How to join handles of threads

#1 
use std::sync::{Arc, Mutex};
use std::thread;
pub fn send_some_data(a: i64) {
    let mut int = 1;
    let arc = Arc::new(Mutex::new(int));
    let mut handles = vec![];
    for i in 0..a {
        let arc_clone = Arc::clone(&arc);
        let handle = thread::spawn(move ||
            {
                let mut val= arc_clone.lock().unwrap();
                (*val) += 1;
            }
            );
        handles.push(handle);    
    }
    for handle in handles.iter()
    {
        handle.join();
        ^^^^^^ move occurs because `*handle` has type `std::thread::JoinHandle<()>`, which does not implement the `Copy` trait
    }
}

Why? I mean, I can see that compiler is trying to explain, but none the less... If somebody could explain that to me in more digestible form. And how to fix that?
Thanks

#2

The root cause of the error is that you're trying to call a method that takes ownership while you only have a borrow. This is OK to do with types that implement Copy, but JoinHandle is not one of those.

Vec::iter returns an iterator of references. In this case, because you have a vec of JoinHandles, the type of handle in the loop is &JoinHandle, a borrow.

Looking at JoinHandle::join, we see that it takes ownership of the JoinHandle it is called with (because the argument is self and not &self or &mut self).

To fix this, you can simply drop the .iter() and the for loop will loop over owned JoinHandles instead.

1 Like
#3

Ef me, fan-bloody-tastic!
Thanks man!