How do I clone a HashMap containing a boxed trait object?

Question

I have HashMap with custom hasher. Items of this HashMap without implemented trait Clone (it's a trait), but there is function to clone items like this:

use std::collections::HashMap;
use std::hash::BuildHasherDefault;

use fnv::FnvHasher;

trait Item {
    fn get_id(&self) -> i32;
    fn cloned(&self) -> Box<Item>;
}

#[derive(Clone)]
struct ItemImpl {
    id: i32,
    foo: i32
}

impl Item for ItemImpl {
    fn get_id(&self) -> i32 { self.id }
    fn cloned(&self) -> Box<Item> { Box::new(self.clone()) }
}

fn main() {
    let hash_map = HashMap::<i32, Box<Item>, BuildHasherDefault<FnvHasher>>::default();
}

How I could clone hash_map shortly (in code) and efficiently (without creating a temporary collection)?

Answer 1

You can implement Clone for the boxed trait object itself:

impl Clone for Box<Item> {
    fn clone(&self) -> Self {
        self.cloned()
    }
}

Now you may clone the HashMap . Note that the custom hasher has nothing to do with the problem.

See also

• Why would I implement methods on a trait instead of as part of the trait?