Arc in alloc::sync - Rust
Description: A thread-safe reference-counting pointer. ‘Arc’ stands for ‘Atomically Reference Counted’.

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## Arc

# Struct Arc

1.36.0 · Source

    pub struct Arc<T: ?Sized, A: Allocator = Global> { /* private fields */ }

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

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

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

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

    use std::sync::Arc;

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

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

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

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

### §Thread Safety

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

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

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

### §Breaking cycles with `Weak`

The `downgrade` method can be used to create a non-owning `Weak` pointer. A `Weak` pointer can be `upgrade`d 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.

## Implementations§

Source§

### impl<T> Arc<T>

1.0.0 · 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 {
        /// 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()
        }
    }

1.82.0 · Source

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

Constructs a new `Arc` with uninitialized contents.

##### §Examples

    use std::sync::Arc;

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

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

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

    assert_eq!(*five, 5)

1.92.0 · Source

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

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

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

##### §Examples

    use std::sync::Arc;

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

    assert_eq!(*zero, 0)

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` #32838)

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` #32838)

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` #32838)

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

##### §Examples

    #![feature(allocator_api)]

    use std::sync::Arc;

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

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

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

    assert_eq!(*five, 5);

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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);

Source

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

🔬This is a nightly-only experimental API. (`smart_pointer_try_map` #144419)

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

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

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

##### §Examples

    #![feature(smart_pointer_try_map)]

    use std::sync::Arc;

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

Source

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

🔬This is a nightly-only experimental API. (`smart_pointer_try_map` #144419)

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

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

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

##### §Examples

    #![feature(smart_pointer_try_map)]

    use std::sync::Arc;

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

Source§

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

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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);

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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)

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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)

Source

#### 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` #32838)

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`

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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.

Source

#### 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` #32838)

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

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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)?;

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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);

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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);

1.4.0 · Source

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

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

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

This will succeed even if there are outstanding weak references.

It is strongly recommended to use `Arc::into_inner` instead if you don’t keep the `Arc` in the `Err` case. Immediately dropping the `Err`-value, as the expression `Arc::try_unwrap(this).ok()` does, can cause the strong count to drop to zero and the inner value of the `Arc` to be dropped. For instance, if two threads execute such an expression in parallel, there is a race condition without the possibility of unsafety: The threads could first both check whether they own the last instance in `Arc::try_unwrap`, determine that they both do not, and then both discard and drop their instance in the call to `ok`. 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);

1.70.0 · Source

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

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

Otherwise, `None` is returned and the `Arc` is dropped.

This will succeed even if there are outstanding weak references.

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

`Arc::try_unwrap` is conceptually similar to `Arc::into_inner`, but it is meant for different use-cases. If used as a direct replacement for `Arc::into_inner` anyway, such as with the expression `Arc::try_unwrap(this).ok()`, 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();

Source§

### impl<T> Arc<[T]>

1.82.0 · Source

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

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

##### §Examples

    use std::sync::Arc;

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

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

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

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

1.92.0 · Source

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

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

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

##### §Examples

    use std::sync::Arc;

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

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

Source§

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

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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])

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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])

Source

#### pub fn into_array<const N: usize>(self) -> Result<Arc<[T; N], A>, Self>

🔬This is a nightly-only experimental API. (`alloc_slice_into_array` #148082)

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

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

##### §Errors

Returns the original `Arc<[T]>` in the `Err` variant if `self.len()` does not equal `N`.

##### §Examples

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

    let arc_slice: Arc<[i32]> = Arc::new([1, 2, 3]);

    let arc_array: Arc<[i32; 3]> = arc_slice.into_array().unwrap();

Source§

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

1.82.0 · Source

#### 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

    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: ?Sized + CloneToUninit> Arc<T>

Source

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

🔬This is a nightly-only experimental API. (`clone_from_ref` #149075)

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

##### §Examples

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

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

Source

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

🔬This is a nightly-only experimental API. (`clone_from_ref` #149075)

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

##### §Examples

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

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

Source§

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

Source

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

🔬This is a nightly-only experimental API. (`clone_from_ref` #149075)

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

##### §Examples

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

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

Source

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

🔬This is a nightly-only experimental API. (`clone_from_ref` #149075)

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

##### §Examples

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

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

Source§

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

1.82.0 · Source

#### 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

    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: ?Sized> Arc<T>

1.17.0 · Source

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

Constructs an `Arc<T>` from a raw pointer.

The raw pointer must have been previously returned by a call to `Arc<U>::into_raw` or `Arc<U>::into_raw_with_allocator`.

##### §Safety

- Creating a `Arc<T>` from a pointer other than one returned from `Arc<U>::into_raw` or `Arc<U>::into_raw_with_allocator` is undefined behavior.
- If `U` is sized, it must have the same size and alignment as `T`. This is trivially true if `U` is `T`.
- If `U` is unsized, its data pointer must have the same size and alignment as `T`. This is trivially true if `Arc<U>` was constructed through `Arc<T>` and then converted to `Arc<U>` through an unsized coercion.
- Note that if `U` or `U`’s data pointer is not `T` but has the same size and alignment, this is basically like transmuting references of different types. See `mem::transmute` for more information on what restrictions apply in this case.
- The raw pointer must point to a block of memory allocated by the global allocator.
- The user of `from_raw` has to make sure a specific value of `T` is only dropped once.

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

##### §Examples

    use std::sync::Arc;

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

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

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

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

Convert a slice back into its original array:

    use std::sync::Arc;

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

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

1.17.0 · Source

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

Consumes the `Arc`, returning the wrapped pointer.

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

##### §Examples

    use std::sync::Arc;

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

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

##### §Examples

    use std::sync::Arc;

    let five = Arc::new(5);

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

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

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

##### §Examples

    use std::sync::Arc;

    let five = Arc::new(5);

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

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

Source§

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

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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.

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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");

1.45.0 · Source

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

Provides a raw pointer to the data.

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

##### §Examples

    use std::sync::Arc;

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

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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` or `Arc<U, A>::into_raw_with_allocator`.

##### §Safety

- Creating a `Arc<T, A>` from a pointer other than one returned from `Arc<U, A>::into_raw` or `Arc<U, A>::into_raw_with_allocator` is undefined behavior.
- 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, A>` was constructed through `Arc<T, A>` and then converted to `Arc<U, A>` through an unsized coercion.
- Note that if `U` or `U`’s data pointer is not `T` but has the same size and alignment, this is basically like transmuting references of different types. See `mem::transmute` for more information on what restrictions apply in this case.
- The raw pointer must point to a block of memory allocated by `alloc`
- The user of `from_raw` has to make sure a specific value of `T` is only dropped once.

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

##### §Examples

    #![feature(allocator_api)]

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

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

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

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

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

Convert a slice back into its original array:

    #![feature(allocator_api)]

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

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

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

1.4.0 · Source

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

Creates a new `Weak` pointer to this allocation.

##### §Examples

    use std::sync::Arc;

    let five = Arc::new(5);

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

1.15.0 · Source

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

Gets the number of `Weak` pointers to this allocation.

##### §Safety

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

##### §Examples

    use std::sync::Arc;

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

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

1.15.0 · Source

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

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

##### §Safety

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

##### §Examples

    use std::sync::Arc;

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

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

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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

##### §Safety

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

##### §Examples

    #![feature(allocator_api)]

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

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

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

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

Source

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

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

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

##### §Safety

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

##### §Examples

    #![feature(allocator_api)]

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

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

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

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

1.17.0 · Source

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

Returns `true` if the two `Arc`s 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));

Source§

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

1.4.0 · Source

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

Makes a mutable reference into the given `Arc`.

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

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

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

##### §Examples

    use std::sync::Arc;

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

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

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

`Weak` pointers will be dissociated:

    use std::sync::Arc;

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

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

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

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

Source§

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

1.76.0 · Source

#### pub fn unwrap_or_clone(this: Self) -> T

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

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

##### §Examples

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

1.4.0 · Source

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

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

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

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

##### §Examples

    use std::sync::Arc;

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

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

Source

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

🔬This is a nightly-only experimental API. (`get_mut_unchecked` #63292)

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

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

##### §Safety

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

##### §Examples

    #![feature(get_mut_unchecked)]

    use std::sync::Arc;

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

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

    #![feature(get_mut_unchecked)]

    use std::sync::Arc;

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

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

    #![feature(get_mut_unchecked)]

    use std::sync::Arc;

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

Source

#### pub fn is_unique(this: &Self) -> bool

🔬This is a nightly-only experimental API. (`arc_is_unique` #138938)

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

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

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

##### §Examples

    #![feature(arc_is_unique)]

    use std::sync::Arc;

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

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

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

##### §Pointer invalidation

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

    #![feature(arc_is_unique)]

    use std::sync::Arc;

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

##### §Atomic orderings

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

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

Source§

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

1.29.0 · Source

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

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

##### §Examples

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

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

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

Source

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

🔬This is a nightly-only experimental API. (`downcast_unchecked` #90850)

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§

Source§

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

Source§

#### fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>

🔬This is a nightly-only experimental API. (`allocator_api` #32838)
Attempts to allocate a block of memory. Read more
Source§

#### fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>

🔬This is a nightly-only experimental API. (`allocator_api` #32838)
Behaves like `allocate`, but also ensures that the returned memory is zero-initialized. Read more
Source§

#### unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout)

🔬This is a nightly-only experimental API. (`allocator_api` #32838)
Deallocates the memory referenced by `ptr`. Read more
Source§

#### unsafe fn grow( &self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout, ) -> Result<NonNull<[u8]>, AllocError>

🔬This is a nightly-only experimental API. (`allocator_api` #32838)
Attempts to extend the memory block. Read more
Source§

#### unsafe fn grow_zeroed( &self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout, ) -> Result<NonNull<[u8]>, AllocError>

🔬This is a nightly-only experimental API. (`allocator_api` #32838)
Behaves like `grow`, but also ensures that the new contents are set to zero before being returned. Read more
Source§

#### unsafe fn shrink( &self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout, ) -> Result<NonNull<[u8]>, AllocError>

🔬This is a nightly-only experimental API. (`allocator_api` #32838)
Attempts to shrink the memory block. Read more
Source§

#### fn by_ref(&self) -> &Self
where Self: Sized,

🔬This is a nightly-only experimental API. (`allocator_api` #32838)
Creates a “by reference” adapter for this instance of `Allocator`. Read more
1.5.0 · Source§

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

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#### fn as_ref(&self) -> &T

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

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

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
1.0.0 · Source§

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

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#### 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);

1.0.0 (const: unstable) · Source§

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

Performs copy-assignment from `source`. Read more
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### impl<T: ?Sized> CloneFromCell for Arc<T>

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

1.0.0 · Source§

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

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

Formats the value using the given formatter. Read more
1.0.0 · Source§

### impl<T: Default> Default for Arc<T>

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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);

1.80.0 · Source§

### impl Default for Arc<str>

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#### fn default() -> Self

Creates an empty str inside an Arc

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

1.80.0 · Source§

### impl Default for Arc<CStr>

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#### fn default() -> Self

Creates an empty CStr inside an Arc

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

1.80.0 · Source§

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

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#### fn default() -> Self

Creates an empty `[T]` inside an Arc

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

1.91.0 · Source§

### impl<T> Default for Pin<Arc<T>>
where T: ?Sized, Arc<T>: Default,

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#### fn default() -> Self

Returns the “default value” for a type. Read more
1.0.0 · Source§

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

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#### type Target = T

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

Dereferences the value.
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### impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A>

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

1.0.0 · Source§

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

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

Formats the value using the given formatter. Read more
1.0.0 · Source§

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

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#### 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!"

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#### fn pin_drop(self: Pin<&mut Self>)

🔬This is a nightly-only experimental API. (`pin_ergonomics` #130494)
Execute the destructor for this type, but different to `Drop::drop`, it requires `self` to be pinned. Read more
1.0.0 · Source§

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

1.52.0 · Source§

### impl<T: Error + ?Sized> Error for Arc<T>

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

👎Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

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

Returns the lower-level source of this error, if any. Read more
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#### fn provide<'a>(&'a self, req: &mut Request<'a>)

🔬This is a nightly-only experimental API. (`error_generic_member_access` #99301)
Provides type-based access to context intended for error reports. Read more
1.0.0 · Source§

#### fn description(&self) -> &str

👎Deprecated since 1.42.0:

use the Display impl or to_string()

Read more
1.24.0 · Source§

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

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

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

1.21.0 · Source§

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

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#### 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[..]);

1.84.0 · Source§

### impl From<&mut CStr> for Arc<CStr>

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

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

1.84.0 · Source§

### impl<T: Clone> From<&mut [T]> for Arc<[T]>

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#### 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[..]);

1.84.0 · Source§

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

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#### 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[..]);

1.21.0 · Source§

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

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#### 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[..]);

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### impl From<Arc<ByteStr>> for Arc<[u8]>

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

Converts to this type from the input type.
1.51.0 · Source§

### impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for Waker

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#### 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.

1.51.0 · Source§

### impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for RawWaker

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#### 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.

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### impl From<Arc<[u8]>> for Arc<ByteStr>

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

Converts to this type from the input type.
1.62.0 · Source§

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

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

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: ?Sized, A: Allocator> From<Box<T, A>> for Arc<T, A>

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#### 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[..]);

1.24.0 · Source§

### impl From<CString> for Arc<CStr>

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#### 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::Owned>,

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#### 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[..]);

1.21.0 · Source§

### impl From<String> for Arc<str>

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#### 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[..]);

1.6.0 · Source§

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

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#### fn from(t: T) -> Self

Converts a `T` into an `Arc<T>`

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

##### §Example

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

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

1.21.0 · Source§

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

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#### 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[..]);

1.74.0 · Source§

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

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#### 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[..]);

1.37.0 · Source§

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

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

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

##### §Performance characteristics

###### §The general case

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

    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.

1.0.0 · Source§

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

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

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
1.0.0 · Source§

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

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

Comparison for two `Arc`s.

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 (const: unstable) · Source§

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

Compares and returns the maximum of two values. Read more
1.21.0 (const: unstable) · Source§

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

Compares and returns the minimum of two values. Read more
1.50.0 (const: unstable) · Source§

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

Restrict a value to a certain interval. Read more
1.0.0 · Source§

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

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

Equality for two `Arc`s.

Two `Arc`s 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 `Arc`s that point to the same allocation are always equal.

##### §Examples

    use std::sync::Arc;

    let five = Arc::new(5);

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

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

Inequality for two `Arc`s.

Two `Arc`s are not equal if their inner values are not equal.

If `T` also implements `Eq` (implying reflexivity of equality), two `Arc`s that point to the same value are always equal.

##### §Examples

    use std::sync::Arc;

    let five = Arc::new(5);

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

1.0.0 · Source§

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

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

Partial comparison for two `Arc`s.

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)));

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

Less-than comparison for two `Arc`s.

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, A>) -> bool

‘Less than or equal to’ comparison for two `Arc`s.

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, A>) -> bool

Greater-than comparison for two `Arc`s.

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));

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

‘Greater than or equal to’ comparison for two `Arc`s.

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

1.0.0 · Source§

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

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

Formats the value using the given formatter. Read more
1.0.0 · Source§

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

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

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#### final fn share(&self) -> Self

🔬This is a nightly-only experimental API. (`share_trait` #156756)
Creates another alias to the same underlying resource or shared state. Read more
1.0.0 · Source§

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

1.43.0 · Source§

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

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#### type Error = Arc<[T], A>

The type returned in the event of a conversion error.
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#### fn try_from(boxed_slice: Arc<[T], A>) -> Result<Self, Self::Error>

Performs the conversion.
1.33.0 · Source§

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

1.9.0 · Source§

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

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

## Auto Trait Implementations§

§

### impl<T, A> Freeze for Arc<T, A>
where A: Freeze, T: ?Sized,

§

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

§

### impl<T, A> UnsafeUnpin for Arc<T, A>
where A: UnsafeUnpin, T: ?Sized,

## Blanket Implementations§

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

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

Source§

#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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

🔬This is a nightly-only experimental API. (`clone_to_uninit` #126799)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<T> From<!> for T

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#### fn from(t: !) -> T

Converts to this type from the input type.
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### 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<P, T> Receiver for P
where P: Deref<Target = T> + ?Sized, T: ?Sized,

Source§

#### type Target = T

🔬This is a nightly-only experimental API. (`arbitrary_self_types` #44874)
The target type on which the method may be called.
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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### 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 T
where T: Display + ?Sized,

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#### fn to_string(&self) -> String

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

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### 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 T
where U: TryFrom<T>,

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

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.