Struct Arc

pub struct Arc<T> {
    ptr: NonNull<ArcInner<T>>,
    phantom: PhantomData<ArcInner<T>>,
}

A thread-safe reference-counting pointer. ‘Arc’ stands for ‘Atomically Reference Counted’.

The type Arc<T> provides shared ownership of a value of type T, allocated in the heap. Invoking clone on Arc produces a new Arc instance, which points to the same allocation on the heap as the source Arc, while increasing a reference count. When the last Arc pointer to a given allocation is destroyed, the value stored in that allocation (often referred to as “inner value”) is also dropped.

Shared references in Rust disallow mutation by default, and Arc is no exception: you cannot generally obtain a mutable reference to something inside an Arc. If you need to mutate through an Arc, use Mutex, RwLock, or one of the Atomic types.

Thread Safety

Unlike Rc<T>, Arc<T> uses atomic operations for its reference counting. This means that it is thread-safe. The disadvantage is that atomic operations are more expensive than ordinary memory accesses. If you are not sharing reference-counted allocations between threads, consider using Rc<T> for lower overhead. Rc<T> is a safe default, because the compiler will catch any attempt to send an Rc<T> between threads. However, a library might choose Arc<T> in order to give library consumers more flexibility.

Arc<T> will implement Send and Sync as long as the T implements Send and Sync. Why can’t you put a non-thread-safe type T in an Arc<T> to make it thread-safe? This may be a bit counter-intuitive at first: after all, isn’t the point of Arc<T> thread safety? The key is this: Arc<T> makes it thread safe to have multiple ownership of the same data, but it doesn’t add thread safety to its data. Consider Arc<RefCell<T>>. RefCell<T> isn’t Sync, and if Arc<T> was always Send, Arc<RefCell<T>> would be as well. But then we’d have a problem: RefCell<T> is not thread safe; it keeps track of the borrowing count using non-atomic operations.

In the end, this means that you may need to pair Arc<T> with some sort of std::sync type, usually Mutex<T>.

Cloning references

Creating a new reference from an existing reference-counted pointer is done using the Clone trait implemented for Arc<T>.

use cs431_homework::Arc;
let foo = Arc::new(vec![1.0, 2.0, 3.0]);
// The two syntaxes below are equivalent.
let a = foo.clone();
let b = Arc::clone(&foo);
// a, b, and foo are all Arcs that point to the same memory location

Deref behavior

Arc<T> automatically dereferences to T (via the Deref trait), so you can call T’s methods on a value of type Arc<T>. To avoid name clashes with T’s methods, the methods of Arc<T> itself are associated functions, called using fully qualified syntax:

use cs431_homework::Arc;

let my_arc = Arc::new(5);
let my_five = Arc::try_unwrap(my_arc).unwrap();

Arc<T>’s implementations of traits like Clone may also be called using fully qualified syntax. Some people prefer to use fully qualified syntax, while others prefer using method-call syntax.

use cs431_homework::Arc;

let arc = Arc::new(());
// Method-call syntax
let arc2 = arc.clone();
// Fully qualified syntax
let arc3 = Arc::clone(&arc);

Examples

Sharing some immutable data between threads:

use cs431_homework::Arc;
use std::thread;

let five = Arc::new(5);

for _ in 0..10 {
    let five = Arc::clone(&five);

    thread::spawn(move || {
        println!("{five:?}");
    });
}

Sharing a mutable AtomicUsize:

use cs431_homework::Arc;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::thread;

let val = Arc::new(AtomicUsize::new(5));

for _ in 0..10 {
    let val = Arc::clone(&val);

    thread::spawn(move || {
        let v = val.fetch_add(1, Ordering::SeqCst);
        println!("{v:?}");
    });
}

See the rc documentation for more examples of reference counting in general.

Fields

ptr: NonNull<ArcInner<T>>

phantom: PhantomData<ArcInner<T>>

Implementations

impl<T> Arc<T>

fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self

impl<T> Arc<T>

pub fn new(data: T) -> Arc<T>

Constructs a new Arc<T>.

pub fn get_mut(this: &mut Self) -> Option<&mut T>

Returns a mutable reference into the given Arc if there are no other Arc. Otherwise, return None.

Examples

use cs431_homework::Arc;

let mut x = Arc::new(3);
*Arc::get_mut(&mut x).unwrap() = 4;
assert_eq!(*x, 4);

let y = Arc::clone(&x);
assert!(Arc::get_mut(&mut x).is_none());

drop(y);
assert!(Arc::get_mut(&mut x).is_some());

fn is_unique(&mut self) -> bool

pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T

Returns a mutable reference into the given Arc without any check.

Safety

Any other Arc to the same allocation must not be dereferenced for the duration of the returned borrow. Specifically, call to this function must happen-after destruction of all the other Arc to the same allocation.

Examples

use cs431_homework::Arc;

let mut x = Arc::new(String::new());
unsafe {
    Arc::get_mut_unchecked(&mut x).push_str("foo")
}
assert_eq!(*x, "foo");

pub fn count(this: &Self) -> usize

Gets the number of Arcs to this allocation. In addition, synchronize with the update that this function reads from.

Safety

This method by itself is safe, but using it correctly requires extra care. Another thread can change the reference count at any time, including potentially between calling this method and acting on the result.

Examples

use cs431_homework::Arc;

let five = Arc::new(5);
let _also_five = Arc::clone(&five);

// This assertion is deterministic because we haven't shared
// the `Arc` between threads.
assert_eq!(2, Arc::count(&five));

fn inner(&self) -> &ArcInner<T>

pub fn ptr_eq(this: &Self, other: &Self) -> bool

Returns true if the two Arcs point to the same allocation (in a vein similar to ptr::eq).

Examples

use cs431_homework::Arc;

let five = Arc::new(5);
let same_five = Arc::clone(&five);
let other_five = Arc::new(5);

assert!(Arc::ptr_eq(&five, &same_five));
assert!(!Arc::ptr_eq(&five, &other_five));

pub fn try_unwrap(this: Self) -> Result<T, Self>

Returns the inner value, if the given Arc is unique.

Otherwise, an Err is returned with the same Arc that was passed in.

Examples

use cs431_homework::Arc;

let x = Arc::new(3);
assert_eq!(Arc::try_unwrap(x).unwrap(), 3);

let x = Arc::new(4);
let _y = Arc::clone(&x);
assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);

impl<T: Clone> Arc<T>

pub fn make_mut(this: &mut Self) -> &mut T

Makes a mutable reference into the given Arc.

If there are other Arc to the same allocation, then make_mut will create a new allocation and invoke clone on the inner value to ensure unique ownership. This is also referred to as clone-on-write.

See also get_mut, which will fail rather than cloning.

Examples

use cs431_homework::Arc;

let mut data = Arc::new(5);

*Arc::make_mut(&mut data) += 1;         // Won't clone anything
let mut other_data = Arc::clone(&data); // Won't clone inner data
*Arc::make_mut(&mut data) += 1;         // Clones inner data
*Arc::make_mut(&mut data) += 1;         // Won't clone anything
*Arc::make_mut(&mut other_data) *= 2;   // Won't clone anything

// Now `data` and `other_data` point to different allocations.
assert_eq!(*data, 8);
assert_eq!(*other_data, 12);

Trait Implementations

impl<T> Clone for Arc<T>

fn clone(&self) -> Arc<T>

Makes a clone of the Arc pointer.

This creates another pointer to the same allocation, increasing the reference count.

Panics

This panics if the number of Arcs is larger than isize::Max.

Examples

use cs431_homework::Arc;

let five = Arc::new(5);

let _ = Arc::clone(&five);

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: Debug> Debug for Arc<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T> Deref for Arc<T>

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

impl<T: Display> Display for Arc<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T> Drop for Arc<T>

fn drop(&mut self)

Drops the Arc.

This will decrement the reference count. If the reference count reaches zero, we drop the inner value.

Examples

use cs431_homework::Arc;

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("dropped!");
    }
}

let foo  = Arc::new(Foo);
let foo2 = Arc::clone(&foo);

drop(foo);    // Doesn't print anything
drop(foo2);   // Prints "dropped!"

impl<T> Pointer for Arc<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: Sync + Send> Send for Arc<T>

impl<T: Sync + Send> Sync for Arc<T>