1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][arc] documentation for more details.
7 //! [arc]: struct.Arc.html
10 use core
::array
::LengthAtMost32
;
11 use core
::sync
::atomic
;
12 use core
::sync
::atomic
::Ordering
::{Acquire, Relaxed, Release, SeqCst}
;
15 use core
::cmp
::{self, Ordering}
;
17 use core
::intrinsics
::abort
;
18 use core
::mem
::{self, align_of, align_of_val, size_of_val}
;
19 use core
::ops
::{Deref, Receiver, CoerceUnsized, DispatchFromDyn}
;
21 use core
::ptr
::{self, NonNull}
;
22 use core
::marker
::{Unpin, Unsize, PhantomData}
;
23 use core
::hash
::{Hash, Hasher}
;
24 use core
::{isize, usize}
;
25 use core
::convert
::{From, TryFrom}
;
26 use core
::slice
::{self, from_raw_parts_mut}
;
28 use crate::alloc
::{Global, Alloc, Layout, box_free, handle_alloc_error}
;
29 use crate::boxed
::Box
;
30 use crate::rc
::is_dangling
;
31 use crate::string
::String
;
37 /// A soft limit on the amount of references that may be made to an `Arc`.
39 /// Going above this limit will abort your program (although not
40 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
41 const MAX_REFCOUNT
: usize = (isize::MAX
) as usize;
43 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
44 /// Reference Counted'.
46 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
47 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
48 /// a new `Arc` instance, which points to the same value on the heap as the
49 /// source `Arc`, while increasing a reference count. When the last `Arc`
50 /// pointer to a given value is destroyed, the pointed-to value is also
53 /// Shared references in Rust disallow mutation by default, and `Arc` is no
54 /// exception: you cannot generally obtain a mutable reference to something
55 /// inside an `Arc`. If you need to mutate through an `Arc`, use
56 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
61 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
62 /// counting. This means that it is thread-safe. The disadvantage is that
63 /// atomic operations are more expensive than ordinary memory accesses. If you
64 /// are not sharing reference-counted values between threads, consider using
65 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
66 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
67 /// However, a library might choose `Arc<T>` in order to give library consumers
70 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
71 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
72 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
73 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
74 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
75 /// data, but it doesn't add thread safety to its data. Consider
76 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
77 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
78 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
79 /// non-atomic operations.
81 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
82 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
84 /// ## Breaking cycles with `Weak`
86 /// The [`downgrade`][downgrade] method can be used to create a non-owning
87 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
88 /// to an `Arc`, but this will return [`None`] if the value has already been
91 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
92 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
93 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
94 /// pointers from children back to their parents.
96 /// # Cloning references
98 /// Creating a new reference from an existing reference counted pointer is done using the
99 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
102 /// use std::sync::Arc;
103 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
104 /// // The two syntaxes below are equivalent.
105 /// let a = foo.clone();
106 /// let b = Arc::clone(&foo);
107 /// // a, b, and foo are all Arcs that point to the same memory location
110 /// ## `Deref` behavior
112 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
113 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
114 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
115 /// functions, called using function-like syntax:
118 /// use std::sync::Arc;
119 /// let my_arc = Arc::new(());
121 /// Arc::downgrade(&my_arc);
124 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have
125 /// already been destroyed.
127 /// [arc]: struct.Arc.html
128 /// [weak]: struct.Weak.html
129 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
130 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
131 /// [mutex]: ../../std/sync/struct.Mutex.html
132 /// [rwlock]: ../../std/sync/struct.RwLock.html
133 /// [atomic]: ../../std/sync/atomic/index.html
134 /// [`Send`]: ../../std/marker/trait.Send.html
135 /// [`Sync`]: ../../std/marker/trait.Sync.html
136 /// [deref]: ../../std/ops/trait.Deref.html
137 /// [downgrade]: struct.Arc.html#method.downgrade
138 /// [upgrade]: struct.Weak.html#method.upgrade
139 /// [`None`]: ../../std/option/enum.Option.html#variant.None
140 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
141 /// [`std::sync`]: ../../std/sync/index.html
142 /// [`Arc::clone(&from)`]: #method.clone
146 /// Sharing some immutable data between threads:
148 // Note that we **do not** run these tests here. The windows builders get super
149 // unhappy if a thread outlives the main thread and then exits at the same time
150 // (something deadlocks) so we just avoid this entirely by not running these
153 /// use std::sync::Arc;
156 /// let five = Arc::new(5);
159 /// let five = Arc::clone(&five);
161 /// thread::spawn(move || {
162 /// println!("{:?}", five);
167 /// Sharing a mutable [`AtomicUsize`]:
169 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
172 /// use std::sync::Arc;
173 /// use std::sync::atomic::{AtomicUsize, Ordering};
176 /// let val = Arc::new(AtomicUsize::new(5));
179 /// let val = Arc::clone(&val);
181 /// thread::spawn(move || {
182 /// let v = val.fetch_add(1, Ordering::SeqCst);
183 /// println!("{:?}", v);
188 /// See the [`rc` documentation][rc_examples] for more examples of reference
189 /// counting in general.
191 /// [rc_examples]: ../../std/rc/index.html#examples
192 #[cfg_attr(not(test), lang = "arc")]
193 #[stable(feature = "rust1", since = "1.0.0")]
194 pub struct Arc
<T
: ?Sized
> {
195 ptr
: NonNull
<ArcInner
<T
>>,
196 phantom
: PhantomData
<T
>,
199 #[stable(feature = "rust1", since = "1.0.0")]
200 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Send
for Arc
<T
> {}
201 #[stable(feature = "rust1", since = "1.0.0")]
202 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Sync
for Arc
<T
> {}
204 #[unstable(feature = "coerce_unsized", issue = "27732")]
205 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Arc
<U
>> for Arc
<T
> {}
207 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
208 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> DispatchFromDyn
<Arc
<U
>> for Arc
<T
> {}
210 impl<T
: ?Sized
> Arc
<T
> {
211 fn from_inner(ptr
: NonNull
<ArcInner
<T
>>) -> Self {
214 phantom
: PhantomData
,
218 unsafe fn from_ptr(ptr
: *mut ArcInner
<T
>) -> Self {
219 Self::from_inner(NonNull
::new_unchecked(ptr
))
223 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
224 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
225 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
227 /// Since a `Weak` reference does not count towards ownership, it will not
228 /// prevent the inner value from being dropped, and `Weak` itself makes no
229 /// guarantees about the value still being present and may return [`None`]
230 /// when [`upgrade`]d.
232 /// A `Weak` pointer is useful for keeping a temporary reference to the value
233 /// within [`Arc`] without extending its lifetime. It is also used to prevent
234 /// circular references between [`Arc`] pointers, since mutual owning references
235 /// would never allow either [`Arc`] to be dropped. For example, a tree could
236 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
237 /// pointers from children back to their parents.
239 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
241 /// [`Arc`]: struct.Arc.html
242 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
243 /// [`upgrade`]: struct.Weak.html#method.upgrade
244 /// [`Option`]: ../../std/option/enum.Option.html
245 /// [`None`]: ../../std/option/enum.Option.html#variant.None
246 #[stable(feature = "arc_weak", since = "1.4.0")]
247 pub struct Weak
<T
: ?Sized
> {
248 // This is a `NonNull` to allow optimizing the size of this type in enums,
249 // but it is not necessarily a valid pointer.
250 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
251 // to allocate space on the heap. That's not a value a real pointer
252 // will ever have because RcBox has alignment at least 2.
253 ptr
: NonNull
<ArcInner
<T
>>,
256 #[stable(feature = "arc_weak", since = "1.4.0")]
257 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Send
for Weak
<T
> {}
258 #[stable(feature = "arc_weak", since = "1.4.0")]
259 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Sync
for Weak
<T
> {}
261 #[unstable(feature = "coerce_unsized", issue = "27732")]
262 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Weak
<U
>> for Weak
<T
> {}
263 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
264 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> DispatchFromDyn
<Weak
<U
>> for Weak
<T
> {}
266 #[stable(feature = "arc_weak", since = "1.4.0")]
267 impl<T
: ?Sized
+ fmt
::Debug
> fmt
::Debug
for Weak
<T
> {
268 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
273 struct ArcInner
<T
: ?Sized
> {
274 strong
: atomic
::AtomicUsize
,
276 // the value usize::MAX acts as a sentinel for temporarily "locking" the
277 // ability to upgrade weak pointers or downgrade strong ones; this is used
278 // to avoid races in `make_mut` and `get_mut`.
279 weak
: atomic
::AtomicUsize
,
284 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Send
for ArcInner
<T
> {}
285 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Sync
for ArcInner
<T
> {}
288 /// Constructs a new `Arc<T>`.
293 /// use std::sync::Arc;
295 /// let five = Arc::new(5);
298 #[stable(feature = "rust1", since = "1.0.0")]
299 pub fn new(data
: T
) -> Arc
<T
> {
300 // Start the weak pointer count as 1 which is the weak pointer that's
301 // held by all the strong pointers (kinda), see std/rc.rs for more info
302 let x
: Box
<_
> = box ArcInner
{
303 strong
: atomic
::AtomicUsize
::new(1),
304 weak
: atomic
::AtomicUsize
::new(1),
307 Self::from_inner(Box
::into_raw_non_null(x
))
310 /// Constructs a new `Arc` with uninitialized contents.
315 /// #![feature(new_uninit)]
316 /// #![feature(get_mut_unchecked)]
318 /// use std::sync::Arc;
320 /// let mut five = Arc::<u32>::new_uninit();
322 /// let five = unsafe {
323 /// // Deferred initialization:
324 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
326 /// five.assume_init()
329 /// assert_eq!(*five, 5)
331 #[unstable(feature = "new_uninit", issue = "63291")]
332 pub fn new_uninit() -> Arc
<mem
::MaybeUninit
<T
>> {
334 Arc
::from_ptr(Arc
::allocate_for_layout(
336 |mem
| mem
as *mut ArcInner
<mem
::MaybeUninit
<T
>>,
341 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
342 /// `data` will be pinned in memory and unable to be moved.
343 #[stable(feature = "pin", since = "1.33.0")]
344 pub fn pin(data
: T
) -> Pin
<Arc
<T
>> {
345 unsafe { Pin::new_unchecked(Arc::new(data)) }
348 /// Returns the contained value, if the `Arc` has exactly one strong reference.
350 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
353 /// This will succeed even if there are outstanding weak references.
355 /// [result]: ../../std/result/enum.Result.html
360 /// use std::sync::Arc;
362 /// let x = Arc::new(3);
363 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
365 /// let x = Arc::new(4);
366 /// let _y = Arc::clone(&x);
367 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
370 #[stable(feature = "arc_unique", since = "1.4.0")]
371 pub fn try_unwrap(this
: Self) -> Result
<T
, Self> {
372 // See `drop` for why all these atomics are like this
373 if this
.inner().strong
.compare_exchange(1, 0, Release
, Relaxed
).is_err() {
377 atomic
::fence(Acquire
);
380 let elem
= ptr
::read(&this
.ptr
.as_ref().data
);
382 // Make a weak pointer to clean up the implicit strong-weak reference
383 let _weak
= Weak { ptr: this.ptr }
;
392 /// Constructs a new reference-counted slice with uninitialized contents.
397 /// #![feature(new_uninit)]
398 /// #![feature(get_mut_unchecked)]
400 /// use std::sync::Arc;
402 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
404 /// let values = unsafe {
405 /// // Deferred initialization:
406 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
407 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
408 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
410 /// values.assume_init()
413 /// assert_eq!(*values, [1, 2, 3])
415 #[unstable(feature = "new_uninit", issue = "63291")]
416 pub fn new_uninit_slice(len
: usize) -> Arc
<[mem
::MaybeUninit
<T
>]> {
418 Arc
::from_ptr(Arc
::allocate_for_slice(len
))
423 impl<T
> Arc
<mem
::MaybeUninit
<T
>> {
424 /// Converts to `Arc<T>`.
428 /// As with [`MaybeUninit::assume_init`],
429 /// it is up to the caller to guarantee that the value
430 /// really is in an initialized state.
431 /// Calling this when the content is not yet fully initialized
432 /// causes immediate undefined behavior.
434 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
439 /// #![feature(new_uninit)]
440 /// #![feature(get_mut_unchecked)]
442 /// use std::sync::Arc;
444 /// let mut five = Arc::<u32>::new_uninit();
446 /// let five = unsafe {
447 /// // Deferred initialization:
448 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
450 /// five.assume_init()
453 /// assert_eq!(*five, 5)
455 #[unstable(feature = "new_uninit", issue = "63291")]
457 pub unsafe fn assume_init(self) -> Arc
<T
> {
458 Arc
::from_inner(mem
::ManuallyDrop
::new(self).ptr
.cast())
462 impl<T
> Arc
<[mem
::MaybeUninit
<T
>]> {
463 /// Converts to `Arc<[T]>`.
467 /// As with [`MaybeUninit::assume_init`],
468 /// it is up to the caller to guarantee that the value
469 /// really is in an initialized state.
470 /// Calling this when the content is not yet fully initialized
471 /// causes immediate undefined behavior.
473 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
478 /// #![feature(new_uninit)]
479 /// #![feature(get_mut_unchecked)]
481 /// use std::sync::Arc;
483 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
485 /// let values = unsafe {
486 /// // Deferred initialization:
487 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
488 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
489 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
491 /// values.assume_init()
494 /// assert_eq!(*values, [1, 2, 3])
496 #[unstable(feature = "new_uninit", issue = "63291")]
498 pub unsafe fn assume_init(self) -> Arc
<[T
]> {
499 Arc
::from_ptr(mem
::ManuallyDrop
::new(self).ptr
.as_ptr() as _
)
503 impl<T
: ?Sized
> Arc
<T
> {
504 /// Consumes the `Arc`, returning the wrapped pointer.
506 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
507 /// [`Arc::from_raw`][from_raw].
509 /// [from_raw]: struct.Arc.html#method.from_raw
514 /// use std::sync::Arc;
516 /// let x = Arc::new("hello".to_owned());
517 /// let x_ptr = Arc::into_raw(x);
518 /// assert_eq!(unsafe { &*x_ptr }, "hello");
520 #[stable(feature = "rc_raw", since = "1.17.0")]
521 pub fn into_raw(this
: Self) -> *const T
{
522 let ptr
: *const T
= &*this
;
527 /// Constructs an `Arc` from a raw pointer.
529 /// The raw pointer must have been previously returned by a call to a
530 /// [`Arc::into_raw`][into_raw].
532 /// This function is unsafe because improper use may lead to memory problems. For example, a
533 /// double-free may occur if the function is called twice on the same raw pointer.
535 /// [into_raw]: struct.Arc.html#method.into_raw
540 /// use std::sync::Arc;
542 /// let x = Arc::new("hello".to_owned());
543 /// let x_ptr = Arc::into_raw(x);
546 /// // Convert back to an `Arc` to prevent leak.
547 /// let x = Arc::from_raw(x_ptr);
548 /// assert_eq!(&*x, "hello");
550 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
553 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
555 #[stable(feature = "rc_raw", since = "1.17.0")]
556 pub unsafe fn from_raw(ptr
: *const T
) -> Self {
557 let offset
= data_offset(ptr
);
559 // Reverse the offset to find the original ArcInner.
560 let fake_ptr
= ptr
as *mut ArcInner
<T
>;
561 let arc_ptr
= set_data_ptr(fake_ptr
, (ptr
as *mut u8).offset(-offset
));
563 Self::from_ptr(arc_ptr
)
566 /// Consumes the `Arc`, returning the wrapped pointer as `NonNull<T>`.
571 /// #![feature(rc_into_raw_non_null)]
573 /// use std::sync::Arc;
575 /// let x = Arc::new("hello".to_owned());
576 /// let ptr = Arc::into_raw_non_null(x);
577 /// let deref = unsafe { ptr.as_ref() };
578 /// assert_eq!(deref, "hello");
580 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
582 pub fn into_raw_non_null(this
: Self) -> NonNull
<T
> {
583 // safe because Arc guarantees its pointer is non-null
584 unsafe { NonNull::new_unchecked(Arc::into_raw(this) as *mut _) }
587 /// Creates a new [`Weak`][weak] pointer to this value.
589 /// [weak]: struct.Weak.html
594 /// use std::sync::Arc;
596 /// let five = Arc::new(5);
598 /// let weak_five = Arc::downgrade(&five);
600 #[stable(feature = "arc_weak", since = "1.4.0")]
601 pub fn downgrade(this
: &Self) -> Weak
<T
> {
602 // This Relaxed is OK because we're checking the value in the CAS
604 let mut cur
= this
.inner().weak
.load(Relaxed
);
607 // check if the weak counter is currently "locked"; if so, spin.
608 if cur
== usize::MAX
{
609 cur
= this
.inner().weak
.load(Relaxed
);
613 // NOTE: this code currently ignores the possibility of overflow
614 // into usize::MAX; in general both Rc and Arc need to be adjusted
615 // to deal with overflow.
617 // Unlike with Clone(), we need this to be an Acquire read to
618 // synchronize with the write coming from `is_unique`, so that the
619 // events prior to that write happen before this read.
620 match this
.inner().weak
.compare_exchange_weak(cur
, cur
+ 1, Acquire
, Relaxed
) {
622 // Make sure we do not create a dangling Weak
623 debug_assert
!(!is_dangling(this
.ptr
));
624 return Weak { ptr: this.ptr }
;
626 Err(old
) => cur
= old
,
631 /// Gets the number of [`Weak`][weak] pointers to this value.
633 /// [weak]: struct.Weak.html
637 /// This method by itself is safe, but using it correctly requires extra care.
638 /// Another thread can change the weak count at any time,
639 /// including potentially between calling this method and acting on the result.
644 /// use std::sync::Arc;
646 /// let five = Arc::new(5);
647 /// let _weak_five = Arc::downgrade(&five);
649 /// // This assertion is deterministic because we haven't shared
650 /// // the `Arc` or `Weak` between threads.
651 /// assert_eq!(1, Arc::weak_count(&five));
654 #[stable(feature = "arc_counts", since = "1.15.0")]
655 pub fn weak_count(this
: &Self) -> usize {
656 let cnt
= this
.inner().weak
.load(SeqCst
);
657 // If the weak count is currently locked, the value of the
658 // count was 0 just before taking the lock.
659 if cnt
== usize::MAX { 0 }
else { cnt - 1 }
662 /// Gets the number of strong (`Arc`) pointers to this value.
666 /// This method by itself is safe, but using it correctly requires extra care.
667 /// Another thread can change the strong count at any time,
668 /// including potentially between calling this method and acting on the result.
673 /// use std::sync::Arc;
675 /// let five = Arc::new(5);
676 /// let _also_five = Arc::clone(&five);
678 /// // This assertion is deterministic because we haven't shared
679 /// // the `Arc` between threads.
680 /// assert_eq!(2, Arc::strong_count(&five));
683 #[stable(feature = "arc_counts", since = "1.15.0")]
684 pub fn strong_count(this
: &Self) -> usize {
685 this
.inner().strong
.load(SeqCst
)
689 fn inner(&self) -> &ArcInner
<T
> {
690 // This unsafety is ok because while this arc is alive we're guaranteed
691 // that the inner pointer is valid. Furthermore, we know that the
692 // `ArcInner` structure itself is `Sync` because the inner data is
693 // `Sync` as well, so we're ok loaning out an immutable pointer to these
695 unsafe { self.ptr.as_ref() }
698 // Non-inlined part of `drop`.
700 unsafe fn drop_slow(&mut self) {
701 // Destroy the data at this time, even though we may not free the box
702 // allocation itself (there may still be weak pointers lying around).
703 ptr
::drop_in_place(&mut self.ptr
.as_mut().data
);
705 if self.inner().weak
.fetch_sub(1, Release
) == 1 {
706 atomic
::fence(Acquire
);
707 Global
.dealloc(self.ptr
.cast(), Layout
::for_value(self.ptr
.as_ref()))
712 #[stable(feature = "ptr_eq", since = "1.17.0")]
713 /// Returns `true` if the two `Arc`s point to the same value (not
714 /// just values that compare as equal).
719 /// use std::sync::Arc;
721 /// let five = Arc::new(5);
722 /// let same_five = Arc::clone(&five);
723 /// let other_five = Arc::new(5);
725 /// assert!(Arc::ptr_eq(&five, &same_five));
726 /// assert!(!Arc::ptr_eq(&five, &other_five));
728 pub fn ptr_eq(this
: &Self, other
: &Self) -> bool
{
729 this
.ptr
.as_ptr() == other
.ptr
.as_ptr()
733 impl<T
: ?Sized
> Arc
<T
> {
734 /// Allocates an `ArcInner<T>` with sufficient space for
735 /// a possibly-unsized value where the value has the layout provided.
737 /// The function `mem_to_arcinner` is called with the data pointer
738 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
739 unsafe fn allocate_for_layout(
740 value_layout
: Layout
,
741 mem_to_arcinner
: impl FnOnce(*mut u8) -> *mut ArcInner
<T
>
742 ) -> *mut ArcInner
<T
> {
743 // Calculate layout using the given value layout.
744 // Previously, layout was calculated on the expression
745 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
746 // reference (see #54908).
747 let layout
= Layout
::new
::<ArcInner
<()>>()
748 .extend(value_layout
).unwrap().0
749 .pad_to_align().unwrap();
751 let mem
= Global
.alloc(layout
)
752 .unwrap_or_else(|_
| handle_alloc_error(layout
));
754 // Initialize the ArcInner
755 let inner
= mem_to_arcinner(mem
.as_ptr());
756 debug_assert_eq
!(Layout
::for_value(&*inner
), layout
);
758 ptr
::write(&mut (*inner
).strong
, atomic
::AtomicUsize
::new(1));
759 ptr
::write(&mut (*inner
).weak
, atomic
::AtomicUsize
::new(1));
764 /// Allocates an `ArcInner<T>` with sufficient space for an unsized value.
765 unsafe fn allocate_for_ptr(ptr
: *const T
) -> *mut ArcInner
<T
> {
766 // Allocate for the `ArcInner<T>` using the given value.
767 Self::allocate_for_layout(
768 Layout
::for_value(&*ptr
),
769 |mem
| set_data_ptr(ptr
as *mut T
, mem
) as *mut ArcInner
<T
>,
773 fn from_box(v
: Box
<T
>) -> Arc
<T
> {
775 let box_unique
= Box
::into_unique(v
);
776 let bptr
= box_unique
.as_ptr();
778 let value_size
= size_of_val(&*bptr
);
779 let ptr
= Self::allocate_for_ptr(bptr
);
781 // Copy value as bytes
782 ptr
::copy_nonoverlapping(
783 bptr
as *const T
as *const u8,
784 &mut (*ptr
).data
as *mut _
as *mut u8,
787 // Free the allocation without dropping its contents
788 box_free(box_unique
);
796 /// Allocates an `ArcInner<[T]>` with the given length.
797 unsafe fn allocate_for_slice(len
: usize) -> *mut ArcInner
<[T
]> {
798 Self::allocate_for_layout(
799 Layout
::array
::<T
>(len
).unwrap(),
800 |mem
| ptr
::slice_from_raw_parts_mut(mem
as *mut T
, len
) as *mut ArcInner
<[T
]>,
805 /// Sets the data pointer of a `?Sized` raw pointer.
807 /// For a slice/trait object, this sets the `data` field and leaves the rest
808 /// unchanged. For a sized raw pointer, this simply sets the pointer.
809 unsafe fn set_data_ptr
<T
: ?Sized
, U
>(mut ptr
: *mut T
, data
: *mut U
) -> *mut T
{
810 ptr
::write(&mut ptr
as *mut _
as *mut *mut u8, data
as *mut u8);
815 /// Copy elements from slice into newly allocated Arc<[T]>
817 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
818 unsafe fn copy_from_slice(v
: &[T
]) -> Arc
<[T
]> {
819 let ptr
= Self::allocate_for_slice(v
.len());
821 ptr
::copy_nonoverlapping(
823 &mut (*ptr
).data
as *mut [T
] as *mut T
,
829 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
831 /// Behavior is undefined should the size be wrong.
832 unsafe fn from_iter_exact(iter
: impl iter
::Iterator
<Item
= T
>, len
: usize) -> Arc
<[T
]> {
833 // Panic guard while cloning T elements.
834 // In the event of a panic, elements that have been written
835 // into the new ArcInner will be dropped, then the memory freed.
843 impl<T
> Drop
for Guard
<T
> {
846 let slice
= from_raw_parts_mut(self.elems
, self.n_elems
);
847 ptr
::drop_in_place(slice
);
849 Global
.dealloc(self.mem
.cast(), self.layout
);
854 let ptr
= Self::allocate_for_slice(len
);
856 let mem
= ptr
as *mut _
as *mut u8;
857 let layout
= Layout
::for_value(&*ptr
);
859 // Pointer to first element
860 let elems
= &mut (*ptr
).data
as *mut [T
] as *mut T
;
862 let mut guard
= Guard
{
863 mem
: NonNull
::new_unchecked(mem
),
869 for (i
, item
) in iter
.enumerate() {
870 ptr
::write(elems
.add(i
), item
);
874 // All clear. Forget the guard so it doesn't free the new ArcInner.
881 /// Specialization trait used for `From<&[T]>`.
882 trait ArcFromSlice
<T
> {
883 fn from_slice(slice
: &[T
]) -> Self;
886 impl<T
: Clone
> ArcFromSlice
<T
> for Arc
<[T
]> {
888 default fn from_slice(v
: &[T
]) -> Self {
890 Self::from_iter_exact(v
.iter().cloned(), v
.len())
895 impl<T
: Copy
> ArcFromSlice
<T
> for Arc
<[T
]> {
897 fn from_slice(v
: &[T
]) -> Self {
898 unsafe { Arc::copy_from_slice(v) }
902 #[stable(feature = "rust1", since = "1.0.0")]
903 impl<T
: ?Sized
> Clone
for Arc
<T
> {
904 /// Makes a clone of the `Arc` pointer.
906 /// This creates another pointer to the same inner value, increasing the
907 /// strong reference count.
912 /// use std::sync::Arc;
914 /// let five = Arc::new(5);
916 /// let _ = Arc::clone(&five);
919 fn clone(&self) -> Arc
<T
> {
920 // Using a relaxed ordering is alright here, as knowledge of the
921 // original reference prevents other threads from erroneously deleting
924 // As explained in the [Boost documentation][1], Increasing the
925 // reference counter can always be done with memory_order_relaxed: New
926 // references to an object can only be formed from an existing
927 // reference, and passing an existing reference from one thread to
928 // another must already provide any required synchronization.
930 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
931 let old_size
= self.inner().strong
.fetch_add(1, Relaxed
);
933 // However we need to guard against massive refcounts in case someone
934 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
935 // and users will use-after free. We racily saturate to `isize::MAX` on
936 // the assumption that there aren't ~2 billion threads incrementing
937 // the reference count at once. This branch will never be taken in
938 // any realistic program.
940 // We abort because such a program is incredibly degenerate, and we
941 // don't care to support it.
942 if old_size
> MAX_REFCOUNT
{
948 Self::from_inner(self.ptr
)
952 #[stable(feature = "rust1", since = "1.0.0")]
953 impl<T
: ?Sized
> Deref
for Arc
<T
> {
957 fn deref(&self) -> &T
{
962 #[unstable(feature = "receiver_trait", issue = "0")]
963 impl<T
: ?Sized
> Receiver
for Arc
<T
> {}
965 impl<T
: Clone
> Arc
<T
> {
966 /// Makes a mutable reference into the given `Arc`.
968 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
969 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
970 /// ensure unique ownership. This is also referred to as clone-on-write.
972 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
974 /// [weak]: struct.Weak.html
975 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
976 /// [get_mut]: struct.Arc.html#method.get_mut
981 /// use std::sync::Arc;
983 /// let mut data = Arc::new(5);
985 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
986 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
987 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
988 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
989 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
991 /// // Now `data` and `other_data` point to different values.
992 /// assert_eq!(*data, 8);
993 /// assert_eq!(*other_data, 12);
996 #[stable(feature = "arc_unique", since = "1.4.0")]
997 pub fn make_mut(this
: &mut Self) -> &mut T
{
998 // Note that we hold both a strong reference and a weak reference.
999 // Thus, releasing our strong reference only will not, by itself, cause
1000 // the memory to be deallocated.
1002 // Use Acquire to ensure that we see any writes to `weak` that happen
1003 // before release writes (i.e., decrements) to `strong`. Since we hold a
1004 // weak count, there's no chance the ArcInner itself could be
1006 if this
.inner().strong
.compare_exchange(1, 0, Acquire
, Relaxed
).is_err() {
1007 // Another strong pointer exists; clone
1008 *this
= Arc
::new((**this
).clone());
1009 } else if this
.inner().weak
.load(Relaxed
) != 1 {
1010 // Relaxed suffices in the above because this is fundamentally an
1011 // optimization: we are always racing with weak pointers being
1012 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1014 // We removed the last strong ref, but there are additional weak
1015 // refs remaining. We'll move the contents to a new Arc, and
1016 // invalidate the other weak refs.
1018 // Note that it is not possible for the read of `weak` to yield
1019 // usize::MAX (i.e., locked), since the weak count can only be
1020 // locked by a thread with a strong reference.
1022 // Materialize our own implicit weak pointer, so that it can clean
1023 // up the ArcInner as needed.
1024 let weak
= Weak { ptr: this.ptr }
;
1026 // mark the data itself as already deallocated
1028 // there is no data race in the implicit write caused by `read`
1029 // here (due to zeroing) because data is no longer accessed by
1030 // other threads (due to there being no more strong refs at this
1032 let mut swap
= Arc
::new(ptr
::read(&weak
.ptr
.as_ref().data
));
1033 mem
::swap(this
, &mut swap
);
1037 // We were the sole reference of either kind; bump back up the
1038 // strong ref count.
1039 this
.inner().strong
.store(1, Release
);
1042 // As with `get_mut()`, the unsafety is ok because our reference was
1043 // either unique to begin with, or became one upon cloning the contents.
1045 &mut this
.ptr
.as_mut().data
1050 impl<T
: ?Sized
> Arc
<T
> {
1051 /// Returns a mutable reference to the inner value, if there are
1052 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
1054 /// Returns [`None`][option] otherwise, because it is not safe to
1055 /// mutate a shared value.
1057 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1058 /// the inner value when it's shared.
1060 /// [weak]: struct.Weak.html
1061 /// [option]: ../../std/option/enum.Option.html
1062 /// [make_mut]: struct.Arc.html#method.make_mut
1063 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1068 /// use std::sync::Arc;
1070 /// let mut x = Arc::new(3);
1071 /// *Arc::get_mut(&mut x).unwrap() = 4;
1072 /// assert_eq!(*x, 4);
1074 /// let _y = Arc::clone(&x);
1075 /// assert!(Arc::get_mut(&mut x).is_none());
1078 #[stable(feature = "arc_unique", since = "1.4.0")]
1079 pub fn get_mut(this
: &mut Self) -> Option
<&mut T
> {
1080 if this
.is_unique() {
1081 // This unsafety is ok because we're guaranteed that the pointer
1082 // returned is the *only* pointer that will ever be returned to T. Our
1083 // reference count is guaranteed to be 1 at this point, and we required
1084 // the Arc itself to be `mut`, so we're returning the only possible
1085 // reference to the inner data.
1087 Some(Arc
::get_mut_unchecked(this
))
1094 /// Returns a mutable reference to the inner value,
1095 /// without any check.
1097 /// See also [`get_mut`], which is safe and does appropriate checks.
1099 /// [`get_mut`]: struct.Arc.html#method.get_mut
1103 /// Any other `Arc` or [`Weak`] pointers to the same value must not be dereferenced
1104 /// for the duration of the returned borrow.
1105 /// This is trivially the case if no such pointers exist,
1106 /// for example immediately after `Arc::new`.
1111 /// #![feature(get_mut_unchecked)]
1113 /// use std::sync::Arc;
1115 /// let mut x = Arc::new(String::new());
1117 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1119 /// assert_eq!(*x, "foo");
1122 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1123 pub unsafe fn get_mut_unchecked(this
: &mut Self) -> &mut T
{
1124 &mut this
.ptr
.as_mut().data
1127 /// Determine whether this is the unique reference (including weak refs) to
1128 /// the underlying data.
1130 /// Note that this requires locking the weak ref count.
1131 fn is_unique(&mut self) -> bool
{
1132 // lock the weak pointer count if we appear to be the sole weak pointer
1135 // The acquire label here ensures a happens-before relationship with any
1136 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1137 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1138 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1139 if self.inner().weak
.compare_exchange(1, usize::MAX
, Acquire
, Relaxed
).is_ok() {
1140 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1141 // counter in `drop` -- the only access that happens when any but the last reference
1142 // is being dropped.
1143 let unique
= self.inner().strong
.load(Acquire
) == 1;
1145 // The release write here synchronizes with a read in `downgrade`,
1146 // effectively preventing the above read of `strong` from happening
1148 self.inner().weak
.store(1, Release
); // release the lock
1156 #[stable(feature = "rust1", since = "1.0.0")]
1157 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1158 /// Drops the `Arc`.
1160 /// This will decrement the strong reference count. If the strong reference
1161 /// count reaches zero then the only other references (if any) are
1162 /// [`Weak`], so we `drop` the inner value.
1167 /// use std::sync::Arc;
1171 /// impl Drop for Foo {
1172 /// fn drop(&mut self) {
1173 /// println!("dropped!");
1177 /// let foo = Arc::new(Foo);
1178 /// let foo2 = Arc::clone(&foo);
1180 /// drop(foo); // Doesn't print anything
1181 /// drop(foo2); // Prints "dropped!"
1184 /// [`Weak`]: ../../std/sync/struct.Weak.html
1186 fn drop(&mut self) {
1187 // Because `fetch_sub` is already atomic, we do not need to synchronize
1188 // with other threads unless we are going to delete the object. This
1189 // same logic applies to the below `fetch_sub` to the `weak` count.
1190 if self.inner().strong
.fetch_sub(1, Release
) != 1 {
1194 // This fence is needed to prevent reordering of use of the data and
1195 // deletion of the data. Because it is marked `Release`, the decreasing
1196 // of the reference count synchronizes with this `Acquire` fence. This
1197 // means that use of the data happens before decreasing the reference
1198 // count, which happens before this fence, which happens before the
1199 // deletion of the data.
1201 // As explained in the [Boost documentation][1],
1203 // > It is important to enforce any possible access to the object in one
1204 // > thread (through an existing reference) to *happen before* deleting
1205 // > the object in a different thread. This is achieved by a "release"
1206 // > operation after dropping a reference (any access to the object
1207 // > through this reference must obviously happened before), and an
1208 // > "acquire" operation before deleting the object.
1210 // In particular, while the contents of an Arc are usually immutable, it's
1211 // possible to have interior writes to something like a Mutex<T>. Since a
1212 // Mutex is not acquired when it is deleted, we can't rely on its
1213 // synchronization logic to make writes in thread A visible to a destructor
1214 // running in thread B.
1216 // Also note that the Acquire fence here could probably be replaced with an
1217 // Acquire load, which could improve performance in highly-contended
1218 // situations. See [2].
1220 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1221 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1222 atomic
::fence(Acquire
);
1230 impl Arc
<dyn Any
+ Send
+ Sync
> {
1232 #[stable(feature = "rc_downcast", since = "1.29.0")]
1233 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1238 /// use std::any::Any;
1239 /// use std::sync::Arc;
1241 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1242 /// if let Ok(string) = value.downcast::<String>() {
1243 /// println!("String ({}): {}", string.len(), string);
1248 /// let my_string = "Hello World".to_string();
1249 /// print_if_string(Arc::new(my_string));
1250 /// print_if_string(Arc::new(0i8));
1253 pub fn downcast
<T
>(self) -> Result
<Arc
<T
>, Self>
1255 T
: Any
+ Send
+ Sync
+ '
static,
1257 if (*self).is
::<T
>() {
1258 let ptr
= self.ptr
.cast
::<ArcInner
<T
>>();
1260 Ok(Arc
::from_inner(ptr
))
1268 /// Constructs a new `Weak<T>`, without allocating any memory.
1269 /// Calling [`upgrade`] on the return value always gives [`None`].
1271 /// [`upgrade`]: struct.Weak.html#method.upgrade
1272 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1277 /// use std::sync::Weak;
1279 /// let empty: Weak<i64> = Weak::new();
1280 /// assert!(empty.upgrade().is_none());
1282 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1283 pub fn new() -> Weak
<T
> {
1285 ptr
: NonNull
::new(usize::MAX
as *mut ArcInner
<T
>).expect("MAX is not 0"),
1289 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1291 /// It is up to the caller to ensure that the object is still alive when accessing it through
1294 /// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
1299 /// #![feature(weak_into_raw)]
1301 /// use std::sync::Arc;
1304 /// let strong = Arc::new("hello".to_owned());
1305 /// let weak = Arc::downgrade(&strong);
1306 /// // Both point to the same object
1307 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1308 /// // The strong here keeps it alive, so we can still access the object.
1309 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1312 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1313 /// // undefined behaviour.
1314 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1317 /// [`null`]: ../../std/ptr/fn.null.html
1318 #[unstable(feature = "weak_into_raw", issue = "60728")]
1319 pub fn as_raw(&self) -> *const T
{
1320 match self.inner() {
1321 None
=> ptr
::null(),
1323 let offset
= data_offset_sized
::<T
>();
1324 let ptr
= inner
as *const ArcInner
<T
>;
1325 // Note: while the pointer we create may already point to dropped value, the
1326 // allocation still lives (it must hold the weak point as long as we are alive).
1327 // Therefore, the offset is OK to do, it won't get out of the allocation.
1328 let ptr
= unsafe { (ptr as *const u8).offset(offset) }
;
1334 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1336 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1337 /// can be turned back into the `Weak<T>` with [`from_raw`].
1339 /// The same restrictions of accessing the target of the pointer as with
1340 /// [`as_raw`] apply.
1345 /// #![feature(weak_into_raw)]
1347 /// use std::sync::{Arc, Weak};
1349 /// let strong = Arc::new("hello".to_owned());
1350 /// let weak = Arc::downgrade(&strong);
1351 /// let raw = weak.into_raw();
1353 /// assert_eq!(1, Arc::weak_count(&strong));
1354 /// assert_eq!("hello", unsafe { &*raw });
1356 /// drop(unsafe { Weak::from_raw(raw) });
1357 /// assert_eq!(0, Arc::weak_count(&strong));
1360 /// [`from_raw`]: struct.Weak.html#method.from_raw
1361 /// [`as_raw`]: struct.Weak.html#method.as_raw
1362 #[unstable(feature = "weak_into_raw", issue = "60728")]
1363 pub fn into_raw(self) -> *const T
{
1364 let result
= self.as_raw();
1369 /// Converts a raw pointer previously created by [`into_raw`] back into
1372 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1373 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1375 /// It takes ownership of one weak count. In case a [`null`] is passed, a dangling [`Weak`] is
1380 /// The pointer must represent one valid weak count. In other words, it must point to `T` which
1381 /// is or *was* managed by an [`Arc`] and the weak count of that [`Arc`] must not have reached
1382 /// 0. It is allowed for the strong count to be 0.
1387 /// #![feature(weak_into_raw)]
1389 /// use std::sync::{Arc, Weak};
1391 /// let strong = Arc::new("hello".to_owned());
1393 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1394 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1396 /// assert_eq!(2, Arc::weak_count(&strong));
1398 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1399 /// assert_eq!(1, Arc::weak_count(&strong));
1403 /// // Decrement the last weak count.
1404 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1407 /// [`null`]: ../../std/ptr/fn.null.html
1408 /// [`into_raw`]: struct.Weak.html#method.into_raw
1409 /// [`upgrade`]: struct.Weak.html#method.upgrade
1410 /// [`Weak`]: struct.Weak.html
1411 /// [`Arc`]: struct.Arc.html
1412 #[unstable(feature = "weak_into_raw", issue = "60728")]
1413 pub unsafe fn from_raw(ptr
: *const T
) -> Self {
1417 // See Arc::from_raw for details
1418 let offset
= data_offset(ptr
);
1419 let fake_ptr
= ptr
as *mut ArcInner
<T
>;
1420 let ptr
= set_data_ptr(fake_ptr
, (ptr
as *mut u8).offset(-offset
));
1422 ptr
: NonNull
::new(ptr
).expect("Invalid pointer passed to from_raw"),
1428 impl<T
: ?Sized
> Weak
<T
> {
1429 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1430 /// the lifetime of the value if successful.
1432 /// Returns [`None`] if the value has since been dropped.
1434 /// [`Arc`]: struct.Arc.html
1435 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1440 /// use std::sync::Arc;
1442 /// let five = Arc::new(5);
1444 /// let weak_five = Arc::downgrade(&five);
1446 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1447 /// assert!(strong_five.is_some());
1449 /// // Destroy all strong pointers.
1450 /// drop(strong_five);
1453 /// assert!(weak_five.upgrade().is_none());
1455 #[stable(feature = "arc_weak", since = "1.4.0")]
1456 pub fn upgrade(&self) -> Option
<Arc
<T
>> {
1457 // We use a CAS loop to increment the strong count instead of a
1458 // fetch_add because once the count hits 0 it must never be above 0.
1459 let inner
= self.inner()?
;
1461 // Relaxed load because any write of 0 that we can observe
1462 // leaves the field in a permanently zero state (so a
1463 // "stale" read of 0 is fine), and any other value is
1464 // confirmed via the CAS below.
1465 let mut n
= inner
.strong
.load(Relaxed
);
1472 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1473 if n
> MAX_REFCOUNT
{
1479 // Relaxed is valid for the same reason it is on Arc's Clone impl
1480 match inner
.strong
.compare_exchange_weak(n
, n
+ 1, Relaxed
, Relaxed
) {
1481 Ok(_
) => return Some(Arc
::from_inner(self.ptr
)), // null checked above
1482 Err(old
) => n
= old
,
1487 /// Gets the number of strong (`Arc`) pointers pointing to this value.
1489 /// If `self` was created using [`Weak::new`], this will return 0.
1491 /// [`Weak::new`]: #method.new
1492 #[unstable(feature = "weak_counts", issue = "57977")]
1493 pub fn strong_count(&self) -> usize {
1494 if let Some(inner
) = self.inner() {
1495 inner
.strong
.load(SeqCst
)
1501 /// Gets an approximation of the number of `Weak` pointers pointing to this
1504 /// If `self` was created using [`Weak::new`], this will return 0. If not,
1505 /// the returned value is at least 1, since `self` still points to the
1510 /// Due to implementation details, the returned value can be off by 1 in
1511 /// either direction when other threads are manipulating any `Arc`s or
1512 /// `Weak`s pointing to the same value.
1514 /// [`Weak::new`]: #method.new
1515 #[unstable(feature = "weak_counts", issue = "57977")]
1516 pub fn weak_count(&self) -> Option
<usize> {
1517 // Due to the implicit weak pointer added when any strong pointers are
1518 // around, we cannot implement `weak_count` correctly since it
1519 // necessarily requires accessing the strong count and weak count in an
1520 // unsynchronized fashion. So this version is a bit racy.
1521 self.inner().map(|inner
| {
1522 let strong
= inner
.strong
.load(SeqCst
);
1523 let weak
= inner
.weak
.load(SeqCst
);
1525 // If the last `Arc` has *just* been dropped, it might not yet
1526 // have removed the implicit weak count, so the value we get
1527 // here might be 1 too high.
1530 // As long as there's still at least 1 `Arc` around, subtract
1531 // the implicit weak pointer.
1532 // Note that the last `Arc` might get dropped between the 2
1533 // loads we do above, removing the implicit weak pointer. This
1534 // means that the value might be 1 too low here. In order to not
1535 // return 0 here (which would happen if we're the only weak
1536 // pointer), we guard against that specifically.
1537 cmp
::max(1, weak
- 1)
1542 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1543 /// (i.e., when this `Weak` was created by `Weak::new`).
1545 fn inner(&self) -> Option
<&ArcInner
<T
>> {
1546 if is_dangling(self.ptr
) {
1549 Some(unsafe { self.ptr.as_ref() }
)
1553 /// Returns `true` if the two `Weak`s point to the same value (not just
1554 /// values that compare as equal), or if both don't point to any value
1555 /// (because they were created with `Weak::new()`).
1559 /// Since this compares pointers it means that `Weak::new()` will equal each
1560 /// other, even though they don't point to any value.
1565 /// use std::sync::Arc;
1567 /// let first_rc = Arc::new(5);
1568 /// let first = Arc::downgrade(&first_rc);
1569 /// let second = Arc::downgrade(&first_rc);
1571 /// assert!(first.ptr_eq(&second));
1573 /// let third_rc = Arc::new(5);
1574 /// let third = Arc::downgrade(&third_rc);
1576 /// assert!(!first.ptr_eq(&third));
1579 /// Comparing `Weak::new`.
1582 /// use std::sync::{Arc, Weak};
1584 /// let first = Weak::new();
1585 /// let second = Weak::new();
1586 /// assert!(first.ptr_eq(&second));
1588 /// let third_rc = Arc::new(());
1589 /// let third = Arc::downgrade(&third_rc);
1590 /// assert!(!first.ptr_eq(&third));
1593 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1594 pub fn ptr_eq(&self, other
: &Self) -> bool
{
1595 self.ptr
.as_ptr() == other
.ptr
.as_ptr()
1599 #[stable(feature = "arc_weak", since = "1.4.0")]
1600 impl<T
: ?Sized
> Clone
for Weak
<T
> {
1601 /// Makes a clone of the `Weak` pointer that points to the same value.
1606 /// use std::sync::{Arc, Weak};
1608 /// let weak_five = Arc::downgrade(&Arc::new(5));
1610 /// let _ = Weak::clone(&weak_five);
1613 fn clone(&self) -> Weak
<T
> {
1614 let inner
= if let Some(inner
) = self.inner() {
1617 return Weak { ptr: self.ptr }
;
1619 // See comments in Arc::clone() for why this is relaxed. This can use a
1620 // fetch_add (ignoring the lock) because the weak count is only locked
1621 // where are *no other* weak pointers in existence. (So we can't be
1622 // running this code in that case).
1623 let old_size
= inner
.weak
.fetch_add(1, Relaxed
);
1625 // See comments in Arc::clone() for why we do this (for mem::forget).
1626 if old_size
> MAX_REFCOUNT
{
1632 return Weak { ptr: self.ptr }
;
1636 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1637 impl<T
> Default
for Weak
<T
> {
1638 /// Constructs a new `Weak<T>`, without allocating memory.
1639 /// Calling [`upgrade`] on the return value always
1642 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1643 /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
1648 /// use std::sync::Weak;
1650 /// let empty: Weak<i64> = Default::default();
1651 /// assert!(empty.upgrade().is_none());
1653 fn default() -> Weak
<T
> {
1658 #[stable(feature = "arc_weak", since = "1.4.0")]
1659 impl<T
: ?Sized
> Drop
for Weak
<T
> {
1660 /// Drops the `Weak` pointer.
1665 /// use std::sync::{Arc, Weak};
1669 /// impl Drop for Foo {
1670 /// fn drop(&mut self) {
1671 /// println!("dropped!");
1675 /// let foo = Arc::new(Foo);
1676 /// let weak_foo = Arc::downgrade(&foo);
1677 /// let other_weak_foo = Weak::clone(&weak_foo);
1679 /// drop(weak_foo); // Doesn't print anything
1680 /// drop(foo); // Prints "dropped!"
1682 /// assert!(other_weak_foo.upgrade().is_none());
1684 fn drop(&mut self) {
1685 // If we find out that we were the last weak pointer, then its time to
1686 // deallocate the data entirely. See the discussion in Arc::drop() about
1687 // the memory orderings
1689 // It's not necessary to check for the locked state here, because the
1690 // weak count can only be locked if there was precisely one weak ref,
1691 // meaning that drop could only subsequently run ON that remaining weak
1692 // ref, which can only happen after the lock is released.
1693 let inner
= if let Some(inner
) = self.inner() {
1699 if inner
.weak
.fetch_sub(1, Release
) == 1 {
1700 atomic
::fence(Acquire
);
1702 Global
.dealloc(self.ptr
.cast(), Layout
::for_value(self.ptr
.as_ref()))
1708 #[stable(feature = "rust1", since = "1.0.0")]
1709 trait ArcEqIdent
<T
: ?Sized
+ PartialEq
> {
1710 fn eq(&self, other
: &Arc
<T
>) -> bool
;
1711 fn ne(&self, other
: &Arc
<T
>) -> bool
;
1714 #[stable(feature = "rust1", since = "1.0.0")]
1715 impl<T
: ?Sized
+ PartialEq
> ArcEqIdent
<T
> for Arc
<T
> {
1717 default fn eq(&self, other
: &Arc
<T
>) -> bool
{
1721 default fn ne(&self, other
: &Arc
<T
>) -> bool
{
1726 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1727 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
1728 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1729 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
1730 /// the same value, than two `&T`s.
1731 #[stable(feature = "rust1", since = "1.0.0")]
1732 impl<T
: ?Sized
+ Eq
> ArcEqIdent
<T
> for Arc
<T
> {
1734 fn eq(&self, other
: &Arc
<T
>) -> bool
{
1735 Arc
::ptr_eq(self, other
) || **self == **other
1739 fn ne(&self, other
: &Arc
<T
>) -> bool
{
1740 !Arc
::ptr_eq(self, other
) && **self != **other
1744 #[stable(feature = "rust1", since = "1.0.0")]
1745 impl<T
: ?Sized
+ PartialEq
> PartialEq
for Arc
<T
> {
1746 /// Equality for two `Arc`s.
1748 /// Two `Arc`s are equal if their inner values are equal.
1750 /// If `T` also implements `Eq`, two `Arc`s that point to the same value are
1756 /// use std::sync::Arc;
1758 /// let five = Arc::new(5);
1760 /// assert!(five == Arc::new(5));
1763 fn eq(&self, other
: &Arc
<T
>) -> bool
{
1764 ArcEqIdent
::eq(self, other
)
1767 /// Inequality for two `Arc`s.
1769 /// Two `Arc`s are unequal if their inner values are unequal.
1771 /// If `T` also implements `Eq`, two `Arc`s that point to the same value are
1777 /// use std::sync::Arc;
1779 /// let five = Arc::new(5);
1781 /// assert!(five != Arc::new(6));
1784 fn ne(&self, other
: &Arc
<T
>) -> bool
{
1785 ArcEqIdent
::ne(self, other
)
1789 #[stable(feature = "rust1", since = "1.0.0")]
1790 impl<T
: ?Sized
+ PartialOrd
> PartialOrd
for Arc
<T
> {
1791 /// Partial comparison for two `Arc`s.
1793 /// The two are compared by calling `partial_cmp()` on their inner values.
1798 /// use std::sync::Arc;
1799 /// use std::cmp::Ordering;
1801 /// let five = Arc::new(5);
1803 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1805 fn partial_cmp(&self, other
: &Arc
<T
>) -> Option
<Ordering
> {
1806 (**self).partial_cmp(&**other
)
1809 /// Less-than comparison for two `Arc`s.
1811 /// The two are compared by calling `<` on their inner values.
1816 /// use std::sync::Arc;
1818 /// let five = Arc::new(5);
1820 /// assert!(five < Arc::new(6));
1822 fn lt(&self, other
: &Arc
<T
>) -> bool
{
1823 *(*self) < *(*other
)
1826 /// 'Less than or equal to' comparison for two `Arc`s.
1828 /// The two are compared by calling `<=` on their inner values.
1833 /// use std::sync::Arc;
1835 /// let five = Arc::new(5);
1837 /// assert!(five <= Arc::new(5));
1839 fn le(&self, other
: &Arc
<T
>) -> bool
{
1840 *(*self) <= *(*other
)
1843 /// Greater-than comparison for two `Arc`s.
1845 /// The two are compared by calling `>` on their inner values.
1850 /// use std::sync::Arc;
1852 /// let five = Arc::new(5);
1854 /// assert!(five > Arc::new(4));
1856 fn gt(&self, other
: &Arc
<T
>) -> bool
{
1857 *(*self) > *(*other
)
1860 /// 'Greater than or equal to' comparison for two `Arc`s.
1862 /// The two are compared by calling `>=` on their inner values.
1867 /// use std::sync::Arc;
1869 /// let five = Arc::new(5);
1871 /// assert!(five >= Arc::new(5));
1873 fn ge(&self, other
: &Arc
<T
>) -> bool
{
1874 *(*self) >= *(*other
)
1877 #[stable(feature = "rust1", since = "1.0.0")]
1878 impl<T
: ?Sized
+ Ord
> Ord
for Arc
<T
> {
1879 /// Comparison for two `Arc`s.
1881 /// The two are compared by calling `cmp()` on their inner values.
1886 /// use std::sync::Arc;
1887 /// use std::cmp::Ordering;
1889 /// let five = Arc::new(5);
1891 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1893 fn cmp(&self, other
: &Arc
<T
>) -> Ordering
{
1894 (**self).cmp(&**other
)
1897 #[stable(feature = "rust1", since = "1.0.0")]
1898 impl<T
: ?Sized
+ Eq
> Eq
for Arc
<T
> {}
1900 #[stable(feature = "rust1", since = "1.0.0")]
1901 impl<T
: ?Sized
+ fmt
::Display
> fmt
::Display
for Arc
<T
> {
1902 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1903 fmt
::Display
::fmt(&**self, f
)
1907 #[stable(feature = "rust1", since = "1.0.0")]
1908 impl<T
: ?Sized
+ fmt
::Debug
> fmt
::Debug
for Arc
<T
> {
1909 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1910 fmt
::Debug
::fmt(&**self, f
)
1914 #[stable(feature = "rust1", since = "1.0.0")]
1915 impl<T
: ?Sized
> fmt
::Pointer
for Arc
<T
> {
1916 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1917 fmt
::Pointer
::fmt(&(&**self as *const T
), f
)
1921 #[stable(feature = "rust1", since = "1.0.0")]
1922 impl<T
: Default
> Default
for Arc
<T
> {
1923 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1928 /// use std::sync::Arc;
1930 /// let x: Arc<i32> = Default::default();
1931 /// assert_eq!(*x, 0);
1933 fn default() -> Arc
<T
> {
1934 Arc
::new(Default
::default())
1938 #[stable(feature = "rust1", since = "1.0.0")]
1939 impl<T
: ?Sized
+ Hash
> Hash
for Arc
<T
> {
1940 fn hash
<H
: Hasher
>(&self, state
: &mut H
) {
1941 (**self).hash(state
)
1945 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1946 impl<T
> From
<T
> for Arc
<T
> {
1947 fn from(t
: T
) -> Self {
1952 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1953 impl<T
: Clone
> From
<&[T
]> for Arc
<[T
]> {
1955 fn from(v
: &[T
]) -> Arc
<[T
]> {
1956 <Self as ArcFromSlice
<T
>>::from_slice(v
)
1960 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1961 impl From
<&str> for Arc
<str> {
1963 fn from(v
: &str) -> Arc
<str> {
1964 let arc
= Arc
::<[u8]>::from(v
.as_bytes());
1965 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1969 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1970 impl From
<String
> for Arc
<str> {
1972 fn from(v
: String
) -> Arc
<str> {
1977 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1978 impl<T
: ?Sized
> From
<Box
<T
>> for Arc
<T
> {
1980 fn from(v
: Box
<T
>) -> Arc
<T
> {
1985 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1986 impl<T
> From
<Vec
<T
>> for Arc
<[T
]> {
1988 fn from(mut v
: Vec
<T
>) -> Arc
<[T
]> {
1990 let arc
= Arc
::copy_from_slice(&v
);
1992 // Allow the Vec to free its memory, but not destroy its contents
2000 #[unstable(feature = "boxed_slice_try_from", issue = "0")]
2001 impl<T
, const N
: usize> TryFrom
<Arc
<[T
]>> for Arc
<[T
; N
]>
2003 [T
; N
]: LengthAtMost32
,
2005 type Error
= Arc
<[T
]>;
2007 fn try_from(boxed_slice
: Arc
<[T
]>) -> Result
<Self, Self::Error
> {
2008 if boxed_slice
.len() == N
{
2009 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) }
)
2016 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2017 impl<T
> iter
::FromIterator
<T
> for Arc
<[T
]> {
2018 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2020 /// # Performance characteristics
2022 /// ## The general case
2024 /// In the general case, collecting into `Arc<[T]>` is done by first
2025 /// collecting into a `Vec<T>`. That is, when writing the following:
2028 /// # use std::sync::Arc;
2029 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2030 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2033 /// this behaves as if we wrote:
2036 /// # use std::sync::Arc;
2037 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2038 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2039 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2040 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2043 /// This will allocate as many times as needed for constructing the `Vec<T>`
2044 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2046 /// ## Iterators of known length
2048 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2049 /// a single allocation will be made for the `Arc<[T]>`. For example:
2052 /// # use std::sync::Arc;
2053 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2054 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2056 fn from_iter
<I
: iter
::IntoIterator
<Item
= T
>>(iter
: I
) -> Self {
2057 ArcFromIter
::from_iter(iter
.into_iter())
2061 /// Specialization trait used for collecting into `Arc<[T]>`.
2062 trait ArcFromIter
<T
, I
> {
2063 fn from_iter(iter
: I
) -> Self;
2066 impl<T
, I
: Iterator
<Item
= T
>> ArcFromIter
<T
, I
> for Arc
<[T
]> {
2067 default fn from_iter(iter
: I
) -> Self {
2068 iter
.collect
::<Vec
<T
>>().into()
2072 impl<T
, I
: iter
::TrustedLen
<Item
= T
>> ArcFromIter
<T
, I
> for Arc
<[T
]> {
2073 default fn from_iter(iter
: I
) -> Self {
2074 // This is the case for a `TrustedLen` iterator.
2075 let (low
, high
) = iter
.size_hint();
2076 if let Some(high
) = high
{
2079 "TrustedLen iterator's size hint is not exact: {:?}",
2084 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2085 Arc
::from_iter_exact(iter
, low
)
2088 // Fall back to normal implementation.
2089 iter
.collect
::<Vec
<T
>>().into()
2094 impl<'a
, T
: 'a
+ Clone
> ArcFromIter
<&'a T
, slice
::Iter
<'a
, T
>> for Arc
<[T
]> {
2095 fn from_iter(iter
: slice
::Iter
<'a
, T
>) -> Self {
2096 // Delegate to `impl<T: Clone> From<&[T]> for Arc<[T]>`.
2098 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
2099 // which is even more performant.
2101 // In the fall-back case we have `T: Clone`. This is still better
2102 // than the `TrustedLen` implementation as slices have a known length
2103 // and so we get to avoid calling `size_hint` and avoid the branching.
2104 iter
.as_slice().into()
2108 #[stable(feature = "rust1", since = "1.0.0")]
2109 impl<T
: ?Sized
> borrow
::Borrow
<T
> for Arc
<T
> {
2110 fn borrow(&self) -> &T
{
2115 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2116 impl<T
: ?Sized
> AsRef
<T
> for Arc
<T
> {
2117 fn as_ref(&self) -> &T
{
2122 #[stable(feature = "pin", since = "1.33.0")]
2123 impl<T
: ?Sized
> Unpin
for Arc
<T
> { }
2125 /// Computes the offset of the data field within `ArcInner`.
2126 unsafe fn data_offset
<T
: ?Sized
>(ptr
: *const T
) -> isize {
2127 // Align the unsized value to the end of the `ArcInner`.
2128 // Because it is `?Sized`, it will always be the last field in memory.
2129 data_offset_align(align_of_val(&*ptr
))
2132 /// Computes the offset of the data field within `ArcInner`.
2134 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2135 fn data_offset_sized
<T
>() -> isize {
2136 data_offset_align(align_of
::<T
>())
2140 fn data_offset_align(align
: usize) -> isize {
2141 let layout
= Layout
::new
::<ArcInner
<()>>();
2142 (layout
.size() + layout
.padding_needed_for(align
)) as isize