Struct alloc::sync::Arc

#[lang = "arc"]
pub struct Arc<T: ?Sized> { /* fields omitted */ }

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 value on the heap as the source Arc, while increasing a reference count. When the last Arc pointer to a given value is destroyed, the pointed-to value is also destroyed.

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 values 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>.

Breaking cycles with Weak

The downgrade method can be used to create a non-owning Weak pointer. A Weak pointer can be upgraded to an Arc, but this will return None if the value has already been dropped.

A cycle between Arc pointers will never be deallocated. For this reason, Weak is used to break cycles. For example, a tree could have strong Arc pointers from parent nodes to children, and Weak pointers from children back to their parents.

Cloning references

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

use std::sync::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

The Arc::clone(&from) syntax is the most idiomatic because it conveys more explicitly the meaning of the code. In the example above, this syntax makes it easier to see that this code is creating a new reference rather than copying the whole content of foo.

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 function-like syntax:

use std::sync::Arc;
let my_arc = Arc::new(());

Arc::downgrade(&my_arc);

Weak<T> does not auto-dereference to T, because the value may have already been destroyed.

Examples

Sharing some immutable data between threads:

use std::sync::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 std::sync::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.

Methods

impl<T> Arc<T>

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

Constructs a new Arc<T>.

Examples

use std::sync::Arc;

let five = Arc::new(5);

pub fn pin(data: T) -> Pin<Arc<T>>

Constructs a new Pin<Arc<T>>. If T does not implement Unpin, then data will be pinned in memory and unable to be moved.

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

Returns the contained value, if the Arc has exactly one strong reference.

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

This will succeed even if there are outstanding weak references.

Examples

use std::sync::Arc;

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

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

impl<T: ?Sized> Arc<T>

pub fn into_raw(this: Self) -> *const T

Consumes the Arc, returning the wrapped pointer.

To avoid a memory leak the pointer must be converted back to an Arc using Arc::from_raw.

Examples

use std::sync::Arc;

let x = Arc::new(10);
let x_ptr = Arc::into_raw(x);
assert_eq!(unsafe { *x_ptr }, 10);

pub unsafe fn from_raw(ptr: *const T) -> Self

Constructs an Arc from a raw pointer.

The raw pointer must have been previously returned by a call to a Arc::into_raw.

This function is unsafe because improper use may lead to memory problems. For example, a double-free may occur if the function is called twice on the same raw pointer.

Examples

use std::sync::Arc;

let x = Arc::new(10);
let x_ptr = Arc::into_raw(x);

unsafe {
    // Convert back to an `Arc` to prevent leak.
    let x = Arc::from_raw(x_ptr);
    assert_eq!(*x, 10);

    // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
}

// The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!

pub fn into_raw_non_null(this: Self) -> NonNull<T>

🔬 This is a nightly-only experimental API. (rc_into_raw_non_null #47336)

Consumes the Arc, returning the wrapped pointer as NonNull<T>.

Examples

#![feature(rc_into_raw_non_null)]

use std::sync::Arc;

let x = Arc::new(10);
let ptr = Arc::into_raw_non_null(x);
let deref = unsafe { *ptr.as_ref() };
assert_eq!(deref, 10);

pub fn downgrade(this: &Self) -> Weak<T>

Creates a new Weak pointer to this value.

Examples

use std::sync::Arc;

let five = Arc::new(5);

let weak_five = Arc::downgrade(&five);

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

Gets the number of Weak pointers to this value.

Safety

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

Examples

use std::sync::Arc;

let five = Arc::new(5);
let _weak_five = Arc::downgrade(&five);

// This assertion is deterministic because we haven't shared
// the `Arc` or `Weak` between threads.
assert_eq!(1, Arc::weak_count(&five));

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

Gets the number of strong (Arc) pointers to this value.

Safety

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

Examples

use std::sync::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::strong_count(&five));

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

Returns true if the two Arcs point to the same value (not just values that compare as equal).

Examples

use std::sync::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));

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 or Weak pointers to the same value, then make_mut will 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 std::sync::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 values.
assert_eq!(*data, 8);
assert_eq!(*other_data, 12);

impl<T: ?Sized> Arc<T>

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

Returns a mutable reference to the inner value, if there are no other Arc or Weak pointers to the same value.

Returns None otherwise, because it is not safe to mutate a shared value.

See also make_mut, which will clone the inner value when it's shared.

Examples

use std::sync::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());

impl Arc<dyn Any + Send + Sync>

pub fn downcast<T>(self) -> Result<Arc<T>, Self> where
    T: Any + Send + Sync + 'static, 

Attempt to downcast the Arc<dyn Any + Send + Sync> to a concrete type.

Examples

use std::any::Any;
use std::sync::Arc;

fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
    if let Ok(string) = value.downcast::<String>() {
        println!("String ({}): {}", string.len(), string);
    }
}

fn main() {
    let my_string = "Hello World".to_string();
    print_if_string(Arc::new(my_string));
    print_if_string(Arc::new(0i8));
}

Trait Implementations

impl<T: ?Sized> Deref for Arc<T>

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

impl<T: ?Sized + Display> Display for Arc<T>

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

Formats the value using the given formatter.

impl<T: ?Sized + Debug> Debug for Arc<T>

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

Formats the value using the given formatter.

impl<T: ?Sized + PartialEq> PartialEq<Arc<T>> for Arc<T>

fn eq(&self, other: &Arc<T>) -> bool

Equality for two Arcs.

Two Arcs are equal if their inner values are equal.

If T also implements Eq, two Arcs that point to the same value are always equal.

Examples

use std::sync::Arc;

let five = Arc::new(5);

assert!(five == Arc::new(5));

fn ne(&self, other: &Arc<T>) -> bool

Inequality for two Arcs.

Two Arcs are unequal if their inner values are unequal.

If T also implements Eq, two Arcs that point to the same value are never unequal.

Examples

use std::sync::Arc;

let five = Arc::new(5);

assert!(five != Arc::new(6));

impl<T: ?Sized + Eq> Eq for Arc<T>

impl<T: ?Sized + Ord> Ord for Arc<T>

fn cmp(&self, other: &Arc<T>) -> Ordering

Comparison for two Arcs.

The two are compared by calling cmp() on their inner values.

Examples

use std::sync::Arc;
use std::cmp::Ordering;

let five = Arc::new(5);

assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

impl<T: ?Sized + PartialOrd> PartialOrd<Arc<T>> for Arc<T>

fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering>

Partial comparison for two Arcs.

The two are compared by calling partial_cmp() on their inner values.

Examples

use std::sync::Arc;
use std::cmp::Ordering;

let five = Arc::new(5);

assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));

fn lt(&self, other: &Arc<T>) -> bool

Less-than comparison for two Arcs.

The two are compared by calling < on their inner values.

Examples

use std::sync::Arc;

let five = Arc::new(5);

assert!(five < Arc::new(6));

fn le(&self, other: &Arc<T>) -> bool

'Less than or equal to' comparison for two Arcs.

The two are compared by calling <= on their inner values.

Examples

use std::sync::Arc;

let five = Arc::new(5);

assert!(five <= Arc::new(5));

fn gt(&self, other: &Arc<T>) -> bool

Greater-than comparison for two Arcs.

The two are compared by calling > on their inner values.

Examples

use std::sync::Arc;

let five = Arc::new(5);

assert!(five > Arc::new(4));

fn ge(&self, other: &Arc<T>) -> bool

'Greater than or equal to' comparison for two Arcs.

The two are compared by calling >= on their inner values.

Examples

use std::sync::Arc;

let five = Arc::new(5);

assert!(five >= Arc::new(5));

impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T>

impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T>

impl<T> From<T> for Arc<T>

fn from(t: T) -> Self

Performs the conversion.

impl<T: Clone, '_> From<&'_ [T]> for Arc<[T]>

fn from(v: &[T]) -> Arc<[T]>

Performs the conversion.

impl<'_> From<&'_ str> for Arc<str>

fn from(v: &str) -> Arc<str>

Performs the conversion.

impl From<String> for Arc<str>

fn from(v: String) -> Arc<str>

Performs the conversion.

impl<T: ?Sized> From<Box<T>> for Arc<T>

fn from(v: Box<T>) -> Arc<T>

Performs the conversion.

impl<T> From<Vec<T>> for Arc<[T]>

fn from(v: Vec<T>) -> Arc<[T]>

Performs the conversion.

impl<T: ?Sized + Hash> Hash for Arc<T>

fn hash<H: Hasher>(&self, state: &mut H)

Feeds this value into the given [Hasher].

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given [Hasher].

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

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

impl<T: ?Sized> Unpin for Arc<T>

impl<T: ?Sized> Receiver for Arc<T>

impl<T: ?Sized> Drop for Arc<T>

fn drop(&mut self)

Drops the Arc.

This will decrement the strong reference count. If the strong reference count reaches zero then the only other references (if any) are Weak, so we drop the inner value.

Examples

use std::sync::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: ?Sized> Pointer for Arc<T>

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

Formats the value using the given formatter.

impl<T: ?Sized> Clone for Arc<T>

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

Makes a clone of the Arc pointer.

This creates another pointer to the same inner value, increasing the strong reference count.

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

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

Performs copy-assignment from source.

impl<T: Default> Default for Arc<T>

fn default() -> Arc<T>

Creates a new Arc<T>, with the Default value for T.

Examples

use std::sync::Arc;

let x: Arc<i32> = Default::default();
assert_eq!(*x, 0);

impl<T: ?Sized> AsRef<T> for Arc<T>

fn as_ref(&self) -> &T

Performs the conversion.

impl<T: ?Sized> Borrow<T> for Arc<T>

fn borrow(&self) -> &T

Immutably borrows from an owned value.