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Arc in std::sync - Rust

pub struct Arc<T, A = Global>{  }

A thread-safe reference-counting pointer. ‘Arc’ stands for ‘Atomically Reference Counted’.

The type Arc<T> provides shared ownership of a value of type T, allocated in the heap. Invoking clone on Arc produces a new Arc instance, which points to the same allocation on the heap as the source Arc, while increasing a reference count. When the last Arc pointer to a given allocation is destroyed, the value stored in that allocation (often referred to as “inner value”) is also dropped.

Shared references in Rust disallow mutation by default, and Arc is no exception: you cannot generally obtain a mutable reference to something inside an Arc. If you do need to mutate through an Arc, you have several options:

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

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

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

use std::sync::Arc;

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

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")].

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>.

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.

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]);
let a = foo.clone();
let b = Arc::clone(&foo);

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(());
let arc2 = arc.clone();
let arc3 = Arc::clone(&arc);

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

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.

Constructs a new Arc<T>.

use std::sync::Arc;

let five = Arc::new(5);

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.

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

use std::sync::{Arc, Weak};

struct Gadget {
    me: Weak<Gadget>,
}

impl Gadget {
    fn new() -> Arc<Self> {
        Arc::new_cyclic(|me| {
            Gadget { me: me.clone() }
        })
    }

    fn me(&self) -> Arc<Self> {
        self.me.upgrade().unwrap()
    }
}

Constructs a new Arc with uninitialized contents.

#![feature(get_mut_unchecked)]

use std::sync::Arc;

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

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

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

assert_eq!(*five, 5)

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.

#![feature(new_zeroed_alloc)]

use std::sync::Arc;

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

assert_eq!(*zero, 0)

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

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

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

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

let five = Arc::try_new(5)?;

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

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

use std::sync::Arc;

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

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

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

assert_eq!(*five, 5);

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.

#![feature( allocator_api)]

use std::sync::Arc;

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

assert_eq!(*zero, 0);

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

#![feature(allocator_api)]

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

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

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

#![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 {
    Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5)

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.

#![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)

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.

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

See new_cyclic

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.

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

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

#![feature(allocator_api)]

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

let five = Arc::try_new_in(5, System)?;

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

#![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 {
    Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5);

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.

#![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);

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.

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);

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.

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

use std::sync::Arc;

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

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();

assert!(matches!(
    (x_inner_value, y_inner_value),
    (None, Some(3)) | (Some(3), None)
));

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

use std::sync::Arc;

#[derive(Clone)]
struct LinkedList<T>(Option<Arc<Node<T>>>);
struct Node<T>(T, Option<Arc<Node<T>>>);

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;
            }
        }
    }
}

impl<T> LinkedList<T> {
    }

let mut x = LinkedList::new();
let size = 100000;
for i in 0..size {
    x.push(i); }
let y = x.clone();

let x_thread = std::thread::spawn(|| drop(x));
let y_thread = std::thread::spawn(|| drop(y));
x_thread.join().unwrap();
y_thread.join().unwrap();

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

#![feature(get_mut_unchecked)]

use std::sync::Arc;

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

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])

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.

#![feature(new_zeroed_alloc)]

use std::sync::Arc;

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

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

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

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

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

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

#![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 {
    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])

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.

#![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])

Converts to Arc<T>.

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.

#![feature(get_mut_unchecked)]

use std::sync::Arc;

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

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

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

assert_eq!(*five, 5)

Converts to Arc<[T]>.

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.

#![feature(get_mut_unchecked)]

use std::sync::Arc;

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

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])

Constructs an Arc<T> from a raw pointer.

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

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

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

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

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

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

use std::sync::Arc;

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

unsafe {
    let x = Arc::from_raw(x_ptr);
    assert_eq!(&*x, "hello");

    }

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]);
}

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

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.

use std::sync::Arc;

let five = Arc::new(5);

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

    let five = Arc::from_raw(ptr);
    assert_eq!(2, Arc::strong_count(&five));
}

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

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.

use std::sync::Arc;

let five = Arc::new(5);

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

    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));
}

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.

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.

use std::sync::Arc;

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

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.

#![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");

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.

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");

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

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

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

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

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

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

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

#![feature(allocator_api)]

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

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

unsafe {
    let x = Arc::from_raw_in(x_ptr, System);
    assert_eq!(&*x, "hello");

    }

Convert a slice back into its original array:

#![feature(allocator_api)]

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

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

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

Creates a new Weak pointer to this allocation.

use std::sync::Arc;

let five = Arc::new(5);

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

Gets the number of Weak pointers to this allocation.

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.

use std::sync::Arc;

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

assert_eq!(1, Arc::weak_count(&five));

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

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.

use std::sync::Arc;

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

assert_eq!(2, Arc::strong_count(&five));

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

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.

#![feature(allocator_api)]

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

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

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

    let five = Arc::from_raw_in(ptr, System);
    assert_eq!(2, Arc::strong_count(&five));
}

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

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.

#![feature(allocator_api)]

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

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

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

    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));
}

Returns true if the two Arcs point to the same allocation in a vein similar to ptr::eq. This function ignores the metadata of dyn Trait pointers.

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));

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.

use std::sync::Arc;

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

*Arc::make_mut(&mut data) += 1;         let mut other_data = Arc::clone(&data); *Arc::make_mut(&mut data) += 1;         *Arc::make_mut(&mut data) += 1;         *Arc::make_mut(&mut other_data) *= 2;   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());

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.

let inner = String::from("test");
let ptr = inner.as_ptr();

let arc = Arc::new(inner);
let inner = Arc::unwrap_or_clone(arc);
assert!(ptr::eq(ptr, inner.as_ptr()));

let arc = Arc::new(inner);
let arc2 = arc.clone();
let inner = Arc::unwrap_or_clone(arc);
assert!(!ptr::eq(ptr, inner.as_ptr()));
let inner = Arc::unwrap_or_clone(arc2);
assert!(ptr::eq(ptr, inner.as_ptr()));

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.

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());

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

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

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.

#![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 {
    Arc::get_mut_unchecked(&mut y).fill(0xff); }
println!("{}", &*x); 

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 {
        *Arc::get_mut_unchecked(&mut y) = &s;
    }
}
println!("{}", &*x); 

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.

#![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);

let z = Arc::downgrade(&x);
assert!(!Arc::is_unique(&x));

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);

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.

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

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));

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

For a safe alternative see downcast.

#![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);
}

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

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> {}

This impl allows implementing traits that require AsHandle on Arc.

use std::fs::File;
use std::sync::Arc;

trait MyTrait: AsHandle {}
impl MyTrait for Arc<File> {}
impl MyTrait for Box<File> {}

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> {}

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

This impl allows implementing traits that require AsSocket on Arc.

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

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

Makes a clone of the Arc pointer.

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

use std::sync::Arc;

let five = Arc::new(5);

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

Performs copy-assignment from

.

Creates an empty [T] inside an Arc

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

Creates an empty CStr inside an Arc

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

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

use std::sync::Arc;

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

Creates an empty str inside an Arc

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

The resulting type after dereferencing.

Dereferences the value.

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.

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);    drop(foo2);   

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

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

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

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

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

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

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[..]);

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

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

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

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

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

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

The conversion moves the array into a newly allocated Arc.

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

Converts to this type from the input type.

Converts to this type from the input type.

Use a Wake-able type as a RawWaker.

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

Use a Wake-able type as a Waker.

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

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

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

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

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

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

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

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

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

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).

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

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

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

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

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

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<_>>() .into(); 

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]>.

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(); 

Comparison for two Arcs.

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

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

let five = Arc::new(5);

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

Compares and returns the maximum of two values.

Compares and returns the minimum of two values.

Restrict a value to a certain interval.

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.

use std::sync::Arc;

let five = Arc::new(5);

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

Inequality for two Arcs.

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

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

use std::sync::Arc;

let five = Arc::new(5);

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

Partial comparison for two Arcs.

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

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

let five = Arc::new(5);

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

Less-than comparison for two Arcs.

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

use std::sync::Arc;

let five = Arc::new(5);

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

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

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

use std::sync::Arc;

let five = Arc::new(5);

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

Greater-than comparison for two Arcs.

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

use std::sync::Arc;

let five = Arc::new(5);

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

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

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

use std::sync::Arc;

let five = Arc::new(5);

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

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

Like

, except that it reads into a slice of buffers.

Pull some bytes from this source into the specified buffer.

Determines if this

er has an efficient

implementation.

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

.

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

.

Reads the exact number of bytes required to fill

.

Reads the exact number of bytes required to fill

.

Creates a “by reference” adaptor for this instance of

.

Creates an adapter which will chain this stream with another.

Creates an adapter which will read at most

bytes from it.

The type returned in the event of a conversion error.

Performs the conversion.

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

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

Attempts to write an entire buffer into this writer.

Attempts to write multiple buffers into this writer.

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

Creates a “by reference” adapter for this instance of

.

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