Rust
PhantomData

Usando PhantomData como un marcador de tipo

El uso del tipo PhantomData como este le permite usar un tipo específico sin necesidad de que sea parte de Struct.

use std::marker::PhantomData;

struct Authenticator<T: GetInstance> {
    _marker: PhantomData<*const T>, // Using `*const T` indicates that we do not own a T
}

impl<T: GetInstance> Authenticator<T> {
    fn new() -> Authenticator<T> {
        Authenticator {
            _marker: PhantomData,
        }
    }
    
    fn auth(&self, id: i64) -> bool {
        T::get_instance(id).is_some()
    }
}

trait GetInstance {
    type Output; // Using nightly this could be defaulted to `Self`
    fn get_instance(id: i64) -> Option<Self::Output>;
}

struct Foo;

impl GetInstance for Foo {
    type Output = Self; 
    fn get_instance(id: i64) -> Option<Foo> {
        // Here you could do something like a Database lookup or similarly
        if id == 1 {
            Some(Foo)
        } else {
            None
        }
    }
}

struct User;

impl GetInstance for User {
    type Output = Self;
    fn get_instance(id: i64) -> Option<User> {
        // Here you could do something like a Database lookup or similarly
        if id == 2 {
            Some(User)
        } else {
            None
        }
    }
}

fn main() {
    let user_auth = Authenticator::<User>::new();
    let other_auth = Authenticator::<Foo>::new();
    
    assert!(user_auth.auth(2));
    assert!(!user_auth.auth(1));
    
    assert!(other_auth.auth(1));
    assert!(!other_auth.auth(2));
    
}