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 use core::panic::{RefUnwindSafe, UnwindSafe};
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};
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 /// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
103 /// [`Send`], <code>Arc<[RefCell\<T>]></code> 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 /// let my_weak = 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 /// [`RefCell<T>`]: core::cell::RefCell
181 /// [`std::sync`]: ../../std/sync/index.html
182 /// [`Arc::clone(&from)`]: Arc::clone
183 /// [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
187 /// Sharing some immutable data between threads:
189 // Note that we **do not** run these tests here. The windows builders get super
190 // unhappy if a thread outlives the main thread and then exits at the same time
191 // (something deadlocks) so we just avoid this entirely by not running these
194 /// use std::sync::Arc;
197 /// let five = Arc::new(5);
200 /// let five = Arc::clone(&five);
202 /// thread::spawn(move || {
203 /// println!("{five:?}");
208 /// Sharing a mutable [`AtomicUsize`]:
210 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
213 /// use std::sync::Arc;
214 /// use std::sync::atomic::{AtomicUsize, Ordering};
217 /// let val = Arc::new(AtomicUsize::new(5));
220 /// let val = Arc::clone(&val);
222 /// thread::spawn(move || {
223 /// let v = val.fetch_add(1, Ordering::SeqCst);
224 /// println!("{v:?}");
229 /// See the [`rc` documentation][rc_examples] for more examples of reference
230 /// counting in general.
232 /// [rc_examples]: crate::rc#examples
233 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
234 #[stable(feature = "rust1", since = "1.0.0")]
235 pub struct Arc<T: ?Sized> {
236 ptr: NonNull<ArcInner<T>>,
237 phantom: PhantomData<ArcInner<T>>,
240 #[stable(feature = "rust1", since = "1.0.0")]
241 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
242 #[stable(feature = "rust1", since = "1.0.0")]
243 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
245 #[stable(feature = "catch_unwind", since = "1.9.0")]
246 impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}
248 #[unstable(feature = "coerce_unsized", issue = "27732")]
249 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
251 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
252 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
254 impl<T: ?Sized> Arc<T> {
255 unsafe fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
256 Self { ptr, phantom: PhantomData }
259 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
260 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
264 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
265 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
266 /// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
268 /// Since a `Weak` reference does not count towards ownership, it will not
269 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
270 /// guarantees about the value still being present. Thus it may return [`None`]
271 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
272 /// itself (the backing store) from being deallocated.
274 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
275 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
276 /// prevent circular references between [`Arc`] pointers, since mutual owning references
277 /// would never allow either [`Arc`] to be dropped. For example, a tree could
278 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
279 /// pointers from children back to their parents.
281 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
283 /// [`upgrade`]: Weak::upgrade
284 #[stable(feature = "arc_weak", since = "1.4.0")]
285 pub struct Weak<T: ?Sized> {
286 // This is a `NonNull` to allow optimizing the size of this type in enums,
287 // but it is not necessarily a valid pointer.
288 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
289 // to allocate space on the heap. That's not a value a real pointer
290 // will ever have because RcBox has alignment at least 2.
291 // This is only possible when `T: Sized`; unsized `T` never dangle.
292 ptr: NonNull<ArcInner<T>>,
295 #[stable(feature = "arc_weak", since = "1.4.0")]
296 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
297 #[stable(feature = "arc_weak", since = "1.4.0")]
298 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
300 #[unstable(feature = "coerce_unsized", issue = "27732")]
301 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
302 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
303 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
305 #[stable(feature = "arc_weak", since = "1.4.0")]
306 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
307 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
312 // This is repr(C) to future-proof against possible field-reordering, which
313 // would interfere with otherwise safe [into|from]_raw() of transmutable
316 struct ArcInner<T: ?Sized> {
317 strong: atomic::AtomicUsize,
319 // the value usize::MAX acts as a sentinel for temporarily "locking" the
320 // ability to upgrade weak pointers or downgrade strong ones; this is used
321 // to avoid races in `make_mut` and `get_mut`.
322 weak: atomic::AtomicUsize,
327 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
328 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
331 /// Constructs a new `Arc<T>`.
336 /// use std::sync::Arc;
338 /// let five = Arc::new(5);
340 #[cfg(not(no_global_oom_handling))]
342 #[stable(feature = "rust1", since = "1.0.0")]
343 pub fn new(data: T) -> Arc<T> {
344 // Start the weak pointer count as 1 which is the weak pointer that's
345 // held by all the strong pointers (kinda), see std/rc.rs for more info
346 let x: Box<_> = Box::new(ArcInner {
347 strong: atomic::AtomicUsize::new(1),
348 weak: atomic::AtomicUsize::new(1),
351 unsafe { Self::from_inner(Box::leak(x).into()) }
354 /// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
355 /// to allow you to construct a `T` which holds a weak pointer to itself.
357 /// Generally, a structure circularly referencing itself, either directly or
358 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
359 /// Using this function, you get access to the weak pointer during the
360 /// initialization of `T`, before the `Arc<T>` is created, such that you can
361 /// clone and store it inside the `T`.
363 /// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
364 /// then calls your closure, giving it a `Weak<T>` to this allocation,
365 /// and only afterwards completes the construction of the `Arc<T>` by placing
366 /// the `T` returned from your closure into the allocation.
368 /// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
369 /// returns, calling [`upgrade`] on the weak reference inside your closure will
370 /// fail and result in a `None` value.
374 /// If `data_fn` panics, the panic is propagated to the caller, and the
375 /// temporary [`Weak<T>`] is dropped normally.
380 /// # #![allow(dead_code)]
381 /// use std::sync::{Arc, Weak};
384 /// me: Weak<Gadget>,
388 /// /// Construct a reference counted Gadget.
389 /// fn new() -> Arc<Self> {
390 /// // `me` is a `Weak<Gadget>` pointing at the new allocation of the
391 /// // `Arc` we're constructing.
392 /// Arc::new_cyclic(|me| {
393 /// // Create the actual struct here.
394 /// Gadget { me: me.clone() }
398 /// /// Return a reference counted pointer to Self.
399 /// fn me(&self) -> Arc<Self> {
400 /// self.me.upgrade().unwrap()
404 /// [`upgrade`]: Weak::upgrade
405 #[cfg(not(no_global_oom_handling))]
407 #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
408 pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
410 F: FnOnce(&Weak<T>) -> T,
412 // Construct the inner in the "uninitialized" state with a single
414 let uninit_ptr: NonNull<_> = Box::leak(Box::new(ArcInner {
415 strong: atomic::AtomicUsize::new(0),
416 weak: atomic::AtomicUsize::new(1),
417 data: mem::MaybeUninit::<T>::uninit(),
420 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
422 let weak = Weak { ptr: init_ptr };
424 // It's important we don't give up ownership of the weak pointer, or
425 // else the memory might be freed by the time `data_fn` returns. If
426 // we really wanted to pass ownership, we could create an additional
427 // weak pointer for ourselves, but this would result in additional
428 // updates to the weak reference count which might not be necessary
430 let data = data_fn(&weak);
432 // Now we can properly initialize the inner value and turn our weak
433 // reference into a strong reference.
434 let strong = unsafe {
435 let inner = init_ptr.as_ptr();
436 ptr::write(ptr::addr_of_mut!((*inner).data), data);
438 // The above write to the data field must be visible to any threads which
439 // observe a non-zero strong count. Therefore we need at least "Release" ordering
440 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
442 // "Acquire" ordering is not required. When considering the possible behaviours
443 // of `data_fn` we only need to look at what it could do with a reference to a
444 // non-upgradeable `Weak`:
445 // - It can *clone* the `Weak`, increasing the weak reference count.
446 // - It can drop those clones, decreasing the weak reference count (but never to zero).
448 // These side effects do not impact us in any way, and no other side effects are
449 // possible with safe code alone.
450 let prev_value = (*inner).strong.fetch_add(1, Release);
451 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
453 Arc::from_inner(init_ptr)
456 // Strong references should collectively own a shared weak reference,
457 // so don't run the destructor for our old weak reference.
462 /// Constructs a new `Arc` with uninitialized contents.
467 /// #![feature(new_uninit)]
468 /// #![feature(get_mut_unchecked)]
470 /// use std::sync::Arc;
472 /// let mut five = Arc::<u32>::new_uninit();
474 /// // Deferred initialization:
475 /// Arc::get_mut(&mut five).unwrap().write(5);
477 /// let five = unsafe { five.assume_init() };
479 /// assert_eq!(*five, 5)
481 #[cfg(not(no_global_oom_handling))]
482 #[unstable(feature = "new_uninit", issue = "63291")]
484 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
486 Arc::from_ptr(Arc::allocate_for_layout(
488 |layout| Global.allocate(layout),
489 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
494 /// Constructs a new `Arc` with uninitialized contents, with the memory
495 /// being filled with `0` bytes.
497 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
503 /// #![feature(new_uninit)]
505 /// use std::sync::Arc;
507 /// let zero = Arc::<u32>::new_zeroed();
508 /// let zero = unsafe { zero.assume_init() };
510 /// assert_eq!(*zero, 0)
513 /// [zeroed]: mem::MaybeUninit::zeroed
514 #[cfg(not(no_global_oom_handling))]
515 #[unstable(feature = "new_uninit", issue = "63291")]
517 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
519 Arc::from_ptr(Arc::allocate_for_layout(
521 |layout| Global.allocate_zeroed(layout),
522 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
527 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
528 /// `data` will be pinned in memory and unable to be moved.
529 #[cfg(not(no_global_oom_handling))]
530 #[stable(feature = "pin", since = "1.33.0")]
532 pub fn pin(data: T) -> Pin<Arc<T>> {
533 unsafe { Pin::new_unchecked(Arc::new(data)) }
536 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
537 #[unstable(feature = "allocator_api", issue = "32838")]
539 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
540 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
543 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
548 /// #![feature(allocator_api)]
549 /// use std::sync::Arc;
551 /// let five = Arc::try_new(5)?;
552 /// # Ok::<(), std::alloc::AllocError>(())
554 #[unstable(feature = "allocator_api", issue = "32838")]
556 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
557 // Start the weak pointer count as 1 which is the weak pointer that's
558 // held by all the strong pointers (kinda), see std/rc.rs for more info
559 let x: Box<_> = Box::try_new(ArcInner {
560 strong: atomic::AtomicUsize::new(1),
561 weak: atomic::AtomicUsize::new(1),
564 unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
567 /// Constructs a new `Arc` with uninitialized contents, returning an error
568 /// if allocation fails.
573 /// #![feature(new_uninit, allocator_api)]
574 /// #![feature(get_mut_unchecked)]
576 /// use std::sync::Arc;
578 /// let mut five = Arc::<u32>::try_new_uninit()?;
580 /// // Deferred initialization:
581 /// Arc::get_mut(&mut five).unwrap().write(5);
583 /// let five = unsafe { five.assume_init() };
585 /// assert_eq!(*five, 5);
586 /// # Ok::<(), std::alloc::AllocError>(())
588 #[unstable(feature = "allocator_api", issue = "32838")]
589 // #[unstable(feature = "new_uninit", issue = "63291")]
590 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
592 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
594 |layout| Global.allocate(layout),
595 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
600 /// Constructs a new `Arc` with uninitialized contents, with the memory
601 /// being filled with `0` bytes, returning an error if allocation fails.
603 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
609 /// #![feature(new_uninit, allocator_api)]
611 /// use std::sync::Arc;
613 /// let zero = Arc::<u32>::try_new_zeroed()?;
614 /// let zero = unsafe { zero.assume_init() };
616 /// assert_eq!(*zero, 0);
617 /// # Ok::<(), std::alloc::AllocError>(())
620 /// [zeroed]: mem::MaybeUninit::zeroed
621 #[unstable(feature = "allocator_api", issue = "32838")]
622 // #[unstable(feature = "new_uninit", issue = "63291")]
623 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
625 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
627 |layout| Global.allocate_zeroed(layout),
628 |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
632 /// Returns the inner value, if the `Arc` has exactly one strong reference.
634 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
637 /// This will succeed even if there are outstanding weak references.
642 /// use std::sync::Arc;
644 /// let x = Arc::new(3);
645 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
647 /// let x = Arc::new(4);
648 /// let _y = Arc::clone(&x);
649 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
652 #[stable(feature = "arc_unique", since = "1.4.0")]
653 pub fn try_unwrap(this: Self) -> Result<T, Self> {
654 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
658 acquire!(this.inner().strong);
661 let elem = ptr::read(&this.ptr.as_ref().data);
663 // Make a weak pointer to clean up the implicit strong-weak reference
664 let _weak = Weak { ptr: this.ptr };
673 /// Constructs a new atomically reference-counted slice with uninitialized contents.
678 /// #![feature(new_uninit)]
679 /// #![feature(get_mut_unchecked)]
681 /// use std::sync::Arc;
683 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
685 /// // Deferred initialization:
686 /// let data = Arc::get_mut(&mut values).unwrap();
687 /// data[0].write(1);
688 /// data[1].write(2);
689 /// data[2].write(3);
691 /// let values = unsafe { values.assume_init() };
693 /// assert_eq!(*values, [1, 2, 3])
695 #[cfg(not(no_global_oom_handling))]
696 #[unstable(feature = "new_uninit", issue = "63291")]
698 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
699 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
702 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
703 /// filled with `0` bytes.
705 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
706 /// incorrect usage of this method.
711 /// #![feature(new_uninit)]
713 /// use std::sync::Arc;
715 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
716 /// let values = unsafe { values.assume_init() };
718 /// assert_eq!(*values, [0, 0, 0])
721 /// [zeroed]: mem::MaybeUninit::zeroed
722 #[cfg(not(no_global_oom_handling))]
723 #[unstable(feature = "new_uninit", issue = "63291")]
725 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
727 Arc::from_ptr(Arc::allocate_for_layout(
728 Layout::array::<T>(len).unwrap(),
729 |layout| Global.allocate_zeroed(layout),
731 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
732 as *mut ArcInner<[mem::MaybeUninit<T>]>
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`]: mem::MaybeUninit::assume_init
755 /// #![feature(new_uninit)]
756 /// #![feature(get_mut_unchecked)]
758 /// use std::sync::Arc;
760 /// let mut five = Arc::<u32>::new_uninit();
762 /// // Deferred initialization:
763 /// Arc::get_mut(&mut five).unwrap().write(5);
765 /// let five = unsafe { five.assume_init() };
767 /// assert_eq!(*five, 5)
769 #[unstable(feature = "new_uninit", issue = "63291")]
770 #[must_use = "`self` will be dropped if the result is not used"]
772 pub unsafe fn assume_init(self) -> Arc<T> {
773 unsafe { Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast()) }
777 impl<T> Arc<[mem::MaybeUninit<T>]> {
778 /// Converts to `Arc<[T]>`.
782 /// As with [`MaybeUninit::assume_init`],
783 /// it is up to the caller to guarantee that the inner value
784 /// really is in an initialized state.
785 /// Calling this when the content is not yet fully initialized
786 /// causes immediate undefined behavior.
788 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
793 /// #![feature(new_uninit)]
794 /// #![feature(get_mut_unchecked)]
796 /// use std::sync::Arc;
798 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
800 /// // Deferred initialization:
801 /// let data = Arc::get_mut(&mut values).unwrap();
802 /// data[0].write(1);
803 /// data[1].write(2);
804 /// data[2].write(3);
806 /// let values = unsafe { values.assume_init() };
808 /// assert_eq!(*values, [1, 2, 3])
810 #[unstable(feature = "new_uninit", issue = "63291")]
811 #[must_use = "`self` will be dropped if the result is not used"]
813 pub unsafe fn assume_init(self) -> Arc<[T]> {
814 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
818 impl<T: ?Sized> Arc<T> {
819 /// Consumes the `Arc`, returning the wrapped pointer.
821 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
822 /// [`Arc::from_raw`].
827 /// use std::sync::Arc;
829 /// let x = Arc::new("hello".to_owned());
830 /// let x_ptr = Arc::into_raw(x);
831 /// assert_eq!(unsafe { &*x_ptr }, "hello");
833 #[must_use = "losing the pointer will leak memory"]
834 #[stable(feature = "rc_raw", since = "1.17.0")]
835 pub fn into_raw(this: Self) -> *const T {
836 let ptr = Self::as_ptr(&this);
841 /// Provides a raw pointer to the data.
843 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
844 /// as long as there are strong counts in the `Arc`.
849 /// use std::sync::Arc;
851 /// let x = Arc::new("hello".to_owned());
852 /// let y = Arc::clone(&x);
853 /// let x_ptr = Arc::as_ptr(&x);
854 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
855 /// assert_eq!(unsafe { &*x_ptr }, "hello");
858 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
859 pub fn as_ptr(this: &Self) -> *const T {
860 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
862 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
863 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
864 // write through the pointer after the Rc is recovered through `from_raw`.
865 unsafe { ptr::addr_of_mut!((*ptr).data) }
868 /// Constructs an `Arc<T>` from a raw pointer.
870 /// The raw pointer must have been previously returned by a call to
871 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
872 /// alignment as `T`. This is trivially true if `U` is `T`.
873 /// Note that if `U` is not `T` but has the same size and alignment, this is
874 /// basically like transmuting references of different types. See
875 /// [`mem::transmute`][transmute] for more information on what
876 /// restrictions apply in this case.
878 /// The user of `from_raw` has to make sure a specific value of `T` is only
881 /// This function is unsafe because improper use may lead to memory unsafety,
882 /// even if the returned `Arc<T>` is never accessed.
884 /// [into_raw]: Arc::into_raw
885 /// [transmute]: core::mem::transmute
890 /// use std::sync::Arc;
892 /// let x = Arc::new("hello".to_owned());
893 /// let x_ptr = Arc::into_raw(x);
896 /// // Convert back to an `Arc` to prevent leak.
897 /// let x = Arc::from_raw(x_ptr);
898 /// assert_eq!(&*x, "hello");
900 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
903 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
905 #[stable(feature = "rc_raw", since = "1.17.0")]
906 pub unsafe fn from_raw(ptr: *const T) -> Self {
908 let offset = data_offset(ptr);
910 // Reverse the offset to find the original ArcInner.
912 (ptr as *mut u8).offset(-offset).with_metadata_of(ptr as *mut ArcInner<T>);
914 Self::from_ptr(arc_ptr)
918 /// Creates a new [`Weak`] pointer to this allocation.
923 /// use std::sync::Arc;
925 /// let five = Arc::new(5);
927 /// let weak_five = Arc::downgrade(&five);
929 #[must_use = "this returns a new `Weak` pointer, \
930 without modifying the original `Arc`"]
931 #[stable(feature = "arc_weak", since = "1.4.0")]
932 pub fn downgrade(this: &Self) -> Weak<T> {
933 // This Relaxed is OK because we're checking the value in the CAS
935 let mut cur = this.inner().weak.load(Relaxed);
938 // check if the weak counter is currently "locked"; if so, spin.
939 if cur == usize::MAX {
941 cur = this.inner().weak.load(Relaxed);
945 // NOTE: this code currently ignores the possibility of overflow
946 // into usize::MAX; in general both Rc and Arc need to be adjusted
947 // to deal with overflow.
949 // Unlike with Clone(), we need this to be an Acquire read to
950 // synchronize with the write coming from `is_unique`, so that the
951 // events prior to that write happen before this read.
952 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
954 // Make sure we do not create a dangling Weak
955 debug_assert!(!is_dangling(this.ptr.as_ptr()));
956 return Weak { ptr: this.ptr };
958 Err(old) => cur = old,
963 /// Gets the number of [`Weak`] pointers to this allocation.
967 /// This method by itself is safe, but using it correctly requires extra care.
968 /// Another thread can change the weak count at any time,
969 /// including potentially between calling this method and acting on the result.
974 /// use std::sync::Arc;
976 /// let five = Arc::new(5);
977 /// let _weak_five = Arc::downgrade(&five);
979 /// // This assertion is deterministic because we haven't shared
980 /// // the `Arc` or `Weak` between threads.
981 /// assert_eq!(1, Arc::weak_count(&five));
985 #[stable(feature = "arc_counts", since = "1.15.0")]
986 pub fn weak_count(this: &Self) -> usize {
987 let cnt = this.inner().weak.load(Acquire);
988 // If the weak count is currently locked, the value of the
989 // count was 0 just before taking the lock.
990 if cnt == usize::MAX { 0 } else { cnt - 1 }
993 /// Gets the number of strong (`Arc`) pointers to this allocation.
997 /// This method by itself is safe, but using it correctly requires extra care.
998 /// Another thread can change the strong count at any time,
999 /// including potentially between calling this method and acting on the result.
1004 /// use std::sync::Arc;
1006 /// let five = Arc::new(5);
1007 /// let _also_five = Arc::clone(&five);
1009 /// // This assertion is deterministic because we haven't shared
1010 /// // the `Arc` between threads.
1011 /// assert_eq!(2, Arc::strong_count(&five));
1015 #[stable(feature = "arc_counts", since = "1.15.0")]
1016 pub fn strong_count(this: &Self) -> usize {
1017 this.inner().strong.load(Acquire)
1020 /// Increments the strong reference count on the `Arc<T>` associated with the
1021 /// provided pointer by one.
1025 /// The pointer must have been obtained through `Arc::into_raw`, and the
1026 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1027 /// least 1) for the duration of this method.
1032 /// use std::sync::Arc;
1034 /// let five = Arc::new(5);
1037 /// let ptr = Arc::into_raw(five);
1038 /// Arc::increment_strong_count(ptr);
1040 /// // This assertion is deterministic because we haven't shared
1041 /// // the `Arc` between threads.
1042 /// let five = Arc::from_raw(ptr);
1043 /// assert_eq!(2, Arc::strong_count(&five));
1047 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1048 pub unsafe fn increment_strong_count(ptr: *const T) {
1049 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1050 let arc = unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) };
1051 // Now increase refcount, but don't drop new refcount either
1052 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1055 /// Decrements the strong reference count on the `Arc<T>` associated with the
1056 /// provided pointer by one.
1060 /// The pointer must have been obtained through `Arc::into_raw`, and the
1061 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1062 /// least 1) when invoking this method. This method can be used to release the final
1063 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1069 /// use std::sync::Arc;
1071 /// let five = Arc::new(5);
1074 /// let ptr = Arc::into_raw(five);
1075 /// Arc::increment_strong_count(ptr);
1077 /// // Those assertions are deterministic because we haven't shared
1078 /// // the `Arc` between threads.
1079 /// let five = Arc::from_raw(ptr);
1080 /// assert_eq!(2, Arc::strong_count(&five));
1081 /// Arc::decrement_strong_count(ptr);
1082 /// assert_eq!(1, Arc::strong_count(&five));
1086 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1087 pub unsafe fn decrement_strong_count(ptr: *const T) {
1088 unsafe { mem::drop(Arc::from_raw(ptr)) };
1092 fn inner(&self) -> &ArcInner<T> {
1093 // This unsafety is ok because while this arc is alive we're guaranteed
1094 // that the inner pointer is valid. Furthermore, we know that the
1095 // `ArcInner` structure itself is `Sync` because the inner data is
1096 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1098 unsafe { self.ptr.as_ref() }
1101 // Non-inlined part of `drop`.
1103 unsafe fn drop_slow(&mut self) {
1104 // Destroy the data at this time, even though we must not free the box
1105 // allocation itself (there might still be weak pointers lying around).
1106 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1108 // Drop the weak ref collectively held by all strong references
1109 drop(Weak { ptr: self.ptr });
1112 /// Returns `true` if the two `Arc`s point to the same allocation
1113 /// (in a vein similar to [`ptr::eq`]).
1118 /// use std::sync::Arc;
1120 /// let five = Arc::new(5);
1121 /// let same_five = Arc::clone(&five);
1122 /// let other_five = Arc::new(5);
1124 /// assert!(Arc::ptr_eq(&five, &same_five));
1125 /// assert!(!Arc::ptr_eq(&five, &other_five));
1128 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1131 #[stable(feature = "ptr_eq", since = "1.17.0")]
1132 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1133 this.ptr.as_ptr() == other.ptr.as_ptr()
1137 impl<T: ?Sized> Arc<T> {
1138 /// Allocates an `ArcInner<T>` with sufficient space for
1139 /// a possibly-unsized inner value where the value has the layout provided.
1141 /// The function `mem_to_arcinner` is called with the data pointer
1142 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1143 #[cfg(not(no_global_oom_handling))]
1144 unsafe fn allocate_for_layout(
1145 value_layout: Layout,
1146 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1147 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1148 ) -> *mut ArcInner<T> {
1149 // Calculate layout using the given value layout.
1150 // Previously, layout was calculated on the expression
1151 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1152 // reference (see #54908).
1153 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1155 Arc::try_allocate_for_layout(value_layout, allocate, mem_to_arcinner)
1156 .unwrap_or_else(|_| handle_alloc_error(layout))
1160 /// Allocates an `ArcInner<T>` with sufficient space for
1161 /// a possibly-unsized inner value where the value has the layout provided,
1162 /// returning an error if allocation fails.
1164 /// The function `mem_to_arcinner` is called with the data pointer
1165 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1166 unsafe fn try_allocate_for_layout(
1167 value_layout: Layout,
1168 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1169 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1170 ) -> Result<*mut ArcInner<T>, AllocError> {
1171 // Calculate layout using the given value layout.
1172 // Previously, layout was calculated on the expression
1173 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
1174 // reference (see #54908).
1175 let layout = Layout::new::<ArcInner<()>>().extend(value_layout).unwrap().0.pad_to_align();
1177 let ptr = allocate(layout)?;
1179 // Initialize the ArcInner
1180 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1181 debug_assert_eq!(unsafe { Layout::for_value(&*inner) }, layout);
1184 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
1185 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
1191 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1192 #[cfg(not(no_global_oom_handling))]
1193 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
1194 // Allocate for the `ArcInner<T>` using the given value.
1196 Self::allocate_for_layout(
1197 Layout::for_value(&*ptr),
1198 |layout| Global.allocate(layout),
1199 |mem| mem.with_metadata_of(ptr as *mut ArcInner<T>),
1204 #[cfg(not(no_global_oom_handling))]
1205 fn from_box(v: Box<T>) -> Arc<T> {
1207 let (box_unique, alloc) = Box::into_unique(v);
1208 let bptr = box_unique.as_ptr();
1210 let value_size = size_of_val(&*bptr);
1211 let ptr = Self::allocate_for_ptr(bptr);
1213 // Copy value as bytes
1214 ptr::copy_nonoverlapping(
1215 bptr as *const T as *const u8,
1216 &mut (*ptr).data as *mut _ as *mut u8,
1220 // Free the allocation without dropping its contents
1221 box_free(box_unique, alloc);
1229 /// Allocates an `ArcInner<[T]>` with the given length.
1230 #[cfg(not(no_global_oom_handling))]
1231 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
1233 Self::allocate_for_layout(
1234 Layout::array::<T>(len).unwrap(),
1235 |layout| Global.allocate(layout),
1236 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
1241 /// Copy elements from slice into newly allocated Arc<\[T\]>
1243 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1244 #[cfg(not(no_global_oom_handling))]
1245 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
1247 let ptr = Self::allocate_for_slice(v.len());
1249 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).data as *mut [T] as *mut T, v.len());
1255 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1257 /// Behavior is undefined should the size be wrong.
1258 #[cfg(not(no_global_oom_handling))]
1259 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
1260 // Panic guard while cloning T elements.
1261 // In the event of a panic, elements that have been written
1262 // into the new ArcInner will be dropped, then the memory freed.
1270 impl<T> Drop for Guard<T> {
1271 fn drop(&mut self) {
1273 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1274 ptr::drop_in_place(slice);
1276 Global.deallocate(self.mem, self.layout);
1282 let ptr = Self::allocate_for_slice(len);
1284 let mem = ptr as *mut _ as *mut u8;
1285 let layout = Layout::for_value(&*ptr);
1287 // Pointer to first element
1288 let elems = &mut (*ptr).data as *mut [T] as *mut T;
1290 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1292 for (i, item) in iter.enumerate() {
1293 ptr::write(elems.add(i), item);
1297 // All clear. Forget the guard so it doesn't free the new ArcInner.
1305 /// Specialization trait used for `From<&[T]>`.
1306 #[cfg(not(no_global_oom_handling))]
1307 trait ArcFromSlice<T> {
1308 fn from_slice(slice: &[T]) -> Self;
1311 #[cfg(not(no_global_oom_handling))]
1312 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
1314 default fn from_slice(v: &[T]) -> Self {
1315 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1319 #[cfg(not(no_global_oom_handling))]
1320 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
1322 fn from_slice(v: &[T]) -> Self {
1323 unsafe { Arc::copy_from_slice(v) }
1327 #[stable(feature = "rust1", since = "1.0.0")]
1328 impl<T: ?Sized> Clone for Arc<T> {
1329 /// Makes a clone of the `Arc` pointer.
1331 /// This creates another pointer to the same allocation, increasing the
1332 /// strong reference count.
1337 /// use std::sync::Arc;
1339 /// let five = Arc::new(5);
1341 /// let _ = Arc::clone(&five);
1344 fn clone(&self) -> Arc<T> {
1345 // Using a relaxed ordering is alright here, as knowledge of the
1346 // original reference prevents other threads from erroneously deleting
1349 // As explained in the [Boost documentation][1], Increasing the
1350 // reference counter can always be done with memory_order_relaxed: New
1351 // references to an object can only be formed from an existing
1352 // reference, and passing an existing reference from one thread to
1353 // another must already provide any required synchronization.
1355 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1356 let old_size = self.inner().strong.fetch_add(1, Relaxed);
1358 // However we need to guard against massive refcounts in case someone is `mem::forget`ing
1359 // Arcs. If we don't do this the count can overflow and users will use-after free. This
1360 // branch will never be taken in any realistic program. We abort because such a program is
1361 // incredibly degenerate, and we don't care to support it.
1363 // This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
1364 // But we do that check *after* having done the increment, so there is a chance here that
1365 // the worst already happened and we actually do overflow the `usize` counter. However, that
1366 // requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
1367 // above and the `abort` below, which seems exceedingly unlikely.
1368 if old_size > MAX_REFCOUNT {
1372 unsafe { Self::from_inner(self.ptr) }
1376 #[stable(feature = "rust1", since = "1.0.0")]
1377 impl<T: ?Sized> Deref for Arc<T> {
1381 fn deref(&self) -> &T {
1386 #[unstable(feature = "receiver_trait", issue = "none")]
1387 impl<T: ?Sized> Receiver for Arc<T> {}
1389 impl<T: Clone> Arc<T> {
1390 /// Makes a mutable reference into the given `Arc`.
1392 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
1393 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1394 /// referred to as clone-on-write.
1396 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
1397 /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
1400 /// See also [`get_mut`], which will fail rather than cloning the inner value
1401 /// or dissociating [`Weak`] pointers.
1403 /// [`clone`]: Clone::clone
1404 /// [`get_mut`]: Arc::get_mut
1409 /// use std::sync::Arc;
1411 /// let mut data = Arc::new(5);
1413 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1414 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1415 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1416 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1417 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1419 /// // Now `data` and `other_data` point to different allocations.
1420 /// assert_eq!(*data, 8);
1421 /// assert_eq!(*other_data, 12);
1424 /// [`Weak`] pointers will be dissociated:
1427 /// use std::sync::Arc;
1429 /// let mut data = Arc::new(75);
1430 /// let weak = Arc::downgrade(&data);
1432 /// assert!(75 == *data);
1433 /// assert!(75 == *weak.upgrade().unwrap());
1435 /// *Arc::make_mut(&mut data) += 1;
1437 /// assert!(76 == *data);
1438 /// assert!(weak.upgrade().is_none());
1440 #[cfg(not(no_global_oom_handling))]
1442 #[stable(feature = "arc_unique", since = "1.4.0")]
1443 pub fn make_mut(this: &mut Self) -> &mut T {
1444 // Note that we hold both a strong reference and a weak reference.
1445 // Thus, releasing our strong reference only will not, by itself, cause
1446 // the memory to be deallocated.
1448 // Use Acquire to ensure that we see any writes to `weak` that happen
1449 // before release writes (i.e., decrements) to `strong`. Since we hold a
1450 // weak count, there's no chance the ArcInner itself could be
1452 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
1453 // Another strong pointer exists, so we must clone.
1454 // Pre-allocate memory to allow writing the cloned value directly.
1455 let mut arc = Self::new_uninit();
1457 let data = Arc::get_mut_unchecked(&mut arc);
1458 (**this).write_clone_into_raw(data.as_mut_ptr());
1459 *this = arc.assume_init();
1461 } else if this.inner().weak.load(Relaxed) != 1 {
1462 // Relaxed suffices in the above because this is fundamentally an
1463 // optimization: we are always racing with weak pointers being
1464 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1466 // We removed the last strong ref, but there are additional weak
1467 // refs remaining. We'll move the contents to a new Arc, and
1468 // invalidate the other weak refs.
1470 // Note that it is not possible for the read of `weak` to yield
1471 // usize::MAX (i.e., locked), since the weak count can only be
1472 // locked by a thread with a strong reference.
1474 // Materialize our own implicit weak pointer, so that it can clean
1475 // up the ArcInner as needed.
1476 let _weak = Weak { ptr: this.ptr };
1478 // Can just steal the data, all that's left is Weaks
1479 let mut arc = Self::new_uninit();
1481 let data = Arc::get_mut_unchecked(&mut arc);
1482 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1483 ptr::write(this, arc.assume_init());
1486 // We were the sole reference of either kind; bump back up the
1487 // strong ref count.
1488 this.inner().strong.store(1, Release);
1491 // As with `get_mut()`, the unsafety is ok because our reference was
1492 // either unique to begin with, or became one upon cloning the contents.
1493 unsafe { Self::get_mut_unchecked(this) }
1496 /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
1499 /// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
1500 /// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
1505 /// #![feature(arc_unwrap_or_clone)]
1506 /// # use std::{ptr, sync::Arc};
1507 /// let inner = String::from("test");
1508 /// let ptr = inner.as_ptr();
1510 /// let arc = Arc::new(inner);
1511 /// let inner = Arc::unwrap_or_clone(arc);
1512 /// // The inner value was not cloned
1513 /// assert!(ptr::eq(ptr, inner.as_ptr()));
1515 /// let arc = Arc::new(inner);
1516 /// let arc2 = arc.clone();
1517 /// let inner = Arc::unwrap_or_clone(arc);
1518 /// // Because there were 2 references, we had to clone the inner value.
1519 /// assert!(!ptr::eq(ptr, inner.as_ptr()));
1520 /// // `arc2` is the last reference, so when we unwrap it we get back
1521 /// // the original `String`.
1522 /// let inner = Arc::unwrap_or_clone(arc2);
1523 /// assert!(ptr::eq(ptr, inner.as_ptr()));
1526 #[unstable(feature = "arc_unwrap_or_clone", issue = "93610")]
1527 pub fn unwrap_or_clone(this: Self) -> T {
1528 Arc::try_unwrap(this).unwrap_or_else(|arc| (*arc).clone())
1532 impl<T: ?Sized> Arc<T> {
1533 /// Returns a mutable reference into the given `Arc`, if there are
1534 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1536 /// Returns [`None`] otherwise, because it is not safe to
1537 /// mutate a shared value.
1539 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1540 /// the inner value when there are other `Arc` pointers.
1542 /// [make_mut]: Arc::make_mut
1543 /// [clone]: Clone::clone
1548 /// use std::sync::Arc;
1550 /// let mut x = Arc::new(3);
1551 /// *Arc::get_mut(&mut x).unwrap() = 4;
1552 /// assert_eq!(*x, 4);
1554 /// let _y = Arc::clone(&x);
1555 /// assert!(Arc::get_mut(&mut x).is_none());
1558 #[stable(feature = "arc_unique", since = "1.4.0")]
1559 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1560 if this.is_unique() {
1561 // This unsafety is ok because we're guaranteed that the pointer
1562 // returned is the *only* pointer that will ever be returned to T. Our
1563 // reference count is guaranteed to be 1 at this point, and we required
1564 // the Arc itself to be `mut`, so we're returning the only possible
1565 // reference to the inner data.
1566 unsafe { Some(Arc::get_mut_unchecked(this)) }
1572 /// Returns a mutable reference into the given `Arc`,
1573 /// without any check.
1575 /// See also [`get_mut`], which is safe and does appropriate checks.
1577 /// [`get_mut`]: Arc::get_mut
1581 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1582 /// for the duration of the returned borrow.
1583 /// This is trivially the case if no such pointers exist,
1584 /// for example immediately after `Arc::new`.
1589 /// #![feature(get_mut_unchecked)]
1591 /// use std::sync::Arc;
1593 /// let mut x = Arc::new(String::new());
1595 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1597 /// assert_eq!(*x, "foo");
1600 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1601 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1602 // We are careful to *not* create a reference covering the "count" fields, as
1603 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1604 unsafe { &mut (*this.ptr.as_ptr()).data }
1607 /// Determine whether this is the unique reference (including weak refs) to
1608 /// the underlying data.
1610 /// Note that this requires locking the weak ref count.
1611 fn is_unique(&mut self) -> bool {
1612 // lock the weak pointer count if we appear to be the sole weak pointer
1615 // The acquire label here ensures a happens-before relationship with any
1616 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1617 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1618 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1619 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
1620 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1621 // counter in `drop` -- the only access that happens when any but the last reference
1622 // is being dropped.
1623 let unique = self.inner().strong.load(Acquire) == 1;
1625 // The release write here synchronizes with a read in `downgrade`,
1626 // effectively preventing the above read of `strong` from happening
1628 self.inner().weak.store(1, Release); // release the lock
1636 #[stable(feature = "rust1", since = "1.0.0")]
1637 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1638 /// Drops the `Arc`.
1640 /// This will decrement the strong reference count. If the strong reference
1641 /// count reaches zero then the only other references (if any) are
1642 /// [`Weak`], so we `drop` the inner value.
1647 /// use std::sync::Arc;
1651 /// impl Drop for Foo {
1652 /// fn drop(&mut self) {
1653 /// println!("dropped!");
1657 /// let foo = Arc::new(Foo);
1658 /// let foo2 = Arc::clone(&foo);
1660 /// drop(foo); // Doesn't print anything
1661 /// drop(foo2); // Prints "dropped!"
1664 fn drop(&mut self) {
1665 // Because `fetch_sub` is already atomic, we do not need to synchronize
1666 // with other threads unless we are going to delete the object. This
1667 // same logic applies to the below `fetch_sub` to the `weak` count.
1668 if self.inner().strong.fetch_sub(1, Release) != 1 {
1672 // This fence is needed to prevent reordering of use of the data and
1673 // deletion of the data. Because it is marked `Release`, the decreasing
1674 // of the reference count synchronizes with this `Acquire` fence. This
1675 // means that use of the data happens before decreasing the reference
1676 // count, which happens before this fence, which happens before the
1677 // deletion of the data.
1679 // As explained in the [Boost documentation][1],
1681 // > It is important to enforce any possible access to the object in one
1682 // > thread (through an existing reference) to *happen before* deleting
1683 // > the object in a different thread. This is achieved by a "release"
1684 // > operation after dropping a reference (any access to the object
1685 // > through this reference must obviously happened before), and an
1686 // > "acquire" operation before deleting the object.
1688 // In particular, while the contents of an Arc are usually immutable, it's
1689 // possible to have interior writes to something like a Mutex<T>. Since a
1690 // Mutex is not acquired when it is deleted, we can't rely on its
1691 // synchronization logic to make writes in thread A visible to a destructor
1692 // running in thread B.
1694 // Also note that the Acquire fence here could probably be replaced with an
1695 // Acquire load, which could improve performance in highly-contended
1696 // situations. See [2].
1698 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1699 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1700 acquire!(self.inner().strong);
1708 impl Arc<dyn Any + Send + Sync> {
1709 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1714 /// use std::any::Any;
1715 /// use std::sync::Arc;
1717 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1718 /// if let Ok(string) = value.downcast::<String>() {
1719 /// println!("String ({}): {}", string.len(), string);
1723 /// let my_string = "Hello World".to_string();
1724 /// print_if_string(Arc::new(my_string));
1725 /// print_if_string(Arc::new(0i8));
1728 #[stable(feature = "rc_downcast", since = "1.29.0")]
1729 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1731 T: Any + Send + Sync,
1733 if (*self).is::<T>() {
1735 let ptr = self.ptr.cast::<ArcInner<T>>();
1737 Ok(Arc::from_inner(ptr))
1744 /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
1746 /// For a safe alternative see [`downcast`].
1751 /// #![feature(downcast_unchecked)]
1753 /// use std::any::Any;
1754 /// use std::sync::Arc;
1756 /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
1759 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1765 /// The contained value must be of type `T`. Calling this method
1766 /// with the incorrect type is *undefined behavior*.
1769 /// [`downcast`]: Self::downcast
1771 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1772 pub unsafe fn downcast_unchecked<T>(self) -> Arc<T>
1774 T: Any + Send + Sync,
1777 let ptr = self.ptr.cast::<ArcInner<T>>();
1779 Arc::from_inner(ptr)
1785 /// Constructs a new `Weak<T>`, without allocating any memory.
1786 /// Calling [`upgrade`] on the return value always gives [`None`].
1788 /// [`upgrade`]: Weak::upgrade
1793 /// use std::sync::Weak;
1795 /// let empty: Weak<i64> = Weak::new();
1796 /// assert!(empty.upgrade().is_none());
1798 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1799 #[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
1801 pub const fn new() -> Weak<T> {
1802 Weak { ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<ArcInner<T>>(usize::MAX)) } }
1806 /// Helper type to allow accessing the reference counts without
1807 /// making any assertions about the data field.
1808 struct WeakInner<'a> {
1809 weak: &'a atomic::AtomicUsize,
1810 strong: &'a atomic::AtomicUsize,
1813 impl<T: ?Sized> Weak<T> {
1814 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1816 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1817 /// unaligned or even [`null`] otherwise.
1822 /// use std::sync::Arc;
1825 /// let strong = Arc::new("hello".to_owned());
1826 /// let weak = Arc::downgrade(&strong);
1827 /// // Both point to the same object
1828 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1829 /// // The strong here keeps it alive, so we can still access the object.
1830 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1833 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1834 /// // undefined behaviour.
1835 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1838 /// [`null`]: core::ptr::null "ptr::null"
1840 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1841 pub fn as_ptr(&self) -> *const T {
1842 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
1844 if is_dangling(ptr) {
1845 // If the pointer is dangling, we return the sentinel directly. This cannot be
1846 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
1849 // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
1850 // The payload may be dropped at this point, and we have to maintain provenance,
1851 // so use raw pointer manipulation.
1852 unsafe { ptr::addr_of_mut!((*ptr).data) }
1856 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1858 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1859 /// one weak reference (the weak count is not modified by this operation). It can be turned
1860 /// back into the `Weak<T>` with [`from_raw`].
1862 /// The same restrictions of accessing the target of the pointer as with
1863 /// [`as_ptr`] apply.
1868 /// use std::sync::{Arc, Weak};
1870 /// let strong = Arc::new("hello".to_owned());
1871 /// let weak = Arc::downgrade(&strong);
1872 /// let raw = weak.into_raw();
1874 /// assert_eq!(1, Arc::weak_count(&strong));
1875 /// assert_eq!("hello", unsafe { &*raw });
1877 /// drop(unsafe { Weak::from_raw(raw) });
1878 /// assert_eq!(0, Arc::weak_count(&strong));
1881 /// [`from_raw`]: Weak::from_raw
1882 /// [`as_ptr`]: Weak::as_ptr
1883 #[must_use = "`self` will be dropped if the result is not used"]
1884 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1885 pub fn into_raw(self) -> *const T {
1886 let result = self.as_ptr();
1891 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1893 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1894 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1896 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1897 /// as these don't own anything; the method still works on them).
1901 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1904 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1905 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1906 /// count is not modified by this operation) and therefore it must be paired with a previous
1907 /// call to [`into_raw`].
1911 /// use std::sync::{Arc, Weak};
1913 /// let strong = Arc::new("hello".to_owned());
1915 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1916 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1918 /// assert_eq!(2, Arc::weak_count(&strong));
1920 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1921 /// assert_eq!(1, Arc::weak_count(&strong));
1925 /// // Decrement the last weak count.
1926 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1929 /// [`new`]: Weak::new
1930 /// [`into_raw`]: Weak::into_raw
1931 /// [`upgrade`]: Weak::upgrade
1932 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1933 pub unsafe fn from_raw(ptr: *const T) -> Self {
1934 // See Weak::as_ptr for context on how the input pointer is derived.
1936 let ptr = if is_dangling(ptr as *mut T) {
1937 // This is a dangling Weak.
1938 ptr as *mut ArcInner<T>
1940 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
1941 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
1942 let offset = unsafe { data_offset(ptr) };
1943 // Thus, we reverse the offset to get the whole RcBox.
1944 // SAFETY: the pointer originated from a Weak, so this offset is safe.
1945 unsafe { (ptr as *mut u8).offset(-offset).with_metadata_of(ptr as *mut ArcInner<T>) }
1948 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
1949 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
1953 impl<T: ?Sized> Weak<T> {
1954 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1955 /// dropping of the inner value if successful.
1957 /// Returns [`None`] if the inner value has since been dropped.
1962 /// use std::sync::Arc;
1964 /// let five = Arc::new(5);
1966 /// let weak_five = Arc::downgrade(&five);
1968 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1969 /// assert!(strong_five.is_some());
1971 /// // Destroy all strong pointers.
1972 /// drop(strong_five);
1975 /// assert!(weak_five.upgrade().is_none());
1977 #[must_use = "this returns a new `Arc`, \
1978 without modifying the original weak pointer"]
1979 #[stable(feature = "arc_weak", since = "1.4.0")]
1980 pub fn upgrade(&self) -> Option<Arc<T>> {
1981 // We use a CAS loop to increment the strong count instead of a
1982 // fetch_add as this function should never take the reference count
1983 // from zero to one.
1984 let inner = self.inner()?;
1986 // Relaxed load because any write of 0 that we can observe
1987 // leaves the field in a permanently zero state (so a
1988 // "stale" read of 0 is fine), and any other value is
1989 // confirmed via the CAS below.
1990 let mut n = inner.strong.load(Relaxed);
1997 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1998 if n > MAX_REFCOUNT {
2002 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
2003 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
2004 // value can be initialized after `Weak` references have already been created. In that case, we
2005 // expect to observe the fully initialized value.
2006 match inner.strong.compare_exchange_weak(n, n + 1, Acquire, Relaxed) {
2007 Ok(_) => return Some(unsafe { Arc::from_inner(self.ptr) }), // null checked above
2008 Err(old) => n = old,
2013 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
2015 /// If `self` was created using [`Weak::new`], this will return 0.
2017 #[stable(feature = "weak_counts", since = "1.41.0")]
2018 pub fn strong_count(&self) -> usize {
2019 if let Some(inner) = self.inner() { inner.strong.load(Acquire) } else { 0 }
2022 /// Gets an approximation of the number of `Weak` pointers pointing to this
2025 /// If `self` was created using [`Weak::new`], or if there are no remaining
2026 /// strong pointers, this will return 0.
2030 /// Due to implementation details, the returned value can be off by 1 in
2031 /// either direction when other threads are manipulating any `Arc`s or
2032 /// `Weak`s pointing to the same allocation.
2034 #[stable(feature = "weak_counts", since = "1.41.0")]
2035 pub fn weak_count(&self) -> usize {
2038 let weak = inner.weak.load(Acquire);
2039 let strong = inner.strong.load(Acquire);
2043 // Since we observed that there was at least one strong pointer
2044 // after reading the weak count, we know that the implicit weak
2045 // reference (present whenever any strong references are alive)
2046 // was still around when we observed the weak count, and can
2047 // therefore safely subtract it.
2054 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
2055 /// (i.e., when this `Weak` was created by `Weak::new`).
2057 fn inner(&self) -> Option<WeakInner<'_>> {
2058 if is_dangling(self.ptr.as_ptr()) {
2061 // We are careful to *not* create a reference covering the "data" field, as
2062 // the field may be mutated concurrently (for example, if the last `Arc`
2063 // is dropped, the data field will be dropped in-place).
2065 let ptr = self.ptr.as_ptr();
2066 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
2071 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
2072 /// [`ptr::eq`]), or if both don't point to any allocation
2073 /// (because they were created with `Weak::new()`).
2077 /// Since this compares pointers it means that `Weak::new()` will equal each
2078 /// other, even though they don't point to any allocation.
2083 /// use std::sync::Arc;
2085 /// let first_rc = Arc::new(5);
2086 /// let first = Arc::downgrade(&first_rc);
2087 /// let second = Arc::downgrade(&first_rc);
2089 /// assert!(first.ptr_eq(&second));
2091 /// let third_rc = Arc::new(5);
2092 /// let third = Arc::downgrade(&third_rc);
2094 /// assert!(!first.ptr_eq(&third));
2097 /// Comparing `Weak::new`.
2100 /// use std::sync::{Arc, Weak};
2102 /// let first = Weak::new();
2103 /// let second = Weak::new();
2104 /// assert!(first.ptr_eq(&second));
2106 /// let third_rc = Arc::new(());
2107 /// let third = Arc::downgrade(&third_rc);
2108 /// assert!(!first.ptr_eq(&third));
2111 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
2114 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
2115 pub fn ptr_eq(&self, other: &Self) -> bool {
2116 self.ptr.as_ptr() == other.ptr.as_ptr()
2120 #[stable(feature = "arc_weak", since = "1.4.0")]
2121 impl<T: ?Sized> Clone for Weak<T> {
2122 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2127 /// use std::sync::{Arc, Weak};
2129 /// let weak_five = Arc::downgrade(&Arc::new(5));
2131 /// let _ = Weak::clone(&weak_five);
2134 fn clone(&self) -> Weak<T> {
2135 let inner = if let Some(inner) = self.inner() {
2138 return Weak { ptr: self.ptr };
2140 // See comments in Arc::clone() for why this is relaxed. This can use a
2141 // fetch_add (ignoring the lock) because the weak count is only locked
2142 // where are *no other* weak pointers in existence. (So we can't be
2143 // running this code in that case).
2144 let old_size = inner.weak.fetch_add(1, Relaxed);
2146 // See comments in Arc::clone() for why we do this (for mem::forget).
2147 if old_size > MAX_REFCOUNT {
2151 Weak { ptr: self.ptr }
2155 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2156 impl<T> Default for Weak<T> {
2157 /// Constructs a new `Weak<T>`, without allocating memory.
2158 /// Calling [`upgrade`] on the return value always
2161 /// [`upgrade`]: Weak::upgrade
2166 /// use std::sync::Weak;
2168 /// let empty: Weak<i64> = Default::default();
2169 /// assert!(empty.upgrade().is_none());
2171 fn default() -> Weak<T> {
2176 #[stable(feature = "arc_weak", since = "1.4.0")]
2177 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2178 /// Drops the `Weak` pointer.
2183 /// use std::sync::{Arc, Weak};
2187 /// impl Drop for Foo {
2188 /// fn drop(&mut self) {
2189 /// println!("dropped!");
2193 /// let foo = Arc::new(Foo);
2194 /// let weak_foo = Arc::downgrade(&foo);
2195 /// let other_weak_foo = Weak::clone(&weak_foo);
2197 /// drop(weak_foo); // Doesn't print anything
2198 /// drop(foo); // Prints "dropped!"
2200 /// assert!(other_weak_foo.upgrade().is_none());
2202 fn drop(&mut self) {
2203 // If we find out that we were the last weak pointer, then its time to
2204 // deallocate the data entirely. See the discussion in Arc::drop() about
2205 // the memory orderings
2207 // It's not necessary to check for the locked state here, because the
2208 // weak count can only be locked if there was precisely one weak ref,
2209 // meaning that drop could only subsequently run ON that remaining weak
2210 // ref, which can only happen after the lock is released.
2211 let inner = if let Some(inner) = self.inner() { inner } else { return };
2213 if inner.weak.fetch_sub(1, Release) == 1 {
2214 acquire!(inner.weak);
2215 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2220 #[stable(feature = "rust1", since = "1.0.0")]
2221 trait ArcEqIdent<T: ?Sized + PartialEq> {
2222 fn eq(&self, other: &Arc<T>) -> bool;
2223 fn ne(&self, other: &Arc<T>) -> bool;
2226 #[stable(feature = "rust1", since = "1.0.0")]
2227 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
2229 default fn eq(&self, other: &Arc<T>) -> bool {
2233 default fn ne(&self, other: &Arc<T>) -> bool {
2238 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2239 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2240 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2241 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2242 /// the same value, than two `&T`s.
2244 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2245 #[stable(feature = "rust1", since = "1.0.0")]
2246 impl<T: ?Sized + crate::rc::MarkerEq> ArcEqIdent<T> for Arc<T> {
2248 fn eq(&self, other: &Arc<T>) -> bool {
2249 Arc::ptr_eq(self, other) || **self == **other
2253 fn ne(&self, other: &Arc<T>) -> bool {
2254 !Arc::ptr_eq(self, other) && **self != **other
2258 #[stable(feature = "rust1", since = "1.0.0")]
2259 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
2260 /// Equality for two `Arc`s.
2262 /// Two `Arc`s are equal if their inner values are equal, even if they are
2263 /// stored in different allocation.
2265 /// If `T` also implements `Eq` (implying reflexivity of equality),
2266 /// two `Arc`s that point to the same allocation are always equal.
2271 /// use std::sync::Arc;
2273 /// let five = Arc::new(5);
2275 /// assert!(five == Arc::new(5));
2278 fn eq(&self, other: &Arc<T>) -> bool {
2279 ArcEqIdent::eq(self, other)
2282 /// Inequality for two `Arc`s.
2284 /// Two `Arc`s are unequal if their inner values are unequal.
2286 /// If `T` also implements `Eq` (implying reflexivity of equality),
2287 /// two `Arc`s that point to the same value are never unequal.
2292 /// use std::sync::Arc;
2294 /// let five = Arc::new(5);
2296 /// assert!(five != Arc::new(6));
2299 fn ne(&self, other: &Arc<T>) -> bool {
2300 ArcEqIdent::ne(self, other)
2304 #[stable(feature = "rust1", since = "1.0.0")]
2305 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
2306 /// Partial comparison for two `Arc`s.
2308 /// The two are compared by calling `partial_cmp()` on their inner values.
2313 /// use std::sync::Arc;
2314 /// use std::cmp::Ordering;
2316 /// let five = Arc::new(5);
2318 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2320 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
2321 (**self).partial_cmp(&**other)
2324 /// Less-than comparison for two `Arc`s.
2326 /// The two are compared by calling `<` on their inner values.
2331 /// use std::sync::Arc;
2333 /// let five = Arc::new(5);
2335 /// assert!(five < Arc::new(6));
2337 fn lt(&self, other: &Arc<T>) -> bool {
2338 *(*self) < *(*other)
2341 /// 'Less than or equal to' comparison for two `Arc`s.
2343 /// The two are compared by calling `<=` on their inner values.
2348 /// use std::sync::Arc;
2350 /// let five = Arc::new(5);
2352 /// assert!(five <= Arc::new(5));
2354 fn le(&self, other: &Arc<T>) -> bool {
2355 *(*self) <= *(*other)
2358 /// Greater-than comparison for two `Arc`s.
2360 /// The two are compared by calling `>` on their inner values.
2365 /// use std::sync::Arc;
2367 /// let five = Arc::new(5);
2369 /// assert!(five > Arc::new(4));
2371 fn gt(&self, other: &Arc<T>) -> bool {
2372 *(*self) > *(*other)
2375 /// 'Greater than or equal to' comparison for two `Arc`s.
2377 /// The two are compared by calling `>=` on their inner values.
2382 /// use std::sync::Arc;
2384 /// let five = Arc::new(5);
2386 /// assert!(five >= Arc::new(5));
2388 fn ge(&self, other: &Arc<T>) -> bool {
2389 *(*self) >= *(*other)
2392 #[stable(feature = "rust1", since = "1.0.0")]
2393 impl<T: ?Sized + Ord> Ord for Arc<T> {
2394 /// Comparison for two `Arc`s.
2396 /// The two are compared by calling `cmp()` on their inner values.
2401 /// use std::sync::Arc;
2402 /// use std::cmp::Ordering;
2404 /// let five = Arc::new(5);
2406 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2408 fn cmp(&self, other: &Arc<T>) -> Ordering {
2409 (**self).cmp(&**other)
2412 #[stable(feature = "rust1", since = "1.0.0")]
2413 impl<T: ?Sized + Eq> Eq for Arc<T> {}
2415 #[stable(feature = "rust1", since = "1.0.0")]
2416 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
2417 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2418 fmt::Display::fmt(&**self, f)
2422 #[stable(feature = "rust1", since = "1.0.0")]
2423 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
2424 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2425 fmt::Debug::fmt(&**self, f)
2429 #[stable(feature = "rust1", since = "1.0.0")]
2430 impl<T: ?Sized> fmt::Pointer for Arc<T> {
2431 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2432 fmt::Pointer::fmt(&(&**self as *const T), f)
2436 #[cfg(not(no_global_oom_handling))]
2437 #[stable(feature = "rust1", since = "1.0.0")]
2438 impl<T: Default> Default for Arc<T> {
2439 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2444 /// use std::sync::Arc;
2446 /// let x: Arc<i32> = Default::default();
2447 /// assert_eq!(*x, 0);
2449 fn default() -> Arc<T> {
2450 Arc::new(Default::default())
2454 #[stable(feature = "rust1", since = "1.0.0")]
2455 impl<T: ?Sized + Hash> Hash for Arc<T> {
2456 fn hash<H: Hasher>(&self, state: &mut H) {
2457 (**self).hash(state)
2461 #[cfg(not(no_global_oom_handling))]
2462 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2463 impl<T> From<T> for Arc<T> {
2464 /// Converts a `T` into an `Arc<T>`
2466 /// The conversion moves the value into a
2467 /// newly allocated `Arc`. It is equivalent to
2468 /// calling `Arc::new(t)`.
2472 /// # use std::sync::Arc;
2474 /// let arc = Arc::new(5);
2476 /// assert_eq!(Arc::from(x), arc);
2478 fn from(t: T) -> Self {
2483 #[cfg(not(no_global_oom_handling))]
2484 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2485 impl<T: Clone> From<&[T]> for Arc<[T]> {
2486 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2491 /// # use std::sync::Arc;
2492 /// let original: &[i32] = &[1, 2, 3];
2493 /// let shared: Arc<[i32]> = Arc::from(original);
2494 /// assert_eq!(&[1, 2, 3], &shared[..]);
2497 fn from(v: &[T]) -> Arc<[T]> {
2498 <Self as ArcFromSlice<T>>::from_slice(v)
2502 #[cfg(not(no_global_oom_handling))]
2503 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2504 impl From<&str> for Arc<str> {
2505 /// Allocate a reference-counted `str` and copy `v` into it.
2510 /// # use std::sync::Arc;
2511 /// let shared: Arc<str> = Arc::from("eggplant");
2512 /// assert_eq!("eggplant", &shared[..]);
2515 fn from(v: &str) -> Arc<str> {
2516 let arc = Arc::<[u8]>::from(v.as_bytes());
2517 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2521 #[cfg(not(no_global_oom_handling))]
2522 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2523 impl From<String> for Arc<str> {
2524 /// Allocate a reference-counted `str` and copy `v` into it.
2529 /// # use std::sync::Arc;
2530 /// let unique: String = "eggplant".to_owned();
2531 /// let shared: Arc<str> = Arc::from(unique);
2532 /// assert_eq!("eggplant", &shared[..]);
2535 fn from(v: String) -> Arc<str> {
2540 #[cfg(not(no_global_oom_handling))]
2541 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2542 impl<T: ?Sized> From<Box<T>> for Arc<T> {
2543 /// Move a boxed object to a new, reference-counted allocation.
2548 /// # use std::sync::Arc;
2549 /// let unique: Box<str> = Box::from("eggplant");
2550 /// let shared: Arc<str> = Arc::from(unique);
2551 /// assert_eq!("eggplant", &shared[..]);
2554 fn from(v: Box<T>) -> Arc<T> {
2559 #[cfg(not(no_global_oom_handling))]
2560 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2561 impl<T> From<Vec<T>> for Arc<[T]> {
2562 /// Allocate a reference-counted slice and move `v`'s items into it.
2567 /// # use std::sync::Arc;
2568 /// let unique: Vec<i32> = vec![1, 2, 3];
2569 /// let shared: Arc<[i32]> = Arc::from(unique);
2570 /// assert_eq!(&[1, 2, 3], &shared[..]);
2573 fn from(mut v: Vec<T>) -> Arc<[T]> {
2575 let arc = Arc::copy_from_slice(&v);
2577 // Allow the Vec to free its memory, but not destroy its contents
2585 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2586 impl<'a, B> From<Cow<'a, B>> for Arc<B>
2588 B: ToOwned + ?Sized,
2589 Arc<B>: From<&'a B> + From<B::Owned>,
2591 /// Create an atomically reference-counted pointer from
2592 /// a clone-on-write pointer by copying its content.
2597 /// # use std::sync::Arc;
2598 /// # use std::borrow::Cow;
2599 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2600 /// let shared: Arc<str> = Arc::from(cow);
2601 /// assert_eq!("eggplant", &shared[..]);
2604 fn from(cow: Cow<'a, B>) -> Arc<B> {
2606 Cow::Borrowed(s) => Arc::from(s),
2607 Cow::Owned(s) => Arc::from(s),
2612 #[stable(feature = "shared_from_str", since = "1.62.0")]
2613 impl From<Arc<str>> for Arc<[u8]> {
2614 /// Converts an atomically reference-counted string slice into a byte slice.
2619 /// # use std::sync::Arc;
2620 /// let string: Arc<str> = Arc::from("eggplant");
2621 /// let bytes: Arc<[u8]> = Arc::from(string);
2622 /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
2625 fn from(rc: Arc<str>) -> Self {
2626 // SAFETY: `str` has the same layout as `[u8]`.
2627 unsafe { Arc::from_raw(Arc::into_raw(rc) as *const [u8]) }
2631 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2632 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]> {
2633 type Error = Arc<[T]>;
2635 fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
2636 if boxed_slice.len() == N {
2637 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
2644 #[cfg(not(no_global_oom_handling))]
2645 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2646 impl<T> iter::FromIterator<T> for Arc<[T]> {
2647 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2649 /// # Performance characteristics
2651 /// ## The general case
2653 /// In the general case, collecting into `Arc<[T]>` is done by first
2654 /// collecting into a `Vec<T>`. That is, when writing the following:
2657 /// # use std::sync::Arc;
2658 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2659 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2662 /// this behaves as if we wrote:
2665 /// # use std::sync::Arc;
2666 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2667 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2668 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2669 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2672 /// This will allocate as many times as needed for constructing the `Vec<T>`
2673 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2675 /// ## Iterators of known length
2677 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2678 /// a single allocation will be made for the `Arc<[T]>`. For example:
2681 /// # use std::sync::Arc;
2682 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2683 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2685 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2686 ToArcSlice::to_arc_slice(iter.into_iter())
2690 /// Specialization trait used for collecting into `Arc<[T]>`.
2691 trait ToArcSlice<T>: Iterator<Item = T> + Sized {
2692 fn to_arc_slice(self) -> Arc<[T]>;
2695 #[cfg(not(no_global_oom_handling))]
2696 impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
2697 default fn to_arc_slice(self) -> Arc<[T]> {
2698 self.collect::<Vec<T>>().into()
2702 #[cfg(not(no_global_oom_handling))]
2703 impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
2704 fn to_arc_slice(self) -> Arc<[T]> {
2705 // This is the case for a `TrustedLen` iterator.
2706 let (low, high) = self.size_hint();
2707 if let Some(high) = high {
2711 "TrustedLen iterator's size hint is not exact: {:?}",
2716 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2717 Arc::from_iter_exact(self, low)
2720 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2721 // length exceeding `usize::MAX`.
2722 // The default implementation would collect into a vec which would panic.
2723 // Thus we panic here immediately without invoking `Vec` code.
2724 panic!("capacity overflow");
2729 #[stable(feature = "rust1", since = "1.0.0")]
2730 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2731 fn borrow(&self) -> &T {
2736 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2737 impl<T: ?Sized> AsRef<T> for Arc<T> {
2738 fn as_ref(&self) -> &T {
2743 #[stable(feature = "pin", since = "1.33.0")]
2744 impl<T: ?Sized> Unpin for Arc<T> {}
2746 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2750 /// The pointer must point to (and have valid metadata for) a previously
2751 /// valid instance of T, but the T is allowed to be dropped.
2752 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2753 // Align the unsized value to the end of the ArcInner.
2754 // Because RcBox is repr(C), it will always be the last field in memory.
2755 // SAFETY: since the only unsized types possible are slices, trait objects,
2756 // and extern types, the input safety requirement is currently enough to
2757 // satisfy the requirements of align_of_val_raw; this is an implementation
2758 // detail of the language that must not be relied upon outside of std.
2759 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2763 fn data_offset_align(align: usize) -> isize {
2764 let layout = Layout::new::<ArcInner<()>>();
2765 (layout.size() + layout.padding_needed_for(align)) as isize