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协变与逆变

在 PHP 7.2.0 里,通过对子类方法里参数的类型放宽限制,实现对逆变的部分支持。 自 PHP 7.4.0 起开始支持完整的协变和逆变。

协变使子类比父类方法能返回更具体的类型; 逆变使子类比父类方法参数类型能接受更模糊的类型。

在以下情况下,类型声明被认为更具体:

如果情况相反,则类型类被认为是模糊的。

协变

创建一个名为 Animal 的简单的抽象父类,用于演示什么是协变。 两个子类:CatDog 扩展(extended)了 Animal

<?php

abstract class Animal
{
protected
string $name;

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

abstract public function
speak();
}

class
Dog extends Animal
{
public function
speak()
{
echo
$this->name . " barks";
}
}

class
Cat extends Animal
{
public function
speak()
{
echo
$this->name . " meows";
}
}

注意:在这个例子中,没有方法返回了值。 将通过添加个别工厂方法,创建并返回 AnimalCatDog 类型的新对象。

<?php

interface AnimalShelter
{
public function
adopt(string $name): Animal;
}

class
CatShelter implements AnimalShelter
{
public function
adopt(string $name): Cat // 返回类的类型不仅限于 Animal,还可以是 Cat 类型
{
return new
Cat($name);
}
}

class
DogShelter implements AnimalShelter
{
public function
adopt(string $name): Dog // 返回类的类型不仅限于 Animal,还可以是 Dog 类型
{
return new
Dog($name);
}
}

$kitty = (new CatShelter)->adopt("Ricky");
$kitty->speak();
echo
"\n";

$doggy = (new DogShelter)->adopt("Mavrick");
$doggy->speak();

以上例程会输出:

Ricky meows
Mavrick barks

逆变

继续上一个例子,除了 AnimalCatDog,我们还添加了 FoodAnimalFood 类, 同时为抽象类 Animal 添加了一个 eat(AnimalFood $food) 方法。

<?php

class Food {}

class
AnimalFood extends Food {}

abstract class
Animal
{
protected
string $name;

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

public function
eat(AnimalFood $food)
{
echo
$this->name . " eats " . get_class($food);
}
}

为了演示什么是逆变,Dog 类重写(overridden)了 eat 方法, 允许传入任意 Food 类型的对象。 而 Cat 类保持不变。

<?php

class Dog extends Animal
{
public function
eat(Food $food) {
echo
$this->name . " eats " . get_class($food);
}
}

下面的例子展示了逆变。

<?php

$kitty
= (new CatShelter)->adopt("Ricky");
$catFood = new AnimalFood();
$kitty->eat($catFood);
echo
"\n";

$doggy = (new DogShelter)->adopt("Mavrick");
$banana = new Food();
$doggy->eat($banana);

以上例程会输出:

Ricky eats AnimalFood
Mavrick eats Food

$kitty 若尝试 eat $banana 会发生什么呢?

$kitty->eat($banana);

以上例程会输出:

Fatal error: Uncaught TypeError: Argument 1 passed to Animal::eat() must be an instance of AnimalFood, instance of Food given
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User Contributed Notes 7 notes

up
74
xedin dot unknown at gmail dot com
3 years ago
I would like to explain why covariance and contravariance are important, and why they apply to return types and parameter types respectively, and not the other way around.

Covariance is probably easiest to understand, and is directly related to the Liskov Substitution Principle. Using the above example, let's say that we receive an `AnimalShelter` object, and then we want to use it by invoking its `adopt()` method. We know that it returns an `Animal` object, and no matter what exactly that object is, i.e. whether it is a `Cat` or a `Dog`, we can treat them the same. Therefore, it is OK to specialize the return type: we know at least the common interface of any thing that can be returned, and we can treat all of those values in the same way.

Contravariance is slightly more complicated. It is related very much to the practicality of increasing the flexibility of a method. Using the above example again, perhaps the "base" method `eat()` accepts a specific type of food; however, a _particular_ animal may want to support a _wider range_ of food types. Maybe it, like in the above example, adds functionality to the original method that allows it to consume _any_ kind of food, not just that meant for animals. The "base" method in `Animal` already implements the functionality allowing it to consume food specialized for animals. The overriding method in the `Dog` class can check if the parameter is of type `AnimalFood`, and simply invoke `parent::eat($food)`. If the parameter is _not_ of the specialized type, it can perform additional or even completely different processing of that parameter - without breaking the original signature, because it _still_ handles the specialized type, but also more. That's why it is also related closely to the Liskov Substitution: consumers may still pass a specialized food type to the `Animal` without knowing exactly whether it is a `Cat` or `Dog`.
up
6
jotaelesalinas at example dot com
9 months ago
Bear in mind that, although the PHP syntax does not allow stricter method parameter types, you can achieve it programmatically:

<?php
class AnimalFood { ... }
class
CatFood extends AnimalFood { ... }

class
Animal
{
    public function
eat(AnimalFood $food) {
        echo
$this->name . " eats " . get_class($food);
    }
}

class
Cat extends Animal
{
    public function
eat(AnimalFood $food) {
        if (!
$food instanceof CatFood) {
           throw new \
InvalidArgumentException(...);
        }
       
parent::eat($food);
    }
}
?>
up
6
Anonymous
2 years ago
Covariance also works with general type-hinting, note also the interface:

interface xInterface
{
    public function y() : object;
}

abstract class x implements xInterface
{
    abstract public function y() : object;
}

class a extends x
{
    public function y() : \DateTime
    {
        return new \DateTime("now");
    }
}

$a = new a;
echo '<pre>';
var_dump($a->y());
echo '</pre>';
up
10
phpnet-at-kennel17-dotco-dotuk
2 years ago
Following the examples above, you might assume the following would be possible.

<?php

class CatFood extends AnimalFood { ... }

class
Cat extends Animal
{
    public function
eat(CatFood $food) {
        echo
$this->name . " eats " . get_class($food);
    }
}

?>

However, the Liskov Substitution Prinicpal, and therefore PHP, forbids this.  There's no way for cats to eat cat food, if animals are defined as eating animal food.

There are a large number of legitimate abstractions that are forbidden by PHP, due to this restriction.
up
1
Hayley Watson
3 months ago
The gist of how the Liskov Substition Princple applies to class types is, basically: "If an object is an instance of something, it should be possible to use it wherever an instance of something is allowed". The Co- and Contravariance rules come from this expectation when you remember that "something" could be a parent class of the object.

For the Cat/Animal example of the text, Cats are Animals, so it should be possible for Cats to go anywhere Animals can go. The variance rules formalise this.

Covariance: A subclass can override a method in the parent class with one that has a narrower return type. (Return values can be more specific in more specific subclasses; they "vary in the same direction", hence "covariant").
If an object has a method you expect to produce Animals, you should be able to replace it with an object where that method produces only Cats. You'll only get Cats from it but Cats are Animals, which are what you expected from the object.

Contravariance: A subclass can override a method in the parent class with one that has a parameter with a wider type. (Parameters can be less specific in more specific subclasses; they "vary in the opposite direction", hence "contravariant").
If an object has a method you expect to take Cats, you should be able to replace it with an object where that method takes any sort of Animal. You'll only be giving it Cats but Cats are Animals, which are what the object expected from you.

So, if your code is working with an object of a certain class, and it's given an instance of a subclass to work with, it shouldn't cause any trouble:
It might accept any sort of Animal where you're only giving it Cats, or it might only return Cats when you're happy to receive any sort of Animal, but LSP says "so what? Cats are Animals so you should both be satisfied."
up
0
maxim dot kainov at gmail dot com
1 year ago
This example will not work:

<?php

class CatFood extends AnimalFood { }

class
Cat extends Animal
{
    public function
eat(CatFood $food) {
        echo
$this->name . " eats " . get_class($food);
    }
}

?>

The reason is:

<?php
   
class DogFood extends AnimalFood { }
  
    function
feedAnimal(Animal $animal, AnimalFood $food) {
       
$animal->eat($food);  
    }

   
$cat = new Cat();
   
$dogFood = new DogFood();  

   
feedAnimal($cat, $dogFood);   
?>

But you can do it with traits, like this:

<?php

trait AnimalTrait
{
    public function
eat(AnimalFood $food)
    {
        echo
$this->name . " ест " . get_class($food);
    }
}

class
Cat
{
    use
AnimalTrait;

    public function
eat(CatFood $food) {
        echo
$this->name . " eats " . get_class($food);
    }
}

?>
up
-1
hrustbb2 at gmail dot com
8 months ago
Почему это не работает с интерфейсами? Почему я не могу в дочернем методе ограничить тип возвращаемого значения более конкретным интерфейсом?
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