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;
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][assoc], 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 /// [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
150 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
151 /// [`std::sync`]: ../../std/sync/index.html
152 /// [`Arc::clone(&from)`]: #method.clone
156 /// Sharing some immutable data between threads:
158 // Note that we **do not** run these tests here. The windows builders get super
159 // unhappy if a thread outlives the main thread and then exits at the same time
160 // (something deadlocks) so we just avoid this entirely by not running these
163 /// use std::sync::Arc;
166 /// let five = Arc::new(5);
169 /// let five = Arc::clone(&five);
171 /// thread::spawn(move || {
172 /// println!("{:?}", five);
177 /// Sharing a mutable [`AtomicUsize`]:
179 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
182 /// use std::sync::Arc;
183 /// use std::sync::atomic::{AtomicUsize, Ordering};
186 /// let val = Arc::new(AtomicUsize::new(5));
189 /// let val = Arc::clone(&val);
191 /// thread::spawn(move || {
192 /// let v = val.fetch_add(1, Ordering::SeqCst);
193 /// println!("{:?}", v);
198 /// See the [`rc` documentation][rc_examples] for more examples of reference
199 /// counting in general.
201 /// [rc_examples]: ../../std/rc/index.html#examples
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 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
217 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
218 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
220 /// Since a `Weak` reference does not count towards ownership, it will not
221 /// prevent the inner value from being dropped, and `Weak` itself makes no
222 /// guarantees about the value still being present and may return [`None`]
223 /// when [`upgrade`]d.
225 /// A `Weak` pointer is useful for keeping a temporary reference to the value
226 /// within [`Arc`] without extending its lifetime. It is also used to prevent
227 /// circular references between [`Arc`] pointers, since mutual owning references
228 /// would never allow either [`Arc`] to be dropped. For example, a tree could
229 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
230 /// pointers from children back to their parents.
232 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
234 /// [`Arc`]: struct.Arc.html
235 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
236 /// [`upgrade`]: struct.Weak.html#method.upgrade
237 /// [`Option`]: ../../std/option/enum.Option.html
238 /// [`None`]: ../../std/option/enum.Option.html#variant.None
239 #[stable(feature = "arc_weak", since = "1.4.0")]
240 pub struct Weak<T: ?Sized> {
241 // This is a `NonNull` to allow optimizing the size of this type in enums,
242 // but it is not necessarily a valid pointer.
243 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
244 // to allocate space on the heap. That's not a value a real pointer
245 // will ever have because RcBox has alignment at least 2.
246 ptr: NonNull<ArcInner<T>>,
249 #[stable(feature = "arc_weak", since = "1.4.0")]
250 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
251 #[stable(feature = "arc_weak", since = "1.4.0")]
252 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
254 #[unstable(feature = "coerce_unsized", issue = "27732")]
255 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
257 #[stable(feature = "arc_weak", since = "1.4.0")]
258 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
259 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
264 struct ArcInner<T: ?Sized> {
265 strong: atomic::AtomicUsize,
267 // the value usize::MAX acts as a sentinel for temporarily "locking" the
268 // ability to upgrade weak pointers or downgrade strong ones; this is used
269 // to avoid races in `make_mut` and `get_mut`.
270 weak: atomic::AtomicUsize,
275 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
276 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
279 /// Constructs a new `Arc<T>`.
284 /// use std::sync::Arc;
286 /// let five = Arc::new(5);
289 #[stable(feature = "rust1", since = "1.0.0")]
290 pub fn new(data: T) -> Arc<T> {
291 // Start the weak pointer count as 1 which is the weak pointer that's
292 // held by all the strong pointers (kinda), see std/rc.rs for more info
293 let x: Box<_> = box ArcInner {
294 strong: atomic::AtomicUsize::new(1),
295 weak: atomic::AtomicUsize::new(1),
298 Arc { ptr: Box::into_raw_non_null(x), phantom: PhantomData }
301 #[unstable(feature = "pin", issue = "49150")]
302 pub fn pinned(data: T) -> Pin<Arc<T>> {
303 unsafe { Pin::new_unchecked(Arc::new(data)) }
306 /// Returns the contained value, if the `Arc` has exactly one strong reference.
308 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
311 /// This will succeed even if there are outstanding weak references.
313 /// [result]: ../../std/result/enum.Result.html
318 /// use std::sync::Arc;
320 /// let x = Arc::new(3);
321 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
323 /// let x = Arc::new(4);
324 /// let _y = Arc::clone(&x);
325 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
328 #[stable(feature = "arc_unique", since = "1.4.0")]
329 pub fn try_unwrap(this: Self) -> Result<T, Self> {
330 // See `drop` for why all these atomics are like this
331 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
335 atomic::fence(Acquire);
338 let elem = ptr::read(&this.ptr.as_ref().data);
340 // Make a weak pointer to clean up the implicit strong-weak reference
341 let _weak = Weak { ptr: this.ptr };
349 impl<T: ?Sized> Arc<T> {
350 /// Consumes the `Arc`, returning the wrapped pointer.
352 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
353 /// [`Arc::from_raw`][from_raw].
355 /// [from_raw]: struct.Arc.html#method.from_raw
360 /// use std::sync::Arc;
362 /// let x = Arc::new(10);
363 /// let x_ptr = Arc::into_raw(x);
364 /// assert_eq!(unsafe { *x_ptr }, 10);
366 #[stable(feature = "rc_raw", since = "1.17.0")]
367 pub fn into_raw(this: Self) -> *const T {
368 let ptr: *const T = &*this;
373 /// Constructs an `Arc` from a raw pointer.
375 /// The raw pointer must have been previously returned by a call to a
376 /// [`Arc::into_raw`][into_raw].
378 /// This function is unsafe because improper use may lead to memory problems. For example, a
379 /// double-free may occur if the function is called twice on the same raw pointer.
381 /// [into_raw]: struct.Arc.html#method.into_raw
386 /// use std::sync::Arc;
388 /// let x = Arc::new(10);
389 /// let x_ptr = Arc::into_raw(x);
392 /// // Convert back to an `Arc` to prevent leak.
393 /// let x = Arc::from_raw(x_ptr);
394 /// assert_eq!(*x, 10);
396 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
399 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
401 #[stable(feature = "rc_raw", since = "1.17.0")]
402 pub unsafe fn from_raw(ptr: *const T) -> Self {
403 // Align the unsized value to the end of the ArcInner.
404 // Because it is ?Sized, it will always be the last field in memory.
405 let align = align_of_val(&*ptr);
406 let layout = Layout::new::<ArcInner<()>>();
407 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
409 // Reverse the offset to find the original ArcInner.
410 let fake_ptr = ptr as *mut ArcInner<T>;
411 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
414 ptr: NonNull::new_unchecked(arc_ptr),
415 phantom: PhantomData,
419 /// Creates a new [`Weak`][weak] pointer to this value.
421 /// [weak]: struct.Weak.html
426 /// use std::sync::Arc;
428 /// let five = Arc::new(5);
430 /// let weak_five = Arc::downgrade(&five);
432 #[stable(feature = "arc_weak", since = "1.4.0")]
433 pub fn downgrade(this: &Self) -> Weak<T> {
434 // This Relaxed is OK because we're checking the value in the CAS
436 let mut cur = this.inner().weak.load(Relaxed);
439 // check if the weak counter is currently "locked"; if so, spin.
440 if cur == usize::MAX {
441 cur = this.inner().weak.load(Relaxed);
445 // NOTE: this code currently ignores the possibility of overflow
446 // into usize::MAX; in general both Rc and Arc need to be adjusted
447 // to deal with overflow.
449 // Unlike with Clone(), we need this to be an Acquire read to
450 // synchronize with the write coming from `is_unique`, so that the
451 // events prior to that write happen before this read.
452 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
454 // Make sure we do not create a dangling Weak
455 debug_assert!(!is_dangling(this.ptr));
456 return Weak { ptr: this.ptr };
458 Err(old) => cur = old,
463 /// Gets the number of [`Weak`][weak] pointers to this value.
465 /// [weak]: struct.Weak.html
469 /// This method by itself is safe, but using it correctly requires extra care.
470 /// Another thread can change the weak count at any time,
471 /// including potentially between calling this method and acting on the result.
476 /// use std::sync::Arc;
478 /// let five = Arc::new(5);
479 /// let _weak_five = Arc::downgrade(&five);
481 /// // This assertion is deterministic because we haven't shared
482 /// // the `Arc` or `Weak` between threads.
483 /// assert_eq!(1, Arc::weak_count(&five));
486 #[stable(feature = "arc_counts", since = "1.15.0")]
487 pub fn weak_count(this: &Self) -> usize {
488 let cnt = this.inner().weak.load(SeqCst);
489 // If the weak count is currently locked, the value of the
490 // count was 0 just before taking the lock.
491 if cnt == usize::MAX { 0 } else { cnt - 1 }
494 /// Gets the number of strong (`Arc`) pointers to this value.
498 /// This method by itself is safe, but using it correctly requires extra care.
499 /// Another thread can change the strong count at any time,
500 /// including potentially between calling this method and acting on the result.
505 /// use std::sync::Arc;
507 /// let five = Arc::new(5);
508 /// let _also_five = Arc::clone(&five);
510 /// // This assertion is deterministic because we haven't shared
511 /// // the `Arc` between threads.
512 /// assert_eq!(2, Arc::strong_count(&five));
515 #[stable(feature = "arc_counts", since = "1.15.0")]
516 pub fn strong_count(this: &Self) -> usize {
517 this.inner().strong.load(SeqCst)
521 fn inner(&self) -> &ArcInner<T> {
522 // This unsafety is ok because while this arc is alive we're guaranteed
523 // that the inner pointer is valid. Furthermore, we know that the
524 // `ArcInner` structure itself is `Sync` because the inner data is
525 // `Sync` as well, so we're ok loaning out an immutable pointer to these
527 unsafe { self.ptr.as_ref() }
530 // Non-inlined part of `drop`.
532 unsafe fn drop_slow(&mut self) {
533 // Destroy the data at this time, even though we may not free the box
534 // allocation itself (there may still be weak pointers lying around).
535 ptr::drop_in_place(&mut self.ptr.as_mut().data);
537 if self.inner().weak.fetch_sub(1, Release) == 1 {
538 atomic::fence(Acquire);
539 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
544 #[stable(feature = "ptr_eq", since = "1.17.0")]
545 /// Returns true if the two `Arc`s point to the same value (not
546 /// just values that compare as equal).
551 /// use std::sync::Arc;
553 /// let five = Arc::new(5);
554 /// let same_five = Arc::clone(&five);
555 /// let other_five = Arc::new(5);
557 /// assert!(Arc::ptr_eq(&five, &same_five));
558 /// assert!(!Arc::ptr_eq(&five, &other_five));
560 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
561 this.ptr.as_ptr() == other.ptr.as_ptr()
565 impl<T: ?Sized> Arc<T> {
566 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
567 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
568 // Create a fake ArcInner to find allocation size and alignment
569 let fake_ptr = ptr as *mut ArcInner<T>;
571 let layout = Layout::for_value(&*fake_ptr);
573 let mem = Global.alloc(layout)
574 .unwrap_or_else(|_| handle_alloc_error(layout));
576 // Initialize the real ArcInner
577 let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;
579 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
580 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
585 fn from_box(v: Box<T>) -> Arc<T> {
587 let box_unique = Box::into_unique(v);
588 let bptr = box_unique.as_ptr();
590 let value_size = size_of_val(&*bptr);
591 let ptr = Self::allocate_for_ptr(bptr);
593 // Copy value as bytes
594 ptr::copy_nonoverlapping(
595 bptr as *const T as *const u8,
596 &mut (*ptr).data as *mut _ as *mut u8,
599 // Free the allocation without dropping its contents
600 box_free(box_unique);
602 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
607 // Sets the data pointer of a `?Sized` raw pointer.
609 // For a slice/trait object, this sets the `data` field and leaves the rest
610 // unchanged. For a sized raw pointer, this simply sets the pointer.
611 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
612 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
617 // Copy elements from slice into newly allocated Arc<[T]>
619 // Unsafe because the caller must either take ownership or bind `T: Copy`
620 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
621 let v_ptr = v as *const [T];
622 let ptr = Self::allocate_for_ptr(v_ptr);
624 ptr::copy_nonoverlapping(
626 &mut (*ptr).data as *mut [T] as *mut T,
629 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
633 // Specialization trait used for From<&[T]>
634 trait ArcFromSlice<T> {
635 fn from_slice(slice: &[T]) -> Self;
638 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
640 default fn from_slice(v: &[T]) -> Self {
641 // Panic guard while cloning T elements.
642 // In the event of a panic, elements that have been written
643 // into the new ArcInner will be dropped, then the memory freed.
651 impl<T> Drop for Guard<T> {
653 use core::slice::from_raw_parts_mut;
656 let slice = from_raw_parts_mut(self.elems, self.n_elems);
657 ptr::drop_in_place(slice);
659 Global.dealloc(self.mem.cast(), self.layout.clone());
665 let v_ptr = v as *const [T];
666 let ptr = Self::allocate_for_ptr(v_ptr);
668 let mem = ptr as *mut _ as *mut u8;
669 let layout = Layout::for_value(&*ptr);
671 // Pointer to first element
672 let elems = &mut (*ptr).data as *mut [T] as *mut T;
674 let mut guard = Guard{
675 mem: NonNull::new_unchecked(mem),
681 for (i, item) in v.iter().enumerate() {
682 ptr::write(elems.add(i), item.clone());
686 // All clear. Forget the guard so it doesn't free the new ArcInner.
689 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
694 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
696 fn from_slice(v: &[T]) -> Self {
697 unsafe { Arc::copy_from_slice(v) }
701 #[stable(feature = "rust1", since = "1.0.0")]
702 impl<T: ?Sized> Clone for Arc<T> {
703 /// Makes a clone of the `Arc` pointer.
705 /// This creates another pointer to the same inner value, increasing the
706 /// strong reference count.
711 /// use std::sync::Arc;
713 /// let five = Arc::new(5);
715 /// Arc::clone(&five);
718 fn clone(&self) -> Arc<T> {
719 // Using a relaxed ordering is alright here, as knowledge of the
720 // original reference prevents other threads from erroneously deleting
723 // As explained in the [Boost documentation][1], Increasing the
724 // reference counter can always be done with memory_order_relaxed: New
725 // references to an object can only be formed from an existing
726 // reference, and passing an existing reference from one thread to
727 // another must already provide any required synchronization.
729 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
730 let old_size = self.inner().strong.fetch_add(1, Relaxed);
732 // However we need to guard against massive refcounts in case someone
733 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
734 // and users will use-after free. We racily saturate to `isize::MAX` on
735 // the assumption that there aren't ~2 billion threads incrementing
736 // the reference count at once. This branch will never be taken in
737 // any realistic program.
739 // We abort because such a program is incredibly degenerate, and we
740 // don't care to support it.
741 if old_size > MAX_REFCOUNT {
747 Arc { ptr: self.ptr, phantom: PhantomData }
751 #[stable(feature = "rust1", since = "1.0.0")]
752 impl<T: ?Sized> Deref for Arc<T> {
756 fn deref(&self) -> &T {
761 impl<T: Clone> Arc<T> {
762 /// Makes a mutable reference into the given `Arc`.
764 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
765 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
766 /// ensure unique ownership. This is also referred to as clone-on-write.
768 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
770 /// [weak]: struct.Weak.html
771 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
772 /// [get_mut]: struct.Arc.html#method.get_mut
777 /// use std::sync::Arc;
779 /// let mut data = Arc::new(5);
781 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
782 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
783 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
784 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
785 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
787 /// // Now `data` and `other_data` point to different values.
788 /// assert_eq!(*data, 8);
789 /// assert_eq!(*other_data, 12);
792 #[stable(feature = "arc_unique", since = "1.4.0")]
793 pub fn make_mut(this: &mut Self) -> &mut T {
794 // Note that we hold both a strong reference and a weak reference.
795 // Thus, releasing our strong reference only will not, by itself, cause
796 // the memory to be deallocated.
798 // Use Acquire to ensure that we see any writes to `weak` that happen
799 // before release writes (i.e., decrements) to `strong`. Since we hold a
800 // weak count, there's no chance the ArcInner itself could be
802 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
803 // Another strong pointer exists; clone
804 *this = Arc::new((**this).clone());
805 } else if this.inner().weak.load(Relaxed) != 1 {
806 // Relaxed suffices in the above because this is fundamentally an
807 // optimization: we are always racing with weak pointers being
808 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
810 // We removed the last strong ref, but there are additional weak
811 // refs remaining. We'll move the contents to a new Arc, and
812 // invalidate the other weak refs.
814 // Note that it is not possible for the read of `weak` to yield
815 // usize::MAX (i.e., locked), since the weak count can only be
816 // locked by a thread with a strong reference.
818 // Materialize our own implicit weak pointer, so that it can clean
819 // up the ArcInner as needed.
820 let weak = Weak { ptr: this.ptr };
822 // mark the data itself as already deallocated
824 // there is no data race in the implicit write caused by `read`
825 // here (due to zeroing) because data is no longer accessed by
826 // other threads (due to there being no more strong refs at this
828 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
829 mem::swap(this, &mut swap);
833 // We were the sole reference of either kind; bump back up the
835 this.inner().strong.store(1, Release);
838 // As with `get_mut()`, the unsafety is ok because our reference was
839 // either unique to begin with, or became one upon cloning the contents.
841 &mut this.ptr.as_mut().data
846 impl<T: ?Sized> Arc<T> {
847 /// Returns a mutable reference to the inner value, if there are
848 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
850 /// Returns [`None`][option] otherwise, because it is not safe to
851 /// mutate a shared value.
853 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
854 /// the inner value when it's shared.
856 /// [weak]: struct.Weak.html
857 /// [option]: ../../std/option/enum.Option.html
858 /// [make_mut]: struct.Arc.html#method.make_mut
859 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
864 /// use std::sync::Arc;
866 /// let mut x = Arc::new(3);
867 /// *Arc::get_mut(&mut x).unwrap() = 4;
868 /// assert_eq!(*x, 4);
870 /// let _y = Arc::clone(&x);
871 /// assert!(Arc::get_mut(&mut x).is_none());
874 #[stable(feature = "arc_unique", since = "1.4.0")]
875 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
876 if this.is_unique() {
877 // This unsafety is ok because we're guaranteed that the pointer
878 // returned is the *only* pointer that will ever be returned to T. Our
879 // reference count is guaranteed to be 1 at this point, and we required
880 // the Arc itself to be `mut`, so we're returning the only possible
881 // reference to the inner data.
883 Some(&mut this.ptr.as_mut().data)
890 /// Determine whether this is the unique reference (including weak refs) to
891 /// the underlying data.
893 /// Note that this requires locking the weak ref count.
894 fn is_unique(&mut self) -> bool {
895 // lock the weak pointer count if we appear to be the sole weak pointer
898 // The acquire label here ensures a happens-before relationship with any
899 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
900 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
901 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
902 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
903 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
904 // counter in `drop` -- the only access that happens when any but the last reference
906 let unique = self.inner().strong.load(Acquire) == 1;
908 // The release write here synchronizes with a read in `downgrade`,
909 // effectively preventing the above read of `strong` from happening
911 self.inner().weak.store(1, Release); // release the lock
919 #[stable(feature = "rust1", since = "1.0.0")]
920 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
923 /// This will decrement the strong reference count. If the strong reference
924 /// count reaches zero then the only other references (if any) are
925 /// [`Weak`], so we `drop` the inner value.
930 /// use std::sync::Arc;
934 /// impl Drop for Foo {
935 /// fn drop(&mut self) {
936 /// println!("dropped!");
940 /// let foo = Arc::new(Foo);
941 /// let foo2 = Arc::clone(&foo);
943 /// drop(foo); // Doesn't print anything
944 /// drop(foo2); // Prints "dropped!"
948 // Because `fetch_sub` is already atomic, we do not need to synchronize
949 // with other threads unless we are going to delete the object. This
950 // same logic applies to the below `fetch_sub` to the `weak` count.
951 if self.inner().strong.fetch_sub(1, Release) != 1 {
955 // This fence is needed to prevent reordering of use of the data and
956 // deletion of the data. Because it is marked `Release`, the decreasing
957 // of the reference count synchronizes with this `Acquire` fence. This
958 // means that use of the data happens before decreasing the reference
959 // count, which happens before this fence, which happens before the
960 // deletion of the data.
962 // As explained in the [Boost documentation][1],
964 // > It is important to enforce any possible access to the object in one
965 // > thread (through an existing reference) to *happen before* deleting
966 // > the object in a different thread. This is achieved by a "release"
967 // > operation after dropping a reference (any access to the object
968 // > through this reference must obviously happened before), and an
969 // > "acquire" operation before deleting the object.
971 // In particular, while the contents of an Arc are usually immutable, it's
972 // possible to have interior writes to something like a Mutex<T>. Since a
973 // Mutex is not acquired when it is deleted, we can't rely on its
974 // synchronization logic to make writes in thread A visible to a destructor
975 // running in thread B.
977 // Also note that the Acquire fence here could probably be replaced with an
978 // Acquire load, which could improve performance in highly-contended
979 // situations. See [2].
981 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
982 // [2]: (https://github.com/rust-lang/rust/pull/41714)
983 atomic::fence(Acquire);
991 impl Arc<dyn Any + Send + Sync> {
993 #[stable(feature = "rc_downcast", since = "1.29.0")]
994 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
999 /// use std::any::Any;
1000 /// use std::sync::Arc;
1002 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1003 /// if let Ok(string) = value.downcast::<String>() {
1004 /// println!("String ({}): {}", string.len(), string);
1009 /// let my_string = "Hello World".to_string();
1010 /// print_if_string(Arc::new(my_string));
1011 /// print_if_string(Arc::new(0i8));
1014 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1016 T: Any + Send + Sync + 'static,
1018 if (*self).is::<T>() {
1019 let ptr = self.ptr.cast::<ArcInner<T>>();
1021 Ok(Arc { ptr, phantom: PhantomData })
1029 /// Constructs a new `Weak<T>`, without allocating any memory.
1030 /// Calling [`upgrade`] on the return value always gives [`None`].
1032 /// [`upgrade`]: struct.Weak.html#method.upgrade
1033 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1038 /// use std::sync::Weak;
1040 /// let empty: Weak<i64> = Weak::new();
1041 /// assert!(empty.upgrade().is_none());
1043 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1044 pub fn new() -> Weak<T> {
1046 ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0"),
1051 impl<T: ?Sized> Weak<T> {
1052 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1053 /// the lifetime of the value if successful.
1055 /// Returns [`None`] if the value has since been dropped.
1057 /// [`Arc`]: struct.Arc.html
1058 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1063 /// use std::sync::Arc;
1065 /// let five = Arc::new(5);
1067 /// let weak_five = Arc::downgrade(&five);
1069 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1070 /// assert!(strong_five.is_some());
1072 /// // Destroy all strong pointers.
1073 /// drop(strong_five);
1076 /// assert!(weak_five.upgrade().is_none());
1078 #[stable(feature = "arc_weak", since = "1.4.0")]
1079 pub fn upgrade(&self) -> Option<Arc<T>> {
1080 // We use a CAS loop to increment the strong count instead of a
1081 // fetch_add because once the count hits 0 it must never be above 0.
1082 let inner = self.inner()?;
1084 // Relaxed load because any write of 0 that we can observe
1085 // leaves the field in a permanently zero state (so a
1086 // "stale" read of 0 is fine), and any other value is
1087 // confirmed via the CAS below.
1088 let mut n = inner.strong.load(Relaxed);
1095 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1096 if n > MAX_REFCOUNT {
1102 // Relaxed is valid for the same reason it is on Arc's Clone impl
1103 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1104 Ok(_) => return Some(Arc {
1105 // null checked above
1107 phantom: PhantomData,
1109 Err(old) => n = old,
1114 /// Return `None` when the pointer is dangling and there is no allocated `ArcInner`,
1115 /// i.e. this `Weak` was created by `Weak::new`
1117 fn inner(&self) -> Option<&ArcInner<T>> {
1118 if is_dangling(self.ptr) {
1121 Some(unsafe { self.ptr.as_ref() })
1126 #[stable(feature = "arc_weak", since = "1.4.0")]
1127 impl<T: ?Sized> Clone for Weak<T> {
1128 /// Makes a clone of the `Weak` pointer that points to the same value.
1133 /// use std::sync::{Arc, Weak};
1135 /// let weak_five = Arc::downgrade(&Arc::new(5));
1137 /// Weak::clone(&weak_five);
1140 fn clone(&self) -> Weak<T> {
1141 let inner = if let Some(inner) = self.inner() {
1144 return Weak { ptr: self.ptr };
1146 // See comments in Arc::clone() for why this is relaxed. This can use a
1147 // fetch_add (ignoring the lock) because the weak count is only locked
1148 // where are *no other* weak pointers in existence. (So we can't be
1149 // running this code in that case).
1150 let old_size = inner.weak.fetch_add(1, Relaxed);
1152 // See comments in Arc::clone() for why we do this (for mem::forget).
1153 if old_size > MAX_REFCOUNT {
1159 return Weak { ptr: self.ptr };
1163 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1164 impl<T> Default for Weak<T> {
1165 /// Constructs a new `Weak<T>`, without allocating memory.
1166 /// Calling [`upgrade`][Weak::upgrade] on the return value always
1169 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1174 /// use std::sync::Weak;
1176 /// let empty: Weak<i64> = Default::default();
1177 /// assert!(empty.upgrade().is_none());
1179 fn default() -> Weak<T> {
1184 #[stable(feature = "arc_weak", since = "1.4.0")]
1185 impl<T: ?Sized> Drop for Weak<T> {
1186 /// Drops the `Weak` pointer.
1191 /// use std::sync::{Arc, Weak};
1195 /// impl Drop for Foo {
1196 /// fn drop(&mut self) {
1197 /// println!("dropped!");
1201 /// let foo = Arc::new(Foo);
1202 /// let weak_foo = Arc::downgrade(&foo);
1203 /// let other_weak_foo = Weak::clone(&weak_foo);
1205 /// drop(weak_foo); // Doesn't print anything
1206 /// drop(foo); // Prints "dropped!"
1208 /// assert!(other_weak_foo.upgrade().is_none());
1210 fn drop(&mut self) {
1211 // If we find out that we were the last weak pointer, then its time to
1212 // deallocate the data entirely. See the discussion in Arc::drop() about
1213 // the memory orderings
1215 // It's not necessary to check for the locked state here, because the
1216 // weak count can only be locked if there was precisely one weak ref,
1217 // meaning that drop could only subsequently run ON that remaining weak
1218 // ref, which can only happen after the lock is released.
1219 let inner = if let Some(inner) = self.inner() {
1225 if inner.weak.fetch_sub(1, Release) == 1 {
1226 atomic::fence(Acquire);
1228 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
1234 #[stable(feature = "rust1", since = "1.0.0")]
1235 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1236 /// Equality for two `Arc`s.
1238 /// Two `Arc`s are equal if their inner values are equal.
1243 /// use std::sync::Arc;
1245 /// let five = Arc::new(5);
1247 /// assert!(five == Arc::new(5));
1249 fn eq(&self, other: &Arc<T>) -> bool {
1250 *(*self) == *(*other)
1253 /// Inequality for two `Arc`s.
1255 /// Two `Arc`s are unequal if their inner values are unequal.
1260 /// use std::sync::Arc;
1262 /// let five = Arc::new(5);
1264 /// assert!(five != Arc::new(6));
1266 fn ne(&self, other: &Arc<T>) -> bool {
1267 *(*self) != *(*other)
1270 #[stable(feature = "rust1", since = "1.0.0")]
1271 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1272 /// Partial comparison for two `Arc`s.
1274 /// The two are compared by calling `partial_cmp()` on their inner values.
1279 /// use std::sync::Arc;
1280 /// use std::cmp::Ordering;
1282 /// let five = Arc::new(5);
1284 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1286 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1287 (**self).partial_cmp(&**other)
1290 /// Less-than comparison for two `Arc`s.
1292 /// The two are compared by calling `<` on their inner values.
1297 /// use std::sync::Arc;
1299 /// let five = Arc::new(5);
1301 /// assert!(five < Arc::new(6));
1303 fn lt(&self, other: &Arc<T>) -> bool {
1304 *(*self) < *(*other)
1307 /// 'Less than or equal to' comparison for two `Arc`s.
1309 /// The two are compared by calling `<=` on their inner values.
1314 /// use std::sync::Arc;
1316 /// let five = Arc::new(5);
1318 /// assert!(five <= Arc::new(5));
1320 fn le(&self, other: &Arc<T>) -> bool {
1321 *(*self) <= *(*other)
1324 /// Greater-than comparison for two `Arc`s.
1326 /// The two are compared by calling `>` on their inner values.
1331 /// use std::sync::Arc;
1333 /// let five = Arc::new(5);
1335 /// assert!(five > Arc::new(4));
1337 fn gt(&self, other: &Arc<T>) -> bool {
1338 *(*self) > *(*other)
1341 /// 'Greater than or equal to' comparison for two `Arc`s.
1343 /// The two are compared by calling `>=` on their inner values.
1348 /// use std::sync::Arc;
1350 /// let five = Arc::new(5);
1352 /// assert!(five >= Arc::new(5));
1354 fn ge(&self, other: &Arc<T>) -> bool {
1355 *(*self) >= *(*other)
1358 #[stable(feature = "rust1", since = "1.0.0")]
1359 impl<T: ?Sized + Ord> Ord for Arc<T> {
1360 /// Comparison for two `Arc`s.
1362 /// The two are compared by calling `cmp()` on their inner values.
1367 /// use std::sync::Arc;
1368 /// use std::cmp::Ordering;
1370 /// let five = Arc::new(5);
1372 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1374 fn cmp(&self, other: &Arc<T>) -> Ordering {
1375 (**self).cmp(&**other)
1378 #[stable(feature = "rust1", since = "1.0.0")]
1379 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1381 #[stable(feature = "rust1", since = "1.0.0")]
1382 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1383 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1384 fmt::Display::fmt(&**self, f)
1388 #[stable(feature = "rust1", since = "1.0.0")]
1389 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1390 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1391 fmt::Debug::fmt(&**self, f)
1395 #[stable(feature = "rust1", since = "1.0.0")]
1396 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1397 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1398 fmt::Pointer::fmt(&(&**self as *const T), f)
1402 #[stable(feature = "rust1", since = "1.0.0")]
1403 impl<T: Default> Default for Arc<T> {
1404 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1409 /// use std::sync::Arc;
1411 /// let x: Arc<i32> = Default::default();
1412 /// assert_eq!(*x, 0);
1414 fn default() -> Arc<T> {
1415 Arc::new(Default::default())
1419 #[stable(feature = "rust1", since = "1.0.0")]
1420 impl<T: ?Sized + Hash> Hash for Arc<T> {
1421 fn hash<H: Hasher>(&self, state: &mut H) {
1422 (**self).hash(state)
1426 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1427 impl<T> From<T> for Arc<T> {
1428 fn from(t: T) -> Self {
1433 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1434 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1436 fn from(v: &[T]) -> Arc<[T]> {
1437 <Self as ArcFromSlice<T>>::from_slice(v)
1441 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1442 impl<'a> From<&'a str> for Arc<str> {
1444 fn from(v: &str) -> Arc<str> {
1445 let arc = Arc::<[u8]>::from(v.as_bytes());
1446 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1450 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1451 impl From<String> for Arc<str> {
1453 fn from(v: String) -> Arc<str> {
1458 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1459 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1461 fn from(v: Box<T>) -> Arc<T> {
1466 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1467 impl<T> From<Vec<T>> for Arc<[T]> {
1469 fn from(mut v: Vec<T>) -> Arc<[T]> {
1471 let arc = Arc::copy_from_slice(&v);
1473 // Allow the Vec to free its memory, but not destroy its contents
1483 use std::boxed::Box;
1484 use std::clone::Clone;
1485 use std::sync::mpsc::channel;
1488 use std::option::Option;
1489 use std::option::Option::{None, Some};
1490 use std::sync::atomic;
1491 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1493 use std::sync::Mutex;
1494 use std::convert::From;
1496 use super::{Arc, Weak};
1499 struct Canary(*mut atomic::AtomicUsize);
1501 impl Drop for Canary {
1502 fn drop(&mut self) {
1506 (*c).fetch_add(1, SeqCst);
1514 #[cfg_attr(target_os = "emscripten", ignore)]
1515 fn manually_share_arc() {
1516 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1517 let arc_v = Arc::new(v);
1519 let (tx, rx) = channel();
1521 let _t = thread::spawn(move || {
1522 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1523 assert_eq!((*arc_v)[3], 4);
1526 tx.send(arc_v.clone()).unwrap();
1528 assert_eq!((*arc_v)[2], 3);
1529 assert_eq!((*arc_v)[4], 5);
1533 fn test_arc_get_mut() {
1534 let mut x = Arc::new(3);
1535 *Arc::get_mut(&mut x).unwrap() = 4;
1538 assert!(Arc::get_mut(&mut x).is_none());
1540 assert!(Arc::get_mut(&mut x).is_some());
1541 let _w = Arc::downgrade(&x);
1542 assert!(Arc::get_mut(&mut x).is_none());
1547 let x = Arc::new(3);
1548 assert_eq!(Arc::try_unwrap(x), Ok(3));
1549 let x = Arc::new(4);
1551 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1552 let x = Arc::new(5);
1553 let _w = Arc::downgrade(&x);
1554 assert_eq!(Arc::try_unwrap(x), Ok(5));
1558 fn into_from_raw() {
1559 let x = Arc::new(box "hello");
1562 let x_ptr = Arc::into_raw(x);
1565 assert_eq!(**x_ptr, "hello");
1567 let x = Arc::from_raw(x_ptr);
1568 assert_eq!(**x, "hello");
1570 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1575 fn test_into_from_raw_unsized() {
1576 use std::fmt::Display;
1577 use std::string::ToString;
1579 let arc: Arc<str> = Arc::from("foo");
1581 let ptr = Arc::into_raw(arc.clone());
1582 let arc2 = unsafe { Arc::from_raw(ptr) };
1584 assert_eq!(unsafe { &*ptr }, "foo");
1585 assert_eq!(arc, arc2);
1587 let arc: Arc<dyn Display> = Arc::new(123);
1589 let ptr = Arc::into_raw(arc.clone());
1590 let arc2 = unsafe { Arc::from_raw(ptr) };
1592 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1593 assert_eq!(arc2.to_string(), "123");
1597 fn test_cowarc_clone_make_mut() {
1598 let mut cow0 = Arc::new(75);
1599 let mut cow1 = cow0.clone();
1600 let mut cow2 = cow1.clone();
1602 assert!(75 == *Arc::make_mut(&mut cow0));
1603 assert!(75 == *Arc::make_mut(&mut cow1));
1604 assert!(75 == *Arc::make_mut(&mut cow2));
1606 *Arc::make_mut(&mut cow0) += 1;
1607 *Arc::make_mut(&mut cow1) += 2;
1608 *Arc::make_mut(&mut cow2) += 3;
1610 assert!(76 == *cow0);
1611 assert!(77 == *cow1);
1612 assert!(78 == *cow2);
1614 // none should point to the same backing memory
1615 assert!(*cow0 != *cow1);
1616 assert!(*cow0 != *cow2);
1617 assert!(*cow1 != *cow2);
1621 fn test_cowarc_clone_unique2() {
1622 let mut cow0 = Arc::new(75);
1623 let cow1 = cow0.clone();
1624 let cow2 = cow1.clone();
1626 assert!(75 == *cow0);
1627 assert!(75 == *cow1);
1628 assert!(75 == *cow2);
1630 *Arc::make_mut(&mut cow0) += 1;
1631 assert!(76 == *cow0);
1632 assert!(75 == *cow1);
1633 assert!(75 == *cow2);
1635 // cow1 and cow2 should share the same contents
1636 // cow0 should have a unique reference
1637 assert!(*cow0 != *cow1);
1638 assert!(*cow0 != *cow2);
1639 assert!(*cow1 == *cow2);
1643 fn test_cowarc_clone_weak() {
1644 let mut cow0 = Arc::new(75);
1645 let cow1_weak = Arc::downgrade(&cow0);
1647 assert!(75 == *cow0);
1648 assert!(75 == *cow1_weak.upgrade().unwrap());
1650 *Arc::make_mut(&mut cow0) += 1;
1652 assert!(76 == *cow0);
1653 assert!(cow1_weak.upgrade().is_none());
1658 let x = Arc::new(5);
1659 let y = Arc::downgrade(&x);
1660 assert!(y.upgrade().is_some());
1665 let x = Arc::new(5);
1666 let y = Arc::downgrade(&x);
1668 assert!(y.upgrade().is_none());
1672 fn weak_self_cyclic() {
1674 x: Mutex<Option<Weak<Cycle>>>,
1677 let a = Arc::new(Cycle { x: Mutex::new(None) });
1678 let b = Arc::downgrade(&a.clone());
1679 *a.x.lock().unwrap() = Some(b);
1681 // hopefully we don't double-free (or leak)...
1686 let mut canary = atomic::AtomicUsize::new(0);
1687 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1689 assert!(canary.load(Acquire) == 1);
1693 fn drop_arc_weak() {
1694 let mut canary = atomic::AtomicUsize::new(0);
1695 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1696 let arc_weak = Arc::downgrade(&arc);
1697 assert!(canary.load(Acquire) == 0);
1699 assert!(canary.load(Acquire) == 1);
1704 fn test_strong_count() {
1705 let a = Arc::new(0);
1706 assert!(Arc::strong_count(&a) == 1);
1707 let w = Arc::downgrade(&a);
1708 assert!(Arc::strong_count(&a) == 1);
1709 let b = w.upgrade().expect("");
1710 assert!(Arc::strong_count(&b) == 2);
1711 assert!(Arc::strong_count(&a) == 2);
1714 assert!(Arc::strong_count(&b) == 1);
1716 assert!(Arc::strong_count(&b) == 2);
1717 assert!(Arc::strong_count(&c) == 2);
1721 fn test_weak_count() {
1722 let a = Arc::new(0);
1723 assert!(Arc::strong_count(&a) == 1);
1724 assert!(Arc::weak_count(&a) == 0);
1725 let w = Arc::downgrade(&a);
1726 assert!(Arc::strong_count(&a) == 1);
1727 assert!(Arc::weak_count(&a) == 1);
1729 assert!(Arc::weak_count(&a) == 2);
1732 assert!(Arc::strong_count(&a) == 1);
1733 assert!(Arc::weak_count(&a) == 0);
1735 assert!(Arc::strong_count(&a) == 2);
1736 assert!(Arc::weak_count(&a) == 0);
1737 let d = Arc::downgrade(&c);
1738 assert!(Arc::weak_count(&c) == 1);
1739 assert!(Arc::strong_count(&c) == 2);
1748 let a = Arc::new(5);
1749 assert_eq!(format!("{:?}", a), "5");
1752 // Make sure deriving works with Arc<T>
1753 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1760 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1761 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1762 let y = Arc::downgrade(&x.clone());
1764 assert!(y.upgrade().is_none());
1768 fn test_from_owned() {
1770 let foo_arc = Arc::from(foo);
1771 assert!(123 == *foo_arc);
1775 fn test_new_weak() {
1776 let foo: Weak<usize> = Weak::new();
1777 assert!(foo.upgrade().is_none());
1782 let five = Arc::new(5);
1783 let same_five = five.clone();
1784 let other_five = Arc::new(5);
1786 assert!(Arc::ptr_eq(&five, &same_five));
1787 assert!(!Arc::ptr_eq(&five, &other_five));
1791 #[cfg_attr(target_os = "emscripten", ignore)]
1792 fn test_weak_count_locked() {
1793 let mut a = Arc::new(atomic::AtomicBool::new(false));
1795 let t = thread::spawn(move || {
1796 for _i in 0..1000000 {
1797 Arc::get_mut(&mut a);
1799 a.store(true, SeqCst);
1802 while !a2.load(SeqCst) {
1803 let n = Arc::weak_count(&a2);
1804 assert!(n < 2, "bad weak count: {}", n);
1810 fn test_from_str() {
1811 let r: Arc<str> = Arc::from("foo");
1813 assert_eq!(&r[..], "foo");
1817 fn test_copy_from_slice() {
1818 let s: &[u32] = &[1, 2, 3];
1819 let r: Arc<[u32]> = Arc::from(s);
1821 assert_eq!(&r[..], [1, 2, 3]);
1825 fn test_clone_from_slice() {
1826 #[derive(Clone, Debug, Eq, PartialEq)]
1829 let s: &[X] = &[X(1), X(2), X(3)];
1830 let r: Arc<[X]> = Arc::from(s);
1832 assert_eq!(&r[..], s);
1837 fn test_clone_from_slice_panic() {
1838 use std::string::{String, ToString};
1840 struct Fail(u32, String);
1842 impl Clone for Fail {
1843 fn clone(&self) -> Fail {
1847 Fail(self.0, self.1.clone())
1852 Fail(0, "foo".to_string()),
1853 Fail(1, "bar".to_string()),
1854 Fail(2, "baz".to_string()),
1857 // Should panic, but not cause memory corruption
1858 let _r: Arc<[Fail]> = Arc::from(s);
1862 fn test_from_box() {
1863 let b: Box<u32> = box 123;
1864 let r: Arc<u32> = Arc::from(b);
1866 assert_eq!(*r, 123);
1870 fn test_from_box_str() {
1871 use std::string::String;
1873 let s = String::from("foo").into_boxed_str();
1874 let r: Arc<str> = Arc::from(s);
1876 assert_eq!(&r[..], "foo");
1880 fn test_from_box_slice() {
1881 let s = vec![1, 2, 3].into_boxed_slice();
1882 let r: Arc<[u32]> = Arc::from(s);
1884 assert_eq!(&r[..], [1, 2, 3]);
1888 fn test_from_box_trait() {
1889 use std::fmt::Display;
1890 use std::string::ToString;
1892 let b: Box<dyn Display> = box 123;
1893 let r: Arc<dyn Display> = Arc::from(b);
1895 assert_eq!(r.to_string(), "123");
1899 fn test_from_box_trait_zero_sized() {
1900 use std::fmt::Debug;
1902 let b: Box<dyn Debug> = box ();
1903 let r: Arc<dyn Debug> = Arc::from(b);
1905 assert_eq!(format!("{:?}", r), "()");
1909 fn test_from_vec() {
1910 let v = vec![1, 2, 3];
1911 let r: Arc<[u32]> = Arc::from(v);
1913 assert_eq!(&r[..], [1, 2, 3]);
1917 fn test_downcast() {
1920 let r1: Arc<dyn Any + Send + Sync> = Arc::new(i32::max_value());
1921 let r2: Arc<dyn Any + Send + Sync> = Arc::new("abc");
1923 assert!(r1.clone().downcast::<u32>().is_err());
1925 let r1i32 = r1.downcast::<i32>();
1926 assert!(r1i32.is_ok());
1927 assert_eq!(r1i32.unwrap(), Arc::new(i32::max_value()));
1929 assert!(r2.clone().downcast::<i32>().is_err());
1931 let r2str = r2.downcast::<&'static str>();
1932 assert!(r2str.is_ok());
1933 assert_eq!(r2str.unwrap(), Arc::new("abc"));
1937 #[stable(feature = "rust1", since = "1.0.0")]
1938 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1939 fn borrow(&self) -> &T {
1944 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1945 impl<T: ?Sized> AsRef<T> for Arc<T> {
1946 fn as_ref(&self) -> &T {
1951 #[unstable(feature = "pin", issue = "49150")]
1952 impl<T: ?Sized> Unpin for Arc<T> { }