'Internal call to SomeClass::$name : '.SomeClass::returnClassNameWith('This is a ').'<br />'; ?>

There are two issues with this workaround :
- if you call a static method from global env, you need to declare the name of the class BEFORE calling the method, otherwise the workaround won't work (see 3rd and 4th examples). But I assume good programming makes few calls to static methods from global scope, so this shouldn't be long to fix if you use it.
- the workaround fails to access to private or protected static vars, as it uses get_class_vars(). If you find any better solution, let us know.

With Php 5.3.0, upgrading will be easy : just delete the methods from the basic class, and search/replace any call to getStatic() and setNextStatic() by static:: - or one could use a selector on PHP_VERSION value to include either the BaseClass file with workaround or a BaseClass file using static::

[#19] Anonymous [2008-07-12 12:49:11]

Trying to recreate an inheritable static part for an object through a singleton pattern.

<?php

abstract class Inheritable_Static extends Singleton
{
}

abstract class Inheritable
{
public static function getStatic($className)
{
// Use an abstract Singleton
return Singleton::getInstance($className . '_Static') ;
}

public function goStatic()
{
return self::getStatic(get_class($this)) ;
}
}

abstract class SayIt_Abstract_Static extends Inheritable_Static
{
public $format ;
}

abstract class SayIt_Abstract extends Inheritable
{
protected $_name ;

public function __construct($name)
{
$this->_name = $name ;
}

final public function sayIt()
{
echo sprintf($this->goStatic()->format, $this->_name) . "\n" ;
}

}

class SayIt_Hello_Static extends SayIt_Abstract_Static
{
}

class SayIt_Hello extends SayIt_Abstract
{
public static function getStatic() { return parent::getStatic(__CLASS__) ; }
}

SayIt_Hello::getStatic()->format = 'Hello %s' ;

$w = new SayIt_Hello('World') ;
$j = new SayIt_Hello('Joe') ;

echo $w->sayIt() ; // Hello World
echo $j->sayIt() ; // Hello Joe

[#20] Andrea Giammarchi [2008-06-21 13:06:09]

About static parameters, these work as expected.
<?php class A { protected static $__CLASS__ = __CLASS__; public static function constructor(){ return static::$__CLASS__; } } class B extends A { protected static $__CLASS__ = __CLASS__; } echo B::constructor(); // B ?>

[#21] martinpauly [at] google mail [dot] com [2008-06-18 05:54:09]

will this work for variables as well?

it would be great, if the following worked:

<?php class A { protected static $table = "table"; public static function connect(){ //do some stuff here echo static::$table; return static::getInstance(); //function getInstance() now can return classes A or B depending on the context it was called } ... } class B extends A { protected static $table = "subtable"; ... } $table = B::connect(); //hopefully the output will be: subtable ?>

[#22] deadimp at gmail dot com [2008-06-05 10:39:32]

I think this will be pretty helpful too.
My question is, can just 'static' by itself resolve to the late static class?
I ask this because it could help in making new instances of the derived class, from a base class, by calling a derived class's static method instead of having to create a new instance of the derived class - or explicitly defining a 'getClass' method for each derived class.
Example:
<?php //There isn't really any purpose for this example I posted //Just a random implementation class Base {     static function useful() {         //Create a list of instances of the derived class         $list=array();         for ($i=0;$i<10;$i++) $list[]=new static(); //Here's the point in question         return $list;     } } class Derived extends Base {     static function somethingElse() {         //...         $list=static::useful();     } } ?>
I'm not sure what kind of lexical / whatever-it's-called problems this would make with parsing. I don't think it could really collide with any contexts where you would use static otherwise - variable / method declaration.

Even more so, is there a way to get the class's name to which the keywords 'self', 'parent', or 'static' refer?
Example:
<?php class Base {     static function stuff() {         echo "Self: ".get_class(self);         echo "Parent: ".get_class(parent);         echo "Derived: ".get_class(static);     } } class Derived extends Base {     static function stuff() {         static::stuff();     } } ?>

I don't think there should be a massive bloat in the PHP core to support all of this, but it would be nice to take advantage of the dynamic nature of PHP.

And yet another side note:
If you're in the instance-level scope in a method of a base, and you want to get a top-level static, here's an ugly workaround (from Thacmus /lib/core.php - see SVN repo):
<?php //Get reference [?] to static from class     //$class - Class name OR object (uses get_class())     //$var - Not gonna say function& get_static($class,$var) { //'static_get'?     if (!is_string($class)) $class=get_class($class);     if (!@property_exists($class,$var)) {         trigger_error("Static property does not exist: $class::\$$var");         //debug_callstack(); //This is just a wrapper for debug_backtrace() for HTML         return null;     }     //Store a reference so that the base data can be referred to         //The code [[ return eval('return &'.$class.'::$'.$var.';') ]] does not work - can not return references...         //To establish the reference, use [[ $ref=&get_static(...) ]]     eval('$temp=&'.$class.'::$'.$var.';'); //using     return $temp; } ?>

[#23] tyler AT canfone [dot] COM [2008-06-04 22:48:34]

@ php at mikebird

You can pass arguments to your constructor through your getInstance method, assuming you are running php5.

        public static function getInstance($params = null) {
            if (self::$objInstance == null) {
                $strClass = static::getClass();
                self::$objInstance = new $strClass($params);
            }
            return self::$objInstance;
        }

This would pass the params to your constructor. Love for php.

[#24] sergei at 2440media dot com [2008-05-28 13:22:38]

Finally we can implement some ActiveRecord methods:

<?php class Model { public static function find() { echo static::$name; } } class Product extends Model { protected static $name = 'Product'; } Product::find(); ?>

Output: 'Product'

[#25] php at mikebird dot co dot uk [2008-04-23 08:39:48]

This should make life easier and neater if you have a project with a lot of singleton classes e.g.

<?php class Singleton { public static $objInstance; public static function &getInstance() { if (self::$objInstance == null) { $strClass = static::getClass(); self::$objInstance = new $strClass; } return self::$objInstance; } public static function getClass() { return __CLASS__; } } class Foo extends Singleton { public $intBar; public function __construct() { $this->intBar = 1; } public static function getClass() { return __CLASS__; } } $objFooTwo = Foo::getInstance(); $objFooTwo->intBar = 2; $objFooOne = Foo::getInstance(); if ($objFooOne->intBar == $objFooTwo->intBar) { echo 'it is a singleton'; } else { echo 'it is not a singleton'; } ?>

The above will output 'it is a singleton'. The obvious downfall to this method is not being able to give arguments to the constructor.

[#26] max at mastershrimp dot com [2008-04-10 15:24:31]

If you are using PHP < 5.3.0 you might be interested in the following workaround for late static binding: http://de2.php.net/manual/de/function.get-class.php#77698