Covariance and Contravariance
In PHP 7.2.0, partial contravariance was introduced by removing type restrictions on parameters in a child method. As of PHP 7.4.0, full covariance and contravariance support was added.
Covariance allows a child's method to return a more specific type than the return type of its parent's method. Whereas, contravariance allows a parameter type to be less specific in a child method, than that of its parent.
A type declaration is considered more specific in the following case:
- A type is removed from a union type
- A type is added to an intersection type
- A class type is changed to a child class type
- iterable is changed to array or Traversable
Covariance
To illustrate how covariance works, a simple abstract parent class, Animal is created. Animal will be extended by children classes, Cat, and Dog.
<?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";
}
}
Note that there aren't any methods which return values in this example. A few factories will be added which return a new object of class type Animal, Cat, or Dog.
<?php
interface AnimalShelter
{
public function adopt(string $name): Animal;
}
class CatShelter implements AnimalShelter
{
public function adopt(string $name): Cat // instead of returning class type Animal, it can return class type Cat
{
return new Cat($name);
}
}
class DogShelter implements AnimalShelter
{
public function adopt(string $name): Dog // instead of returning class type Animal, it can return class type Dog
{
return new Dog($name);
}
}
$kitty = (new CatShelter)->adopt("Ricky");
$kitty->speak();
echo "\n";
$doggy = (new DogShelter)->adopt("Mavrick");
$doggy->speak();
The above example will output:
Ricky meows Mavrick barks
Contravariance
Continuing with the previous example with the classes Animal, Cat, and Dog, a class called Food and AnimalFood will be included, and a method eat(AnimalFood $food) is added to the Animal abstract class.
<?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);
}
}
In order to see the behavior of contravariance, the eat method is overridden in the Dog class to allow any Food type object. The Cat class remains unchanged.
<?php
class Dog extends Animal
{
public function eat(Food $food) {
echo $this->name . " eats " . get_class($food);
}
}
The next example will show the behavior of contravariance.
<?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);
The above example will output:
Ricky eats AnimalFood Mavrick eats Food
But what happens if $kitty tries to eat the $banana?
$kitty->eat($banana);
The above example will output:
Fatal error: Uncaught TypeError: Argument 1 passed to Animal::eat() must be an instance of AnimalFood, instance of Food given
