1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][Arc] documentation for more details.
9 use core::cmp::Ordering;
10 use core::convert::{From, TryFrom};
12 use core::hash::{Hash, Hasher};
14 use core::intrinsics::abort;
15 #[cfg(not(no_global_oom_handling))]
17 use core::marker::{PhantomData, Unpin, Unsize};
18 #[cfg(not(no_global_oom_handling))]
19 use core::mem::size_of_val;
20 use core::mem::{self, align_of_val_raw};
21 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
23 use core::ptr::{self, NonNull};
24 #[cfg(not(no_global_oom_handling))]
25 use core::slice::from_raw_parts_mut;
26 use core::sync::atomic;
27 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
29 #[cfg(not(no_global_oom_handling))]
30 use crate::alloc::handle_alloc_error;
31 #[cfg(not(no_global_oom_handling))]
32 use crate::alloc::{box_free, WriteCloneIntoRaw};
33 use crate::alloc::{AllocError, Allocator, Global, Layout};
34 use crate::borrow::{Cow, ToOwned};
35 use crate::boxed::Box;
36 use crate::rc::is_dangling;
37 #[cfg(not(no_global_oom_handling))]
38 use crate::string::String;
39 #[cfg(not(no_global_oom_handling))]
45 /// A soft limit on the amount of references that may be made to an `Arc`.
47 /// Going above this limit will abort your program (although not
48 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
49 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
51 #[cfg(not(sanitize = "thread"))]
52 macro_rules! acquire {
54 atomic::fence(Acquire)
58 // ThreadSanitizer does not support memory fences. To avoid false positive
59 // reports in Arc / Weak implementation use atomic loads for synchronization
61 #[cfg(sanitize = "thread")]
62 macro_rules! acquire {
68 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
69 /// Reference Counted'.
71 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
72 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
73 /// a new `Arc` instance, which points to the same allocation on the heap as the
74 /// source `Arc`, while increasing a reference count. When the last `Arc`
75 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
76 /// referred to as "inner value") is also dropped.
78 /// Shared references in Rust disallow mutation by default, and `Arc` is no
79 /// exception: you cannot generally obtain a mutable reference to something
80 /// inside an `Arc`. If you need to mutate through an `Arc`, use
81 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
86 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
87 /// counting. This means that it is thread-safe. The disadvantage is that
88 /// atomic operations are more expensive than ordinary memory accesses. If you
89 /// are not sharing reference-counted allocations between threads, consider using
90 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
91 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
92 /// However, a library might choose `Arc<T>` in order to give library consumers
95 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
96 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
97 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
98 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
99 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
100 /// data, but it doesn't add thread safety to its data. Consider
101 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
102 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
103 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
104 /// non-atomic operations.
106 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
107 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
109 /// ## Breaking cycles with `Weak`
111 /// The [`downgrade`][downgrade] method can be used to create a non-owning
112 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
113 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
114 /// already been dropped. In other words, `Weak` pointers do not keep the value
115 /// inside the allocation alive; however, they *do* keep the allocation
116 /// (the backing store for the value) alive.
118 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
119 /// [`Weak`] is used to break cycles. For example, a tree could have
120 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
121 /// pointers from children back to their parents.
123 /// # Cloning references
125 /// Creating a new reference from an existing reference-counted pointer is done using the
126 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
129 /// use std::sync::Arc;
130 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
131 /// // The two syntaxes below are equivalent.
132 /// let a = foo.clone();
133 /// let b = Arc::clone(&foo);
134 /// // a, b, and foo are all Arcs that point to the same memory location
137 /// ## `Deref` behavior
139 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
140 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
141 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
142 /// functions, called using [fully qualified syntax]:
145 /// use std::sync::Arc;
147 /// let my_arc = Arc::new(());
148 /// Arc::downgrade(&my_arc);
151 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
152 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
153 /// while others prefer using method-call syntax.
156 /// use std::sync::Arc;
158 /// let arc = Arc::new(());
159 /// // Method-call syntax
160 /// let arc2 = arc.clone();
161 /// // Fully qualified syntax
162 /// let arc3 = Arc::clone(&arc);
165 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
166 /// already been dropped.
168 /// [`Rc<T>`]: crate::rc::Rc
169 /// [clone]: Clone::clone
170 /// [mutex]: ../../std/sync/struct.Mutex.html
171 /// [rwlock]: ../../std/sync/struct.RwLock.html
172 /// [atomic]: core::sync::atomic
173 /// [`Send`]: core::marker::Send
174 /// [`Sync`]: core::marker::Sync
175 /// [deref]: core::ops::Deref
176 /// [downgrade]: Arc::downgrade
177 /// [upgrade]: Weak::upgrade
178 /// [`RefCell<T>`]: core::cell::RefCell
179 /// [`std::sync`]: ../../std/sync/index.html
180 /// [`Arc::clone(&from)`]: Arc::clone
181 /// [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
185 /// Sharing some immutable data between threads:
187 // Note that we **do not** run these tests here. The windows builders get super
188 // unhappy if a thread outlives the main thread and then exits at the same time
189 // (something deadlocks) so we just avoid this entirely by not running these
192 /// use std::sync::Arc;
195 /// let five = Arc::new(5);
198 /// let five = Arc::clone(&five);
200 /// thread::spawn(move || {
201 /// println!("{:?}", five);
206 /// Sharing a mutable [`AtomicUsize`]:
208 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize
211 /// use std::sync::Arc;
212 /// use std::sync::atomic::{AtomicUsize, Ordering};
215 /// let val = Arc::new(AtomicUsize::new(5));
218 /// let val = Arc::clone(&val);
220 /// thread::spawn(move || {
221 /// let v = val.fetch_add(1, Ordering::SeqCst);
222 /// println!("{:?}", v);
227 /// See the [`rc` documentation][rc_examples] for more examples of reference
228 /// counting in general.
230 /// [rc_examples]: crate::rc#examples
231 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
232 #[stable(feature = "rust1", since = "1.0.0")]
233 pub struct Arc<T: ?Sized> {
234 ptr: NonNull<ArcInner<T>>,
235 phantom: PhantomData<ArcInner<T>>,
238 #[stable(feature = "rust1", since = "1.0.0")]
239 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
240 #[stable(feature = "rust1", since = "1.0.0")]
241 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
243 #[unstable(feature = "coerce_unsized", issue = "27732")]
244 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
246 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
247 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
249 impl<T: ?Sized> Arc<T> {
250 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
251 Self { ptr, phantom: PhantomData }
254 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
255 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
259 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
260 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
261 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
263 /// Since a `Weak` reference does not count towards ownership, it will not
264 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
265 /// guarantees about the value still being present. Thus it may return [`None`]
266 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
267 /// itself (the backing store) from being deallocated.
269 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
270 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
271 /// prevent circular references between [`Arc`] pointers, since mutual owning references
272 /// would never allow either [`Arc`] to be dropped. For example, a tree could
273 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
274 /// pointers from children back to their parents.
276 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
278 /// [`upgrade`]: Weak::upgrade
279 #[stable(feature = "arc_weak", since = "1.4.0")]
280 pub struct Weak<T: ?Sized> {
281 // This is a `NonNull` to allow optimizing the size of this type in enums,
282 // but it is not necessarily a valid pointer.
283 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
284 // to allocate space on the heap. That's not a value a real pointer
285 // will ever have because RcBox has alignment at least 2.
286 // This is only possible when `T: Sized`; unsized `T` never dangle.
287 ptr: NonNull<ArcInner<T>>,
290 #[stable(feature = "arc_weak", since = "1.4.0")]
291 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
292 #[stable(feature = "arc_weak", since = "1.4.0")]
293 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
295 #[unstable(feature = "coerce_unsized", issue = "27732")]
296 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
297 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
298 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
300 #[stable(feature = "arc_weak", since = "1.4.0")]
301 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
302 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
307 // This is repr(C) to future-proof against possible field-reordering, which
308 // would interfere with otherwise safe [into|from]_raw() of transmutable
311 struct ArcInner<T: ?Sized> {
312 strong: atomic::AtomicUsize,
314 // the value usize::MAX acts as a sentinel for temporarily "locking" the
315 // ability to upgrade weak pointers or downgrade strong ones; this is used
316 // to avoid races in `make_mut` and `get_mut`.
317 weak: atomic::AtomicUsize,
322 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
323 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
326 /// Constructs a new `Arc<T>`.
331 /// use std::sync::Arc;
333 /// let five = Arc::new(5);
335 #[cfg(not(no_global_oom_handling))]
337 #[stable(feature = "rust1", since = "1.0.0")]
338 pub fn new(data: T) -> Arc<T> {
339 // Start the weak pointer count as 1 which is the weak pointer that's
340 // held by all the strong pointers (kinda), see std/rc.rs for more info
341 let x: Box<_> = box ArcInner {
342 strong: atomic::AtomicUsize::new(1),
343 weak: atomic::AtomicUsize::new(1),
346 Self::from_inner(Box::leak(x).into())
349 /// Constructs a new `Arc<T>` using a weak reference to itself. Attempting
350 /// to upgrade the weak reference before this function returns will result
351 /// in a `None` value. However, the weak reference may be cloned freely and
352 /// stored for use at a later time.
356 /// #![feature(arc_new_cyclic)]
357 /// #![allow(dead_code)]
359 /// use std::sync::{Arc, Weak};
365 /// let foo = Arc::new_cyclic(|me| Foo {
369 #[cfg(not(no_global_oom_handling))]
371 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
372 pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Arc<T> {
373 // Construct the inner in the "uninitialized" state with a single
375 let uninit_ptr: NonNull<_> = Box::leak(box ArcInner {
376 strong: atomic::AtomicUsize::new(0),
377 weak: atomic::AtomicUsize::new(1),
378 data: mem::MaybeUninit::<T>::uninit(),
381 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
383 let weak = Weak { ptr: init_ptr };
385 // It's important we don't give up ownership of the weak pointer, or
386 // else the memory might be freed by the time `data_fn` returns. If
387 // we really wanted to pass ownership, we could create an additional
388 // weak pointer for ourselves, but this would result in additional
389 // updates to the weak reference count which might not be necessary
391 let data = data_fn(&weak);
393 // Now we can properly initialize the inner value and turn our weak
394 // reference into a strong reference.
396 let inner = init_ptr.as_ptr();
397 ptr::write(ptr::addr_of_mut!((*inner).data), data);
399 // The above write to the data field must be visible to any threads which
400 // observe a non-zero strong count. Therefore we need at least "Release" ordering
401 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
403 // "Acquire" ordering is not required. When considering the possible behaviours
404 // of `data_fn` we only need to look at what it could do with a reference to a
405 // non-upgradeable `Weak`:
406 // - It can *clone* the `Weak`, increasing the weak reference count.
407 // - It can drop those clones, decreasing the weak reference count (but never to zero).
409 // These side effects do not impact us in any way, and no other side effects are
410 // possible with safe code alone.
411 let prev_value = (*inner).strong.fetch_add(1, Release);
412 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
415 let strong = Arc::from_inner(init_ptr);
417 // Strong references should collectively own a shared weak reference,
418 // so don't run the destructor for our old weak reference.
423 /// Constructs a new `Arc` with uninitialized contents.
428 /// #![feature(new_uninit)]
429 /// #![feature(get_mut_unchecked)]
431 /// use std::sync::Arc;
433 /// let mut five = Arc::<u32>::new_uninit();
435 /// let five = unsafe {
436 /// // Deferred initialization:
437 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
439 /// five.assume_init()
442 /// assert_eq!(*five, 5)
444 #[cfg(not(no_global_oom_handling))]
445 #[unstable(feature = "new_uninit", issue = "63291")]
446 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
448 Arc::from_ptr(Arc::allocate_for_layout(
450 |layout| Global.allocate(layout),
451 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
456 /// Constructs a new `Arc` with uninitialized contents, with the memory
457 /// being filled with `0` bytes.
459 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
465 /// #![feature(new_uninit)]
467 /// use std::sync::Arc;
469 /// let zero = Arc::<u32>::new_zeroed();
470 /// let zero = unsafe { zero.assume_init() };
472 /// assert_eq!(*zero, 0)
475 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
476 #[cfg(not(no_global_oom_handling))]
477 #[unstable(feature = "new_uninit", issue = "63291")]
478 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
480 Arc::from_ptr(Arc::allocate_for_layout(
482 |layout| Global.allocate_zeroed(layout),
483 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
488 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
489 /// `data` will be pinned in memory and unable to be moved.
490 #[cfg(not(no_global_oom_handling))]
491 #[stable(feature = "pin", since = "1.33.0")]
492 pub fn pin(data: T) -> Pin<Arc<T>> {
493 unsafe { Pin::new_unchecked(Arc::new(data)) }
496 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
497 #[unstable(feature = "allocator_api", issue = "32838")]
499 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
500 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
503 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
508 /// #![feature(allocator_api)]
509 /// use std::sync::Arc;
511 /// let five = Arc::try_new(5)?;
512 /// # Ok::<(), std::alloc::AllocError>(())
514 #[unstable(feature = "allocator_api", issue = "32838")]
516 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
517 // Start the weak pointer count as 1 which is the weak pointer that's
518 // held by all the strong pointers (kinda), see std/rc.rs for more info
519 let x: Box<_> = Box::try_new(ArcInner {
520 strong: atomic::AtomicUsize::new(1),
521 weak: atomic::AtomicUsize::new(1),
524 Ok(Self::from_inner(Box::leak(x).into()))
527 /// Constructs a new `Arc` with uninitialized contents, returning an error
528 /// if allocation fails.
533 /// #![feature(new_uninit, allocator_api)]
534 /// #![feature(get_mut_unchecked)]
536 /// use std::sync::Arc;
538 /// let mut five = Arc::<u32>::try_new_uninit()?;
540 /// let five = unsafe {
541 /// // Deferred initialization:
542 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
544 /// five.assume_init()
547 /// assert_eq!(*five, 5);
548 /// # Ok::<(), std::alloc::AllocError>(())
550 #[unstable(feature = "allocator_api", issue = "32838")]
551 // #[unstable(feature = "new_uninit", issue = "63291")]
552 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
554 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
556 |layout| Global.allocate(layout),
557 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
562 /// Constructs a new `Arc` with uninitialized contents, with the memory
563 /// being filled with `0` bytes, returning an error if allocation fails.
565 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
571 /// #![feature(new_uninit, allocator_api)]
573 /// use std::sync::Arc;
575 /// let zero = Arc::<u32>::try_new_zeroed()?;
576 /// let zero = unsafe { zero.assume_init() };
578 /// assert_eq!(*zero, 0);
579 /// # Ok::<(), std::alloc::AllocError>(())
582 /// [zeroed]: mem::MaybeUninit::zeroed
583 #[unstable(feature = "allocator_api", issue = "32838")]
584 // #[unstable(feature = "new_uninit", issue = "63291")]
585 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
587 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
589 |layout| Global.allocate_zeroed(layout),
590 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
594 /// Returns the inner value, if the `Arc` has exactly one strong reference.
596 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
599 /// This will succeed even if there are outstanding weak references.
604 /// use std::sync::Arc;
606 /// let x = Arc::new(3);
607 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
609 /// let x = Arc::new(4);
610 /// let _y = Arc::clone(&x);
611 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
614 #[stable(feature = "arc_unique", since = "1.4.0")]
615 pub fn try_unwrap(this: Self) -> Result<T, Self> {
616 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
620 acquire!(this.inner().strong);
623 let elem = ptr::read(&this.ptr.as_ref().data);
625 // Make a weak pointer to clean up the implicit strong-weak reference
626 let _weak = Weak { ptr: this.ptr };
635 /// Constructs a new atomically reference-counted slice with uninitialized contents.
640 /// #![feature(new_uninit)]
641 /// #![feature(get_mut_unchecked)]
643 /// use std::sync::Arc;
645 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
647 /// let values = unsafe {
648 /// // Deferred initialization:
649 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
650 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
651 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
653 /// values.assume_init()
656 /// assert_eq!(*values, [1, 2, 3])
658 #[cfg(not(no_global_oom_handling))]
659 #[unstable(feature = "new_uninit", issue = "63291")]
660 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
661 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
664 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
665 /// filled with `0` bytes.
667 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
668 /// incorrect usage of this method.
673 /// #![feature(new_uninit)]
675 /// use std::sync::Arc;
677 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
678 /// let values = unsafe { values.assume_init() };
680 /// assert_eq!(*values, [0, 0, 0])
683 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
684 #[cfg(not(no_global_oom_handling))]
685 #[unstable(feature = "new_uninit", issue = "63291")]
686 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
688 Arc::from_ptr(Arc::allocate_for_layout(
689 Layout::array::<T>(len).unwrap(),
690 |layout| Global.allocate_zeroed(layout),
692 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
693 as *mut ArcInner<[mem::MaybeUninit<T>]>
700 impl<T> Arc<mem::MaybeUninit<T>> {
701 /// Converts to `Arc<T>`.
705 /// As with [`MaybeUninit::assume_init`],
706 /// it is up to the caller to guarantee that the inner value
707 /// really is in an initialized state.
708 /// Calling this when the content is not yet fully initialized
709 /// causes immediate undefined behavior.
711 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
716 /// #![feature(new_uninit)]
717 /// #![feature(get_mut_unchecked)]
719 /// use std::sync::Arc;
721 /// let mut five = Arc::<u32>::new_uninit();
723 /// let five = unsafe {
724 /// // Deferred initialization:
725 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
727 /// five.assume_init()
730 /// assert_eq!(*five, 5)
732 #[unstable(feature = "new_uninit", issue = "63291")]
734 pub unsafe fn assume_init(self) -> Arc<T> {
735 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
739 impl<T> Arc<[mem::MaybeUninit<T>]> {
740 /// Converts to `Arc<[T]>`.
744 /// As with [`MaybeUninit::assume_init`],
745 /// it is up to the caller to guarantee that the inner value
746 /// really is in an initialized state.
747 /// Calling this when the content is not yet fully initialized
748 /// causes immediate undefined behavior.
750 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
755 /// #![feature(new_uninit)]
756 /// #![feature(get_mut_unchecked)]
758 /// use std::sync::Arc;
760 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
762 /// let values = unsafe {
763 /// // Deferred initialization:
764 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
765 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
766 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
768 /// values.assume_init()
771 /// assert_eq!(*values, [1, 2, 3])
773 #[unstable(feature = "new_uninit", issue = "63291")]
775 pub unsafe fn assume_init(self) -> Arc<[T]> {
776 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
780 impl<T: ?Sized> Arc<T> {
781 /// Consumes the `Arc`, returning the wrapped pointer.
783 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
784 /// [`Arc::from_raw`].
789 /// use std::sync::Arc;
791 /// let x = Arc::new("hello".to_owned());
792 /// let x_ptr = Arc::into_raw(x);
793 /// assert_eq!(unsafe { &*x_ptr }, "hello");
795 #[stable(feature = "rc_raw", since = "1.17.0")]
796 pub fn into_raw(this: Self) -> *const T {
797 let ptr = Self::as_ptr(&this);
802 /// Provides a raw pointer to the data.
804 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
805 /// as long as there are strong counts in the `Arc`.
810 /// use std::sync::Arc;
812 /// let x = Arc::new("hello".to_owned());
813 /// let y = Arc::clone(&x);
814 /// let x_ptr = Arc::as_ptr(&x);
815 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
816 /// assert_eq!(unsafe { &*x_ptr }, "hello");
818 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
819 pub fn as_ptr(this: &Self) -> *const T {
820 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
822 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
823 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
824 // write through the pointer after the Rc is recovered through `from_raw`.
825 unsafe { ptr::addr_of_mut!((*ptr).data) }
828 /// Constructs an `Arc<T>` from a raw pointer.
830 /// The raw pointer must have been previously returned by a call to
831 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
832 /// alignment as `T`. This is trivially true if `U` is `T`.
833 /// Note that if `U` is not `T` but has the same size and alignment, this is
834 /// basically like transmuting references of different types. See
835 /// [`mem::transmute`][transmute] for more information on what
836 /// restrictions apply in this case.
838 /// The user of `from_raw` has to make sure a specific value of `T` is only
841 /// This function is unsafe because improper use may lead to memory unsafety,
842 /// even if the returned `Arc<T>` is never accessed.
844 /// [into_raw]: Arc::into_raw
845 /// [transmute]: core::mem::transmute
850 /// use std::sync::Arc;
852 /// let x = Arc::new("hello".to_owned());
853 /// let x_ptr = Arc::into_raw(x);
856 /// // Convert back to an `Arc` to prevent leak.
857 /// let x = Arc::from_raw(x_ptr);
858 /// assert_eq!(&*x, "hello");
860 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
863 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
865 #[stable(feature = "rc_raw", since = "1.17.0")]
866 pub unsafe fn from_raw(ptr: *const T) -> Self {
868 let offset = data_offset(ptr);
870 // Reverse the offset to find the original ArcInner.
871 let arc_ptr = (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset));
873 Self::from_ptr(arc_ptr)
877 /// Creates a new [`Weak`] pointer to this allocation.
882 /// use std::sync::Arc;
884 /// let five = Arc::new(5);
886 /// let weak_five = Arc::downgrade(&five);
888 #[stable(feature = "arc_weak", since = "1.4.0")]
889 pub fn downgrade(this: &Self) -> Weak<T> {
890 // This Relaxed is OK because we're checking the value in the CAS
892 let mut cur = this.inner().weak.load(Relaxed);
895 // check if the weak counter is currently "locked"; if so, spin.
896 if cur == usize::MAX {
898 cur = this.inner().weak.load(Relaxed);
902 // NOTE: this code currently ignores the possibility of overflow
903 // into usize::MAX; in general both Rc and Arc need to be adjusted
904 // to deal with overflow.
906 // Unlike with Clone(), we need this to be an Acquire read to
907 // synchronize with the write coming from `is_unique`, so that the
908 // events prior to that write happen before this read.
909 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
911 // Make sure we do not create a dangling Weak
912 debug_assert!(!is_dangling(this.ptr.as_ptr()));
913 return Weak { ptr: this.ptr };
915 Err(old) => cur = old,
920 /// Gets the number of [`Weak`] pointers to this allocation.
924 /// This method by itself is safe, but using it correctly requires extra care.
925 /// Another thread can change the weak count at any time,
926 /// including potentially between calling this method and acting on the result.
931 /// use std::sync::Arc;
933 /// let five = Arc::new(5);
934 /// let _weak_five = Arc::downgrade(&five);
936 /// // This assertion is deterministic because we haven't shared
937 /// // the `Arc` or `Weak` between threads.
938 /// assert_eq!(1, Arc::weak_count(&five));
941 #[stable(feature = "arc_counts", since = "1.15.0")]
942 pub fn weak_count(this: &Self) -> usize {
943 let cnt = this.inner().weak.load(SeqCst);
944 // If the weak count is currently locked, the value of the
945 // count was 0 just before taking the lock.
946 if cnt == usize::MAX { 0 } else { cnt - 1 }
949 /// Gets the number of strong (`Arc`) pointers to this allocation.
953 /// This method by itself is safe, but using it correctly requires extra care.
954 /// Another thread can change the strong count at any time,
955 /// including potentially between calling this method and acting on the result.
960 /// use std::sync::Arc;
962 /// let five = Arc::new(5);
963 /// let _also_five = Arc::clone(&five);
965 /// // This assertion is deterministic because we haven't shared
966 /// // the `Arc` between threads.
967 /// assert_eq!(2, Arc::strong_count(&five));
970 #[stable(feature = "arc_counts", since = "1.15.0")]
971 pub fn strong_count(this: &Self) -> usize {
972 this.inner().strong.load(SeqCst)
975 /// Increments the strong reference count on the `Arc<T>` associated with the
976 /// provided pointer by one.
980 /// The pointer must have been obtained through `Arc::into_raw`, and the
981 /// associated `Arc` instance must be valid (i.e. the strong count must be at
982 /// least 1) for the duration of this method.
987 /// use std::sync::Arc;
989 /// let five = Arc::new(5);
992 /// let ptr = Arc::into_raw(five);
993 /// Arc::increment_strong_count(ptr);
995 /// // This assertion is deterministic because we haven't shared
996 /// // the `Arc` between threads.
997 /// let five = Arc::from_raw(ptr);
998 /// assert_eq!(2, Arc::strong_count(&five));
1002 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1003 pub unsafe fn increment_strong_count(ptr: *const T) {
1004 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1005 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
1006 // Now increase refcount, but don't drop new refcount either
1007 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1010 /// Decrements the strong reference count on the `Arc<T>` associated with the
1011 /// provided pointer by one.
1015 /// The pointer must have been obtained through `Arc::into_raw`, and the
1016 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1017 /// least 1) when invoking this method. This method can be used to release the final
1018 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1024 /// use std::sync::Arc;
1026 /// let five = Arc::new(5);
1029 /// let ptr = Arc::into_raw(five);
1030 /// Arc::increment_strong_count(ptr);
1032 /// // Those assertions are deterministic because we haven't shared
1033 /// // the `Arc` between threads.
1034 /// let five = Arc::from_raw(ptr);
1035 /// assert_eq!(2, Arc::strong_count(&five));
1036 /// Arc::decrement_strong_count(ptr);
1037 /// assert_eq!(1, Arc::strong_count(&five));
1041 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1042 pub unsafe fn decrement_strong_count(ptr: *const T) {
1043 unsafe { mem::drop(Arc::from_raw(ptr)) };
1047 fn inner(&self) -> &ArcInner<T> {
1048 // This unsafety is ok because while this arc is alive we're guaranteed
1049 // that the inner pointer is valid. Furthermore, we know that the
1050 // `ArcInner` structure itself is `Sync` because the inner data is
1051 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1053 unsafe { self.ptr.as_ref() }
1056 // Non-inlined part of `drop`.
1058 unsafe fn drop_slow(&mut self) {
1059 // Destroy the data at this time, even though we may not free the box
1060 // allocation itself (there may still be weak pointers lying around).
1061 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1063 // Drop the weak ref collectively held by all strong references
1064 drop(Weak { ptr: self.ptr });
1068 #[stable(feature = "ptr_eq", since = "1.17.0")]
1069 /// Returns `true` if the two `Arc`s point to the same allocation
1070 /// (in a vein similar to [`ptr::eq`]).
1075 /// use std::sync::Arc;
1077 /// let five = Arc::new(5);
1078 /// let same_five = Arc::clone(&five);
1079 /// let other_five = Arc::new(5);
1081 /// assert!(Arc::ptr_eq(&five, &same_five));
1082 /// assert!(!Arc::ptr_eq(&five, &other_five));
1085 /// [`ptr::eq`]: core::ptr::eq
1086 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1087 this.ptr.as_ptr() == other.ptr.as_ptr()
1091 impl<T: ?Sized> Arc<T> {
1092 /// Allocates an `ArcInner<T>` with sufficient space for
1093 /// a possibly-unsized inner value where the value has the layout provided.
1095 /// The function `mem_to_arcinner` is called with the data pointer
1096 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1097 #[cfg(not(no_global_oom_handling))]
1098 unsafe fn allocate_for_layout(
1099 value_layout: Layout,
1100 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1101 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1102 ) -> *mut ArcInner<T> {
1103 // Calculate layout using the given value layout.
1104 // Previously, layout was calculated on the expression
1105 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1106 // reference (see #54908).
1107 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1109 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1110 .unwrap_or_else(|_| handle_alloc_error(layout))
1114 /// Allocates an `ArcInner<T>` with sufficient space for
1115 /// a possibly-unsized inner value where the value has the layout provided,
1116 /// returning an error if allocation fails.
1118 /// The function `mem_to_arcinner` is called with the data pointer
1119 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1120 unsafe fn try_allocate_for_layout(
1121 value_layout: Layout,
1122 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1123 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1124 ) -> Result<*mut ArcInner<T>, AllocError> {
1125 // Calculate layout using the given value layout.
1126 // Previously, layout was calculated on the expression
1127 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1128 // reference (see #54908).
1129 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1131 let ptr = allocate(layout)?;
1133 // Initialize the ArcInner
1134 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1135 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1138 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1139 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1145 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1146 #[cfg(not(no_global_oom_handling))]
1147 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1148 // Allocate for the `ArcInner<T>` using the given value.
1150 Self::allocate_for_layout(
1151 Layout::for_value(&*ptr),
1152 |layout| Global.allocate(layout),
1153 |mem| (ptr as *mut ArcInner<T>).set_ptr_value(mem) as *mut ArcInner<T>,
1158 #[cfg(not(no_global_oom_handling))]
1159 fn from_box(v: Box<T>) -> Arc<T> {
1161 let (box_unique, alloc) = Box::into_unique(v);
1162 let bptr = box_unique.as_ptr();
1164 let value_size = size_of_val(&*bptr);
1165 let ptr = Self::allocate_for_ptr(bptr);
1167 // Copy value as bytes
1168 ptr::copy_nonoverlapping(
1169 bptr as *const T as *const u8,
1170 &mut (*ptr).data as *mut _ as *mut u8,
1174 // Free the allocation without dropping its contents
1175 box_free(box_unique, alloc);
1183 /// Allocates an `ArcInner<[T]>` with the given length.
1184 #[cfg(not(no_global_oom_handling))]
1185 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1187 Self::allocate_for_layout(
1188 Layout::array::<T>(len).unwrap(),
1189 |layout| Global.allocate(layout),
1190 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1195 /// Copy elements from slice into newly allocated Arc<\[T\]>
1197 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1198 #[cfg(not(no_global_oom_handling))]
1199 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1201 let ptr = Self::allocate_for_slice(v.len());
1203 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1209 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1211 /// Behavior is undefined should the size be wrong.
1212 #[cfg(not(no_global_oom_handling))]
1213 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1214 // Panic guard while cloning T elements.
1215 // In the event of a panic, elements that have been written
1216 // into the new ArcInner will be dropped, then the memory freed.
1224 impl<T> Drop for Guard<T> {
1225 fn drop(&mut self) {
1227 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1228 ptr::drop_in_place(slice);
1230 Global.deallocate(self.mem, self.layout);
1236 let ptr = Self::allocate_for_slice(len);
1238 let mem = ptr as *mut _ as *mut u8;
1239 let layout = Layout::for_value(&*ptr);
1241 // Pointer to first element
1242 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1244 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1246 for (i, item) in iter.enumerate() {
1247 ptr::write(elems.add(i), item);
1251 // All clear. Forget the guard so it doesn't free the new ArcInner.
1259 /// Specialization trait used for `From<&[T]>`.
1260 #[cfg(not(no_global_oom_handling))]
1261 trait ArcFromSlice<T> {
1262 fn from_slice(slice: &[T]) -> Self;
1265 #[cfg(not(no_global_oom_handling))]
1266 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1268 default fn from_slice(v: &[T]) -> Self {
1269 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1273 #[cfg(not(no_global_oom_handling))]
1274 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1276 fn from_slice(v: &[T]) -> Self {
1277 unsafe { Arc::copy_from_slice(v) }
1281 #[stable(feature = "rust1", since = "1.0.0")]
1282 impl<T: ?Sized> Clone for Arc<T> {
1283 /// Makes a clone of the `Arc` pointer.
1285 /// This creates another pointer to the same allocation, increasing the
1286 /// strong reference count.
1291 /// use std::sync::Arc;
1293 /// let five = Arc::new(5);
1295 /// let _ = Arc::clone(&five);
1298 fn clone(&self) -> Arc<T> {
1299 // Using a relaxed ordering is alright here, as knowledge of the
1300 // original reference prevents other threads from erroneously deleting
1303 // As explained in the [Boost documentation][1], Increasing the
1304 // reference counter can always be done with memory_order_relaxed: New
1305 // references to an object can only be formed from an existing
1306 // reference, and passing an existing reference from one thread to
1307 // another must already provide any required synchronization.
1309 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1310 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1312 // However we need to guard against massive refcounts in case someone
1313 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1314 // and users will use-after free. We racily saturate to `isize::MAX` on
1315 // the assumption that there aren't ~2 billion threads incrementing
1316 // the reference count at once. This branch will never be taken in
1317 // any realistic program.
1319 // We abort because such a program is incredibly degenerate, and we
1320 // don't care to support it.
1321 if old_size > MAX_REFCOUNT {
1325 Self::from_inner(self.ptr)
1329 #[stable(feature = "rust1", since = "1.0.0")]
1330 impl<T: ?Sized> Deref for Arc<T> {
1334 fn deref(&self) -> &T {
1339 #[unstable(feature = "receiver_trait", issue = "none")]
1340 impl<T: ?Sized> Receiver for Arc<T> {}
1342 impl<T: Clone> Arc<T> {
1343 /// Makes a mutable reference into the given `Arc`.
1345 /// If there are other `Arc` or [`Weak`] pointers to the same allocation,
1346 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
1347 /// to ensure unique ownership. This is also referred to as clone-on-write.
1349 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
1350 /// any remaining `Weak` pointers.
1352 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
1354 /// [clone]: Clone::clone
1355 /// [get_mut]: Arc::get_mut
1356 /// [`Rc::make_mut`]: super::rc::Rc::make_mut
1361 /// use std::sync::Arc;
1363 /// let mut data = Arc::new(5);
1365 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1366 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1367 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1368 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1369 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1371 /// // Now `data` and `other_data` point to different allocations.
1372 /// assert_eq!(*data, 8);
1373 /// assert_eq!(*other_data, 12);
1375 #[cfg(not(no_global_oom_handling))]
1377 #[stable(feature = "arc_unique", since = "1.4.0")]
1378 pub fn make_mut(this: &mut Self) -> &mut T {
1379 // Note that we hold both a strong reference and a weak reference.
1380 // Thus, releasing our strong reference only will not, by itself, cause
1381 // the memory to be deallocated.
1383 // Use Acquire to ensure that we see any writes to `weak` that happen
1384 // before release writes (i.e., decrements) to `strong`. Since we hold a
1385 // weak count, there's no chance the ArcInner itself could be
1387 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1388 // Another strong pointer exists, so we must clone.
1389 // Pre-allocate memory to allow writing the cloned value directly.
1390 let mut arc = Self::new_uninit();
1392 let data = Arc::get_mut_unchecked(&mut arc);
1393 (**this).write_clone_into_raw(data.as_mut_ptr());
1394 *this = arc.assume_init();
1396 } else if this.inner().weak.load(Relaxed) != 1 {
1397 // Relaxed suffices in the above because this is fundamentally an
1398 // optimization: we are always racing with weak pointers being
1399 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1401 // We removed the last strong ref, but there are additional weak
1402 // refs remaining. We'll move the contents to a new Arc, and
1403 // invalidate the other weak refs.
1405 // Note that it is not possible for the read of `weak` to yield
1406 // usize::MAX (i.e., locked), since the weak count can only be
1407 // locked by a thread with a strong reference.
1409 // Materialize our own implicit weak pointer, so that it can clean
1410 // up the ArcInner as needed.
1411 let _weak = Weak { ptr: this.ptr };
1413 // Can just steal the data, all that's left is Weaks
1414 let mut arc = Self::new_uninit();
1416 let data = Arc::get_mut_unchecked(&mut arc);
1417 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1418 ptr::write(this, arc.assume_init());
1421 // We were the sole reference of either kind; bump back up the
1422 // strong ref count.
1423 this.inner().strong.store(1, Release);
1426 // As with `get_mut()`, the unsafety is ok because our reference was
1427 // either unique to begin with, or became one upon cloning the contents.
1428 unsafe { Self::get_mut_unchecked(this) }
1432 impl<T: ?Sized> Arc<T> {
1433 /// Returns a mutable reference into the given `Arc`, if there are
1434 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1436 /// Returns [`None`] otherwise, because it is not safe to
1437 /// mutate a shared value.
1439 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1440 /// the inner value when there are other pointers.
1442 /// [make_mut]: Arc::make_mut
1443 /// [clone]: Clone::clone
1448 /// use std::sync::Arc;
1450 /// let mut x = Arc::new(3);
1451 /// *Arc::get_mut(&mut x).unwrap() = 4;
1452 /// assert_eq!(*x, 4);
1454 /// let _y = Arc::clone(&x);
1455 /// assert!(Arc::get_mut(&mut x).is_none());
1458 #[stable(feature = "arc_unique", since = "1.4.0")]
1459 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1460 if this.is_unique() {
1461 // This unsafety is ok because we're guaranteed that the pointer
1462 // returned is the *only* pointer that will ever be returned to T. Our
1463 // reference count is guaranteed to be 1 at this point, and we required
1464 // the Arc itself to be `mut`, so we're returning the only possible
1465 // reference to the inner data.
1466 unsafe { Some(Arc::get_mut_unchecked(this)) }
1472 /// Returns a mutable reference into the given `Arc`,
1473 /// without any check.
1475 /// See also [`get_mut`], which is safe and does appropriate checks.
1477 /// [`get_mut`]: Arc::get_mut
1481 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1482 /// for the duration of the returned borrow.
1483 /// This is trivially the case if no such pointers exist,
1484 /// for example immediately after `Arc::new`.
1489 /// #![feature(get_mut_unchecked)]
1491 /// use std::sync::Arc;
1493 /// let mut x = Arc::new(String::new());
1495 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1497 /// assert_eq!(*x, "foo");
1500 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1501 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1502 // We are careful to *not* create a reference covering the "count" fields, as
1503 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1504 unsafe { &mut (*this.ptr.as_ptr()).data }
1507 /// Determine whether this is the unique reference (including weak refs) to
1508 /// the underlying data.
1510 /// Note that this requires locking the weak ref count.
1511 fn is_unique(&mut self) -> bool {
1512 // lock the weak pointer count if we appear to be the sole weak pointer
1515 // The acquire label here ensures a happens-before relationship with any
1516 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1517 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1518 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1519 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1520 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1521 // counter in `drop` -- the only access that happens when any but the last reference
1522 // is being dropped.
1523 let unique = self.inner().strong.load(Acquire) == 1;
1525 // The release write here synchronizes with a read in `downgrade`,
1526 // effectively preventing the above read of `strong` from happening
1528 self.inner().weak.store(1, Release); // release the lock
1536 #[stable(feature = "rust1", since = "1.0.0")]
1537 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1538 /// Drops the `Arc`.
1540 /// This will decrement the strong reference count. If the strong reference
1541 /// count reaches zero then the only other references (if any) are
1542 /// [`Weak`], so we `drop` the inner value.
1547 /// use std::sync::Arc;
1551 /// impl Drop for Foo {
1552 /// fn drop(&mut self) {
1553 /// println!("dropped!");
1557 /// let foo = Arc::new(Foo);
1558 /// let foo2 = Arc::clone(&foo);
1560 /// drop(foo); // Doesn't print anything
1561 /// drop(foo2); // Prints "dropped!"
1564 fn drop(&mut self) {
1565 // Because `fetch_sub` is already atomic, we do not need to synchronize
1566 // with other threads unless we are going to delete the object. This
1567 // same logic applies to the below `fetch_sub` to the `weak` count.
1568 if self.inner().strong.fetch_sub(1, Release) != 1 {
1572 // This fence is needed to prevent reordering of use of the data and
1573 // deletion of the data. Because it is marked `Release`, the decreasing
1574 // of the reference count synchronizes with this `Acquire` fence. This
1575 // means that use of the data happens before decreasing the reference
1576 // count, which happens before this fence, which happens before the
1577 // deletion of the data.
1579 // As explained in the [Boost documentation][1],
1581 // > It is important to enforce any possible access to the object in one
1582 // > thread (through an existing reference) to *happen before* deleting
1583 // > the object in a different thread. This is achieved by a "release"
1584 // > operation after dropping a reference (any access to the object
1585 // > through this reference must obviously happened before), and an
1586 // > "acquire" operation before deleting the object.
1588 // In particular, while the contents of an Arc are usually immutable, it's
1589 // possible to have interior writes to something like a Mutex<T>. Since a
1590 // Mutex is not acquired when it is deleted, we can't rely on its
1591 // synchronization logic to make writes in thread A visible to a destructor
1592 // running in thread B.
1594 // Also note that the Acquire fence here could probably be replaced with an
1595 // Acquire load, which could improve performance in highly-contended
1596 // situations. See [2].
1598 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1599 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1600 acquire!(self.inner().strong);
1608 impl Arc<dyn Any + Send + Sync> {
1610 #[stable(feature = "rc_downcast", since = "1.29.0")]
1611 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1616 /// use std::any::Any;
1617 /// use std::sync::Arc;
1619 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1620 /// if let Ok(string) = value.downcast::<String>() {
1621 /// println!("String ({}): {}", string.len(), string);
1625 /// let my_string = "Hello World".to_string();
1626 /// print_if_string(Arc::new(my_string));
1627 /// print_if_string(Arc::new(0i8));
1629 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1631 T: Any + Send + Sync + 'static,
1633 if (*self).is::<T>() {
1634 let ptr = self.ptr.cast::<ArcInner<T>>();
1636 Ok(Arc::from_inner(ptr))
1644 /// Constructs a new `Weak<T>`, without allocating any memory.
1645 /// Calling [`upgrade`] on the return value always gives [`None`].
1647 /// [`upgrade`]: Weak::upgrade
1652 /// use std::sync::Weak;
1654 /// let empty: Weak<i64> = Weak::new();
1655 /// assert!(empty.upgrade().is_none());
1657 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1658 pub fn new() -> Weak<T> {
1659 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1663 /// Helper type to allow accessing the reference counts without
1664 /// making any assertions about the data field.
1665 struct WeakInner<'a> {
1666 weak: &'a atomic::AtomicUsize,
1667 strong: &'a atomic::AtomicUsize,
1670 impl<T: ?Sized> Weak<T> {
1671 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1673 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1674 /// unaligned or even [`null`] otherwise.
1679 /// use std::sync::Arc;
1682 /// let strong = Arc::new("hello".to_owned());
1683 /// let weak = Arc::downgrade(&strong);
1684 /// // Both point to the same object
1685 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1686 /// // The strong here keeps it alive, so we can still access the object.
1687 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1690 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1691 /// // undefined behaviour.
1692 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1695 /// [`null`]: core::ptr::null
1696 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1697 pub fn as_ptr(&self) -> *const T {
1698 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1700 if is_dangling(ptr) {
1701 // If the pointer is dangling, we return the sentinel directly. This cannot be
1702 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1705 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
1706 // The payload may be dropped at this point, and we have to maintain provenance,
1707 // so use raw pointer manipulation.
1708 unsafe { ptr::addr_of_mut!((*ptr).data) }
1712 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1714 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1715 /// one weak reference (the weak count is not modified by this operation). It can be turned
1716 /// back into the `Weak<T>` with [`from_raw`].
1718 /// The same restrictions of accessing the target of the pointer as with
1719 /// [`as_ptr`] apply.
1724 /// use std::sync::{Arc, Weak};
1726 /// let strong = Arc::new("hello".to_owned());
1727 /// let weak = Arc::downgrade(&strong);
1728 /// let raw = weak.into_raw();
1730 /// assert_eq!(1, Arc::weak_count(&strong));
1731 /// assert_eq!("hello", unsafe { &*raw });
1733 /// drop(unsafe { Weak::from_raw(raw) });
1734 /// assert_eq!(0, Arc::weak_count(&strong));
1737 /// [`from_raw`]: Weak::from_raw
1738 /// [`as_ptr`]: Weak::as_ptr
1739 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1740 pub fn into_raw(self) -> *const T {
1741 let result = self.as_ptr();
1746 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1748 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1749 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1751 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1752 /// as these don't own anything; the method still works on them).
1756 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1759 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1760 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1761 /// count is not modified by this operation) and therefore it must be paired with a previous
1762 /// call to [`into_raw`].
1766 /// use std::sync::{Arc, Weak};
1768 /// let strong = Arc::new("hello".to_owned());
1770 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1771 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1773 /// assert_eq!(2, Arc::weak_count(&strong));
1775 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1776 /// assert_eq!(1, Arc::weak_count(&strong));
1780 /// // Decrement the last weak count.
1781 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1784 /// [`new`]: Weak::new
1785 /// [`into_raw`]: Weak::into_raw
1786 /// [`upgrade`]: Weak::upgrade
1787 /// [`forget`]: std::mem::forget
1788 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1789 pub unsafe fn from_raw(ptr: *const T) -> Self {
1790 // See Weak::as_ptr for context on how the input pointer is derived.
1792 let ptr = if is_dangling(ptr as *mut T) {
1793 // This is a dangling Weak.
1794 ptr as *mut ArcInner<T>
1796 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1797 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1798 let offset = unsafe { data_offset(ptr) };
1799 // Thus, we reverse the offset to get the whole RcBox.
1800 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1801 unsafe { (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
1804 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1805 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1809 impl<T: ?Sized> Weak<T> {
1810 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1811 /// dropping of the inner value if successful.
1813 /// Returns [`None`] if the inner value has since been dropped.
1818 /// use std::sync::Arc;
1820 /// let five = Arc::new(5);
1822 /// let weak_five = Arc::downgrade(&five);
1824 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1825 /// assert!(strong_five.is_some());
1827 /// // Destroy all strong pointers.
1828 /// drop(strong_five);
1831 /// assert!(weak_five.upgrade().is_none());
1833 #[stable(feature = "arc_weak", since = "1.4.0")]
1834 pub fn upgrade(&self) -> Option<Arc<T>> {
1835 // We use a CAS loop to increment the strong count instead of a
1836 // fetch_add as this function should never take the reference count
1837 // from zero to one.
1838 let inner = self.inner()?;
1840 // Relaxed load because any write of 0 that we can observe
1841 // leaves the field in a permanently zero state (so a
1842 // "stale" read of 0 is fine), and any other value is
1843 // confirmed via the CAS below.
1844 let mut n = inner.strong.load(Relaxed);
1851 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1852 if n > MAX_REFCOUNT {
1856 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
1857 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
1858 // value can be initialized after `Weak` references have already been created. In that case, we
1859 // expect to observe the fully initialized value.
1860 match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
1861 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1862 Err(old) => n = old,
1867 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1869 /// If `self` was created using [`Weak::new`], this will return 0.
1870 #[stable(feature = "weak_counts", since = "1.41.0")]
1871 pub fn strong_count(&self) -> usize {
1872 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1875 /// Gets an approximation of the number of `Weak` pointers pointing to this
1878 /// If `self` was created using [`Weak::new`], or if there are no remaining
1879 /// strong pointers, this will return 0.
1883 /// Due to implementation details, the returned value can be off by 1 in
1884 /// either direction when other threads are manipulating any `Arc`s or
1885 /// `Weak`s pointing to the same allocation.
1886 #[stable(feature = "weak_counts", since = "1.41.0")]
1887 pub fn weak_count(&self) -> usize {
1890 let weak = inner.weak.load(SeqCst);
1891 let strong = inner.strong.load(SeqCst);
1895 // Since we observed that there was at least one strong pointer
1896 // after reading the weak count, we know that the implicit weak
1897 // reference (present whenever any strong references are alive)
1898 // was still around when we observed the weak count, and can
1899 // therefore safely subtract it.
1906 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1907 /// (i.e., when this `Weak` was created by `Weak::new`).
1909 fn inner(&self) -> Option<WeakInner<'_>> {
1910 if is_dangling(self.ptr.as_ptr()) {
1913 // We are careful to *not* create a reference covering the "data" field, as
1914 // the field may be mutated concurrently (for example, if the last `Arc`
1915 // is dropped, the data field will be dropped in-place).
1917 let ptr = self.ptr.as_ptr();
1918 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1923 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1924 /// [`ptr::eq`]), or if both don't point to any allocation
1925 /// (because they were created with `Weak::new()`).
1929 /// Since this compares pointers it means that `Weak::new()` will equal each
1930 /// other, even though they don't point to any allocation.
1935 /// use std::sync::Arc;
1937 /// let first_rc = Arc::new(5);
1938 /// let first = Arc::downgrade(&first_rc);
1939 /// let second = Arc::downgrade(&first_rc);
1941 /// assert!(first.ptr_eq(&second));
1943 /// let third_rc = Arc::new(5);
1944 /// let third = Arc::downgrade(&third_rc);
1946 /// assert!(!first.ptr_eq(&third));
1949 /// Comparing `Weak::new`.
1952 /// use std::sync::{Arc, Weak};
1954 /// let first = Weak::new();
1955 /// let second = Weak::new();
1956 /// assert!(first.ptr_eq(&second));
1958 /// let third_rc = Arc::new(());
1959 /// let third = Arc::downgrade(&third_rc);
1960 /// assert!(!first.ptr_eq(&third));
1963 /// [`ptr::eq`]: core::ptr::eq
1965 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1966 pub fn ptr_eq(&self, other: &Self) -> bool {
1967 self.ptr.as_ptr() == other.ptr.as_ptr()
1971 #[stable(feature = "arc_weak", since = "1.4.0")]
1972 impl<T: ?Sized> Clone for Weak<T> {
1973 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1978 /// use std::sync::{Arc, Weak};
1980 /// let weak_five = Arc::downgrade(&Arc::new(5));
1982 /// let _ = Weak::clone(&weak_five);
1985 fn clone(&self) -> Weak<T> {
1986 let inner = if let Some(inner) = self.inner() {
1989 return Weak { ptr: self.ptr };
1991 // See comments in Arc::clone() for why this is relaxed. This can use a
1992 // fetch_add (ignoring the lock) because the weak count is only locked
1993 // where are *no other* weak pointers in existence. (So we can't be
1994 // running this code in that case).
1995 let old_size = inner.weak.fetch_add(1, Relaxed);
1997 // See comments in Arc::clone() for why we do this (for mem::forget).
1998 if old_size > MAX_REFCOUNT {
2002 Weak { ptr: self.ptr }
2006 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2007 impl<T> Default for Weak<T> {
2008 /// Constructs a new `Weak<T>`, without allocating memory.
2009 /// Calling [`upgrade`] on the return value always
2012 /// [`upgrade`]: Weak::upgrade
2017 /// use std::sync::Weak;
2019 /// let empty: Weak<i64> = Default::default();
2020 /// assert!(empty.upgrade().is_none());
2022 fn default() -> Weak<T> {
2027 #[stable(feature = "arc_weak", since = "1.4.0")]
2028 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2029 /// Drops the `Weak` pointer.
2034 /// use std::sync::{Arc, Weak};
2038 /// impl Drop for Foo {
2039 /// fn drop(&mut self) {
2040 /// println!("dropped!");
2044 /// let foo = Arc::new(Foo);
2045 /// let weak_foo = Arc::downgrade(&foo);
2046 /// let other_weak_foo = Weak::clone(&weak_foo);
2048 /// drop(weak_foo); // Doesn't print anything
2049 /// drop(foo); // Prints "dropped!"
2051 /// assert!(other_weak_foo.upgrade().is_none());
2053 fn drop(&mut self) {
2054 // If we find out that we were the last weak pointer, then its time to
2055 // deallocate the data entirely. See the discussion in Arc::drop() about
2056 // the memory orderings
2058 // It's not necessary to check for the locked state here, because the
2059 // weak count can only be locked if there was precisely one weak ref,
2060 // meaning that drop could only subsequently run ON that remaining weak
2061 // ref, which can only happen after the lock is released.
2062 let inner = if let Some(inner) = self.inner() { inner } else { return };
2064 if inner.weak.fetch_sub(1, Release) == 1 {
2065 acquire!(inner.weak);
2066 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2071 #[stable(feature = "rust1", since = "1.0.0")]
2072 trait ArcEqIdent<T: ?Sized + PartialEq> {
2073 fn eq(&self, other: &Arc<T>) -> bool;
2074 fn ne(&self, other: &Arc<T>) -> bool;
2077 #[stable(feature = "rust1", since = "1.0.0")]
2078 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2080 default fn eq(&self, other: &Arc<T>) -> bool {
2084 default fn ne(&self, other: &Arc<T>) -> bool {
2089 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2090 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2091 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2092 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2093 /// the same value, than two `&T`s.
2095 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2096 #[stable(feature = "rust1", since = "1.0.0")]
2097 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2099 fn eq(&self, other: &Arc<T>) -> bool {
2100 Arc::ptr_eq(self, other) || **self == **other
2104 fn ne(&self, other: &Arc<T>) -> bool {
2105 !Arc::ptr_eq(self, other) && **self != **other
2109 #[stable(feature = "rust1", since = "1.0.0")]
2110 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2111 /// Equality for two `Arc`s.
2113 /// Two `Arc`s are equal if their inner values are equal, even if they are
2114 /// stored in different allocation.
2116 /// If `T` also implements `Eq` (implying reflexivity of equality),
2117 /// two `Arc`s that point to the same allocation are always equal.
2122 /// use std::sync::Arc;
2124 /// let five = Arc::new(5);
2126 /// assert!(five == Arc::new(5));
2129 fn eq(&self, other: &Arc<T>) -> bool {
2130 ArcEqIdent::eq(self, other)
2133 /// Inequality for two `Arc`s.
2135 /// Two `Arc`s are unequal if their inner values are unequal.
2137 /// If `T` also implements `Eq` (implying reflexivity of equality),
2138 /// two `Arc`s that point to the same value are never unequal.
2143 /// use std::sync::Arc;
2145 /// let five = Arc::new(5);
2147 /// assert!(five != Arc::new(6));
2150 fn ne(&self, other: &Arc<T>) -> bool {
2151 ArcEqIdent::ne(self, other)
2155 #[stable(feature = "rust1", since = "1.0.0")]
2156 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2157 /// Partial comparison for two `Arc`s.
2159 /// The two are compared by calling `partial_cmp()` on their inner values.
2164 /// use std::sync::Arc;
2165 /// use std::cmp::Ordering;
2167 /// let five = Arc::new(5);
2169 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2171 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2172 (**self).partial_cmp(&**other)
2175 /// Less-than comparison for two `Arc`s.
2177 /// The two are compared by calling `<` on their inner values.
2182 /// use std::sync::Arc;
2184 /// let five = Arc::new(5);
2186 /// assert!(five < Arc::new(6));
2188 fn lt(&self, other: &Arc<T>) -> bool {
2189 *(*self) < *(*other)
2192 /// 'Less than or equal to' comparison for two `Arc`s.
2194 /// The two are compared by calling `<=` on their inner values.
2199 /// use std::sync::Arc;
2201 /// let five = Arc::new(5);
2203 /// assert!(five <= Arc::new(5));
2205 fn le(&self, other: &Arc<T>) -> bool {
2206 *(*self) <= *(*other)
2209 /// Greater-than comparison for two `Arc`s.
2211 /// The two are compared by calling `>` on their inner values.
2216 /// use std::sync::Arc;
2218 /// let five = Arc::new(5);
2220 /// assert!(five > Arc::new(4));
2222 fn gt(&self, other: &Arc<T>) -> bool {
2223 *(*self) > *(*other)
2226 /// 'Greater than or equal to' comparison for two `Arc`s.
2228 /// The two are compared by calling `>=` on their inner values.
2233 /// use std::sync::Arc;
2235 /// let five = Arc::new(5);
2237 /// assert!(five >= Arc::new(5));
2239 fn ge(&self, other: &Arc<T>) -> bool {
2240 *(*self) >= *(*other)
2243 #[stable(feature = "rust1", since = "1.0.0")]
2244 impl<T: ?Sized + Ord> Ord for Arc<T> {
2245 /// Comparison for two `Arc`s.
2247 /// The two are compared by calling `cmp()` on their inner values.
2252 /// use std::sync::Arc;
2253 /// use std::cmp::Ordering;
2255 /// let five = Arc::new(5);
2257 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2259 fn cmp(&self, other: &Arc<T>) -> Ordering {
2260 (**self).cmp(&**other)
2263 #[stable(feature = "rust1", since = "1.0.0")]
2264 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2266 #[stable(feature = "rust1", since = "1.0.0")]
2267 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2268 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2269 fmt::Display::fmt(&**self, f)
2273 #[stable(feature = "rust1", since = "1.0.0")]
2274 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2275 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2276 fmt::Debug::fmt(&**self, f)
2280 #[stable(feature = "rust1", since = "1.0.0")]
2281 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2282 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2283 fmt::Pointer::fmt(&(&**self as *const T), f)
2287 #[cfg(not(no_global_oom_handling))]
2288 #[stable(feature = "rust1", since = "1.0.0")]
2289 impl<T: Default> Default for Arc<T> {
2290 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2295 /// use std::sync::Arc;
2297 /// let x: Arc<i32> = Default::default();
2298 /// assert_eq!(*x, 0);
2300 fn default() -> Arc<T> {
2301 Arc::new(Default::default())
2305 #[stable(feature = "rust1", since = "1.0.0")]
2306 impl<T: ?Sized + Hash> Hash for Arc<T> {
2307 fn hash<H: Hasher>(&self, state: &mut H) {
2308 (**self).hash(state)
2312 #[cfg(not(no_global_oom_handling))]
2313 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2314 impl<T> From<T> for Arc<T> {
2315 /// Converts a `T` into an `Arc<T>`
2317 /// The conversion moves the value into a
2318 /// newly allocated `Arc`. It is equivalent to
2319 /// calling `Arc::new(t)`.
2323 /// # use std::sync::Arc;
2325 /// let arc = Arc::new(5);
2327 /// assert_eq!(Arc::from(x), arc);
2329 fn from(t: T) -> Self {
2334 #[cfg(not(no_global_oom_handling))]
2335 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2336 impl<T: Clone> From<&[T]> for Arc<[T]> {
2337 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2342 /// # use std::sync::Arc;
2343 /// let original: &[i32] = &[1, 2, 3];
2344 /// let shared: Arc<[i32]> = Arc::from(original);
2345 /// assert_eq!(&[1, 2, 3], &shared[..]);
2348 fn from(v: &[T]) -> Arc<[T]> {
2349 <Self as ArcFromSlice<T>>::from_slice(v)
2353 #[cfg(not(no_global_oom_handling))]
2354 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2355 impl From<&str> for Arc<str> {
2356 /// Allocate a reference-counted `str` and copy `v` into it.
2361 /// # use std::sync::Arc;
2362 /// let shared: Arc<str> = Arc::from("eggplant");
2363 /// assert_eq!("eggplant", &shared[..]);
2366 fn from(v: &str) -> Arc<str> {
2367 let arc = Arc::<[u8]>::from(v.as_bytes());
2368 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2372 #[cfg(not(no_global_oom_handling))]
2373 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2374 impl From<String> for Arc<str> {
2375 /// Allocate a reference-counted `str` and copy `v` into it.
2380 /// # use std::sync::Arc;
2381 /// let unique: String = "eggplant".to_owned();
2382 /// let shared: Arc<str> = Arc::from(unique);
2383 /// assert_eq!("eggplant", &shared[..]);
2386 fn from(v: String) -> Arc<str> {
2391 #[cfg(not(no_global_oom_handling))]
2392 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2393 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2394 /// Move a boxed object to a new, reference-counted allocation.
2399 /// # use std::sync::Arc;
2400 /// let unique: Box<str> = Box::from("eggplant");
2401 /// let shared: Arc<str> = Arc::from(unique);
2402 /// assert_eq!("eggplant", &shared[..]);
2405 fn from(v: Box<T>) -> Arc<T> {
2410 #[cfg(not(no_global_oom_handling))]
2411 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2412 impl<T> From<Vec<T>> for Arc<[T]> {
2413 /// Allocate a reference-counted slice and move `v`'s items into it.
2418 /// # use std::sync::Arc;
2419 /// let unique: Vec<i32> = vec![1, 2, 3];
2420 /// let shared: Arc<[i32]> = Arc::from(unique);
2421 /// assert_eq!(&[1, 2, 3], &shared[..]);
2424 fn from(mut v: Vec<T>) -> Arc<[T]> {
2426 let arc = Arc::copy_from_slice(&v);
2428 // Allow the Vec to free its memory, but not destroy its contents
2436 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2437 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2439 B: ToOwned + ?Sized,
2440 Arc<B>: From<&'a B> + From<B::Owned>,
2442 /// Create an atomically reference-counted pointer from
2443 /// a clone-on-write pointer by copying its content.
2448 /// # use std::sync::Arc;
2449 /// # use std::borrow::Cow;
2450 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2451 /// let shared: Arc<str> = Arc::from(cow);
2452 /// assert_eq!("eggplant", &shared[..]);
2455 fn from(cow: Cow<'a, B>) -> Arc<B> {
2457 Cow::Borrowed(s) => Arc::from(s),
2458 Cow::Owned(s) => Arc::from(s),
2463 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2464 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2465 type Error = Arc<[T]>;
2467 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2468 if boxed_slice.len() == N {
2469 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2476 #[cfg(not(no_global_oom_handling))]
2477 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2478 impl<T> iter::FromIterator<T> for Arc<[T]> {
2479 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2481 /// # Performance characteristics
2483 /// ## The general case
2485 /// In the general case, collecting into `Arc<[T]>` is done by first
2486 /// collecting into a `Vec<T>`. That is, when writing the following:
2489 /// # use std::sync::Arc;
2490 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2491 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2494 /// this behaves as if we wrote:
2497 /// # use std::sync::Arc;
2498 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2499 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2500 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2501 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2504 /// This will allocate as many times as needed for constructing the `Vec<T>`
2505 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2507 /// ## Iterators of known length
2509 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2510 /// a single allocation will be made for the `Arc<[T]>`. For example:
2513 /// # use std::sync::Arc;
2514 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2515 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2517 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2518 ToArcSlice::to_arc_slice(iter.into_iter())
2522 /// Specialization trait used for collecting into `Arc<[T]>`.
2523 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2524 fn to_arc_slice(self) -> Arc<[T]>;
2527 #[cfg(not(no_global_oom_handling))]
2528 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2529 default fn to_arc_slice(self) -> Arc<[T]> {
2530 self.collect::<Vec<T>>().into()
2534 #[cfg(not(no_global_oom_handling))]
2535 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2536 fn to_arc_slice(self) -> Arc<[T]> {
2537 // This is the case for a `TrustedLen` iterator.
2538 let (low, high) = self.size_hint();
2539 if let Some(high) = high {
2543 "TrustedLen iterator's size hint is not exact: {:?}",
2548 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2549 Arc::from_iter_exact(self, low)
2552 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2553 // length exceeding `usize::MAX`.
2554 // The default implementation would collect into a vec which would panic.
2555 // Thus we panic here immediately without invoking `Vec` code.
2556 panic!("capacity overflow");
2561 #[stable(feature = "rust1", since = "1.0.0")]
2562 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2563 fn borrow(&self) -> &T {
2568 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2569 impl<T: ?Sized> AsRef<T> for Arc<T> {
2570 fn as_ref(&self) -> &T {
2575 #[stable(feature = "pin", since = "1.33.0")]
2576 impl<T: ?Sized> Unpin for Arc<T> {}
2578 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2582 /// The pointer must point to (and have valid metadata for) a previously
2583 /// valid instance of T, but the T is allowed to be dropped.
2584 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2585 // Align the unsized value to the end of the ArcInner.
2586 // Because RcBox is repr(C), it will always be the last field in memory.
2587 // SAFETY: since the only unsized types possible are slices, trait objects,
2588 // and extern types, the input safety requirement is currently enough to
2589 // satisfy the requirements of align_of_val_raw; this is an implementation
2590 // detail of the language that may not be relied upon outside of std.
2591 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2595 fn data_offset_align(align: usize) -> isize {
2596 let layout = Layout::new::<ArcInner<()>>();
2597 (layout.size() + layout.padding_needed_for(align)) as isize