Struct Lrc

pub struct Lrc<T, A = Global>
where
    A: Allocator,
    T: ?Sized,
{
    ptr: NonNull<ArcInner<T>>,
    phantom: PhantomData<ArcInner<T>>,
    alloc: A,
}

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.

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> 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

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> 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.

Fields

ptr: NonNull<ArcInner<T>>

phantom: PhantomData<ArcInner<T>>

alloc: A

Implementations

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 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

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()
    }
}

pub fn new_uninit() -> Arc<MaybeUninit<T>>

Constructs a new Arc with uninitialized contents.

Examples

#![feature(get_mut_unchecked)]

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)

pub fn new_zeroed() -> Arc<MaybeUninit<T>>

🔬This is a nightly-only experimental API. (new_zeroed_alloc)

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

#![feature(new_zeroed_alloc)]

use std::sync::Arc;

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

assert_eq!(*zero, 0)

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

🔬This is a nightly-only experimental API. (allocator_api)

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

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

🔬This is a nightly-only experimental API. (allocator_api)

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)?;

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

🔬This is a nightly-only experimental API. (allocator_api)

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

Examples

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

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);

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

🔬This is a nightly-only experimental API. (allocator_api)

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);

impl<T, A> Arc<T, A>

where A: Allocator,

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

🔬This is a nightly-only experimental API. (allocator_api)

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);

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

🔬This is a nightly-only experimental API. (allocator_api)

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)

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

🔬This is a nightly-only experimental API. (allocator_api)

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)

pub fn new_cyclic_in<F>(data_fn: F, alloc: A) -> Arc<T, A>

where F: FnOnce(&Weak<T, A>) -> T,

🔬This is a nightly-only experimental API. (allocator_api)

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

pub fn pin_in(data: T, alloc: A) -> Pin<Arc<T, A>>

where A: 'static,

🔬This is a nightly-only experimental API. (allocator_api)

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.

pub fn try_pin_in(data: T, alloc: A) -> Result<Pin<Arc<T, A>>, AllocError>

where A: 'static,

🔬This is a nightly-only experimental API. (allocator_api)

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

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

🔬This is a nightly-only experimental API. (allocator_api)

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)?;

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

🔬This is a nightly-only experimental API. (allocator_api)

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);

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

🔬This is a nightly-only experimental API. (allocator_api)

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);

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

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. 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);

pub fn into_inner(this: Arc<T, A>) -> 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(), 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> {
    /* ... */
}

// 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;
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> Arc<[T]>

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

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

Examples

#![feature(get_mut_unchecked)]

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])

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

🔬This is a nightly-only experimental API. (new_zeroed_alloc)

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

#![feature(new_zeroed_alloc)]

use std::sync::Arc;

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

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

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

where A: Allocator,

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

🔬This is a nightly-only experimental API. (allocator_api)

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])

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

🔬This is a nightly-only experimental API. (allocator_api)

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> Arc<MaybeUninit<T>, A>

where A: Allocator,

pub unsafe fn assume_init(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

#![feature(get_mut_unchecked)]

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, A> Arc<[MaybeUninit<T>], A>

where A: Allocator,

pub unsafe fn assume_init(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

#![feature(get_mut_unchecked)]

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> Arc<T>

where T: ?Sized,

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

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 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]);
}

pub 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 the associated Arc instance must be valid (i.e. the strong count must be at least 1) for the duration of this method.

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));
}

pub 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 the associated Arc instance must be valid (i.e. the strong count must be at least 1) when invoking this method. 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, A> Arc<T, A>

where A: Allocator, T: ?Sized,

pub fn allocator(this: &Arc<T, A>) -> &A

🔬This is a nightly-only experimental API. (allocator_api)

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.

pub fn into_raw(this: Arc<T, A>) -> *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");

pub fn into_raw_with_allocator(this: Arc<T, A>) -> (*const T, A)

🔬This is a nightly-only experimental API. (allocator_api)

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");

pub fn as_ptr(this: &Arc<T, A>) -> *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");

pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Arc<T, A>

🔬This is a nightly-only experimental API. (allocator_api)

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 = Arc::into_raw(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(x);

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

pub fn downgrade(this: &Arc<T, A>) -> 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);

pub fn weak_count(this: &Arc<T, A>) -> 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));

pub fn strong_count(this: &Arc<T, A>) -> 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));

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

where A: Clone,

🔬This is a nightly-only experimental API. (allocator_api)

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 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 = Arc::into_raw(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));
}

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

🔬This is a nightly-only experimental API. (allocator_api)

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, 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 = Arc::into_raw(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));
}

pub fn ptr_eq(this: &Arc<T, A>, other: &Arc<T, A>) -> 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, A> Arc<T, A>

where T: CloneToUninit + ?Sized, A: Allocator + Clone,

pub fn make_mut(this: &mut Arc<T, A>) -> &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, A> Arc<T, A>

where T: Clone, A: Allocator,

pub fn unwrap_or_clone(this: Arc<T, A>) -> 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

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<T, A> Arc<T, A>

where A: Allocator, T: ?Sized,

pub fn get_mut(this: &mut Arc<T, A>) -> 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());

pub unsafe fn get_mut_unchecked(this: &mut Arc<T, A>) -> &mut T

🔬This is a nightly-only experimental API. (get_mut_unchecked)

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 Rc (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().into();
    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

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

where A: Allocator,

pub fn downcast<T>(self) -> Result<Arc<T, A>, Arc<dyn Any + Send + Sync, A>>

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));

pub unsafe fn downcast_unchecked<T>(self) -> Arc<T, A>

where T: Any + Send + Sync,

🔬This is a nightly-only experimental API. (downcast_unchecked)

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.

Trait Implementations

impl<T> AsFd for Arc<T>

where T: AsFd + ?Sized,

This impl allows implementing traits that require AsFd on Arc.

use std::net::UdpSocket;
use std::sync::Arc;

trait MyTrait: AsFd {}
impl MyTrait for Arc<UdpSocket> {}
impl MyTrait for Box<UdpSocket> {}

fn as_fd(&self) -> BorrowedFd<'_>

Borrows the file descriptor.

impl<T> AsRawFd for Arc<T>

where T: AsRawFd,

This impl allows implementing traits that require AsRawFd on Arc.

use std::net::UdpSocket;
use std::sync::Arc;
trait MyTrait: AsRawFd {
}
impl MyTrait for Arc<UdpSocket> {}
impl MyTrait for Box<UdpSocket> {}

fn as_raw_fd(&self) -> i32

Extracts the raw file descriptor.

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

where A: Allocator, T: ?Sized,

fn as_ref(&self) -> &T

Converts this type into a shared reference of the (usually inferred) input type.

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

where A: Allocator, T: ?Sized,

fn borrow(&self) -> &T

Immutably borrows from an owned value.

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

where A: Allocator + Clone, T: ?Sized,

fn clone(&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);

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

Performs copy-assignment from source.

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

where T: Debug + ?Sized, A: Allocator,

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

Formats the value using the given formatter.

impl<D, T> Decodable<D> for Arc<[T]>

where D: Decoder, T: Decodable<D>,

fn decode(d: &mut D) -> Arc<[T]>

impl<D, T> Decodable<D> for Arc<T>

where D: Decoder, T: Decodable<D>,

fn decode(d: &mut D) -> Arc<T>

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

fn default() -> Arc<[T]>

Creates an empty [T] inside an Arc

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

impl Default for Arc<CStr>

fn default() -> Arc<CStr>

Creates an empty CStr inside an Arc

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

impl<T> Default for Arc<T>

where T: Default,

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 Default for Arc<str>

fn default() -> Arc<str>

Creates an empty str inside an Arc

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

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

where A: Allocator, T: ?Sized,

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

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

where T: Display + ?Sized, A: Allocator,

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

Formats the value using the given formatter.

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

where A: Allocator, T: ?Sized,

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<E, T> Encodable<E> for Arc<[T]>

where E: Encoder, T: Encodable<E>,

fn encode(&self, s: &mut E)

impl<S, T> Encodable<S> for Arc<T>

where S: Encoder, T: Encodable<S>,

fn encode(&self, s: &mut S)

impl<T> Error for Arc<T>

where T: Error + ?Sized,

fn description(&self) -> &str

👎Deprecated since 1.42.0: use the Display impl or to_string()

fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0: replaced by Error::source, which can support downcasting

fn source(&self) -> Option<&(dyn Error + 'static)>

Returns the lower-level source of this error, if any.

fn provide<'a>(&'a self, req: &mut Request<'a>)

🔬This is a nightly-only experimental API. (error_generic_member_access)

Provides type-based access to context intended for error reports.

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

where T: Clone,

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

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

Example

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

impl From<&CStr> for Arc<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<&OsStr> for Arc<OsStr>

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

Copies the string into a newly allocated Arc<OsStr>.

impl From<&Path> for Arc<Path>

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

Converts a Path into an Arc by copying the Path data into a new Arc buffer.

impl<T> From<&mut [T]> for Arc<[T]>

where T: Clone,

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

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

Example

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 From<&mut CStr> for Arc<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<&mut OsStr> for Arc<OsStr>

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

Copies the string into a newly allocated Arc<OsStr>.

impl From<&mut Path> for Arc<Path>

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

Converts a Path into an Arc by copying the Path data into a new Arc buffer.

impl From<&mut str> for Arc<str>

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

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

Example

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<&str> for Arc<str>

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

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

Example

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

impl<T, const N: usize> From<[T; N]> 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

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

impl From<Arc<str>> for Arc<[u8]>

fn from(rc: Arc<str>) -> Arc<[u8]>

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

Example

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

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

where A: Allocator, T: ?Sized,

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

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

Example

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

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

where B: ToOwned + ?Sized, Arc<B>: From<&'a B> + From<<B as ToOwned>::Owned>,

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

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

impl From<OsString> for Arc<OsStr>

fn from(s: OsString) -> Arc<OsStr>

Converts an OsString into an Arc<OsStr> by moving the OsString data into a new Arc buffer.

impl From<PathBuf> for Arc<Path>

fn from(s: PathBuf) -> Arc<Path>

Converts a PathBuf into an Arc<Path> by moving the PathBuf data into a new Arc buffer.

impl From<String> for Arc<str>

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

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

Example

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

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

fn from(t: T) -> Arc<T>

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

let x = 5;
let arc = Arc::new(5);

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

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

where A: Allocator + Clone,

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

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

Example

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

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

fn from_iter<I>(iter: I) -> Arc<[T]>

where I: IntoIterator<Item = T>,

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:

let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();

this behaves as if we wrote:

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.

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:

let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.

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

where T: Hash + ?Sized, A: Allocator,

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

where H: Hasher,

Feeds this value into the given Hasher.

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

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<T: ?Sized + HashStable<CTX>, CTX> HashStable<CTX> for Arc<T>

fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher)

impl<T> Log for Arc<T>

where T: Log + ?Sized,

fn enabled(&self, metadata: &Metadata<'_>) -> bool

Determines if a log message with the specified metadata would be logged.

fn log(&self, record: &Record<'_>)

Logs the Record.

fn flush(&self)

Flushes any buffered records.

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

where T: Ord + ?Sized, A: Allocator,

fn cmp(&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)));

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

where Self: Sized,

Compares and returns the maximum of two values.

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

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

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

where T: PartialEq + ?Sized, A: Allocator,

fn eq(&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, 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, A> PartialOrd for Arc<T, A>

where T: PartialOrd + ?Sized, A: Allocator,

fn partial_cmp(&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, 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, 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, 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, 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 + Aligned> Pointer for Arc<T>

const BITS: u32 = _

Number of unused (always zero) least-significant bits in this pointer, usually related to the pointees alignment.

fn into_ptr(self) -> NonNull<T>

Turns this pointer into a raw, non-null pointer.

unsafe fn from_ptr(ptr: NonNull<T>) -> Self

Re-creates the original pointer, from a raw pointer returned by into_ptr.

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

where A: Allocator, T: ?Sized,

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

Formats the value using the given formatter.

impl Read for Arc<File>

fn read(&mut self, buf: &mut [u8]) -> Result<usize, Error>

Pull some bytes from this source into the specified buffer, returning how many bytes were read.

fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize, Error>

Like read, except that it reads into a slice of buffers.

fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> Result<(), Error>

🔬This is a nightly-only experimental API. (read_buf)

Pull some bytes from this source into the specified buffer.

fn is_read_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Reader has an efficient read_vectored implementation.

fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize, Error>

Reads all bytes until EOF in this source, placing them into buf.

fn read_to_string(&mut self, buf: &mut String) -> Result<usize, Error>

Reads all bytes until EOF in this source, appending them to buf.

fn read_exact(&mut self, buf: &mut [u8]) -> Result<(), Error>

Reads the exact number of bytes required to fill buf.

fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<(), Error>

🔬This is a nightly-only experimental API. (read_buf)

Reads the exact number of bytes required to fill cursor.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adaptor for this instance of Read.

fn bytes(self) -> Bytes<Self>

where Self: Sized,

Transforms this Read instance to an Iterator over its bytes.

fn chain<R>(self, next: R) -> Chain<Self, R>

where R: Read, Self: Sized,

Creates an adapter which will chain this stream with another.

fn take(self, limit: u64) -> Take<Self>

where Self: Sized,

Creates an adapter which will read at most limit bytes from it.

impl Seek for Arc<File>

fn seek(&mut self, pos: SeekFrom) -> Result<u64, Error>

Seek to an offset, in bytes, in a stream.

fn rewind(&mut self) -> Result<(), Error>

Rewind to the beginning of a stream.

fn stream_len(&mut self) -> Result<u64, Error>

🔬This is a nightly-only experimental API. (seek_stream_len)

Returns the length of this stream (in bytes).

fn stream_position(&mut self) -> Result<u64, Error>

Returns the current seek position from the start of the stream.

fn seek_relative(&mut self, offset: i64) -> Result<(), Error>

Seeks relative to the current position.

impl<S> Subscriber for Arc<S>

where S: Subscriber + ?Sized,

fn register_callsite(&self, metadata: &'static Metadata<'static>) -> Interest

Registers a new callsite with this subscriber, returning whether or not the subscriber is interested in being notified about the callsite.

fn enabled(&self, metadata: &Metadata<'_>) -> bool

Returns true if a span or event with the specified metadata would be recorded.

fn max_level_hint(&self) -> Option<LevelFilter>

Returns the highest verbosity level that this Subscriber will enable, or None, if the subscriber does not implement level-based filtering or chooses not to implement this method.

fn new_span(&self, span: &Attributes<'_>) -> Id

Visit the construction of a new span, returning a new span ID for the span being constructed.

fn record(&self, span: &Id, values: &Record<'_>)

Record a set of values on a span.

fn record_follows_from(&self, span: &Id, follows: &Id)

Adds an indication that span follows from the span with the id follows.

fn event_enabled(&self, event: &Event<'_>) -> bool

Determine if an Event should be recorded.

fn event(&self, event: &Event<'_>)

Records that an Event has occurred.

fn enter(&self, span: &Id)

Records that a span has been entered.

fn exit(&self, span: &Id)

Records that a span has been exited.

fn clone_span(&self, id: &Id) -> Id

Notifies the subscriber that a span ID has been cloned.

fn try_close(&self, id: Id) -> bool

Notifies the subscriber that a span ID has been dropped, and returns true if there are now 0 IDs that refer to that span.

fn drop_span(&self, id: Id)

👎Deprecated since 0.1.2: use Subscriber::try_close instead

This method is deprecated.

fn current_span(&self) -> Current

Returns a type representing this subscriber’s view of the current span.

unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()>

If self is the same type as the provided TypeId, returns an untyped *const pointer to that type. Otherwise, returns None.

fn on_register_dispatch(&self, subscriber: &Dispatch)

Invoked when this subscriber becomes a Dispatch.

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

where A: Allocator,

type Error = Arc<[T], A>

The type returned in the event of a conversion error.

fn try_from(boxed_slice: Arc<[T], A>) -> Result<Arc<[T; N], A>, Arc<[T], A>>

Performs the conversion.

impl Write for Arc<File>

fn write(&mut self, buf: &[u8]) -> Result<usize, Error>

Writes a buffer into this writer, returning how many bytes were written.

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize, Error>

Like write, except that it writes from a slice of buffers.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector)

Determines if this Writer has an efficient write_vectored implementation.

fn flush(&mut self) -> Result<(), Error>

Flushes this output stream, ensuring that all intermediately buffered contents reach their destination.

fn write_all(&mut self, buf: &[u8]) -> Result<(), Error>

Attempts to write an entire buffer into this writer.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<(), Error>

🔬This is a nightly-only experimental API. (write_all_vectored)

Attempts to write multiple buffers into this writer.

fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<(), Error>

Writes a formatted string into this writer, returning any error encountered.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adapter for this instance of Write.

impl<T> CloneStableDeref for Arc<T>

where T: ?Sized,

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

where T: Unsize<U> + ?Sized, A: Allocator, U: ?Sized,

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

where A: Allocator, T: ?Sized,

impl<T, U> DispatchFromDyn<Arc<U>> for Arc<T>

where T: Unsize<U> + ?Sized, U: ?Sized,

impl<T: ?Sized + DynSync + DynSend> DynSend for Arc<T>

impl<T: ?Sized + DynSync + DynSend> DynSync for Arc<T>

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

where T: Eq + ?Sized, A: Allocator,

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

where A: Allocator, T: ?Sized,

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

where T: Sync + Send + ?Sized, A: Allocator + Send,

impl<T> StableDeref for Arc<T>

where T: ?Sized,

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

where T: Sync + Send + ?Sized, A: Allocator + Sync,

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

where A: Allocator, T: ?Sized,

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

where T: RefUnwindSafe + ?Sized, A: Allocator + UnwindSafe,

Layout

Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.