Struct Arc

struct Arc<T: ?Sized, A: Allocator = crate::alloc::Global> { ... }

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 do need to mutate through an Arc, you have several options:

  1. Use interior mutability with synchronization primitives like Mutex, RwLock, or one of the Atomic types.

  2. Use clone-on-write semantics with Arc::make_mut which provides efficient mutation without requiring interior mutability. This approach clones the data only when needed (when there are multiple references) and can be more efficient when mutations are infrequent.

  3. Use Arc::get_mut when you know your Arc is not shared (has a reference count of 1), which provides direct mutable access to the inner value without any cloning.

use std::sync::Arc;

let mut data = Arc::new(vec![1, 2, 3]);

// This will clone the vector only if there are other references to it
Arc::make_mut(&mut data).push(4);

assert_eq!(*data, vec![1, 2, 3, 4]);

Note: This type is only available on platforms that support atomic loads and stores of pointers, which includes all platforms that support the std crate but not all those which only support alloc. This may be detected at compile time using #[cfg(target_has_atomic = "ptr")].

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>.

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 stored in the allocation has already been dropped. In other words, Weak pointers do not keep the value inside the allocation alive; however, they do keep the allocation (the backing store for the value) alive.

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>][Arc] and [Weak<T>][Weak].

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

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 std::sync::Arc;

let my_arc = Arc::new(());
let my_weak = Arc::downgrade(&my_arc);

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 std::sync::Arc;

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

[Weak<T>][Weak] does not auto-dereference to T, because the inner value may have already been dropped.

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::Relaxed);
        println!("{v:?}");
    });
}

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

Implementations

impl<A: Allocator> Arc<dyn Any + Send + Sync, A>

fn downcast<T>(self: Self) -> Result<Arc<T, A>, Self>
where
    T: Any + Send + Sync

Attempts 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);
    }
}

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

unsafe fn downcast_unchecked<T>(self: Self) -> Arc<T, A>
where
    T: Any + Send + Sync

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

For a safe alternative see downcast.

Examples

#![feature(downcast_unchecked)]

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

let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);

unsafe {
    assert_eq!(*x.downcast_unchecked::<usize>(), 1);
}

Safety

The contained value must be of type T. Calling this method with the incorrect type is undefined behavior.

impl<T> Arc<T>

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

Constructs a new Arc<T>.

Examples

use std::sync::Arc;

let five = Arc::new(5);

fn new_cyclic<F>(data_fn: F) -> Arc<T>
where
    F: FnOnce(&Weak<T>) -> T

Constructs a new Arc<T> while giving you a Weak<T> to the allocation, to allow you to construct a T which holds a weak pointer to itself.

Generally, a structure circularly referencing itself, either directly or indirectly, should not hold a strong reference to itself to prevent a memory leak. Using this function, you get access to the weak pointer during the initialization of T, before the Arc<T> is created, such that you can clone and store it inside the T.

new_cyclic first allocates the managed allocation for the Arc<T>, then calls your closure, giving it a Weak<T> to this allocation, and only afterwards completes the construction of the Arc<T> by placing the T returned from your closure into the allocation.

Since the new Arc<T> is not fully-constructed until Arc<T>::new_cyclic returns, calling upgrade on the weak reference inside your closure will fail and result in a None value.

Panics

If data_fn panics, the panic is propagated to the caller, and the temporary [Weak<T>] is dropped normally.

Example

# #![allow(dead_code)]
use std::sync::{Arc, Weak};

struct Gadget {
    me: Weak<Gadget>,
}

impl Gadget {
    /// Constructs a reference counted Gadget.
    fn new() -> Arc<Self> {
        // `me` is a `Weak<Gadget>` pointing at the new allocation of the
        // `Arc` we're constructing.
        Arc::new_cyclic(|me| {
            // Create the actual struct here.
            Gadget { me: me.clone() }
        })
    }

    /// Returns a reference counted pointer to Self.
    fn me(&self) -> Arc<Self> {
        self.me.upgrade().unwrap()
    }
}

fn new_uninit() -> Arc<mem::MaybeUninit<T>>

Constructs a new Arc with uninitialized contents.

Examples

use std::sync::Arc;

let mut five = Arc::<u32>::new_uninit();

// Deferred initialization:
Arc::get_mut(&mut five).unwrap().write(5);

let five = unsafe { five.assume_init() };

assert_eq!(*five, 5)

fn new_zeroed() -> Arc<mem::MaybeUninit<T>>

Constructs a new Arc with uninitialized contents, with the memory being filled with 0 bytes.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

use std::sync::Arc;

let zero = Arc::<u32>::new_zeroed();
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0)

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.

fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError>

Constructs a new Pin<Arc<T>>, return an error if allocation fails.

fn try_new(data: T) -> Result<Arc<T>, AllocError>

Constructs a new Arc<T>, returning an error if allocation fails.

Examples

#![feature(allocator_api)]
use std::sync::Arc;

let five = Arc::try_new(5)?;
# Ok::<(), std::alloc::AllocError>(())

fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError>

Constructs a new Arc with uninitialized contents, returning an error if allocation fails.

Examples

#![feature(allocator_api)]

use std::sync::Arc;

let mut five = Arc::<u32>::try_new_uninit()?;

// Deferred initialization:
Arc::get_mut(&mut five).unwrap().write(5);

let five = unsafe { five.assume_init() };

assert_eq!(*five, 5);
# Ok::<(), std::alloc::AllocError>(())

fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError>

Constructs a new Arc with uninitialized contents, with the memory being filled with 0 bytes, returning an error if allocation fails.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature( allocator_api)]

use std::sync::Arc;

let zero = Arc::<u32>::try_new_zeroed()?;
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0);
# Ok::<(), std::alloc::AllocError>(())

fn map<U, impl FnOnce(&T) -> U: FnOnce(&T) -> U>(this: Self, f: impl FnOnce(&T) -> U) -> Arc<U>

Maps the value in an Arc, reusing the allocation if possible.

f is called on a reference to the value in the Arc, and the result is returned, also in an Arc.

Note: this is an associated function, which means that you have to call it as Arc::map(a, f) instead of r.map(a). This is so that there is no conflict with a method on the inner type.

Examples

#![feature(smart_pointer_try_map)]

use std::sync::Arc;

let r = Arc::new(7);
let new = Arc::map(r, |i| i + 7);
assert_eq!(*new, 14);

fn try_map<R, impl FnOnce(&T) -> R: FnOnce(&T) -> R>(this: Self, f: impl FnOnce(&T) -> R) -> <<R as >::Residual as Residual<Arc<<R as >::Output>>>::TryType
where
    R: Try,
    <R as >::Residual: Residual<Arc<<R as >::Output>>

Attempts to map the value in an Arc, reusing the allocation if possible.

f is called on a reference to the value in the Arc, and if the operation succeeds, the result is returned, also in an Arc.

Note: this is an associated function, which means that you have to call it as Arc::try_map(a, f) instead of a.try_map(f). This is so that there is no conflict with a method on the inner type.

Examples

#![feature(smart_pointer_try_map)]

use std::sync::Arc;

let b = Arc::new(7);
let new = Arc::try_map(b, |&i| u32::try_from(i)).unwrap();
assert_eq!(*new, 7);

impl<T> Arc<[T]>

fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]>

Constructs a new atomically reference-counted slice with uninitialized contents.

Examples

use std::sync::Arc;

let mut values = Arc::<[u32]>::new_uninit_slice(3);

// Deferred initialization:
let data = Arc::get_mut(&mut values).unwrap();
data[0].write(1);
data[1].write(2);
data[2].write(3);

let values = unsafe { values.assume_init() };

assert_eq!(*values, [1, 2, 3])

fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]>

Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being filled with 0 bytes.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

use std::sync::Arc;

let values = Arc::<[u32]>::new_zeroed_slice(3);
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0])

fn into_array<N: usize>(self: Self) -> Option<Arc<[T; N]>>

Converts the reference-counted slice into a reference-counted array.

This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.

If N is not exactly equal to the length of self, then this method returns None.

impl<T, A: Allocator> Arc<T, A>

fn new_in(data: T, alloc: A) -> Arc<T, A>

Constructs a new Arc<T> in the provided allocator.

Examples

#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let five = Arc::new_in(5, System);

fn new_uninit_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A>

Constructs a new Arc with uninitialized contents in the provided allocator.

Examples

#![feature(get_mut_unchecked)]
#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let mut five = Arc::<u32, _>::new_uninit_in(System);

let five = unsafe {
    // Deferred initialization:
    Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5)

fn new_zeroed_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A>

Constructs a new Arc with uninitialized contents, with the memory being filled with 0 bytes, in the provided allocator.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let zero = Arc::<u32, _>::new_zeroed_in(System);
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0)

fn new_cyclic_in<F>(data_fn: F, alloc: A) -> Arc<T, A>
where
    F: FnOnce(&Weak<T, A>) -> T

Constructs a new Arc<T, A> in the given allocator while giving you a Weak<T, A> to the allocation, to allow you to construct a T which holds a weak pointer to itself.

Generally, a structure circularly referencing itself, either directly or indirectly, should not hold a strong reference to itself to prevent a memory leak. Using this function, you get access to the weak pointer during the initialization of T, before the Arc<T, A> is created, such that you can clone and store it inside the T.

new_cyclic_in first allocates the managed allocation for the Arc<T, A>, then calls your closure, giving it a Weak<T, A> to this allocation, and only afterwards completes the construction of the Arc<T, A> by placing the T returned from your closure into the allocation.

Since the new Arc<T, A> is not fully-constructed until Arc<T, A>::new_cyclic_in returns, calling upgrade on the weak reference inside your closure will fail and result in a None value.

Panics

If data_fn panics, the panic is propagated to the caller, and the temporary [Weak<T>] is dropped normally.

Example

See new_cyclic

fn pin_in(data: T, alloc: A) -> Pin<Arc<T, A>>
where
    A: 'static

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

fn try_pin_in(data: T, alloc: A) -> Result<Pin<Arc<T, A>>, AllocError>
where
    A: 'static

Constructs a new Pin<Arc<T, A>> in the provided allocator, return an error if allocation fails.

fn try_new_in(data: T, alloc: A) -> Result<Arc<T, A>, AllocError>

Constructs a new Arc<T, A> in the provided allocator, returning an error if allocation fails.

Examples

#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let five = Arc::try_new_in(5, System)?;
# Ok::<(), std::alloc::AllocError>(())

fn try_new_uninit_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError>

Constructs a new Arc with uninitialized contents, in the provided allocator, returning an error if allocation fails.

Examples

#![feature(allocator_api)]
#![feature(get_mut_unchecked)]

use std::sync::Arc;
use std::alloc::System;

let mut five = Arc::<u32, _>::try_new_uninit_in(System)?;

let five = unsafe {
    // Deferred initialization:
    Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5);
# Ok::<(), std::alloc::AllocError>(())

fn try_new_zeroed_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError>

Constructs a new Arc with uninitialized contents, with the memory being filled with 0 bytes, in the provided allocator, returning an error if allocation fails.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let zero = Arc::<u32, _>::try_new_zeroed_in(System)?;
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0);
# Ok::<(), std::alloc::AllocError>(())

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

Returns the inner 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.

It is strongly recommended to use Arc::into_inner instead if you don't keep the Arc in the Err case. Immediately dropping the Err-value, as the expression Arc::try_unwrap(this).ok() does, can cause the strong count to drop to zero and the inner value of the Arc to be dropped. For instance, if two threads execute such an expression in parallel, there is a race condition without the possibility of unsafety: The threads could first both check whether they own the last instance in Arc::try_unwrap, determine that they both do not, and then both discard and drop their instance in the call to [ok]Result::ok. In this scenario, the value inside the Arc is safely destroyed by exactly one of the threads, but neither thread will ever be able to use the value.

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);

fn into_inner(this: Self) -> Option<T>

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

Otherwise, None is returned and the Arc is dropped.

This will succeed even if there are outstanding weak references.

If Arc::into_inner is called on every clone of this Arc, it is guaranteed that exactly one of the calls returns the inner value. This means in particular that the inner value is not dropped.

Arc::try_unwrap is conceptually similar to Arc::into_inner, but it is meant for different use-cases. If used as a direct replacement for Arc::into_inner anyway, such as with the expression [Arc::try_unwrap](this).[ok]Result::ok, then it does not give the same guarantee as described in the previous paragraph. For more information, see the examples below and read the documentation of Arc::try_unwrap.

Examples

Minimal example demonstrating the guarantee that Arc::into_inner gives.

use std::sync::Arc;

let x = Arc::new(3);
let y = Arc::clone(&x);

// Two threads calling `Arc::into_inner` on both clones of an `Arc`:
let x_thread = std::thread::spawn(|| Arc::into_inner(x));
let y_thread = std::thread::spawn(|| Arc::into_inner(y));

let x_inner_value = x_thread.join().unwrap();
let y_inner_value = y_thread.join().unwrap();

// One of the threads is guaranteed to receive the inner value:
assert!(matches!(
    (x_inner_value, y_inner_value),
    (None, Some(3)) | (Some(3), None)
));
// The result could also be `(None, None)` if the threads called
// `Arc::try_unwrap(x).ok()` and `Arc::try_unwrap(y).ok()` instead.

A more practical example demonstrating the need for Arc::into_inner:

use std::sync::Arc;

// Definition of a simple singly linked list using `Arc`:
#[derive(Clone)]
struct LinkedList<T>(Option<Arc<Node<T>>>);
struct Node<T>(T, Option<Arc<Node<T>>>);

// Dropping a long `LinkedList<T>` relying on the destructor of `Arc`
// can cause a stack overflow. To prevent this, we can provide a
// manual `Drop` implementation that does the destruction in a loop:
impl<T> Drop for LinkedList<T> {
    fn drop(&mut self) {
        let mut link = self.0.take();
        while let Some(arc_node) = link.take() {
            if let Some(Node(_value, next)) = Arc::into_inner(arc_node) {
                link = next;
            }
        }
    }
}

// Implementation of `new` and `push` omitted
impl<T> LinkedList<T> {
    /* ... */
#   fn new() -> Self {
#       LinkedList(None)
#   }
#   fn push(&mut self, x: T) {
#       self.0 = Some(Arc::new(Node(x, self.0.take())));
#   }
}

// The following code could have still caused a stack overflow
// despite the manual `Drop` impl if that `Drop` impl had used
// `Arc::try_unwrap(arc).ok()` instead of `Arc::into_inner(arc)`.

// Create a long list and clone it
let mut x = LinkedList::new();
let size = 100000;
# let size = if cfg!(miri) { 100 } else { size };
for i in 0..size {
    x.push(i); // Adds i to the front of x
}
let y = x.clone();

// Drop the clones in parallel
let x_thread = std::thread::spawn(|| drop(x));
let y_thread = std::thread::spawn(|| drop(y));
x_thread.join().unwrap();
y_thread.join().unwrap();

impl<T, A: Allocator> Arc<[T], A>

fn new_uninit_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A>

Constructs a new atomically reference-counted slice with uninitialized contents in the provided allocator.

Examples

#![feature(get_mut_unchecked)]
#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let mut values = Arc::<[u32], _>::new_uninit_slice_in(3, System);

let values = unsafe {
    // Deferred initialization:
    Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
    Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
    Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);

    values.assume_init()
};

assert_eq!(*values, [1, 2, 3])

fn new_zeroed_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A>

Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being filled with 0 bytes, in the provided allocator.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let values = Arc::<[u32], _>::new_zeroed_slice_in(3, System);
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0])

impl<T, A: Allocator> Arc<[mem::MaybeUninit<T>], A>

unsafe fn assume_init(self: Self) -> Arc<[T], A>

Converts to Arc<[T]>.

Safety

As with MaybeUninit::assume_init, it is up to the caller to guarantee that the inner value really is in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior.

Examples

use std::sync::Arc;

let mut values = Arc::<[u32]>::new_uninit_slice(3);

// Deferred initialization:
let data = Arc::get_mut(&mut values).unwrap();
data[0].write(1);
data[1].write(2);
data[2].write(3);

let values = unsafe { values.assume_init() };

assert_eq!(*values, [1, 2, 3])

impl<T, A: Allocator> Arc<mem::MaybeUninit<T>, A>

unsafe fn assume_init(self: Self) -> Arc<T, A>

Converts to Arc<T>.

Safety

As with MaybeUninit::assume_init, it is up to the caller to guarantee that the inner value really is in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior.

Examples

use std::sync::Arc;

let mut five = Arc::<u32>::new_uninit();

// Deferred initialization:
Arc::get_mut(&mut five).unwrap().write(5);

let five = unsafe { five.assume_init() };

assert_eq!(*five, 5)

impl<T: ?Sized + CloneToUninit> Arc<T>

fn clone_from_ref(value: &T) -> Arc<T>

Constructs a new Arc<T> with a clone of value.

Examples

#![feature(clone_from_ref)]
use std::sync::Arc;

let hello: Arc<str> = Arc::clone_from_ref("hello");

fn try_clone_from_ref(value: &T) -> Result<Arc<T>, AllocError>

Constructs a new Arc<T> with a clone of value, returning an error if allocation fails

Examples

#![feature(clone_from_ref)]
#![feature(allocator_api)]
use std::sync::Arc;

let hello: Arc<str> = Arc::try_clone_from_ref("hello")?;
# Ok::<(), std::alloc::AllocError>(())

impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A>

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

Makes a mutable reference into the given Arc.

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

However, if there are no other Arc pointers to this allocation, but some Weak pointers, then the Weak pointers will be dissociated and the inner value will not be cloned.

See also get_mut, which will fail rather than cloning the inner value or dissociating Weak pointers.

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 allocations.
assert_eq!(*data, 8);
assert_eq!(*other_data, 12);

Weak pointers will be dissociated:

use std::sync::Arc;

let mut data = Arc::new(75);
let weak = Arc::downgrade(&data);

assert!(75 == *data);
assert!(75 == *weak.upgrade().unwrap());

*Arc::make_mut(&mut data) += 1;

assert!(76 == *data);
assert!(weak.upgrade().is_none());

impl<T: ?Sized + CloneToUninit, A: Allocator> Arc<T, A>

fn clone_from_ref_in(value: &T, alloc: A) -> Arc<T, A>

Constructs a new Arc<T> with a clone of value in the provided allocator.

Examples

#![feature(clone_from_ref)]
#![feature(allocator_api)]
use std::sync::Arc;
use std::alloc::System;

let hello: Arc<str, System> = Arc::clone_from_ref_in("hello", System);

fn try_clone_from_ref_in(value: &T, alloc: A) -> Result<Arc<T, A>, AllocError>

Constructs a new Arc<T> with a clone of value in the provided allocator, returning an error if allocation fails

Examples

#![feature(clone_from_ref)]
#![feature(allocator_api)]
use std::sync::Arc;
use std::alloc::System;

let hello: Arc<str, System> = Arc::try_clone_from_ref_in("hello", System)?;
# Ok::<(), std::alloc::AllocError>(())

impl<T: ?Sized> Arc<T>

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

Constructs an Arc<T> from a raw pointer.

The raw pointer must have been previously returned by a call to Arc<U>::into_raw with the following requirements:

  • If U is sized, it must have the same size and alignment as T. This is trivially true if U is T.
  • If U is unsized, its data pointer must have the same size and alignment as T. This is trivially true if Arc<U> was constructed through Arc<T> and then converted to Arc<U> through an unsized coercion.

Note that if U or U's data pointer is not T but has the same size and alignment, this is basically like transmuting references of different types. See mem::transmute for more information on what restrictions apply in this case.

The raw pointer must point to a block of memory allocated by the global allocator.

The user of from_raw has to make sure a specific value of T is only dropped once.

This function is unsafe because improper use may lead to memory unsafety, even if the returned Arc<T> is never accessed.

Examples

use std::sync::Arc;

let x = Arc::new("hello".to_owned());
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, "hello");

    // 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!

Convert a slice back into its original array:

use std::sync::Arc;

let x: Arc<[u32]> = Arc::new([1, 2, 3]);
let x_ptr: *const [u32] = Arc::into_raw(x);

unsafe {
    let x: Arc<[u32; 3]> = Arc::from_raw(x_ptr.cast::<[u32; 3]>());
    assert_eq!(&*x, &[1, 2, 3]);
}

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("hello".to_owned());
let x_ptr = Arc::into_raw(x);
assert_eq!(unsafe { &*x_ptr }, "hello");
# // Prevent leaks for Miri.
# drop(unsafe { Arc::from_raw(x_ptr) });

unsafe fn increment_strong_count(ptr: *const T)

Increments the strong reference count on the Arc<T> associated with the provided pointer by one.

Safety

The pointer must have been obtained through Arc::into_raw and must satisfy the same layout requirements specified in Arc::from_raw_in. The associated Arc instance must be valid (i.e. the strong count must be at least 1) for the duration of this method, and ptr must point to a block of memory allocated by the global allocator.

Examples

use std::sync::Arc;

let five = Arc::new(5);

unsafe {
    let ptr = Arc::into_raw(five);
    Arc::increment_strong_count(ptr);

    // This assertion is deterministic because we haven't shared
    // the `Arc` between threads.
    let five = Arc::from_raw(ptr);
    assert_eq!(2, Arc::strong_count(&five));
#   // Prevent leaks for Miri.
#   Arc::decrement_strong_count(ptr);
}

unsafe fn decrement_strong_count(ptr: *const T)

Decrements the strong reference count on the Arc<T> associated with the provided pointer by one.

Safety

The pointer must have been obtained through Arc::into_raw and must satisfy the same layout requirements specified in Arc::from_raw_in. The associated Arc instance must be valid (i.e. the strong count must be at least 1) when invoking this method, and ptr must point to a block of memory allocated by the global allocator. This method can be used to release the final Arc and backing storage, but should not be called after the final Arc has been released.

Examples

use std::sync::Arc;

let five = Arc::new(5);

unsafe {
    let ptr = Arc::into_raw(five);
    Arc::increment_strong_count(ptr);

    // Those assertions are deterministic because we haven't shared
    // the `Arc` between threads.
    let five = Arc::from_raw(ptr);
    assert_eq!(2, Arc::strong_count(&five));
    Arc::decrement_strong_count(ptr);
    assert_eq!(1, Arc::strong_count(&five));
}

impl<T: ?Sized, A: Allocator> Arc<T, A>

fn allocator(this: &Self) -> &A

Returns a reference to the underlying allocator.

Note: this is an associated function, which means that you have to call it as Arc::allocator(&a) instead of a.allocator(). This is so that there is no conflict with a method on the inner type.

fn into_raw_with_allocator(this: Self) -> (*const T, A)

Consumes the Arc, returning the wrapped pointer and allocator.

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

Examples

#![feature(allocator_api)]
use std::sync::Arc;
use std::alloc::System;

let x = Arc::new_in("hello".to_owned(), System);
let (ptr, alloc) = Arc::into_raw_with_allocator(x);
assert_eq!(unsafe { &*ptr }, "hello");
let x = unsafe { Arc::from_raw_in(ptr, alloc) };
assert_eq!(&*x, "hello");

fn as_ptr(this: &Self) -> *const T

Provides a raw pointer to the data.

The counts are not affected in any way and the Arc is not consumed. The pointer is valid for as long as there are strong counts in the Arc.

Examples

use std::sync::Arc;

let x = Arc::new("hello".to_owned());
let y = Arc::clone(&x);
let x_ptr = Arc::as_ptr(&x);
assert_eq!(x_ptr, Arc::as_ptr(&y));
assert_eq!(unsafe { &*x_ptr }, "hello");

unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self

Constructs an Arc<T, A> from a raw pointer.

The raw pointer must have been previously returned by a call to Arc<U, A>::into_raw with the following requirements:

  • If U is sized, it must have the same size and alignment as T. This is trivially true if U is T.
  • If U is unsized, its data pointer must have the same size and alignment as T. This is trivially true if Arc<U> was constructed through Arc<T> and then converted to Arc<U> through an unsized coercion.

Note that if U or U's data pointer is not T but has the same size and alignment, this is basically like transmuting references of different types. See mem::transmute for more information on what restrictions apply in this case.

The raw pointer must point to a block of memory allocated by alloc

The user of from_raw has to make sure a specific value of T is only dropped once.

This function is unsafe because improper use may lead to memory unsafety, even if the returned Arc<T> is never accessed.

Examples

#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let x = Arc::new_in("hello".to_owned(), System);
let (x_ptr, alloc) = Arc::into_raw_with_allocator(x);

unsafe {
    // Convert back to an `Arc` to prevent leak.
    let x = Arc::from_raw_in(x_ptr, System);
    assert_eq!(&*x, "hello");

    // 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!

Convert a slice back into its original array:

#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let x: Arc<[u32], _> = Arc::new_in([1, 2, 3], System);
let x_ptr: *const [u32] = Arc::into_raw_with_allocator(x).0;

unsafe {
    let x: Arc<[u32; 3], _> = Arc::from_raw_in(x_ptr.cast::<[u32; 3]>(), System);
    assert_eq!(&*x, &[1, 2, 3]);
}

fn downgrade(this: &Self) -> Weak<T, A>
where
    A: Clone

Creates a new Weak pointer to this allocation.

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

fn weak_count(this: &Self) -> usize

Gets the number of Weak pointers to this allocation.

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));

fn strong_count(this: &Self) -> usize

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

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));

unsafe fn increment_strong_count_in(ptr: *const T, alloc: A)
where
    A: Clone

Increments the strong reference count on the Arc<T> associated with the provided pointer by one.

Safety

The pointer must have been obtained through Arc::into_raw and must satisfy the same layout requirements specified in Arc::from_raw_in. The associated Arc instance must be valid (i.e. the strong count must be at least 1) for the duration of this method, and ptr must point to a block of memory allocated by alloc.

Examples

#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let five = Arc::new_in(5, System);

unsafe {
    let (ptr, _alloc) = Arc::into_raw_with_allocator(five);
    Arc::increment_strong_count_in(ptr, System);

    // This assertion is deterministic because we haven't shared
    // the `Arc` between threads.
    let five = Arc::from_raw_in(ptr, System);
    assert_eq!(2, Arc::strong_count(&five));
#   // Prevent leaks for Miri.
#   Arc::decrement_strong_count_in(ptr, System);
}

unsafe fn decrement_strong_count_in(ptr: *const T, alloc: A)

Decrements the strong reference count on the Arc<T> associated with the provided pointer by one.

Safety

The pointer must have been obtained through Arc::into_raw and must satisfy the same layout requirements specified in Arc::from_raw_in. The associated Arc instance must be valid (i.e. the strong count must be at least 1) when invoking this method, and ptr must point to a block of memory allocated by alloc. This method can be used to release the final Arc and backing storage, but should not be called after the final Arc has been released.

Examples

#![feature(allocator_api)]

use std::sync::Arc;
use std::alloc::System;

let five = Arc::new_in(5, System);

unsafe {
    let (ptr, _alloc) = Arc::into_raw_with_allocator(five);
    Arc::increment_strong_count_in(ptr, System);

    // Those assertions are deterministic because we haven't shared
    // the `Arc` between threads.
    let five = Arc::from_raw_in(ptr, System);
    assert_eq!(2, Arc::strong_count(&five));
    Arc::decrement_strong_count_in(ptr, System);
    assert_eq!(1, Arc::strong_count(&five));
}

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. This function ignores the metadata of dyn Trait pointers.

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: ?Sized, A: Allocator> Arc<T, A>

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

Returns a mutable reference into the given Arc, if there are no other Arc or Weak pointers to the same allocation.

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

See also make_mut, which will clone the inner value when there are other Arc pointers.

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());

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

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

See also get_mut, which is safe and does appropriate checks.

Safety

If any other Arc or Weak pointers to the same allocation exist, then they must not be dereferenced or have active borrows for the duration of the returned borrow, and their inner type must be exactly the same as the inner type of this Arc (including lifetimes). This is trivially the case if no such pointers exist, for example immediately after Arc::new.

Examples

#![feature(get_mut_unchecked)]

use std::sync::Arc;

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

Other Arc pointers to the same allocation must be to the same type.

#![feature(get_mut_unchecked)]

use std::sync::Arc;

let x: Arc<str> = Arc::from("Hello, world!");
let mut y: Arc<[u8]> = x.clone().into();
unsafe {
    // this is Undefined Behavior, because x's inner type is str, not [u8]
    Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
}
println!("{}", &*x); // Invalid UTF-8 in a str

Other Arc pointers to the same allocation must be to the exact same type, including lifetimes.

#![feature(get_mut_unchecked)]

use std::sync::Arc;

let x: Arc<&str> = Arc::new("Hello, world!");
{
    let s = String::from("Oh, no!");
    let mut y: Arc<&str> = x.clone();
    unsafe {
        // this is Undefined Behavior, because x's inner type
        // is &'long str, not &'short str
        *Arc::get_mut_unchecked(&mut y) = &s;
    }
}
println!("{}", &*x); // Use-after-free

fn is_unique(this: &Self) -> bool

Determine whether this is the unique reference to the underlying data.

Returns true if there are no other Arc or Weak pointers to the same allocation; returns false otherwise.

If this function returns true, then is guaranteed to be safe to call get_mut_unchecked on this Arc, so long as no clones occur in between.

Examples

#![feature(arc_is_unique)]

use std::sync::Arc;

let x = Arc::new(3);
assert!(Arc::is_unique(&x));

let y = Arc::clone(&x);
assert!(!Arc::is_unique(&x));
drop(y);

// Weak references also count, because they could be upgraded at any time.
let z = Arc::downgrade(&x);
assert!(!Arc::is_unique(&x));

Pointer invalidation

This function will always return the same value as Arc::get_mut(arc).is_some(). However, unlike that operation it does not produce any mutable references to the underlying data, meaning no pointers to the data inside the Arc are invalidated by the call. Thus, the following code is valid, even though it would be UB if it used Arc::get_mut:

#![feature(arc_is_unique)]

use std::sync::Arc;

let arc = Arc::new(5);
let pointer: *const i32 = &*arc;
assert!(Arc::is_unique(&arc));
assert_eq!(unsafe { *pointer }, 5);

Atomic orderings

Concurrent drops to other Arc pointers to the same allocation will synchronize with this call - that is, this call performs an Acquire operation on the underlying strong and weak ref counts. This ensures that calling get_mut_unchecked is safe.

Note that this operation requires locking the weak ref count, so concurrent calls to downgrade may spin-loop for a short period of time.

impl<T: Clone, A: Allocator> Arc<T, A>

fn unwrap_or_clone(this: Self) -> T

If we have the only reference to T then unwrap it. Otherwise, clone T and return the clone.

Assuming arc_t is of type Arc<T>, this function is functionally equivalent to (*arc_t).clone(), but will avoid cloning the inner value where possible.

Examples

# use std::{ptr, sync::Arc};
let inner = String::from("test");
let ptr = inner.as_ptr();

let arc = Arc::new(inner);
let inner = Arc::unwrap_or_clone(arc);
// The inner value was not cloned
assert!(ptr::eq(ptr, inner.as_ptr()));

let arc = Arc::new(inner);
let arc2 = arc.clone();
let inner = Arc::unwrap_or_clone(arc);
// Because there were 2 references, we had to clone the inner value.
assert!(!ptr::eq(ptr, inner.as_ptr()));
// `arc2` is the last reference, so when we unwrap it we get back
// the original `String`.
let inner = Arc::unwrap_or_clone(arc2);
assert!(ptr::eq(ptr, inner.as_ptr()));

impl Default for Arc<core::ffi::CStr>

fn default() -> Self

Creates an empty CStr inside an Arc

This may or may not share an allocation with other Arcs.

impl Default for Arc<str>

fn default() -> Self

Creates an empty str inside an Arc

This may or may not share an allocation with other Arcs.

impl From for Arc<[u8]>

fn from(rc: Arc<str>) -> Self

Converts an atomically reference-counted string slice into a byte slice.

Example

# use std::sync::Arc;
let string: Arc<str> = Arc::from("eggplant");
let bytes: Arc<[u8]> = Arc::from(string);
assert_eq!("eggplant".as_bytes(), bytes.as_ref());

impl From for Arc<str>

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

Allocates a reference-counted str and copies v into it.

Example

# use std::sync::Arc;
let unique: String = "eggplant".to_owned();
let shared: Arc<str> = Arc::from(unique);
assert_eq!("eggplant", &shared[..]);

impl From for Arc<str>

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

Allocates a reference-counted str and copies v into it.

Example

# use std::sync::Arc;
let shared: Arc<str> = Arc::from("eggplant");
assert_eq!("eggplant", &shared[..]);

impl From for Arc<str>

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

Allocates a reference-counted str and copies v into it.

Example

# use std::sync::Arc;
let mut original = String::from("eggplant");
let original: &mut str = &mut original;
let shared: Arc<str> = Arc::from(original);
assert_eq!("eggplant", &shared[..]);

impl From for crate::sync::Arc<ByteStr>

fn from(s: Arc<[u8]>) -> Arc<ByteStr>

impl From for crate::sync::Arc<core::ffi::CStr>

fn from(s: &mut CStr) -> Arc<CStr>

Converts a &mut CStr into a Arc<CStr>, by copying the contents into a newly allocated Arc.

impl From for crate::sync::Arc<core::ffi::CStr>

fn from(s: &CStr) -> Arc<CStr>

Converts a &CStr into a Arc<CStr>, by copying the contents into a newly allocated Arc.

impl From for crate::sync::Arc<core::ffi::CStr>

fn from(s: CString) -> Arc<CStr>

Converts a CString into an [Arc]<[CStr]> by moving the CString data into a new Arc buffer.

impl From for crate::sync::Arc<[u8]>

fn from(s: Arc<ByteStr>) -> Arc<[u8]>

impl<'a, B> From for Arc<B>

fn from(cow: Cow<'a, B>) -> Arc<B>

Creates an atomically reference-counted pointer from a clone-on-write pointer by copying its content.

Example

# use std::sync::Arc;
# use std::borrow::Cow;
let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
let shared: Arc<str> = Arc::from(cow);
assert_eq!("eggplant", &shared[..]);

impl<P, T> Receiver for Arc<T, A>

impl<T> Any for Arc<T, A>

fn type_id(self: &Self) -> TypeId

impl<T> Borrow for Arc<T, A>

fn borrow(self: &Self) -> &T

impl<T> BorrowMut for Arc<T, A>

fn borrow_mut(self: &mut Self) -> &mut T

impl<T> CloneToUninit for Arc<T, A>

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

impl<T> Default for Arc<[T]>

fn default() -> Self

Creates an empty [T] inside an Arc

This may or may not share an allocation with other Arcs.

impl<T> From for Arc<T>

fn from(t: T) -> Self

Converts a T into an Arc<T>

The conversion moves the value into a newly allocated Arc. It is equivalent to calling Arc::new(t).

Example

# use std::sync::Arc;
let x = 5;
let arc = Arc::new(5);

assert_eq!(Arc::from(x), arc);

impl<T> From for Arc<T, A>

fn from(t: T) -> T

Returns the argument unchanged.

impl<T> From for Arc<T, A>

fn from(t: never) -> T

impl<T> FromIterator for Arc<[T]>

fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self

Takes each element in the Iterator and collects it into an Arc<[T]>.

Performance characteristics

The general case

In the general case, collecting into Arc<[T]> is done by first collecting into a Vec<T>. That is, when writing the following:

# use std::sync::Arc;
let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
# assert_eq!(&*evens, &[0, 2, 4, 6, 8]);

this behaves as if we wrote:

# use std::sync::Arc;
let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
    .collect::<Vec<_>>() // The first set of allocations happens here.
    .into(); // A second allocation for `Arc<[T]>` happens here.
# assert_eq!(&*evens, &[0, 2, 4, 6, 8]);

This will allocate as many times as needed for constructing the Vec<T> and then it will allocate once for turning the Vec<T> into the Arc<[T]>.

Iterators of known length

When your Iterator implements TrustedLen and is of an exact size, a single allocation will be made for the Arc<[T]>. For example:

# use std::sync::Arc;
let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
# assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());

impl<T> ToOwned for Arc<T, A>

fn to_owned(self: &Self) -> T
fn clone_into(self: &Self, target: &mut T)

impl<T> ToString for Arc<T, A>

fn to_string(self: &Self) -> String

impl<T, A> Freeze for Arc<T, A>

impl<T, A> RefUnwindSafe for Arc<T, A>

impl<T, A: Allocator + Clone> From for Arc<[T], A>

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

Allocates a reference-counted slice and moves v's items into it.

Example

# use std::sync::Arc;
let unique: Vec<i32> = vec![1, 2, 3];
let shared: Arc<[i32]> = Arc::from(unique);
assert_eq!(&[1, 2, 3], &shared[..]);

impl<T, A: Allocator, N: usize> TryFrom for Arc<[T; N], A>

fn try_from(boxed_slice: Arc<[T], A>) -> Result<Self, <Self as >::Error>

impl<T, N: usize> From for Arc<[T]>

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

Converts a [T; N] into an Arc<[T]>.

The conversion moves the array into a newly allocated Arc.

Example

# use std::sync::Arc;
let original: [i32; 3] = [1, 2, 3];
let shared: Arc<[i32]> = Arc::from(original);
assert_eq!(&[1, 2, 3], &shared[..]);

impl<T, U> Into for Arc<T, A>

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

impl<T, U> TryFrom for Arc<T, A>

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto for Arc<T, A>

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>

impl<T: ?Sized + Allocator, A: Allocator> Allocator for Arc<T, A>

fn allocate(self: &Self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>
fn allocate_zeroed(self: &Self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>
unsafe fn deallocate(self: &Self, ptr: NonNull<u8>, layout: Layout)
unsafe fn grow(self: &Self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout) -> Result<NonNull<[u8]>, AllocError>
unsafe fn grow_zeroed(self: &Self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout) -> Result<NonNull<[u8]>, AllocError>
unsafe fn shrink(self: &Self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout) -> Result<NonNull<[u8]>, AllocError>

impl<T: ?Sized + Eq, A: Allocator> Eq for Arc<T, A>

impl<T: ?Sized + Hash, A: Allocator> Hash for Arc<T, A>

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

impl<T: ?Sized + Ord, A: Allocator> Ord for Arc<T, A>

fn cmp(self: &Self, other: &Arc<T, A>) -> 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)));

impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Arc<T, A>

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

Equality for two Arcs.

Two Arcs are equal if their inner values are equal, even if they are stored in different allocation.

If T also implements Eq (implying reflexivity of equality), two Arcs that point to the same allocation are always equal.

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

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

Inequality for two Arcs.

Two Arcs are not equal if their inner values are not equal.

If T also implements Eq (implying reflexivity of equality), 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(6));

impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Arc<T, A>

fn partial_cmp(self: &Self, other: &Arc<T, A>) -> 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: &Self, other: &Arc<T, A>) -> 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: &Self, other: &Arc<T, A>) -> 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: &Self, other: &Arc<T, A>) -> 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: &Self, other: &Arc<T, A>) -> 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 + Sync + Send, A: Allocator + Send> Send for Arc<T, A>

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

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

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

impl<T: ?Sized + fmt::Debug, A: Allocator> Debug for Arc<T, A>

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

impl<T: ?Sized + fmt::Display, A: Allocator> Display for Arc<T, A>

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

impl<T: ?Sized> CloneFromCell for Arc<T>

impl<T: ?Sized, A: Allocator + Clone> Clone for Arc<T, A>

fn clone(self: &Self) -> Arc<T, A>

Makes a clone of the Arc pointer.

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

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

impl<T: ?Sized, A: Allocator + Clone> UseCloned for Arc<T, A>

impl<T: ?Sized, A: Allocator> AsRef for Arc<T, A>

fn as_ref(self: &Self) -> &T

impl<T: ?Sized, A: Allocator> Borrow for Arc<T, A>

fn borrow(self: &Self) -> &T

impl<T: ?Sized, A: Allocator> Deref for Arc<T, A>

fn deref(self: &Self) -> &T

impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A>

impl<T: ?Sized, A: Allocator> Drop for Arc<T, A>

fn drop(self: &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, A: Allocator> From for Arc<T, A>

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

Move a boxed object to a new, reference-counted allocation.

Example

# use std::sync::Arc;
let unique: Box<str> = Box::from("eggplant");
let shared: Arc<str> = Arc::from(unique);
assert_eq!("eggplant", &shared[..]);

impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Arc<T, A>

impl<T: ?Sized, A: Allocator> Pointer for Arc<T, A>

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

impl<T: ?Sized, A: Allocator> Unpin for Arc<T, A>

impl<T: Clone> From for Arc<[T]>

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

Allocates a reference-counted slice and fills it by cloning v's items.

Example

# use std::sync::Arc;
let mut original = [1, 2, 3];
let original: &mut [i32] = &mut original;
let shared: Arc<[i32]> = Arc::from(original);
assert_eq!(&[1, 2, 3], &shared[..]);

impl<T: Clone> From for Arc<[T]>

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

Allocates a reference-counted slice and fills it by cloning v's items.

Example

# use std::sync::Arc;
let original: &[i32] = &[1, 2, 3];
let shared: Arc<[i32]> = Arc::from(original);
assert_eq!(&[1, 2, 3], &shared[..]);

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: RefUnwindSafe + ?Sized, A: Allocator + UnwindSafe> UnwindSafe for Arc<T, A>

impl<T: core::error::Error + ?Sized> Error for Arc<T>

fn cause(self: &Self) -> Option<&dyn core::error::Error>
fn source(self: &Self) -> Option<&dyn core::error::Error + 'static>
fn provide<'a>(self: &'a Self, req: &mut core::error::Request<'a>)