Rust
PhantomData

Utilisation de PhantomData comme marqueur de type

L'utilisation du type PhantomData comme ceci vous permet d'utiliser un type spécifique sans avoir besoin de faire partie 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));
    
}