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};
22 #[cfg(not(no_global_oom_handling))]
24 use core::ptr::{self, NonNull};
25 #[cfg(not(no_global_oom_handling))]
26 use core::slice::from_raw_parts_mut;
27 use core::sync::atomic;
28 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
30 #[cfg(not(no_global_oom_handling))]
31 use crate::alloc::handle_alloc_error;
32 #[cfg(not(no_global_oom_handling))]
33 use crate::alloc::{box_free, WriteCloneIntoRaw};
34 use crate::alloc::{AllocError, Allocator, Global, Layout};
35 use crate::borrow::{Cow, ToOwned};
36 use crate::boxed::Box;
37 use crate::rc::is_dangling;
38 #[cfg(not(no_global_oom_handling))]
39 use crate::string::String;
40 #[cfg(not(no_global_oom_handling))]
46 /// A soft limit on the amount of references that may be made to an `Arc`.
48 /// Going above this limit will abort your program (although not
49 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
50 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
52 #[cfg(not(sanitize = "thread"))]
53 macro_rules! acquire {
55 atomic::fence(Acquire)
59 // ThreadSanitizer does not support memory fences. To avoid false positive
60 // reports in Arc / Weak implementation use atomic loads for synchronization
62 #[cfg(sanitize = "thread")]
63 macro_rules! acquire {
69 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
70 /// Reference Counted'.
72 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
73 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
74 /// a new `Arc` instance, which points to the same allocation on the heap as the
75 /// source `Arc`, while increasing a reference count. When the last `Arc`
76 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
77 /// referred to as "inner value") is also dropped.
79 /// Shared references in Rust disallow mutation by default, and `Arc` is no
80 /// exception: you cannot generally obtain a mutable reference to something
81 /// inside an `Arc`. If you need to mutate through an `Arc`, use
82 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
87 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
88 /// counting. This means that it is thread-safe. The disadvantage is that
89 /// atomic operations are more expensive than ordinary memory accesses. If you
90 /// are not sharing reference-counted allocations between threads, consider using
91 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
92 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
93 /// However, a library might choose `Arc<T>` in order to give library consumers
96 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
97 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
98 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
99 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
100 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
101 /// data, but it doesn't add thread safety to its data. Consider
102 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
103 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
104 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
105 /// non-atomic operations.
107 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
108 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
110 /// ## Breaking cycles with `Weak`
112 /// The [`downgrade`][downgrade] method can be used to create a non-owning
113 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
114 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
115 /// already been dropped. In other words, `Weak` pointers do not keep the value
116 /// inside the allocation alive; however, they *do* keep the allocation
117 /// (the backing store for the value) alive.
119 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
120 /// [`Weak`] is used to break cycles. For example, a tree could have
121 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
122 /// pointers from children back to their parents.
124 /// # Cloning references
126 /// Creating a new reference from an existing reference-counted pointer is done using the
127 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
130 /// use std::sync::Arc;
131 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
132 /// // The two syntaxes below are equivalent.
133 /// let a = foo.clone();
134 /// let b = Arc::clone(&foo);
135 /// // a, b, and foo are all Arcs that point to the same memory location
138 /// ## `Deref` behavior
140 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
141 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
142 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
143 /// functions, called using [fully qualified syntax]:
146 /// use std::sync::Arc;
148 /// let my_arc = Arc::new(());
149 /// Arc::downgrade(&my_arc);
152 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
153 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
154 /// while others prefer using method-call syntax.
157 /// use std::sync::Arc;
159 /// let arc = Arc::new(());
160 /// // Method-call syntax
161 /// let arc2 = arc.clone();
162 /// // Fully qualified syntax
163 /// let arc3 = Arc::clone(&arc);
166 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
167 /// already been dropped.
169 /// [`Rc<T>`]: crate::rc::Rc
170 /// [clone]: Clone::clone
171 /// [mutex]: ../../std/sync/struct.Mutex.html
172 /// [rwlock]: ../../std/sync/struct.RwLock.html
173 /// [atomic]: core::sync::atomic
174 /// [`Send`]: core::marker::Send
175 /// [`Sync`]: core::marker::Sync
176 /// [deref]: core::ops::Deref
177 /// [downgrade]: Arc::downgrade
178 /// [upgrade]: Weak::upgrade
179 /// [`RefCell<T>`]: core::cell::RefCell
180 /// [`std::sync`]: ../../std/sync/index.html
181 /// [`Arc::clone(&from)`]: Arc::clone
182 /// [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
186 /// Sharing some immutable data between threads:
188 // Note that we **do not** run these tests here. The windows builders get super
189 // unhappy if a thread outlives the main thread and then exits at the same time
190 // (something deadlocks) so we just avoid this entirely by not running these
193 /// use std::sync::Arc;
196 /// let five = Arc::new(5);
199 /// let five = Arc::clone(&five);
201 /// thread::spawn(move || {
202 /// println!("{:?}", five);
207 /// Sharing a mutable [`AtomicUsize`]:
209 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize
212 /// use std::sync::Arc;
213 /// use std::sync::atomic::{AtomicUsize, Ordering};
216 /// let val = Arc::new(AtomicUsize::new(5));
219 /// let val = Arc::clone(&val);
221 /// thread::spawn(move || {
222 /// let v = val.fetch_add(1, Ordering::SeqCst);
223 /// println!("{:?}", v);
228 /// See the [`rc` documentation][rc_examples] for more examples of reference
229 /// counting in general.
231 /// [rc_examples]: crate::rc#examples
232 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
233 #[stable(feature = "rust1", since = "1.0.0")]
234 pub struct Arc<T: ?Sized> {
235 ptr: NonNull<ArcInner<T>>,
236 phantom: PhantomData<ArcInner<T>>,
239 #[stable(feature = "rust1", since = "1.0.0")]
240 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
241 #[stable(feature = "rust1", since = "1.0.0")]
242 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
244 #[unstable(feature = "coerce_unsized", issue = "27732")]
245 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
247 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
248 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
250 impl<T: ?Sized> Arc<T> {
251 fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
252 Self { ptr, phantom: PhantomData }
255 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
256 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
260 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
261 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
262 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
264 /// Since a `Weak` reference does not count towards ownership, it will not
265 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
266 /// guarantees about the value still being present. Thus it may return [`None`]
267 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
268 /// itself (the backing store) from being deallocated.
270 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
271 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
272 /// prevent circular references between [`Arc`] pointers, since mutual owning references
273 /// would never allow either [`Arc`] to be dropped. For example, a tree could
274 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
275 /// pointers from children back to their parents.
277 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
279 /// [`upgrade`]: Weak::upgrade
280 #[stable(feature = "arc_weak", since = "1.4.0")]
281 pub struct Weak<T: ?Sized> {
282 // This is a `NonNull` to allow optimizing the size of this type in enums,
283 // but it is not necessarily a valid pointer.
284 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
285 // to allocate space on the heap. That's not a value a real pointer
286 // will ever have because RcBox has alignment at least 2.
287 // This is only possible when `T: Sized`; unsized `T` never dangle.
288 ptr: NonNull<ArcInner<T>>,
291 #[stable(feature = "arc_weak", since = "1.4.0")]
292 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
293 #[stable(feature = "arc_weak", since = "1.4.0")]
294 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
296 #[unstable(feature = "coerce_unsized", issue = "27732")]
297 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
298 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
299 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
301 #[stable(feature = "arc_weak", since = "1.4.0")]
302 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
303 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
308 // This is repr(C) to future-proof against possible field-reordering, which
309 // would interfere with otherwise safe [into|from]_raw() of transmutable
312 struct ArcInner<T: ?Sized> {
313 strong: atomic::AtomicUsize,
315 // the value usize::MAX acts as a sentinel for temporarily "locking" the
316 // ability to upgrade weak pointers or downgrade strong ones; this is used
317 // to avoid races in `make_mut` and `get_mut`.
318 weak: atomic::AtomicUsize,
323 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
324 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
327 /// Constructs a new `Arc<T>`.
332 /// use std::sync::Arc;
334 /// let five = Arc::new(5);
336 #[cfg(not(no_global_oom_handling))]
338 #[stable(feature = "rust1", since = "1.0.0")]
339 pub fn new(data: T) -> Arc<T> {
340 // Start the weak pointer count as 1 which is the weak pointer that's
341 // held by all the strong pointers (kinda), see std/rc.rs for more info
342 let x: Box<_> = box ArcInner {
343 strong: atomic::AtomicUsize::new(1),
344 weak: atomic::AtomicUsize::new(1),
347 Self::from_inner(Box::leak(x).into())
350 /// Constructs a new `Arc<T>` using a weak reference to itself. Attempting
351 /// to upgrade the weak reference before this function returns will result
352 /// in a `None` value. However, the weak reference may be cloned freely and
353 /// stored for use at a later time.
357 /// #![feature(arc_new_cyclic)]
358 /// #![allow(dead_code)]
360 /// use std::sync::{Arc, Weak};
366 /// let foo = Arc::new_cyclic(|me| Foo {
370 #[cfg(not(no_global_oom_handling))]
372 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
373 pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Arc<T> {
374 // Construct the inner in the "uninitialized" state with a single
376 let uninit_ptr: NonNull<_> = Box::leak(box ArcInner {
377 strong: atomic::AtomicUsize::new(0),
378 weak: atomic::AtomicUsize::new(1),
379 data: mem::MaybeUninit::<T>::uninit(),
382 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
384 let weak = Weak { ptr: init_ptr };
386 // It's important we don't give up ownership of the weak pointer, or
387 // else the memory might be freed by the time `data_fn` returns. If
388 // we really wanted to pass ownership, we could create an additional
389 // weak pointer for ourselves, but this would result in additional
390 // updates to the weak reference count which might not be necessary
392 let data = data_fn(&weak);
394 // Now we can properly initialize the inner value and turn our weak
395 // reference into a strong reference.
397 let inner = init_ptr.as_ptr();
398 ptr::write(ptr::addr_of_mut!((*inner).data), data);
400 // The above write to the data field must be visible to any threads which
401 // observe a non-zero strong count. Therefore we need at least "Release" ordering
402 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
404 // "Acquire" ordering is not required. When considering the possible behaviours
405 // of `data_fn` we only need to look at what it could do with a reference to a
406 // non-upgradeable `Weak`:
407 // - It can *clone* the `Weak`, increasing the weak reference count.
408 // - It can drop those clones, decreasing the weak reference count (but never to zero).
410 // These side effects do not impact us in any way, and no other side effects are
411 // possible with safe code alone.
412 let prev_value = (*inner).strong.fetch_add(1, Release);
413 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
416 let strong = Arc::from_inner(init_ptr);
418 // Strong references should collectively own a shared weak reference,
419 // so don't run the destructor for our old weak reference.
424 /// Constructs a new `Arc` with uninitialized contents.
429 /// #![feature(new_uninit)]
430 /// #![feature(get_mut_unchecked)]
432 /// use std::sync::Arc;
434 /// let mut five = Arc::<u32>::new_uninit();
436 /// let five = unsafe {
437 /// // Deferred initialization:
438 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
440 /// five.assume_init()
443 /// assert_eq!(*five, 5)
445 #[cfg(not(no_global_oom_handling))]
446 #[unstable(feature = "new_uninit", issue = "63291")]
447 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
449 Arc::from_ptr(Arc::allocate_for_layout(
451 |layout| Global.allocate(layout),
452 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
457 /// Constructs a new `Arc` with uninitialized contents, with the memory
458 /// being filled with `0` bytes.
460 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
466 /// #![feature(new_uninit)]
468 /// use std::sync::Arc;
470 /// let zero = Arc::<u32>::new_zeroed();
471 /// let zero = unsafe { zero.assume_init() };
473 /// assert_eq!(*zero, 0)
476 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
477 #[cfg(not(no_global_oom_handling))]
478 #[unstable(feature = "new_uninit", issue = "63291")]
479 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
481 Arc::from_ptr(Arc::allocate_for_layout(
483 |layout| Global.allocate_zeroed(layout),
484 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
489 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
490 /// `data` will be pinned in memory and unable to be moved.
491 #[cfg(not(no_global_oom_handling))]
492 #[stable(feature = "pin", since = "1.33.0")]
493 pub fn pin(data: T) -> Pin<Arc<T>> {
494 unsafe { Pin::new_unchecked(Arc::new(data)) }
497 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
502 /// #![feature(allocator_api)]
503 /// use std::sync::Arc;
505 /// let five = Arc::try_new(5)?;
506 /// # Ok::<(), std::alloc::AllocError>(())
508 #[unstable(feature = "allocator_api", issue = "32838")]
510 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
511 // Start the weak pointer count as 1 which is the weak pointer that's
512 // held by all the strong pointers (kinda), see std/rc.rs for more info
513 let x: Box<_> = Box::try_new(ArcInner {
514 strong: atomic::AtomicUsize::new(1),
515 weak: atomic::AtomicUsize::new(1),
518 Ok(Self::from_inner(Box::leak(x).into()))
521 /// Constructs a new `Arc` with uninitialized contents, returning an error
522 /// if allocation fails.
527 /// #![feature(new_uninit, allocator_api)]
528 /// #![feature(get_mut_unchecked)]
530 /// use std::sync::Arc;
532 /// let mut five = Arc::<u32>::try_new_uninit()?;
534 /// let five = unsafe {
535 /// // Deferred initialization:
536 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
538 /// five.assume_init()
541 /// assert_eq!(*five, 5);
542 /// # Ok::<(), std::alloc::AllocError>(())
544 #[unstable(feature = "allocator_api", issue = "32838")]
545 // #[unstable(feature = "new_uninit", issue = "63291")]
546 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
548 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
550 |layout| Global.allocate(layout),
551 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
556 /// Constructs a new `Arc` with uninitialized contents, with the memory
557 /// being filled with `0` bytes, returning an error if allocation fails.
559 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
565 /// #![feature(new_uninit, allocator_api)]
567 /// use std::sync::Arc;
569 /// let zero = Arc::<u32>::try_new_zeroed()?;
570 /// let zero = unsafe { zero.assume_init() };
572 /// assert_eq!(*zero, 0);
573 /// # Ok::<(), std::alloc::AllocError>(())
576 /// [zeroed]: mem::MaybeUninit::zeroed
577 #[unstable(feature = "allocator_api", issue = "32838")]
578 // #[unstable(feature = "new_uninit", issue = "63291")]
579 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
581 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
583 |layout| Global.allocate_zeroed(layout),
584 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
588 /// Returns the inner value, if the `Arc` has exactly one strong reference.
590 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
593 /// This will succeed even if there are outstanding weak references.
598 /// use std::sync::Arc;
600 /// let x = Arc::new(3);
601 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
603 /// let x = Arc::new(4);
604 /// let _y = Arc::clone(&x);
605 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
608 #[stable(feature = "arc_unique", since = "1.4.0")]
609 pub fn try_unwrap(this: Self) -> Result<T, Self> {
610 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
614 acquire!(this.inner().strong);
617 let elem = ptr::read(&this.ptr.as_ref().data);
619 // Make a weak pointer to clean up the implicit strong-weak reference
620 let _weak = Weak { ptr: this.ptr };
629 /// Constructs a new atomically reference-counted slice with uninitialized contents.
634 /// #![feature(new_uninit)]
635 /// #![feature(get_mut_unchecked)]
637 /// use std::sync::Arc;
639 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
641 /// let values = unsafe {
642 /// // Deferred initialization:
643 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
644 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
645 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
647 /// values.assume_init()
650 /// assert_eq!(*values, [1, 2, 3])
652 #[cfg(not(no_global_oom_handling))]
653 #[unstable(feature = "new_uninit", issue = "63291")]
654 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
655 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
658 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
659 /// filled with `0` bytes.
661 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
662 /// incorrect usage of this method.
667 /// #![feature(new_uninit)]
669 /// use std::sync::Arc;
671 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
672 /// let values = unsafe { values.assume_init() };
674 /// assert_eq!(*values, [0, 0, 0])
677 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
678 #[cfg(not(no_global_oom_handling))]
679 #[unstable(feature = "new_uninit", issue = "63291")]
680 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
682 Arc::from_ptr(Arc::allocate_for_layout(
683 Layout::array::<T>(len).unwrap(),
684 |layout| Global.allocate_zeroed(layout),
686 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
687 as *mut ArcInner<[mem::MaybeUninit<T>]>
694 impl<T> Arc<mem::MaybeUninit<T>> {
695 /// Converts to `Arc<T>`.
699 /// As with [`MaybeUninit::assume_init`],
700 /// it is up to the caller to guarantee that the inner value
701 /// really is in an initialized state.
702 /// Calling this when the content is not yet fully initialized
703 /// causes immediate undefined behavior.
705 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
710 /// #![feature(new_uninit)]
711 /// #![feature(get_mut_unchecked)]
713 /// use std::sync::Arc;
715 /// let mut five = Arc::<u32>::new_uninit();
717 /// let five = unsafe {
718 /// // Deferred initialization:
719 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
721 /// five.assume_init()
724 /// assert_eq!(*five, 5)
726 #[unstable(feature = "new_uninit", issue = "63291")]
728 pub unsafe fn assume_init(self) -> Arc<T> {
729 Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
733 impl<T> Arc<[mem::MaybeUninit<T>]> {
734 /// Converts to `Arc<[T]>`.
738 /// As with [`MaybeUninit::assume_init`],
739 /// it is up to the caller to guarantee that the inner value
740 /// really is in an initialized state.
741 /// Calling this when the content is not yet fully initialized
742 /// causes immediate undefined behavior.
744 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
749 /// #![feature(new_uninit)]
750 /// #![feature(get_mut_unchecked)]
752 /// use std::sync::Arc;
754 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
756 /// let values = unsafe {
757 /// // Deferred initialization:
758 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
759 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
760 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
762 /// values.assume_init()
765 /// assert_eq!(*values, [1, 2, 3])
767 #[unstable(feature = "new_uninit", issue = "63291")]
769 pub unsafe fn assume_init(self) -> Arc<[T]> {
770 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
774 impl<T: ?Sized> Arc<T> {
775 /// Consumes the `Arc`, returning the wrapped pointer.
777 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
778 /// [`Arc::from_raw`].
783 /// use std::sync::Arc;
785 /// let x = Arc::new("hello".to_owned());
786 /// let x_ptr = Arc::into_raw(x);
787 /// assert_eq!(unsafe { &*x_ptr }, "hello");
789 #[stable(feature = "rc_raw", since = "1.17.0")]
790 pub fn into_raw(this: Self) -> *const T {
791 let ptr = Self::as_ptr(&this);
796 /// Provides a raw pointer to the data.
798 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
799 /// as long as there are strong counts in the `Arc`.
804 /// use std::sync::Arc;
806 /// let x = Arc::new("hello".to_owned());
807 /// let y = Arc::clone(&x);
808 /// let x_ptr = Arc::as_ptr(&x);
809 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
810 /// assert_eq!(unsafe { &*x_ptr }, "hello");
812 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
813 pub fn as_ptr(this: &Self) -> *const T {
814 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
816 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
817 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
818 // write through the pointer after the Rc is recovered through `from_raw`.
819 unsafe { ptr::addr_of_mut!((*ptr).data) }
822 /// Constructs an `Arc<T>` from a raw pointer.
824 /// The raw pointer must have been previously returned by a call to
825 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
826 /// alignment as `T`. This is trivially true if `U` is `T`.
827 /// Note that if `U` is not `T` but has the same size and alignment, this is
828 /// basically like transmuting references of different types. See
829 /// [`mem::transmute`][transmute] for more information on what
830 /// restrictions apply in this case.
832 /// The user of `from_raw` has to make sure a specific value of `T` is only
835 /// This function is unsafe because improper use may lead to memory unsafety,
836 /// even if the returned `Arc<T>` is never accessed.
838 /// [into_raw]: Arc::into_raw
839 /// [transmute]: core::mem::transmute
844 /// use std::sync::Arc;
846 /// let x = Arc::new("hello".to_owned());
847 /// let x_ptr = Arc::into_raw(x);
850 /// // Convert back to an `Arc` to prevent leak.
851 /// let x = Arc::from_raw(x_ptr);
852 /// assert_eq!(&*x, "hello");
854 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
857 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
859 #[stable(feature = "rc_raw", since = "1.17.0")]
860 pub unsafe fn from_raw(ptr: *const T) -> Self {
862 let offset = data_offset(ptr);
864 // Reverse the offset to find the original ArcInner.
865 let arc_ptr = (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset));
867 Self::from_ptr(arc_ptr)
871 /// Creates a new [`Weak`] pointer to this allocation.
876 /// use std::sync::Arc;
878 /// let five = Arc::new(5);
880 /// let weak_five = Arc::downgrade(&five);
882 #[stable(feature = "arc_weak", since = "1.4.0")]
883 pub fn downgrade(this: &Self) -> Weak<T> {
884 // This Relaxed is OK because we're checking the value in the CAS
886 let mut cur = this.inner().weak.load(Relaxed);
889 // check if the weak counter is currently "locked"; if so, spin.
890 if cur == usize::MAX {
892 cur = this.inner().weak.load(Relaxed);
896 // NOTE: this code currently ignores the possibility of overflow
897 // into usize::MAX; in general both Rc and Arc need to be adjusted
898 // to deal with overflow.
900 // Unlike with Clone(), we need this to be an Acquire read to
901 // synchronize with the write coming from `is_unique`, so that the
902 // events prior to that write happen before this read.
903 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
905 // Make sure we do not create a dangling Weak
906 debug_assert!(!is_dangling(this.ptr.as_ptr()));
907 return Weak { ptr: this.ptr };
909 Err(old) => cur = old,
914 /// Gets the number of [`Weak`] pointers to this allocation.
918 /// This method by itself is safe, but using it correctly requires extra care.
919 /// Another thread can change the weak count at any time,
920 /// including potentially between calling this method and acting on the result.
925 /// use std::sync::Arc;
927 /// let five = Arc::new(5);
928 /// let _weak_five = Arc::downgrade(&five);
930 /// // This assertion is deterministic because we haven't shared
931 /// // the `Arc` or `Weak` between threads.
932 /// assert_eq!(1, Arc::weak_count(&five));
935 #[stable(feature = "arc_counts", since = "1.15.0")]
936 pub fn weak_count(this: &Self) -> usize {
937 let cnt = this.inner().weak.load(SeqCst);
938 // If the weak count is currently locked, the value of the
939 // count was 0 just before taking the lock.
940 if cnt == usize::MAX { 0 } else { cnt - 1 }
943 /// Gets the number of strong (`Arc`) pointers to this allocation.
947 /// This method by itself is safe, but using it correctly requires extra care.
948 /// Another thread can change the strong count at any time,
949 /// including potentially between calling this method and acting on the result.
954 /// use std::sync::Arc;
956 /// let five = Arc::new(5);
957 /// let _also_five = Arc::clone(&five);
959 /// // This assertion is deterministic because we haven't shared
960 /// // the `Arc` between threads.
961 /// assert_eq!(2, Arc::strong_count(&five));
964 #[stable(feature = "arc_counts", since = "1.15.0")]
965 pub fn strong_count(this: &Self) -> usize {
966 this.inner().strong.load(SeqCst)
969 /// Increments the strong reference count on the `Arc<T>` associated with the
970 /// provided pointer by one.
974 /// The pointer must have been obtained through `Arc::into_raw`, and the
975 /// associated `Arc` instance must be valid (i.e. the strong count must be at
976 /// least 1) for the duration of this method.
981 /// use std::sync::Arc;
983 /// let five = Arc::new(5);
986 /// let ptr = Arc::into_raw(five);
987 /// Arc::increment_strong_count(ptr);
989 /// // This assertion is deterministic because we haven't shared
990 /// // the `Arc` between threads.
991 /// let five = Arc::from_raw(ptr);
992 /// assert_eq!(2, Arc::strong_count(&five));
996 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
997 pub unsafe fn increment_strong_count(ptr: *const T) {
998 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
999 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
1000 // Now increase refcount, but don't drop new refcount either
1001 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1004 /// Decrements the strong reference count on the `Arc<T>` associated with the
1005 /// provided pointer by one.
1009 /// The pointer must have been obtained through `Arc::into_raw`, and the
1010 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1011 /// least 1) when invoking this method. This method can be used to release the final
1012 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1018 /// use std::sync::Arc;
1020 /// let five = Arc::new(5);
1023 /// let ptr = Arc::into_raw(five);
1024 /// Arc::increment_strong_count(ptr);
1026 /// // Those assertions are deterministic because we haven't shared
1027 /// // the `Arc` between threads.
1028 /// let five = Arc::from_raw(ptr);
1029 /// assert_eq!(2, Arc::strong_count(&five));
1030 /// Arc::decrement_strong_count(ptr);
1031 /// assert_eq!(1, Arc::strong_count(&five));
1035 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1036 pub unsafe fn decrement_strong_count(ptr: *const T) {
1037 unsafe { mem::drop(Arc::from_raw(ptr)) };
1041 fn inner(&self) -> &ArcInner<T> {
1042 // This unsafety is ok because while this arc is alive we're guaranteed
1043 // that the inner pointer is valid. Furthermore, we know that the
1044 // `ArcInner` structure itself is `Sync` because the inner data is
1045 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1047 unsafe { self.ptr.as_ref() }
1050 // Non-inlined part of `drop`.
1052 unsafe fn drop_slow(&mut self) {
1053 // Destroy the data at this time, even though we may not free the box
1054 // allocation itself (there may still be weak pointers lying around).
1055 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1057 // Drop the weak ref collectively held by all strong references
1058 drop(Weak { ptr: self.ptr });
1062 #[stable(feature = "ptr_eq", since = "1.17.0")]
1063 /// Returns `true` if the two `Arc`s point to the same allocation
1064 /// (in a vein similar to [`ptr::eq`]).
1069 /// use std::sync::Arc;
1071 /// let five = Arc::new(5);
1072 /// let same_five = Arc::clone(&five);
1073 /// let other_five = Arc::new(5);
1075 /// assert!(Arc::ptr_eq(&five, &same_five));
1076 /// assert!(!Arc::ptr_eq(&five, &other_five));
1079 /// [`ptr::eq`]: core::ptr::eq
1080 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1081 this.ptr.as_ptr() == other.ptr.as_ptr()
1085 impl<T: ?Sized> Arc<T> {
1086 /// Allocates an `ArcInner<T>` with sufficient space for
1087 /// a possibly-unsized inner value where the value has the layout provided.
1089 /// The function `mem_to_arcinner` is called with the data pointer
1090 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1091 #[cfg(not(no_global_oom_handling))]
1092 unsafe fn allocate_for_layout(
1093 value_layout: Layout,
1094 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1095 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1096 ) -> *mut ArcInner<T> {
1097 // Calculate layout using the given value layout.
1098 // Previously, layout was calculated on the expression
1099 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1100 // reference (see #54908).
1101 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1103 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1104 .unwrap_or_else(|_| handle_alloc_error(layout))
1108 /// Allocates an `ArcInner<T>` with sufficient space for
1109 /// a possibly-unsized inner value where the value has the layout provided,
1110 /// returning an error if allocation fails.
1112 /// The function `mem_to_arcinner` is called with the data pointer
1113 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1114 unsafe fn try_allocate_for_layout(
1115 value_layout: Layout,
1116 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1117 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1118 ) -> Result<*mut ArcInner<T>, AllocError> {
1119 // Calculate layout using the given value layout.
1120 // Previously, layout was calculated on the expression
1121 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1122 // reference (see #54908).
1123 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1125 let ptr = allocate(layout)?;
1127 // Initialize the ArcInner
1128 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1129 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1132 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1133 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1139 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1140 #[cfg(not(no_global_oom_handling))]
1141 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1142 // Allocate for the `ArcInner<T>` using the given value.
1144 Self::allocate_for_layout(
1145 Layout::for_value(&*ptr),
1146 |layout| Global.allocate(layout),
1147 |mem| (ptr as *mut ArcInner<T>).set_ptr_value(mem) as *mut ArcInner<T>,
1152 #[cfg(not(no_global_oom_handling))]
1153 fn from_box(v: Box<T>) -> Arc<T> {
1155 let (box_unique, alloc) = Box::into_unique(v);
1156 let bptr = box_unique.as_ptr();
1158 let value_size = size_of_val(&*bptr);
1159 let ptr = Self::allocate_for_ptr(bptr);
1161 // Copy value as bytes
1162 ptr::copy_nonoverlapping(
1163 bptr as *const T as *const u8,
1164 &mut (*ptr).data as *mut _ as *mut u8,
1168 // Free the allocation without dropping its contents
1169 box_free(box_unique, alloc);
1177 /// Allocates an `ArcInner<[T]>` with the given length.
1178 #[cfg(not(no_global_oom_handling))]
1179 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1181 Self::allocate_for_layout(
1182 Layout::array::<T>(len).unwrap(),
1183 |layout| Global.allocate(layout),
1184 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1189 /// Copy elements from slice into newly allocated Arc<\[T\]>
1191 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1192 #[cfg(not(no_global_oom_handling))]
1193 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1195 let ptr = Self::allocate_for_slice(v.len());
1197 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1203 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1205 /// Behavior is undefined should the size be wrong.
1206 #[cfg(not(no_global_oom_handling))]
1207 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1208 // Panic guard while cloning T elements.
1209 // In the event of a panic, elements that have been written
1210 // into the new ArcInner will be dropped, then the memory freed.
1218 impl<T> Drop for Guard<T> {
1219 fn drop(&mut self) {
1221 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1222 ptr::drop_in_place(slice);
1224 Global.deallocate(self.mem, self.layout);
1230 let ptr = Self::allocate_for_slice(len);
1232 let mem = ptr as *mut _ as *mut u8;
1233 let layout = Layout::for_value(&*ptr);
1235 // Pointer to first element
1236 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1238 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1240 for (i, item) in iter.enumerate() {
1241 ptr::write(elems.add(i), item);
1245 // All clear. Forget the guard so it doesn't free the new ArcInner.
1253 /// Specialization trait used for `From<&[T]>`.
1254 #[cfg(not(no_global_oom_handling))]
1255 trait ArcFromSlice<T> {
1256 fn from_slice(slice: &[T]) -> Self;
1259 #[cfg(not(no_global_oom_handling))]
1260 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1262 default fn from_slice(v: &[T]) -> Self {
1263 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1267 #[cfg(not(no_global_oom_handling))]
1268 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1270 fn from_slice(v: &[T]) -> Self {
1271 unsafe { Arc::copy_from_slice(v) }
1275 #[stable(feature = "rust1", since = "1.0.0")]
1276 impl<T: ?Sized> Clone for Arc<T> {
1277 /// Makes a clone of the `Arc` pointer.
1279 /// This creates another pointer to the same allocation, increasing the
1280 /// strong reference count.
1285 /// use std::sync::Arc;
1287 /// let five = Arc::new(5);
1289 /// let _ = Arc::clone(&five);
1292 fn clone(&self) -> Arc<T> {
1293 // Using a relaxed ordering is alright here, as knowledge of the
1294 // original reference prevents other threads from erroneously deleting
1297 // As explained in the [Boost documentation][1], Increasing the
1298 // reference counter can always be done with memory_order_relaxed: New
1299 // references to an object can only be formed from an existing
1300 // reference, and passing an existing reference from one thread to
1301 // another must already provide any required synchronization.
1303 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1304 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1306 // However we need to guard against massive refcounts in case someone
1307 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
1308 // and users will use-after free. We racily saturate to `isize::MAX` on
1309 // the assumption that there aren't ~2 billion threads incrementing
1310 // the reference count at once. This branch will never be taken in
1311 // any realistic program.
1313 // We abort because such a program is incredibly degenerate, and we
1314 // don't care to support it.
1315 if old_size > MAX_REFCOUNT {
1319 Self::from_inner(self.ptr)
1323 #[stable(feature = "rust1", since = "1.0.0")]
1324 impl<T: ?Sized> Deref for Arc<T> {
1328 fn deref(&self) -> &T {
1333 #[unstable(feature = "receiver_trait", issue = "none")]
1334 impl<T: ?Sized> Receiver for Arc<T> {}
1336 impl<T: Clone> Arc<T> {
1337 /// Makes a mutable reference into the given `Arc`.
1339 /// If there are other `Arc` or [`Weak`] pointers to the same allocation,
1340 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
1341 /// to ensure unique ownership. This is also referred to as clone-on-write.
1343 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
1344 /// any remaining `Weak` pointers.
1346 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
1348 /// [clone]: Clone::clone
1349 /// [get_mut]: Arc::get_mut
1350 /// [`Rc::make_mut`]: super::rc::Rc::make_mut
1355 /// use std::sync::Arc;
1357 /// let mut data = Arc::new(5);
1359 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1360 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1361 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1362 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1363 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1365 /// // Now `data` and `other_data` point to different allocations.
1366 /// assert_eq!(*data, 8);
1367 /// assert_eq!(*other_data, 12);
1369 #[cfg(not(no_global_oom_handling))]
1371 #[stable(feature = "arc_unique", since = "1.4.0")]
1372 pub fn make_mut(this: &mut Self) -> &mut T {
1373 // Note that we hold both a strong reference and a weak reference.
1374 // Thus, releasing our strong reference only will not, by itself, cause
1375 // the memory to be deallocated.
1377 // Use Acquire to ensure that we see any writes to `weak` that happen
1378 // before release writes (i.e., decrements) to `strong`. Since we hold a
1379 // weak count, there's no chance the ArcInner itself could be
1381 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1382 // Another strong pointer exists, so we must clone.
1383 // Pre-allocate memory to allow writing the cloned value directly.
1384 let mut arc = Self::new_uninit();
1386 let data = Arc::get_mut_unchecked(&mut arc);
1387 (**this).write_clone_into_raw(data.as_mut_ptr());
1388 *this = arc.assume_init();
1390 } else if this.inner().weak.load(Relaxed) != 1 {
1391 // Relaxed suffices in the above because this is fundamentally an
1392 // optimization: we are always racing with weak pointers being
1393 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1395 // We removed the last strong ref, but there are additional weak
1396 // refs remaining. We'll move the contents to a new Arc, and
1397 // invalidate the other weak refs.
1399 // Note that it is not possible for the read of `weak` to yield
1400 // usize::MAX (i.e., locked), since the weak count can only be
1401 // locked by a thread with a strong reference.
1403 // Materialize our own implicit weak pointer, so that it can clean
1404 // up the ArcInner as needed.
1405 let _weak = Weak { ptr: this.ptr };
1407 // Can just steal the data, all that's left is Weaks
1408 let mut arc = Self::new_uninit();
1410 let data = Arc::get_mut_unchecked(&mut arc);
1411 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1412 ptr::write(this, arc.assume_init());
1415 // We were the sole reference of either kind; bump back up the
1416 // strong ref count.
1417 this.inner().strong.store(1, Release);
1420 // As with `get_mut()`, the unsafety is ok because our reference was
1421 // either unique to begin with, or became one upon cloning the contents.
1422 unsafe { Self::get_mut_unchecked(this) }
1426 impl<T: ?Sized> Arc<T> {
1427 /// Returns a mutable reference into the given `Arc`, if there are
1428 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1430 /// Returns [`None`] otherwise, because it is not safe to
1431 /// mutate a shared value.
1433 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1434 /// the inner value when there are other pointers.
1436 /// [make_mut]: Arc::make_mut
1437 /// [clone]: Clone::clone
1442 /// use std::sync::Arc;
1444 /// let mut x = Arc::new(3);
1445 /// *Arc::get_mut(&mut x).unwrap() = 4;
1446 /// assert_eq!(*x, 4);
1448 /// let _y = Arc::clone(&x);
1449 /// assert!(Arc::get_mut(&mut x).is_none());
1452 #[stable(feature = "arc_unique", since = "1.4.0")]
1453 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1454 if this.is_unique() {
1455 // This unsafety is ok because we're guaranteed that the pointer
1456 // returned is the *only* pointer that will ever be returned to T. Our
1457 // reference count is guaranteed to be 1 at this point, and we required
1458 // the Arc itself to be `mut`, so we're returning the only possible
1459 // reference to the inner data.
1460 unsafe { Some(Arc::get_mut_unchecked(this)) }
1466 /// Returns a mutable reference into the given `Arc`,
1467 /// without any check.
1469 /// See also [`get_mut`], which is safe and does appropriate checks.
1471 /// [`get_mut`]: Arc::get_mut
1475 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1476 /// for the duration of the returned borrow.
1477 /// This is trivially the case if no such pointers exist,
1478 /// for example immediately after `Arc::new`.
1483 /// #![feature(get_mut_unchecked)]
1485 /// use std::sync::Arc;
1487 /// let mut x = Arc::new(String::new());
1489 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1491 /// assert_eq!(*x, "foo");
1494 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1495 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1496 // We are careful to *not* create a reference covering the "count" fields, as
1497 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1498 unsafe { &mut (*this.ptr.as_ptr()).data }
1501 /// Determine whether this is the unique reference (including weak refs) to
1502 /// the underlying data.
1504 /// Note that this requires locking the weak ref count.
1505 fn is_unique(&mut self) -> bool {
1506 // lock the weak pointer count if we appear to be the sole weak pointer
1509 // The acquire label here ensures a happens-before relationship with any
1510 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1511 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1512 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1513 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1514 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1515 // counter in `drop` -- the only access that happens when any but the last reference
1516 // is being dropped.
1517 let unique = self.inner().strong.load(Acquire) == 1;
1519 // The release write here synchronizes with a read in `downgrade`,
1520 // effectively preventing the above read of `strong` from happening
1522 self.inner().weak.store(1, Release); // release the lock
1530 #[stable(feature = "rust1", since = "1.0.0")]
1531 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1532 /// Drops the `Arc`.
1534 /// This will decrement the strong reference count. If the strong reference
1535 /// count reaches zero then the only other references (if any) are
1536 /// [`Weak`], so we `drop` the inner value.
1541 /// use std::sync::Arc;
1545 /// impl Drop for Foo {
1546 /// fn drop(&mut self) {
1547 /// println!("dropped!");
1551 /// let foo = Arc::new(Foo);
1552 /// let foo2 = Arc::clone(&foo);
1554 /// drop(foo); // Doesn't print anything
1555 /// drop(foo2); // Prints "dropped!"
1558 fn drop(&mut self) {
1559 // Because `fetch_sub` is already atomic, we do not need to synchronize
1560 // with other threads unless we are going to delete the object. This
1561 // same logic applies to the below `fetch_sub` to the `weak` count.
1562 if self.inner().strong.fetch_sub(1, Release) != 1 {
1566 // This fence is needed to prevent reordering of use of the data and
1567 // deletion of the data. Because it is marked `Release`, the decreasing
1568 // of the reference count synchronizes with this `Acquire` fence. This
1569 // means that use of the data happens before decreasing the reference
1570 // count, which happens before this fence, which happens before the
1571 // deletion of the data.
1573 // As explained in the [Boost documentation][1],
1575 // > It is important to enforce any possible access to the object in one
1576 // > thread (through an existing reference) to *happen before* deleting
1577 // > the object in a different thread. This is achieved by a "release"
1578 // > operation after dropping a reference (any access to the object
1579 // > through this reference must obviously happened before), and an
1580 // > "acquire" operation before deleting the object.
1582 // In particular, while the contents of an Arc are usually immutable, it's
1583 // possible to have interior writes to something like a Mutex<T>. Since a
1584 // Mutex is not acquired when it is deleted, we can't rely on its
1585 // synchronization logic to make writes in thread A visible to a destructor
1586 // running in thread B.
1588 // Also note that the Acquire fence here could probably be replaced with an
1589 // Acquire load, which could improve performance in highly-contended
1590 // situations. See [2].
1592 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1593 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1594 acquire!(self.inner().strong);
1602 impl Arc<dyn Any + Send + Sync> {
1604 #[stable(feature = "rc_downcast", since = "1.29.0")]
1605 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1610 /// use std::any::Any;
1611 /// use std::sync::Arc;
1613 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1614 /// if let Ok(string) = value.downcast::<String>() {
1615 /// println!("String ({}): {}", string.len(), string);
1619 /// let my_string = "Hello World".to_string();
1620 /// print_if_string(Arc::new(my_string));
1621 /// print_if_string(Arc::new(0i8));
1623 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1625 T: Any + Send + Sync + 'static,
1627 if (*self).is::<T>() {
1628 let ptr = self.ptr.cast::<ArcInner<T>>();
1630 Ok(Arc::from_inner(ptr))
1638 /// Constructs a new `Weak<T>`, without allocating any memory.
1639 /// Calling [`upgrade`] on the return value always gives [`None`].
1641 /// [`upgrade`]: Weak::upgrade
1646 /// use std::sync::Weak;
1648 /// let empty: Weak<i64> = Weak::new();
1649 /// assert!(empty.upgrade().is_none());
1651 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1652 pub fn new() -> Weak<T> {
1653 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1657 /// Helper type to allow accessing the reference counts without
1658 /// making any assertions about the data field.
1659 struct WeakInner<'a> {
1660 weak: &'a atomic::AtomicUsize,
1661 strong: &'a atomic::AtomicUsize,
1664 impl<T: ?Sized> Weak<T> {
1665 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1667 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1668 /// unaligned or even [`null`] otherwise.
1673 /// use std::sync::Arc;
1676 /// let strong = Arc::new("hello".to_owned());
1677 /// let weak = Arc::downgrade(&strong);
1678 /// // Both point to the same object
1679 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1680 /// // The strong here keeps it alive, so we can still access the object.
1681 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1684 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1685 /// // undefined behaviour.
1686 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1689 /// [`null`]: core::ptr::null
1690 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1691 pub fn as_ptr(&self) -> *const T {
1692 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1694 if is_dangling(ptr) {
1695 // If the pointer is dangling, we return the sentinel directly. This cannot be
1696 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1699 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
1700 // The payload may be dropped at this point, and we have to maintain provenance,
1701 // so use raw pointer manipulation.
1702 unsafe { ptr::addr_of_mut!((*ptr).data) }
1706 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1708 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1709 /// one weak reference (the weak count is not modified by this operation). It can be turned
1710 /// back into the `Weak<T>` with [`from_raw`].
1712 /// The same restrictions of accessing the target of the pointer as with
1713 /// [`as_ptr`] apply.
1718 /// use std::sync::{Arc, Weak};
1720 /// let strong = Arc::new("hello".to_owned());
1721 /// let weak = Arc::downgrade(&strong);
1722 /// let raw = weak.into_raw();
1724 /// assert_eq!(1, Arc::weak_count(&strong));
1725 /// assert_eq!("hello", unsafe { &*raw });
1727 /// drop(unsafe { Weak::from_raw(raw) });
1728 /// assert_eq!(0, Arc::weak_count(&strong));
1731 /// [`from_raw`]: Weak::from_raw
1732 /// [`as_ptr`]: Weak::as_ptr
1733 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1734 pub fn into_raw(self) -> *const T {
1735 let result = self.as_ptr();
1740 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1742 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1743 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1745 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1746 /// as these don't own anything; the method still works on them).
1750 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1753 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1754 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1755 /// count is not modified by this operation) and therefore it must be paired with a previous
1756 /// call to [`into_raw`].
1760 /// use std::sync::{Arc, Weak};
1762 /// let strong = Arc::new("hello".to_owned());
1764 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1765 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1767 /// assert_eq!(2, Arc::weak_count(&strong));
1769 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1770 /// assert_eq!(1, Arc::weak_count(&strong));
1774 /// // Decrement the last weak count.
1775 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1778 /// [`new`]: Weak::new
1779 /// [`into_raw`]: Weak::into_raw
1780 /// [`upgrade`]: Weak::upgrade
1781 /// [`forget`]: std::mem::forget
1782 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1783 pub unsafe fn from_raw(ptr: *const T) -> Self {
1784 // See Weak::as_ptr for context on how the input pointer is derived.
1786 let ptr = if is_dangling(ptr as *mut T) {
1787 // This is a dangling Weak.
1788 ptr as *mut ArcInner<T>
1790 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1791 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1792 let offset = unsafe { data_offset(ptr) };
1793 // Thus, we reverse the offset to get the whole RcBox.
1794 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1795 unsafe { (ptr as *mut ArcInner<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
1798 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1799 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1803 impl<T: ?Sized> Weak<T> {
1804 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1805 /// dropping of the inner value if successful.
1807 /// Returns [`None`] if the inner value has since been dropped.
1812 /// use std::sync::Arc;
1814 /// let five = Arc::new(5);
1816 /// let weak_five = Arc::downgrade(&five);
1818 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1819 /// assert!(strong_five.is_some());
1821 /// // Destroy all strong pointers.
1822 /// drop(strong_five);
1825 /// assert!(weak_five.upgrade().is_none());
1827 #[stable(feature = "arc_weak", since = "1.4.0")]
1828 pub fn upgrade(&self) -> Option<Arc<T>> {
1829 // We use a CAS loop to increment the strong count instead of a
1830 // fetch_add as this function should never take the reference count
1831 // from zero to one.
1832 let inner = self.inner()?;
1834 // Relaxed load because any write of 0 that we can observe
1835 // leaves the field in a permanently zero state (so a
1836 // "stale" read of 0 is fine), and any other value is
1837 // confirmed via the CAS below.
1838 let mut n = inner.strong.load(Relaxed);
1845 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1846 if n > MAX_REFCOUNT {
1850 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
1851 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
1852 // value can be initialized after `Weak` references have already been created. In that case, we
1853 // expect to observe the fully initialized value.
1854 match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
1855 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1856 Err(old) => n = old,
1861 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1863 /// If `self` was created using [`Weak::new`], this will return 0.
1864 #[stable(feature = "weak_counts", since = "1.41.0")]
1865 pub fn strong_count(&self) -> usize {
1866 if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
1869 /// Gets an approximation of the number of `Weak` pointers pointing to this
1872 /// If `self` was created using [`Weak::new`], or if there are no remaining
1873 /// strong pointers, this will return 0.
1877 /// Due to implementation details, the returned value can be off by 1 in
1878 /// either direction when other threads are manipulating any `Arc`s or
1879 /// `Weak`s pointing to the same allocation.
1880 #[stable(feature = "weak_counts", since = "1.41.0")]
1881 pub fn weak_count(&self) -> usize {
1884 let weak = inner.weak.load(SeqCst);
1885 let strong = inner.strong.load(SeqCst);
1889 // Since we observed that there was at least one strong pointer
1890 // after reading the weak count, we know that the implicit weak
1891 // reference (present whenever any strong references are alive)
1892 // was still around when we observed the weak count, and can
1893 // therefore safely subtract it.
1900 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1901 /// (i.e., when this `Weak` was created by `Weak::new`).
1903 fn inner(&self) -> Option<WeakInner<'_>> {
1904 if is_dangling(self.ptr.as_ptr()) {
1907 // We are careful to *not* create a reference covering the "data" field, as
1908 // the field may be mutated concurrently (for example, if the last `Arc`
1909 // is dropped, the data field will be dropped in-place).
1911 let ptr = self.ptr.as_ptr();
1912 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
1917 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1918 /// [`ptr::eq`]), or if both don't point to any allocation
1919 /// (because they were created with `Weak::new()`).
1923 /// Since this compares pointers it means that `Weak::new()` will equal each
1924 /// other, even though they don't point to any allocation.
1929 /// use std::sync::Arc;
1931 /// let first_rc = Arc::new(5);
1932 /// let first = Arc::downgrade(&first_rc);
1933 /// let second = Arc::downgrade(&first_rc);
1935 /// assert!(first.ptr_eq(&second));
1937 /// let third_rc = Arc::new(5);
1938 /// let third = Arc::downgrade(&third_rc);
1940 /// assert!(!first.ptr_eq(&third));
1943 /// Comparing `Weak::new`.
1946 /// use std::sync::{Arc, Weak};
1948 /// let first = Weak::new();
1949 /// let second = Weak::new();
1950 /// assert!(first.ptr_eq(&second));
1952 /// let third_rc = Arc::new(());
1953 /// let third = Arc::downgrade(&third_rc);
1954 /// assert!(!first.ptr_eq(&third));
1957 /// [`ptr::eq`]: core::ptr::eq
1959 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1960 pub fn ptr_eq(&self, other: &Self) -> bool {
1961 self.ptr.as_ptr() == other.ptr.as_ptr()
1965 #[stable(feature = "arc_weak", since = "1.4.0")]
1966 impl<T: ?Sized> Clone for Weak<T> {
1967 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1972 /// use std::sync::{Arc, Weak};
1974 /// let weak_five = Arc::downgrade(&Arc::new(5));
1976 /// let _ = Weak::clone(&weak_five);
1979 fn clone(&self) -> Weak<T> {
1980 let inner = if let Some(inner) = self.inner() {
1983 return Weak { ptr: self.ptr };
1985 // See comments in Arc::clone() for why this is relaxed. This can use a
1986 // fetch_add (ignoring the lock) because the weak count is only locked
1987 // where are *no other* weak pointers in existence. (So we can't be
1988 // running this code in that case).
1989 let old_size = inner.weak.fetch_add(1, Relaxed);
1991 // See comments in Arc::clone() for why we do this (for mem::forget).
1992 if old_size > MAX_REFCOUNT {
1996 Weak { ptr: self.ptr }
2000 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2001 impl<T> Default for Weak<T> {
2002 /// Constructs a new `Weak<T>`, without allocating memory.
2003 /// Calling [`upgrade`] on the return value always
2006 /// [`upgrade`]: Weak::upgrade
2011 /// use std::sync::Weak;
2013 /// let empty: Weak<i64> = Default::default();
2014 /// assert!(empty.upgrade().is_none());
2016 fn default() -> Weak<T> {
2021 #[stable(feature = "arc_weak", since = "1.4.0")]
2022 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2023 /// Drops the `Weak` pointer.
2028 /// use std::sync::{Arc, Weak};
2032 /// impl Drop for Foo {
2033 /// fn drop(&mut self) {
2034 /// println!("dropped!");
2038 /// let foo = Arc::new(Foo);
2039 /// let weak_foo = Arc::downgrade(&foo);
2040 /// let other_weak_foo = Weak::clone(&weak_foo);
2042 /// drop(weak_foo); // Doesn't print anything
2043 /// drop(foo); // Prints "dropped!"
2045 /// assert!(other_weak_foo.upgrade().is_none());
2047 fn drop(&mut self) {
2048 // If we find out that we were the last weak pointer, then its time to
2049 // deallocate the data entirely. See the discussion in Arc::drop() about
2050 // the memory orderings
2052 // It's not necessary to check for the locked state here, because the
2053 // weak count can only be locked if there was precisely one weak ref,
2054 // meaning that drop could only subsequently run ON that remaining weak
2055 // ref, which can only happen after the lock is released.
2056 let inner = if let Some(inner) = self.inner() { inner } else { return };
2058 if inner.weak.fetch_sub(1, Release) == 1 {
2059 acquire!(inner.weak);
2060 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2065 #[stable(feature = "rust1", since = "1.0.0")]
2066 trait ArcEqIdent<T: ?Sized + PartialEq> {
2067 fn eq(&self, other: &Arc<T>) -> bool;
2068 fn ne(&self, other: &Arc<T>) -> bool;
2071 #[stable(feature = "rust1", since = "1.0.0")]
2072 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2074 default fn eq(&self, other: &Arc<T>) -> bool {
2078 default fn ne(&self, other: &Arc<T>) -> bool {
2083 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2084 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2085 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2086 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2087 /// the same value, than two `&T`s.
2089 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2090 #[stable(feature = "rust1", since = "1.0.0")]
2091 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2093 fn eq(&self, other: &Arc<T>) -> bool {
2094 Arc::ptr_eq(self, other) || **self == **other
2098 fn ne(&self, other: &Arc<T>) -> bool {
2099 !Arc::ptr_eq(self, other) && **self != **other
2103 #[stable(feature = "rust1", since = "1.0.0")]
2104 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2105 /// Equality for two `Arc`s.
2107 /// Two `Arc`s are equal if their inner values are equal, even if they are
2108 /// stored in different allocation.
2110 /// If `T` also implements `Eq` (implying reflexivity of equality),
2111 /// two `Arc`s that point to the same allocation are always equal.
2116 /// use std::sync::Arc;
2118 /// let five = Arc::new(5);
2120 /// assert!(five == Arc::new(5));
2123 fn eq(&self, other: &Arc<T>) -> bool {
2124 ArcEqIdent::eq(self, other)
2127 /// Inequality for two `Arc`s.
2129 /// Two `Arc`s are unequal if their inner values are unequal.
2131 /// If `T` also implements `Eq` (implying reflexivity of equality),
2132 /// two `Arc`s that point to the same value are never unequal.
2137 /// use std::sync::Arc;
2139 /// let five = Arc::new(5);
2141 /// assert!(five != Arc::new(6));
2144 fn ne(&self, other: &Arc<T>) -> bool {
2145 ArcEqIdent::ne(self, other)
2149 #[stable(feature = "rust1", since = "1.0.0")]
2150 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2151 /// Partial comparison for two `Arc`s.
2153 /// The two are compared by calling `partial_cmp()` on their inner values.
2158 /// use std::sync::Arc;
2159 /// use std::cmp::Ordering;
2161 /// let five = Arc::new(5);
2163 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2165 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2166 (**self).partial_cmp(&**other)
2169 /// Less-than comparison for two `Arc`s.
2171 /// The two are compared by calling `<` on their inner values.
2176 /// use std::sync::Arc;
2178 /// let five = Arc::new(5);
2180 /// assert!(five < Arc::new(6));
2182 fn lt(&self, other: &Arc<T>) -> bool {
2183 *(*self) < *(*other)
2186 /// 'Less than or equal to' comparison for two `Arc`s.
2188 /// The two are compared by calling `<=` on their inner values.
2193 /// use std::sync::Arc;
2195 /// let five = Arc::new(5);
2197 /// assert!(five <= Arc::new(5));
2199 fn le(&self, other: &Arc<T>) -> bool {
2200 *(*self) <= *(*other)
2203 /// Greater-than comparison for two `Arc`s.
2205 /// The two are compared by calling `>` on their inner values.
2210 /// use std::sync::Arc;
2212 /// let five = Arc::new(5);
2214 /// assert!(five > Arc::new(4));
2216 fn gt(&self, other: &Arc<T>) -> bool {
2217 *(*self) > *(*other)
2220 /// 'Greater than or equal to' comparison for two `Arc`s.
2222 /// The two are compared by calling `>=` on their inner values.
2227 /// use std::sync::Arc;
2229 /// let five = Arc::new(5);
2231 /// assert!(five >= Arc::new(5));
2233 fn ge(&self, other: &Arc<T>) -> bool {
2234 *(*self) >= *(*other)
2237 #[stable(feature = "rust1", since = "1.0.0")]
2238 impl<T: ?Sized + Ord> Ord for Arc<T> {
2239 /// Comparison for two `Arc`s.
2241 /// The two are compared by calling `cmp()` on their inner values.
2246 /// use std::sync::Arc;
2247 /// use std::cmp::Ordering;
2249 /// let five = Arc::new(5);
2251 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2253 fn cmp(&self, other: &Arc<T>) -> Ordering {
2254 (**self).cmp(&**other)
2257 #[stable(feature = "rust1", since = "1.0.0")]
2258 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2260 #[stable(feature = "rust1", since = "1.0.0")]
2261 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2262 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2263 fmt::Display::fmt(&**self, f)
2267 #[stable(feature = "rust1", since = "1.0.0")]
2268 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2269 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2270 fmt::Debug::fmt(&**self, f)
2274 #[stable(feature = "rust1", since = "1.0.0")]
2275 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2276 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2277 fmt::Pointer::fmt(&(&**self as *const T), f)
2281 #[cfg(not(no_global_oom_handling))]
2282 #[stable(feature = "rust1", since = "1.0.0")]
2283 impl<T: Default> Default for Arc<T> {
2284 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2289 /// use std::sync::Arc;
2291 /// let x: Arc<i32> = Default::default();
2292 /// assert_eq!(*x, 0);
2294 fn default() -> Arc<T> {
2295 Arc::new(Default::default())
2299 #[stable(feature = "rust1", since = "1.0.0")]
2300 impl<T: ?Sized + Hash> Hash for Arc<T> {
2301 fn hash<H: Hasher>(&self, state: &mut H) {
2302 (**self).hash(state)
2306 #[cfg(not(no_global_oom_handling))]
2307 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2308 impl<T> From<T> for Arc<T> {
2309 /// Converts a `T` into an `Arc<T>`
2311 /// The conversion moves the value into a
2312 /// newly allocated `Arc`. It is equivalent to
2313 /// calling `Arc::new(t)`.
2317 /// # use std::sync::Arc;
2319 /// let arc = Arc::new(5);
2321 /// assert_eq!(Arc::from(x), arc);
2323 fn from(t: T) -> Self {
2328 #[cfg(not(no_global_oom_handling))]
2329 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2330 impl<T: Clone> From<&[T]> for Arc<[T]> {
2331 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2336 /// # use std::sync::Arc;
2337 /// let original: &[i32] = &[1, 2, 3];
2338 /// let shared: Arc<[i32]> = Arc::from(original);
2339 /// assert_eq!(&[1, 2, 3], &shared[..]);
2342 fn from(v: &[T]) -> Arc<[T]> {
2343 <Self as ArcFromSlice<T>>::from_slice(v)
2347 #[cfg(not(no_global_oom_handling))]
2348 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2349 impl From<&str> for Arc<str> {
2350 /// Allocate a reference-counted `str` and copy `v` into it.
2355 /// # use std::sync::Arc;
2356 /// let shared: Arc<str> = Arc::from("eggplant");
2357 /// assert_eq!("eggplant", &shared[..]);
2360 fn from(v: &str) -> Arc<str> {
2361 let arc = Arc::<[u8]>::from(v.as_bytes());
2362 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2366 #[cfg(not(no_global_oom_handling))]
2367 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2368 impl From<String> for Arc<str> {
2369 /// Allocate a reference-counted `str` and copy `v` into it.
2374 /// # use std::sync::Arc;
2375 /// let unique: String = "eggplant".to_owned();
2376 /// let shared: Arc<str> = Arc::from(unique);
2377 /// assert_eq!("eggplant", &shared[..]);
2380 fn from(v: String) -> Arc<str> {
2385 #[cfg(not(no_global_oom_handling))]
2386 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2387 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2388 /// Move a boxed object to a new, reference-counted allocation.
2393 /// # use std::sync::Arc;
2394 /// let unique: Box<str> = Box::from("eggplant");
2395 /// let shared: Arc<str> = Arc::from(unique);
2396 /// assert_eq!("eggplant", &shared[..]);
2399 fn from(v: Box<T>) -> Arc<T> {
2404 #[cfg(not(no_global_oom_handling))]
2405 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2406 impl<T> From<Vec<T>> for Arc<[T]> {
2407 /// Allocate a reference-counted slice and move `v`'s items into it.
2412 /// # use std::sync::Arc;
2413 /// let unique: Vec<i32> = vec![1, 2, 3];
2414 /// let shared: Arc<[i32]> = Arc::from(unique);
2415 /// assert_eq!(&[1, 2, 3], &shared[..]);
2418 fn from(mut v: Vec<T>) -> Arc<[T]> {
2420 let arc = Arc::copy_from_slice(&v);
2422 // Allow the Vec to free its memory, but not destroy its contents
2430 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2431 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2433 B: ToOwned + ?Sized,
2434 Arc<B>: From<&'a B> + From<B::Owned>,
2436 /// Create an atomically reference-counted pointer from
2437 /// a clone-on-write pointer by copying its content.
2442 /// # use std::sync::Arc;
2443 /// # use std::borrow::Cow;
2444 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2445 /// let shared: Arc<str> = Arc::from(cow);
2446 /// assert_eq!("eggplant", &shared[..]);
2449 fn from(cow: Cow<'a, B>) -> Arc<B> {
2451 Cow::Borrowed(s) => Arc::from(s),
2452 Cow::Owned(s) => Arc::from(s),
2457 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2458 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2459 type Error = Arc<[T]>;
2461 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2462 if boxed_slice.len() == N {
2463 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2470 #[cfg(not(no_global_oom_handling))]
2471 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2472 impl<T> iter::FromIterator<T> for Arc<[T]> {
2473 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2475 /// # Performance characteristics
2477 /// ## The general case
2479 /// In the general case, collecting into `Arc<[T]>` is done by first
2480 /// collecting into a `Vec<T>`. That is, when writing the following:
2483 /// # use std::sync::Arc;
2484 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2485 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2488 /// this behaves as if we wrote:
2491 /// # use std::sync::Arc;
2492 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2493 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2494 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2495 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2498 /// This will allocate as many times as needed for constructing the `Vec<T>`
2499 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2501 /// ## Iterators of known length
2503 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2504 /// a single allocation will be made for the `Arc<[T]>`. For example:
2507 /// # use std::sync::Arc;
2508 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2509 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2511 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2512 ToArcSlice::to_arc_slice(iter.into_iter())
2516 /// Specialization trait used for collecting into `Arc<[T]>`.
2517 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2518 fn to_arc_slice(self) -> Arc<[T]>;
2521 #[cfg(not(no_global_oom_handling))]
2522 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2523 default fn to_arc_slice(self) -> Arc<[T]> {
2524 self.collect::<Vec<T>>().into()
2528 #[cfg(not(no_global_oom_handling))]
2529 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2530 fn to_arc_slice(self) -> Arc<[T]> {
2531 // This is the case for a `TrustedLen` iterator.
2532 let (low, high) = self.size_hint();
2533 if let Some(high) = high {
2537 "TrustedLen iterator's size hint is not exact: {:?}",
2542 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2543 Arc::from_iter_exact(self, low)
2546 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2547 // length exceeding `usize::MAX`.
2548 // The default implementation would collect into a vec which would panic.
2549 // Thus we panic here immediately without invoking `Vec` code.
2550 panic!("capacity overflow");
2555 #[stable(feature = "rust1", since = "1.0.0")]
2556 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2557 fn borrow(&self) -> &T {
2562 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2563 impl<T: ?Sized> AsRef<T> for Arc<T> {
2564 fn as_ref(&self) -> &T {
2569 #[stable(feature = "pin", since = "1.33.0")]
2570 impl<T: ?Sized> Unpin for Arc<T> {}
2572 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2576 /// The pointer must point to (and have valid metadata for) a previously
2577 /// valid instance of T, but the T is allowed to be dropped.
2578 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2579 // Align the unsized value to the end of the ArcInner.
2580 // Because RcBox is repr(C), it will always be the last field in memory.
2581 // SAFETY: since the only unsized types possible are slices, trait objects,
2582 // and extern types, the input safety requirement is currently enough to
2583 // satisfy the requirements of align_of_val_raw; this is an implementation
2584 // detail of the language that may not be relied upon outside of std.
2585 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2589 fn data_offset_align(align: usize) -> isize {
2590 let layout = Layout::new::<ArcInner<()>>();
2591 (layout.size() + layout.padding_needed_for(align)) as isize