Generics: Introduction #

Classes, interfaces, and functions can be parameterized to work over a range of types rather than a single one. These entities are called generic.

Generics allow programmers to create components that can work over a variety of types, while providing compile-time type safety that allows these components to be used without runtime checks.

Example #

final class Box<T> {
    private readonly T $value;

    public function __construct(T $value) {
        $this->value = $value;
    }

    public function get(): T {
        return $this->value;
    }
}

The Box class is generic over the type T, it has a constructor, which takes a single argument of type T, and has a method named get(), which returns a value of type T.

The Box class can be used with any type:

$box = new Box::<int>(42);

$box->get(); // 42

$box = new Box::<string>("Hello, World!");

$box->get(); // "Hello, World!"

Generic Functions #

Functions can also be generic:

function identity<T>(T $value): T {
    return $value;
}

The identity() function is generic over the type T, it has a single argument of type T, and returns a value of type T.

The identity() function can be used with any type:

identity::<int>(42); // 42

identity::<string>("Hello, World!"); // "Hello, World!"

Generic Interfaces #

Interfaces can also be generic:

interface Boxable<T> {
    public function get(): T;
}

The Boxable interface is generic over the type T, it has a single method named get(), which returns a value of type T.

The Boxable interface can be extended by any class, which must specify the type of T:

final class StringBox implements Boxable<string> {
    private readonly string $value;

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

    public function get(): string {
        return $this->value;
    }
}

final class IntBox implements Boxable<int> {
    private readonly int $value;

    public function __construct(int $value) {
        $this->value = $value;
    }

    public function get(): int {
        return $this->value;
    }
}