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::sync::atomic;
11 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
14 use core::cmp::{self, Ordering};
15 use core::intrinsics::abort;
16 use core::mem::{self, align_of_val, size_of_val};
17 use core::ops::{Deref, Receiver};
18 use core::ops::{CoerceUnsized, DispatchFromDyn};
20 use core::ptr::{self, NonNull};
21 use core::marker::{Unpin, Unsize, PhantomData};
22 use core::hash::{Hash, Hasher};
23 use core::{isize, usize};
24 use core::convert::From;
26 use alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
32 /// A soft limit on the amount of references that may be made to an `Arc`.
34 /// Going above this limit will abort your program (although not
35 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
36 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
38 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
39 /// Reference Counted'.
41 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
42 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
43 /// a new `Arc` instance, which points to the same value on the heap as the
44 /// source `Arc`, while increasing a reference count. When the last `Arc`
45 /// pointer to a given value is destroyed, the pointed-to value is also
48 /// Shared references in Rust disallow mutation by default, and `Arc` is no
49 /// exception: you cannot generally obtain a mutable reference to something
50 /// inside an `Arc`. If you need to mutate through an `Arc`, use
51 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
56 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
57 /// counting. This means that it is thread-safe. The disadvantage is that
58 /// atomic operations are more expensive than ordinary memory accesses. If you
59 /// are not sharing reference-counted values between threads, consider using
60 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
61 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
62 /// However, a library might choose `Arc<T>` in order to give library consumers
65 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
66 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
67 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
68 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
69 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
70 /// data, but it doesn't add thread safety to its data. Consider
71 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
72 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
73 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
74 /// non-atomic operations.
76 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
77 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
79 /// ## Breaking cycles with `Weak`
81 /// The [`downgrade`][downgrade] method can be used to create a non-owning
82 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
83 /// to an `Arc`, but this will return [`None`] if the value has already been
86 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
87 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
88 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
89 /// pointers from children back to their parents.
91 /// # Cloning references
93 /// Creating a new reference from an existing reference counted pointer is done using the
94 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
97 /// use std::sync::Arc;
98 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
99 /// // The two syntaxes below are equivalent.
100 /// let a = foo.clone();
101 /// let b = Arc::clone(&foo);
102 /// // a, b, and foo are all Arcs that point to the same memory location
105 /// The [`Arc::clone(&from)`] syntax is the most idiomatic because it conveys more explicitly
106 /// the meaning of the code. In the example above, this syntax makes it easier to see that
107 /// this code is creating a new reference rather than copying the whole content of foo.
109 /// ## `Deref` behavior
111 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
112 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
113 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
114 /// functions, called using function-like syntax:
117 /// use std::sync::Arc;
118 /// let my_arc = Arc::new(());
120 /// Arc::downgrade(&my_arc);
123 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have
124 /// already been destroyed.
126 /// [arc]: struct.Arc.html
127 /// [weak]: struct.Weak.html
128 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
129 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
130 /// [mutex]: ../../std/sync/struct.Mutex.html
131 /// [rwlock]: ../../std/sync/struct.RwLock.html
132 /// [atomic]: ../../std/sync/atomic/index.html
133 /// [`Send`]: ../../std/marker/trait.Send.html
134 /// [`Sync`]: ../../std/marker/trait.Sync.html
135 /// [deref]: ../../std/ops/trait.Deref.html
136 /// [downgrade]: struct.Arc.html#method.downgrade
137 /// [upgrade]: struct.Weak.html#method.upgrade
138 /// [`None`]: ../../std/option/enum.Option.html#variant.None
139 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
140 /// [`std::sync`]: ../../std/sync/index.html
141 /// [`Arc::clone(&from)`]: #method.clone
145 /// Sharing some immutable data between threads:
147 // Note that we **do not** run these tests here. The windows builders get super
148 // unhappy if a thread outlives the main thread and then exits at the same time
149 // (something deadlocks) so we just avoid this entirely by not running these
152 /// use std::sync::Arc;
155 /// let five = Arc::new(5);
158 /// let five = Arc::clone(&five);
160 /// thread::spawn(move || {
161 /// println!("{:?}", five);
166 /// Sharing a mutable [`AtomicUsize`]:
168 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
171 /// use std::sync::Arc;
172 /// use std::sync::atomic::{AtomicUsize, Ordering};
175 /// let val = Arc::new(AtomicUsize::new(5));
178 /// let val = Arc::clone(&val);
180 /// thread::spawn(move || {
181 /// let v = val.fetch_add(1, Ordering::SeqCst);
182 /// println!("{:?}", v);
187 /// See the [`rc` documentation][rc_examples] for more examples of reference
188 /// counting in general.
190 /// [rc_examples]: ../../std/rc/index.html#examples
191 #[cfg_attr(not(test), lang = "arc")]
192 #[stable(feature = "rust1", since = "1.0.0")]
193 pub struct Arc<T: ?Sized> {
194 ptr: NonNull<ArcInner<T>>,
195 phantom: PhantomData<T>,
198 #[stable(feature = "rust1", since = "1.0.0")]
199 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
200 #[stable(feature = "rust1", since = "1.0.0")]
201 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
203 #[unstable(feature = "coerce_unsized", issue = "27732")]
204 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
206 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
207 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
209 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
210 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
211 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
213 /// Since a `Weak` reference does not count towards ownership, it will not
214 /// prevent the inner value from being dropped, and `Weak` itself makes no
215 /// guarantees about the value still being present and may return [`None`]
216 /// when [`upgrade`]d.
218 /// A `Weak` pointer is useful for keeping a temporary reference to the value
219 /// within [`Arc`] without extending its lifetime. It is also used to prevent
220 /// circular references between [`Arc`] pointers, since mutual owning references
221 /// would never allow either [`Arc`] to be dropped. For example, a tree could
222 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
223 /// pointers from children back to their parents.
225 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
227 /// [`Arc`]: struct.Arc.html
228 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
229 /// [`upgrade`]: struct.Weak.html#method.upgrade
230 /// [`Option`]: ../../std/option/enum.Option.html
231 /// [`None`]: ../../std/option/enum.Option.html#variant.None
232 #[stable(feature = "arc_weak", since = "1.4.0")]
233 pub struct Weak<T: ?Sized> {
234 // This is a `NonNull` to allow optimizing the size of this type in enums,
235 // but it is not necessarily a valid pointer.
236 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
237 // to allocate space on the heap. That's not a value a real pointer
238 // will ever have because RcBox has alignment at least 2.
239 ptr: NonNull<ArcInner<T>>,
242 #[stable(feature = "arc_weak", since = "1.4.0")]
243 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
244 #[stable(feature = "arc_weak", since = "1.4.0")]
245 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
247 #[unstable(feature = "coerce_unsized", issue = "27732")]
248 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
249 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
250 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
252 #[stable(feature = "arc_weak", since = "1.4.0")]
253 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
254 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
259 struct ArcInner<T: ?Sized> {
260 strong: atomic::AtomicUsize,
262 // the value usize::MAX acts as a sentinel for temporarily "locking" the
263 // ability to upgrade weak pointers or downgrade strong ones; this is used
264 // to avoid races in `make_mut` and `get_mut`.
265 weak: atomic::AtomicUsize,
270 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
271 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
274 /// Constructs a new `Arc<T>`.
279 /// use std::sync::Arc;
281 /// let five = Arc::new(5);
284 #[stable(feature = "rust1", since = "1.0.0")]
285 pub fn new(data: T) -> Arc<T> {
286 // Start the weak pointer count as 1 which is the weak pointer that's
287 // held by all the strong pointers (kinda), see std/rc.rs for more info
288 let x: Box<_> = box ArcInner {
289 strong: atomic::AtomicUsize::new(1),
290 weak: atomic::AtomicUsize::new(1),
293 Arc { ptr: Box::into_raw_non_null(x), phantom: PhantomData }
296 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
297 /// `data` will be pinned in memory and unable to be moved.
298 #[stable(feature = "pin", since = "1.33.0")]
299 pub fn pin(data: T) -> Pin<Arc<T>> {
300 unsafe { Pin::new_unchecked(Arc::new(data)) }
303 /// Returns the contained value, if the `Arc` has exactly one strong reference.
305 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
308 /// This will succeed even if there are outstanding weak references.
310 /// [result]: ../../std/result/enum.Result.html
315 /// use std::sync::Arc;
317 /// let x = Arc::new(3);
318 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
320 /// let x = Arc::new(4);
321 /// let _y = Arc::clone(&x);
322 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
325 #[stable(feature = "arc_unique", since = "1.4.0")]
326 pub fn try_unwrap(this: Self) -> Result<T, Self> {
327 // See `drop` for why all these atomics are like this
328 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
332 atomic::fence(Acquire);
335 let elem = ptr::read(&this.ptr.as_ref().data);
337 // Make a weak pointer to clean up the implicit strong-weak reference
338 let _weak = Weak { ptr: this.ptr };
346 impl<T: ?Sized> Arc<T> {
347 /// Consumes the `Arc`, returning the wrapped pointer.
349 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
350 /// [`Arc::from_raw`][from_raw].
352 /// [from_raw]: struct.Arc.html#method.from_raw
357 /// use std::sync::Arc;
359 /// let x = Arc::new(10);
360 /// let x_ptr = Arc::into_raw(x);
361 /// assert_eq!(unsafe { *x_ptr }, 10);
363 #[stable(feature = "rc_raw", since = "1.17.0")]
364 pub fn into_raw(this: Self) -> *const T {
365 let ptr: *const T = &*this;
370 /// Constructs an `Arc` from a raw pointer.
372 /// The raw pointer must have been previously returned by a call to a
373 /// [`Arc::into_raw`][into_raw].
375 /// This function is unsafe because improper use may lead to memory problems. For example, a
376 /// double-free may occur if the function is called twice on the same raw pointer.
378 /// [into_raw]: struct.Arc.html#method.into_raw
383 /// use std::sync::Arc;
385 /// let x = Arc::new(10);
386 /// let x_ptr = Arc::into_raw(x);
389 /// // Convert back to an `Arc` to prevent leak.
390 /// let x = Arc::from_raw(x_ptr);
391 /// assert_eq!(*x, 10);
393 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
396 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
398 #[stable(feature = "rc_raw", since = "1.17.0")]
399 pub unsafe fn from_raw(ptr: *const T) -> Self {
400 // Align the unsized value to the end of the ArcInner.
401 // Because it is ?Sized, it will always be the last field in memory.
402 let align = align_of_val(&*ptr);
403 let layout = Layout::new::<ArcInner<()>>();
404 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
406 // Reverse the offset to find the original ArcInner.
407 let fake_ptr = ptr as *mut ArcInner<T>;
408 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
411 ptr: NonNull::new_unchecked(arc_ptr),
412 phantom: PhantomData,
416 /// Consumes the `Arc`, returning the wrapped pointer as `NonNull<T>`.
421 /// #![feature(rc_into_raw_non_null)]
423 /// use std::sync::Arc;
425 /// let x = Arc::new(10);
426 /// let ptr = Arc::into_raw_non_null(x);
427 /// let deref = unsafe { *ptr.as_ref() };
428 /// assert_eq!(deref, 10);
430 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
432 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
433 // safe because Arc guarantees its pointer is non-null
434 unsafe { NonNull::new_unchecked(Arc::into_raw(this) as *mut _) }
437 /// Creates a new [`Weak`][weak] pointer to this value.
439 /// [weak]: struct.Weak.html
444 /// use std::sync::Arc;
446 /// let five = Arc::new(5);
448 /// let weak_five = Arc::downgrade(&five);
450 #[stable(feature = "arc_weak", since = "1.4.0")]
451 pub fn downgrade(this: &Self) -> Weak<T> {
452 // This Relaxed is OK because we're checking the value in the CAS
454 let mut cur = this.inner().weak.load(Relaxed);
457 // check if the weak counter is currently "locked"; if so, spin.
458 if cur == usize::MAX {
459 cur = this.inner().weak.load(Relaxed);
463 // NOTE: this code currently ignores the possibility of overflow
464 // into usize::MAX; in general both Rc and Arc need to be adjusted
465 // to deal with overflow.
467 // Unlike with Clone(), we need this to be an Acquire read to
468 // synchronize with the write coming from `is_unique`, so that the
469 // events prior to that write happen before this read.
470 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
472 // Make sure we do not create a dangling Weak
473 debug_assert!(!is_dangling(this.ptr));
474 return Weak { ptr: this.ptr };
476 Err(old) => cur = old,
481 /// Gets the number of [`Weak`][weak] pointers to this value.
483 /// [weak]: struct.Weak.html
487 /// This method by itself is safe, but using it correctly requires extra care.
488 /// Another thread can change the weak count at any time,
489 /// including potentially between calling this method and acting on the result.
494 /// use std::sync::Arc;
496 /// let five = Arc::new(5);
497 /// let _weak_five = Arc::downgrade(&five);
499 /// // This assertion is deterministic because we haven't shared
500 /// // the `Arc` or `Weak` between threads.
501 /// assert_eq!(1, Arc::weak_count(&five));
504 #[stable(feature = "arc_counts", since = "1.15.0")]
505 pub fn weak_count(this: &Self) -> usize {
506 let cnt = this.inner().weak.load(SeqCst);
507 // If the weak count is currently locked, the value of the
508 // count was 0 just before taking the lock.
509 if cnt == usize::MAX { 0 } else { cnt - 1 }
512 /// Gets the number of strong (`Arc`) pointers to this value.
516 /// This method by itself is safe, but using it correctly requires extra care.
517 /// Another thread can change the strong count at any time,
518 /// including potentially between calling this method and acting on the result.
523 /// use std::sync::Arc;
525 /// let five = Arc::new(5);
526 /// let _also_five = Arc::clone(&five);
528 /// // This assertion is deterministic because we haven't shared
529 /// // the `Arc` between threads.
530 /// assert_eq!(2, Arc::strong_count(&five));
533 #[stable(feature = "arc_counts", since = "1.15.0")]
534 pub fn strong_count(this: &Self) -> usize {
535 this.inner().strong.load(SeqCst)
539 fn inner(&self) -> &ArcInner<T> {
540 // This unsafety is ok because while this arc is alive we're guaranteed
541 // that the inner pointer is valid. Furthermore, we know that the
542 // `ArcInner` structure itself is `Sync` because the inner data is
543 // `Sync` as well, so we're ok loaning out an immutable pointer to these
545 unsafe { self.ptr.as_ref() }
548 // Non-inlined part of `drop`.
550 unsafe fn drop_slow(&mut self) {
551 // Destroy the data at this time, even though we may not free the box
552 // allocation itself (there may still be weak pointers lying around).
553 ptr::drop_in_place(&mut self.ptr.as_mut().data);
555 if self.inner().weak.fetch_sub(1, Release) == 1 {
556 atomic::fence(Acquire);
557 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
562 #[stable(feature = "ptr_eq", since = "1.17.0")]
563 /// Returns true if the two `Arc`s point to the same value (not
564 /// just values that compare as equal).
569 /// use std::sync::Arc;
571 /// let five = Arc::new(5);
572 /// let same_five = Arc::clone(&five);
573 /// let other_five = Arc::new(5);
575 /// assert!(Arc::ptr_eq(&five, &same_five));
576 /// assert!(!Arc::ptr_eq(&five, &other_five));
578 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
579 this.ptr.as_ptr() == other.ptr.as_ptr()
583 impl<T: ?Sized> Arc<T> {
584 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
585 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
586 // Calculate layout using the given value.
587 // Previously, layout was calculated on the expression
588 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
589 // reference (see #54908).
590 let layout = Layout::new::<ArcInner<()>>()
591 .extend(Layout::for_value(&*ptr)).unwrap().0
592 .pad_to_align().unwrap();
594 let mem = Global.alloc(layout)
595 .unwrap_or_else(|_| handle_alloc_error(layout));
597 // Initialize the ArcInner
598 let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;
599 debug_assert_eq!(Layout::for_value(&*inner), layout);
601 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
602 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
607 fn from_box(v: Box<T>) -> Arc<T> {
609 let box_unique = Box::into_unique(v);
610 let bptr = box_unique.as_ptr();
612 let value_size = size_of_val(&*bptr);
613 let ptr = Self::allocate_for_ptr(bptr);
615 // Copy value as bytes
616 ptr::copy_nonoverlapping(
617 bptr as *const T as *const u8,
618 &mut (*ptr).data as *mut _ as *mut u8,
621 // Free the allocation without dropping its contents
622 box_free(box_unique);
624 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
629 // Sets the data pointer of a `?Sized` raw pointer.
631 // For a slice/trait object, this sets the `data` field and leaves the rest
632 // unchanged. For a sized raw pointer, this simply sets the pointer.
633 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
634 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
639 // Copy elements from slice into newly allocated Arc<[T]>
641 // Unsafe because the caller must either take ownership or bind `T: Copy`
642 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
643 let v_ptr = v as *const [T];
644 let ptr = Self::allocate_for_ptr(v_ptr);
646 ptr::copy_nonoverlapping(
648 &mut (*ptr).data as *mut [T] as *mut T,
651 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
655 // Specialization trait used for From<&[T]>
656 trait ArcFromSlice<T> {
657 fn from_slice(slice: &[T]) -> Self;
660 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
662 default fn from_slice(v: &[T]) -> Self {
663 // Panic guard while cloning T elements.
664 // In the event of a panic, elements that have been written
665 // into the new ArcInner will be dropped, then the memory freed.
673 impl<T> Drop for Guard<T> {
675 use core::slice::from_raw_parts_mut;
678 let slice = from_raw_parts_mut(self.elems, self.n_elems);
679 ptr::drop_in_place(slice);
681 Global.dealloc(self.mem.cast(), self.layout.clone());
687 let v_ptr = v as *const [T];
688 let ptr = Self::allocate_for_ptr(v_ptr);
690 let mem = ptr as *mut _ as *mut u8;
691 let layout = Layout::for_value(&*ptr);
693 // Pointer to first element
694 let elems = &mut (*ptr).data as *mut [T] as *mut T;
696 let mut guard = Guard{
697 mem: NonNull::new_unchecked(mem),
703 for (i, item) in v.iter().enumerate() {
704 ptr::write(elems.add(i), item.clone());
708 // All clear. Forget the guard so it doesn't free the new ArcInner.
711 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
716 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
718 fn from_slice(v: &[T]) -> Self {
719 unsafe { Arc::copy_from_slice(v) }
723 #[stable(feature = "rust1", since = "1.0.0")]
724 impl<T: ?Sized> Clone for Arc<T> {
725 /// Makes a clone of the `Arc` pointer.
727 /// This creates another pointer to the same inner value, increasing the
728 /// strong reference count.
733 /// use std::sync::Arc;
735 /// let five = Arc::new(5);
737 /// let _ = Arc::clone(&five);
740 fn clone(&self) -> Arc<T> {
741 // Using a relaxed ordering is alright here, as knowledge of the
742 // original reference prevents other threads from erroneously deleting
745 // As explained in the [Boost documentation][1], Increasing the
746 // reference counter can always be done with memory_order_relaxed: New
747 // references to an object can only be formed from an existing
748 // reference, and passing an existing reference from one thread to
749 // another must already provide any required synchronization.
751 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
752 let old_size = self.inner().strong.fetch_add(1, Relaxed);
754 // However we need to guard against massive refcounts in case someone
755 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
756 // and users will use-after free. We racily saturate to `isize::MAX` on
757 // the assumption that there aren't ~2 billion threads incrementing
758 // the reference count at once. This branch will never be taken in
759 // any realistic program.
761 // We abort because such a program is incredibly degenerate, and we
762 // don't care to support it.
763 if old_size > MAX_REFCOUNT {
769 Arc { ptr: self.ptr, phantom: PhantomData }
773 #[stable(feature = "rust1", since = "1.0.0")]
774 impl<T: ?Sized> Deref for Arc<T> {
778 fn deref(&self) -> &T {
783 #[unstable(feature = "receiver_trait", issue = "0")]
784 impl<T: ?Sized> Receiver for Arc<T> {}
786 impl<T: Clone> Arc<T> {
787 /// Makes a mutable reference into the given `Arc`.
789 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
790 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
791 /// ensure unique ownership. This is also referred to as clone-on-write.
793 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
795 /// [weak]: struct.Weak.html
796 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
797 /// [get_mut]: struct.Arc.html#method.get_mut
802 /// use std::sync::Arc;
804 /// let mut data = Arc::new(5);
806 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
807 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
808 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
809 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
810 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
812 /// // Now `data` and `other_data` point to different values.
813 /// assert_eq!(*data, 8);
814 /// assert_eq!(*other_data, 12);
817 #[stable(feature = "arc_unique", since = "1.4.0")]
818 pub fn make_mut(this: &mut Self) -> &mut T {
819 // Note that we hold both a strong reference and a weak reference.
820 // Thus, releasing our strong reference only will not, by itself, cause
821 // the memory to be deallocated.
823 // Use Acquire to ensure that we see any writes to `weak` that happen
824 // before release writes (i.e., decrements) to `strong`. Since we hold a
825 // weak count, there's no chance the ArcInner itself could be
827 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
828 // Another strong pointer exists; clone
829 *this = Arc::new((**this).clone());
830 } else if this.inner().weak.load(Relaxed) != 1 {
831 // Relaxed suffices in the above because this is fundamentally an
832 // optimization: we are always racing with weak pointers being
833 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
835 // We removed the last strong ref, but there are additional weak
836 // refs remaining. We'll move the contents to a new Arc, and
837 // invalidate the other weak refs.
839 // Note that it is not possible for the read of `weak` to yield
840 // usize::MAX (i.e., locked), since the weak count can only be
841 // locked by a thread with a strong reference.
843 // Materialize our own implicit weak pointer, so that it can clean
844 // up the ArcInner as needed.
845 let weak = Weak { ptr: this.ptr };
847 // mark the data itself as already deallocated
849 // there is no data race in the implicit write caused by `read`
850 // here (due to zeroing) because data is no longer accessed by
851 // other threads (due to there being no more strong refs at this
853 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
854 mem::swap(this, &mut swap);
858 // We were the sole reference of either kind; bump back up the
860 this.inner().strong.store(1, Release);
863 // As with `get_mut()`, the unsafety is ok because our reference was
864 // either unique to begin with, or became one upon cloning the contents.
866 &mut this.ptr.as_mut().data
871 impl<T: ?Sized> Arc<T> {
872 /// Returns a mutable reference to the inner value, if there are
873 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
875 /// Returns [`None`][option] otherwise, because it is not safe to
876 /// mutate a shared value.
878 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
879 /// the inner value when it's shared.
881 /// [weak]: struct.Weak.html
882 /// [option]: ../../std/option/enum.Option.html
883 /// [make_mut]: struct.Arc.html#method.make_mut
884 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
889 /// use std::sync::Arc;
891 /// let mut x = Arc::new(3);
892 /// *Arc::get_mut(&mut x).unwrap() = 4;
893 /// assert_eq!(*x, 4);
895 /// let _y = Arc::clone(&x);
896 /// assert!(Arc::get_mut(&mut x).is_none());
899 #[stable(feature = "arc_unique", since = "1.4.0")]
900 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
901 if this.is_unique() {
902 // This unsafety is ok because we're guaranteed that the pointer
903 // returned is the *only* pointer that will ever be returned to T. Our
904 // reference count is guaranteed to be 1 at this point, and we required
905 // the Arc itself to be `mut`, so we're returning the only possible
906 // reference to the inner data.
908 Some(&mut this.ptr.as_mut().data)
915 /// Determine whether this is the unique reference (including weak refs) to
916 /// the underlying data.
918 /// Note that this requires locking the weak ref count.
919 fn is_unique(&mut self) -> bool {
920 // lock the weak pointer count if we appear to be the sole weak pointer
923 // The acquire label here ensures a happens-before relationship with any
924 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
925 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
926 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
927 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
928 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
929 // counter in `drop` -- the only access that happens when any but the last reference
931 let unique = self.inner().strong.load(Acquire) == 1;
933 // The release write here synchronizes with a read in `downgrade`,
934 // effectively preventing the above read of `strong` from happening
936 self.inner().weak.store(1, Release); // release the lock
944 #[stable(feature = "rust1", since = "1.0.0")]
945 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
948 /// This will decrement the strong reference count. If the strong reference
949 /// count reaches zero then the only other references (if any) are
950 /// [`Weak`], so we `drop` the inner value.
955 /// use std::sync::Arc;
959 /// impl Drop for Foo {
960 /// fn drop(&mut self) {
961 /// println!("dropped!");
965 /// let foo = Arc::new(Foo);
966 /// let foo2 = Arc::clone(&foo);
968 /// drop(foo); // Doesn't print anything
969 /// drop(foo2); // Prints "dropped!"
972 /// [`Weak`]: ../../std/sync/struct.Weak.html
975 // Because `fetch_sub` is already atomic, we do not need to synchronize
976 // with other threads unless we are going to delete the object. This
977 // same logic applies to the below `fetch_sub` to the `weak` count.
978 if self.inner().strong.fetch_sub(1, Release) != 1 {
982 // This fence is needed to prevent reordering of use of the data and
983 // deletion of the data. Because it is marked `Release`, the decreasing
984 // of the reference count synchronizes with this `Acquire` fence. This
985 // means that use of the data happens before decreasing the reference
986 // count, which happens before this fence, which happens before the
987 // deletion of the data.
989 // As explained in the [Boost documentation][1],
991 // > It is important to enforce any possible access to the object in one
992 // > thread (through an existing reference) to *happen before* deleting
993 // > the object in a different thread. This is achieved by a "release"
994 // > operation after dropping a reference (any access to the object
995 // > through this reference must obviously happened before), and an
996 // > "acquire" operation before deleting the object.
998 // In particular, while the contents of an Arc are usually immutable, it's
999 // possible to have interior writes to something like a Mutex<T>. Since a
1000 // Mutex is not acquired when it is deleted, we can't rely on its
1001 // synchronization logic to make writes in thread A visible to a destructor
1002 // running in thread B.
1004 // Also note that the Acquire fence here could probably be replaced with an
1005 // Acquire load, which could improve performance in highly-contended
1006 // situations. See [2].
1008 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1009 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1010 atomic::fence(Acquire);
1018 impl Arc<dyn Any + Send + Sync> {
1020 #[stable(feature = "rc_downcast", since = "1.29.0")]
1021 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1026 /// use std::any::Any;
1027 /// use std::sync::Arc;
1029 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1030 /// if let Ok(string) = value.downcast::<String>() {
1031 /// println!("String ({}): {}", string.len(), string);
1036 /// let my_string = "Hello World".to_string();
1037 /// print_if_string(Arc::new(my_string));
1038 /// print_if_string(Arc::new(0i8));
1041 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1043 T: Any + Send + Sync + 'static,
1045 if (*self).is::<T>() {
1046 let ptr = self.ptr.cast::<ArcInner<T>>();
1048 Ok(Arc { ptr, phantom: PhantomData })
1056 /// Constructs a new `Weak<T>`, without allocating any memory.
1057 /// Calling [`upgrade`] on the return value always gives [`None`].
1059 /// [`upgrade`]: struct.Weak.html#method.upgrade
1060 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1065 /// use std::sync::Weak;
1067 /// let empty: Weak<i64> = Weak::new();
1068 /// assert!(empty.upgrade().is_none());
1070 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1071 pub fn new() -> Weak<T> {
1073 ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0"),
1078 impl<T: ?Sized> Weak<T> {
1079 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1080 /// the lifetime of the value if successful.
1082 /// Returns [`None`] if the value has since been dropped.
1084 /// [`Arc`]: struct.Arc.html
1085 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1090 /// use std::sync::Arc;
1092 /// let five = Arc::new(5);
1094 /// let weak_five = Arc::downgrade(&five);
1096 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1097 /// assert!(strong_five.is_some());
1099 /// // Destroy all strong pointers.
1100 /// drop(strong_five);
1103 /// assert!(weak_five.upgrade().is_none());
1105 #[stable(feature = "arc_weak", since = "1.4.0")]
1106 pub fn upgrade(&self) -> Option<Arc<T>> {
1107 // We use a CAS loop to increment the strong count instead of a
1108 // fetch_add because once the count hits 0 it must never be above 0.
1109 let inner = self.inner()?;
1111 // Relaxed load because any write of 0 that we can observe
1112 // leaves the field in a permanently zero state (so a
1113 // "stale" read of 0 is fine), and any other value is
1114 // confirmed via the CAS below.
1115 let mut n = inner.strong.load(Relaxed);
1122 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1123 if n > MAX_REFCOUNT {
1129 // Relaxed is valid for the same reason it is on Arc's Clone impl
1130 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1131 Ok(_) => return Some(Arc {
1132 // null checked above
1134 phantom: PhantomData,
1136 Err(old) => n = old,
1141 /// Gets the number of strong (`Arc`) pointers pointing to this value.
1143 /// If `self` was created using [`Weak::new`], this will return 0.
1145 /// [`Weak::new`]: #method.new
1146 #[unstable(feature = "weak_counts", issue = "57977")]
1147 pub fn strong_count(&self) -> usize {
1148 if let Some(inner) = self.inner() {
1149 inner.strong.load(SeqCst)
1155 /// Gets an approximation of the number of `Weak` pointers pointing to this
1158 /// If `self` was created using [`Weak::new`], this will return 0. If not,
1159 /// the returned value is at least 1, since `self` still points to the
1164 /// Due to implementation details, the returned value can be off by 1 in
1165 /// either direction when other threads are manipulating any `Arc`s or
1166 /// `Weak`s pointing to the same value.
1168 /// [`Weak::new`]: #method.new
1169 #[unstable(feature = "weak_counts", issue = "57977")]
1170 pub fn weak_count(&self) -> Option<usize> {
1171 // Due to the implicit weak pointer added when any strong pointers are
1172 // around, we cannot implement `weak_count` correctly since it
1173 // necessarily requires accessing the strong count and weak count in an
1174 // unsynchronized fashion. So this version is a bit racy.
1175 self.inner().map(|inner| {
1176 let strong = inner.strong.load(SeqCst);
1177 let weak = inner.weak.load(SeqCst);
1179 // If the last `Arc` has *just* been dropped, it might not yet
1180 // have removed the implicit weak count, so the value we get
1181 // here might be 1 too high.
1184 // As long as there's still at least 1 `Arc` around, subtract
1185 // the implicit weak pointer.
1186 // Note that the last `Arc` might get dropped between the 2
1187 // loads we do above, removing the implicit weak pointer. This
1188 // means that the value might be 1 too low here. In order to not
1189 // return 0 here (which would happen if we're the only weak
1190 // pointer), we guard against that specifically.
1191 cmp::max(1, weak - 1)
1196 /// Return `None` when the pointer is dangling and there is no allocated `ArcInner`,
1197 /// i.e., this `Weak` was created by `Weak::new`
1199 fn inner(&self) -> Option<&ArcInner<T>> {
1200 if is_dangling(self.ptr) {
1203 Some(unsafe { self.ptr.as_ref() })
1207 /// Returns true if the two `Weak`s point to the same value (not just values
1208 /// that compare as equal).
1212 /// Since this compares pointers it means that `Weak::new()` will equal each
1213 /// other, even though they don't point to any value.
1219 /// #![feature(weak_ptr_eq)]
1220 /// use std::sync::{Arc, Weak};
1222 /// let first_rc = Arc::new(5);
1223 /// let first = Arc::downgrade(&first_rc);
1224 /// let second = Arc::downgrade(&first_rc);
1226 /// assert!(Weak::ptr_eq(&first, &second));
1228 /// let third_rc = Arc::new(5);
1229 /// let third = Arc::downgrade(&third_rc);
1231 /// assert!(!Weak::ptr_eq(&first, &third));
1234 /// Comparing `Weak::new`.
1237 /// #![feature(weak_ptr_eq)]
1238 /// use std::sync::{Arc, Weak};
1240 /// let first = Weak::new();
1241 /// let second = Weak::new();
1242 /// assert!(Weak::ptr_eq(&first, &second));
1244 /// let third_rc = Arc::new(());
1245 /// let third = Arc::downgrade(&third_rc);
1246 /// assert!(!Weak::ptr_eq(&first, &third));
1249 #[unstable(feature = "weak_ptr_eq", issue = "55981")]
1250 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1251 this.ptr.as_ptr() == other.ptr.as_ptr()
1255 #[stable(feature = "arc_weak", since = "1.4.0")]
1256 impl<T: ?Sized> Clone for Weak<T> {
1257 /// Makes a clone of the `Weak` pointer that points to the same value.
1262 /// use std::sync::{Arc, Weak};
1264 /// let weak_five = Arc::downgrade(&Arc::new(5));
1266 /// let _ = Weak::clone(&weak_five);
1269 fn clone(&self) -> Weak<T> {
1270 let inner = if let Some(inner) = self.inner() {
1273 return Weak { ptr: self.ptr };
1275 // See comments in Arc::clone() for why this is relaxed. This can use a
1276 // fetch_add (ignoring the lock) because the weak count is only locked
1277 // where are *no other* weak pointers in existence. (So we can't be
1278 // running this code in that case).
1279 let old_size = inner.weak.fetch_add(1, Relaxed);
1281 // See comments in Arc::clone() for why we do this (for mem::forget).
1282 if old_size > MAX_REFCOUNT {
1288 return Weak { ptr: self.ptr };
1292 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1293 impl<T> Default for Weak<T> {
1294 /// Constructs a new `Weak<T>`, without allocating memory.
1295 /// Calling [`upgrade`] on the return value always
1298 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1299 /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
1304 /// use std::sync::Weak;
1306 /// let empty: Weak<i64> = Default::default();
1307 /// assert!(empty.upgrade().is_none());
1309 fn default() -> Weak<T> {
1314 #[stable(feature = "arc_weak", since = "1.4.0")]
1315 impl<T: ?Sized> Drop for Weak<T> {
1316 /// Drops the `Weak` pointer.
1321 /// use std::sync::{Arc, Weak};
1325 /// impl Drop for Foo {
1326 /// fn drop(&mut self) {
1327 /// println!("dropped!");
1331 /// let foo = Arc::new(Foo);
1332 /// let weak_foo = Arc::downgrade(&foo);
1333 /// let other_weak_foo = Weak::clone(&weak_foo);
1335 /// drop(weak_foo); // Doesn't print anything
1336 /// drop(foo); // Prints "dropped!"
1338 /// assert!(other_weak_foo.upgrade().is_none());
1340 fn drop(&mut self) {
1341 // If we find out that we were the last weak pointer, then its time to
1342 // deallocate the data entirely. See the discussion in Arc::drop() about
1343 // the memory orderings
1345 // It's not necessary to check for the locked state here, because the
1346 // weak count can only be locked if there was precisely one weak ref,
1347 // meaning that drop could only subsequently run ON that remaining weak
1348 // ref, which can only happen after the lock is released.
1349 let inner = if let Some(inner) = self.inner() {
1355 if inner.weak.fetch_sub(1, Release) == 1 {
1356 atomic::fence(Acquire);
1358 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
1364 #[stable(feature = "rust1", since = "1.0.0")]
1365 trait ArcEqIdent<T: ?Sized + PartialEq> {
1366 fn eq(&self, other: &Arc<T>) -> bool;
1367 fn ne(&self, other: &Arc<T>) -> bool;
1370 #[stable(feature = "rust1", since = "1.0.0")]
1371 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
1373 default fn eq(&self, other: &Arc<T>) -> bool {
1377 default fn ne(&self, other: &Arc<T>) -> bool {
1382 #[stable(feature = "rust1", since = "1.0.0")]
1383 impl<T: ?Sized + Eq> ArcEqIdent<T> for Arc<T> {
1385 fn eq(&self, other: &Arc<T>) -> bool {
1386 Arc::ptr_eq(self, other) || **self == **other
1390 fn ne(&self, other: &Arc<T>) -> bool {
1391 !Arc::ptr_eq(self, other) && **self != **other
1395 #[stable(feature = "rust1", since = "1.0.0")]
1396 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1397 /// Equality for two `Arc`s.
1399 /// Two `Arc`s are equal if their inner values are equal.
1401 /// If `T` also implements `Eq`, two `Arc`s that point to the same value are
1407 /// use std::sync::Arc;
1409 /// let five = Arc::new(5);
1411 /// assert!(five == Arc::new(5));
1414 fn eq(&self, other: &Arc<T>) -> bool {
1415 ArcEqIdent::eq(self, other)
1418 /// Inequality for two `Arc`s.
1420 /// Two `Arc`s are unequal if their inner values are unequal.
1422 /// If `T` also implements `Eq`, two `Arc`s that point to the same value are
1428 /// use std::sync::Arc;
1430 /// let five = Arc::new(5);
1432 /// assert!(five != Arc::new(6));
1435 fn ne(&self, other: &Arc<T>) -> bool {
1436 ArcEqIdent::ne(self, other)
1440 #[stable(feature = "rust1", since = "1.0.0")]
1441 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1442 /// Partial comparison for two `Arc`s.
1444 /// The two are compared by calling `partial_cmp()` on their inner values.
1449 /// use std::sync::Arc;
1450 /// use std::cmp::Ordering;
1452 /// let five = Arc::new(5);
1454 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1456 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1457 (**self).partial_cmp(&**other)
1460 /// Less-than comparison for two `Arc`s.
1462 /// The two are compared by calling `<` on their inner values.
1467 /// use std::sync::Arc;
1469 /// let five = Arc::new(5);
1471 /// assert!(five < Arc::new(6));
1473 fn lt(&self, other: &Arc<T>) -> bool {
1474 *(*self) < *(*other)
1477 /// 'Less than or equal to' comparison for two `Arc`s.
1479 /// The two are compared by calling `<=` on their inner values.
1484 /// use std::sync::Arc;
1486 /// let five = Arc::new(5);
1488 /// assert!(five <= Arc::new(5));
1490 fn le(&self, other: &Arc<T>) -> bool {
1491 *(*self) <= *(*other)
1494 /// Greater-than comparison for two `Arc`s.
1496 /// The two are compared by calling `>` on their inner values.
1501 /// use std::sync::Arc;
1503 /// let five = Arc::new(5);
1505 /// assert!(five > Arc::new(4));
1507 fn gt(&self, other: &Arc<T>) -> bool {
1508 *(*self) > *(*other)
1511 /// 'Greater than or equal to' comparison for two `Arc`s.
1513 /// The two are compared by calling `>=` on their inner values.
1518 /// use std::sync::Arc;
1520 /// let five = Arc::new(5);
1522 /// assert!(five >= Arc::new(5));
1524 fn ge(&self, other: &Arc<T>) -> bool {
1525 *(*self) >= *(*other)
1528 #[stable(feature = "rust1", since = "1.0.0")]
1529 impl<T: ?Sized + Ord> Ord for Arc<T> {
1530 /// Comparison for two `Arc`s.
1532 /// The two are compared by calling `cmp()` on their inner values.
1537 /// use std::sync::Arc;
1538 /// use std::cmp::Ordering;
1540 /// let five = Arc::new(5);
1542 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1544 fn cmp(&self, other: &Arc<T>) -> Ordering {
1545 (**self).cmp(&**other)
1548 #[stable(feature = "rust1", since = "1.0.0")]
1549 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1551 #[stable(feature = "rust1", since = "1.0.0")]
1552 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1553 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1554 fmt::Display::fmt(&**self, f)
1558 #[stable(feature = "rust1", since = "1.0.0")]
1559 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1560 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1561 fmt::Debug::fmt(&**self, f)
1565 #[stable(feature = "rust1", since = "1.0.0")]
1566 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1567 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1568 fmt::Pointer::fmt(&(&**self as *const T), f)
1572 #[stable(feature = "rust1", since = "1.0.0")]
1573 impl<T: Default> Default for Arc<T> {
1574 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1579 /// use std::sync::Arc;
1581 /// let x: Arc<i32> = Default::default();
1582 /// assert_eq!(*x, 0);
1584 fn default() -> Arc<T> {
1585 Arc::new(Default::default())
1589 #[stable(feature = "rust1", since = "1.0.0")]
1590 impl<T: ?Sized + Hash> Hash for Arc<T> {
1591 fn hash<H: Hasher>(&self, state: &mut H) {
1592 (**self).hash(state)
1596 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1597 impl<T> From<T> for Arc<T> {
1598 fn from(t: T) -> Self {
1603 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1604 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1606 fn from(v: &[T]) -> Arc<[T]> {
1607 <Self as ArcFromSlice<T>>::from_slice(v)
1611 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1612 impl<'a> From<&'a str> for Arc<str> {
1614 fn from(v: &str) -> Arc<str> {
1615 let arc = Arc::<[u8]>::from(v.as_bytes());
1616 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1620 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1621 impl From<String> for Arc<str> {
1623 fn from(v: String) -> Arc<str> {
1628 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1629 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1631 fn from(v: Box<T>) -> Arc<T> {
1636 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1637 impl<T> From<Vec<T>> for Arc<[T]> {
1639 fn from(mut v: Vec<T>) -> Arc<[T]> {
1641 let arc = Arc::copy_from_slice(&v);
1643 // Allow the Vec to free its memory, but not destroy its contents
1653 use std::boxed::Box;
1654 use std::clone::Clone;
1655 use std::sync::mpsc::channel;
1658 use std::option::Option;
1659 use std::option::Option::{None, Some};
1660 use std::sync::atomic;
1661 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1663 use std::sync::Mutex;
1664 use std::convert::From;
1666 use super::{Arc, Weak};
1669 struct Canary(*mut atomic::AtomicUsize);
1671 impl Drop for Canary {
1672 fn drop(&mut self) {
1676 (*c).fetch_add(1, SeqCst);
1684 #[cfg_attr(target_os = "emscripten", ignore)]
1685 fn manually_share_arc() {
1686 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1687 let arc_v = Arc::new(v);
1689 let (tx, rx) = channel();
1691 let _t = thread::spawn(move || {
1692 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1693 assert_eq!((*arc_v)[3], 4);
1696 tx.send(arc_v.clone()).unwrap();
1698 assert_eq!((*arc_v)[2], 3);
1699 assert_eq!((*arc_v)[4], 5);
1703 fn test_arc_get_mut() {
1704 let mut x = Arc::new(3);
1705 *Arc::get_mut(&mut x).unwrap() = 4;
1708 assert!(Arc::get_mut(&mut x).is_none());
1710 assert!(Arc::get_mut(&mut x).is_some());
1711 let _w = Arc::downgrade(&x);
1712 assert!(Arc::get_mut(&mut x).is_none());
1717 assert_eq!(Weak::weak_count(&Weak::<u64>::new()), None);
1718 assert_eq!(Weak::strong_count(&Weak::<u64>::new()), 0);
1720 let a = Arc::new(0);
1721 let w = Arc::downgrade(&a);
1722 assert_eq!(Weak::strong_count(&w), 1);
1723 assert_eq!(Weak::weak_count(&w), Some(1));
1725 assert_eq!(Weak::strong_count(&w), 1);
1726 assert_eq!(Weak::weak_count(&w), Some(2));
1727 assert_eq!(Weak::strong_count(&w2), 1);
1728 assert_eq!(Weak::weak_count(&w2), Some(2));
1730 assert_eq!(Weak::strong_count(&w2), 1);
1731 assert_eq!(Weak::weak_count(&w2), Some(1));
1733 assert_eq!(Weak::strong_count(&w2), 2);
1734 assert_eq!(Weak::weak_count(&w2), Some(1));
1737 assert_eq!(Weak::strong_count(&w2), 0);
1738 assert_eq!(Weak::weak_count(&w2), Some(1));
1744 let x = Arc::new(3);
1745 assert_eq!(Arc::try_unwrap(x), Ok(3));
1746 let x = Arc::new(4);
1748 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1749 let x = Arc::new(5);
1750 let _w = Arc::downgrade(&x);
1751 assert_eq!(Arc::try_unwrap(x), Ok(5));
1755 fn into_from_raw() {
1756 let x = Arc::new(box "hello");
1759 let x_ptr = Arc::into_raw(x);
1762 assert_eq!(**x_ptr, "hello");
1764 let x = Arc::from_raw(x_ptr);
1765 assert_eq!(**x, "hello");
1767 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1772 fn test_into_from_raw_unsized() {
1773 use std::fmt::Display;
1774 use std::string::ToString;
1776 let arc: Arc<str> = Arc::from("foo");
1778 let ptr = Arc::into_raw(arc.clone());
1779 let arc2 = unsafe { Arc::from_raw(ptr) };
1781 assert_eq!(unsafe { &*ptr }, "foo");
1782 assert_eq!(arc, arc2);
1784 let arc: Arc<dyn Display> = Arc::new(123);
1786 let ptr = Arc::into_raw(arc.clone());
1787 let arc2 = unsafe { Arc::from_raw(ptr) };
1789 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1790 assert_eq!(arc2.to_string(), "123");
1794 fn test_cowarc_clone_make_mut() {
1795 let mut cow0 = Arc::new(75);
1796 let mut cow1 = cow0.clone();
1797 let mut cow2 = cow1.clone();
1799 assert!(75 == *Arc::make_mut(&mut cow0));
1800 assert!(75 == *Arc::make_mut(&mut cow1));
1801 assert!(75 == *Arc::make_mut(&mut cow2));
1803 *Arc::make_mut(&mut cow0) += 1;
1804 *Arc::make_mut(&mut cow1) += 2;
1805 *Arc::make_mut(&mut cow2) += 3;
1807 assert!(76 == *cow0);
1808 assert!(77 == *cow1);
1809 assert!(78 == *cow2);
1811 // none should point to the same backing memory
1812 assert!(*cow0 != *cow1);
1813 assert!(*cow0 != *cow2);
1814 assert!(*cow1 != *cow2);
1818 fn test_cowarc_clone_unique2() {
1819 let mut cow0 = Arc::new(75);
1820 let cow1 = cow0.clone();
1821 let cow2 = cow1.clone();
1823 assert!(75 == *cow0);
1824 assert!(75 == *cow1);
1825 assert!(75 == *cow2);
1827 *Arc::make_mut(&mut cow0) += 1;
1828 assert!(76 == *cow0);
1829 assert!(75 == *cow1);
1830 assert!(75 == *cow2);
1832 // cow1 and cow2 should share the same contents
1833 // cow0 should have a unique reference
1834 assert!(*cow0 != *cow1);
1835 assert!(*cow0 != *cow2);
1836 assert!(*cow1 == *cow2);
1840 fn test_cowarc_clone_weak() {
1841 let mut cow0 = Arc::new(75);
1842 let cow1_weak = Arc::downgrade(&cow0);
1844 assert!(75 == *cow0);
1845 assert!(75 == *cow1_weak.upgrade().unwrap());
1847 *Arc::make_mut(&mut cow0) += 1;
1849 assert!(76 == *cow0);
1850 assert!(cow1_weak.upgrade().is_none());
1855 let x = Arc::new(5);
1856 let y = Arc::downgrade(&x);
1857 assert!(y.upgrade().is_some());
1862 let x = Arc::new(5);
1863 let y = Arc::downgrade(&x);
1865 assert!(y.upgrade().is_none());
1869 fn weak_self_cyclic() {
1871 x: Mutex<Option<Weak<Cycle>>>,
1874 let a = Arc::new(Cycle { x: Mutex::new(None) });
1875 let b = Arc::downgrade(&a.clone());
1876 *a.x.lock().unwrap() = Some(b);
1878 // hopefully we don't double-free (or leak)...
1883 let mut canary = atomic::AtomicUsize::new(0);
1884 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1886 assert!(canary.load(Acquire) == 1);
1890 fn drop_arc_weak() {
1891 let mut canary = atomic::AtomicUsize::new(0);
1892 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1893 let arc_weak = Arc::downgrade(&arc);
1894 assert!(canary.load(Acquire) == 0);
1896 assert!(canary.load(Acquire) == 1);
1901 fn test_strong_count() {
1902 let a = Arc::new(0);
1903 assert!(Arc::strong_count(&a) == 1);
1904 let w = Arc::downgrade(&a);
1905 assert!(Arc::strong_count(&a) == 1);
1906 let b = w.upgrade().expect("");
1907 assert!(Arc::strong_count(&b) == 2);
1908 assert!(Arc::strong_count(&a) == 2);
1911 assert!(Arc::strong_count(&b) == 1);
1913 assert!(Arc::strong_count(&b) == 2);
1914 assert!(Arc::strong_count(&c) == 2);
1918 fn test_weak_count() {
1919 let a = Arc::new(0);
1920 assert!(Arc::strong_count(&a) == 1);
1921 assert!(Arc::weak_count(&a) == 0);
1922 let w = Arc::downgrade(&a);
1923 assert!(Arc::strong_count(&a) == 1);
1924 assert!(Arc::weak_count(&a) == 1);
1926 assert!(Arc::weak_count(&a) == 2);
1929 assert!(Arc::strong_count(&a) == 1);
1930 assert!(Arc::weak_count(&a) == 0);
1932 assert!(Arc::strong_count(&a) == 2);
1933 assert!(Arc::weak_count(&a) == 0);
1934 let d = Arc::downgrade(&c);
1935 assert!(Arc::weak_count(&c) == 1);
1936 assert!(Arc::strong_count(&c) == 2);
1945 let a = Arc::new(5);
1946 assert_eq!(format!("{:?}", a), "5");
1949 // Make sure deriving works with Arc<T>
1950 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1957 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1958 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1959 let y = Arc::downgrade(&x.clone());
1961 assert!(y.upgrade().is_none());
1965 fn test_from_owned() {
1967 let foo_arc = Arc::from(foo);
1968 assert!(123 == *foo_arc);
1972 fn test_new_weak() {
1973 let foo: Weak<usize> = Weak::new();
1974 assert!(foo.upgrade().is_none());
1979 let five = Arc::new(5);
1980 let same_five = five.clone();
1981 let other_five = Arc::new(5);
1983 assert!(Arc::ptr_eq(&five, &same_five));
1984 assert!(!Arc::ptr_eq(&five, &other_five));
1988 #[cfg_attr(target_os = "emscripten", ignore)]
1989 fn test_weak_count_locked() {
1990 let mut a = Arc::new(atomic::AtomicBool::new(false));
1992 let t = thread::spawn(move || {
1993 for _i in 0..1000000 {
1994 Arc::get_mut(&mut a);
1996 a.store(true, SeqCst);
1999 while !a2.load(SeqCst) {
2000 let n = Arc::weak_count(&a2);
2001 assert!(n < 2, "bad weak count: {}", n);
2007 fn test_from_str() {
2008 let r: Arc<str> = Arc::from("foo");
2010 assert_eq!(&r[..], "foo");
2014 fn test_copy_from_slice() {
2015 let s: &[u32] = &[1, 2, 3];
2016 let r: Arc<[u32]> = Arc::from(s);
2018 assert_eq!(&r[..], [1, 2, 3]);
2022 fn test_clone_from_slice() {
2023 #[derive(Clone, Debug, Eq, PartialEq)]
2026 let s: &[X] = &[X(1), X(2), X(3)];
2027 let r: Arc<[X]> = Arc::from(s);
2029 assert_eq!(&r[..], s);
2034 fn test_clone_from_slice_panic() {
2035 use std::string::{String, ToString};
2037 struct Fail(u32, String);
2039 impl Clone for Fail {
2040 fn clone(&self) -> Fail {
2044 Fail(self.0, self.1.clone())
2049 Fail(0, "foo".to_string()),
2050 Fail(1, "bar".to_string()),
2051 Fail(2, "baz".to_string()),
2054 // Should panic, but not cause memory corruption
2055 let _r: Arc<[Fail]> = Arc::from(s);
2059 fn test_from_box() {
2060 let b: Box<u32> = box 123;
2061 let r: Arc<u32> = Arc::from(b);
2063 assert_eq!(*r, 123);
2067 fn test_from_box_str() {
2068 use std::string::String;
2070 let s = String::from("foo").into_boxed_str();
2071 let r: Arc<str> = Arc::from(s);
2073 assert_eq!(&r[..], "foo");
2077 fn test_from_box_slice() {
2078 let s = vec![1, 2, 3].into_boxed_slice();
2079 let r: Arc<[u32]> = Arc::from(s);
2081 assert_eq!(&r[..], [1, 2, 3]);
2085 fn test_from_box_trait() {
2086 use std::fmt::Display;
2087 use std::string::ToString;
2089 let b: Box<dyn Display> = box 123;
2090 let r: Arc<dyn Display> = Arc::from(b);
2092 assert_eq!(r.to_string(), "123");
2096 fn test_from_box_trait_zero_sized() {
2097 use std::fmt::Debug;
2099 let b: Box<dyn Debug> = box ();
2100 let r: Arc<dyn Debug> = Arc::from(b);
2102 assert_eq!(format!("{:?}", r), "()");
2106 fn test_from_vec() {
2107 let v = vec![1, 2, 3];
2108 let r: Arc<[u32]> = Arc::from(v);
2110 assert_eq!(&r[..], [1, 2, 3]);
2114 fn test_downcast() {
2117 let r1: Arc<dyn Any + Send + Sync> = Arc::new(i32::max_value());
2118 let r2: Arc<dyn Any + Send + Sync> = Arc::new("abc");
2120 assert!(r1.clone().downcast::<u32>().is_err());
2122 let r1i32 = r1.downcast::<i32>();
2123 assert!(r1i32.is_ok());
2124 assert_eq!(r1i32.unwrap(), Arc::new(i32::max_value()));
2126 assert!(r2.clone().downcast::<i32>().is_err());
2128 let r2str = r2.downcast::<&'static str>();
2129 assert!(r2str.is_ok());
2130 assert_eq!(r2str.unwrap(), Arc::new("abc"));
2134 #[stable(feature = "rust1", since = "1.0.0")]
2135 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2136 fn borrow(&self) -> &T {
2141 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2142 impl<T: ?Sized> AsRef<T> for Arc<T> {
2143 fn as_ref(&self) -> &T {
2148 #[stable(feature = "pin", since = "1.33.0")]
2149 impl<T: ?Sized> Unpin for Arc<T> { }