downcasting.rs

use std::any::Any;

pub trait ToAny {
    fn as_any(&self) -> &dyn Any;
}

trait A: ToAny {

}

#[derive(Debug, Eq, PartialEq)]
struct B {
    id: i32,
}

impl ToAny for B {
    fn as_any(&self) -> &dyn Any {
        self
    }
}

impl A for B {

}

#[test]
fn a() {
    let b = B {id: 1};
    let boxed: Box<dyn A> = Box::new(b);

    let b: &B = match boxed.as_any().downcast_ref::<B>() {
        Some(b) => b,
        None => panic!(),
    };

    assert_eq!(b, &B{id: 1});
}

#[test]
fn b() {
    let boxeds: Vec<Box<dyn A>> = vec![
        Box::new(B {id: 1}),
        Box::new(B {id: 2}),
        Box::new(B {id: 3}),
        Box::new(B {id: 4}),
    ];

    let mut values: Vec<&B> = vec![];
    for boxed in &boxeds {
        let found = match boxed.as_any().downcast_ref::<B>() {
            Some(b) => b,
            None => panic!(),
        };

        values.push(found);
    }

    assert_eq!(values, vec![
        &B {id: 1},
        &B {id: 2},
        &B {id: 3},
        &B {id: 4}
    ]);
}