How to use Boxes with rayon?

help
#1

Hi !

I'm trying to use rayon with a vector of Boxed structs :

use rayon::prelude::*;

use crate::{entity::Entity, message::Message};

pub struct Orchestrator {
    entities: Vec<Box<dyn Entity>>,
}

impl Orchestrator {
    pub fn new(entities: Vec<Box<dyn Entity>>) -> Orchestrator {
        Orchestrator { entities }
    }

    pub fn operate(&self) {
        loop {
            let results: Vec<Message> = (0..self.entities.len())
                .into_par_iter()
                .flat_map(|i| self.tick_entity(i))
                .collect();
        }
    }

    fn tick_entity(&self, i: usize) -> Vec<Message> {
        vec![]
    }
}

But :

error[E0277]: `(dyn Entity + Send + 'static)` cannot be shared between threads safely
   --> src/orchestrator/mod.rs:18:18
    |
18  |                 .flat_map(|i| self.tick_entity(i))
    |                  ^^^^^^^^ `(dyn Entity + Send + 'static)` cannot be shared between threads safely
    |
    = help: the trait `Sync` is not implemented for `(dyn Entity + Send + 'static)`
    = note: required because of the requirements on the impl of `Sync` for `Unique<(dyn Entity + Send + 'static)>`
    = note: required because it appears within the type `Box<(dyn Entity + Send + 'static)>`
    = note: required because of the requirements on the impl of `Sync` for `Unique<Box<(dyn Entity + Send + 'static)>>`
    = note: required because it appears within the type `alloc::raw_vec::RawVec<Box<(dyn Entity + Send + 'static)>>`
    = note: required because it appears within the type `Vec<Box<(dyn Entity + Send + 'static)>>`
note: required because it appears within the type `Orchestrator`
   --> src/orchestrator/mod.rs:5:12
    |
5   | pub struct Orchestrator {
    |            ^^^^^^^^^^^^
    = note: required because it appears within the type `&Orchestrator`
    = note: required because it appears within the type `&&Orchestrator`
    = note: required because it appears within the type `[closure@src/orchestrator/mod.rs:18:27: 18:50]`
note: required by a bound in `rayon::iter::ParallelIterator::flat_map`
   --> /home/bastiensevajol/.cargo/registry/src/github.com-1ecc6299db9ec823/rayon-1.5.1/src/iter/mod.rs:848:35
    |
848 |         F: Fn(Self::Item) -> PI + Sync + Send,
    |                                   ^^^^ required by this bound in `rayon::iter::ParallelIterator::flat_map`

How can I use rayon::iter::ParallelIterator::flat_map ? I just need to use Box<dyn Entity> as read only. I thought that require only Send ? :thinking:

#2

This hasn't got anything to do with the box. You probably want dyn Entity + Send + Sync in both places where you currently have plain dyn Entity. Playground.

#3

It would require only Send, if you were consuming the Boxes, i.e. if they were moved into the iterator. But you're only borrowing them (as part of &self), i.e. you need &Box<_> to be Send; and &T is Send iff T is Sync.

1 Like
#4

Thanks.

#5

Sorry, I didn't understand this.

#6

The compiler is complaining that your &Box isn't Send, ie. the reference can't be used for shared access across threads. A reference is safe to share across threads if the pointed value ensures synchronization (or is truly read-only), a property which is captured by the Sync trait. Therefore only a T: Sync type's address &T can be Send (ie. a pointer/reference to it can be transferred/copied to a different thread).

#7

Thanks :+1: I will study that :relieved: