1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 #![stable(feature = "rust1", since = "1.0.0")]
13 //! Thread-safe reference-counting pointers.
15 //! See the [`Arc<T>`][arc] documentation for more details.
17 //! [arc]: struct.Arc.html
20 use core::sync::atomic;
21 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
24 use core::cmp::Ordering;
25 use core::intrinsics::abort;
26 use core::mem::{self, align_of_val, size_of_val};
28 use core::ops::{CoerceUnsized, DispatchFromDyn};
30 use core::ptr::{self, NonNull};
31 use core::marker::{Unpin, Unsize, PhantomData};
32 use core::hash::{Hash, Hasher};
33 use core::{isize, usize};
34 use core::convert::From;
36 use alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
42 /// A soft limit on the amount of references that may be made to an `Arc`.
44 /// Going above this limit will abort your program (although not
45 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
46 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
48 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
49 /// Reference Counted'.
51 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
52 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
53 /// a new `Arc` instance, which points to the same value on the heap as the
54 /// source `Arc`, while increasing a reference count. When the last `Arc`
55 /// pointer to a given value is destroyed, the pointed-to value is also
58 /// Shared references in Rust disallow mutation by default, and `Arc` is no
59 /// exception: you cannot generally obtain a mutable reference to something
60 /// inside an `Arc`. If you need to mutate through an `Arc`, use
61 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
66 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
67 /// counting. This means that it is thread-safe. The disadvantage is that
68 /// atomic operations are more expensive than ordinary memory accesses. If you
69 /// are not sharing reference-counted values between threads, consider using
70 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
71 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
72 /// However, a library might choose `Arc<T>` in order to give library consumers
75 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
76 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
77 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
78 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
79 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
80 /// data, but it doesn't add thread safety to its data. Consider
81 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
82 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
83 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
84 /// non-atomic operations.
86 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
87 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
89 /// ## Breaking cycles with `Weak`
91 /// The [`downgrade`][downgrade] method can be used to create a non-owning
92 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
93 /// to an `Arc`, but this will return [`None`] if the value has already been
96 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
97 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
98 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
99 /// pointers from children back to their parents.
101 /// # Cloning references
103 /// Creating a new reference from an existing reference counted pointer is done using the
104 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
107 /// use std::sync::Arc;
108 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
109 /// // The two syntaxes below are equivalent.
110 /// let a = foo.clone();
111 /// let b = Arc::clone(&foo);
112 /// // a, b, and foo are all Arcs that point to the same memory location
115 /// The [`Arc::clone(&from)`] syntax is the most idiomatic because it conveys more explicitly
116 /// the meaning of the code. In the example above, this syntax makes it easier to see that
117 /// this code is creating a new reference rather than copying the whole content of foo.
119 /// ## `Deref` behavior
121 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
122 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
123 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
124 /// functions, called using function-like syntax:
127 /// use std::sync::Arc;
128 /// let my_arc = Arc::new(());
130 /// Arc::downgrade(&my_arc);
133 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have
134 /// already been destroyed.
136 /// [arc]: struct.Arc.html
137 /// [weak]: struct.Weak.html
138 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
139 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
140 /// [mutex]: ../../std/sync/struct.Mutex.html
141 /// [rwlock]: ../../std/sync/struct.RwLock.html
142 /// [atomic]: ../../std/sync/atomic/index.html
143 /// [`Send`]: ../../std/marker/trait.Send.html
144 /// [`Sync`]: ../../std/marker/trait.Sync.html
145 /// [deref]: ../../std/ops/trait.Deref.html
146 /// [downgrade]: struct.Arc.html#method.downgrade
147 /// [upgrade]: struct.Weak.html#method.upgrade
148 /// [`None`]: ../../std/option/enum.Option.html#variant.None
149 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
150 /// [`std::sync`]: ../../std/sync/index.html
151 /// [`Arc::clone(&from)`]: #method.clone
155 /// Sharing some immutable data between threads:
157 // Note that we **do not** run these tests here. The windows builders get super
158 // unhappy if a thread outlives the main thread and then exits at the same time
159 // (something deadlocks) so we just avoid this entirely by not running these
162 /// use std::sync::Arc;
165 /// let five = Arc::new(5);
168 /// let five = Arc::clone(&five);
170 /// thread::spawn(move || {
171 /// println!("{:?}", five);
176 /// Sharing a mutable [`AtomicUsize`]:
178 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
181 /// use std::sync::Arc;
182 /// use std::sync::atomic::{AtomicUsize, Ordering};
185 /// let val = Arc::new(AtomicUsize::new(5));
188 /// let val = Arc::clone(&val);
190 /// thread::spawn(move || {
191 /// let v = val.fetch_add(1, Ordering::SeqCst);
192 /// println!("{:?}", v);
197 /// See the [`rc` documentation][rc_examples] for more examples of reference
198 /// counting in general.
200 /// [rc_examples]: ../../std/rc/index.html#examples
201 #[cfg_attr(not(test), lang = "arc")]
202 #[stable(feature = "rust1", since = "1.0.0")]
203 pub struct Arc<T: ?Sized> {
204 ptr: NonNull<ArcInner<T>>,
205 phantom: PhantomData<T>,
208 #[stable(feature = "rust1", since = "1.0.0")]
209 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
210 #[stable(feature = "rust1", since = "1.0.0")]
211 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
213 #[unstable(feature = "coerce_unsized", issue = "27732")]
214 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
216 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
217 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
219 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
220 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
221 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
223 /// Since a `Weak` reference does not count towards ownership, it will not
224 /// prevent the inner value from being dropped, and `Weak` itself makes no
225 /// guarantees about the value still being present and may return [`None`]
226 /// when [`upgrade`]d.
228 /// A `Weak` pointer is useful for keeping a temporary reference to the value
229 /// within [`Arc`] without extending its lifetime. It is also used to prevent
230 /// circular references between [`Arc`] pointers, since mutual owning references
231 /// would never allow either [`Arc`] to be dropped. For example, a tree could
232 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
233 /// pointers from children back to their parents.
235 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
237 /// [`Arc`]: struct.Arc.html
238 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
239 /// [`upgrade`]: struct.Weak.html#method.upgrade
240 /// [`Option`]: ../../std/option/enum.Option.html
241 /// [`None`]: ../../std/option/enum.Option.html#variant.None
242 #[stable(feature = "arc_weak", since = "1.4.0")]
243 pub struct Weak<T: ?Sized> {
244 // This is a `NonNull` to allow optimizing the size of this type in enums,
245 // but it is not necessarily a valid pointer.
246 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
247 // to allocate space on the heap. That's not a value a real pointer
248 // will ever have because RcBox has alignment at least 2.
249 ptr: NonNull<ArcInner<T>>,
252 #[stable(feature = "arc_weak", since = "1.4.0")]
253 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
254 #[stable(feature = "arc_weak", since = "1.4.0")]
255 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
257 #[unstable(feature = "coerce_unsized", issue = "27732")]
258 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
259 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
260 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
262 #[stable(feature = "arc_weak", since = "1.4.0")]
263 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
264 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
269 struct ArcInner<T: ?Sized> {
270 strong: atomic::AtomicUsize,
272 // the value usize::MAX acts as a sentinel for temporarily "locking" the
273 // ability to upgrade weak pointers or downgrade strong ones; this is used
274 // to avoid races in `make_mut` and `get_mut`.
275 weak: atomic::AtomicUsize,
280 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
281 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
284 /// Constructs a new `Arc<T>`.
289 /// use std::sync::Arc;
291 /// let five = Arc::new(5);
294 #[stable(feature = "rust1", since = "1.0.0")]
295 pub fn new(data: T) -> Arc<T> {
296 // Start the weak pointer count as 1 which is the weak pointer that's
297 // held by all the strong pointers (kinda), see std/rc.rs for more info
298 let x: Box<_> = box ArcInner {
299 strong: atomic::AtomicUsize::new(1),
300 weak: atomic::AtomicUsize::new(1),
303 Arc { ptr: Box::into_raw_non_null(x), phantom: PhantomData }
306 #[unstable(feature = "pin", issue = "49150")]
307 pub fn pinned(data: T) -> Pin<Arc<T>> {
308 unsafe { Pin::new_unchecked(Arc::new(data)) }
311 /// Returns the contained value, if the `Arc` has exactly one strong reference.
313 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
316 /// This will succeed even if there are outstanding weak references.
318 /// [result]: ../../std/result/enum.Result.html
323 /// use std::sync::Arc;
325 /// let x = Arc::new(3);
326 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
328 /// let x = Arc::new(4);
329 /// let _y = Arc::clone(&x);
330 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
333 #[stable(feature = "arc_unique", since = "1.4.0")]
334 pub fn try_unwrap(this: Self) -> Result<T, Self> {
335 // See `drop` for why all these atomics are like this
336 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
340 atomic::fence(Acquire);
343 let elem = ptr::read(&this.ptr.as_ref().data);
345 // Make a weak pointer to clean up the implicit strong-weak reference
346 let _weak = Weak { ptr: this.ptr };
354 impl<T: ?Sized> Arc<T> {
355 /// Consumes the `Arc`, returning the wrapped pointer.
357 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
358 /// [`Arc::from_raw`][from_raw].
360 /// [from_raw]: struct.Arc.html#method.from_raw
365 /// use std::sync::Arc;
367 /// let x = Arc::new(10);
368 /// let x_ptr = Arc::into_raw(x);
369 /// assert_eq!(unsafe { *x_ptr }, 10);
371 #[stable(feature = "rc_raw", since = "1.17.0")]
372 pub fn into_raw(this: Self) -> *const T {
373 let ptr: *const T = &*this;
378 /// Constructs an `Arc` from a raw pointer.
380 /// The raw pointer must have been previously returned by a call to a
381 /// [`Arc::into_raw`][into_raw].
383 /// This function is unsafe because improper use may lead to memory problems. For example, a
384 /// double-free may occur if the function is called twice on the same raw pointer.
386 /// [into_raw]: struct.Arc.html#method.into_raw
391 /// use std::sync::Arc;
393 /// let x = Arc::new(10);
394 /// let x_ptr = Arc::into_raw(x);
397 /// // Convert back to an `Arc` to prevent leak.
398 /// let x = Arc::from_raw(x_ptr);
399 /// assert_eq!(*x, 10);
401 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
404 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
406 #[stable(feature = "rc_raw", since = "1.17.0")]
407 pub unsafe fn from_raw(ptr: *const T) -> Self {
408 // Align the unsized value to the end of the ArcInner.
409 // Because it is ?Sized, it will always be the last field in memory.
410 let align = align_of_val(&*ptr);
411 let layout = Layout::new::<ArcInner<()>>();
412 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
414 // Reverse the offset to find the original ArcInner.
415 let fake_ptr = ptr as *mut ArcInner<T>;
416 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
419 ptr: NonNull::new_unchecked(arc_ptr),
420 phantom: PhantomData,
424 /// Creates a new [`Weak`][weak] pointer to this value.
426 /// [weak]: struct.Weak.html
431 /// use std::sync::Arc;
433 /// let five = Arc::new(5);
435 /// let weak_five = Arc::downgrade(&five);
437 #[stable(feature = "arc_weak", since = "1.4.0")]
438 pub fn downgrade(this: &Self) -> Weak<T> {
439 // This Relaxed is OK because we're checking the value in the CAS
441 let mut cur = this.inner().weak.load(Relaxed);
444 // check if the weak counter is currently "locked"; if so, spin.
445 if cur == usize::MAX {
446 cur = this.inner().weak.load(Relaxed);
450 // NOTE: this code currently ignores the possibility of overflow
451 // into usize::MAX; in general both Rc and Arc need to be adjusted
452 // to deal with overflow.
454 // Unlike with Clone(), we need this to be an Acquire read to
455 // synchronize with the write coming from `is_unique`, so that the
456 // events prior to that write happen before this read.
457 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
459 // Make sure we do not create a dangling Weak
460 debug_assert!(!is_dangling(this.ptr));
461 return Weak { ptr: this.ptr };
463 Err(old) => cur = old,
468 /// Gets the number of [`Weak`][weak] pointers to this value.
470 /// [weak]: struct.Weak.html
474 /// This method by itself is safe, but using it correctly requires extra care.
475 /// Another thread can change the weak count at any time,
476 /// including potentially between calling this method and acting on the result.
481 /// use std::sync::Arc;
483 /// let five = Arc::new(5);
484 /// let _weak_five = Arc::downgrade(&five);
486 /// // This assertion is deterministic because we haven't shared
487 /// // the `Arc` or `Weak` between threads.
488 /// assert_eq!(1, Arc::weak_count(&five));
491 #[stable(feature = "arc_counts", since = "1.15.0")]
492 pub fn weak_count(this: &Self) -> usize {
493 let cnt = this.inner().weak.load(SeqCst);
494 // If the weak count is currently locked, the value of the
495 // count was 0 just before taking the lock.
496 if cnt == usize::MAX { 0 } else { cnt - 1 }
499 /// Gets the number of strong (`Arc`) pointers to this value.
503 /// This method by itself is safe, but using it correctly requires extra care.
504 /// Another thread can change the strong count at any time,
505 /// including potentially between calling this method and acting on the result.
510 /// use std::sync::Arc;
512 /// let five = Arc::new(5);
513 /// let _also_five = Arc::clone(&five);
515 /// // This assertion is deterministic because we haven't shared
516 /// // the `Arc` between threads.
517 /// assert_eq!(2, Arc::strong_count(&five));
520 #[stable(feature = "arc_counts", since = "1.15.0")]
521 pub fn strong_count(this: &Self) -> usize {
522 this.inner().strong.load(SeqCst)
526 fn inner(&self) -> &ArcInner<T> {
527 // This unsafety is ok because while this arc is alive we're guaranteed
528 // that the inner pointer is valid. Furthermore, we know that the
529 // `ArcInner` structure itself is `Sync` because the inner data is
530 // `Sync` as well, so we're ok loaning out an immutable pointer to these
532 unsafe { self.ptr.as_ref() }
535 // Non-inlined part of `drop`.
537 unsafe fn drop_slow(&mut self) {
538 // Destroy the data at this time, even though we may not free the box
539 // allocation itself (there may still be weak pointers lying around).
540 ptr::drop_in_place(&mut self.ptr.as_mut().data);
542 if self.inner().weak.fetch_sub(1, Release) == 1 {
543 atomic::fence(Acquire);
544 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
549 #[stable(feature = "ptr_eq", since = "1.17.0")]
550 /// Returns true if the two `Arc`s point to the same value (not
551 /// just values that compare as equal).
556 /// use std::sync::Arc;
558 /// let five = Arc::new(5);
559 /// let same_five = Arc::clone(&five);
560 /// let other_five = Arc::new(5);
562 /// assert!(Arc::ptr_eq(&five, &same_five));
563 /// assert!(!Arc::ptr_eq(&five, &other_five));
565 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
566 this.ptr.as_ptr() == other.ptr.as_ptr()
570 impl<T: ?Sized> Arc<T> {
571 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
572 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
573 // Calculate layout using the given value.
574 // Previously, layout was calculated on the expression
575 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
576 // reference (see #54908).
577 let layout = Layout::new::<ArcInner<()>>()
578 .extend(Layout::for_value(&*ptr)).unwrap().0
579 .pad_to_align().unwrap();
581 let mem = Global.alloc(layout)
582 .unwrap_or_else(|_| handle_alloc_error(layout));
584 // Initialize the ArcInner
585 let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;
586 debug_assert_eq!(Layout::for_value(&*inner), layout);
588 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
589 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
594 fn from_box(v: Box<T>) -> Arc<T> {
596 let box_unique = Box::into_unique(v);
597 let bptr = box_unique.as_ptr();
599 let value_size = size_of_val(&*bptr);
600 let ptr = Self::allocate_for_ptr(bptr);
602 // Copy value as bytes
603 ptr::copy_nonoverlapping(
604 bptr as *const T as *const u8,
605 &mut (*ptr).data as *mut _ as *mut u8,
608 // Free the allocation without dropping its contents
609 box_free(box_unique);
611 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
616 // Sets the data pointer of a `?Sized` raw pointer.
618 // For a slice/trait object, this sets the `data` field and leaves the rest
619 // unchanged. For a sized raw pointer, this simply sets the pointer.
620 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
621 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
626 // Copy elements from slice into newly allocated Arc<[T]>
628 // Unsafe because the caller must either take ownership or bind `T: Copy`
629 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
630 let v_ptr = v as *const [T];
631 let ptr = Self::allocate_for_ptr(v_ptr);
633 ptr::copy_nonoverlapping(
635 &mut (*ptr).data as *mut [T] as *mut T,
638 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
642 // Specialization trait used for From<&[T]>
643 trait ArcFromSlice<T> {
644 fn from_slice(slice: &[T]) -> Self;
647 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
649 default fn from_slice(v: &[T]) -> Self {
650 // Panic guard while cloning T elements.
651 // In the event of a panic, elements that have been written
652 // into the new ArcInner will be dropped, then the memory freed.
660 impl<T> Drop for Guard<T> {
662 use core::slice::from_raw_parts_mut;
665 let slice = from_raw_parts_mut(self.elems, self.n_elems);
666 ptr::drop_in_place(slice);
668 Global.dealloc(self.mem.cast(), self.layout.clone());
674 let v_ptr = v as *const [T];
675 let ptr = Self::allocate_for_ptr(v_ptr);
677 let mem = ptr as *mut _ as *mut u8;
678 let layout = Layout::for_value(&*ptr);
680 // Pointer to first element
681 let elems = &mut (*ptr).data as *mut [T] as *mut T;
683 let mut guard = Guard{
684 mem: NonNull::new_unchecked(mem),
690 for (i, item) in v.iter().enumerate() {
691 ptr::write(elems.add(i), item.clone());
695 // All clear. Forget the guard so it doesn't free the new ArcInner.
698 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
703 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
705 fn from_slice(v: &[T]) -> Self {
706 unsafe { Arc::copy_from_slice(v) }
710 #[stable(feature = "rust1", since = "1.0.0")]
711 impl<T: ?Sized> Clone for Arc<T> {
712 /// Makes a clone of the `Arc` pointer.
714 /// This creates another pointer to the same inner value, increasing the
715 /// strong reference count.
720 /// use std::sync::Arc;
722 /// let five = Arc::new(5);
724 /// let _ = Arc::clone(&five);
727 fn clone(&self) -> Arc<T> {
728 // Using a relaxed ordering is alright here, as knowledge of the
729 // original reference prevents other threads from erroneously deleting
732 // As explained in the [Boost documentation][1], Increasing the
733 // reference counter can always be done with memory_order_relaxed: New
734 // references to an object can only be formed from an existing
735 // reference, and passing an existing reference from one thread to
736 // another must already provide any required synchronization.
738 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
739 let old_size = self.inner().strong.fetch_add(1, Relaxed);
741 // However we need to guard against massive refcounts in case someone
742 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
743 // and users will use-after free. We racily saturate to `isize::MAX` on
744 // the assumption that there aren't ~2 billion threads incrementing
745 // the reference count at once. This branch will never be taken in
746 // any realistic program.
748 // We abort because such a program is incredibly degenerate, and we
749 // don't care to support it.
750 if old_size > MAX_REFCOUNT {
756 Arc { ptr: self.ptr, phantom: PhantomData }
760 #[stable(feature = "rust1", since = "1.0.0")]
761 impl<T: ?Sized> Deref for Arc<T> {
765 fn deref(&self) -> &T {
770 impl<T: Clone> Arc<T> {
771 /// Makes a mutable reference into the given `Arc`.
773 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
774 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
775 /// ensure unique ownership. This is also referred to as clone-on-write.
777 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
779 /// [weak]: struct.Weak.html
780 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
781 /// [get_mut]: struct.Arc.html#method.get_mut
786 /// use std::sync::Arc;
788 /// let mut data = Arc::new(5);
790 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
791 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
792 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
793 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
794 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
796 /// // Now `data` and `other_data` point to different values.
797 /// assert_eq!(*data, 8);
798 /// assert_eq!(*other_data, 12);
801 #[stable(feature = "arc_unique", since = "1.4.0")]
802 pub fn make_mut(this: &mut Self) -> &mut T {
803 // Note that we hold both a strong reference and a weak reference.
804 // Thus, releasing our strong reference only will not, by itself, cause
805 // the memory to be deallocated.
807 // Use Acquire to ensure that we see any writes to `weak` that happen
808 // before release writes (i.e., decrements) to `strong`. Since we hold a
809 // weak count, there's no chance the ArcInner itself could be
811 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
812 // Another strong pointer exists; clone
813 *this = Arc::new((**this).clone());
814 } else if this.inner().weak.load(Relaxed) != 1 {
815 // Relaxed suffices in the above because this is fundamentally an
816 // optimization: we are always racing with weak pointers being
817 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
819 // We removed the last strong ref, but there are additional weak
820 // refs remaining. We'll move the contents to a new Arc, and
821 // invalidate the other weak refs.
823 // Note that it is not possible for the read of `weak` to yield
824 // usize::MAX (i.e., locked), since the weak count can only be
825 // locked by a thread with a strong reference.
827 // Materialize our own implicit weak pointer, so that it can clean
828 // up the ArcInner as needed.
829 let weak = Weak { ptr: this.ptr };
831 // mark the data itself as already deallocated
833 // there is no data race in the implicit write caused by `read`
834 // here (due to zeroing) because data is no longer accessed by
835 // other threads (due to there being no more strong refs at this
837 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
838 mem::swap(this, &mut swap);
842 // We were the sole reference of either kind; bump back up the
844 this.inner().strong.store(1, Release);
847 // As with `get_mut()`, the unsafety is ok because our reference was
848 // either unique to begin with, or became one upon cloning the contents.
850 &mut this.ptr.as_mut().data
855 impl<T: ?Sized> Arc<T> {
856 /// Returns a mutable reference to the inner value, if there are
857 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
859 /// Returns [`None`][option] otherwise, because it is not safe to
860 /// mutate a shared value.
862 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
863 /// the inner value when it's shared.
865 /// [weak]: struct.Weak.html
866 /// [option]: ../../std/option/enum.Option.html
867 /// [make_mut]: struct.Arc.html#method.make_mut
868 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
873 /// use std::sync::Arc;
875 /// let mut x = Arc::new(3);
876 /// *Arc::get_mut(&mut x).unwrap() = 4;
877 /// assert_eq!(*x, 4);
879 /// let _y = Arc::clone(&x);
880 /// assert!(Arc::get_mut(&mut x).is_none());
883 #[stable(feature = "arc_unique", since = "1.4.0")]
884 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
885 if this.is_unique() {
886 // This unsafety is ok because we're guaranteed that the pointer
887 // returned is the *only* pointer that will ever be returned to T. Our
888 // reference count is guaranteed to be 1 at this point, and we required
889 // the Arc itself to be `mut`, so we're returning the only possible
890 // reference to the inner data.
892 Some(&mut this.ptr.as_mut().data)
899 /// Determine whether this is the unique reference (including weak refs) to
900 /// the underlying data.
902 /// Note that this requires locking the weak ref count.
903 fn is_unique(&mut self) -> bool {
904 // lock the weak pointer count if we appear to be the sole weak pointer
907 // The acquire label here ensures a happens-before relationship with any
908 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
909 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
910 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
911 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
912 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
913 // counter in `drop` -- the only access that happens when any but the last reference
915 let unique = self.inner().strong.load(Acquire) == 1;
917 // The release write here synchronizes with a read in `downgrade`,
918 // effectively preventing the above read of `strong` from happening
920 self.inner().weak.store(1, Release); // release the lock
928 #[stable(feature = "rust1", since = "1.0.0")]
929 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
932 /// This will decrement the strong reference count. If the strong reference
933 /// count reaches zero then the only other references (if any) are
934 /// [`Weak`], so we `drop` the inner value.
939 /// use std::sync::Arc;
943 /// impl Drop for Foo {
944 /// fn drop(&mut self) {
945 /// println!("dropped!");
949 /// let foo = Arc::new(Foo);
950 /// let foo2 = Arc::clone(&foo);
952 /// drop(foo); // Doesn't print anything
953 /// drop(foo2); // Prints "dropped!"
957 // Because `fetch_sub` is already atomic, we do not need to synchronize
958 // with other threads unless we are going to delete the object. This
959 // same logic applies to the below `fetch_sub` to the `weak` count.
960 if self.inner().strong.fetch_sub(1, Release) != 1 {
964 // This fence is needed to prevent reordering of use of the data and
965 // deletion of the data. Because it is marked `Release`, the decreasing
966 // of the reference count synchronizes with this `Acquire` fence. This
967 // means that use of the data happens before decreasing the reference
968 // count, which happens before this fence, which happens before the
969 // deletion of the data.
971 // As explained in the [Boost documentation][1],
973 // > It is important to enforce any possible access to the object in one
974 // > thread (through an existing reference) to *happen before* deleting
975 // > the object in a different thread. This is achieved by a "release"
976 // > operation after dropping a reference (any access to the object
977 // > through this reference must obviously happened before), and an
978 // > "acquire" operation before deleting the object.
980 // In particular, while the contents of an Arc are usually immutable, it's
981 // possible to have interior writes to something like a Mutex<T>. Since a
982 // Mutex is not acquired when it is deleted, we can't rely on its
983 // synchronization logic to make writes in thread A visible to a destructor
984 // running in thread B.
986 // Also note that the Acquire fence here could probably be replaced with an
987 // Acquire load, which could improve performance in highly-contended
988 // situations. See [2].
990 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
991 // [2]: (https://github.com/rust-lang/rust/pull/41714)
992 atomic::fence(Acquire);
1000 impl Arc<dyn Any + Send + Sync> {
1002 #[stable(feature = "rc_downcast", since = "1.29.0")]
1003 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1008 /// use std::any::Any;
1009 /// use std::sync::Arc;
1011 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1012 /// if let Ok(string) = value.downcast::<String>() {
1013 /// println!("String ({}): {}", string.len(), string);
1018 /// let my_string = "Hello World".to_string();
1019 /// print_if_string(Arc::new(my_string));
1020 /// print_if_string(Arc::new(0i8));
1023 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1025 T: Any + Send + Sync + 'static,
1027 if (*self).is::<T>() {
1028 let ptr = self.ptr.cast::<ArcInner<T>>();
1030 Ok(Arc { ptr, phantom: PhantomData })
1038 /// Constructs a new `Weak<T>`, without allocating any memory.
1039 /// Calling [`upgrade`] on the return value always gives [`None`].
1041 /// [`upgrade`]: struct.Weak.html#method.upgrade
1042 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1047 /// use std::sync::Weak;
1049 /// let empty: Weak<i64> = Weak::new();
1050 /// assert!(empty.upgrade().is_none());
1052 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1053 pub fn new() -> Weak<T> {
1055 ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0"),
1060 impl<T: ?Sized> Weak<T> {
1061 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1062 /// the lifetime of the value if successful.
1064 /// Returns [`None`] if the value has since been dropped.
1066 /// [`Arc`]: struct.Arc.html
1067 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1072 /// use std::sync::Arc;
1074 /// let five = Arc::new(5);
1076 /// let weak_five = Arc::downgrade(&five);
1078 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1079 /// assert!(strong_five.is_some());
1081 /// // Destroy all strong pointers.
1082 /// drop(strong_five);
1085 /// assert!(weak_five.upgrade().is_none());
1087 #[stable(feature = "arc_weak", since = "1.4.0")]
1088 pub fn upgrade(&self) -> Option<Arc<T>> {
1089 // We use a CAS loop to increment the strong count instead of a
1090 // fetch_add because once the count hits 0 it must never be above 0.
1091 let inner = self.inner()?;
1093 // Relaxed load because any write of 0 that we can observe
1094 // leaves the field in a permanently zero state (so a
1095 // "stale" read of 0 is fine), and any other value is
1096 // confirmed via the CAS below.
1097 let mut n = inner.strong.load(Relaxed);
1104 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1105 if n > MAX_REFCOUNT {
1111 // Relaxed is valid for the same reason it is on Arc's Clone impl
1112 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1113 Ok(_) => return Some(Arc {
1114 // null checked above
1116 phantom: PhantomData,
1118 Err(old) => n = old,
1123 /// Return `None` when the pointer is dangling and there is no allocated `ArcInner`,
1124 /// i.e. this `Weak` was created by `Weak::new`
1126 fn inner(&self) -> Option<&ArcInner<T>> {
1127 if is_dangling(self.ptr) {
1130 Some(unsafe { self.ptr.as_ref() })
1135 #[stable(feature = "arc_weak", since = "1.4.0")]
1136 impl<T: ?Sized> Clone for Weak<T> {
1137 /// Makes a clone of the `Weak` pointer that points to the same value.
1142 /// use std::sync::{Arc, Weak};
1144 /// let weak_five = Arc::downgrade(&Arc::new(5));
1146 /// let _ = Weak::clone(&weak_five);
1149 fn clone(&self) -> Weak<T> {
1150 let inner = if let Some(inner) = self.inner() {
1153 return Weak { ptr: self.ptr };
1155 // See comments in Arc::clone() for why this is relaxed. This can use a
1156 // fetch_add (ignoring the lock) because the weak count is only locked
1157 // where are *no other* weak pointers in existence. (So we can't be
1158 // running this code in that case).
1159 let old_size = inner.weak.fetch_add(1, Relaxed);
1161 // See comments in Arc::clone() for why we do this (for mem::forget).
1162 if old_size > MAX_REFCOUNT {
1168 return Weak { ptr: self.ptr };
1172 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1173 impl<T> Default for Weak<T> {
1174 /// Constructs a new `Weak<T>`, without allocating memory.
1175 /// Calling [`upgrade`][Weak::upgrade] on the return value always
1178 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1183 /// use std::sync::Weak;
1185 /// let empty: Weak<i64> = Default::default();
1186 /// assert!(empty.upgrade().is_none());
1188 fn default() -> Weak<T> {
1193 #[stable(feature = "arc_weak", since = "1.4.0")]
1194 impl<T: ?Sized> Drop for Weak<T> {
1195 /// Drops the `Weak` pointer.
1200 /// use std::sync::{Arc, Weak};
1204 /// impl Drop for Foo {
1205 /// fn drop(&mut self) {
1206 /// println!("dropped!");
1210 /// let foo = Arc::new(Foo);
1211 /// let weak_foo = Arc::downgrade(&foo);
1212 /// let other_weak_foo = Weak::clone(&weak_foo);
1214 /// drop(weak_foo); // Doesn't print anything
1215 /// drop(foo); // Prints "dropped!"
1217 /// assert!(other_weak_foo.upgrade().is_none());
1219 fn drop(&mut self) {
1220 // If we find out that we were the last weak pointer, then its time to
1221 // deallocate the data entirely. See the discussion in Arc::drop() about
1222 // the memory orderings
1224 // It's not necessary to check for the locked state here, because the
1225 // weak count can only be locked if there was precisely one weak ref,
1226 // meaning that drop could only subsequently run ON that remaining weak
1227 // ref, which can only happen after the lock is released.
1228 let inner = if let Some(inner) = self.inner() {
1234 if inner.weak.fetch_sub(1, Release) == 1 {
1235 atomic::fence(Acquire);
1237 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
1243 #[stable(feature = "rust1", since = "1.0.0")]
1244 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1245 /// Equality for two `Arc`s.
1247 /// Two `Arc`s are equal if their inner values are equal.
1252 /// use std::sync::Arc;
1254 /// let five = Arc::new(5);
1256 /// assert!(five == Arc::new(5));
1258 fn eq(&self, other: &Arc<T>) -> bool {
1259 *(*self) == *(*other)
1262 /// Inequality for two `Arc`s.
1264 /// Two `Arc`s are unequal if their inner values are unequal.
1269 /// use std::sync::Arc;
1271 /// let five = Arc::new(5);
1273 /// assert!(five != Arc::new(6));
1275 fn ne(&self, other: &Arc<T>) -> bool {
1276 *(*self) != *(*other)
1279 #[stable(feature = "rust1", since = "1.0.0")]
1280 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1281 /// Partial comparison for two `Arc`s.
1283 /// The two are compared by calling `partial_cmp()` on their inner values.
1288 /// use std::sync::Arc;
1289 /// use std::cmp::Ordering;
1291 /// let five = Arc::new(5);
1293 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1295 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1296 (**self).partial_cmp(&**other)
1299 /// Less-than comparison for two `Arc`s.
1301 /// The two are compared by calling `<` on their inner values.
1306 /// use std::sync::Arc;
1308 /// let five = Arc::new(5);
1310 /// assert!(five < Arc::new(6));
1312 fn lt(&self, other: &Arc<T>) -> bool {
1313 *(*self) < *(*other)
1316 /// 'Less than or equal to' comparison for two `Arc`s.
1318 /// The two are compared by calling `<=` on their inner values.
1323 /// use std::sync::Arc;
1325 /// let five = Arc::new(5);
1327 /// assert!(five <= Arc::new(5));
1329 fn le(&self, other: &Arc<T>) -> bool {
1330 *(*self) <= *(*other)
1333 /// Greater-than comparison for two `Arc`s.
1335 /// The two are compared by calling `>` on their inner values.
1340 /// use std::sync::Arc;
1342 /// let five = Arc::new(5);
1344 /// assert!(five > Arc::new(4));
1346 fn gt(&self, other: &Arc<T>) -> bool {
1347 *(*self) > *(*other)
1350 /// 'Greater than or equal to' comparison for two `Arc`s.
1352 /// The two are compared by calling `>=` on their inner values.
1357 /// use std::sync::Arc;
1359 /// let five = Arc::new(5);
1361 /// assert!(five >= Arc::new(5));
1363 fn ge(&self, other: &Arc<T>) -> bool {
1364 *(*self) >= *(*other)
1367 #[stable(feature = "rust1", since = "1.0.0")]
1368 impl<T: ?Sized + Ord> Ord for Arc<T> {
1369 /// Comparison for two `Arc`s.
1371 /// The two are compared by calling `cmp()` on their inner values.
1376 /// use std::sync::Arc;
1377 /// use std::cmp::Ordering;
1379 /// let five = Arc::new(5);
1381 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1383 fn cmp(&self, other: &Arc<T>) -> Ordering {
1384 (**self).cmp(&**other)
1387 #[stable(feature = "rust1", since = "1.0.0")]
1388 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1390 #[stable(feature = "rust1", since = "1.0.0")]
1391 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1392 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1393 fmt::Display::fmt(&**self, f)
1397 #[stable(feature = "rust1", since = "1.0.0")]
1398 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1399 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1400 fmt::Debug::fmt(&**self, f)
1404 #[stable(feature = "rust1", since = "1.0.0")]
1405 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1406 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1407 fmt::Pointer::fmt(&(&**self as *const T), f)
1411 #[stable(feature = "rust1", since = "1.0.0")]
1412 impl<T: Default> Default for Arc<T> {
1413 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1418 /// use std::sync::Arc;
1420 /// let x: Arc<i32> = Default::default();
1421 /// assert_eq!(*x, 0);
1423 fn default() -> Arc<T> {
1424 Arc::new(Default::default())
1428 #[stable(feature = "rust1", since = "1.0.0")]
1429 impl<T: ?Sized + Hash> Hash for Arc<T> {
1430 fn hash<H: Hasher>(&self, state: &mut H) {
1431 (**self).hash(state)
1435 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1436 impl<T> From<T> for Arc<T> {
1437 fn from(t: T) -> Self {
1442 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1443 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1445 fn from(v: &[T]) -> Arc<[T]> {
1446 <Self as ArcFromSlice<T>>::from_slice(v)
1450 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1451 impl<'a> From<&'a str> for Arc<str> {
1453 fn from(v: &str) -> Arc<str> {
1454 let arc = Arc::<[u8]>::from(v.as_bytes());
1455 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1459 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1460 impl From<String> for Arc<str> {
1462 fn from(v: String) -> Arc<str> {
1467 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1468 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1470 fn from(v: Box<T>) -> Arc<T> {
1475 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1476 impl<T> From<Vec<T>> for Arc<[T]> {
1478 fn from(mut v: Vec<T>) -> Arc<[T]> {
1480 let arc = Arc::copy_from_slice(&v);
1482 // Allow the Vec to free its memory, but not destroy its contents
1492 use std::boxed::Box;
1493 use std::clone::Clone;
1494 use std::sync::mpsc::channel;
1497 use std::option::Option;
1498 use std::option::Option::{None, Some};
1499 use std::sync::atomic;
1500 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1502 use std::sync::Mutex;
1503 use std::convert::From;
1505 use super::{Arc, Weak};
1508 struct Canary(*mut atomic::AtomicUsize);
1510 impl Drop for Canary {
1511 fn drop(&mut self) {
1515 (*c).fetch_add(1, SeqCst);
1523 #[cfg_attr(target_os = "emscripten", ignore)]
1524 fn manually_share_arc() {
1525 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1526 let arc_v = Arc::new(v);
1528 let (tx, rx) = channel();
1530 let _t = thread::spawn(move || {
1531 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1532 assert_eq!((*arc_v)[3], 4);
1535 tx.send(arc_v.clone()).unwrap();
1537 assert_eq!((*arc_v)[2], 3);
1538 assert_eq!((*arc_v)[4], 5);
1542 fn test_arc_get_mut() {
1543 let mut x = Arc::new(3);
1544 *Arc::get_mut(&mut x).unwrap() = 4;
1547 assert!(Arc::get_mut(&mut x).is_none());
1549 assert!(Arc::get_mut(&mut x).is_some());
1550 let _w = Arc::downgrade(&x);
1551 assert!(Arc::get_mut(&mut x).is_none());
1556 let x = Arc::new(3);
1557 assert_eq!(Arc::try_unwrap(x), Ok(3));
1558 let x = Arc::new(4);
1560 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1561 let x = Arc::new(5);
1562 let _w = Arc::downgrade(&x);
1563 assert_eq!(Arc::try_unwrap(x), Ok(5));
1567 fn into_from_raw() {
1568 let x = Arc::new(box "hello");
1571 let x_ptr = Arc::into_raw(x);
1574 assert_eq!(**x_ptr, "hello");
1576 let x = Arc::from_raw(x_ptr);
1577 assert_eq!(**x, "hello");
1579 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1584 fn test_into_from_raw_unsized() {
1585 use std::fmt::Display;
1586 use std::string::ToString;
1588 let arc: Arc<str> = Arc::from("foo");
1590 let ptr = Arc::into_raw(arc.clone());
1591 let arc2 = unsafe { Arc::from_raw(ptr) };
1593 assert_eq!(unsafe { &*ptr }, "foo");
1594 assert_eq!(arc, arc2);
1596 let arc: Arc<dyn Display> = Arc::new(123);
1598 let ptr = Arc::into_raw(arc.clone());
1599 let arc2 = unsafe { Arc::from_raw(ptr) };
1601 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1602 assert_eq!(arc2.to_string(), "123");
1606 fn test_cowarc_clone_make_mut() {
1607 let mut cow0 = Arc::new(75);
1608 let mut cow1 = cow0.clone();
1609 let mut cow2 = cow1.clone();
1611 assert!(75 == *Arc::make_mut(&mut cow0));
1612 assert!(75 == *Arc::make_mut(&mut cow1));
1613 assert!(75 == *Arc::make_mut(&mut cow2));
1615 *Arc::make_mut(&mut cow0) += 1;
1616 *Arc::make_mut(&mut cow1) += 2;
1617 *Arc::make_mut(&mut cow2) += 3;
1619 assert!(76 == *cow0);
1620 assert!(77 == *cow1);
1621 assert!(78 == *cow2);
1623 // none should point to the same backing memory
1624 assert!(*cow0 != *cow1);
1625 assert!(*cow0 != *cow2);
1626 assert!(*cow1 != *cow2);
1630 fn test_cowarc_clone_unique2() {
1631 let mut cow0 = Arc::new(75);
1632 let cow1 = cow0.clone();
1633 let cow2 = cow1.clone();
1635 assert!(75 == *cow0);
1636 assert!(75 == *cow1);
1637 assert!(75 == *cow2);
1639 *Arc::make_mut(&mut cow0) += 1;
1640 assert!(76 == *cow0);
1641 assert!(75 == *cow1);
1642 assert!(75 == *cow2);
1644 // cow1 and cow2 should share the same contents
1645 // cow0 should have a unique reference
1646 assert!(*cow0 != *cow1);
1647 assert!(*cow0 != *cow2);
1648 assert!(*cow1 == *cow2);
1652 fn test_cowarc_clone_weak() {
1653 let mut cow0 = Arc::new(75);
1654 let cow1_weak = Arc::downgrade(&cow0);
1656 assert!(75 == *cow0);
1657 assert!(75 == *cow1_weak.upgrade().unwrap());
1659 *Arc::make_mut(&mut cow0) += 1;
1661 assert!(76 == *cow0);
1662 assert!(cow1_weak.upgrade().is_none());
1667 let x = Arc::new(5);
1668 let y = Arc::downgrade(&x);
1669 assert!(y.upgrade().is_some());
1674 let x = Arc::new(5);
1675 let y = Arc::downgrade(&x);
1677 assert!(y.upgrade().is_none());
1681 fn weak_self_cyclic() {
1683 x: Mutex<Option<Weak<Cycle>>>,
1686 let a = Arc::new(Cycle { x: Mutex::new(None) });
1687 let b = Arc::downgrade(&a.clone());
1688 *a.x.lock().unwrap() = Some(b);
1690 // hopefully we don't double-free (or leak)...
1695 let mut canary = atomic::AtomicUsize::new(0);
1696 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1698 assert!(canary.load(Acquire) == 1);
1702 fn drop_arc_weak() {
1703 let mut canary = atomic::AtomicUsize::new(0);
1704 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1705 let arc_weak = Arc::downgrade(&arc);
1706 assert!(canary.load(Acquire) == 0);
1708 assert!(canary.load(Acquire) == 1);
1713 fn test_strong_count() {
1714 let a = Arc::new(0);
1715 assert!(Arc::strong_count(&a) == 1);
1716 let w = Arc::downgrade(&a);
1717 assert!(Arc::strong_count(&a) == 1);
1718 let b = w.upgrade().expect("");
1719 assert!(Arc::strong_count(&b) == 2);
1720 assert!(Arc::strong_count(&a) == 2);
1723 assert!(Arc::strong_count(&b) == 1);
1725 assert!(Arc::strong_count(&b) == 2);
1726 assert!(Arc::strong_count(&c) == 2);
1730 fn test_weak_count() {
1731 let a = Arc::new(0);
1732 assert!(Arc::strong_count(&a) == 1);
1733 assert!(Arc::weak_count(&a) == 0);
1734 let w = Arc::downgrade(&a);
1735 assert!(Arc::strong_count(&a) == 1);
1736 assert!(Arc::weak_count(&a) == 1);
1738 assert!(Arc::weak_count(&a) == 2);
1741 assert!(Arc::strong_count(&a) == 1);
1742 assert!(Arc::weak_count(&a) == 0);
1744 assert!(Arc::strong_count(&a) == 2);
1745 assert!(Arc::weak_count(&a) == 0);
1746 let d = Arc::downgrade(&c);
1747 assert!(Arc::weak_count(&c) == 1);
1748 assert!(Arc::strong_count(&c) == 2);
1757 let a = Arc::new(5);
1758 assert_eq!(format!("{:?}", a), "5");
1761 // Make sure deriving works with Arc<T>
1762 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1769 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1770 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1771 let y = Arc::downgrade(&x.clone());
1773 assert!(y.upgrade().is_none());
1777 fn test_from_owned() {
1779 let foo_arc = Arc::from(foo);
1780 assert!(123 == *foo_arc);
1784 fn test_new_weak() {
1785 let foo: Weak<usize> = Weak::new();
1786 assert!(foo.upgrade().is_none());
1791 let five = Arc::new(5);
1792 let same_five = five.clone();
1793 let other_five = Arc::new(5);
1795 assert!(Arc::ptr_eq(&five, &same_five));
1796 assert!(!Arc::ptr_eq(&five, &other_five));
1800 #[cfg_attr(target_os = "emscripten", ignore)]
1801 fn test_weak_count_locked() {
1802 let mut a = Arc::new(atomic::AtomicBool::new(false));
1804 let t = thread::spawn(move || {
1805 for _i in 0..1000000 {
1806 Arc::get_mut(&mut a);
1808 a.store(true, SeqCst);
1811 while !a2.load(SeqCst) {
1812 let n = Arc::weak_count(&a2);
1813 assert!(n < 2, "bad weak count: {}", n);
1819 fn test_from_str() {
1820 let r: Arc<str> = Arc::from("foo");
1822 assert_eq!(&r[..], "foo");
1826 fn test_copy_from_slice() {
1827 let s: &[u32] = &[1, 2, 3];
1828 let r: Arc<[u32]> = Arc::from(s);
1830 assert_eq!(&r[..], [1, 2, 3]);
1834 fn test_clone_from_slice() {
1835 #[derive(Clone, Debug, Eq, PartialEq)]
1838 let s: &[X] = &[X(1), X(2), X(3)];
1839 let r: Arc<[X]> = Arc::from(s);
1841 assert_eq!(&r[..], s);
1846 fn test_clone_from_slice_panic() {
1847 use std::string::{String, ToString};
1849 struct Fail(u32, String);
1851 impl Clone for Fail {
1852 fn clone(&self) -> Fail {
1856 Fail(self.0, self.1.clone())
1861 Fail(0, "foo".to_string()),
1862 Fail(1, "bar".to_string()),
1863 Fail(2, "baz".to_string()),
1866 // Should panic, but not cause memory corruption
1867 let _r: Arc<[Fail]> = Arc::from(s);
1871 fn test_from_box() {
1872 let b: Box<u32> = box 123;
1873 let r: Arc<u32> = Arc::from(b);
1875 assert_eq!(*r, 123);
1879 fn test_from_box_str() {
1880 use std::string::String;
1882 let s = String::from("foo").into_boxed_str();
1883 let r: Arc<str> = Arc::from(s);
1885 assert_eq!(&r[..], "foo");
1889 fn test_from_box_slice() {
1890 let s = vec![1, 2, 3].into_boxed_slice();
1891 let r: Arc<[u32]> = Arc::from(s);
1893 assert_eq!(&r[..], [1, 2, 3]);
1897 fn test_from_box_trait() {
1898 use std::fmt::Display;
1899 use std::string::ToString;
1901 let b: Box<dyn Display> = box 123;
1902 let r: Arc<dyn Display> = Arc::from(b);
1904 assert_eq!(r.to_string(), "123");
1908 fn test_from_box_trait_zero_sized() {
1909 use std::fmt::Debug;
1911 let b: Box<dyn Debug> = box ();
1912 let r: Arc<dyn Debug> = Arc::from(b);
1914 assert_eq!(format!("{:?}", r), "()");
1918 fn test_from_vec() {
1919 let v = vec![1, 2, 3];
1920 let r: Arc<[u32]> = Arc::from(v);
1922 assert_eq!(&r[..], [1, 2, 3]);
1926 fn test_downcast() {
1929 let r1: Arc<dyn Any + Send + Sync> = Arc::new(i32::max_value());
1930 let r2: Arc<dyn Any + Send + Sync> = Arc::new("abc");
1932 assert!(r1.clone().downcast::<u32>().is_err());
1934 let r1i32 = r1.downcast::<i32>();
1935 assert!(r1i32.is_ok());
1936 assert_eq!(r1i32.unwrap(), Arc::new(i32::max_value()));
1938 assert!(r2.clone().downcast::<i32>().is_err());
1940 let r2str = r2.downcast::<&'static str>();
1941 assert!(r2str.is_ok());
1942 assert_eq!(r2str.unwrap(), Arc::new("abc"));
1946 #[stable(feature = "rust1", since = "1.0.0")]
1947 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1948 fn borrow(&self) -> &T {
1953 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1954 impl<T: ?Sized> AsRef<T> for Arc<T> {
1955 fn as_ref(&self) -> &T {
1960 #[unstable(feature = "pin", issue = "49150")]
1961 impl<T: ?Sized> Unpin for Arc<T> { }