Struct serde::lib::core::sync::Arc

1.0.0 · source ·
pub struct Arc<T>where
    T: ?Sized,
{ ptr: NonNull<ArcInner<T>>, phantom: PhantomData<ArcInner<T>>, }
Expand description

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

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

Fields§

§ptr: NonNull<ArcInner<T>>§phantom: PhantomData<ArcInner<T>>

Implementations§

source§

impl<T> Arc<T>

source

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

Constructs a new Arc<T>.

Examples
use std::sync::Arc;

let five = Arc::new(5);
1.60.0 · source

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 {
    /// Construct 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() }
        })
    }

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

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

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

Constructs a new Arc with uninitialized contents.

Examples
#![feature(new_uninit)]
#![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)
source

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

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

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_uninit)]

use std::sync::Arc;

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

assert_eq!(*zero, 0)
1.33.0 · source

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.

source

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.

source

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)?;
source

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(new_uninit, 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);
source

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(new_uninit, allocator_api)]

use std::sync::Arc;

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

assert_eq!(*zero, 0);
1.4.0 · source

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

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.

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);
source

pub fn into_inner(this: Arc<T>) -> Option<T>

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

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.

The similar expression Arc::try_unwrap(this).ok() does not offer such a guarantee. See the last example below.

Examples

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

#![feature(arc_into_inner)]

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:

#![feature(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();
for i in 0..100000 {
    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();
source§

impl<T> Arc<[T]>

source

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

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

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

Examples
#![feature(new_uninit)]
#![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])
source

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

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

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_uninit)]

use std::sync::Arc;

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

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

impl<T> Arc<MaybeUninit<T>>

source

pub unsafe fn assume_init(self) -> Arc<T>

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

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(new_uninit)]
#![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)
source§

impl<T> Arc<[MaybeUninit<T>]>

source

pub unsafe fn assume_init(self) -> Arc<[T]>

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

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(new_uninit)]
#![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])
source§

impl<T> Arc<T>where
    T: ?Sized,

1.17.0 · source

pub fn into_raw(this: Arc<T>) -> *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");
1.45.0 · source

pub fn as_ptr(this: &Arc<T>) -> *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");
1.17.0 · source

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 where U must have the same size and alignment as T. This is trivially true if U is T. Note that if U 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!
1.4.0 · source

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

Creates a new Weak pointer to this allocation.

Examples
use std::sync::Arc;

let five = Arc::new(5);

let weak_five = Arc::downgrade(&five);
1.15.0 · source

pub fn weak_count(this: &Arc<T>) -> 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));
1.15.0 · source

pub fn strong_count(this: &Arc<T>) -> 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));
1.51.0 · source

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));
}
1.51.0 · source

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));
}
1.17.0 · source

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

Returns true if the two Arcs point to the same allocation in a vein similar to ptr::eq. See that function for caveats when comparing 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));
source§

impl<T> Arc<T>where
    T: Clone,

1.4.0 · source

pub fn make_mut(this: &mut Arc<T>) -> &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());
source

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

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

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
#![feature(arc_unwrap_or_clone)]
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()));
source§

impl<T> Arc<T>where
    T: ?Sized,

1.4.0 · source

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

pub unsafe fn get_mut_unchecked(this: &mut Arc<T>) -> &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
source§

impl Arc<dyn Any + Send + Sync + 'static>

1.29.0 · source

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

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

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

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

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

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

1.64.0 · source§

impl<T> AsFd for Arc<T>where
    T: AsFd,

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

Borrows the file descriptor. Read more
1.63.0 · source§

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> {}
source§

fn as_raw_fd(&self) -> i32

Extracts the raw file descriptor. Read more
1.5.0 · source§

impl<T> AsRef<T> for Arc<T>where
    T: ?Sized,

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fn as_ref(&self) -> &T

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

impl<T> Borrow<T> for Arc<T>where
    T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
source§

impl<T> Clone for Arc<T>where
    T: ?Sized,

source§

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

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

Performs copy-assignment from source. Read more
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impl<T> Debug for Arc<T>where
    T: Debug + ?Sized,

source§

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

Formats the value using the given formatter. Read more
source§

impl<T> Default for Arc<T>where
    T: Default,

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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);
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impl<T> Deref for Arc<T>where
    T: ?Sized,

§

type Target = T

The resulting type after dereferencing.
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fn deref(&self) -> &T

Dereferences the value.
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impl<'de, T: ?Sized> Deserialize<'de> for Arc<T>where
    Box<T>: Deserialize<'de>,

This impl requires the "rc" Cargo feature of Serde.

Deserializing a data structure containing Arc will not attempt to deduplicate Arc references to the same data. Every deserialized Arc will end up with a strong count of 1.

source§

fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>where
    D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer. Read more
source§

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

source§

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

Formats the value using the given formatter. Read more
source§

impl<T> Drop for Arc<T>where
    T: ?Sized,

source§

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!"
1.52.0 · source§

impl<T> Error for Arc<T>where
    T: Error + ?Sized,

source§

fn description(&self) -> &str

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

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

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

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

The lower-level source of this error, if any. Read more
source§

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

🔬This is a nightly-only experimental API. (error_generic_member_access)
Provides type based access to context intended for error reports. Read more
1.21.0 · source§

impl<T> From<&[T]> for Arc<[T]>where
    T: Clone,

source§

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

Allocate a reference-counted slice and fill 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[..]);
1.24.0 · source§

impl From<&CStr> for Arc<CStr>

source§

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

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

1.24.0 · source§

impl From<&OsStr> for Arc<OsStr>

source§

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

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

1.24.0 · source§

impl From<&Path> for Arc<Path>

source§

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

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

1.21.0 · source§

impl From<&str> for Arc<str>

source§

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

Allocate a reference-counted str and copy v into it.

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

impl<W> From<Arc<W>> for RawWakerwhere
    W: Wake + Send + Sync + 'static,

source§

fn from(waker: Arc<W>) -> RawWaker

Use a Wake-able type as a RawWaker.

No heap allocations or atomic operations are used for this conversion.

1.51.0 · source§

impl<W> From<Arc<W>> for Wakerwhere
    W: Wake + Send + Sync + 'static,

source§

fn from(waker: Arc<W>) -> Waker

Use a Wake-able type as a Waker.

No heap allocations or atomic operations are used for this conversion.

1.62.0 · source§

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

source§

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());
1.21.0 · source§

impl<T> From<Box<T, Global>> for Arc<T>where
    T: ?Sized,

source§

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

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[..]);
1.24.0 · source§

impl From<CString> for Arc<CStr>

source§

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

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

1.45.0 · source§

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

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

Create 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[..]);
1.24.0 · source§

impl From<OsString> for Arc<OsStr>

source§

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

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

1.24.0 · source§

impl From<PathBuf> for Arc<Path>

source§

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

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

1.21.0 · source§

impl From<String> for Arc<str>

source§

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

Allocate a reference-counted str and copy v into it.

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

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

source§

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);
1.21.0 · source§

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

source§

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

Allocate a reference-counted slice and move 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[..]);
1.37.0 · source§

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

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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.
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impl<T> Hash for Arc<T>where
    T: Hash + ?Sized,

source§

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

Feeds this value into the given Hasher. Read more
1.3.0 · source§

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

Feeds a slice of this type into the given Hasher. Read more
source§

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

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

Comparison for two Arcs.

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

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

let five = Arc::new(5);

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

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

Compares and returns the maximum of two values. Read more
1.21.0 · source§

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

Compares and returns the minimum of two values. Read more
1.50.0 · source§

fn clamp(self, min: Self, max: Self) -> Selfwhere
    Self: Sized + PartialOrd<Self>,

Restrict a value to a certain interval. Read more
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impl<T> PartialEq<Arc<T>> for Arc<T>where
    T: PartialEq<T> + ?Sized,

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fn eq(&self, other: &Arc<T>) -> 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));
source§

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

Inequality for two Arcs.

Two Arcs are unequal if their inner values are unequal.

If T also implements Eq (implying reflexivity of equality), two Arcs that point to the same value are never unequal.

Examples
use std::sync::Arc;

let five = Arc::new(5);

assert!(five != Arc::new(6));
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impl<T> PartialOrd<Arc<T>> for Arc<T>where
    T: PartialOrd<T> + ?Sized,

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

Partial comparison for two Arcs.

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

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

let five = Arc::new(5);

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

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

Less-than comparison for two Arcs.

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

Examples
use std::sync::Arc;

let five = Arc::new(5);

assert!(five < Arc::new(6));
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fn le(&self, other: &Arc<T>) -> bool

‘Less than or equal to’ comparison for two Arcs.

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

Examples
use std::sync::Arc;

let five = Arc::new(5);

assert!(five <= Arc::new(5));
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fn gt(&self, other: &Arc<T>) -> bool

Greater-than comparison for two Arcs.

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

Examples
use std::sync::Arc;

let five = Arc::new(5);

assert!(five > Arc::new(4));
source§

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

‘Greater than or equal to’ comparison for two Arcs.

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

Examples
use std::sync::Arc;

let five = Arc::new(5);

assert!(five >= Arc::new(5));
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impl<T> Pointer for Arc<T>where
    T: ?Sized,

source§

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

Formats the value using the given formatter.
source§

impl<T> Serialize for Arc<T>where
    T: Serialize + ?Sized,

This impl requires the "rc" Cargo feature of Serde.

Serializing a data structure containing Arc will serialize a copy of the contents of the Arc each time the Arc is referenced within the data structure. Serialization will not attempt to deduplicate these repeated data.

source§

fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>where
    S: Serializer,

Serialize this value into the given Serde serializer. Read more
1.43.0 · source§

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

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type Error = Arc<[T]>

The type returned in the event of a conversion error.
source§

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

Performs the conversion.
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impl<T, U> CoerceUnsized<Arc<U>> for Arc<T>where
    T: Unsize<U> + ?Sized,
    U: ?Sized,

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

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

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

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

1.33.0 · source§

impl<T> Unpin for Arc<T>where
    T: ?Sized,

1.9.0 · source§

impl<T> UnwindSafe for Arc<T>where
    T: RefUnwindSafe + ?Sized,

Auto Trait Implementations§

§

impl<T: ?Sized> RefUnwindSafe for Arc<T>where
    T: RefUnwindSafe,

Blanket Implementations§

source§

impl<T> Any for Twhere
    T: 'static + ?Sized,

source§

fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
source§

impl<T> Borrow<T> for Twhere
    T: ?Sized,

const: unstable · source§

fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
source§

impl<T> BorrowMut<T> for Twhere
    T: ?Sized,

const: unstable · source§

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

Mutably borrows from an owned value. Read more
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impl<T> From<!> for T

const: unstable · source§

fn from(t: !) -> T

Converts to this type from the input type.
source§

impl<T> From<T> for T

const: unstable · source§

fn from(t: T) -> T

Returns the argument unchanged.

source§

impl<T, U> Into<U> for Twhere
    U: From<T>,

const: unstable · source§

fn into(self) -> U

Calls U::from(self).

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

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impl<E> Provider for Ewhere
    E: Error + ?Sized,

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fn provide<'a>(&'a self, demand: &mut Demand<'a>)

🔬This is a nightly-only experimental API. (provide_any)
Data providers should implement this method to provide all values they are able to provide by using demand. Read more
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impl<T> ToOwned for Twhere
    T: Clone,

§

type Owned = T

The resulting type after obtaining ownership.
source§

fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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impl<T> ToString for Twhere
    T: Display + ?Sized,

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default fn to_string(&self) -> String

Converts the given value to a String. Read more
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impl<T, U> TryFrom<U> for Twhere
    U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
const: unstable · source§

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

Performs the conversion.
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impl<T, U> TryInto<U> for Twhere
    U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
const: unstable · source§

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

Performs the conversion.
source§

impl<T> DeserializeOwned for Twhere
    T: for<'de> Deserialize<'de>,