Type Alias MemoizedFutureContainer

pub type MemoizedFutureContainer<T, E> = Arc<Mutex<Option<Shared<PinnedFuture<T, E>>>>>;

A mutable container around SharedFuture<T, E> that can be filled in when the stored future is first created.

Aliased Type

struct MemoizedFutureContainer<T, E> { /* private fields */ }

Implementations

impl<T> Arc<T>

where T: ?Sized,

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

Constructs an Arc<T> from a raw pointer.

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

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

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

The 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]);
}

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));
}

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));
}

impl<T> Arc<T>

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

Constructs a new Arc<T>.

Examples

use std::sync::Arc;

let five = Arc::new(5);

pub fn new_cyclic<F>(data_fn: F) -> Arc<T>

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

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

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

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

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

Panics

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

Example

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

struct Gadget {
    me: Weak<Gadget>,
}

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

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

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

Constructs a new Arc with uninitialized contents.

Examples

#![feature(get_mut_unchecked)]

use std::sync::Arc;

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

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

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

assert_eq!(*five, 5)

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

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

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

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

Examples

#![feature(new_zeroed_alloc)]

use std::sync::Arc;

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

assert_eq!(*zero, 0)

pub fn pin(data: T) -> Pin<Arc<T>>

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

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

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

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

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

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

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

Examples

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

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

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

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

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

Examples

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

use std::sync::Arc;

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

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

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

assert_eq!(*five, 5);

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

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

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

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

Examples

#![feature( allocator_api)]

use std::sync::Arc;

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

assert_eq!(*zero, 0);

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

where A: Allocator, T: ?Sized,

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

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

Returns a reference to the underlying allocator.

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

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

Consumes the Arc, returning the wrapped pointer.

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

Examples

use std::sync::Arc;

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

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

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

Consumes the Arc, returning the wrapped pointer and allocator.

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

Examples

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

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

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

Provides a raw pointer to the data.

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

Examples

use std::sync::Arc;

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

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

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

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

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

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

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

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

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

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

Examples

#![feature(allocator_api)]

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

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

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

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

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

Convert a slice back into its original array:

#![feature(allocator_api)]

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

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

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

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

where A: Clone,

Creates a new Weak pointer to this allocation.

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

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

Gets the number of Weak pointers to this allocation.

Safety

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

Examples

use std::sync::Arc;

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

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

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

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

Safety

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

Examples

use std::sync::Arc;

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

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

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

where A: Clone,

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

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

Safety

The pointer must have been obtained through Arc::into_raw and 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 = Arc::into_raw(five);
    Arc::increment_strong_count_in(ptr, System);

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

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

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

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

Safety

The pointer must have been obtained through Arc::into_raw 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 = Arc::into_raw(five);
    Arc::increment_strong_count_in(ptr, System);

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

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

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

Examples

use std::sync::Arc;

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

assert!(Arc::ptr_eq(&five, &same_five));
assert!(!Arc::ptr_eq(&five, &other_five));

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

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

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

Makes a mutable reference into the given Arc.

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

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

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

Examples

use std::sync::Arc;

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

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

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

Weak pointers will be dissociated:

use std::sync::Arc;

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

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

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

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

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

where T: Clone, A: Allocator,

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

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

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

Examples

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

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

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

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

where A: Allocator, T: ?Sized,

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

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

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

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

Examples

use std::sync::Arc;

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

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

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

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

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

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

Safety

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

Examples

#![feature(get_mut_unchecked)]

use std::sync::Arc;

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

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

#![feature(get_mut_unchecked)]

use std::sync::Arc;

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

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

#![feature(get_mut_unchecked)]

use std::sync::Arc;

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

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

where A: Allocator,

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

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

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

Examples

#![feature(allocator_api)]

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

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

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

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

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

Examples

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

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

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

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

    five.assume_init()
};

assert_eq!(*five, 5)

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

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

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

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

Examples

#![feature(allocator_api)]

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

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

assert_eq!(*zero, 0)

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

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

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

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

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

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

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

Panics

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

Example

See new_cyclic

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

where A: 'static,

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

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

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

where A: 'static,

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

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

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

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

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

Examples

#![feature(allocator_api)]

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

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

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

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

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

Examples

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

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

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

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

    five.assume_init()
};

assert_eq!(*five, 5);

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

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

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

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

Examples

#![feature(allocator_api)]

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

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

assert_eq!(*zero, 0);

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

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

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

This will succeed even if there are outstanding weak references.

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

Examples

use std::sync::Arc;

let x = Arc::new(3);
assert_eq!(Arc::try_unwrap(x), Ok(3));

let x = Arc::new(4);
let _y = Arc::clone(&x);
assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);

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

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

Otherwise, None is returned and the Arc is dropped.

This will succeed even if there are outstanding weak references.

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

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

Examples

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

use std::sync::Arc;

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

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

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

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

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

use std::sync::Arc;

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

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

// Implementation of `new` and `push` omitted
impl<T> LinkedList<T> {
    /* ... */
}

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

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

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

Trait Implementations

impl<T> AsFd for Arc<T>

where T: AsFd + ?Sized,

This impl allows implementing traits that require AsFd on Arc.

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

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

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

Borrows the file descriptor.

impl<T> AsRawFd for Arc<T>

where T: AsRawFd,

This impl allows implementing traits that require AsRawFd on Arc.

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

fn as_raw_fd(&self) -> i32

Extracts the raw file descriptor.

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

where A: Allocator, T: ?Sized,

fn as_ref(&self) -> &T

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

impl<T> AsRefDirectory for Arc<T>

where T: Directory,

fn as_ref_directory(&self) -> &dyn Directory

Get a Directory reference.

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

where A: Allocator, T: ?Sized,

fn borrow(&self) -> &T

Immutably borrows from an owned value.

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

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

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

Makes a clone of the Arc pointer.

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

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

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

Performs copy-assignment from source.

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

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

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

Formats the value using the given formatter.

impl<T> Default for Arc<T>

where T: Default,

fn default() -> Arc<T>

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

Examples

use std::sync::Arc;

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

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

where A: Allocator, T: ?Sized,

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

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

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

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

Formats the value using the given formatter.

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

where A: Allocator, T: ?Sized,

fn drop(&mut self)

Drops the Arc.

This will decrement the strong reference count. If the strong reference count reaches zero then the only other references (if any) are Weak, so we drop the inner value.

Examples

use std::sync::Arc;

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("dropped!");
    }
}

let foo  = Arc::new(Foo);
let foo2 = Arc::clone(&foo);

drop(foo);    // Doesn't print anything
drop(foo2);   // Prints "dropped!"

impl<T> Error for Arc<T>

where T: Error + ?Sized,

fn description(&self) -> &str

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

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

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

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

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

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

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

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

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

where A: Allocator, T: ?Sized,

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

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

Example

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

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

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

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

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

Example

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

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

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

Converts a T into an Arc<T>

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

Example

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

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

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

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

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

where H: Hasher,

Feeds this value into the given Hasher.

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

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<Sp> LocalSpawn for Arc<Sp>

where Sp: LocalSpawn + ?Sized,

fn spawn_local_obj( &self, future: LocalFutureObj<'static, ()>, ) -> Result<(), SpawnError>

Spawns a future that will be run to completion.

fn status_local(&self) -> Result<(), SpawnError>

Determines whether the executor is able to spawn new tasks.

impl<T> Log for Arc<T>

where T: Log + ?Sized,

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

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

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

Logs the Record.

fn flush(&self)

Flushes any buffered records.

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

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

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

Comparison for two Arcs.

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

Examples

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

let five = Arc::new(5);

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

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

where Self: Sized,

Compares and returns the maximum of two values.

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

where Self: Sized,

Compares and returns the minimum of two values.

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

where Self: Sized,

Restrict a value to a certain interval.

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

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

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

Equality for two Arcs.

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

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

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

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

Inequality for two Arcs.

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

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

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

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

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

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

Partial comparison for two Arcs.

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

Examples

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

let five = Arc::new(5);

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

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

Less-than comparison for two Arcs.

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

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

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

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

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

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

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

Greater-than comparison for two Arcs.

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

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

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

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

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

Examples

use std::sync::Arc;

let five = Arc::new(5);

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

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

where A: Allocator, T: ?Sized,

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

Formats the value using the given formatter.

impl<S> Source for Arc<S>

where S: Source + ?Sized,

fn visit<'kvs>( &'kvs self, visitor: &mut dyn VisitSource<'kvs>, ) -> Result<(), Error>

Visit key-values.

fn get(&self, key: Key<'_>) -> Option<Value<'_>>

Get the value for a given key.

fn count(&self) -> usize

Count the number of key-values that can be visited.

impl<Sp> Spawn for Arc<Sp>

where Sp: Spawn + ?Sized,

fn spawn_obj(&self, future: FutureObj<'static, ()>) -> Result<(), SpawnError>

Spawns a future that will be run to completion.

fn status(&self) -> Result<(), SpawnError>

Determines whether the executor is able to spawn new tasks.

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

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

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

where A: Allocator, T: ?Sized,

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

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

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

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

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

where A: Allocator, T: ?Sized,

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

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

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

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

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

where A: Allocator, T: ?Sized,

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

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

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

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