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;
29 use core::ptr::{self, NonNull};
30 use core::marker::{Unpin, Unsize, PhantomData};
31 use core::hash::{Hash, Hasher};
32 use core::{isize, usize};
33 use core::convert::From;
35 use alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
41 /// A soft limit on the amount of references that may be made to an `Arc`.
43 /// Going above this limit will abort your program (although not
44 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
45 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
47 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
48 /// Reference Counted'.
50 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
51 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
52 /// a new `Arc` instance, which points to the same value on the heap as the
53 /// source `Arc`, while increasing a reference count. When the last `Arc`
54 /// pointer to a given value is destroyed, the pointed-to value is also
57 /// Shared references in Rust disallow mutation by default, and `Arc` is no
58 /// exception: you cannot generally obtain a mutable reference to something
59 /// inside an `Arc`. If you need to mutate through an `Arc`, use
60 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
65 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
66 /// counting. This means that it is thread-safe. The disadvantage is that
67 /// atomic operations are more expensive than ordinary memory accesses. If you
68 /// are not sharing reference-counted values between threads, consider using
69 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
70 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
71 /// However, a library might choose `Arc<T>` in order to give library consumers
74 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
75 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
76 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
77 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
78 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
79 /// data, but it doesn't add thread safety to its data. Consider
80 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
81 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
82 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
83 /// non-atomic operations.
85 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
86 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
88 /// ## Breaking cycles with `Weak`
90 /// The [`downgrade`][downgrade] method can be used to create a non-owning
91 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
92 /// to an `Arc`, but this will return [`None`] if the value has already been
95 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
96 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
97 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
98 /// pointers from children back to their parents.
100 /// # Cloning references
102 /// Creating a new reference from an existing reference counted pointer is done using the
103 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
106 /// use std::sync::Arc;
107 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
108 /// // The two syntaxes below are equivalent.
109 /// let a = foo.clone();
110 /// let b = Arc::clone(&foo);
111 /// // a, b, and foo are all Arcs that point to the same memory location
114 /// The [`Arc::clone(&from)`] syntax is the most idiomatic because it conveys more explicitly
115 /// the meaning of the code. In the example above, this syntax makes it easier to see that
116 /// this code is creating a new reference rather than copying the whole content of foo.
118 /// ## `Deref` behavior
120 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
121 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
122 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are [associated
123 /// functions][assoc], called using function-like syntax:
126 /// use std::sync::Arc;
127 /// let my_arc = Arc::new(());
129 /// Arc::downgrade(&my_arc);
132 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the value may have
133 /// already been destroyed.
135 /// [arc]: struct.Arc.html
136 /// [weak]: struct.Weak.html
137 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
138 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
139 /// [mutex]: ../../std/sync/struct.Mutex.html
140 /// [rwlock]: ../../std/sync/struct.RwLock.html
141 /// [atomic]: ../../std/sync/atomic/index.html
142 /// [`Send`]: ../../std/marker/trait.Send.html
143 /// [`Sync`]: ../../std/marker/trait.Sync.html
144 /// [deref]: ../../std/ops/trait.Deref.html
145 /// [downgrade]: struct.Arc.html#method.downgrade
146 /// [upgrade]: struct.Weak.html#method.upgrade
147 /// [`None`]: ../../std/option/enum.Option.html#variant.None
148 /// [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
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 #[stable(feature = "rust1", since = "1.0.0")]
202 pub struct Arc<T: ?Sized> {
203 ptr: NonNull<ArcInner<T>>,
204 phantom: PhantomData<T>,
207 #[stable(feature = "rust1", since = "1.0.0")]
208 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
209 #[stable(feature = "rust1", since = "1.0.0")]
210 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
212 #[unstable(feature = "coerce_unsized", issue = "27732")]
213 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
215 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
216 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
217 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
219 /// Since a `Weak` reference does not count towards ownership, it will not
220 /// prevent the inner value from being dropped, and `Weak` itself makes no
221 /// guarantees about the value still being present and may return [`None`]
222 /// when [`upgrade`]d.
224 /// A `Weak` pointer is useful for keeping a temporary reference to the value
225 /// within [`Arc`] without extending its lifetime. It is also used to prevent
226 /// circular references between [`Arc`] pointers, since mutual owning references
227 /// would never allow either [`Arc`] to be dropped. For example, a tree could
228 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
229 /// pointers from children back to their parents.
231 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
233 /// [`Arc`]: struct.Arc.html
234 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
235 /// [`upgrade`]: struct.Weak.html#method.upgrade
236 /// [`Option`]: ../../std/option/enum.Option.html
237 /// [`None`]: ../../std/option/enum.Option.html#variant.None
238 #[stable(feature = "arc_weak", since = "1.4.0")]
239 pub struct Weak<T: ?Sized> {
240 // This is a `NonNull` to allow optimizing the size of this type in enums,
241 // but it is not necessarily a valid pointer.
242 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
243 // to allocate space on the heap. That's not a value a real pointer
244 // will ever have because RcBox has alignment at least 2.
245 ptr: NonNull<ArcInner<T>>,
248 #[stable(feature = "arc_weak", since = "1.4.0")]
249 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
250 #[stable(feature = "arc_weak", since = "1.4.0")]
251 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
253 #[unstable(feature = "coerce_unsized", issue = "27732")]
254 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
256 #[stable(feature = "arc_weak", since = "1.4.0")]
257 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
258 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
263 struct ArcInner<T: ?Sized> {
264 strong: atomic::AtomicUsize,
266 // the value usize::MAX acts as a sentinel for temporarily "locking" the
267 // ability to upgrade weak pointers or downgrade strong ones; this is used
268 // to avoid races in `make_mut` and `get_mut`.
269 weak: atomic::AtomicUsize,
274 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
275 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
278 /// Constructs a new `Arc<T>`.
283 /// use std::sync::Arc;
285 /// let five = Arc::new(5);
288 #[stable(feature = "rust1", since = "1.0.0")]
289 pub fn new(data: T) -> Arc<T> {
290 // Start the weak pointer count as 1 which is the weak pointer that's
291 // held by all the strong pointers (kinda), see std/rc.rs for more info
292 let x: Box<_> = box ArcInner {
293 strong: atomic::AtomicUsize::new(1),
294 weak: atomic::AtomicUsize::new(1),
297 Arc { ptr: Box::into_raw_non_null(x), phantom: PhantomData }
300 /// Returns the contained value, if the `Arc` has exactly one strong reference.
302 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
305 /// This will succeed even if there are outstanding weak references.
307 /// [result]: ../../std/result/enum.Result.html
312 /// use std::sync::Arc;
314 /// let x = Arc::new(3);
315 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
317 /// let x = Arc::new(4);
318 /// let _y = Arc::clone(&x);
319 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
322 #[stable(feature = "arc_unique", since = "1.4.0")]
323 pub fn try_unwrap(this: Self) -> Result<T, Self> {
324 // See `drop` for why all these atomics are like this
325 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
329 atomic::fence(Acquire);
332 let elem = ptr::read(&this.ptr.as_ref().data);
334 // Make a weak pointer to clean up the implicit strong-weak reference
335 let _weak = Weak { ptr: this.ptr };
343 impl<T: ?Sized> Arc<T> {
344 /// Consumes the `Arc`, returning the wrapped pointer.
346 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
347 /// [`Arc::from_raw`][from_raw].
349 /// [from_raw]: struct.Arc.html#method.from_raw
354 /// use std::sync::Arc;
356 /// let x = Arc::new(10);
357 /// let x_ptr = Arc::into_raw(x);
358 /// assert_eq!(unsafe { *x_ptr }, 10);
360 #[stable(feature = "rc_raw", since = "1.17.0")]
361 pub fn into_raw(this: Self) -> *const T {
362 let ptr: *const T = &*this;
367 /// Constructs an `Arc` from a raw pointer.
369 /// The raw pointer must have been previously returned by a call to a
370 /// [`Arc::into_raw`][into_raw].
372 /// This function is unsafe because improper use may lead to memory problems. For example, a
373 /// double-free may occur if the function is called twice on the same raw pointer.
375 /// [into_raw]: struct.Arc.html#method.into_raw
380 /// use std::sync::Arc;
382 /// let x = Arc::new(10);
383 /// let x_ptr = Arc::into_raw(x);
386 /// // Convert back to an `Arc` to prevent leak.
387 /// let x = Arc::from_raw(x_ptr);
388 /// assert_eq!(*x, 10);
390 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
393 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
395 #[stable(feature = "rc_raw", since = "1.17.0")]
396 pub unsafe fn from_raw(ptr: *const T) -> Self {
397 // Align the unsized value to the end of the ArcInner.
398 // Because it is ?Sized, it will always be the last field in memory.
399 let align = align_of_val(&*ptr);
400 let layout = Layout::new::<ArcInner<()>>();
401 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
403 // Reverse the offset to find the original ArcInner.
404 let fake_ptr = ptr as *mut ArcInner<T>;
405 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
408 ptr: NonNull::new_unchecked(arc_ptr),
409 phantom: PhantomData,
413 /// Creates a new [`Weak`][weak] pointer to this value.
415 /// [weak]: struct.Weak.html
420 /// use std::sync::Arc;
422 /// let five = Arc::new(5);
424 /// let weak_five = Arc::downgrade(&five);
426 #[stable(feature = "arc_weak", since = "1.4.0")]
427 pub fn downgrade(this: &Self) -> Weak<T> {
428 // This Relaxed is OK because we're checking the value in the CAS
430 let mut cur = this.inner().weak.load(Relaxed);
433 // check if the weak counter is currently "locked"; if so, spin.
434 if cur == usize::MAX {
435 cur = this.inner().weak.load(Relaxed);
439 // NOTE: this code currently ignores the possibility of overflow
440 // into usize::MAX; in general both Rc and Arc need to be adjusted
441 // to deal with overflow.
443 // Unlike with Clone(), we need this to be an Acquire read to
444 // synchronize with the write coming from `is_unique`, so that the
445 // events prior to that write happen before this read.
446 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
448 // Make sure we do not create a dangling Weak
449 debug_assert!(!is_dangling(this.ptr));
450 return Weak { ptr: this.ptr };
452 Err(old) => cur = old,
457 /// Gets the number of [`Weak`][weak] pointers to this value.
459 /// [weak]: struct.Weak.html
463 /// This method by itself is safe, but using it correctly requires extra care.
464 /// Another thread can change the weak count at any time,
465 /// including potentially between calling this method and acting on the result.
470 /// use std::sync::Arc;
472 /// let five = Arc::new(5);
473 /// let _weak_five = Arc::downgrade(&five);
475 /// // This assertion is deterministic because we haven't shared
476 /// // the `Arc` or `Weak` between threads.
477 /// assert_eq!(1, Arc::weak_count(&five));
480 #[stable(feature = "arc_counts", since = "1.15.0")]
481 pub fn weak_count(this: &Self) -> usize {
482 let cnt = this.inner().weak.load(SeqCst);
483 // If the weak count is currently locked, the value of the
484 // count was 0 just before taking the lock.
485 if cnt == usize::MAX { 0 } else { cnt - 1 }
488 /// Gets the number of strong (`Arc`) pointers to this value.
492 /// This method by itself is safe, but using it correctly requires extra care.
493 /// Another thread can change the strong count at any time,
494 /// including potentially between calling this method and acting on the result.
499 /// use std::sync::Arc;
501 /// let five = Arc::new(5);
502 /// let _also_five = Arc::clone(&five);
504 /// // This assertion is deterministic because we haven't shared
505 /// // the `Arc` between threads.
506 /// assert_eq!(2, Arc::strong_count(&five));
509 #[stable(feature = "arc_counts", since = "1.15.0")]
510 pub fn strong_count(this: &Self) -> usize {
511 this.inner().strong.load(SeqCst)
515 fn inner(&self) -> &ArcInner<T> {
516 // This unsafety is ok because while this arc is alive we're guaranteed
517 // that the inner pointer is valid. Furthermore, we know that the
518 // `ArcInner` structure itself is `Sync` because the inner data is
519 // `Sync` as well, so we're ok loaning out an immutable pointer to these
521 unsafe { self.ptr.as_ref() }
524 // Non-inlined part of `drop`.
526 unsafe fn drop_slow(&mut self) {
527 // Destroy the data at this time, even though we may not free the box
528 // allocation itself (there may still be weak pointers lying around).
529 ptr::drop_in_place(&mut self.ptr.as_mut().data);
531 if self.inner().weak.fetch_sub(1, Release) == 1 {
532 atomic::fence(Acquire);
533 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
538 #[stable(feature = "ptr_eq", since = "1.17.0")]
539 /// Returns true if the two `Arc`s point to the same value (not
540 /// just values that compare as equal).
545 /// use std::sync::Arc;
547 /// let five = Arc::new(5);
548 /// let same_five = Arc::clone(&five);
549 /// let other_five = Arc::new(5);
551 /// assert!(Arc::ptr_eq(&five, &same_five));
552 /// assert!(!Arc::ptr_eq(&five, &other_five));
554 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
555 this.ptr.as_ptr() == other.ptr.as_ptr()
559 impl<T: ?Sized> Arc<T> {
560 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
561 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
562 // Create a fake ArcInner to find allocation size and alignment
563 let fake_ptr = ptr as *mut ArcInner<T>;
565 let layout = Layout::for_value(&*fake_ptr);
567 let mem = Global.alloc(layout)
568 .unwrap_or_else(|_| handle_alloc_error(layout));
570 // Initialize the real ArcInner
571 let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;
573 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
574 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
579 fn from_box(v: Box<T>) -> Arc<T> {
581 let box_unique = Box::into_unique(v);
582 let bptr = box_unique.as_ptr();
584 let value_size = size_of_val(&*bptr);
585 let ptr = Self::allocate_for_ptr(bptr);
587 // Copy value as bytes
588 ptr::copy_nonoverlapping(
589 bptr as *const T as *const u8,
590 &mut (*ptr).data as *mut _ as *mut u8,
593 // Free the allocation without dropping its contents
594 box_free(box_unique);
596 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
601 // Sets the data pointer of a `?Sized` raw pointer.
603 // For a slice/trait object, this sets the `data` field and leaves the rest
604 // unchanged. For a sized raw pointer, this simply sets the pointer.
605 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
606 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
611 // Copy elements from slice into newly allocated Arc<[T]>
613 // Unsafe because the caller must either take ownership or bind `T: Copy`
614 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
615 let v_ptr = v as *const [T];
616 let ptr = Self::allocate_for_ptr(v_ptr);
618 ptr::copy_nonoverlapping(
620 &mut (*ptr).data as *mut [T] as *mut T,
623 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
627 // Specialization trait used for From<&[T]>
628 trait ArcFromSlice<T> {
629 fn from_slice(slice: &[T]) -> Self;
632 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
634 default fn from_slice(v: &[T]) -> Self {
635 // Panic guard while cloning T elements.
636 // In the event of a panic, elements that have been written
637 // into the new ArcInner will be dropped, then the memory freed.
645 impl<T> Drop for Guard<T> {
647 use core::slice::from_raw_parts_mut;
650 let slice = from_raw_parts_mut(self.elems, self.n_elems);
651 ptr::drop_in_place(slice);
653 Global.dealloc(self.mem.cast(), self.layout.clone());
659 let v_ptr = v as *const [T];
660 let ptr = Self::allocate_for_ptr(v_ptr);
662 let mem = ptr as *mut _ as *mut u8;
663 let layout = Layout::for_value(&*ptr);
665 // Pointer to first element
666 let elems = &mut (*ptr).data as *mut [T] as *mut T;
668 let mut guard = Guard{
669 mem: NonNull::new_unchecked(mem),
675 for (i, item) in v.iter().enumerate() {
676 ptr::write(elems.add(i), item.clone());
680 // All clear. Forget the guard so it doesn't free the new ArcInner.
683 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
688 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
690 fn from_slice(v: &[T]) -> Self {
691 unsafe { Arc::copy_from_slice(v) }
695 #[stable(feature = "rust1", since = "1.0.0")]
696 impl<T: ?Sized> Clone for Arc<T> {
697 /// Makes a clone of the `Arc` pointer.
699 /// This creates another pointer to the same inner value, increasing the
700 /// strong reference count.
705 /// use std::sync::Arc;
707 /// let five = Arc::new(5);
709 /// Arc::clone(&five);
712 fn clone(&self) -> Arc<T> {
713 // Using a relaxed ordering is alright here, as knowledge of the
714 // original reference prevents other threads from erroneously deleting
717 // As explained in the [Boost documentation][1], Increasing the
718 // reference counter can always be done with memory_order_relaxed: New
719 // references to an object can only be formed from an existing
720 // reference, and passing an existing reference from one thread to
721 // another must already provide any required synchronization.
723 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
724 let old_size = self.inner().strong.fetch_add(1, Relaxed);
726 // However we need to guard against massive refcounts in case someone
727 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
728 // and users will use-after free. We racily saturate to `isize::MAX` on
729 // the assumption that there aren't ~2 billion threads incrementing
730 // the reference count at once. This branch will never be taken in
731 // any realistic program.
733 // We abort because such a program is incredibly degenerate, and we
734 // don't care to support it.
735 if old_size > MAX_REFCOUNT {
741 Arc { ptr: self.ptr, phantom: PhantomData }
745 #[stable(feature = "rust1", since = "1.0.0")]
746 impl<T: ?Sized> Deref for Arc<T> {
750 fn deref(&self) -> &T {
755 impl<T: Clone> Arc<T> {
756 /// Makes a mutable reference into the given `Arc`.
758 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
759 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
760 /// ensure unique ownership. This is also referred to as clone-on-write.
762 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
764 /// [weak]: struct.Weak.html
765 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
766 /// [get_mut]: struct.Arc.html#method.get_mut
771 /// use std::sync::Arc;
773 /// let mut data = Arc::new(5);
775 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
776 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
777 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
778 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
779 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
781 /// // Now `data` and `other_data` point to different values.
782 /// assert_eq!(*data, 8);
783 /// assert_eq!(*other_data, 12);
786 #[stable(feature = "arc_unique", since = "1.4.0")]
787 pub fn make_mut(this: &mut Self) -> &mut T {
788 // Note that we hold both a strong reference and a weak reference.
789 // Thus, releasing our strong reference only will not, by itself, cause
790 // the memory to be deallocated.
792 // Use Acquire to ensure that we see any writes to `weak` that happen
793 // before release writes (i.e., decrements) to `strong`. Since we hold a
794 // weak count, there's no chance the ArcInner itself could be
796 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
797 // Another strong pointer exists; clone
798 *this = Arc::new((**this).clone());
799 } else if this.inner().weak.load(Relaxed) != 1 {
800 // Relaxed suffices in the above because this is fundamentally an
801 // optimization: we are always racing with weak pointers being
802 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
804 // We removed the last strong ref, but there are additional weak
805 // refs remaining. We'll move the contents to a new Arc, and
806 // invalidate the other weak refs.
808 // Note that it is not possible for the read of `weak` to yield
809 // usize::MAX (i.e., locked), since the weak count can only be
810 // locked by a thread with a strong reference.
812 // Materialize our own implicit weak pointer, so that it can clean
813 // up the ArcInner as needed.
814 let weak = Weak { ptr: this.ptr };
816 // mark the data itself as already deallocated
818 // there is no data race in the implicit write caused by `read`
819 // here (due to zeroing) because data is no longer accessed by
820 // other threads (due to there being no more strong refs at this
822 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
823 mem::swap(this, &mut swap);
827 // We were the sole reference of either kind; bump back up the
829 this.inner().strong.store(1, Release);
832 // As with `get_mut()`, the unsafety is ok because our reference was
833 // either unique to begin with, or became one upon cloning the contents.
835 &mut this.ptr.as_mut().data
840 impl<T: ?Sized> Arc<T> {
841 /// Returns a mutable reference to the inner value, if there are
842 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
844 /// Returns [`None`][option] otherwise, because it is not safe to
845 /// mutate a shared value.
847 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
848 /// the inner value when it's shared.
850 /// [weak]: struct.Weak.html
851 /// [option]: ../../std/option/enum.Option.html
852 /// [make_mut]: struct.Arc.html#method.make_mut
853 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
858 /// use std::sync::Arc;
860 /// let mut x = Arc::new(3);
861 /// *Arc::get_mut(&mut x).unwrap() = 4;
862 /// assert_eq!(*x, 4);
864 /// let _y = Arc::clone(&x);
865 /// assert!(Arc::get_mut(&mut x).is_none());
868 #[stable(feature = "arc_unique", since = "1.4.0")]
869 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
870 if this.is_unique() {
871 // This unsafety is ok because we're guaranteed that the pointer
872 // returned is the *only* pointer that will ever be returned to T. Our
873 // reference count is guaranteed to be 1 at this point, and we required
874 // the Arc itself to be `mut`, so we're returning the only possible
875 // reference to the inner data.
877 Some(&mut this.ptr.as_mut().data)
884 /// Determine whether this is the unique reference (including weak refs) to
885 /// the underlying data.
887 /// Note that this requires locking the weak ref count.
888 fn is_unique(&mut self) -> bool {
889 // lock the weak pointer count if we appear to be the sole weak pointer
892 // The acquire label here ensures a happens-before relationship with any
893 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
894 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
895 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
896 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
897 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
898 // counter in `drop` -- the only access that happens when any but the last reference
900 let unique = self.inner().strong.load(Acquire) == 1;
902 // The release write here synchronizes with a read in `downgrade`,
903 // effectively preventing the above read of `strong` from happening
905 self.inner().weak.store(1, Release); // release the lock
913 #[stable(feature = "rust1", since = "1.0.0")]
914 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
917 /// This will decrement the strong reference count. If the strong reference
918 /// count reaches zero then the only other references (if any) are
919 /// [`Weak`], so we `drop` the inner value.
924 /// use std::sync::Arc;
928 /// impl Drop for Foo {
929 /// fn drop(&mut self) {
930 /// println!("dropped!");
934 /// let foo = Arc::new(Foo);
935 /// let foo2 = Arc::clone(&foo);
937 /// drop(foo); // Doesn't print anything
938 /// drop(foo2); // Prints "dropped!"
942 // Because `fetch_sub` is already atomic, we do not need to synchronize
943 // with other threads unless we are going to delete the object. This
944 // same logic applies to the below `fetch_sub` to the `weak` count.
945 if self.inner().strong.fetch_sub(1, Release) != 1 {
949 // This fence is needed to prevent reordering of use of the data and
950 // deletion of the data. Because it is marked `Release`, the decreasing
951 // of the reference count synchronizes with this `Acquire` fence. This
952 // means that use of the data happens before decreasing the reference
953 // count, which happens before this fence, which happens before the
954 // deletion of the data.
956 // As explained in the [Boost documentation][1],
958 // > It is important to enforce any possible access to the object in one
959 // > thread (through an existing reference) to *happen before* deleting
960 // > the object in a different thread. This is achieved by a "release"
961 // > operation after dropping a reference (any access to the object
962 // > through this reference must obviously happened before), and an
963 // > "acquire" operation before deleting the object.
965 // In particular, while the contents of an Arc are usually immutable, it's
966 // possible to have interior writes to something like a Mutex<T>. Since a
967 // Mutex is not acquired when it is deleted, we can't rely on its
968 // synchronization logic to make writes in thread A visible to a destructor
969 // running in thread B.
971 // Also note that the Acquire fence here could probably be replaced with an
972 // Acquire load, which could improve performance in highly-contended
973 // situations. See [2].
975 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
976 // [2]: (https://github.com/rust-lang/rust/pull/41714)
977 atomic::fence(Acquire);
985 impl Arc<dyn Any + Send + Sync> {
987 #[stable(feature = "rc_downcast", since = "1.29.0")]
988 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
993 /// use std::any::Any;
994 /// use std::sync::Arc;
996 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
997 /// if let Ok(string) = value.downcast::<String>() {
998 /// println!("String ({}): {}", string.len(), string);
1003 /// let my_string = "Hello World".to_string();
1004 /// print_if_string(Arc::new(my_string));
1005 /// print_if_string(Arc::new(0i8));
1008 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1010 T: Any + Send + Sync + 'static,
1012 if (*self).is::<T>() {
1013 let ptr = self.ptr.cast::<ArcInner<T>>();
1015 Ok(Arc { ptr, phantom: PhantomData })
1023 /// Constructs a new `Weak<T>`, without allocating any memory.
1024 /// Calling [`upgrade`] on the return value always gives [`None`].
1026 /// [`upgrade`]: struct.Weak.html#method.upgrade
1027 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1032 /// use std::sync::Weak;
1034 /// let empty: Weak<i64> = Weak::new();
1035 /// assert!(empty.upgrade().is_none());
1037 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1038 pub fn new() -> Weak<T> {
1040 ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0"),
1045 impl<T: ?Sized> Weak<T> {
1046 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1047 /// the lifetime of the value if successful.
1049 /// Returns [`None`] if the value has since been dropped.
1051 /// [`Arc`]: struct.Arc.html
1052 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1057 /// use std::sync::Arc;
1059 /// let five = Arc::new(5);
1061 /// let weak_five = Arc::downgrade(&five);
1063 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1064 /// assert!(strong_five.is_some());
1066 /// // Destroy all strong pointers.
1067 /// drop(strong_five);
1070 /// assert!(weak_five.upgrade().is_none());
1072 #[stable(feature = "arc_weak", since = "1.4.0")]
1073 pub fn upgrade(&self) -> Option<Arc<T>> {
1074 // We use a CAS loop to increment the strong count instead of a
1075 // fetch_add because once the count hits 0 it must never be above 0.
1076 let inner = self.inner()?;
1078 // Relaxed load because any write of 0 that we can observe
1079 // leaves the field in a permanently zero state (so a
1080 // "stale" read of 0 is fine), and any other value is
1081 // confirmed via the CAS below.
1082 let mut n = inner.strong.load(Relaxed);
1089 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1090 if n > MAX_REFCOUNT {
1096 // Relaxed is valid for the same reason it is on Arc's Clone impl
1097 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1098 Ok(_) => return Some(Arc {
1099 // null checked above
1101 phantom: PhantomData,
1103 Err(old) => n = old,
1108 /// Return `None` when the pointer is dangling and there is no allocated `ArcInner`,
1109 /// i.e. this `Weak` was created by `Weak::new`
1111 fn inner(&self) -> Option<&ArcInner<T>> {
1112 if is_dangling(self.ptr) {
1115 Some(unsafe { self.ptr.as_ref() })
1120 #[stable(feature = "arc_weak", since = "1.4.0")]
1121 impl<T: ?Sized> Clone for Weak<T> {
1122 /// Makes a clone of the `Weak` pointer that points to the same value.
1127 /// use std::sync::{Arc, Weak};
1129 /// let weak_five = Arc::downgrade(&Arc::new(5));
1131 /// Weak::clone(&weak_five);
1134 fn clone(&self) -> Weak<T> {
1135 let inner = if let Some(inner) = self.inner() {
1138 return Weak { ptr: self.ptr };
1140 // See comments in Arc::clone() for why this is relaxed. This can use a
1141 // fetch_add (ignoring the lock) because the weak count is only locked
1142 // where are *no other* weak pointers in existence. (So we can't be
1143 // running this code in that case).
1144 let old_size = inner.weak.fetch_add(1, Relaxed);
1146 // See comments in Arc::clone() for why we do this (for mem::forget).
1147 if old_size > MAX_REFCOUNT {
1153 return Weak { ptr: self.ptr };
1157 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1158 impl<T> Default for Weak<T> {
1159 /// Constructs a new `Weak<T>`, without allocating memory.
1160 /// Calling [`upgrade`][Weak::upgrade] on the return value always
1163 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1168 /// use std::sync::Weak;
1170 /// let empty: Weak<i64> = Default::default();
1171 /// assert!(empty.upgrade().is_none());
1173 fn default() -> Weak<T> {
1178 #[stable(feature = "arc_weak", since = "1.4.0")]
1179 impl<T: ?Sized> Drop for Weak<T> {
1180 /// Drops the `Weak` pointer.
1185 /// use std::sync::{Arc, Weak};
1189 /// impl Drop for Foo {
1190 /// fn drop(&mut self) {
1191 /// println!("dropped!");
1195 /// let foo = Arc::new(Foo);
1196 /// let weak_foo = Arc::downgrade(&foo);
1197 /// let other_weak_foo = Weak::clone(&weak_foo);
1199 /// drop(weak_foo); // Doesn't print anything
1200 /// drop(foo); // Prints "dropped!"
1202 /// assert!(other_weak_foo.upgrade().is_none());
1204 fn drop(&mut self) {
1205 // If we find out that we were the last weak pointer, then its time to
1206 // deallocate the data entirely. See the discussion in Arc::drop() about
1207 // the memory orderings
1209 // It's not necessary to check for the locked state here, because the
1210 // weak count can only be locked if there was precisely one weak ref,
1211 // meaning that drop could only subsequently run ON that remaining weak
1212 // ref, which can only happen after the lock is released.
1213 let inner = if let Some(inner) = self.inner() {
1219 if inner.weak.fetch_sub(1, Release) == 1 {
1220 atomic::fence(Acquire);
1222 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
1228 #[stable(feature = "rust1", since = "1.0.0")]
1229 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1230 /// Equality for two `Arc`s.
1232 /// Two `Arc`s are equal if their inner values are equal.
1237 /// use std::sync::Arc;
1239 /// let five = Arc::new(5);
1241 /// assert!(five == Arc::new(5));
1243 fn eq(&self, other: &Arc<T>) -> bool {
1244 *(*self) == *(*other)
1247 /// Inequality for two `Arc`s.
1249 /// Two `Arc`s are unequal if their inner values are unequal.
1254 /// use std::sync::Arc;
1256 /// let five = Arc::new(5);
1258 /// assert!(five != Arc::new(6));
1260 fn ne(&self, other: &Arc<T>) -> bool {
1261 *(*self) != *(*other)
1264 #[stable(feature = "rust1", since = "1.0.0")]
1265 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1266 /// Partial comparison for two `Arc`s.
1268 /// The two are compared by calling `partial_cmp()` on their inner values.
1273 /// use std::sync::Arc;
1274 /// use std::cmp::Ordering;
1276 /// let five = Arc::new(5);
1278 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1280 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1281 (**self).partial_cmp(&**other)
1284 /// Less-than comparison for two `Arc`s.
1286 /// The two are compared by calling `<` on their inner values.
1291 /// use std::sync::Arc;
1293 /// let five = Arc::new(5);
1295 /// assert!(five < Arc::new(6));
1297 fn lt(&self, other: &Arc<T>) -> bool {
1298 *(*self) < *(*other)
1301 /// 'Less than or equal to' comparison for two `Arc`s.
1303 /// The two are compared by calling `<=` on their inner values.
1308 /// use std::sync::Arc;
1310 /// let five = Arc::new(5);
1312 /// assert!(five <= Arc::new(5));
1314 fn le(&self, other: &Arc<T>) -> bool {
1315 *(*self) <= *(*other)
1318 /// Greater-than comparison for two `Arc`s.
1320 /// The two are compared by calling `>` on their inner values.
1325 /// use std::sync::Arc;
1327 /// let five = Arc::new(5);
1329 /// assert!(five > Arc::new(4));
1331 fn gt(&self, other: &Arc<T>) -> bool {
1332 *(*self) > *(*other)
1335 /// 'Greater than or equal to' comparison for two `Arc`s.
1337 /// The two are compared by calling `>=` on their inner values.
1342 /// use std::sync::Arc;
1344 /// let five = Arc::new(5);
1346 /// assert!(five >= Arc::new(5));
1348 fn ge(&self, other: &Arc<T>) -> bool {
1349 *(*self) >= *(*other)
1352 #[stable(feature = "rust1", since = "1.0.0")]
1353 impl<T: ?Sized + Ord> Ord for Arc<T> {
1354 /// Comparison for two `Arc`s.
1356 /// The two are compared by calling `cmp()` on their inner values.
1361 /// use std::sync::Arc;
1362 /// use std::cmp::Ordering;
1364 /// let five = Arc::new(5);
1366 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1368 fn cmp(&self, other: &Arc<T>) -> Ordering {
1369 (**self).cmp(&**other)
1372 #[stable(feature = "rust1", since = "1.0.0")]
1373 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1375 #[stable(feature = "rust1", since = "1.0.0")]
1376 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1377 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1378 fmt::Display::fmt(&**self, f)
1382 #[stable(feature = "rust1", since = "1.0.0")]
1383 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1384 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1385 fmt::Debug::fmt(&**self, f)
1389 #[stable(feature = "rust1", since = "1.0.0")]
1390 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1391 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1392 fmt::Pointer::fmt(&(&**self as *const T), f)
1396 #[stable(feature = "rust1", since = "1.0.0")]
1397 impl<T: Default> Default for Arc<T> {
1398 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1403 /// use std::sync::Arc;
1405 /// let x: Arc<i32> = Default::default();
1406 /// assert_eq!(*x, 0);
1408 fn default() -> Arc<T> {
1409 Arc::new(Default::default())
1413 #[stable(feature = "rust1", since = "1.0.0")]
1414 impl<T: ?Sized + Hash> Hash for Arc<T> {
1415 fn hash<H: Hasher>(&self, state: &mut H) {
1416 (**self).hash(state)
1420 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1421 impl<T> From<T> for Arc<T> {
1422 fn from(t: T) -> Self {
1427 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1428 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1430 fn from(v: &[T]) -> Arc<[T]> {
1431 <Self as ArcFromSlice<T>>::from_slice(v)
1435 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1436 impl<'a> From<&'a str> for Arc<str> {
1438 fn from(v: &str) -> Arc<str> {
1439 let arc = Arc::<[u8]>::from(v.as_bytes());
1440 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1444 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1445 impl From<String> for Arc<str> {
1447 fn from(v: String) -> Arc<str> {
1452 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1453 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1455 fn from(v: Box<T>) -> Arc<T> {
1460 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1461 impl<T> From<Vec<T>> for Arc<[T]> {
1463 fn from(mut v: Vec<T>) -> Arc<[T]> {
1465 let arc = Arc::copy_from_slice(&v);
1467 // Allow the Vec to free its memory, but not destroy its contents
1477 use std::boxed::Box;
1478 use std::clone::Clone;
1479 use std::sync::mpsc::channel;
1482 use std::option::Option;
1483 use std::option::Option::{None, Some};
1484 use std::sync::atomic;
1485 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1487 use std::sync::Mutex;
1488 use std::convert::From;
1490 use super::{Arc, Weak};
1493 struct Canary(*mut atomic::AtomicUsize);
1495 impl Drop for Canary {
1496 fn drop(&mut self) {
1500 (*c).fetch_add(1, SeqCst);
1508 #[cfg_attr(target_os = "emscripten", ignore)]
1509 fn manually_share_arc() {
1510 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1511 let arc_v = Arc::new(v);
1513 let (tx, rx) = channel();
1515 let _t = thread::spawn(move || {
1516 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1517 assert_eq!((*arc_v)[3], 4);
1520 tx.send(arc_v.clone()).unwrap();
1522 assert_eq!((*arc_v)[2], 3);
1523 assert_eq!((*arc_v)[4], 5);
1527 fn test_arc_get_mut() {
1528 let mut x = Arc::new(3);
1529 *Arc::get_mut(&mut x).unwrap() = 4;
1532 assert!(Arc::get_mut(&mut x).is_none());
1534 assert!(Arc::get_mut(&mut x).is_some());
1535 let _w = Arc::downgrade(&x);
1536 assert!(Arc::get_mut(&mut x).is_none());
1541 let x = Arc::new(3);
1542 assert_eq!(Arc::try_unwrap(x), Ok(3));
1543 let x = Arc::new(4);
1545 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1546 let x = Arc::new(5);
1547 let _w = Arc::downgrade(&x);
1548 assert_eq!(Arc::try_unwrap(x), Ok(5));
1552 fn into_from_raw() {
1553 let x = Arc::new(box "hello");
1556 let x_ptr = Arc::into_raw(x);
1559 assert_eq!(**x_ptr, "hello");
1561 let x = Arc::from_raw(x_ptr);
1562 assert_eq!(**x, "hello");
1564 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1569 fn test_into_from_raw_unsized() {
1570 use std::fmt::Display;
1571 use std::string::ToString;
1573 let arc: Arc<str> = Arc::from("foo");
1575 let ptr = Arc::into_raw(arc.clone());
1576 let arc2 = unsafe { Arc::from_raw(ptr) };
1578 assert_eq!(unsafe { &*ptr }, "foo");
1579 assert_eq!(arc, arc2);
1581 let arc: Arc<dyn Display> = Arc::new(123);
1583 let ptr = Arc::into_raw(arc.clone());
1584 let arc2 = unsafe { Arc::from_raw(ptr) };
1586 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1587 assert_eq!(arc2.to_string(), "123");
1591 fn test_cowarc_clone_make_mut() {
1592 let mut cow0 = Arc::new(75);
1593 let mut cow1 = cow0.clone();
1594 let mut cow2 = cow1.clone();
1596 assert!(75 == *Arc::make_mut(&mut cow0));
1597 assert!(75 == *Arc::make_mut(&mut cow1));
1598 assert!(75 == *Arc::make_mut(&mut cow2));
1600 *Arc::make_mut(&mut cow0) += 1;
1601 *Arc::make_mut(&mut cow1) += 2;
1602 *Arc::make_mut(&mut cow2) += 3;
1604 assert!(76 == *cow0);
1605 assert!(77 == *cow1);
1606 assert!(78 == *cow2);
1608 // none should point to the same backing memory
1609 assert!(*cow0 != *cow1);
1610 assert!(*cow0 != *cow2);
1611 assert!(*cow1 != *cow2);
1615 fn test_cowarc_clone_unique2() {
1616 let mut cow0 = Arc::new(75);
1617 let cow1 = cow0.clone();
1618 let cow2 = cow1.clone();
1620 assert!(75 == *cow0);
1621 assert!(75 == *cow1);
1622 assert!(75 == *cow2);
1624 *Arc::make_mut(&mut cow0) += 1;
1625 assert!(76 == *cow0);
1626 assert!(75 == *cow1);
1627 assert!(75 == *cow2);
1629 // cow1 and cow2 should share the same contents
1630 // cow0 should have a unique reference
1631 assert!(*cow0 != *cow1);
1632 assert!(*cow0 != *cow2);
1633 assert!(*cow1 == *cow2);
1637 fn test_cowarc_clone_weak() {
1638 let mut cow0 = Arc::new(75);
1639 let cow1_weak = Arc::downgrade(&cow0);
1641 assert!(75 == *cow0);
1642 assert!(75 == *cow1_weak.upgrade().unwrap());
1644 *Arc::make_mut(&mut cow0) += 1;
1646 assert!(76 == *cow0);
1647 assert!(cow1_weak.upgrade().is_none());
1652 let x = Arc::new(5);
1653 let y = Arc::downgrade(&x);
1654 assert!(y.upgrade().is_some());
1659 let x = Arc::new(5);
1660 let y = Arc::downgrade(&x);
1662 assert!(y.upgrade().is_none());
1666 fn weak_self_cyclic() {
1668 x: Mutex<Option<Weak<Cycle>>>,
1671 let a = Arc::new(Cycle { x: Mutex::new(None) });
1672 let b = Arc::downgrade(&a.clone());
1673 *a.x.lock().unwrap() = Some(b);
1675 // hopefully we don't double-free (or leak)...
1680 let mut canary = atomic::AtomicUsize::new(0);
1681 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1683 assert!(canary.load(Acquire) == 1);
1687 fn drop_arc_weak() {
1688 let mut canary = atomic::AtomicUsize::new(0);
1689 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1690 let arc_weak = Arc::downgrade(&arc);
1691 assert!(canary.load(Acquire) == 0);
1693 assert!(canary.load(Acquire) == 1);
1698 fn test_strong_count() {
1699 let a = Arc::new(0);
1700 assert!(Arc::strong_count(&a) == 1);
1701 let w = Arc::downgrade(&a);
1702 assert!(Arc::strong_count(&a) == 1);
1703 let b = w.upgrade().expect("");
1704 assert!(Arc::strong_count(&b) == 2);
1705 assert!(Arc::strong_count(&a) == 2);
1708 assert!(Arc::strong_count(&b) == 1);
1710 assert!(Arc::strong_count(&b) == 2);
1711 assert!(Arc::strong_count(&c) == 2);
1715 fn test_weak_count() {
1716 let a = Arc::new(0);
1717 assert!(Arc::strong_count(&a) == 1);
1718 assert!(Arc::weak_count(&a) == 0);
1719 let w = Arc::downgrade(&a);
1720 assert!(Arc::strong_count(&a) == 1);
1721 assert!(Arc::weak_count(&a) == 1);
1723 assert!(Arc::weak_count(&a) == 2);
1726 assert!(Arc::strong_count(&a) == 1);
1727 assert!(Arc::weak_count(&a) == 0);
1729 assert!(Arc::strong_count(&a) == 2);
1730 assert!(Arc::weak_count(&a) == 0);
1731 let d = Arc::downgrade(&c);
1732 assert!(Arc::weak_count(&c) == 1);
1733 assert!(Arc::strong_count(&c) == 2);
1742 let a = Arc::new(5);
1743 assert_eq!(format!("{:?}", a), "5");
1746 // Make sure deriving works with Arc<T>
1747 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1754 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1755 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1756 let y = Arc::downgrade(&x.clone());
1758 assert!(y.upgrade().is_none());
1762 fn test_from_owned() {
1764 let foo_arc = Arc::from(foo);
1765 assert!(123 == *foo_arc);
1769 fn test_new_weak() {
1770 let foo: Weak<usize> = Weak::new();
1771 assert!(foo.upgrade().is_none());
1776 let five = Arc::new(5);
1777 let same_five = five.clone();
1778 let other_five = Arc::new(5);
1780 assert!(Arc::ptr_eq(&five, &same_five));
1781 assert!(!Arc::ptr_eq(&five, &other_five));
1785 #[cfg_attr(target_os = "emscripten", ignore)]
1786 fn test_weak_count_locked() {
1787 let mut a = Arc::new(atomic::AtomicBool::new(false));
1789 let t = thread::spawn(move || {
1790 for _i in 0..1000000 {
1791 Arc::get_mut(&mut a);
1793 a.store(true, SeqCst);
1796 while !a2.load(SeqCst) {
1797 let n = Arc::weak_count(&a2);
1798 assert!(n < 2, "bad weak count: {}", n);
1804 fn test_from_str() {
1805 let r: Arc<str> = Arc::from("foo");
1807 assert_eq!(&r[..], "foo");
1811 fn test_copy_from_slice() {
1812 let s: &[u32] = &[1, 2, 3];
1813 let r: Arc<[u32]> = Arc::from(s);
1815 assert_eq!(&r[..], [1, 2, 3]);
1819 fn test_clone_from_slice() {
1820 #[derive(Clone, Debug, Eq, PartialEq)]
1823 let s: &[X] = &[X(1), X(2), X(3)];
1824 let r: Arc<[X]> = Arc::from(s);
1826 assert_eq!(&r[..], s);
1831 fn test_clone_from_slice_panic() {
1832 use std::string::{String, ToString};
1834 struct Fail(u32, String);
1836 impl Clone for Fail {
1837 fn clone(&self) -> Fail {
1841 Fail(self.0, self.1.clone())
1846 Fail(0, "foo".to_string()),
1847 Fail(1, "bar".to_string()),
1848 Fail(2, "baz".to_string()),
1851 // Should panic, but not cause memory corruption
1852 let _r: Arc<[Fail]> = Arc::from(s);
1856 fn test_from_box() {
1857 let b: Box<u32> = box 123;
1858 let r: Arc<u32> = Arc::from(b);
1860 assert_eq!(*r, 123);
1864 fn test_from_box_str() {
1865 use std::string::String;
1867 let s = String::from("foo").into_boxed_str();
1868 let r: Arc<str> = Arc::from(s);
1870 assert_eq!(&r[..], "foo");
1874 fn test_from_box_slice() {
1875 let s = vec![1, 2, 3].into_boxed_slice();
1876 let r: Arc<[u32]> = Arc::from(s);
1878 assert_eq!(&r[..], [1, 2, 3]);
1882 fn test_from_box_trait() {
1883 use std::fmt::Display;
1884 use std::string::ToString;
1886 let b: Box<dyn Display> = box 123;
1887 let r: Arc<dyn Display> = Arc::from(b);
1889 assert_eq!(r.to_string(), "123");
1893 fn test_from_box_trait_zero_sized() {
1894 use std::fmt::Debug;
1896 let b: Box<dyn Debug> = box ();
1897 let r: Arc<dyn Debug> = Arc::from(b);
1899 assert_eq!(format!("{:?}", r), "()");
1903 fn test_from_vec() {
1904 let v = vec![1, 2, 3];
1905 let r: Arc<[u32]> = Arc::from(v);
1907 assert_eq!(&r[..], [1, 2, 3]);
1911 fn test_downcast() {
1914 let r1: Arc<dyn Any + Send + Sync> = Arc::new(i32::max_value());
1915 let r2: Arc<dyn Any + Send + Sync> = Arc::new("abc");
1917 assert!(r1.clone().downcast::<u32>().is_err());
1919 let r1i32 = r1.downcast::<i32>();
1920 assert!(r1i32.is_ok());
1921 assert_eq!(r1i32.unwrap(), Arc::new(i32::max_value()));
1923 assert!(r2.clone().downcast::<i32>().is_err());
1925 let r2str = r2.downcast::<&'static str>();
1926 assert!(r2str.is_ok());
1927 assert_eq!(r2str.unwrap(), Arc::new("abc"));
1931 #[stable(feature = "rust1", since = "1.0.0")]
1932 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1933 fn borrow(&self) -> &T {
1938 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1939 impl<T: ?Sized> AsRef<T> for Arc<T> {
1940 fn as_ref(&self) -> &T {
1945 #[unstable(feature = "pin", issue = "49150")]
1946 impl<T: ?Sized> Unpin for Arc<T> { }