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};
16 use core::intrinsics::abort;
17 use core::mem::{self, align_of, align_of_val, size_of_val};
18 use core::ops::{Deref, Receiver, 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;
25 use core::slice::{self, from_raw_parts_mut};
27 use crate::alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
28 use crate::boxed::Box;
29 use crate::rc::is_dangling;
30 use crate::string::String;
33 /// A soft limit on the amount of references that may be made to an `Arc`.
35 /// Going above this limit will abort your program (although not
36 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
37 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
39 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
40 /// Reference Counted'.
42 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
43 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
44 /// a new `Arc` instance, which points to the same value on the heap as the
45 /// source `Arc`, while increasing a reference count. When the last `Arc`
46 /// pointer to a given value is destroyed, the pointed-to value is also
49 /// Shared references in Rust disallow mutation by default, and `Arc` is no
50 /// exception: you cannot generally obtain a mutable reference to something
51 /// inside an `Arc`. If you need to mutate through an `Arc`, use
52 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
57 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
58 /// counting. This means that it is thread-safe. The disadvantage is that
59 /// atomic operations are more expensive than ordinary memory accesses. If you
60 /// are not sharing reference-counted values between threads, consider using
61 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
62 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
63 /// However, a library might choose `Arc<T>` in order to give library consumers
66 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
67 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
68 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
69 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
70 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
71 /// data, but it doesn't add thread safety to its data. Consider
72 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
73 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
74 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
75 /// non-atomic operations.
77 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
78 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
80 /// ## Breaking cycles with `Weak`
82 /// The [`downgrade`][downgrade] method can be used to create a non-owning
83 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
84 /// to an `Arc`, but this will return [`None`] if the value has already been
87 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
88 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
89 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
90 /// pointers from children back to their parents.
92 /// # Cloning references
94 /// Creating a new reference from an existing reference counted pointer is done using the
95 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
98 /// use std::sync::Arc;
99 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
100 /// // The two syntaxes below are equivalent.
101 /// let a = foo.clone();
102 /// let b = Arc::clone(&foo);
103 /// // a, b, and foo are all Arcs that point to the same memory location
106 /// The [`Arc::clone(&from)`] syntax is the most idiomatic because it conveys more explicitly
107 /// the meaning of the code. In the example above, this syntax makes it easier to see that
108 /// this code is creating a new reference rather than copying the whole content of foo.
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 `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
311 /// `data` will be pinned in memory and unable to be moved.
312 #[stable(feature = "pin", since = "1.33.0")]
313 pub fn pin(data: T) -> Pin<Arc<T>> {
314 unsafe { Pin::new_unchecked(Arc::new(data)) }
317 /// Returns the contained value, if the `Arc` has exactly one strong reference.
319 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
322 /// This will succeed even if there are outstanding weak references.
324 /// [result]: ../../std/result/enum.Result.html
329 /// use std::sync::Arc;
331 /// let x = Arc::new(3);
332 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
334 /// let x = Arc::new(4);
335 /// let _y = Arc::clone(&x);
336 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
339 #[stable(feature = "arc_unique", since = "1.4.0")]
340 pub fn try_unwrap(this: Self) -> Result<T, Self> {
341 // See `drop` for why all these atomics are like this
342 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
346 atomic::fence(Acquire);
349 let elem = ptr::read(&this.ptr.as_ref().data);
351 // Make a weak pointer to clean up the implicit strong-weak reference
352 let _weak = Weak { ptr: this.ptr };
360 impl<T: ?Sized> Arc<T> {
361 /// Consumes the `Arc`, returning the wrapped pointer.
363 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
364 /// [`Arc::from_raw`][from_raw].
366 /// [from_raw]: struct.Arc.html#method.from_raw
371 /// use std::sync::Arc;
373 /// let x = Arc::new("hello".to_owned());
374 /// let x_ptr = Arc::into_raw(x);
375 /// assert_eq!(unsafe { &*x_ptr }, "hello");
377 #[stable(feature = "rc_raw", since = "1.17.0")]
378 pub fn into_raw(this: Self) -> *const T {
379 let ptr: *const T = &*this;
384 /// Constructs an `Arc` from a raw pointer.
386 /// The raw pointer must have been previously returned by a call to a
387 /// [`Arc::into_raw`][into_raw].
389 /// This function is unsafe because improper use may lead to memory problems. For example, a
390 /// double-free may occur if the function is called twice on the same raw pointer.
392 /// [into_raw]: struct.Arc.html#method.into_raw
397 /// use std::sync::Arc;
399 /// let x = Arc::new("hello".to_owned());
400 /// let x_ptr = Arc::into_raw(x);
403 /// // Convert back to an `Arc` to prevent leak.
404 /// let x = Arc::from_raw(x_ptr);
405 /// assert_eq!(&*x, "hello");
407 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
410 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
412 #[stable(feature = "rc_raw", since = "1.17.0")]
413 pub unsafe fn from_raw(ptr: *const T) -> Self {
414 let offset = data_offset(ptr);
416 // Reverse the offset to find the original ArcInner.
417 let fake_ptr = ptr as *mut ArcInner<T>;
418 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
420 Self::from_ptr(arc_ptr)
423 /// Consumes the `Arc`, returning the wrapped pointer as `NonNull<T>`.
428 /// #![feature(rc_into_raw_non_null)]
430 /// use std::sync::Arc;
432 /// let x = Arc::new("hello".to_owned());
433 /// let ptr = Arc::into_raw_non_null(x);
434 /// let deref = unsafe { ptr.as_ref() };
435 /// assert_eq!(deref, "hello");
437 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
439 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
440 // safe because Arc guarantees its pointer is non-null
441 unsafe { NonNull::new_unchecked(Arc::into_raw(this) as *mut _) }
444 /// Creates a new [`Weak`][weak] pointer to this value.
446 /// [weak]: struct.Weak.html
451 /// use std::sync::Arc;
453 /// let five = Arc::new(5);
455 /// let weak_five = Arc::downgrade(&five);
457 #[stable(feature = "arc_weak", since = "1.4.0")]
458 pub fn downgrade(this: &Self) -> Weak<T> {
459 // This Relaxed is OK because we're checking the value in the CAS
461 let mut cur = this.inner().weak.load(Relaxed);
464 // check if the weak counter is currently "locked"; if so, spin.
465 if cur == usize::MAX {
466 cur = this.inner().weak.load(Relaxed);
470 // NOTE: this code currently ignores the possibility of overflow
471 // into usize::MAX; in general both Rc and Arc need to be adjusted
472 // to deal with overflow.
474 // Unlike with Clone(), we need this to be an Acquire read to
475 // synchronize with the write coming from `is_unique`, so that the
476 // events prior to that write happen before this read.
477 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
479 // Make sure we do not create a dangling Weak
480 debug_assert!(!is_dangling(this.ptr));
481 return Weak { ptr: this.ptr };
483 Err(old) => cur = old,
488 /// Gets the number of [`Weak`][weak] pointers to this value.
490 /// [weak]: struct.Weak.html
494 /// This method by itself is safe, but using it correctly requires extra care.
495 /// Another thread can change the weak count at any time,
496 /// including potentially between calling this method and acting on the result.
501 /// use std::sync::Arc;
503 /// let five = Arc::new(5);
504 /// let _weak_five = Arc::downgrade(&five);
506 /// // This assertion is deterministic because we haven't shared
507 /// // the `Arc` or `Weak` between threads.
508 /// assert_eq!(1, Arc::weak_count(&five));
511 #[stable(feature = "arc_counts", since = "1.15.0")]
512 pub fn weak_count(this: &Self) -> usize {
513 let cnt = this.inner().weak.load(SeqCst);
514 // If the weak count is currently locked, the value of the
515 // count was 0 just before taking the lock.
516 if cnt == usize::MAX { 0 } else { cnt - 1 }
519 /// Gets the number of strong (`Arc`) pointers to this value.
523 /// This method by itself is safe, but using it correctly requires extra care.
524 /// Another thread can change the strong count at any time,
525 /// including potentially between calling this method and acting on the result.
530 /// use std::sync::Arc;
532 /// let five = Arc::new(5);
533 /// let _also_five = Arc::clone(&five);
535 /// // This assertion is deterministic because we haven't shared
536 /// // the `Arc` between threads.
537 /// assert_eq!(2, Arc::strong_count(&five));
540 #[stable(feature = "arc_counts", since = "1.15.0")]
541 pub fn strong_count(this: &Self) -> usize {
542 this.inner().strong.load(SeqCst)
546 fn inner(&self) -> &ArcInner<T> {
547 // This unsafety is ok because while this arc is alive we're guaranteed
548 // that the inner pointer is valid. Furthermore, we know that the
549 // `ArcInner` structure itself is `Sync` because the inner data is
550 // `Sync` as well, so we're ok loaning out an immutable pointer to these
552 unsafe { self.ptr.as_ref() }
555 // Non-inlined part of `drop`.
557 unsafe fn drop_slow(&mut self) {
558 // Destroy the data at this time, even though we may not free the box
559 // allocation itself (there may still be weak pointers lying around).
560 ptr::drop_in_place(&mut self.ptr.as_mut().data);
562 if self.inner().weak.fetch_sub(1, Release) == 1 {
563 atomic::fence(Acquire);
564 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
569 #[stable(feature = "ptr_eq", since = "1.17.0")]
570 /// Returns `true` if the two `Arc`s point to the same value (not
571 /// just values that compare as equal).
576 /// use std::sync::Arc;
578 /// let five = Arc::new(5);
579 /// let same_five = Arc::clone(&five);
580 /// let other_five = Arc::new(5);
582 /// assert!(Arc::ptr_eq(&five, &same_five));
583 /// assert!(!Arc::ptr_eq(&five, &other_five));
585 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
586 this.ptr.as_ptr() == other.ptr.as_ptr()
590 impl<T: ?Sized> Arc<T> {
591 /// Allocates an `ArcInner<T>` with sufficient space for
592 /// an unsized value where the value has the layout provided.
594 /// The function `mem_to_arcinner` is called with the data pointer
595 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
596 unsafe fn allocate_for_unsized(
597 value_layout: Layout,
598 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>
599 ) -> *mut ArcInner<T> {
600 // Calculate layout using the given value layout.
601 // Previously, layout was calculated on the expression
602 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
603 // reference (see #54908).
604 let layout = Layout::new::<ArcInner<()>>()
605 .extend(value_layout).unwrap().0
606 .pad_to_align().unwrap();
608 let mem = Global.alloc(layout)
609 .unwrap_or_else(|_| handle_alloc_error(layout));
611 // Initialize the ArcInner
612 let inner = mem_to_arcinner(mem.as_ptr());
613 debug_assert_eq!(Layout::for_value(&*inner), layout);
615 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
616 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
621 /// Allocates an `ArcInner<T>` with sufficient space for an unsized value.
622 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
623 // Allocate for the `ArcInner<T>` using the given value.
624 Self::allocate_for_unsized(
625 Layout::for_value(&*ptr),
626 |mem| set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>,
630 fn from_box(v: Box<T>) -> Arc<T> {
632 let box_unique = Box::into_unique(v);
633 let bptr = box_unique.as_ptr();
635 let value_size = size_of_val(&*bptr);
636 let ptr = Self::allocate_for_ptr(bptr);
638 // Copy value as bytes
639 ptr::copy_nonoverlapping(
640 bptr as *const T as *const u8,
641 &mut (*ptr).data as *mut _ as *mut u8,
644 // Free the allocation without dropping its contents
645 box_free(box_unique);
653 /// Allocates an `ArcInner<[T]>` with the given length.
654 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
655 Self::allocate_for_unsized(
656 Layout::array::<T>(len).unwrap(),
657 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
662 /// Sets the data pointer of a `?Sized` raw pointer.
664 /// For a slice/trait object, this sets the `data` field and leaves the rest
665 /// unchanged. For a sized raw pointer, this simply sets the pointer.
666 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
667 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
672 /// Copy elements from slice into newly allocated Arc<[T]>
674 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
675 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
676 let ptr = Self::allocate_for_slice(v.len());
678 ptr::copy_nonoverlapping(
680 &mut (*ptr).data as *mut [T] as *mut T,
686 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
688 /// Behavior is undefined should the size be wrong.
689 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
690 // Panic guard while cloning T elements.
691 // In the event of a panic, elements that have been written
692 // into the new ArcInner will be dropped, then the memory freed.
700 impl<T> Drop for Guard<T> {
703 let slice = from_raw_parts_mut(self.elems, self.n_elems);
704 ptr::drop_in_place(slice);
706 Global.dealloc(self.mem.cast(), self.layout.clone());
711 let ptr = Self::allocate_for_slice(len);
713 let mem = ptr as *mut _ as *mut u8;
714 let layout = Layout::for_value(&*ptr);
716 // Pointer to first element
717 let elems = &mut (*ptr).data as *mut [T] as *mut T;
719 let mut guard = Guard {
720 mem: NonNull::new_unchecked(mem),
726 for (i, item) in iter.enumerate() {
727 ptr::write(elems.add(i), item);
731 // All clear. Forget the guard so it doesn't free the new ArcInner.
738 /// Specialization trait used for `From<&[T]>`.
739 trait ArcFromSlice<T> {
740 fn from_slice(slice: &[T]) -> Self;
743 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
745 default fn from_slice(v: &[T]) -> Self {
747 Self::from_iter_exact(v.iter().cloned(), v.len())
752 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
754 fn from_slice(v: &[T]) -> Self {
755 unsafe { Arc::copy_from_slice(v) }
759 #[stable(feature = "rust1", since = "1.0.0")]
760 impl<T: ?Sized> Clone for Arc<T> {
761 /// Makes a clone of the `Arc` pointer.
763 /// This creates another pointer to the same inner value, increasing the
764 /// strong reference count.
769 /// use std::sync::Arc;
771 /// let five = Arc::new(5);
773 /// let _ = Arc::clone(&five);
776 fn clone(&self) -> Arc<T> {
777 // Using a relaxed ordering is alright here, as knowledge of the
778 // original reference prevents other threads from erroneously deleting
781 // As explained in the [Boost documentation][1], Increasing the
782 // reference counter can always be done with memory_order_relaxed: New
783 // references to an object can only be formed from an existing
784 // reference, and passing an existing reference from one thread to
785 // another must already provide any required synchronization.
787 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
788 let old_size = self.inner().strong.fetch_add(1, Relaxed);
790 // However we need to guard against massive refcounts in case someone
791 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
792 // and users will use-after free. We racily saturate to `isize::MAX` on
793 // the assumption that there aren't ~2 billion threads incrementing
794 // the reference count at once. This branch will never be taken in
795 // any realistic program.
797 // We abort because such a program is incredibly degenerate, and we
798 // don't care to support it.
799 if old_size > MAX_REFCOUNT {
805 Self::from_inner(self.ptr)
809 #[stable(feature = "rust1", since = "1.0.0")]
810 impl<T: ?Sized> Deref for Arc<T> {
814 fn deref(&self) -> &T {
819 #[unstable(feature = "receiver_trait", issue = "0")]
820 impl<T: ?Sized> Receiver for Arc<T> {}
822 impl<T: Clone> Arc<T> {
823 /// Makes a mutable reference into the given `Arc`.
825 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
826 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
827 /// ensure unique ownership. This is also referred to as clone-on-write.
829 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
831 /// [weak]: struct.Weak.html
832 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
833 /// [get_mut]: struct.Arc.html#method.get_mut
838 /// use std::sync::Arc;
840 /// let mut data = Arc::new(5);
842 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
843 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
844 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
845 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
846 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
848 /// // Now `data` and `other_data` point to different values.
849 /// assert_eq!(*data, 8);
850 /// assert_eq!(*other_data, 12);
853 #[stable(feature = "arc_unique", since = "1.4.0")]
854 pub fn make_mut(this: &mut Self) -> &mut T {
855 // Note that we hold both a strong reference and a weak reference.
856 // Thus, releasing our strong reference only will not, by itself, cause
857 // the memory to be deallocated.
859 // Use Acquire to ensure that we see any writes to `weak` that happen
860 // before release writes (i.e., decrements) to `strong`. Since we hold a
861 // weak count, there's no chance the ArcInner itself could be
863 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
864 // Another strong pointer exists; clone
865 *this = Arc::new((**this).clone());
866 } else if this.inner().weak.load(Relaxed) != 1 {
867 // Relaxed suffices in the above because this is fundamentally an
868 // optimization: we are always racing with weak pointers being
869 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
871 // We removed the last strong ref, but there are additional weak
872 // refs remaining. We'll move the contents to a new Arc, and
873 // invalidate the other weak refs.
875 // Note that it is not possible for the read of `weak` to yield
876 // usize::MAX (i.e., locked), since the weak count can only be
877 // locked by a thread with a strong reference.
879 // Materialize our own implicit weak pointer, so that it can clean
880 // up the ArcInner as needed.
881 let weak = Weak { ptr: this.ptr };
883 // mark the data itself as already deallocated
885 // there is no data race in the implicit write caused by `read`
886 // here (due to zeroing) because data is no longer accessed by
887 // other threads (due to there being no more strong refs at this
889 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
890 mem::swap(this, &mut swap);
894 // We were the sole reference of either kind; bump back up the
896 this.inner().strong.store(1, Release);
899 // As with `get_mut()`, the unsafety is ok because our reference was
900 // either unique to begin with, or became one upon cloning the contents.
902 &mut this.ptr.as_mut().data
907 impl<T: ?Sized> Arc<T> {
908 /// Returns a mutable reference to the inner value, if there are
909 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
911 /// Returns [`None`][option] otherwise, because it is not safe to
912 /// mutate a shared value.
914 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
915 /// the inner value when it's shared.
917 /// [weak]: struct.Weak.html
918 /// [option]: ../../std/option/enum.Option.html
919 /// [make_mut]: struct.Arc.html#method.make_mut
920 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
925 /// use std::sync::Arc;
927 /// let mut x = Arc::new(3);
928 /// *Arc::get_mut(&mut x).unwrap() = 4;
929 /// assert_eq!(*x, 4);
931 /// let _y = Arc::clone(&x);
932 /// assert!(Arc::get_mut(&mut x).is_none());
935 #[stable(feature = "arc_unique", since = "1.4.0")]
936 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
937 if this.is_unique() {
938 // This unsafety is ok because we're guaranteed that the pointer
939 // returned is the *only* pointer that will ever be returned to T. Our
940 // reference count is guaranteed to be 1 at this point, and we required
941 // the Arc itself to be `mut`, so we're returning the only possible
942 // reference to the inner data.
944 Some(&mut this.ptr.as_mut().data)
951 /// Determine whether this is the unique reference (including weak refs) to
952 /// the underlying data.
954 /// Note that this requires locking the weak ref count.
955 fn is_unique(&mut self) -> bool {
956 // lock the weak pointer count if we appear to be the sole weak pointer
959 // The acquire label here ensures a happens-before relationship with any
960 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
961 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
962 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
963 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
964 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
965 // counter in `drop` -- the only access that happens when any but the last reference
967 let unique = self.inner().strong.load(Acquire) == 1;
969 // The release write here synchronizes with a read in `downgrade`,
970 // effectively preventing the above read of `strong` from happening
972 self.inner().weak.store(1, Release); // release the lock
980 #[stable(feature = "rust1", since = "1.0.0")]
981 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
984 /// This will decrement the strong reference count. If the strong reference
985 /// count reaches zero then the only other references (if any) are
986 /// [`Weak`], so we `drop` the inner value.
991 /// use std::sync::Arc;
995 /// impl Drop for Foo {
996 /// fn drop(&mut self) {
997 /// println!("dropped!");
1001 /// let foo = Arc::new(Foo);
1002 /// let foo2 = Arc::clone(&foo);
1004 /// drop(foo); // Doesn't print anything
1005 /// drop(foo2); // Prints "dropped!"
1008 /// [`Weak`]: ../../std/sync/struct.Weak.html
1010 fn drop(&mut self) {
1011 // Because `fetch_sub` is already atomic, we do not need to synchronize
1012 // with other threads unless we are going to delete the object. This
1013 // same logic applies to the below `fetch_sub` to the `weak` count.
1014 if self.inner().strong.fetch_sub(1, Release) != 1 {
1018 // This fence is needed to prevent reordering of use of the data and
1019 // deletion of the data. Because it is marked `Release`, the decreasing
1020 // of the reference count synchronizes with this `Acquire` fence. This
1021 // means that use of the data happens before decreasing the reference
1022 // count, which happens before this fence, which happens before the
1023 // deletion of the data.
1025 // As explained in the [Boost documentation][1],
1027 // > It is important to enforce any possible access to the object in one
1028 // > thread (through an existing reference) to *happen before* deleting
1029 // > the object in a different thread. This is achieved by a "release"
1030 // > operation after dropping a reference (any access to the object
1031 // > through this reference must obviously happened before), and an
1032 // > "acquire" operation before deleting the object.
1034 // In particular, while the contents of an Arc are usually immutable, it's
1035 // possible to have interior writes to something like a Mutex<T>. Since a
1036 // Mutex is not acquired when it is deleted, we can't rely on its
1037 // synchronization logic to make writes in thread A visible to a destructor
1038 // running in thread B.
1040 // Also note that the Acquire fence here could probably be replaced with an
1041 // Acquire load, which could improve performance in highly-contended
1042 // situations. See [2].
1044 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1045 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1046 atomic::fence(Acquire);
1054 impl Arc<dyn Any + Send + Sync> {
1056 #[stable(feature = "rc_downcast", since = "1.29.0")]
1057 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1062 /// use std::any::Any;
1063 /// use std::sync::Arc;
1065 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1066 /// if let Ok(string) = value.downcast::<String>() {
1067 /// println!("String ({}): {}", string.len(), string);
1072 /// let my_string = "Hello World".to_string();
1073 /// print_if_string(Arc::new(my_string));
1074 /// print_if_string(Arc::new(0i8));
1077 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1079 T: Any + Send + Sync + 'static,
1081 if (*self).is::<T>() {
1082 let ptr = self.ptr.cast::<ArcInner<T>>();
1084 Ok(Arc::from_inner(ptr))
1092 /// Constructs a new `Weak<T>`, without allocating any memory.
1093 /// Calling [`upgrade`] on the return value always gives [`None`].
1095 /// [`upgrade`]: struct.Weak.html#method.upgrade
1096 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1101 /// use std::sync::Weak;
1103 /// let empty: Weak<i64> = Weak::new();
1104 /// assert!(empty.upgrade().is_none());
1106 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1107 pub fn new() -> Weak<T> {
1109 ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0"),
1113 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1115 /// It is up to the caller to ensure that the object is still alive when accessing it through
1118 /// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
1123 /// #![feature(weak_into_raw)]
1125 /// use std::sync::Arc;
1128 /// let strong = Arc::new("hello".to_owned());
1129 /// let weak = Arc::downgrade(&strong);
1130 /// // Both point to the same object
1131 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1132 /// // The strong here keeps it alive, so we can still access the object.
1133 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1136 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1137 /// // undefined behaviour.
1138 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1141 /// [`null`]: ../../std/ptr/fn.null.html
1142 #[unstable(feature = "weak_into_raw", issue = "60728")]
1143 pub fn as_raw(&self) -> *const T {
1144 match self.inner() {
1145 None => ptr::null(),
1147 let offset = data_offset_sized::<T>();
1148 let ptr = inner as *const ArcInner<T>;
1149 // Note: while the pointer we create may already point to dropped value, the
1150 // allocation still lives (it must hold the weak point as long as we are alive).
1151 // Therefore, the offset is OK to do, it won't get out of the allocation.
1152 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1158 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1160 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1161 /// can be turned back into the `Weak<T>` with [`from_raw`].
1163 /// The same restrictions of accessing the target of the pointer as with
1164 /// [`as_raw`] apply.
1169 /// #![feature(weak_into_raw)]
1171 /// use std::sync::{Arc, Weak};
1173 /// let strong = Arc::new("hello".to_owned());
1174 /// let weak = Arc::downgrade(&strong);
1175 /// let raw = weak.into_raw();
1177 /// assert_eq!(1, Arc::weak_count(&strong));
1178 /// assert_eq!("hello", unsafe { &*raw });
1180 /// drop(unsafe { Weak::from_raw(raw) });
1181 /// assert_eq!(0, Arc::weak_count(&strong));
1184 /// [`from_raw`]: struct.Weak.html#method.from_raw
1185 /// [`as_raw`]: struct.Weak.html#method.as_raw
1186 #[unstable(feature = "weak_into_raw", issue = "60728")]
1187 pub fn into_raw(self) -> *const T {
1188 let result = self.as_raw();
1193 /// Converts a raw pointer previously created by [`into_raw`] back into
1196 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1197 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1199 /// It takes ownership of one weak count. In case a [`null`] is passed, a dangling [`Weak`] is
1204 /// The pointer must represent one valid weak count. In other words, it must point to `T` which
1205 /// is or *was* managed by an [`Arc`] and the weak count of that [`Arc`] must not have reached
1206 /// 0. It is allowed for the strong count to be 0.
1211 /// #![feature(weak_into_raw)]
1213 /// use std::sync::{Arc, Weak};
1215 /// let strong = Arc::new("hello".to_owned());
1217 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1218 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1220 /// assert_eq!(2, Arc::weak_count(&strong));
1222 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1223 /// assert_eq!(1, Arc::weak_count(&strong));
1227 /// // Decrement the last weak count.
1228 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1231 /// [`null`]: ../../std/ptr/fn.null.html
1232 /// [`into_raw`]: struct.Weak.html#method.into_raw
1233 /// [`upgrade`]: struct.Weak.html#method.upgrade
1234 /// [`Weak`]: struct.Weak.html
1235 /// [`Arc`]: struct.Arc.html
1236 #[unstable(feature = "weak_into_raw", issue = "60728")]
1237 pub unsafe fn from_raw(ptr: *const T) -> Self {
1241 // See Arc::from_raw for details
1242 let offset = data_offset(ptr);
1243 let fake_ptr = ptr as *mut ArcInner<T>;
1244 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1246 ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
1252 impl<T: ?Sized> Weak<T> {
1253 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1254 /// the lifetime of the value if successful.
1256 /// Returns [`None`] if the value has since been dropped.
1258 /// [`Arc`]: struct.Arc.html
1259 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1264 /// use std::sync::Arc;
1266 /// let five = Arc::new(5);
1268 /// let weak_five = Arc::downgrade(&five);
1270 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1271 /// assert!(strong_five.is_some());
1273 /// // Destroy all strong pointers.
1274 /// drop(strong_five);
1277 /// assert!(weak_five.upgrade().is_none());
1279 #[stable(feature = "arc_weak", since = "1.4.0")]
1280 pub fn upgrade(&self) -> Option<Arc<T>> {
1281 // We use a CAS loop to increment the strong count instead of a
1282 // fetch_add because once the count hits 0 it must never be above 0.
1283 let inner = self.inner()?;
1285 // Relaxed load because any write of 0 that we can observe
1286 // leaves the field in a permanently zero state (so a
1287 // "stale" read of 0 is fine), and any other value is
1288 // confirmed via the CAS below.
1289 let mut n = inner.strong.load(Relaxed);
1296 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1297 if n > MAX_REFCOUNT {
1303 // Relaxed is valid for the same reason it is on Arc's Clone impl
1304 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1305 Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
1306 Err(old) => n = old,
1311 /// Gets the number of strong (`Arc`) pointers pointing to this value.
1313 /// If `self` was created using [`Weak::new`], this will return 0.
1315 /// [`Weak::new`]: #method.new
1316 #[unstable(feature = "weak_counts", issue = "57977")]
1317 pub fn strong_count(&self) -> usize {
1318 if let Some(inner) = self.inner() {
1319 inner.strong.load(SeqCst)
1325 /// Gets an approximation of the number of `Weak` pointers pointing to this
1328 /// If `self` was created using [`Weak::new`], this will return 0. If not,
1329 /// the returned value is at least 1, since `self` still points to the
1334 /// Due to implementation details, the returned value can be off by 1 in
1335 /// either direction when other threads are manipulating any `Arc`s or
1336 /// `Weak`s pointing to the same value.
1338 /// [`Weak::new`]: #method.new
1339 #[unstable(feature = "weak_counts", issue = "57977")]
1340 pub fn weak_count(&self) -> Option<usize> {
1341 // Due to the implicit weak pointer added when any strong pointers are
1342 // around, we cannot implement `weak_count` correctly since it
1343 // necessarily requires accessing the strong count and weak count in an
1344 // unsynchronized fashion. So this version is a bit racy.
1345 self.inner().map(|inner| {
1346 let strong = inner.strong.load(SeqCst);
1347 let weak = inner.weak.load(SeqCst);
1349 // If the last `Arc` has *just* been dropped, it might not yet
1350 // have removed the implicit weak count, so the value we get
1351 // here might be 1 too high.
1354 // As long as there's still at least 1 `Arc` around, subtract
1355 // the implicit weak pointer.
1356 // Note that the last `Arc` might get dropped between the 2
1357 // loads we do above, removing the implicit weak pointer. This
1358 // means that the value might be 1 too low here. In order to not
1359 // return 0 here (which would happen if we're the only weak
1360 // pointer), we guard against that specifically.
1361 cmp::max(1, weak - 1)
1366 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1367 /// (i.e., when this `Weak` was created by `Weak::new`).
1369 fn inner(&self) -> Option<&ArcInner<T>> {
1370 if is_dangling(self.ptr) {
1373 Some(unsafe { self.ptr.as_ref() })
1377 /// Returns `true` if the two `Weak`s point to the same value (not just values
1378 /// that compare as equal).
1382 /// Since this compares pointers it means that `Weak::new()` will equal each
1383 /// other, even though they don't point to any value.
1389 /// #![feature(weak_ptr_eq)]
1390 /// use std::sync::Arc;
1392 /// let first_rc = Arc::new(5);
1393 /// let first = Arc::downgrade(&first_rc);
1394 /// let second = Arc::downgrade(&first_rc);
1396 /// assert!(first.ptr_eq(&second));
1398 /// let third_rc = Arc::new(5);
1399 /// let third = Arc::downgrade(&third_rc);
1401 /// assert!(!first.ptr_eq(&third));
1404 /// Comparing `Weak::new`.
1407 /// #![feature(weak_ptr_eq)]
1408 /// use std::sync::{Arc, Weak};
1410 /// let first = Weak::new();
1411 /// let second = Weak::new();
1412 /// assert!(first.ptr_eq(&second));
1414 /// let third_rc = Arc::new(());
1415 /// let third = Arc::downgrade(&third_rc);
1416 /// assert!(!first.ptr_eq(&third));
1419 #[unstable(feature = "weak_ptr_eq", issue = "55981")]
1420 pub fn ptr_eq(&self, other: &Self) -> bool {
1421 self.ptr.as_ptr() == other.ptr.as_ptr()
1425 #[stable(feature = "arc_weak", since = "1.4.0")]
1426 impl<T: ?Sized> Clone for Weak<T> {
1427 /// Makes a clone of the `Weak` pointer that points to the same value.
1432 /// use std::sync::{Arc, Weak};
1434 /// let weak_five = Arc::downgrade(&Arc::new(5));
1436 /// let _ = Weak::clone(&weak_five);
1439 fn clone(&self) -> Weak<T> {
1440 let inner = if let Some(inner) = self.inner() {
1443 return Weak { ptr: self.ptr };
1445 // See comments in Arc::clone() for why this is relaxed. This can use a
1446 // fetch_add (ignoring the lock) because the weak count is only locked
1447 // where are *no other* weak pointers in existence. (So we can't be
1448 // running this code in that case).
1449 let old_size = inner.weak.fetch_add(1, Relaxed);
1451 // See comments in Arc::clone() for why we do this (for mem::forget).
1452 if old_size > MAX_REFCOUNT {
1458 return Weak { ptr: self.ptr };
1462 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1463 impl<T> Default for Weak<T> {
1464 /// Constructs a new `Weak<T>`, without allocating memory.
1465 /// Calling [`upgrade`] on the return value always
1468 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1469 /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
1474 /// use std::sync::Weak;
1476 /// let empty: Weak<i64> = Default::default();
1477 /// assert!(empty.upgrade().is_none());
1479 fn default() -> Weak<T> {
1484 #[stable(feature = "arc_weak", since = "1.4.0")]
1485 impl<T: ?Sized> Drop for Weak<T> {
1486 /// Drops the `Weak` pointer.
1491 /// use std::sync::{Arc, Weak};
1495 /// impl Drop for Foo {
1496 /// fn drop(&mut self) {
1497 /// println!("dropped!");
1501 /// let foo = Arc::new(Foo);
1502 /// let weak_foo = Arc::downgrade(&foo);
1503 /// let other_weak_foo = Weak::clone(&weak_foo);
1505 /// drop(weak_foo); // Doesn't print anything
1506 /// drop(foo); // Prints "dropped!"
1508 /// assert!(other_weak_foo.upgrade().is_none());
1510 fn drop(&mut self) {
1511 // If we find out that we were the last weak pointer, then its time to
1512 // deallocate the data entirely. See the discussion in Arc::drop() about
1513 // the memory orderings
1515 // It's not necessary to check for the locked state here, because the
1516 // weak count can only be locked if there was precisely one weak ref,
1517 // meaning that drop could only subsequently run ON that remaining weak
1518 // ref, which can only happen after the lock is released.
1519 let inner = if let Some(inner) = self.inner() {
1525 if inner.weak.fetch_sub(1, Release) == 1 {
1526 atomic::fence(Acquire);
1528 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
1534 #[stable(feature = "rust1", since = "1.0.0")]
1535 trait ArcEqIdent<T: ?Sized + PartialEq> {
1536 fn eq(&self, other: &Arc<T>) -> bool;
1537 fn ne(&self, other: &Arc<T>) -> bool;
1540 #[stable(feature = "rust1", since = "1.0.0")]
1541 impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
1543 default fn eq(&self, other: &Arc<T>) -> bool {
1547 default fn ne(&self, other: &Arc<T>) -> bool {
1552 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1553 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
1554 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1555 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
1556 /// the same value, than two `&T`s.
1557 #[stable(feature = "rust1", since = "1.0.0")]
1558 impl<T: ?Sized + Eq> ArcEqIdent<T> for Arc<T> {
1560 fn eq(&self, other: &Arc<T>) -> bool {
1561 Arc::ptr_eq(self, other) || **self == **other
1565 fn ne(&self, other: &Arc<T>) -> bool {
1566 !Arc::ptr_eq(self, other) && **self != **other
1570 #[stable(feature = "rust1", since = "1.0.0")]
1571 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1572 /// Equality for two `Arc`s.
1574 /// Two `Arc`s are equal if their inner values are equal.
1576 /// If `T` also implements `Eq`, two `Arc`s that point to the same value are
1582 /// use std::sync::Arc;
1584 /// let five = Arc::new(5);
1586 /// assert!(five == Arc::new(5));
1589 fn eq(&self, other: &Arc<T>) -> bool {
1590 ArcEqIdent::eq(self, other)
1593 /// Inequality for two `Arc`s.
1595 /// Two `Arc`s are unequal if their inner values are unequal.
1597 /// If `T` also implements `Eq`, two `Arc`s that point to the same value are
1603 /// use std::sync::Arc;
1605 /// let five = Arc::new(5);
1607 /// assert!(five != Arc::new(6));
1610 fn ne(&self, other: &Arc<T>) -> bool {
1611 ArcEqIdent::ne(self, other)
1615 #[stable(feature = "rust1", since = "1.0.0")]
1616 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1617 /// Partial comparison for two `Arc`s.
1619 /// The two are compared by calling `partial_cmp()` on their inner values.
1624 /// use std::sync::Arc;
1625 /// use std::cmp::Ordering;
1627 /// let five = Arc::new(5);
1629 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1631 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1632 (**self).partial_cmp(&**other)
1635 /// Less-than comparison for two `Arc`s.
1637 /// The two are compared by calling `<` on their inner values.
1642 /// use std::sync::Arc;
1644 /// let five = Arc::new(5);
1646 /// assert!(five < Arc::new(6));
1648 fn lt(&self, other: &Arc<T>) -> bool {
1649 *(*self) < *(*other)
1652 /// 'Less than or equal to' comparison for two `Arc`s.
1654 /// The two are compared by calling `<=` on their inner values.
1659 /// use std::sync::Arc;
1661 /// let five = Arc::new(5);
1663 /// assert!(five <= Arc::new(5));
1665 fn le(&self, other: &Arc<T>) -> bool {
1666 *(*self) <= *(*other)
1669 /// Greater-than comparison for two `Arc`s.
1671 /// The two are compared by calling `>` on their inner values.
1676 /// use std::sync::Arc;
1678 /// let five = Arc::new(5);
1680 /// assert!(five > Arc::new(4));
1682 fn gt(&self, other: &Arc<T>) -> bool {
1683 *(*self) > *(*other)
1686 /// 'Greater than or equal to' comparison for two `Arc`s.
1688 /// The two are compared by calling `>=` on their inner values.
1693 /// use std::sync::Arc;
1695 /// let five = Arc::new(5);
1697 /// assert!(five >= Arc::new(5));
1699 fn ge(&self, other: &Arc<T>) -> bool {
1700 *(*self) >= *(*other)
1703 #[stable(feature = "rust1", since = "1.0.0")]
1704 impl<T: ?Sized + Ord> Ord for Arc<T> {
1705 /// Comparison for two `Arc`s.
1707 /// The two are compared by calling `cmp()` on their inner values.
1712 /// use std::sync::Arc;
1713 /// use std::cmp::Ordering;
1715 /// let five = Arc::new(5);
1717 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1719 fn cmp(&self, other: &Arc<T>) -> Ordering {
1720 (**self).cmp(&**other)
1723 #[stable(feature = "rust1", since = "1.0.0")]
1724 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1726 #[stable(feature = "rust1", since = "1.0.0")]
1727 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1728 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1729 fmt::Display::fmt(&**self, f)
1733 #[stable(feature = "rust1", since = "1.0.0")]
1734 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1735 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1736 fmt::Debug::fmt(&**self, f)
1740 #[stable(feature = "rust1", since = "1.0.0")]
1741 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1742 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1743 fmt::Pointer::fmt(&(&**self as *const T), f)
1747 #[stable(feature = "rust1", since = "1.0.0")]
1748 impl<T: Default> Default for Arc<T> {
1749 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1754 /// use std::sync::Arc;
1756 /// let x: Arc<i32> = Default::default();
1757 /// assert_eq!(*x, 0);
1759 fn default() -> Arc<T> {
1760 Arc::new(Default::default())
1764 #[stable(feature = "rust1", since = "1.0.0")]
1765 impl<T: ?Sized + Hash> Hash for Arc<T> {
1766 fn hash<H: Hasher>(&self, state: &mut H) {
1767 (**self).hash(state)
1771 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1772 impl<T> From<T> for Arc<T> {
1773 fn from(t: T) -> Self {
1778 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1779 impl<T: Clone> From<&[T]> for Arc<[T]> {
1781 fn from(v: &[T]) -> Arc<[T]> {
1782 <Self as ArcFromSlice<T>>::from_slice(v)
1786 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1787 impl From<&str> for Arc<str> {
1789 fn from(v: &str) -> Arc<str> {
1790 let arc = Arc::<[u8]>::from(v.as_bytes());
1791 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1795 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1796 impl From<String> for Arc<str> {
1798 fn from(v: String) -> Arc<str> {
1803 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1804 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1806 fn from(v: Box<T>) -> Arc<T> {
1811 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1812 impl<T> From<Vec<T>> for Arc<[T]> {
1814 fn from(mut v: Vec<T>) -> Arc<[T]> {
1816 let arc = Arc::copy_from_slice(&v);
1818 // Allow the Vec to free its memory, but not destroy its contents
1826 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1827 impl<T> iter::FromIterator<T> for Arc<[T]> {
1828 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
1830 /// # Performance characteristics
1832 /// ## The general case
1834 /// In the general case, collecting into `Arc<[T]>` is done by first
1835 /// collecting into a `Vec<T>`. That is, when writing the following:
1838 /// # use std::sync::Arc;
1839 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1840 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1843 /// this behaves as if we wrote:
1846 /// # use std::sync::Arc;
1847 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1848 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1849 /// .into(); // A second allocation for `Arc<[T]>` happens here.
1850 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1853 /// This will allocate as many times as needed for constructing the `Vec<T>`
1854 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
1856 /// ## Iterators of known length
1858 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1859 /// a single allocation will be made for the `Arc<[T]>`. For example:
1862 /// # use std::sync::Arc;
1863 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1864 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1866 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1867 ArcFromIter::from_iter(iter.into_iter())
1871 /// Specialization trait used for collecting into `Arc<[T]>`.
1872 trait ArcFromIter<T, I> {
1873 fn from_iter(iter: I) -> Self;
1876 impl<T, I: Iterator<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
1877 default fn from_iter(iter: I) -> Self {
1878 iter.collect::<Vec<T>>().into()
1882 impl<T, I: iter::TrustedLen<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
1883 default fn from_iter(iter: I) -> Self {
1884 // This is the case for a `TrustedLen` iterator.
1885 let (low, high) = iter.size_hint();
1886 if let Some(high) = high {
1889 "TrustedLen iterator's size hint is not exact: {:?}",
1894 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1895 Arc::from_iter_exact(iter, low)
1898 // Fall back to normal implementation.
1899 iter.collect::<Vec<T>>().into()
1904 impl<'a, T: 'a + Clone> ArcFromIter<&'a T, slice::Iter<'a, T>> for Arc<[T]> {
1905 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
1906 // Delegate to `impl<T: Clone> From<&[T]> for Arc<[T]>`.
1908 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
1909 // which is even more performant.
1911 // In the fall-back case we have `T: Clone`. This is still better
1912 // than the `TrustedLen` implementation as slices have a known length
1913 // and so we get to avoid calling `size_hint` and avoid the branching.
1914 iter.as_slice().into()
1920 use std::boxed::Box;
1921 use std::clone::Clone;
1922 use std::sync::mpsc::channel;
1925 use std::option::Option::{self, None, Some};
1926 use std::sync::atomic::{self, Ordering::{Acquire, SeqCst}};
1928 use std::sync::Mutex;
1929 use std::convert::From;
1931 use super::{Arc, Weak};
1932 use crate::vec::Vec;
1934 struct Canary(*mut atomic::AtomicUsize);
1936 impl Drop for Canary {
1937 fn drop(&mut self) {
1941 (*c).fetch_add(1, SeqCst);
1949 #[cfg_attr(target_os = "emscripten", ignore)]
1950 #[cfg(not(miri))] // Miri does not support threads
1951 fn manually_share_arc() {
1952 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1953 let arc_v = Arc::new(v);
1955 let (tx, rx) = channel();
1957 let _t = thread::spawn(move || {
1958 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1959 assert_eq!((*arc_v)[3], 4);
1962 tx.send(arc_v.clone()).unwrap();
1964 assert_eq!((*arc_v)[2], 3);
1965 assert_eq!((*arc_v)[4], 5);
1969 fn test_arc_get_mut() {
1970 let mut x = Arc::new(3);
1971 *Arc::get_mut(&mut x).unwrap() = 4;
1974 assert!(Arc::get_mut(&mut x).is_none());
1976 assert!(Arc::get_mut(&mut x).is_some());
1977 let _w = Arc::downgrade(&x);
1978 assert!(Arc::get_mut(&mut x).is_none());
1983 assert_eq!(Weak::weak_count(&Weak::<u64>::new()), None);
1984 assert_eq!(Weak::strong_count(&Weak::<u64>::new()), 0);
1986 let a = Arc::new(0);
1987 let w = Arc::downgrade(&a);
1988 assert_eq!(Weak::strong_count(&w), 1);
1989 assert_eq!(Weak::weak_count(&w), Some(1));
1991 assert_eq!(Weak::strong_count(&w), 1);
1992 assert_eq!(Weak::weak_count(&w), Some(2));
1993 assert_eq!(Weak::strong_count(&w2), 1);
1994 assert_eq!(Weak::weak_count(&w2), Some(2));
1996 assert_eq!(Weak::strong_count(&w2), 1);
1997 assert_eq!(Weak::weak_count(&w2), Some(1));
1999 assert_eq!(Weak::strong_count(&w2), 2);
2000 assert_eq!(Weak::weak_count(&w2), Some(1));
2003 assert_eq!(Weak::strong_count(&w2), 0);
2004 assert_eq!(Weak::weak_count(&w2), Some(1));
2010 let x = Arc::new(3);
2011 assert_eq!(Arc::try_unwrap(x), Ok(3));
2012 let x = Arc::new(4);
2014 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
2015 let x = Arc::new(5);
2016 let _w = Arc::downgrade(&x);
2017 assert_eq!(Arc::try_unwrap(x), Ok(5));
2021 fn into_from_raw() {
2022 let x = Arc::new(box "hello");
2025 let x_ptr = Arc::into_raw(x);
2028 assert_eq!(**x_ptr, "hello");
2030 let x = Arc::from_raw(x_ptr);
2031 assert_eq!(**x, "hello");
2033 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
2038 fn test_into_from_raw_unsized() {
2039 use std::fmt::Display;
2040 use std::string::ToString;
2042 let arc: Arc<str> = Arc::from("foo");
2044 let ptr = Arc::into_raw(arc.clone());
2045 let arc2 = unsafe { Arc::from_raw(ptr) };
2047 assert_eq!(unsafe { &*ptr }, "foo");
2048 assert_eq!(arc, arc2);
2050 let arc: Arc<dyn Display> = Arc::new(123);
2052 let ptr = Arc::into_raw(arc.clone());
2053 let arc2 = unsafe { Arc::from_raw(ptr) };
2055 assert_eq!(unsafe { &*ptr }.to_string(), "123");
2056 assert_eq!(arc2.to_string(), "123");
2060 fn test_cowarc_clone_make_mut() {
2061 let mut cow0 = Arc::new(75);
2062 let mut cow1 = cow0.clone();
2063 let mut cow2 = cow1.clone();
2065 assert!(75 == *Arc::make_mut(&mut cow0));
2066 assert!(75 == *Arc::make_mut(&mut cow1));
2067 assert!(75 == *Arc::make_mut(&mut cow2));
2069 *Arc::make_mut(&mut cow0) += 1;
2070 *Arc::make_mut(&mut cow1) += 2;
2071 *Arc::make_mut(&mut cow2) += 3;
2073 assert!(76 == *cow0);
2074 assert!(77 == *cow1);
2075 assert!(78 == *cow2);
2077 // none should point to the same backing memory
2078 assert!(*cow0 != *cow1);
2079 assert!(*cow0 != *cow2);
2080 assert!(*cow1 != *cow2);
2084 fn test_cowarc_clone_unique2() {
2085 let mut cow0 = Arc::new(75);
2086 let cow1 = cow0.clone();
2087 let cow2 = cow1.clone();
2089 assert!(75 == *cow0);
2090 assert!(75 == *cow1);
2091 assert!(75 == *cow2);
2093 *Arc::make_mut(&mut cow0) += 1;
2094 assert!(76 == *cow0);
2095 assert!(75 == *cow1);
2096 assert!(75 == *cow2);
2098 // cow1 and cow2 should share the same contents
2099 // cow0 should have a unique reference
2100 assert!(*cow0 != *cow1);
2101 assert!(*cow0 != *cow2);
2102 assert!(*cow1 == *cow2);
2106 fn test_cowarc_clone_weak() {
2107 let mut cow0 = Arc::new(75);
2108 let cow1_weak = Arc::downgrade(&cow0);
2110 assert!(75 == *cow0);
2111 assert!(75 == *cow1_weak.upgrade().unwrap());
2113 *Arc::make_mut(&mut cow0) += 1;
2115 assert!(76 == *cow0);
2116 assert!(cow1_weak.upgrade().is_none());
2121 let x = Arc::new(5);
2122 let y = Arc::downgrade(&x);
2123 assert!(y.upgrade().is_some());
2128 let x = Arc::new(5);
2129 let y = Arc::downgrade(&x);
2131 assert!(y.upgrade().is_none());
2135 fn weak_self_cyclic() {
2137 x: Mutex<Option<Weak<Cycle>>>,
2140 let a = Arc::new(Cycle { x: Mutex::new(None) });
2141 let b = Arc::downgrade(&a.clone());
2142 *a.x.lock().unwrap() = Some(b);
2144 // hopefully we don't double-free (or leak)...
2149 let mut canary = atomic::AtomicUsize::new(0);
2150 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
2152 assert!(canary.load(Acquire) == 1);
2156 fn drop_arc_weak() {
2157 let mut canary = atomic::AtomicUsize::new(0);
2158 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
2159 let arc_weak = Arc::downgrade(&arc);
2160 assert!(canary.load(Acquire) == 0);
2162 assert!(canary.load(Acquire) == 1);
2167 fn test_strong_count() {
2168 let a = Arc::new(0);
2169 assert!(Arc::strong_count(&a) == 1);
2170 let w = Arc::downgrade(&a);
2171 assert!(Arc::strong_count(&a) == 1);
2172 let b = w.upgrade().expect("");
2173 assert!(Arc::strong_count(&b) == 2);
2174 assert!(Arc::strong_count(&a) == 2);
2177 assert!(Arc::strong_count(&b) == 1);
2179 assert!(Arc::strong_count(&b) == 2);
2180 assert!(Arc::strong_count(&c) == 2);
2184 fn test_weak_count() {
2185 let a = Arc::new(0);
2186 assert!(Arc::strong_count(&a) == 1);
2187 assert!(Arc::weak_count(&a) == 0);
2188 let w = Arc::downgrade(&a);
2189 assert!(Arc::strong_count(&a) == 1);
2190 assert!(Arc::weak_count(&a) == 1);
2192 assert!(Arc::weak_count(&a) == 2);
2195 assert!(Arc::strong_count(&a) == 1);
2196 assert!(Arc::weak_count(&a) == 0);
2198 assert!(Arc::strong_count(&a) == 2);
2199 assert!(Arc::weak_count(&a) == 0);
2200 let d = Arc::downgrade(&c);
2201 assert!(Arc::weak_count(&c) == 1);
2202 assert!(Arc::strong_count(&c) == 2);
2211 let a = Arc::new(5);
2212 assert_eq!(format!("{:?}", a), "5");
2215 // Make sure deriving works with Arc<T>
2216 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
2223 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
2224 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
2225 let y = Arc::downgrade(&x.clone());
2227 assert!(y.upgrade().is_none());
2231 fn test_from_owned() {
2233 let foo_arc = Arc::from(foo);
2234 assert!(123 == *foo_arc);
2238 fn test_new_weak() {
2239 let foo: Weak<usize> = Weak::new();
2240 assert!(foo.upgrade().is_none());
2245 let five = Arc::new(5);
2246 let same_five = five.clone();
2247 let other_five = Arc::new(5);
2249 assert!(Arc::ptr_eq(&five, &same_five));
2250 assert!(!Arc::ptr_eq(&five, &other_five));
2254 #[cfg_attr(target_os = "emscripten", ignore)]
2255 #[cfg(not(miri))] // Miri does not support threads
2256 fn test_weak_count_locked() {
2257 let mut a = Arc::new(atomic::AtomicBool::new(false));
2259 let t = thread::spawn(move || {
2260 for _i in 0..1000000 {
2261 Arc::get_mut(&mut a);
2263 a.store(true, SeqCst);
2266 while !a2.load(SeqCst) {
2267 let n = Arc::weak_count(&a2);
2268 assert!(n < 2, "bad weak count: {}", n);
2274 fn test_from_str() {
2275 let r: Arc<str> = Arc::from("foo");
2277 assert_eq!(&r[..], "foo");
2281 fn test_copy_from_slice() {
2282 let s: &[u32] = &[1, 2, 3];
2283 let r: Arc<[u32]> = Arc::from(s);
2285 assert_eq!(&r[..], [1, 2, 3]);
2289 fn test_clone_from_slice() {
2290 #[derive(Clone, Debug, Eq, PartialEq)]
2293 let s: &[X] = &[X(1), X(2), X(3)];
2294 let r: Arc<[X]> = Arc::from(s);
2296 assert_eq!(&r[..], s);
2301 fn test_clone_from_slice_panic() {
2302 use std::string::{String, ToString};
2304 struct Fail(u32, String);
2306 impl Clone for Fail {
2307 fn clone(&self) -> Fail {
2311 Fail(self.0, self.1.clone())
2316 Fail(0, "foo".to_string()),
2317 Fail(1, "bar".to_string()),
2318 Fail(2, "baz".to_string()),
2321 // Should panic, but not cause memory corruption
2322 let _r: Arc<[Fail]> = Arc::from(s);
2326 fn test_from_box() {
2327 let b: Box<u32> = box 123;
2328 let r: Arc<u32> = Arc::from(b);
2330 assert_eq!(*r, 123);
2334 fn test_from_box_str() {
2335 use std::string::String;
2337 let s = String::from("foo").into_boxed_str();
2338 let r: Arc<str> = Arc::from(s);
2340 assert_eq!(&r[..], "foo");
2344 fn test_from_box_slice() {
2345 let s = vec![1, 2, 3].into_boxed_slice();
2346 let r: Arc<[u32]> = Arc::from(s);
2348 assert_eq!(&r[..], [1, 2, 3]);
2352 fn test_from_box_trait() {
2353 use std::fmt::Display;
2354 use std::string::ToString;
2356 let b: Box<dyn Display> = box 123;
2357 let r: Arc<dyn Display> = Arc::from(b);
2359 assert_eq!(r.to_string(), "123");
2363 fn test_from_box_trait_zero_sized() {
2364 use std::fmt::Debug;
2366 let b: Box<dyn Debug> = box ();
2367 let r: Arc<dyn Debug> = Arc::from(b);
2369 assert_eq!(format!("{:?}", r), "()");
2373 fn test_from_vec() {
2374 let v = vec![1, 2, 3];
2375 let r: Arc<[u32]> = Arc::from(v);
2377 assert_eq!(&r[..], [1, 2, 3]);
2381 fn test_downcast() {
2384 let r1: Arc<dyn Any + Send + Sync> = Arc::new(i32::max_value());
2385 let r2: Arc<dyn Any + Send + Sync> = Arc::new("abc");
2387 assert!(r1.clone().downcast::<u32>().is_err());
2389 let r1i32 = r1.downcast::<i32>();
2390 assert!(r1i32.is_ok());
2391 assert_eq!(r1i32.unwrap(), Arc::new(i32::max_value()));
2393 assert!(r2.clone().downcast::<i32>().is_err());
2395 let r2str = r2.downcast::<&'static str>();
2396 assert!(r2str.is_ok());
2397 assert_eq!(r2str.unwrap(), Arc::new("abc"));
2401 #[stable(feature = "rust1", since = "1.0.0")]
2402 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
2403 fn borrow(&self) -> &T {
2408 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2409 impl<T: ?Sized> AsRef<T> for Arc<T> {
2410 fn as_ref(&self) -> &T {
2415 #[stable(feature = "pin", since = "1.33.0")]
2416 impl<T: ?Sized> Unpin for Arc<T> { }
2418 /// Computes the offset of the data field within `ArcInner`.
2419 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2420 // Align the unsized value to the end of the `ArcInner`.
2421 // Because it is `?Sized`, it will always be the last field in memory.
2422 data_offset_align(align_of_val(&*ptr))
2425 /// Computes the offset of the data field within `ArcInner`.
2427 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2428 fn data_offset_sized<T>() -> isize {
2429 data_offset_align(align_of::<T>())
2433 fn data_offset_align(align: usize) -> isize {
2434 let layout = Layout::new::<ArcInner<()>>();
2435 (layout.size() + layout.padding_needed_for(align)) as isize