What exactly are late static bindings in PHP?

问题

回答1:

You definitely need to read Late Static Bindings in the PHP manual. However, I'll try to give you a quick summary.

Basically, it boils down to the fact that the self keyword does not follow the same rules of inheritance. self always resolves to the class in which it is used. This means that if you make a method in a parent class and call it from a child class, self will not reference the child as you might expect.

Late static binding introduces a new use for the static keyword, which addresses this particular shortcoming. When you use static, it represents the class where you first use it, ie. it 'binds' to the runtime class.

Those are the two basic concepts behind it. The way self, parent and static operate when static is in play can be subtle, so rather than go in to more detail, I'd strongly recommend that you study the manual page examples. Once you understand the basics of each keyword, the examples are quite necessary to see what kind of results you're going to get.

回答2:

As of PHP 5.3.0, PHP implements a feature called late static binding which can be used to reference the called class in the context of static inheritance.

Late static binding tries to solve that limitation by introducing a keyword that references the class that was initially called at runtime. It was decided not to introduce a new keyword, but rather use static that was already reserved.

Let's see an example:

<?php
    class Car
    {
        public static function run()
        {
            return static::getName();
        }

        private static function getName()
        {
            return 'Car';
        }
    }

    class Toyota extends Car
    {
        public static function getName()
        {
            return 'Toyota';
        }
    }

    echo Car::run(); // Output: Car
    echo Toyota::run(); // Output: Toyota
?>

late static bindings work by storing the class named in the last "non-forwarding call". In case of static method calls, this is the class explicitly named (usually the one on the left of the :: operator); in case of non-static method calls, it is the class of the object.

A "forwarding call" is a static one that is introduced by self::, parent::, static::, or, if going up in the class hierarchy, forward_static_call().

The function get_called_class() can be used to retrieve a string with the name of the called class and static:: introduces its scope.

回答3:

There is not very obvious behavior:

The following code produces 'alphabeta'.

class alpha {

    function classname(){
        return __CLASS__;
    }

    function selfname(){
        return self::classname();
    }

    function staticname(){
        return static::classname();
    }
}

class beta extends alpha {

    function classname(){
        return __CLASS__;
    }
}

$beta = new beta();
echo $beta->selfname(); // Output: alpha
echo $beta->staticname(); // Output: beta

However, if we remove the declaration of the classname function from the beta class, we get 'alphaalpha' as the result.

回答4:

I'm quoting from the book: "PHP Master write cutting-edge code".

Late static binding was a feature introduced with php 5.3. It allows us to inherit static methods from a parent class, and to reference the child class being called.

This means you can have an abstract class with static methods, and reference the child class's concrete implementations by using the static::method() notation instead of the self::method().

Feel free to take a look at the official php documentation as well: http://php.net/manual/en/language.oop5.late-static-bindings.php

The clearest way to explain Late Static Binding is with a simple example. Take a look at the two class definitions below, and read on.

class Vehicle {
    public static function invokeDriveByStatic() {
        return static::drive(); // Late Static Binding
    }
    public static function invokeStopBySelf() {
        return self::stop(); // NOT Late Static Binding
    }
    private static function drive(){
        return "I'm driving a vehicle";
    }
    private static function stop(){
        return "I'm stopping a vehicle";
    }
}

class Car extends Vehicle  {
    protected static function drive(){
        return "I'm driving a CAR";
    }
    private static function stop(){
        return "I'm stopping a CAR";
    }
}

We see a Parent Class (Vehicle) and a Child Class (Car). The Parent Class has 2 public methods:

invokeDriveByStatic
invokeStopBySelf

The Parent Class also has 2 private methods:

drive
stop

The Child Class overrides 2 methods:

drive
stop

Now let's invoke the public methods:

invokeDriveByStatic
invokeStopBySelf

Ask yourself: Which class invokes invokeDriveByStatic / invokeStopBySelf? The Parent or Child class?

Take a look below:

// This is NOT Late Static Binding
// Parent class invokes from Parent. In this case Vehicle.
echo Vehicle::invokeDriveByStatic(); // I'm driving a vehicle
echo Vehicle::invokeStopBySelf(); // I'm stopping a vehicle

// This is Late Static Binding.
// Child class invokes an inherited method from Parent.
// Child class = Car, Inherited method = invokeDriveByStatic().
// ...
// The inherited method invokes a method that is overridden by the Child class.
// Overridden method = drive()
echo Car::invokeDriveByStatic(); // I'm driving a CAR

// This is NOT Late Static Binding
// Child class invokes an inherited method from Parent.
// The inherited method invokes a method inside the Vehicle context.
echo Car::invokeStopBySelf(); // I'm stopping a vehicle

The static keyword is used in a Singleton design pattern. See link: https://refactoring.guru/design-patterns/singleton/php/example

回答5:

The simplest example to show the difference.
Note, self::$c

class A
{
    static $c = 7;

    public static function getVal()
    {
        return self::$c;
    }
}

class B extends A
{
    static $c = 8;
}

B::getVal(); // 7

Late static binding, note static::$c

class A
{
    static $c = 7;

    public static function getVal()
    {
        return static::$c;
    }
}

class B extends A
{
    static $c = 8;
}

B::getVal(); // 8

回答6:

For example:

abstract class Builder {
    public static function build() {
        return new static;
    }
}

class Member extends Builder {
    public function who_am_i() {
         echo 'Member';
    }
}

Member::build()->who_am_i();

回答7:

Looking at it from a "why would I use this?" perspective, it's basically a way to change the context from which the static method is being interpreted/run.

With self, the context is the one where you defined the method originally. With static, it's the one you're calling it from.

回答8:

Also, watch if you update static variables in child classes. I found this (somewhat) unexpected result where child B updates child C:

class A{
    protected static $things;
}

class B extends A {
    public static function things(){
        static::$things[1] = 'Thing B';
        return static::$things; 
    }
}

class C extends A{
    public static function things(){
        static::$things[2] = 'Thing C';
        return static::$things;        
    }
}

print_r(C::things());
// Array (
//   [2] => Thing C
// )

B::things();

print_r(C::things()); 
// Array (
//    [2] => Thing C
//    [1] => Thing B
// )

You can fix it by declaring the same variable in each child class, for example:

class C extends A{
    protected static $things; // add this and B will not interfere!

    public static function things(){
        static::$things[2] = 'Thing C';
        return static::$things;        
    }
}

来源：https://stackoverflow.com/questions/1912902/what-exactly-are-late-static-bindings-in-php

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php

late-binding

late-static-binding