Getting reference to shared data in async closure [duplicate] - concurrency

Question

I have something like this:
use std::sync::Arc;
fn main() {
let arc = Arc::new(42);
move || { arc.clone() };
move || { arc.clone() };
}
I am getting:
error[E0382]: capture of moved value: `arc`
--> src/main.rs:6:19
|
5 | move || { arc.clone() };
| ------- value moved (into closure) here
6 | move || { arc.clone() };
| ^^^ value captured here after move
|
= note: move occurs because `arc` has type `std::sync::Arc<i32>`, which does not implement the `Copy` trait
I understand why I am getting this: the clone isn't called before arc is passed to the closure. I can fix this by defining each closure in a function and clone the Arc before passing it to the closure, but is there another option?

Answer 1

There is no way around it. You should clone the Arc before it is used in a closure. The common pattern is to re-bind the cloned Arc to the same name in a nested scope:
use std::sync::Arc;
fn main() {
let arc = Arc::new(42);
{
let arc = arc.clone();
move || { /* do something with arc */ };
}
{
let arc = arc.clone();
move || { /* do something else with arc */ };
}
}
This is usually done together with thread::spawn():
use std::sync::{Arc, Mutex};
use std::thread;
const NUM_THREADS: usize = 4;
fn main() {
let arc = Arc::new(Mutex::new(42));
for _ in 0..NUM_THREADS {
let arc = arc.clone();
thread::spawn(move || {
let mut shared_data = arc.lock().unwrap();
*shared_data += 1;
});
}
}

Answer 2

is there another option?
Because this pattern of cloning things before defining a closure is somewhat common, some people have proposed adding something like clone || as an analog to move ||. I wouldn't hold out hope for this happening, but a number of comments there point out that macros can solve the case fairly well.
Several crates provide some form of this macro:
closet
capture
clone_all
It's likely that many projects define their own macro to do something similar. For example, the WASM example rust-todomvc defines:
macro_rules! enclose {
( ($( $x:ident ),*) $y:expr ) => {
{
$(let $x = $x.clone();)*
$y
}
};
}
Which can be used as:
fn main() {
let arc = Arc::new(42);
enclose! { (arc) move || arc };
enclose! { (arc) move || arc };
}