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, CoerceSized};
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 #[cfg_attr(not(test), lang = "arc")]
203 #[stable(feature = "rust1", since = "1.0.0")]
204 pub struct Arc<T: ?Sized> {
205 ptr: NonNull<ArcInner<T>>,
206 phantom: PhantomData<T>,
209 #[stable(feature = "rust1", since = "1.0.0")]
210 unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
211 #[stable(feature = "rust1", since = "1.0.0")]
212 unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
214 #[unstable(feature = "coerce_unsized", issue = "27732")]
215 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
217 #[unstable(feature = "coerce_sized", issue = "0")]
218 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Arc<T>> for Arc<U> {}
220 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
221 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
222 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
224 /// Since a `Weak` reference does not count towards ownership, it will not
225 /// prevent the inner value from being dropped, and `Weak` itself makes no
226 /// guarantees about the value still being present and may return [`None`]
227 /// when [`upgrade`]d.
229 /// A `Weak` pointer is useful for keeping a temporary reference to the value
230 /// within [`Arc`] without extending its lifetime. It is also used to prevent
231 /// circular references between [`Arc`] pointers, since mutual owning references
232 /// would never allow either [`Arc`] to be dropped. For example, a tree could
233 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
234 /// pointers from children back to their parents.
236 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
238 /// [`Arc`]: struct.Arc.html
239 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
240 /// [`upgrade`]: struct.Weak.html#method.upgrade
241 /// [`Option`]: ../../std/option/enum.Option.html
242 /// [`None`]: ../../std/option/enum.Option.html#variant.None
243 #[stable(feature = "arc_weak", since = "1.4.0")]
244 pub struct Weak<T: ?Sized> {
245 // This is a `NonNull` to allow optimizing the size of this type in enums,
246 // but it is not necessarily a valid pointer.
247 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
248 // to allocate space on the heap. That's not a value a real pointer
249 // will ever have because RcBox has alignment at least 2.
250 ptr: NonNull<ArcInner<T>>,
253 #[stable(feature = "arc_weak", since = "1.4.0")]
254 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
255 #[stable(feature = "arc_weak", since = "1.4.0")]
256 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
258 #[unstable(feature = "coerce_unsized", issue = "27732")]
259 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
260 #[unstable(feature = "coerce_sized", issue = "0")]
261 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Weak<T>> for Weak<U> {}
263 #[stable(feature = "arc_weak", since = "1.4.0")]
264 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
265 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
270 struct ArcInner<T: ?Sized> {
271 strong: atomic::AtomicUsize,
273 // the value usize::MAX acts as a sentinel for temporarily "locking" the
274 // ability to upgrade weak pointers or downgrade strong ones; this is used
275 // to avoid races in `make_mut` and `get_mut`.
276 weak: atomic::AtomicUsize,
281 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
282 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
285 /// Constructs a new `Arc<T>`.
290 /// use std::sync::Arc;
292 /// let five = Arc::new(5);
295 #[stable(feature = "rust1", since = "1.0.0")]
296 pub fn new(data: T) -> Arc<T> {
297 // Start the weak pointer count as 1 which is the weak pointer that's
298 // held by all the strong pointers (kinda), see std/rc.rs for more info
299 let x: Box<_> = box ArcInner {
300 strong: atomic::AtomicUsize::new(1),
301 weak: atomic::AtomicUsize::new(1),
304 Arc { ptr: Box::into_raw_non_null(x), phantom: PhantomData }
307 #[unstable(feature = "pin", issue = "49150")]
308 pub fn pinned(data: T) -> Pin<Arc<T>> {
309 unsafe { Pin::new_unchecked(Arc::new(data)) }
312 /// Returns the contained value, if the `Arc` has exactly one strong reference.
314 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
317 /// This will succeed even if there are outstanding weak references.
319 /// [result]: ../../std/result/enum.Result.html
324 /// use std::sync::Arc;
326 /// let x = Arc::new(3);
327 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
329 /// let x = Arc::new(4);
330 /// let _y = Arc::clone(&x);
331 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
334 #[stable(feature = "arc_unique", since = "1.4.0")]
335 pub fn try_unwrap(this: Self) -> Result<T, Self> {
336 // See `drop` for why all these atomics are like this
337 if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
341 atomic::fence(Acquire);
344 let elem = ptr::read(&this.ptr.as_ref().data);
346 // Make a weak pointer to clean up the implicit strong-weak reference
347 let _weak = Weak { ptr: this.ptr };
355 impl<T: ?Sized> Arc<T> {
356 /// Consumes the `Arc`, returning the wrapped pointer.
358 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
359 /// [`Arc::from_raw`][from_raw].
361 /// [from_raw]: struct.Arc.html#method.from_raw
366 /// use std::sync::Arc;
368 /// let x = Arc::new(10);
369 /// let x_ptr = Arc::into_raw(x);
370 /// assert_eq!(unsafe { *x_ptr }, 10);
372 #[stable(feature = "rc_raw", since = "1.17.0")]
373 pub fn into_raw(this: Self) -> *const T {
374 let ptr: *const T = &*this;
379 /// Constructs an `Arc` from a raw pointer.
381 /// The raw pointer must have been previously returned by a call to a
382 /// [`Arc::into_raw`][into_raw].
384 /// This function is unsafe because improper use may lead to memory problems. For example, a
385 /// double-free may occur if the function is called twice on the same raw pointer.
387 /// [into_raw]: struct.Arc.html#method.into_raw
392 /// use std::sync::Arc;
394 /// let x = Arc::new(10);
395 /// let x_ptr = Arc::into_raw(x);
398 /// // Convert back to an `Arc` to prevent leak.
399 /// let x = Arc::from_raw(x_ptr);
400 /// assert_eq!(*x, 10);
402 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory unsafe.
405 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
407 #[stable(feature = "rc_raw", since = "1.17.0")]
408 pub unsafe fn from_raw(ptr: *const T) -> Self {
409 // Align the unsized value to the end of the ArcInner.
410 // Because it is ?Sized, it will always be the last field in memory.
411 let align = align_of_val(&*ptr);
412 let layout = Layout::new::<ArcInner<()>>();
413 let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
415 // Reverse the offset to find the original ArcInner.
416 let fake_ptr = ptr as *mut ArcInner<T>;
417 let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
420 ptr: NonNull::new_unchecked(arc_ptr),
421 phantom: PhantomData,
425 /// Creates a new [`Weak`][weak] pointer to this value.
427 /// [weak]: struct.Weak.html
432 /// use std::sync::Arc;
434 /// let five = Arc::new(5);
436 /// let weak_five = Arc::downgrade(&five);
438 #[stable(feature = "arc_weak", since = "1.4.0")]
439 pub fn downgrade(this: &Self) -> Weak<T> {
440 // This Relaxed is OK because we're checking the value in the CAS
442 let mut cur = this.inner().weak.load(Relaxed);
445 // check if the weak counter is currently "locked"; if so, spin.
446 if cur == usize::MAX {
447 cur = this.inner().weak.load(Relaxed);
451 // NOTE: this code currently ignores the possibility of overflow
452 // into usize::MAX; in general both Rc and Arc need to be adjusted
453 // to deal with overflow.
455 // Unlike with Clone(), we need this to be an Acquire read to
456 // synchronize with the write coming from `is_unique`, so that the
457 // events prior to that write happen before this read.
458 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
460 // Make sure we do not create a dangling Weak
461 debug_assert!(!is_dangling(this.ptr));
462 return Weak { ptr: this.ptr };
464 Err(old) => cur = old,
469 /// Gets the number of [`Weak`][weak] pointers to this value.
471 /// [weak]: struct.Weak.html
475 /// This method by itself is safe, but using it correctly requires extra care.
476 /// Another thread can change the weak count at any time,
477 /// including potentially between calling this method and acting on the result.
482 /// use std::sync::Arc;
484 /// let five = Arc::new(5);
485 /// let _weak_five = Arc::downgrade(&five);
487 /// // This assertion is deterministic because we haven't shared
488 /// // the `Arc` or `Weak` between threads.
489 /// assert_eq!(1, Arc::weak_count(&five));
492 #[stable(feature = "arc_counts", since = "1.15.0")]
493 pub fn weak_count(this: &Self) -> usize {
494 let cnt = this.inner().weak.load(SeqCst);
495 // If the weak count is currently locked, the value of the
496 // count was 0 just before taking the lock.
497 if cnt == usize::MAX { 0 } else { cnt - 1 }
500 /// Gets the number of strong (`Arc`) pointers to this value.
504 /// This method by itself is safe, but using it correctly requires extra care.
505 /// Another thread can change the strong count at any time,
506 /// including potentially between calling this method and acting on the result.
511 /// use std::sync::Arc;
513 /// let five = Arc::new(5);
514 /// let _also_five = Arc::clone(&five);
516 /// // This assertion is deterministic because we haven't shared
517 /// // the `Arc` between threads.
518 /// assert_eq!(2, Arc::strong_count(&five));
521 #[stable(feature = "arc_counts", since = "1.15.0")]
522 pub fn strong_count(this: &Self) -> usize {
523 this.inner().strong.load(SeqCst)
527 fn inner(&self) -> &ArcInner<T> {
528 // This unsafety is ok because while this arc is alive we're guaranteed
529 // that the inner pointer is valid. Furthermore, we know that the
530 // `ArcInner` structure itself is `Sync` because the inner data is
531 // `Sync` as well, so we're ok loaning out an immutable pointer to these
533 unsafe { self.ptr.as_ref() }
536 // Non-inlined part of `drop`.
538 unsafe fn drop_slow(&mut self) {
539 // Destroy the data at this time, even though we may not free the box
540 // allocation itself (there may still be weak pointers lying around).
541 ptr::drop_in_place(&mut self.ptr.as_mut().data);
543 if self.inner().weak.fetch_sub(1, Release) == 1 {
544 atomic::fence(Acquire);
545 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
550 #[stable(feature = "ptr_eq", since = "1.17.0")]
551 /// Returns true if the two `Arc`s point to the same value (not
552 /// just values that compare as equal).
557 /// use std::sync::Arc;
559 /// let five = Arc::new(5);
560 /// let same_five = Arc::clone(&five);
561 /// let other_five = Arc::new(5);
563 /// assert!(Arc::ptr_eq(&five, &same_five));
564 /// assert!(!Arc::ptr_eq(&five, &other_five));
566 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
567 this.ptr.as_ptr() == other.ptr.as_ptr()
571 impl<T: ?Sized> Arc<T> {
572 // Allocates an `ArcInner<T>` with sufficient space for an unsized value
573 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
574 // Create a fake ArcInner to find allocation size and alignment
575 let fake_ptr = ptr as *mut ArcInner<T>;
577 let layout = Layout::for_value(&*fake_ptr);
579 let mem = Global.alloc(layout)
580 .unwrap_or_else(|_| handle_alloc_error(layout));
582 // Initialize the real ArcInner
583 let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;
585 ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
586 ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
591 fn from_box(v: Box<T>) -> Arc<T> {
593 let box_unique = Box::into_unique(v);
594 let bptr = box_unique.as_ptr();
596 let value_size = size_of_val(&*bptr);
597 let ptr = Self::allocate_for_ptr(bptr);
599 // Copy value as bytes
600 ptr::copy_nonoverlapping(
601 bptr as *const T as *const u8,
602 &mut (*ptr).data as *mut _ as *mut u8,
605 // Free the allocation without dropping its contents
606 box_free(box_unique);
608 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
613 // Sets the data pointer of a `?Sized` raw pointer.
615 // For a slice/trait object, this sets the `data` field and leaves the rest
616 // unchanged. For a sized raw pointer, this simply sets the pointer.
617 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
618 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
623 // Copy elements from slice into newly allocated Arc<[T]>
625 // Unsafe because the caller must either take ownership or bind `T: Copy`
626 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
627 let v_ptr = v as *const [T];
628 let ptr = Self::allocate_for_ptr(v_ptr);
630 ptr::copy_nonoverlapping(
632 &mut (*ptr).data as *mut [T] as *mut T,
635 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
639 // Specialization trait used for From<&[T]>
640 trait ArcFromSlice<T> {
641 fn from_slice(slice: &[T]) -> Self;
644 impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
646 default fn from_slice(v: &[T]) -> Self {
647 // Panic guard while cloning T elements.
648 // In the event of a panic, elements that have been written
649 // into the new ArcInner will be dropped, then the memory freed.
657 impl<T> Drop for Guard<T> {
659 use core::slice::from_raw_parts_mut;
662 let slice = from_raw_parts_mut(self.elems, self.n_elems);
663 ptr::drop_in_place(slice);
665 Global.dealloc(self.mem.cast(), self.layout.clone());
671 let v_ptr = v as *const [T];
672 let ptr = Self::allocate_for_ptr(v_ptr);
674 let mem = ptr as *mut _ as *mut u8;
675 let layout = Layout::for_value(&*ptr);
677 // Pointer to first element
678 let elems = &mut (*ptr).data as *mut [T] as *mut T;
680 let mut guard = Guard{
681 mem: NonNull::new_unchecked(mem),
687 for (i, item) in v.iter().enumerate() {
688 ptr::write(elems.add(i), item.clone());
692 // All clear. Forget the guard so it doesn't free the new ArcInner.
695 Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
700 impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
702 fn from_slice(v: &[T]) -> Self {
703 unsafe { Arc::copy_from_slice(v) }
707 #[stable(feature = "rust1", since = "1.0.0")]
708 impl<T: ?Sized> Clone for Arc<T> {
709 /// Makes a clone of the `Arc` pointer.
711 /// This creates another pointer to the same inner value, increasing the
712 /// strong reference count.
717 /// use std::sync::Arc;
719 /// let five = Arc::new(5);
721 /// let _ = Arc::clone(&five);
724 fn clone(&self) -> Arc<T> {
725 // Using a relaxed ordering is alright here, as knowledge of the
726 // original reference prevents other threads from erroneously deleting
729 // As explained in the [Boost documentation][1], Increasing the
730 // reference counter can always be done with memory_order_relaxed: New
731 // references to an object can only be formed from an existing
732 // reference, and passing an existing reference from one thread to
733 // another must already provide any required synchronization.
735 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
736 let old_size = self.inner().strong.fetch_add(1, Relaxed);
738 // However we need to guard against massive refcounts in case someone
739 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
740 // and users will use-after free. We racily saturate to `isize::MAX` on
741 // the assumption that there aren't ~2 billion threads incrementing
742 // the reference count at once. This branch will never be taken in
743 // any realistic program.
745 // We abort because such a program is incredibly degenerate, and we
746 // don't care to support it.
747 if old_size > MAX_REFCOUNT {
753 Arc { ptr: self.ptr, phantom: PhantomData }
757 #[stable(feature = "rust1", since = "1.0.0")]
758 impl<T: ?Sized> Deref for Arc<T> {
762 fn deref(&self) -> &T {
767 impl<T: Clone> Arc<T> {
768 /// Makes a mutable reference into the given `Arc`.
770 /// If there are other `Arc` or [`Weak`][weak] pointers to the same value,
771 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
772 /// ensure unique ownership. This is also referred to as clone-on-write.
774 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
776 /// [weak]: struct.Weak.html
777 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
778 /// [get_mut]: struct.Arc.html#method.get_mut
783 /// use std::sync::Arc;
785 /// let mut data = Arc::new(5);
787 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
788 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
789 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
790 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
791 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
793 /// // Now `data` and `other_data` point to different values.
794 /// assert_eq!(*data, 8);
795 /// assert_eq!(*other_data, 12);
798 #[stable(feature = "arc_unique", since = "1.4.0")]
799 pub fn make_mut(this: &mut Self) -> &mut T {
800 // Note that we hold both a strong reference and a weak reference.
801 // Thus, releasing our strong reference only will not, by itself, cause
802 // the memory to be deallocated.
804 // Use Acquire to ensure that we see any writes to `weak` that happen
805 // before release writes (i.e., decrements) to `strong`. Since we hold a
806 // weak count, there's no chance the ArcInner itself could be
808 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
809 // Another strong pointer exists; clone
810 *this = Arc::new((**this).clone());
811 } else if this.inner().weak.load(Relaxed) != 1 {
812 // Relaxed suffices in the above because this is fundamentally an
813 // optimization: we are always racing with weak pointers being
814 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
816 // We removed the last strong ref, but there are additional weak
817 // refs remaining. We'll move the contents to a new Arc, and
818 // invalidate the other weak refs.
820 // Note that it is not possible for the read of `weak` to yield
821 // usize::MAX (i.e., locked), since the weak count can only be
822 // locked by a thread with a strong reference.
824 // Materialize our own implicit weak pointer, so that it can clean
825 // up the ArcInner as needed.
826 let weak = Weak { ptr: this.ptr };
828 // mark the data itself as already deallocated
830 // there is no data race in the implicit write caused by `read`
831 // here (due to zeroing) because data is no longer accessed by
832 // other threads (due to there being no more strong refs at this
834 let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
835 mem::swap(this, &mut swap);
839 // We were the sole reference of either kind; bump back up the
841 this.inner().strong.store(1, Release);
844 // As with `get_mut()`, the unsafety is ok because our reference was
845 // either unique to begin with, or became one upon cloning the contents.
847 &mut this.ptr.as_mut().data
852 impl<T: ?Sized> Arc<T> {
853 /// Returns a mutable reference to the inner value, if there are
854 /// no other `Arc` or [`Weak`][weak] pointers to the same value.
856 /// Returns [`None`][option] otherwise, because it is not safe to
857 /// mutate a shared value.
859 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
860 /// the inner value when it's shared.
862 /// [weak]: struct.Weak.html
863 /// [option]: ../../std/option/enum.Option.html
864 /// [make_mut]: struct.Arc.html#method.make_mut
865 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
870 /// use std::sync::Arc;
872 /// let mut x = Arc::new(3);
873 /// *Arc::get_mut(&mut x).unwrap() = 4;
874 /// assert_eq!(*x, 4);
876 /// let _y = Arc::clone(&x);
877 /// assert!(Arc::get_mut(&mut x).is_none());
880 #[stable(feature = "arc_unique", since = "1.4.0")]
881 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
882 if this.is_unique() {
883 // This unsafety is ok because we're guaranteed that the pointer
884 // returned is the *only* pointer that will ever be returned to T. Our
885 // reference count is guaranteed to be 1 at this point, and we required
886 // the Arc itself to be `mut`, so we're returning the only possible
887 // reference to the inner data.
889 Some(&mut this.ptr.as_mut().data)
896 /// Determine whether this is the unique reference (including weak refs) to
897 /// the underlying data.
899 /// Note that this requires locking the weak ref count.
900 fn is_unique(&mut self) -> bool {
901 // lock the weak pointer count if we appear to be the sole weak pointer
904 // The acquire label here ensures a happens-before relationship with any
905 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
906 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
907 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
908 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
909 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
910 // counter in `drop` -- the only access that happens when any but the last reference
912 let unique = self.inner().strong.load(Acquire) == 1;
914 // The release write here synchronizes with a read in `downgrade`,
915 // effectively preventing the above read of `strong` from happening
917 self.inner().weak.store(1, Release); // release the lock
925 #[stable(feature = "rust1", since = "1.0.0")]
926 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
929 /// This will decrement the strong reference count. If the strong reference
930 /// count reaches zero then the only other references (if any) are
931 /// [`Weak`], so we `drop` the inner value.
936 /// use std::sync::Arc;
940 /// impl Drop for Foo {
941 /// fn drop(&mut self) {
942 /// println!("dropped!");
946 /// let foo = Arc::new(Foo);
947 /// let foo2 = Arc::clone(&foo);
949 /// drop(foo); // Doesn't print anything
950 /// drop(foo2); // Prints "dropped!"
954 // Because `fetch_sub` is already atomic, we do not need to synchronize
955 // with other threads unless we are going to delete the object. This
956 // same logic applies to the below `fetch_sub` to the `weak` count.
957 if self.inner().strong.fetch_sub(1, Release) != 1 {
961 // This fence is needed to prevent reordering of use of the data and
962 // deletion of the data. Because it is marked `Release`, the decreasing
963 // of the reference count synchronizes with this `Acquire` fence. This
964 // means that use of the data happens before decreasing the reference
965 // count, which happens before this fence, which happens before the
966 // deletion of the data.
968 // As explained in the [Boost documentation][1],
970 // > It is important to enforce any possible access to the object in one
971 // > thread (through an existing reference) to *happen before* deleting
972 // > the object in a different thread. This is achieved by a "release"
973 // > operation after dropping a reference (any access to the object
974 // > through this reference must obviously happened before), and an
975 // > "acquire" operation before deleting the object.
977 // In particular, while the contents of an Arc are usually immutable, it's
978 // possible to have interior writes to something like a Mutex<T>. Since a
979 // Mutex is not acquired when it is deleted, we can't rely on its
980 // synchronization logic to make writes in thread A visible to a destructor
981 // running in thread B.
983 // Also note that the Acquire fence here could probably be replaced with an
984 // Acquire load, which could improve performance in highly-contended
985 // situations. See [2].
987 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
988 // [2]: (https://github.com/rust-lang/rust/pull/41714)
989 atomic::fence(Acquire);
997 impl Arc<dyn Any + Send + Sync> {
999 #[stable(feature = "rc_downcast", since = "1.29.0")]
1000 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1005 /// use std::any::Any;
1006 /// use std::sync::Arc;
1008 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1009 /// if let Ok(string) = value.downcast::<String>() {
1010 /// println!("String ({}): {}", string.len(), string);
1015 /// let my_string = "Hello World".to_string();
1016 /// print_if_string(Arc::new(my_string));
1017 /// print_if_string(Arc::new(0i8));
1020 pub fn downcast<T>(self) -> Result<Arc<T>, Self>
1022 T: Any + Send + Sync + 'static,
1024 if (*self).is::<T>() {
1025 let ptr = self.ptr.cast::<ArcInner<T>>();
1027 Ok(Arc { ptr, phantom: PhantomData })
1035 /// Constructs a new `Weak<T>`, without allocating any memory.
1036 /// Calling [`upgrade`] on the return value always gives [`None`].
1038 /// [`upgrade`]: struct.Weak.html#method.upgrade
1039 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1044 /// use std::sync::Weak;
1046 /// let empty: Weak<i64> = Weak::new();
1047 /// assert!(empty.upgrade().is_none());
1049 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1050 pub fn new() -> Weak<T> {
1052 ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0"),
1057 impl<T: ?Sized> Weak<T> {
1058 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
1059 /// the lifetime of the value if successful.
1061 /// Returns [`None`] if the value has since been dropped.
1063 /// [`Arc`]: struct.Arc.html
1064 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1069 /// use std::sync::Arc;
1071 /// let five = Arc::new(5);
1073 /// let weak_five = Arc::downgrade(&five);
1075 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1076 /// assert!(strong_five.is_some());
1078 /// // Destroy all strong pointers.
1079 /// drop(strong_five);
1082 /// assert!(weak_five.upgrade().is_none());
1084 #[stable(feature = "arc_weak", since = "1.4.0")]
1085 pub fn upgrade(&self) -> Option<Arc<T>> {
1086 // We use a CAS loop to increment the strong count instead of a
1087 // fetch_add because once the count hits 0 it must never be above 0.
1088 let inner = self.inner()?;
1090 // Relaxed load because any write of 0 that we can observe
1091 // leaves the field in a permanently zero state (so a
1092 // "stale" read of 0 is fine), and any other value is
1093 // confirmed via the CAS below.
1094 let mut n = inner.strong.load(Relaxed);
1101 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1102 if n > MAX_REFCOUNT {
1108 // Relaxed is valid for the same reason it is on Arc's Clone impl
1109 match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
1110 Ok(_) => return Some(Arc {
1111 // null checked above
1113 phantom: PhantomData,
1115 Err(old) => n = old,
1120 /// Return `None` when the pointer is dangling and there is no allocated `ArcInner`,
1121 /// i.e. this `Weak` was created by `Weak::new`
1123 fn inner(&self) -> Option<&ArcInner<T>> {
1124 if is_dangling(self.ptr) {
1127 Some(unsafe { self.ptr.as_ref() })
1132 #[stable(feature = "arc_weak", since = "1.4.0")]
1133 impl<T: ?Sized> Clone for Weak<T> {
1134 /// Makes a clone of the `Weak` pointer that points to the same value.
1139 /// use std::sync::{Arc, Weak};
1141 /// let weak_five = Arc::downgrade(&Arc::new(5));
1143 /// let _ = Weak::clone(&weak_five);
1146 fn clone(&self) -> Weak<T> {
1147 let inner = if let Some(inner) = self.inner() {
1150 return Weak { ptr: self.ptr };
1152 // See comments in Arc::clone() for why this is relaxed. This can use a
1153 // fetch_add (ignoring the lock) because the weak count is only locked
1154 // where are *no other* weak pointers in existence. (So we can't be
1155 // running this code in that case).
1156 let old_size = inner.weak.fetch_add(1, Relaxed);
1158 // See comments in Arc::clone() for why we do this (for mem::forget).
1159 if old_size > MAX_REFCOUNT {
1165 return Weak { ptr: self.ptr };
1169 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1170 impl<T> Default for Weak<T> {
1171 /// Constructs a new `Weak<T>`, without allocating memory.
1172 /// Calling [`upgrade`][Weak::upgrade] on the return value always
1175 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1180 /// use std::sync::Weak;
1182 /// let empty: Weak<i64> = Default::default();
1183 /// assert!(empty.upgrade().is_none());
1185 fn default() -> Weak<T> {
1190 #[stable(feature = "arc_weak", since = "1.4.0")]
1191 impl<T: ?Sized> Drop for Weak<T> {
1192 /// Drops the `Weak` pointer.
1197 /// use std::sync::{Arc, Weak};
1201 /// impl Drop for Foo {
1202 /// fn drop(&mut self) {
1203 /// println!("dropped!");
1207 /// let foo = Arc::new(Foo);
1208 /// let weak_foo = Arc::downgrade(&foo);
1209 /// let other_weak_foo = Weak::clone(&weak_foo);
1211 /// drop(weak_foo); // Doesn't print anything
1212 /// drop(foo); // Prints "dropped!"
1214 /// assert!(other_weak_foo.upgrade().is_none());
1216 fn drop(&mut self) {
1217 // If we find out that we were the last weak pointer, then its time to
1218 // deallocate the data entirely. See the discussion in Arc::drop() about
1219 // the memory orderings
1221 // It's not necessary to check for the locked state here, because the
1222 // weak count can only be locked if there was precisely one weak ref,
1223 // meaning that drop could only subsequently run ON that remaining weak
1224 // ref, which can only happen after the lock is released.
1225 let inner = if let Some(inner) = self.inner() {
1231 if inner.weak.fetch_sub(1, Release) == 1 {
1232 atomic::fence(Acquire);
1234 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
1240 #[stable(feature = "rust1", since = "1.0.0")]
1241 impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
1242 /// Equality for two `Arc`s.
1244 /// Two `Arc`s are equal if their inner values are equal.
1249 /// use std::sync::Arc;
1251 /// let five = Arc::new(5);
1253 /// assert!(five == Arc::new(5));
1255 fn eq(&self, other: &Arc<T>) -> bool {
1256 *(*self) == *(*other)
1259 /// Inequality for two `Arc`s.
1261 /// Two `Arc`s are unequal if their inner values are unequal.
1266 /// use std::sync::Arc;
1268 /// let five = Arc::new(5);
1270 /// assert!(five != Arc::new(6));
1272 fn ne(&self, other: &Arc<T>) -> bool {
1273 *(*self) != *(*other)
1276 #[stable(feature = "rust1", since = "1.0.0")]
1277 impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
1278 /// Partial comparison for two `Arc`s.
1280 /// The two are compared by calling `partial_cmp()` on their inner values.
1285 /// use std::sync::Arc;
1286 /// use std::cmp::Ordering;
1288 /// let five = Arc::new(5);
1290 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1292 fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
1293 (**self).partial_cmp(&**other)
1296 /// Less-than comparison for two `Arc`s.
1298 /// The two are compared by calling `<` on their inner values.
1303 /// use std::sync::Arc;
1305 /// let five = Arc::new(5);
1307 /// assert!(five < Arc::new(6));
1309 fn lt(&self, other: &Arc<T>) -> bool {
1310 *(*self) < *(*other)
1313 /// 'Less than or equal to' comparison for two `Arc`s.
1315 /// The two are compared by calling `<=` on their inner values.
1320 /// use std::sync::Arc;
1322 /// let five = Arc::new(5);
1324 /// assert!(five <= Arc::new(5));
1326 fn le(&self, other: &Arc<T>) -> bool {
1327 *(*self) <= *(*other)
1330 /// Greater-than comparison for two `Arc`s.
1332 /// The two are compared by calling `>` on their inner values.
1337 /// use std::sync::Arc;
1339 /// let five = Arc::new(5);
1341 /// assert!(five > Arc::new(4));
1343 fn gt(&self, other: &Arc<T>) -> bool {
1344 *(*self) > *(*other)
1347 /// 'Greater than or equal to' comparison for two `Arc`s.
1349 /// The two are compared by calling `>=` on their inner values.
1354 /// use std::sync::Arc;
1356 /// let five = Arc::new(5);
1358 /// assert!(five >= Arc::new(5));
1360 fn ge(&self, other: &Arc<T>) -> bool {
1361 *(*self) >= *(*other)
1364 #[stable(feature = "rust1", since = "1.0.0")]
1365 impl<T: ?Sized + Ord> Ord for Arc<T> {
1366 /// Comparison for two `Arc`s.
1368 /// The two are compared by calling `cmp()` on their inner values.
1373 /// use std::sync::Arc;
1374 /// use std::cmp::Ordering;
1376 /// let five = Arc::new(5);
1378 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1380 fn cmp(&self, other: &Arc<T>) -> Ordering {
1381 (**self).cmp(&**other)
1384 #[stable(feature = "rust1", since = "1.0.0")]
1385 impl<T: ?Sized + Eq> Eq for Arc<T> {}
1387 #[stable(feature = "rust1", since = "1.0.0")]
1388 impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
1389 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1390 fmt::Display::fmt(&**self, f)
1394 #[stable(feature = "rust1", since = "1.0.0")]
1395 impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
1396 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1397 fmt::Debug::fmt(&**self, f)
1401 #[stable(feature = "rust1", since = "1.0.0")]
1402 impl<T: ?Sized> fmt::Pointer for Arc<T> {
1403 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1404 fmt::Pointer::fmt(&(&**self as *const T), f)
1408 #[stable(feature = "rust1", since = "1.0.0")]
1409 impl<T: Default> Default for Arc<T> {
1410 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1415 /// use std::sync::Arc;
1417 /// let x: Arc<i32> = Default::default();
1418 /// assert_eq!(*x, 0);
1420 fn default() -> Arc<T> {
1421 Arc::new(Default::default())
1425 #[stable(feature = "rust1", since = "1.0.0")]
1426 impl<T: ?Sized + Hash> Hash for Arc<T> {
1427 fn hash<H: Hasher>(&self, state: &mut H) {
1428 (**self).hash(state)
1432 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1433 impl<T> From<T> for Arc<T> {
1434 fn from(t: T) -> Self {
1439 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1440 impl<'a, T: Clone> From<&'a [T]> for Arc<[T]> {
1442 fn from(v: &[T]) -> Arc<[T]> {
1443 <Self as ArcFromSlice<T>>::from_slice(v)
1447 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1448 impl<'a> From<&'a str> for Arc<str> {
1450 fn from(v: &str) -> Arc<str> {
1451 let arc = Arc::<[u8]>::from(v.as_bytes());
1452 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1456 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1457 impl From<String> for Arc<str> {
1459 fn from(v: String) -> Arc<str> {
1464 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1465 impl<T: ?Sized> From<Box<T>> for Arc<T> {
1467 fn from(v: Box<T>) -> Arc<T> {
1472 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1473 impl<T> From<Vec<T>> for Arc<[T]> {
1475 fn from(mut v: Vec<T>) -> Arc<[T]> {
1477 let arc = Arc::copy_from_slice(&v);
1479 // Allow the Vec to free its memory, but not destroy its contents
1489 use std::boxed::Box;
1490 use std::clone::Clone;
1491 use std::sync::mpsc::channel;
1494 use std::option::Option;
1495 use std::option::Option::{None, Some};
1496 use std::sync::atomic;
1497 use std::sync::atomic::Ordering::{Acquire, SeqCst};
1499 use std::sync::Mutex;
1500 use std::convert::From;
1502 use super::{Arc, Weak};
1505 struct Canary(*mut atomic::AtomicUsize);
1507 impl Drop for Canary {
1508 fn drop(&mut self) {
1512 (*c).fetch_add(1, SeqCst);
1520 #[cfg_attr(target_os = "emscripten", ignore)]
1521 fn manually_share_arc() {
1522 let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
1523 let arc_v = Arc::new(v);
1525 let (tx, rx) = channel();
1527 let _t = thread::spawn(move || {
1528 let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
1529 assert_eq!((*arc_v)[3], 4);
1532 tx.send(arc_v.clone()).unwrap();
1534 assert_eq!((*arc_v)[2], 3);
1535 assert_eq!((*arc_v)[4], 5);
1539 fn test_arc_get_mut() {
1540 let mut x = Arc::new(3);
1541 *Arc::get_mut(&mut x).unwrap() = 4;
1544 assert!(Arc::get_mut(&mut x).is_none());
1546 assert!(Arc::get_mut(&mut x).is_some());
1547 let _w = Arc::downgrade(&x);
1548 assert!(Arc::get_mut(&mut x).is_none());
1553 let x = Arc::new(3);
1554 assert_eq!(Arc::try_unwrap(x), Ok(3));
1555 let x = Arc::new(4);
1557 assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
1558 let x = Arc::new(5);
1559 let _w = Arc::downgrade(&x);
1560 assert_eq!(Arc::try_unwrap(x), Ok(5));
1564 fn into_from_raw() {
1565 let x = Arc::new(box "hello");
1568 let x_ptr = Arc::into_raw(x);
1571 assert_eq!(**x_ptr, "hello");
1573 let x = Arc::from_raw(x_ptr);
1574 assert_eq!(**x, "hello");
1576 assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
1581 fn test_into_from_raw_unsized() {
1582 use std::fmt::Display;
1583 use std::string::ToString;
1585 let arc: Arc<str> = Arc::from("foo");
1587 let ptr = Arc::into_raw(arc.clone());
1588 let arc2 = unsafe { Arc::from_raw(ptr) };
1590 assert_eq!(unsafe { &*ptr }, "foo");
1591 assert_eq!(arc, arc2);
1593 let arc: Arc<dyn Display> = Arc::new(123);
1595 let ptr = Arc::into_raw(arc.clone());
1596 let arc2 = unsafe { Arc::from_raw(ptr) };
1598 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1599 assert_eq!(arc2.to_string(), "123");
1603 fn test_cowarc_clone_make_mut() {
1604 let mut cow0 = Arc::new(75);
1605 let mut cow1 = cow0.clone();
1606 let mut cow2 = cow1.clone();
1608 assert!(75 == *Arc::make_mut(&mut cow0));
1609 assert!(75 == *Arc::make_mut(&mut cow1));
1610 assert!(75 == *Arc::make_mut(&mut cow2));
1612 *Arc::make_mut(&mut cow0) += 1;
1613 *Arc::make_mut(&mut cow1) += 2;
1614 *Arc::make_mut(&mut cow2) += 3;
1616 assert!(76 == *cow0);
1617 assert!(77 == *cow1);
1618 assert!(78 == *cow2);
1620 // none should point to the same backing memory
1621 assert!(*cow0 != *cow1);
1622 assert!(*cow0 != *cow2);
1623 assert!(*cow1 != *cow2);
1627 fn test_cowarc_clone_unique2() {
1628 let mut cow0 = Arc::new(75);
1629 let cow1 = cow0.clone();
1630 let cow2 = cow1.clone();
1632 assert!(75 == *cow0);
1633 assert!(75 == *cow1);
1634 assert!(75 == *cow2);
1636 *Arc::make_mut(&mut cow0) += 1;
1637 assert!(76 == *cow0);
1638 assert!(75 == *cow1);
1639 assert!(75 == *cow2);
1641 // cow1 and cow2 should share the same contents
1642 // cow0 should have a unique reference
1643 assert!(*cow0 != *cow1);
1644 assert!(*cow0 != *cow2);
1645 assert!(*cow1 == *cow2);
1649 fn test_cowarc_clone_weak() {
1650 let mut cow0 = Arc::new(75);
1651 let cow1_weak = Arc::downgrade(&cow0);
1653 assert!(75 == *cow0);
1654 assert!(75 == *cow1_weak.upgrade().unwrap());
1656 *Arc::make_mut(&mut cow0) += 1;
1658 assert!(76 == *cow0);
1659 assert!(cow1_weak.upgrade().is_none());
1664 let x = Arc::new(5);
1665 let y = Arc::downgrade(&x);
1666 assert!(y.upgrade().is_some());
1671 let x = Arc::new(5);
1672 let y = Arc::downgrade(&x);
1674 assert!(y.upgrade().is_none());
1678 fn weak_self_cyclic() {
1680 x: Mutex<Option<Weak<Cycle>>>,
1683 let a = Arc::new(Cycle { x: Mutex::new(None) });
1684 let b = Arc::downgrade(&a.clone());
1685 *a.x.lock().unwrap() = Some(b);
1687 // hopefully we don't double-free (or leak)...
1692 let mut canary = atomic::AtomicUsize::new(0);
1693 let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1695 assert!(canary.load(Acquire) == 1);
1699 fn drop_arc_weak() {
1700 let mut canary = atomic::AtomicUsize::new(0);
1701 let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
1702 let arc_weak = Arc::downgrade(&arc);
1703 assert!(canary.load(Acquire) == 0);
1705 assert!(canary.load(Acquire) == 1);
1710 fn test_strong_count() {
1711 let a = Arc::new(0);
1712 assert!(Arc::strong_count(&a) == 1);
1713 let w = Arc::downgrade(&a);
1714 assert!(Arc::strong_count(&a) == 1);
1715 let b = w.upgrade().expect("");
1716 assert!(Arc::strong_count(&b) == 2);
1717 assert!(Arc::strong_count(&a) == 2);
1720 assert!(Arc::strong_count(&b) == 1);
1722 assert!(Arc::strong_count(&b) == 2);
1723 assert!(Arc::strong_count(&c) == 2);
1727 fn test_weak_count() {
1728 let a = Arc::new(0);
1729 assert!(Arc::strong_count(&a) == 1);
1730 assert!(Arc::weak_count(&a) == 0);
1731 let w = Arc::downgrade(&a);
1732 assert!(Arc::strong_count(&a) == 1);
1733 assert!(Arc::weak_count(&a) == 1);
1735 assert!(Arc::weak_count(&a) == 2);
1738 assert!(Arc::strong_count(&a) == 1);
1739 assert!(Arc::weak_count(&a) == 0);
1741 assert!(Arc::strong_count(&a) == 2);
1742 assert!(Arc::weak_count(&a) == 0);
1743 let d = Arc::downgrade(&c);
1744 assert!(Arc::weak_count(&c) == 1);
1745 assert!(Arc::strong_count(&c) == 2);
1754 let a = Arc::new(5);
1755 assert_eq!(format!("{:?}", a), "5");
1758 // Make sure deriving works with Arc<T>
1759 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
1766 let x: Arc<[i32]> = Arc::new([1, 2, 3]);
1767 assert_eq!(format!("{:?}", x), "[1, 2, 3]");
1768 let y = Arc::downgrade(&x.clone());
1770 assert!(y.upgrade().is_none());
1774 fn test_from_owned() {
1776 let foo_arc = Arc::from(foo);
1777 assert!(123 == *foo_arc);
1781 fn test_new_weak() {
1782 let foo: Weak<usize> = Weak::new();
1783 assert!(foo.upgrade().is_none());
1788 let five = Arc::new(5);
1789 let same_five = five.clone();
1790 let other_five = Arc::new(5);
1792 assert!(Arc::ptr_eq(&five, &same_five));
1793 assert!(!Arc::ptr_eq(&five, &other_five));
1797 #[cfg_attr(target_os = "emscripten", ignore)]
1798 fn test_weak_count_locked() {
1799 let mut a = Arc::new(atomic::AtomicBool::new(false));
1801 let t = thread::spawn(move || {
1802 for _i in 0..1000000 {
1803 Arc::get_mut(&mut a);
1805 a.store(true, SeqCst);
1808 while !a2.load(SeqCst) {
1809 let n = Arc::weak_count(&a2);
1810 assert!(n < 2, "bad weak count: {}", n);
1816 fn test_from_str() {
1817 let r: Arc<str> = Arc::from("foo");
1819 assert_eq!(&r[..], "foo");
1823 fn test_copy_from_slice() {
1824 let s: &[u32] = &[1, 2, 3];
1825 let r: Arc<[u32]> = Arc::from(s);
1827 assert_eq!(&r[..], [1, 2, 3]);
1831 fn test_clone_from_slice() {
1832 #[derive(Clone, Debug, Eq, PartialEq)]
1835 let s: &[X] = &[X(1), X(2), X(3)];
1836 let r: Arc<[X]> = Arc::from(s);
1838 assert_eq!(&r[..], s);
1843 fn test_clone_from_slice_panic() {
1844 use std::string::{String, ToString};
1846 struct Fail(u32, String);
1848 impl Clone for Fail {
1849 fn clone(&self) -> Fail {
1853 Fail(self.0, self.1.clone())
1858 Fail(0, "foo".to_string()),
1859 Fail(1, "bar".to_string()),
1860 Fail(2, "baz".to_string()),
1863 // Should panic, but not cause memory corruption
1864 let _r: Arc<[Fail]> = Arc::from(s);
1868 fn test_from_box() {
1869 let b: Box<u32> = box 123;
1870 let r: Arc<u32> = Arc::from(b);
1872 assert_eq!(*r, 123);
1876 fn test_from_box_str() {
1877 use std::string::String;
1879 let s = String::from("foo").into_boxed_str();
1880 let r: Arc<str> = Arc::from(s);
1882 assert_eq!(&r[..], "foo");
1886 fn test_from_box_slice() {
1887 let s = vec![1, 2, 3].into_boxed_slice();
1888 let r: Arc<[u32]> = Arc::from(s);
1890 assert_eq!(&r[..], [1, 2, 3]);
1894 fn test_from_box_trait() {
1895 use std::fmt::Display;
1896 use std::string::ToString;
1898 let b: Box<dyn Display> = box 123;
1899 let r: Arc<dyn Display> = Arc::from(b);
1901 assert_eq!(r.to_string(), "123");
1905 fn test_from_box_trait_zero_sized() {
1906 use std::fmt::Debug;
1908 let b: Box<dyn Debug> = box ();
1909 let r: Arc<dyn Debug> = Arc::from(b);
1911 assert_eq!(format!("{:?}", r), "()");
1915 fn test_from_vec() {
1916 let v = vec![1, 2, 3];
1917 let r: Arc<[u32]> = Arc::from(v);
1919 assert_eq!(&r[..], [1, 2, 3]);
1923 fn test_downcast() {
1926 let r1: Arc<dyn Any + Send + Sync> = Arc::new(i32::max_value());
1927 let r2: Arc<dyn Any + Send + Sync> = Arc::new("abc");
1929 assert!(r1.clone().downcast::<u32>().is_err());
1931 let r1i32 = r1.downcast::<i32>();
1932 assert!(r1i32.is_ok());
1933 assert_eq!(r1i32.unwrap(), Arc::new(i32::max_value()));
1935 assert!(r2.clone().downcast::<i32>().is_err());
1937 let r2str = r2.downcast::<&'static str>();
1938 assert!(r2str.is_ok());
1939 assert_eq!(r2str.unwrap(), Arc::new("abc"));
1943 #[stable(feature = "rust1", since = "1.0.0")]
1944 impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
1945 fn borrow(&self) -> &T {
1950 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1951 impl<T: ?Sized> AsRef<T> for Arc<T> {
1952 fn as_ref(&self) -> &T {
1957 #[unstable(feature = "pin", issue = "49150")]
1958 impl<T: ?Sized> Unpin for Arc<T> { }