Filtering a struct field Vec containing Box instances

[ShamylZakariya]

I have a Vec of Box of a trait, and I want to be able to filter it on some predicate, mutating the vec (which is a field of a long lived struct).

struct Environment {
    entities: Vec<Box<dyn Entity>> 
}
impl Environment {
    fn prune(&mut self) {
        self.entities = self.entities.into_iter().filter(|e| e.is_alive()).collect();
    }
}

And here's the specific error I get.

self.entities = self.entities.into_iter().filter(|e| e.is_alive()).collect();
                ^^^^^^^^^^^^^ move occurs because `self.entities` has type `Vec<Box<dyn Entity>>`, which does not implement the `Copy` trait

Here's a link to a playground showing a minimal example: Rust Playground

Anyway, the error makes sense! But I don't know how to resolve it. I feel like I would want one of a couple options:

• Use some kind of variant on filter which mutates the vec in place
• Transfer ownership from the old self.entities into the new self.entities after the filter operation.

[quinedot]

@ShamylZakariya

Use some kind of variant on filter which mutates the vec in place

You want Vec::retain():

    self.entities.retain(|e| e.is_alive());

@ShamylZakariya

Transfer ownership from the old self.entities into the new self.entities after the filter operation.

Another approach is to temporarily replace self.entities, do your operation, and then replace self.entities again. Something like (untested):

    let tmp = std::mem::replace(&mut self.entities, Vec::new());
    tmp.into_iter().filter(|e| e.is_alive()).collect();
    self.entities = tmp;

[scottmcm]

@quinedot

let tmp = std::mem::replace(&mut self.entities, Vec::new());

You can also shorten that a bit by using take (new back in 1.40.0):

let tmp = std::mem::take(&mut self.entities);

(Since Vec<T>: Default. And constructing an empty Vec is essentially free -- no memory allocation required -- so the replace/take approach is still highly efficient.)

[ShamylZakariya]

Vec::retain, holy smokes that's precisely what I was hoping for. Thanks!