CODEWITHSUNDEEP
rustdereference
Chris Cioffi
Chris Cioffi

Reputation: 51

Rustlings thread exercise, why do I NOT dereference Mutex(Struct)?

I'm learning Rust and have no experience with threads. I'm going through the Rustlings course and I've solved the threads1.rs exercise, but I don't understand why my Mutex struct doesn't need to be dereferenced.

use std::sync::{Arc, Mutex};
use std::thread;
use std::time::Duration;

struct JobStatus {
    jobs_completed: u32,
}

fn main() {
    let status = Arc::new(Mutex::new(JobStatus { jobs_completed: 0 }));
    let status_shared = Arc::clone(&status);
    thread::spawn(move || {
        for _ in 0..10 {
            thread::sleep(Duration::from_millis(250));
            let mut status_shared = status_shared.lock().unwrap();
            status_shared.jobs_completed += 1;  // why not *status_shared?
        }
    });

    let mut jobs_completed: u32;
    loop {
        jobs_completed = status.lock().unwrap().jobs_completed;
        if jobs_completed < 10 {
            println!("waiting... ({} jobs done)", jobs_completed);
            thread::sleep(Duration::from_millis(500));
        } else {
            break;
        }
    }
}

Based on Chapter 16.3 of The Book, I would have expected to need to assign to

*status_shared.jobs_completed

in order to get to the jobs_completed field, but that generates the error:

error[E0614]: type `u32` cannot be dereferenced
  --> src/main.rs:16:13
   |
16 |             *status_shared.jobs_completed += 1;
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Is the difference that the book gives a pointer to a simple type and the above code gives a reference to a struct?

Upvotes: 5

Views: 2439

Answers (3)

Jmb
Jmb

Reputation: 23244

As @Matthias247 said, dereferencing is automatic when behind a .

Additionally, your attempt at explicit dereference fails because of operator precedence: *status_shared.jobs_completed is equivalent to *(status_shared.jobs_completed) so it attempts to dereference an u32 and fails, but you want (*status_shared).jobs_completed in order to dereference the Arc<JobStatus>.

Upvotes: 3

Matthias247
Matthias247

Reputation: 10396

status_shared is of type MutexGuard. MutexGuard implements the DerefMut and Deref traits, with a deref target of T (the type which is stored inside the Mutex - JobStatus in your case.

When you use behind a . behind an object the rust compiler will automatically try to deref it into something where the requested operation can be performed. Therefore the explicit dereferencing is not necessary here. This behavior is described in the Rust book in the Deref chapter

Upvotes: 7

zerkms
zerkms

Reputation: 254886

Arc<T> automatically dereferences via the Deref trait.

References:

Upvotes: 1

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