1#![stable(feature = "rust1", since = "1.0.0")]
2
3//! Thread-safe reference-counting pointers.
4//!
5//! See the [`Arc<T>`][Arc] documentation for more details.
6//!
7//! **Note**: This module is only available on platforms that support atomic
8//! loads and stores of pointers. This may be detected at compile time using
9//! `#[cfg(target_has_atomic = "ptr")]`.
10
11use core::any::Any;
12#[cfg(not(no_global_oom_handling))]
13use core::clone::CloneToUninit;
14use core::clone::UseCloned;
15use core::cmp::Ordering;
16use core::hash::{Hash, Hasher};
17use core::intrinsics::abort;
18#[cfg(not(no_global_oom_handling))]
19use core::iter;
20use core::marker::{PhantomData, Unsize};
21use core::mem::{self, ManuallyDrop, align_of_val_raw};
22use core::num::NonZeroUsize;
23use core::ops::{CoerceUnsized, Deref, DerefPure, DispatchFromDyn, LegacyReceiver};
24use core::panic::{RefUnwindSafe, UnwindSafe};
25use core::pin::{Pin, PinCoerceUnsized};
26use core::ptr::{self, NonNull};
27#[cfg(not(no_global_oom_handling))]
28use core::slice::from_raw_parts_mut;
29use core::sync::atomic;
30use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
31use core::{borrow, fmt, hint};
32
33#[cfg(not(no_global_oom_handling))]
34use crate::alloc::handle_alloc_error;
35use crate::alloc::{AllocError, Allocator, Global, Layout};
36use crate::borrow::{Cow, ToOwned};
37use crate::boxed::Box;
38use crate::rc::is_dangling;
39#[cfg(not(no_global_oom_handling))]
40use crate::string::String;
41#[cfg(not(no_global_oom_handling))]
42use crate::vec::Vec;
43
44/// A soft limit on the amount of references that may be made to an `Arc`.
45///
46/// Going above this limit will abort your program (although not
47/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
48/// Trying to go above it might call a `panic` (if not actually going above it).
49///
50/// This is a global invariant, and also applies when using a compare-exchange loop.
51///
52/// See comment in `Arc::clone`.
53const MAX_REFCOUNT: usize = (isize::MAX) as usize;
54
55/// The error in case either counter reaches above `MAX_REFCOUNT`, and we can `panic` safely.
56const INTERNAL_OVERFLOW_ERROR: &str = "Arc counter overflow";
57
58#[cfg(not(sanitize = "thread"))]
59macro_rules! acquire {
60 ($x:expr) => {
61 atomic::fence(Acquire)
62 };
63}
64
65// ThreadSanitizer does not support memory fences. To avoid false positive
66// reports in Arc / Weak implementation use atomic loads for synchronization
67// instead.
68#[cfg(sanitize = "thread")]
69macro_rules! acquire {
70 ($x:expr) => {
71 $x.load(Acquire)
72 };
73}
74
75/// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
76/// Reference Counted'.
77///
78/// The type `Arc<T>` provides shared ownership of a value of type `T`,
79/// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
80/// a new `Arc` instance, which points to the same allocation on the heap as the
81/// source `Arc`, while increasing a reference count. When the last `Arc`
82/// pointer to a given allocation is destroyed, the value stored in that allocation (often
83/// referred to as "inner value") is also dropped.
84///
85/// Shared references in Rust disallow mutation by default, and `Arc` is no
86/// exception: you cannot generally obtain a mutable reference to something
87/// inside an `Arc`. If you do need to mutate through an `Arc`, you have several options:
88///
89/// 1. Use interior mutability with synchronization primitives like [`Mutex`][mutex],
90/// [`RwLock`][rwlock], or one of the [`Atomic`][atomic] types.
91///
92/// 2. Use clone-on-write semantics with [`Arc::make_mut`] which provides efficient mutation
93/// without requiring interior mutability. This approach clones the data only when
94/// needed (when there are multiple references) and can be more efficient when mutations
95/// are infrequent.
96///
97/// 3. Use [`Arc::get_mut`] when you know your `Arc` is not shared (has a reference count of 1),
98/// which provides direct mutable access to the inner value without any cloning.
99///
100/// ```
101/// use std::sync::Arc;
102///
103/// let mut data = Arc::new(vec![1, 2, 3]);
104///
105/// // This will clone the vector only if there are other references to it
106/// Arc::make_mut(&mut data).push(4);
107///
108/// assert_eq!(*data, vec![1, 2, 3, 4]);
109/// ```
110///
111/// **Note**: This type is only available on platforms that support atomic
112/// loads and stores of pointers, which includes all platforms that support
113/// the `std` crate but not all those which only support [`alloc`](crate).
114/// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
115///
116/// ## Thread Safety
117///
118/// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
119/// counting. This means that it is thread-safe. The disadvantage is that
120/// atomic operations are more expensive than ordinary memory accesses. If you
121/// are not sharing reference-counted allocations between threads, consider using
122/// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
123/// compiler will catch any attempt to send an [`Rc<T>`] between threads.
124/// However, a library might choose `Arc<T>` in order to give library consumers
125/// more flexibility.
126///
127/// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
128/// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
129/// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
130/// first: after all, isn't the point of `Arc<T>` thread safety? The key is
131/// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
132/// data, but it doesn't add thread safety to its data. Consider
133/// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
134/// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
135/// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
136/// non-atomic operations.
137///
138/// In the end, this means that you may need to pair `Arc<T>` with some sort of
139/// [`std::sync`] type, usually [`Mutex<T>`][mutex].
140///
141/// ## Breaking cycles with `Weak`
142///
143/// The [`downgrade`][downgrade] method can be used to create a non-owning
144/// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
145/// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
146/// already been dropped. In other words, `Weak` pointers do not keep the value
147/// inside the allocation alive; however, they *do* keep the allocation
148/// (the backing store for the value) alive.
149///
150/// A cycle between `Arc` pointers will never be deallocated. For this reason,
151/// [`Weak`] is used to break cycles. For example, a tree could have
152/// strong `Arc` pointers from parent nodes to children, and [`Weak`]
153/// pointers from children back to their parents.
154///
155/// # Cloning references
156///
157/// Creating a new reference from an existing reference-counted pointer is done using the
158/// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
159///
160/// ```
161/// use std::sync::Arc;
162/// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
163/// // The two syntaxes below are equivalent.
164/// let a = foo.clone();
165/// let b = Arc::clone(&foo);
166/// // a, b, and foo are all Arcs that point to the same memory location
167/// ```
168///
169/// ## `Deref` behavior
170///
171/// `Arc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
172/// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
173/// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
174/// functions, called using [fully qualified syntax]:
175///
176/// ```
177/// use std::sync::Arc;
178///
179/// let my_arc = Arc::new(());
180/// let my_weak = Arc::downgrade(&my_arc);
181/// ```
182///
183/// `Arc<T>`'s implementations of traits like `Clone` may also be called using
184/// fully qualified syntax. Some people prefer to use fully qualified syntax,
185/// while others prefer using method-call syntax.
186///
187/// ```
188/// use std::sync::Arc;
189///
190/// let arc = Arc::new(());
191/// // Method-call syntax
192/// let arc2 = arc.clone();
193/// // Fully qualified syntax
194/// let arc3 = Arc::clone(&arc);
195/// ```
196///
197/// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
198/// already been dropped.
199///
200/// [`Rc<T>`]: crate::rc::Rc
201/// [clone]: Clone::clone
202/// [mutex]: ../../std/sync/struct.Mutex.html
203/// [rwlock]: ../../std/sync/struct.RwLock.html
204/// [atomic]: core::sync::atomic
205/// [downgrade]: Arc::downgrade
206/// [upgrade]: Weak::upgrade
207/// [RefCell\<T>]: core::cell::RefCell
208/// [`RefCell<T>`]: core::cell::RefCell
209/// [`std::sync`]: ../../std/sync/index.html
210/// [`Arc::clone(&from)`]: Arc::clone
211/// [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
212///
213/// # Examples
214///
215/// Sharing some immutable data between threads:
216///
217/// ```
218/// use std::sync::Arc;
219/// use std::thread;
220///
221/// let five = Arc::new(5);
222///
223/// for _ in 0..10 {
224/// let five = Arc::clone(&five);
225///
226/// thread::spawn(move || {
227/// println!("{five:?}");
228/// });
229/// }
230/// ```
231///
232/// Sharing a mutable [`AtomicUsize`]:
233///
234/// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
235///
236/// ```
237/// use std::sync::Arc;
238/// use std::sync::atomic::{AtomicUsize, Ordering};
239/// use std::thread;
240///
241/// let val = Arc::new(AtomicUsize::new(5));
242///
243/// for _ in 0..10 {
244/// let val = Arc::clone(&val);
245///
246/// thread::spawn(move || {
247/// let v = val.fetch_add(1, Ordering::Relaxed);
248/// println!("{v:?}");
249/// });
250/// }
251/// ```
252///
253/// See the [`rc` documentation][rc_examples] for more examples of reference
254/// counting in general.
255///
256/// [rc_examples]: crate::rc#examples
257#[doc(search_unbox)]
258#[rustc_diagnostic_item = "Arc"]
259#[stable(feature = "rust1", since = "1.0.0")]
260#[rustc_insignificant_dtor]
261pub struct Arc<
262 T: ?Sized,
263 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
264> {
265 ptr: NonNull<ArcInner<T>>,
266 phantom: PhantomData<ArcInner<T>>,
267 alloc: A,
268}
269
270#[stable(feature = "rust1", since = "1.0.0")]
271unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Arc<T, A> {}
272#[stable(feature = "rust1", since = "1.0.0")]
273unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Arc<T, A> {}
274
275#[stable(feature = "catch_unwind", since = "1.9.0")]
276impl<T: RefUnwindSafe + ?Sized, A: Allocator + UnwindSafe> UnwindSafe for Arc<T, A> {}
277
278#[unstable(feature = "coerce_unsized", issue = "18598")]
279impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Arc<U, A>> for Arc<T, A> {}
280
281#[unstable(feature = "dispatch_from_dyn", issue = "none")]
282impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
283
284impl<T: ?Sized> Arc<T> {
285 unsafe fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
286 unsafe { Self::from_inner_in(ptr, alloc:Global) }
287 }
288
289 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
290 unsafe { Self::from_ptr_in(ptr, alloc:Global) }
291 }
292}
293
294impl<T: ?Sized, A: Allocator> Arc<T, A> {
295 #[inline]
296 fn into_inner_with_allocator(this: Self) -> (NonNull<ArcInner<T>>, A) {
297 let this: ManuallyDrop> = mem::ManuallyDrop::new(this);
298 (this.ptr, unsafe { ptr::read(&this.alloc) })
299 }
300
301 #[inline]
302 unsafe fn from_inner_in(ptr: NonNull<ArcInner<T>>, alloc: A) -> Self {
303 Self { ptr, phantom: PhantomData, alloc }
304 }
305
306 #[inline]
307 unsafe fn from_ptr_in(ptr: *mut ArcInner<T>, alloc: A) -> Self {
308 unsafe { Self::from_inner_in(ptr:NonNull::new_unchecked(ptr), alloc) }
309 }
310}
311
312/// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
313/// managed allocation.
314///
315/// The allocation is accessed by calling [`upgrade`] on the `Weak`
316/// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
317///
318/// Since a `Weak` reference does not count towards ownership, it will not
319/// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
320/// guarantees about the value still being present. Thus it may return [`None`]
321/// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
322/// itself (the backing store) from being deallocated.
323///
324/// A `Weak` pointer is useful for keeping a temporary reference to the allocation
325/// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
326/// prevent circular references between [`Arc`] pointers, since mutual owning references
327/// would never allow either [`Arc`] to be dropped. For example, a tree could
328/// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
329/// pointers from children back to their parents.
330///
331/// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
332///
333/// [`upgrade`]: Weak::upgrade
334#[stable(feature = "arc_weak", since = "1.4.0")]
335#[rustc_diagnostic_item = "ArcWeak"]
336pub struct Weak<
337 T: ?Sized,
338 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
339> {
340 // This is a `NonNull` to allow optimizing the size of this type in enums,
341 // but it is not necessarily a valid pointer.
342 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
343 // to allocate space on the heap. That's not a value a real pointer
344 // will ever have because RcInner has alignment at least 2.
345 // This is only possible when `T: Sized`; unsized `T` never dangle.
346 ptr: NonNull<ArcInner<T>>,
347 alloc: A,
348}
349
350#[stable(feature = "arc_weak", since = "1.4.0")]
351unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Weak<T, A> {}
352#[stable(feature = "arc_weak", since = "1.4.0")]
353unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Weak<T, A> {}
354
355#[unstable(feature = "coerce_unsized", issue = "18598")]
356impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Weak<U, A>> for Weak<T, A> {}
357#[unstable(feature = "dispatch_from_dyn", issue = "none")]
358impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
359
360#[stable(feature = "arc_weak", since = "1.4.0")]
361impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
362 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
363 write!(f, "(Weak)")
364 }
365}
366
367// This is repr(C) to future-proof against possible field-reordering, which
368// would interfere with otherwise safe [into|from]_raw() of transmutable
369// inner types.
370#[repr(C)]
371struct ArcInner<T: ?Sized> {
372 strong: atomic::AtomicUsize,
373
374 // the value usize::MAX acts as a sentinel for temporarily "locking" the
375 // ability to upgrade weak pointers or downgrade strong ones; this is used
376 // to avoid races in `make_mut` and `get_mut`.
377 weak: atomic::AtomicUsize,
378
379 data: T,
380}
381
382/// Calculate layout for `ArcInner<T>` using the inner value's layout
383fn arcinner_layout_for_value_layout(layout: Layout) -> Layout {
384 // Calculate layout using the given value layout.
385 // Previously, layout was calculated on the expression
386 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
387 // reference (see #54908).
388 Layout::new::<ArcInner<()>>().extend(next:layout).unwrap().0.pad_to_align()
389}
390
391unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
392unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
393
394impl<T> Arc<T> {
395 /// Constructs a new `Arc<T>`.
396 ///
397 /// # Examples
398 ///
399 /// ```
400 /// use std::sync::Arc;
401 ///
402 /// let five = Arc::new(5);
403 /// ```
404 #[cfg(not(no_global_oom_handling))]
405 #[inline]
406 #[stable(feature = "rust1", since = "1.0.0")]
407 pub fn new(data: T) -> Arc<T> {
408 // Start the weak pointer count as 1 which is the weak pointer that's
409 // held by all the strong pointers (kinda), see std/rc.rs for more info
410 let x: Box<_> = Box::new(ArcInner {
411 strong: atomic::AtomicUsize::new(1),
412 weak: atomic::AtomicUsize::new(1),
413 data,
414 });
415 unsafe { Self::from_inner(Box::leak(x).into()) }
416 }
417
418 /// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
419 /// to allow you to construct a `T` which holds a weak pointer to itself.
420 ///
421 /// Generally, a structure circularly referencing itself, either directly or
422 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
423 /// Using this function, you get access to the weak pointer during the
424 /// initialization of `T`, before the `Arc<T>` is created, such that you can
425 /// clone and store it inside the `T`.
426 ///
427 /// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
428 /// then calls your closure, giving it a `Weak<T>` to this allocation,
429 /// and only afterwards completes the construction of the `Arc<T>` by placing
430 /// the `T` returned from your closure into the allocation.
431 ///
432 /// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
433 /// returns, calling [`upgrade`] on the weak reference inside your closure will
434 /// fail and result in a `None` value.
435 ///
436 /// # Panics
437 ///
438 /// If `data_fn` panics, the panic is propagated to the caller, and the
439 /// temporary [`Weak<T>`] is dropped normally.
440 ///
441 /// # Example
442 ///
443 /// ```
444 /// # #![allow(dead_code)]
445 /// use std::sync::{Arc, Weak};
446 ///
447 /// struct Gadget {
448 /// me: Weak<Gadget>,
449 /// }
450 ///
451 /// impl Gadget {
452 /// /// Constructs a reference counted Gadget.
453 /// fn new() -> Arc<Self> {
454 /// // `me` is a `Weak<Gadget>` pointing at the new allocation of the
455 /// // `Arc` we're constructing.
456 /// Arc::new_cyclic(|me| {
457 /// // Create the actual struct here.
458 /// Gadget { me: me.clone() }
459 /// })
460 /// }
461 ///
462 /// /// Returns a reference counted pointer to Self.
463 /// fn me(&self) -> Arc<Self> {
464 /// self.me.upgrade().unwrap()
465 /// }
466 /// }
467 /// ```
468 /// [`upgrade`]: Weak::upgrade
469 #[cfg(not(no_global_oom_handling))]
470 #[inline]
471 #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
472 pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
473 where
474 F: FnOnce(&Weak<T>) -> T,
475 {
476 Self::new_cyclic_in(data_fn, Global)
477 }
478
479 /// Constructs a new `Arc` with uninitialized contents.
480 ///
481 /// # Examples
482 ///
483 /// ```
484 /// #![feature(get_mut_unchecked)]
485 ///
486 /// use std::sync::Arc;
487 ///
488 /// let mut five = Arc::<u32>::new_uninit();
489 ///
490 /// // Deferred initialization:
491 /// Arc::get_mut(&mut five).unwrap().write(5);
492 ///
493 /// let five = unsafe { five.assume_init() };
494 ///
495 /// assert_eq!(*five, 5)
496 /// ```
497 #[cfg(not(no_global_oom_handling))]
498 #[inline]
499 #[stable(feature = "new_uninit", since = "1.82.0")]
500 #[must_use]
501 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
502 unsafe {
503 Arc::from_ptr(Arc::allocate_for_layout(
504 Layout::new::<T>(),
505 |layout| Global.allocate(layout),
506 <*mut u8>::cast,
507 ))
508 }
509 }
510
511 /// Constructs a new `Arc` with uninitialized contents, with the memory
512 /// being filled with `0` bytes.
513 ///
514 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
515 /// of this method.
516 ///
517 /// # Examples
518 ///
519 /// ```
520 /// #![feature(new_zeroed_alloc)]
521 ///
522 /// use std::sync::Arc;
523 ///
524 /// let zero = Arc::<u32>::new_zeroed();
525 /// let zero = unsafe { zero.assume_init() };
526 ///
527 /// assert_eq!(*zero, 0)
528 /// ```
529 ///
530 /// [zeroed]: mem::MaybeUninit::zeroed
531 #[cfg(not(no_global_oom_handling))]
532 #[inline]
533 #[unstable(feature = "new_zeroed_alloc", issue = "129396")]
534 #[must_use]
535 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
536 unsafe {
537 Arc::from_ptr(Arc::allocate_for_layout(
538 Layout::new::<T>(),
539 |layout| Global.allocate_zeroed(layout),
540 <*mut u8>::cast,
541 ))
542 }
543 }
544
545 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
546 /// `data` will be pinned in memory and unable to be moved.
547 #[cfg(not(no_global_oom_handling))]
548 #[stable(feature = "pin", since = "1.33.0")]
549 #[must_use]
550 pub fn pin(data: T) -> Pin<Arc<T>> {
551 unsafe { Pin::new_unchecked(Arc::new(data)) }
552 }
553
554 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
555 #[unstable(feature = "allocator_api", issue = "32838")]
556 #[inline]
557 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
558 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
559 }
560
561 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
562 ///
563 /// # Examples
564 ///
565 /// ```
566 /// #![feature(allocator_api)]
567 /// use std::sync::Arc;
568 ///
569 /// let five = Arc::try_new(5)?;
570 /// # Ok::<(), std::alloc::AllocError>(())
571 /// ```
572 #[unstable(feature = "allocator_api", issue = "32838")]
573 #[inline]
574 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
575 // Start the weak pointer count as 1 which is the weak pointer that's
576 // held by all the strong pointers (kinda), see std/rc.rs for more info
577 let x: Box<_> = Box::try_new(ArcInner {
578 strong: atomic::AtomicUsize::new(1),
579 weak: atomic::AtomicUsize::new(1),
580 data,
581 })?;
582 unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
583 }
584
585 /// Constructs a new `Arc` with uninitialized contents, returning an error
586 /// if allocation fails.
587 ///
588 /// # Examples
589 ///
590 /// ```
591 /// #![feature(allocator_api)]
592 /// #![feature(get_mut_unchecked)]
593 ///
594 /// use std::sync::Arc;
595 ///
596 /// let mut five = Arc::<u32>::try_new_uninit()?;
597 ///
598 /// // Deferred initialization:
599 /// Arc::get_mut(&mut five).unwrap().write(5);
600 ///
601 /// let five = unsafe { five.assume_init() };
602 ///
603 /// assert_eq!(*five, 5);
604 /// # Ok::<(), std::alloc::AllocError>(())
605 /// ```
606 #[unstable(feature = "allocator_api", issue = "32838")]
607 // #[unstable(feature = "new_uninit", issue = "63291")]
608 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
609 unsafe {
610 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
611 Layout::new::<T>(),
612 |layout| Global.allocate(layout),
613 <*mut u8>::cast,
614 )?))
615 }
616 }
617
618 /// Constructs a new `Arc` with uninitialized contents, with the memory
619 /// being filled with `0` bytes, returning an error if allocation fails.
620 ///
621 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
622 /// of this method.
623 ///
624 /// # Examples
625 ///
626 /// ```
627 /// #![feature( allocator_api)]
628 ///
629 /// use std::sync::Arc;
630 ///
631 /// let zero = Arc::<u32>::try_new_zeroed()?;
632 /// let zero = unsafe { zero.assume_init() };
633 ///
634 /// assert_eq!(*zero, 0);
635 /// # Ok::<(), std::alloc::AllocError>(())
636 /// ```
637 ///
638 /// [zeroed]: mem::MaybeUninit::zeroed
639 #[unstable(feature = "allocator_api", issue = "32838")]
640 // #[unstable(feature = "new_uninit", issue = "63291")]
641 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
642 unsafe {
643 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
644 Layout::new::<T>(),
645 |layout| Global.allocate_zeroed(layout),
646 <*mut u8>::cast,
647 )?))
648 }
649 }
650}
651
652impl<T, A: Allocator> Arc<T, A> {
653 /// Constructs a new `Arc<T>` in the provided allocator.
654 ///
655 /// # Examples
656 ///
657 /// ```
658 /// #![feature(allocator_api)]
659 ///
660 /// use std::sync::Arc;
661 /// use std::alloc::System;
662 ///
663 /// let five = Arc::new_in(5, System);
664 /// ```
665 #[inline]
666 #[cfg(not(no_global_oom_handling))]
667 #[unstable(feature = "allocator_api", issue = "32838")]
668 pub fn new_in(data: T, alloc: A) -> Arc<T, A> {
669 // Start the weak pointer count as 1 which is the weak pointer that's
670 // held by all the strong pointers (kinda), see std/rc.rs for more info
671 let x = Box::new_in(
672 ArcInner {
673 strong: atomic::AtomicUsize::new(1),
674 weak: atomic::AtomicUsize::new(1),
675 data,
676 },
677 alloc,
678 );
679 let (ptr, alloc) = Box::into_unique(x);
680 unsafe { Self::from_inner_in(ptr.into(), alloc) }
681 }
682
683 /// Constructs a new `Arc` with uninitialized contents in the provided allocator.
684 ///
685 /// # Examples
686 ///
687 /// ```
688 /// #![feature(get_mut_unchecked)]
689 /// #![feature(allocator_api)]
690 ///
691 /// use std::sync::Arc;
692 /// use std::alloc::System;
693 ///
694 /// let mut five = Arc::<u32, _>::new_uninit_in(System);
695 ///
696 /// let five = unsafe {
697 /// // Deferred initialization:
698 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
699 ///
700 /// five.assume_init()
701 /// };
702 ///
703 /// assert_eq!(*five, 5)
704 /// ```
705 #[cfg(not(no_global_oom_handling))]
706 #[unstable(feature = "allocator_api", issue = "32838")]
707 // #[unstable(feature = "new_uninit", issue = "63291")]
708 #[inline]
709 pub fn new_uninit_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A> {
710 unsafe {
711 Arc::from_ptr_in(
712 Arc::allocate_for_layout(
713 Layout::new::<T>(),
714 |layout| alloc.allocate(layout),
715 <*mut u8>::cast,
716 ),
717 alloc,
718 )
719 }
720 }
721
722 /// Constructs a new `Arc` with uninitialized contents, with the memory
723 /// being filled with `0` bytes, in the provided allocator.
724 ///
725 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
726 /// of this method.
727 ///
728 /// # Examples
729 ///
730 /// ```
731 /// #![feature(allocator_api)]
732 ///
733 /// use std::sync::Arc;
734 /// use std::alloc::System;
735 ///
736 /// let zero = Arc::<u32, _>::new_zeroed_in(System);
737 /// let zero = unsafe { zero.assume_init() };
738 ///
739 /// assert_eq!(*zero, 0)
740 /// ```
741 ///
742 /// [zeroed]: mem::MaybeUninit::zeroed
743 #[cfg(not(no_global_oom_handling))]
744 #[unstable(feature = "allocator_api", issue = "32838")]
745 // #[unstable(feature = "new_uninit", issue = "63291")]
746 #[inline]
747 pub fn new_zeroed_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A> {
748 unsafe {
749 Arc::from_ptr_in(
750 Arc::allocate_for_layout(
751 Layout::new::<T>(),
752 |layout| alloc.allocate_zeroed(layout),
753 <*mut u8>::cast,
754 ),
755 alloc,
756 )
757 }
758 }
759
760 /// Constructs a new `Arc<T, A>` in the given allocator while giving you a `Weak<T, A>` to the allocation,
761 /// to allow you to construct a `T` which holds a weak pointer to itself.
762 ///
763 /// Generally, a structure circularly referencing itself, either directly or
764 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
765 /// Using this function, you get access to the weak pointer during the
766 /// initialization of `T`, before the `Arc<T, A>` is created, such that you can
767 /// clone and store it inside the `T`.
768 ///
769 /// `new_cyclic_in` first allocates the managed allocation for the `Arc<T, A>`,
770 /// then calls your closure, giving it a `Weak<T, A>` to this allocation,
771 /// and only afterwards completes the construction of the `Arc<T, A>` by placing
772 /// the `T` returned from your closure into the allocation.
773 ///
774 /// Since the new `Arc<T, A>` is not fully-constructed until `Arc<T, A>::new_cyclic_in`
775 /// returns, calling [`upgrade`] on the weak reference inside your closure will
776 /// fail and result in a `None` value.
777 ///
778 /// # Panics
779 ///
780 /// If `data_fn` panics, the panic is propagated to the caller, and the
781 /// temporary [`Weak<T>`] is dropped normally.
782 ///
783 /// # Example
784 ///
785 /// See [`new_cyclic`]
786 ///
787 /// [`new_cyclic`]: Arc::new_cyclic
788 /// [`upgrade`]: Weak::upgrade
789 #[cfg(not(no_global_oom_handling))]
790 #[inline]
791 #[unstable(feature = "allocator_api", issue = "32838")]
792 pub fn new_cyclic_in<F>(data_fn: F, alloc: A) -> Arc<T, A>
793 where
794 F: FnOnce(&Weak<T, A>) -> T,
795 {
796 // Construct the inner in the "uninitialized" state with a single
797 // weak reference.
798 let (uninit_raw_ptr, alloc) = Box::into_raw_with_allocator(Box::new_in(
799 ArcInner {
800 strong: atomic::AtomicUsize::new(0),
801 weak: atomic::AtomicUsize::new(1),
802 data: mem::MaybeUninit::<T>::uninit(),
803 },
804 alloc,
805 ));
806 let uninit_ptr: NonNull<_> = (unsafe { &mut *uninit_raw_ptr }).into();
807 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
808
809 let weak = Weak { ptr: init_ptr, alloc };
810
811 // It's important we don't give up ownership of the weak pointer, or
812 // else the memory might be freed by the time `data_fn` returns. If
813 // we really wanted to pass ownership, we could create an additional
814 // weak pointer for ourselves, but this would result in additional
815 // updates to the weak reference count which might not be necessary
816 // otherwise.
817 let data = data_fn(&weak);
818
819 // Now we can properly initialize the inner value and turn our weak
820 // reference into a strong reference.
821 let strong = unsafe {
822 let inner = init_ptr.as_ptr();
823 ptr::write(&raw mut (*inner).data, data);
824
825 // The above write to the data field must be visible to any threads which
826 // observe a non-zero strong count. Therefore we need at least "Release" ordering
827 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
828 //
829 // "Acquire" ordering is not required. When considering the possible behaviors
830 // of `data_fn` we only need to look at what it could do with a reference to a
831 // non-upgradeable `Weak`:
832 // - It can *clone* the `Weak`, increasing the weak reference count.
833 // - It can drop those clones, decreasing the weak reference count (but never to zero).
834 //
835 // These side effects do not impact us in any way, and no other side effects are
836 // possible with safe code alone.
837 let prev_value = (*inner).strong.fetch_add(1, Release);
838 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
839
840 // Strong references should collectively own a shared weak reference,
841 // so don't run the destructor for our old weak reference.
842 // Calling into_raw_with_allocator has the double effect of giving us back the allocator,
843 // and forgetting the weak reference.
844 let alloc = weak.into_raw_with_allocator().1;
845
846 Arc::from_inner_in(init_ptr, alloc)
847 };
848
849 strong
850 }
851
852 /// Constructs a new `Pin<Arc<T, A>>` in the provided allocator. If `T` does not implement `Unpin`,
853 /// then `data` will be pinned in memory and unable to be moved.
854 #[cfg(not(no_global_oom_handling))]
855 #[unstable(feature = "allocator_api", issue = "32838")]
856 #[inline]
857 pub fn pin_in(data: T, alloc: A) -> Pin<Arc<T, A>>
858 where
859 A: 'static,
860 {
861 unsafe { Pin::new_unchecked(Arc::new_in(data, alloc)) }
862 }
863
864 /// Constructs a new `Pin<Arc<T, A>>` in the provided allocator, return an error if allocation
865 /// fails.
866 #[inline]
867 #[unstable(feature = "allocator_api", issue = "32838")]
868 pub fn try_pin_in(data: T, alloc: A) -> Result<Pin<Arc<T, A>>, AllocError>
869 where
870 A: 'static,
871 {
872 unsafe { Ok(Pin::new_unchecked(Arc::try_new_in(data, alloc)?)) }
873 }
874
875 /// Constructs a new `Arc<T, A>` in the provided allocator, returning an error if allocation fails.
876 ///
877 /// # Examples
878 ///
879 /// ```
880 /// #![feature(allocator_api)]
881 ///
882 /// use std::sync::Arc;
883 /// use std::alloc::System;
884 ///
885 /// let five = Arc::try_new_in(5, System)?;
886 /// # Ok::<(), std::alloc::AllocError>(())
887 /// ```
888 #[inline]
889 #[unstable(feature = "allocator_api", issue = "32838")]
890 #[inline]
891 pub fn try_new_in(data: T, alloc: A) -> Result<Arc<T, A>, AllocError> {
892 // Start the weak pointer count as 1 which is the weak pointer that's
893 // held by all the strong pointers (kinda), see std/rc.rs for more info
894 let x = Box::try_new_in(
895 ArcInner {
896 strong: atomic::AtomicUsize::new(1),
897 weak: atomic::AtomicUsize::new(1),
898 data,
899 },
900 alloc,
901 )?;
902 let (ptr, alloc) = Box::into_unique(x);
903 Ok(unsafe { Self::from_inner_in(ptr.into(), alloc) })
904 }
905
906 /// Constructs a new `Arc` with uninitialized contents, in the provided allocator, returning an
907 /// error if allocation fails.
908 ///
909 /// # Examples
910 ///
911 /// ```
912 /// #![feature(allocator_api)]
913 /// #![feature(get_mut_unchecked)]
914 ///
915 /// use std::sync::Arc;
916 /// use std::alloc::System;
917 ///
918 /// let mut five = Arc::<u32, _>::try_new_uninit_in(System)?;
919 ///
920 /// let five = unsafe {
921 /// // Deferred initialization:
922 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
923 ///
924 /// five.assume_init()
925 /// };
926 ///
927 /// assert_eq!(*five, 5);
928 /// # Ok::<(), std::alloc::AllocError>(())
929 /// ```
930 #[unstable(feature = "allocator_api", issue = "32838")]
931 // #[unstable(feature = "new_uninit", issue = "63291")]
932 #[inline]
933 pub fn try_new_uninit_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError> {
934 unsafe {
935 Ok(Arc::from_ptr_in(
936 Arc::try_allocate_for_layout(
937 Layout::new::<T>(),
938 |layout| alloc.allocate(layout),
939 <*mut u8>::cast,
940 )?,
941 alloc,
942 ))
943 }
944 }
945
946 /// Constructs a new `Arc` with uninitialized contents, with the memory
947 /// being filled with `0` bytes, in the provided allocator, returning an error if allocation
948 /// fails.
949 ///
950 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
951 /// of this method.
952 ///
953 /// # Examples
954 ///
955 /// ```
956 /// #![feature(allocator_api)]
957 ///
958 /// use std::sync::Arc;
959 /// use std::alloc::System;
960 ///
961 /// let zero = Arc::<u32, _>::try_new_zeroed_in(System)?;
962 /// let zero = unsafe { zero.assume_init() };
963 ///
964 /// assert_eq!(*zero, 0);
965 /// # Ok::<(), std::alloc::AllocError>(())
966 /// ```
967 ///
968 /// [zeroed]: mem::MaybeUninit::zeroed
969 #[unstable(feature = "allocator_api", issue = "32838")]
970 // #[unstable(feature = "new_uninit", issue = "63291")]
971 #[inline]
972 pub fn try_new_zeroed_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError> {
973 unsafe {
974 Ok(Arc::from_ptr_in(
975 Arc::try_allocate_for_layout(
976 Layout::new::<T>(),
977 |layout| alloc.allocate_zeroed(layout),
978 <*mut u8>::cast,
979 )?,
980 alloc,
981 ))
982 }
983 }
984 /// Returns the inner value, if the `Arc` has exactly one strong reference.
985 ///
986 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
987 /// passed in.
988 ///
989 /// This will succeed even if there are outstanding weak references.
990 ///
991 /// It is strongly recommended to use [`Arc::into_inner`] instead if you don't
992 /// keep the `Arc` in the [`Err`] case.
993 /// Immediately dropping the [`Err`]-value, as the expression
994 /// `Arc::try_unwrap(this).ok()` does, can cause the strong count to
995 /// drop to zero and the inner value of the `Arc` to be dropped.
996 /// For instance, if two threads execute such an expression in parallel,
997 /// there is a race condition without the possibility of unsafety:
998 /// The threads could first both check whether they own the last instance
999 /// in `Arc::try_unwrap`, determine that they both do not, and then both
1000 /// discard and drop their instance in the call to [`ok`][`Result::ok`].
1001 /// In this scenario, the value inside the `Arc` is safely destroyed
1002 /// by exactly one of the threads, but neither thread will ever be able
1003 /// to use the value.
1004 ///
1005 /// # Examples
1006 ///
1007 /// ```
1008 /// use std::sync::Arc;
1009 ///
1010 /// let x = Arc::new(3);
1011 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
1012 ///
1013 /// let x = Arc::new(4);
1014 /// let _y = Arc::clone(&x);
1015 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
1016 /// ```
1017 #[inline]
1018 #[stable(feature = "arc_unique", since = "1.4.0")]
1019 pub fn try_unwrap(this: Self) -> Result<T, Self> {
1020 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
1021 return Err(this);
1022 }
1023
1024 acquire!(this.inner().strong);
1025
1026 let this = ManuallyDrop::new(this);
1027 let elem: T = unsafe { ptr::read(&this.ptr.as_ref().data) };
1028 let alloc: A = unsafe { ptr::read(&this.alloc) }; // copy the allocator
1029
1030 // Make a weak pointer to clean up the implicit strong-weak reference
1031 let _weak = Weak { ptr: this.ptr, alloc };
1032
1033 Ok(elem)
1034 }
1035
1036 /// Returns the inner value, if the `Arc` has exactly one strong reference.
1037 ///
1038 /// Otherwise, [`None`] is returned and the `Arc` is dropped.
1039 ///
1040 /// This will succeed even if there are outstanding weak references.
1041 ///
1042 /// If `Arc::into_inner` is called on every clone of this `Arc`,
1043 /// it is guaranteed that exactly one of the calls returns the inner value.
1044 /// This means in particular that the inner value is not dropped.
1045 ///
1046 /// [`Arc::try_unwrap`] is conceptually similar to `Arc::into_inner`, but it
1047 /// is meant for different use-cases. If used as a direct replacement
1048 /// for `Arc::into_inner` anyway, such as with the expression
1049 /// <code>[Arc::try_unwrap]\(this).[ok][Result::ok]()</code>, then it does
1050 /// **not** give the same guarantee as described in the previous paragraph.
1051 /// For more information, see the examples below and read the documentation
1052 /// of [`Arc::try_unwrap`].
1053 ///
1054 /// # Examples
1055 ///
1056 /// Minimal example demonstrating the guarantee that `Arc::into_inner` gives.
1057 /// ```
1058 /// use std::sync::Arc;
1059 ///
1060 /// let x = Arc::new(3);
1061 /// let y = Arc::clone(&x);
1062 ///
1063 /// // Two threads calling `Arc::into_inner` on both clones of an `Arc`:
1064 /// let x_thread = std::thread::spawn(|| Arc::into_inner(x));
1065 /// let y_thread = std::thread::spawn(|| Arc::into_inner(y));
1066 ///
1067 /// let x_inner_value = x_thread.join().unwrap();
1068 /// let y_inner_value = y_thread.join().unwrap();
1069 ///
1070 /// // One of the threads is guaranteed to receive the inner value:
1071 /// assert!(matches!(
1072 /// (x_inner_value, y_inner_value),
1073 /// (None, Some(3)) | (Some(3), None)
1074 /// ));
1075 /// // The result could also be `(None, None)` if the threads called
1076 /// // `Arc::try_unwrap(x).ok()` and `Arc::try_unwrap(y).ok()` instead.
1077 /// ```
1078 ///
1079 /// A more practical example demonstrating the need for `Arc::into_inner`:
1080 /// ```
1081 /// use std::sync::Arc;
1082 ///
1083 /// // Definition of a simple singly linked list using `Arc`:
1084 /// #[derive(Clone)]
1085 /// struct LinkedList<T>(Option<Arc<Node<T>>>);
1086 /// struct Node<T>(T, Option<Arc<Node<T>>>);
1087 ///
1088 /// // Dropping a long `LinkedList<T>` relying on the destructor of `Arc`
1089 /// // can cause a stack overflow. To prevent this, we can provide a
1090 /// // manual `Drop` implementation that does the destruction in a loop:
1091 /// impl<T> Drop for LinkedList<T> {
1092 /// fn drop(&mut self) {
1093 /// let mut link = self.0.take();
1094 /// while let Some(arc_node) = link.take() {
1095 /// if let Some(Node(_value, next)) = Arc::into_inner(arc_node) {
1096 /// link = next;
1097 /// }
1098 /// }
1099 /// }
1100 /// }
1101 ///
1102 /// // Implementation of `new` and `push` omitted
1103 /// impl<T> LinkedList<T> {
1104 /// /* ... */
1105 /// # fn new() -> Self {
1106 /// # LinkedList(None)
1107 /// # }
1108 /// # fn push(&mut self, x: T) {
1109 /// # self.0 = Some(Arc::new(Node(x, self.0.take())));
1110 /// # }
1111 /// }
1112 ///
1113 /// // The following code could have still caused a stack overflow
1114 /// // despite the manual `Drop` impl if that `Drop` impl had used
1115 /// // `Arc::try_unwrap(arc).ok()` instead of `Arc::into_inner(arc)`.
1116 ///
1117 /// // Create a long list and clone it
1118 /// let mut x = LinkedList::new();
1119 /// let size = 100000;
1120 /// # let size = if cfg!(miri) { 100 } else { size };
1121 /// for i in 0..size {
1122 /// x.push(i); // Adds i to the front of x
1123 /// }
1124 /// let y = x.clone();
1125 ///
1126 /// // Drop the clones in parallel
1127 /// let x_thread = std::thread::spawn(|| drop(x));
1128 /// let y_thread = std::thread::spawn(|| drop(y));
1129 /// x_thread.join().unwrap();
1130 /// y_thread.join().unwrap();
1131 /// ```
1132 #[inline]
1133 #[stable(feature = "arc_into_inner", since = "1.70.0")]
1134 pub fn into_inner(this: Self) -> Option<T> {
1135 // Make sure that the ordinary `Drop` implementation isn’t called as well
1136 let mut this = mem::ManuallyDrop::new(this);
1137
1138 // Following the implementation of `drop` and `drop_slow`
1139 if this.inner().strong.fetch_sub(1, Release) != 1 {
1140 return None;
1141 }
1142
1143 acquire!(this.inner().strong);
1144
1145 // SAFETY: This mirrors the line
1146 //
1147 // unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1148 //
1149 // in `drop_slow`. Instead of dropping the value behind the pointer,
1150 // it is read and eventually returned; `ptr::read` has the same
1151 // safety conditions as `ptr::drop_in_place`.
1152
1153 let inner = unsafe { ptr::read(Self::get_mut_unchecked(&mut this)) };
1154 let alloc = unsafe { ptr::read(&this.alloc) };
1155
1156 drop(Weak { ptr: this.ptr, alloc });
1157
1158 Some(inner)
1159 }
1160}
1161
1162impl<T> Arc<[T]> {
1163 /// Constructs a new atomically reference-counted slice with uninitialized contents.
1164 ///
1165 /// # Examples
1166 ///
1167 /// ```
1168 /// #![feature(get_mut_unchecked)]
1169 ///
1170 /// use std::sync::Arc;
1171 ///
1172 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
1173 ///
1174 /// // Deferred initialization:
1175 /// let data = Arc::get_mut(&mut values).unwrap();
1176 /// data[0].write(1);
1177 /// data[1].write(2);
1178 /// data[2].write(3);
1179 ///
1180 /// let values = unsafe { values.assume_init() };
1181 ///
1182 /// assert_eq!(*values, [1, 2, 3])
1183 /// ```
1184 #[cfg(not(no_global_oom_handling))]
1185 #[inline]
1186 #[stable(feature = "new_uninit", since = "1.82.0")]
1187 #[must_use]
1188 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
1189 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
1190 }
1191
1192 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
1193 /// filled with `0` bytes.
1194 ///
1195 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
1196 /// incorrect usage of this method.
1197 ///
1198 /// # Examples
1199 ///
1200 /// ```
1201 /// #![feature(new_zeroed_alloc)]
1202 ///
1203 /// use std::sync::Arc;
1204 ///
1205 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
1206 /// let values = unsafe { values.assume_init() };
1207 ///
1208 /// assert_eq!(*values, [0, 0, 0])
1209 /// ```
1210 ///
1211 /// [zeroed]: mem::MaybeUninit::zeroed
1212 #[cfg(not(no_global_oom_handling))]
1213 #[inline]
1214 #[unstable(feature = "new_zeroed_alloc", issue = "129396")]
1215 #[must_use]
1216 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
1217 unsafe {
1218 Arc::from_ptr(Arc::allocate_for_layout(
1219 Layout::array::<T>(len).unwrap(),
1220 |layout| Global.allocate_zeroed(layout),
1221 |mem| {
1222 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
1223 as *mut ArcInner<[mem::MaybeUninit<T>]>
1224 },
1225 ))
1226 }
1227 }
1228
1229 /// Converts the reference-counted slice into a reference-counted array.
1230 ///
1231 /// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.
1232 ///
1233 /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
1234 #[unstable(feature = "slice_as_array", issue = "133508")]
1235 #[inline]
1236 #[must_use]
1237 pub fn into_array<const N: usize>(self) -> Option<Arc<[T; N]>> {
1238 if self.len() == N {
1239 let ptr = Self::into_raw(self) as *const [T; N];
1240
1241 // SAFETY: The underlying array of a slice has the exact same layout as an actual array `[T; N]` if `N` is equal to the slice's length.
1242 let me = unsafe { Arc::from_raw(ptr) };
1243 Some(me)
1244 } else {
1245 None
1246 }
1247 }
1248}
1249
1250impl<T, A: Allocator> Arc<[T], A> {
1251 /// Constructs a new atomically reference-counted slice with uninitialized contents in the
1252 /// provided allocator.
1253 ///
1254 /// # Examples
1255 ///
1256 /// ```
1257 /// #![feature(get_mut_unchecked)]
1258 /// #![feature(allocator_api)]
1259 ///
1260 /// use std::sync::Arc;
1261 /// use std::alloc::System;
1262 ///
1263 /// let mut values = Arc::<[u32], _>::new_uninit_slice_in(3, System);
1264 ///
1265 /// let values = unsafe {
1266 /// // Deferred initialization:
1267 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
1268 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
1269 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
1270 ///
1271 /// values.assume_init()
1272 /// };
1273 ///
1274 /// assert_eq!(*values, [1, 2, 3])
1275 /// ```
1276 #[cfg(not(no_global_oom_handling))]
1277 #[unstable(feature = "allocator_api", issue = "32838")]
1278 #[inline]
1279 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A> {
1280 unsafe { Arc::from_ptr_in(Arc::allocate_for_slice_in(len, &alloc), alloc) }
1281 }
1282
1283 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
1284 /// filled with `0` bytes, in the provided allocator.
1285 ///
1286 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
1287 /// incorrect usage of this method.
1288 ///
1289 /// # Examples
1290 ///
1291 /// ```
1292 /// #![feature(allocator_api)]
1293 ///
1294 /// use std::sync::Arc;
1295 /// use std::alloc::System;
1296 ///
1297 /// let values = Arc::<[u32], _>::new_zeroed_slice_in(3, System);
1298 /// let values = unsafe { values.assume_init() };
1299 ///
1300 /// assert_eq!(*values, [0, 0, 0])
1301 /// ```
1302 ///
1303 /// [zeroed]: mem::MaybeUninit::zeroed
1304 #[cfg(not(no_global_oom_handling))]
1305 #[unstable(feature = "allocator_api", issue = "32838")]
1306 #[inline]
1307 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A> {
1308 unsafe {
1309 Arc::from_ptr_in(
1310 Arc::allocate_for_layout(
1311 Layout::array::<T>(len).unwrap(),
1312 |layout| alloc.allocate_zeroed(layout),
1313 |mem| {
1314 ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len)
1315 as *mut ArcInner<[mem::MaybeUninit<T>]>
1316 },
1317 ),
1318 alloc,
1319 )
1320 }
1321 }
1322}
1323
1324impl<T, A: Allocator> Arc<mem::MaybeUninit<T>, A> {
1325 /// Converts to `Arc<T>`.
1326 ///
1327 /// # Safety
1328 ///
1329 /// As with [`MaybeUninit::assume_init`],
1330 /// it is up to the caller to guarantee that the inner value
1331 /// really is in an initialized state.
1332 /// Calling this when the content is not yet fully initialized
1333 /// causes immediate undefined behavior.
1334 ///
1335 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
1336 ///
1337 /// # Examples
1338 ///
1339 /// ```
1340 /// #![feature(get_mut_unchecked)]
1341 ///
1342 /// use std::sync::Arc;
1343 ///
1344 /// let mut five = Arc::<u32>::new_uninit();
1345 ///
1346 /// // Deferred initialization:
1347 /// Arc::get_mut(&mut five).unwrap().write(5);
1348 ///
1349 /// let five = unsafe { five.assume_init() };
1350 ///
1351 /// assert_eq!(*five, 5)
1352 /// ```
1353 #[stable(feature = "new_uninit", since = "1.82.0")]
1354 #[must_use = "`self` will be dropped if the result is not used"]
1355 #[inline]
1356 pub unsafe fn assume_init(self) -> Arc<T, A> {
1357 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
1358 unsafe { Arc::from_inner_in(ptr.cast(), alloc) }
1359 }
1360}
1361
1362impl<T, A: Allocator> Arc<[mem::MaybeUninit<T>], A> {
1363 /// Converts to `Arc<[T]>`.
1364 ///
1365 /// # Safety
1366 ///
1367 /// As with [`MaybeUninit::assume_init`],
1368 /// it is up to the caller to guarantee that the inner value
1369 /// really is in an initialized state.
1370 /// Calling this when the content is not yet fully initialized
1371 /// causes immediate undefined behavior.
1372 ///
1373 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
1374 ///
1375 /// # Examples
1376 ///
1377 /// ```
1378 /// #![feature(get_mut_unchecked)]
1379 ///
1380 /// use std::sync::Arc;
1381 ///
1382 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
1383 ///
1384 /// // Deferred initialization:
1385 /// let data = Arc::get_mut(&mut values).unwrap();
1386 /// data[0].write(1);
1387 /// data[1].write(2);
1388 /// data[2].write(3);
1389 ///
1390 /// let values = unsafe { values.assume_init() };
1391 ///
1392 /// assert_eq!(*values, [1, 2, 3])
1393 /// ```
1394 #[stable(feature = "new_uninit", since = "1.82.0")]
1395 #[must_use = "`self` will be dropped if the result is not used"]
1396 #[inline]
1397 pub unsafe fn assume_init(self) -> Arc<[T], A> {
1398 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
1399 unsafe { Arc::from_ptr_in(ptr.as_ptr() as _, alloc) }
1400 }
1401}
1402
1403impl<T: ?Sized> Arc<T> {
1404 /// Constructs an `Arc<T>` from a raw pointer.
1405 ///
1406 /// The raw pointer must have been previously returned by a call to
1407 /// [`Arc<U>::into_raw`][into_raw] with the following requirements:
1408 ///
1409 /// * If `U` is sized, it must have the same size and alignment as `T`. This
1410 /// is trivially true if `U` is `T`.
1411 /// * If `U` is unsized, its data pointer must have the same size and
1412 /// alignment as `T`. This is trivially true if `Arc<U>` was constructed
1413 /// through `Arc<T>` and then converted to `Arc<U>` through an [unsized
1414 /// coercion].
1415 ///
1416 /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
1417 /// and alignment, this is basically like transmuting references of
1418 /// different types. See [`mem::transmute`][transmute] for more information
1419 /// on what restrictions apply in this case.
1420 ///
1421 /// The raw pointer must point to a block of memory allocated by the global allocator.
1422 ///
1423 /// The user of `from_raw` has to make sure a specific value of `T` is only
1424 /// dropped once.
1425 ///
1426 /// This function is unsafe because improper use may lead to memory unsafety,
1427 /// even if the returned `Arc<T>` is never accessed.
1428 ///
1429 /// [into_raw]: Arc::into_raw
1430 /// [transmute]: core::mem::transmute
1431 /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
1432 ///
1433 /// # Examples
1434 ///
1435 /// ```
1436 /// use std::sync::Arc;
1437 ///
1438 /// let x = Arc::new("hello".to_owned());
1439 /// let x_ptr = Arc::into_raw(x);
1440 ///
1441 /// unsafe {
1442 /// // Convert back to an `Arc` to prevent leak.
1443 /// let x = Arc::from_raw(x_ptr);
1444 /// assert_eq!(&*x, "hello");
1445 ///
1446 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1447 /// }
1448 ///
1449 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1450 /// ```
1451 ///
1452 /// Convert a slice back into its original array:
1453 ///
1454 /// ```
1455 /// use std::sync::Arc;
1456 ///
1457 /// let x: Arc<[u32]> = Arc::new([1, 2, 3]);
1458 /// let x_ptr: *const [u32] = Arc::into_raw(x);
1459 ///
1460 /// unsafe {
1461 /// let x: Arc<[u32; 3]> = Arc::from_raw(x_ptr.cast::<[u32; 3]>());
1462 /// assert_eq!(&*x, &[1, 2, 3]);
1463 /// }
1464 /// ```
1465 #[inline]
1466 #[stable(feature = "rc_raw", since = "1.17.0")]
1467 pub unsafe fn from_raw(ptr: *const T) -> Self {
1468 unsafe { Arc::from_raw_in(ptr, Global) }
1469 }
1470
1471 /// Increments the strong reference count on the `Arc<T>` associated with the
1472 /// provided pointer by one.
1473 ///
1474 /// # Safety
1475 ///
1476 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1477 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1478 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1479 /// least 1) for the duration of this method, and `ptr` must point to a block of memory
1480 /// allocated by the global allocator.
1481 ///
1482 /// [from_raw_in]: Arc::from_raw_in
1483 ///
1484 /// # Examples
1485 ///
1486 /// ```
1487 /// use std::sync::Arc;
1488 ///
1489 /// let five = Arc::new(5);
1490 ///
1491 /// unsafe {
1492 /// let ptr = Arc::into_raw(five);
1493 /// Arc::increment_strong_count(ptr);
1494 ///
1495 /// // This assertion is deterministic because we haven't shared
1496 /// // the `Arc` between threads.
1497 /// let five = Arc::from_raw(ptr);
1498 /// assert_eq!(2, Arc::strong_count(&five));
1499 /// # // Prevent leaks for Miri.
1500 /// # Arc::decrement_strong_count(ptr);
1501 /// }
1502 /// ```
1503 #[inline]
1504 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1505 pub unsafe fn increment_strong_count(ptr: *const T) {
1506 unsafe { Arc::increment_strong_count_in(ptr, Global) }
1507 }
1508
1509 /// Decrements the strong reference count on the `Arc<T>` associated with the
1510 /// provided pointer by one.
1511 ///
1512 /// # Safety
1513 ///
1514 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1515 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1516 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1517 /// least 1) when invoking this method, and `ptr` must point to a block of memory
1518 /// allocated by the global allocator. This method can be used to release the final
1519 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1520 /// released.
1521 ///
1522 /// [from_raw_in]: Arc::from_raw_in
1523 ///
1524 /// # Examples
1525 ///
1526 /// ```
1527 /// use std::sync::Arc;
1528 ///
1529 /// let five = Arc::new(5);
1530 ///
1531 /// unsafe {
1532 /// let ptr = Arc::into_raw(five);
1533 /// Arc::increment_strong_count(ptr);
1534 ///
1535 /// // Those assertions are deterministic because we haven't shared
1536 /// // the `Arc` between threads.
1537 /// let five = Arc::from_raw(ptr);
1538 /// assert_eq!(2, Arc::strong_count(&five));
1539 /// Arc::decrement_strong_count(ptr);
1540 /// assert_eq!(1, Arc::strong_count(&five));
1541 /// }
1542 /// ```
1543 #[inline]
1544 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1545 pub unsafe fn decrement_strong_count(ptr: *const T) {
1546 unsafe { Arc::decrement_strong_count_in(ptr, Global) }
1547 }
1548}
1549
1550impl<T: ?Sized, A: Allocator> Arc<T, A> {
1551 /// Returns a reference to the underlying allocator.
1552 ///
1553 /// Note: this is an associated function, which means that you have
1554 /// to call it as `Arc::allocator(&a)` instead of `a.allocator()`. This
1555 /// is so that there is no conflict with a method on the inner type.
1556 #[inline]
1557 #[unstable(feature = "allocator_api", issue = "32838")]
1558 pub fn allocator(this: &Self) -> &A {
1559 &this.alloc
1560 }
1561
1562 /// Consumes the `Arc`, returning the wrapped pointer.
1563 ///
1564 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
1565 /// [`Arc::from_raw`].
1566 ///
1567 /// # Examples
1568 ///
1569 /// ```
1570 /// use std::sync::Arc;
1571 ///
1572 /// let x = Arc::new("hello".to_owned());
1573 /// let x_ptr = Arc::into_raw(x);
1574 /// assert_eq!(unsafe { &*x_ptr }, "hello");
1575 /// # // Prevent leaks for Miri.
1576 /// # drop(unsafe { Arc::from_raw(x_ptr) });
1577 /// ```
1578 #[must_use = "losing the pointer will leak memory"]
1579 #[stable(feature = "rc_raw", since = "1.17.0")]
1580 #[rustc_never_returns_null_ptr]
1581 pub fn into_raw(this: Self) -> *const T {
1582 let this = ManuallyDrop::new(this);
1583 Self::as_ptr(&*this)
1584 }
1585
1586 /// Consumes the `Arc`, returning the wrapped pointer and allocator.
1587 ///
1588 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
1589 /// [`Arc::from_raw_in`].
1590 ///
1591 /// # Examples
1592 ///
1593 /// ```
1594 /// #![feature(allocator_api)]
1595 /// use std::sync::Arc;
1596 /// use std::alloc::System;
1597 ///
1598 /// let x = Arc::new_in("hello".to_owned(), System);
1599 /// let (ptr, alloc) = Arc::into_raw_with_allocator(x);
1600 /// assert_eq!(unsafe { &*ptr }, "hello");
1601 /// let x = unsafe { Arc::from_raw_in(ptr, alloc) };
1602 /// assert_eq!(&*x, "hello");
1603 /// ```
1604 #[must_use = "losing the pointer will leak memory"]
1605 #[unstable(feature = "allocator_api", issue = "32838")]
1606 pub fn into_raw_with_allocator(this: Self) -> (*const T, A) {
1607 let this = mem::ManuallyDrop::new(this);
1608 let ptr = Self::as_ptr(&this);
1609 // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
1610 let alloc = unsafe { ptr::read(&this.alloc) };
1611 (ptr, alloc)
1612 }
1613
1614 /// Provides a raw pointer to the data.
1615 ///
1616 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
1617 /// as long as there are strong counts in the `Arc`.
1618 ///
1619 /// # Examples
1620 ///
1621 /// ```
1622 /// use std::sync::Arc;
1623 ///
1624 /// let x = Arc::new("hello".to_owned());
1625 /// let y = Arc::clone(&x);
1626 /// let x_ptr = Arc::as_ptr(&x);
1627 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
1628 /// assert_eq!(unsafe { &*x_ptr }, "hello");
1629 /// ```
1630 #[must_use]
1631 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
1632 #[rustc_never_returns_null_ptr]
1633 pub fn as_ptr(this: &Self) -> *const T {
1634 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
1635
1636 // SAFETY: This cannot go through Deref::deref or RcInnerPtr::inner because
1637 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
1638 // write through the pointer after the Rc is recovered through `from_raw`.
1639 unsafe { &raw mut (*ptr).data }
1640 }
1641
1642 /// Constructs an `Arc<T, A>` from a raw pointer.
1643 ///
1644 /// The raw pointer must have been previously returned by a call to [`Arc<U,
1645 /// A>::into_raw`][into_raw] with the following requirements:
1646 ///
1647 /// * If `U` is sized, it must have the same size and alignment as `T`. This
1648 /// is trivially true if `U` is `T`.
1649 /// * If `U` is unsized, its data pointer must have the same size and
1650 /// alignment as `T`. This is trivially true if `Arc<U>` was constructed
1651 /// through `Arc<T>` and then converted to `Arc<U>` through an [unsized
1652 /// coercion].
1653 ///
1654 /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
1655 /// and alignment, this is basically like transmuting references of
1656 /// different types. See [`mem::transmute`][transmute] for more information
1657 /// on what restrictions apply in this case.
1658 ///
1659 /// The raw pointer must point to a block of memory allocated by `alloc`
1660 ///
1661 /// The user of `from_raw` has to make sure a specific value of `T` is only
1662 /// dropped once.
1663 ///
1664 /// This function is unsafe because improper use may lead to memory unsafety,
1665 /// even if the returned `Arc<T>` is never accessed.
1666 ///
1667 /// [into_raw]: Arc::into_raw
1668 /// [transmute]: core::mem::transmute
1669 /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
1670 ///
1671 /// # Examples
1672 ///
1673 /// ```
1674 /// #![feature(allocator_api)]
1675 ///
1676 /// use std::sync::Arc;
1677 /// use std::alloc::System;
1678 ///
1679 /// let x = Arc::new_in("hello".to_owned(), System);
1680 /// let x_ptr = Arc::into_raw(x);
1681 ///
1682 /// unsafe {
1683 /// // Convert back to an `Arc` to prevent leak.
1684 /// let x = Arc::from_raw_in(x_ptr, System);
1685 /// assert_eq!(&*x, "hello");
1686 ///
1687 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1688 /// }
1689 ///
1690 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1691 /// ```
1692 ///
1693 /// Convert a slice back into its original array:
1694 ///
1695 /// ```
1696 /// #![feature(allocator_api)]
1697 ///
1698 /// use std::sync::Arc;
1699 /// use std::alloc::System;
1700 ///
1701 /// let x: Arc<[u32], _> = Arc::new_in([1, 2, 3], System);
1702 /// let x_ptr: *const [u32] = Arc::into_raw(x);
1703 ///
1704 /// unsafe {
1705 /// let x: Arc<[u32; 3], _> = Arc::from_raw_in(x_ptr.cast::<[u32; 3]>(), System);
1706 /// assert_eq!(&*x, &[1, 2, 3]);
1707 /// }
1708 /// ```
1709 #[inline]
1710 #[unstable(feature = "allocator_api", issue = "32838")]
1711 pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self {
1712 unsafe {
1713 let offset = data_offset(ptr);
1714
1715 // Reverse the offset to find the original ArcInner.
1716 let arc_ptr = ptr.byte_sub(offset) as *mut ArcInner<T>;
1717
1718 Self::from_ptr_in(arc_ptr, alloc)
1719 }
1720 }
1721
1722 /// Creates a new [`Weak`] pointer to this allocation.
1723 ///
1724 /// # Examples
1725 ///
1726 /// ```
1727 /// use std::sync::Arc;
1728 ///
1729 /// let five = Arc::new(5);
1730 ///
1731 /// let weak_five = Arc::downgrade(&five);
1732 /// ```
1733 #[must_use = "this returns a new `Weak` pointer, \
1734 without modifying the original `Arc`"]
1735 #[stable(feature = "arc_weak", since = "1.4.0")]
1736 pub fn downgrade(this: &Self) -> Weak<T, A>
1737 where
1738 A: Clone,
1739 {
1740 // This Relaxed is OK because we're checking the value in the CAS
1741 // below.
1742 let mut cur = this.inner().weak.load(Relaxed);
1743
1744 loop {
1745 // check if the weak counter is currently "locked"; if so, spin.
1746 if cur == usize::MAX {
1747 hint::spin_loop();
1748 cur = this.inner().weak.load(Relaxed);
1749 continue;
1750 }
1751
1752 // We can't allow the refcount to increase much past `MAX_REFCOUNT`.
1753 assert!(cur <= MAX_REFCOUNT, "{}", INTERNAL_OVERFLOW_ERROR);
1754
1755 // NOTE: this code currently ignores the possibility of overflow
1756 // into usize::MAX; in general both Rc and Arc need to be adjusted
1757 // to deal with overflow.
1758
1759 // Unlike with Clone(), we need this to be an Acquire read to
1760 // synchronize with the write coming from `is_unique`, so that the
1761 // events prior to that write happen before this read.
1762 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
1763 Ok(_) => {
1764 // Make sure we do not create a dangling Weak
1765 debug_assert!(!is_dangling(this.ptr.as_ptr()));
1766 return Weak { ptr: this.ptr, alloc: this.alloc.clone() };
1767 }
1768 Err(old) => cur = old,
1769 }
1770 }
1771 }
1772
1773 /// Gets the number of [`Weak`] pointers to this allocation.
1774 ///
1775 /// # Safety
1776 ///
1777 /// This method by itself is safe, but using it correctly requires extra care.
1778 /// Another thread can change the weak count at any time,
1779 /// including potentially between calling this method and acting on the result.
1780 ///
1781 /// # Examples
1782 ///
1783 /// ```
1784 /// use std::sync::Arc;
1785 ///
1786 /// let five = Arc::new(5);
1787 /// let _weak_five = Arc::downgrade(&five);
1788 ///
1789 /// // This assertion is deterministic because we haven't shared
1790 /// // the `Arc` or `Weak` between threads.
1791 /// assert_eq!(1, Arc::weak_count(&five));
1792 /// ```
1793 #[inline]
1794 #[must_use]
1795 #[stable(feature = "arc_counts", since = "1.15.0")]
1796 pub fn weak_count(this: &Self) -> usize {
1797 let cnt = this.inner().weak.load(Relaxed);
1798 // If the weak count is currently locked, the value of the
1799 // count was 0 just before taking the lock.
1800 if cnt == usize::MAX { 0 } else { cnt - 1 }
1801 }
1802
1803 /// Gets the number of strong (`Arc`) pointers to this allocation.
1804 ///
1805 /// # Safety
1806 ///
1807 /// This method by itself is safe, but using it correctly requires extra care.
1808 /// Another thread can change the strong count at any time,
1809 /// including potentially between calling this method and acting on the result.
1810 ///
1811 /// # Examples
1812 ///
1813 /// ```
1814 /// use std::sync::Arc;
1815 ///
1816 /// let five = Arc::new(5);
1817 /// let _also_five = Arc::clone(&five);
1818 ///
1819 /// // This assertion is deterministic because we haven't shared
1820 /// // the `Arc` between threads.
1821 /// assert_eq!(2, Arc::strong_count(&five));
1822 /// ```
1823 #[inline]
1824 #[must_use]
1825 #[stable(feature = "arc_counts", since = "1.15.0")]
1826 pub fn strong_count(this: &Self) -> usize {
1827 this.inner().strong.load(Relaxed)
1828 }
1829
1830 /// Increments the strong reference count on the `Arc<T>` associated with the
1831 /// provided pointer by one.
1832 ///
1833 /// # Safety
1834 ///
1835 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1836 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1837 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1838 /// least 1) for the duration of this method, and `ptr` must point to a block of memory
1839 /// allocated by `alloc`.
1840 ///
1841 /// [from_raw_in]: Arc::from_raw_in
1842 ///
1843 /// # Examples
1844 ///
1845 /// ```
1846 /// #![feature(allocator_api)]
1847 ///
1848 /// use std::sync::Arc;
1849 /// use std::alloc::System;
1850 ///
1851 /// let five = Arc::new_in(5, System);
1852 ///
1853 /// unsafe {
1854 /// let ptr = Arc::into_raw(five);
1855 /// Arc::increment_strong_count_in(ptr, System);
1856 ///
1857 /// // This assertion is deterministic because we haven't shared
1858 /// // the `Arc` between threads.
1859 /// let five = Arc::from_raw_in(ptr, System);
1860 /// assert_eq!(2, Arc::strong_count(&five));
1861 /// # // Prevent leaks for Miri.
1862 /// # Arc::decrement_strong_count_in(ptr, System);
1863 /// }
1864 /// ```
1865 #[inline]
1866 #[unstable(feature = "allocator_api", issue = "32838")]
1867 pub unsafe fn increment_strong_count_in(ptr: *const T, alloc: A)
1868 where
1869 A: Clone,
1870 {
1871 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1872 let arc = unsafe { mem::ManuallyDrop::new(Arc::from_raw_in(ptr, alloc)) };
1873 // Now increase refcount, but don't drop new refcount either
1874 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1875 }
1876
1877 /// Decrements the strong reference count on the `Arc<T>` associated with the
1878 /// provided pointer by one.
1879 ///
1880 /// # Safety
1881 ///
1882 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1883 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1884 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1885 /// least 1) when invoking this method, and `ptr` must point to a block of memory
1886 /// allocated by `alloc`. This method can be used to release the final
1887 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1888 /// released.
1889 ///
1890 /// [from_raw_in]: Arc::from_raw_in
1891 ///
1892 /// # Examples
1893 ///
1894 /// ```
1895 /// #![feature(allocator_api)]
1896 ///
1897 /// use std::sync::Arc;
1898 /// use std::alloc::System;
1899 ///
1900 /// let five = Arc::new_in(5, System);
1901 ///
1902 /// unsafe {
1903 /// let ptr = Arc::into_raw(five);
1904 /// Arc::increment_strong_count_in(ptr, System);
1905 ///
1906 /// // Those assertions are deterministic because we haven't shared
1907 /// // the `Arc` between threads.
1908 /// let five = Arc::from_raw_in(ptr, System);
1909 /// assert_eq!(2, Arc::strong_count(&five));
1910 /// Arc::decrement_strong_count_in(ptr, System);
1911 /// assert_eq!(1, Arc::strong_count(&five));
1912 /// }
1913 /// ```
1914 #[inline]
1915 #[unstable(feature = "allocator_api", issue = "32838")]
1916 pub unsafe fn decrement_strong_count_in(ptr: *const T, alloc: A) {
1917 unsafe { drop(Arc::from_raw_in(ptr, alloc)) };
1918 }
1919
1920 #[inline]
1921 fn inner(&self) -> &ArcInner<T> {
1922 // This unsafety is ok because while this arc is alive we're guaranteed
1923 // that the inner pointer is valid. Furthermore, we know that the
1924 // `ArcInner` structure itself is `Sync` because the inner data is
1925 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1926 // contents.
1927 unsafe { self.ptr.as_ref() }
1928 }
1929
1930 // Non-inlined part of `drop`.
1931 #[inline(never)]
1932 unsafe fn drop_slow(&mut self) {
1933 // Drop the weak ref collectively held by all strong references when this
1934 // variable goes out of scope. This ensures that the memory is deallocated
1935 // even if the destructor of `T` panics.
1936 // Take a reference to `self.alloc` instead of cloning because 1. it'll last long
1937 // enough, and 2. you should be able to drop `Arc`s with unclonable allocators
1938 let _weak = Weak { ptr: self.ptr, alloc: &self.alloc };
1939
1940 // Destroy the data at this time, even though we must not free the box
1941 // allocation itself (there might still be weak pointers lying around).
1942 // We cannot use `get_mut_unchecked` here, because `self.alloc` is borrowed.
1943 unsafe { ptr::drop_in_place(&mut (*self.ptr.as_ptr()).data) };
1944 }
1945
1946 /// Returns `true` if the two `Arc`s point to the same allocation in a vein similar to
1947 /// [`ptr::eq`]. This function ignores the metadata of `dyn Trait` pointers.
1948 ///
1949 /// # Examples
1950 ///
1951 /// ```
1952 /// use std::sync::Arc;
1953 ///
1954 /// let five = Arc::new(5);
1955 /// let same_five = Arc::clone(&five);
1956 /// let other_five = Arc::new(5);
1957 ///
1958 /// assert!(Arc::ptr_eq(&five, &same_five));
1959 /// assert!(!Arc::ptr_eq(&five, &other_five));
1960 /// ```
1961 ///
1962 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1963 #[inline]
1964 #[must_use]
1965 #[stable(feature = "ptr_eq", since = "1.17.0")]
1966 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1967 ptr::addr_eq(this.ptr.as_ptr(), other.ptr.as_ptr())
1968 }
1969}
1970
1971impl<T: ?Sized> Arc<T> {
1972 /// Allocates an `ArcInner<T>` with sufficient space for
1973 /// a possibly-unsized inner value where the value has the layout provided.
1974 ///
1975 /// The function `mem_to_arcinner` is called with the data pointer
1976 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1977 #[cfg(not(no_global_oom_handling))]
1978 unsafe fn allocate_for_layout(
1979 value_layout: Layout,
1980 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1981 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1982 ) -> *mut ArcInner<T> {
1983 let layout = arcinner_layout_for_value_layout(value_layout);
1984
1985 let ptr = allocate(layout).unwrap_or_else(|_| handle_alloc_error(layout));
1986
1987 unsafe { Self::initialize_arcinner(ptr, layout, mem_to_arcinner) }
1988 }
1989
1990 /// Allocates an `ArcInner<T>` with sufficient space for
1991 /// a possibly-unsized inner value where the value has the layout provided,
1992 /// returning an error if allocation fails.
1993 ///
1994 /// The function `mem_to_arcinner` is called with the data pointer
1995 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1996 unsafe fn try_allocate_for_layout(
1997 value_layout: Layout,
1998 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1999 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
2000 ) -> Result<*mut ArcInner<T>, AllocError> {
2001 let layout = arcinner_layout_for_value_layout(value_layout);
2002
2003 let ptr = allocate(layout)?;
2004
2005 let inner = unsafe { Self::initialize_arcinner(ptr, layout, mem_to_arcinner) };
2006
2007 Ok(inner)
2008 }
2009
2010 unsafe fn initialize_arcinner(
2011 ptr: NonNull<[u8]>,
2012 layout: Layout,
2013 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
2014 ) -> *mut ArcInner<T> {
2015 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
2016 debug_assert_eq!(unsafe { Layout::for_value_raw(inner) }, layout);
2017
2018 unsafe {
2019 (&raw mut (*inner).strong).write(atomic::AtomicUsize::new(1));
2020 (&raw mut (*inner).weak).write(atomic::AtomicUsize::new(1));
2021 }
2022
2023 inner
2024 }
2025}
2026
2027impl<T: ?Sized, A: Allocator> Arc<T, A> {
2028 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
2029 #[inline]
2030 #[cfg(not(no_global_oom_handling))]
2031 unsafe fn allocate_for_ptr_in(ptr: *const T, alloc: &A) -> *mut ArcInner<T> {
2032 // Allocate for the `ArcInner<T>` using the given value.
2033 unsafe {
2034 Arc::allocate_for_layout(
2035 Layout::for_value_raw(ptr),
2036 |layout| alloc.allocate(layout),
2037 |mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
2038 )
2039 }
2040 }
2041
2042 #[cfg(not(no_global_oom_handling))]
2043 fn from_box_in(src: Box<T, A>) -> Arc<T, A> {
2044 unsafe {
2045 let value_size = size_of_val(&*src);
2046 let ptr = Self::allocate_for_ptr_in(&*src, Box::allocator(&src));
2047
2048 // Copy value as bytes
2049 ptr::copy_nonoverlapping(
2050 (&raw const *src) as *const u8,
2051 (&raw mut (*ptr).data) as *mut u8,
2052 value_size,
2053 );
2054
2055 // Free the allocation without dropping its contents
2056 let (bptr, alloc) = Box::into_raw_with_allocator(src);
2057 let src = Box::from_raw_in(bptr as *mut mem::ManuallyDrop<T>, alloc.by_ref());
2058 drop(src);
2059
2060 Self::from_ptr_in(ptr, alloc)
2061 }
2062 }
2063}
2064
2065impl<T> Arc<[T]> {
2066 /// Allocates an `ArcInner<[T]>` with the given length.
2067 #[cfg(not(no_global_oom_handling))]
2068 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
2069 unsafe {
2070 Self::allocate_for_layout(
2071 Layout::array::<T>(len).unwrap(),
2072 |layout| Global.allocate(layout),
2073 |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut ArcInner<[T]>,
2074 )
2075 }
2076 }
2077
2078 /// Copy elements from slice into newly allocated `Arc<[T]>`
2079 ///
2080 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
2081 #[cfg(not(no_global_oom_handling))]
2082 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
2083 unsafe {
2084 let ptr = Self::allocate_for_slice(v.len());
2085
2086 ptr::copy_nonoverlapping(v.as_ptr(), (&raw mut (*ptr).data) as *mut T, v.len());
2087
2088 Self::from_ptr(ptr)
2089 }
2090 }
2091
2092 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
2093 ///
2094 /// Behavior is undefined should the size be wrong.
2095 #[cfg(not(no_global_oom_handling))]
2096 unsafe fn from_iter_exact(iter: impl Iterator<Item = T>, len: usize) -> Arc<[T]> {
2097 // Panic guard while cloning T elements.
2098 // In the event of a panic, elements that have been written
2099 // into the new ArcInner will be dropped, then the memory freed.
2100 struct Guard<T> {
2101 mem: NonNull<u8>,
2102 elems: *mut T,
2103 layout: Layout,
2104 n_elems: usize,
2105 }
2106
2107 impl<T> Drop for Guard<T> {
2108 fn drop(&mut self) {
2109 unsafe {
2110 let slice = from_raw_parts_mut(self.elems, self.n_elems);
2111 ptr::drop_in_place(slice);
2112
2113 Global.deallocate(self.mem, self.layout);
2114 }
2115 }
2116 }
2117
2118 unsafe {
2119 let ptr = Self::allocate_for_slice(len);
2120
2121 let mem = ptr as *mut _ as *mut u8;
2122 let layout = Layout::for_value_raw(ptr);
2123
2124 // Pointer to first element
2125 let elems = (&raw mut (*ptr).data) as *mut T;
2126
2127 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
2128
2129 for (i, item) in iter.enumerate() {
2130 ptr::write(elems.add(i), item);
2131 guard.n_elems += 1;
2132 }
2133
2134 // All clear. Forget the guard so it doesn't free the new ArcInner.
2135 mem::forget(guard);
2136
2137 Self::from_ptr(ptr)
2138 }
2139 }
2140}
2141
2142impl<T, A: Allocator> Arc<[T], A> {
2143 /// Allocates an `ArcInner<[T]>` with the given length.
2144 #[inline]
2145 #[cfg(not(no_global_oom_handling))]
2146 unsafe fn allocate_for_slice_in(len: usize, alloc: &A) -> *mut ArcInner<[T]> {
2147 unsafe {
2148 Arc::allocate_for_layout(
2149 value_layout:Layout::array::<T>(len).unwrap(),
2150 |layout| alloc.allocate(layout),
2151 |mem: *mut u8| ptr::slice_from_raw_parts_mut(data:mem.cast::<T>(), len) as *mut ArcInner<[T]>,
2152 )
2153 }
2154 }
2155}
2156
2157/// Specialization trait used for `From<&[T]>`.
2158#[cfg(not(no_global_oom_handling))]
2159trait ArcFromSlice<T> {
2160 fn from_slice(slice: &[T]) -> Self;
2161}
2162
2163#[cfg(not(no_global_oom_handling))]
2164impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
2165 #[inline]
2166 default fn from_slice(v: &[T]) -> Self {
2167 unsafe { Self::from_iter_exact(iter:v.iter().cloned(), v.len()) }
2168 }
2169}
2170
2171#[cfg(not(no_global_oom_handling))]
2172impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
2173 #[inline]
2174 fn from_slice(v: &[T]) -> Self {
2175 unsafe { Arc::copy_from_slice(v) }
2176 }
2177}
2178
2179#[stable(feature = "rust1", since = "1.0.0")]
2180impl<T: ?Sized, A: Allocator + Clone> Clone for Arc<T, A> {
2181 /// Makes a clone of the `Arc` pointer.
2182 ///
2183 /// This creates another pointer to the same allocation, increasing the
2184 /// strong reference count.
2185 ///
2186 /// # Examples
2187 ///
2188 /// ```
2189 /// use std::sync::Arc;
2190 ///
2191 /// let five = Arc::new(5);
2192 ///
2193 /// let _ = Arc::clone(&five);
2194 /// ```
2195 #[inline]
2196 fn clone(&self) -> Arc<T, A> {
2197 // Using a relaxed ordering is alright here, as knowledge of the
2198 // original reference prevents other threads from erroneously deleting
2199 // the object.
2200 //
2201 // As explained in the [Boost documentation][1], Increasing the
2202 // reference counter can always be done with memory_order_relaxed: New
2203 // references to an object can only be formed from an existing
2204 // reference, and passing an existing reference from one thread to
2205 // another must already provide any required synchronization.
2206 //
2207 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
2208 let old_size = self.inner().strong.fetch_add(1, Relaxed);
2209
2210 // However we need to guard against massive refcounts in case someone is `mem::forget`ing
2211 // Arcs. If we don't do this the count can overflow and users will use-after free. This
2212 // branch will never be taken in any realistic program. We abort because such a program is
2213 // incredibly degenerate, and we don't care to support it.
2214 //
2215 // This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
2216 // But we do that check *after* having done the increment, so there is a chance here that
2217 // the worst already happened and we actually do overflow the `usize` counter. However, that
2218 // requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
2219 // above and the `abort` below, which seems exceedingly unlikely.
2220 //
2221 // This is a global invariant, and also applies when using a compare-exchange loop to increment
2222 // counters in other methods.
2223 // Otherwise, the counter could be brought to an almost-overflow using a compare-exchange loop,
2224 // and then overflow using a few `fetch_add`s.
2225 if old_size > MAX_REFCOUNT {
2226 abort();
2227 }
2228
2229 unsafe { Self::from_inner_in(self.ptr, self.alloc.clone()) }
2230 }
2231}
2232
2233#[unstable(feature = "ergonomic_clones", issue = "132290")]
2234impl<T: ?Sized, A: Allocator + Clone> UseCloned for Arc<T, A> {}
2235
2236#[stable(feature = "rust1", since = "1.0.0")]
2237impl<T: ?Sized, A: Allocator> Deref for Arc<T, A> {
2238 type Target = T;
2239
2240 #[inline]
2241 fn deref(&self) -> &T {
2242 &self.inner().data
2243 }
2244}
2245
2246#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2247unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Arc<T, A> {}
2248
2249#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2250unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Weak<T, A> {}
2251
2252#[unstable(feature = "deref_pure_trait", issue = "87121")]
2253unsafe impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A> {}
2254
2255#[unstable(feature = "legacy_receiver_trait", issue = "none")]
2256impl<T: ?Sized> LegacyReceiver for Arc<T> {}
2257
2258#[cfg(not(no_global_oom_handling))]
2259impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A> {
2260 /// Makes a mutable reference into the given `Arc`.
2261 ///
2262 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
2263 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
2264 /// referred to as clone-on-write.
2265 ///
2266 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
2267 /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
2268 /// be cloned.
2269 ///
2270 /// See also [`get_mut`], which will fail rather than cloning the inner value
2271 /// or dissociating [`Weak`] pointers.
2272 ///
2273 /// [`clone`]: Clone::clone
2274 /// [`get_mut`]: Arc::get_mut
2275 ///
2276 /// # Examples
2277 ///
2278 /// ```
2279 /// use std::sync::Arc;
2280 ///
2281 /// let mut data = Arc::new(5);
2282 ///
2283 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
2284 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
2285 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
2286 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
2287 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
2288 ///
2289 /// // Now `data` and `other_data` point to different allocations.
2290 /// assert_eq!(*data, 8);
2291 /// assert_eq!(*other_data, 12);
2292 /// ```
2293 ///
2294 /// [`Weak`] pointers will be dissociated:
2295 ///
2296 /// ```
2297 /// use std::sync::Arc;
2298 ///
2299 /// let mut data = Arc::new(75);
2300 /// let weak = Arc::downgrade(&data);
2301 ///
2302 /// assert!(75 == *data);
2303 /// assert!(75 == *weak.upgrade().unwrap());
2304 ///
2305 /// *Arc::make_mut(&mut data) += 1;
2306 ///
2307 /// assert!(76 == *data);
2308 /// assert!(weak.upgrade().is_none());
2309 /// ```
2310 #[inline]
2311 #[stable(feature = "arc_unique", since = "1.4.0")]
2312 pub fn make_mut(this: &mut Self) -> &mut T {
2313 let size_of_val = size_of_val::<T>(&**this);
2314
2315 // Note that we hold both a strong reference and a weak reference.
2316 // Thus, releasing our strong reference only will not, by itself, cause
2317 // the memory to be deallocated.
2318 //
2319 // Use Acquire to ensure that we see any writes to `weak` that happen
2320 // before release writes (i.e., decrements) to `strong`. Since we hold a
2321 // weak count, there's no chance the ArcInner itself could be
2322 // deallocated.
2323 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
2324 // Another strong pointer exists, so we must clone.
2325
2326 let this_data_ref: &T = &**this;
2327 // `in_progress` drops the allocation if we panic before finishing initializing it.
2328 let mut in_progress: UniqueArcUninit<T, A> =
2329 UniqueArcUninit::new(this_data_ref, this.alloc.clone());
2330
2331 let initialized_clone = unsafe {
2332 // Clone. If the clone panics, `in_progress` will be dropped and clean up.
2333 this_data_ref.clone_to_uninit(in_progress.data_ptr().cast());
2334 // Cast type of pointer, now that it is initialized.
2335 in_progress.into_arc()
2336 };
2337 *this = initialized_clone;
2338 } else if this.inner().weak.load(Relaxed) != 1 {
2339 // Relaxed suffices in the above because this is fundamentally an
2340 // optimization: we are always racing with weak pointers being
2341 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
2342
2343 // We removed the last strong ref, but there are additional weak
2344 // refs remaining. We'll move the contents to a new Arc, and
2345 // invalidate the other weak refs.
2346
2347 // Note that it is not possible for the read of `weak` to yield
2348 // usize::MAX (i.e., locked), since the weak count can only be
2349 // locked by a thread with a strong reference.
2350
2351 // Materialize our own implicit weak pointer, so that it can clean
2352 // up the ArcInner as needed.
2353 let _weak = Weak { ptr: this.ptr, alloc: this.alloc.clone() };
2354
2355 // Can just steal the data, all that's left is Weaks
2356 //
2357 // We don't need panic-protection like the above branch does, but we might as well
2358 // use the same mechanism.
2359 let mut in_progress: UniqueArcUninit<T, A> =
2360 UniqueArcUninit::new(&**this, this.alloc.clone());
2361 unsafe {
2362 // Initialize `in_progress` with move of **this.
2363 // We have to express this in terms of bytes because `T: ?Sized`; there is no
2364 // operation that just copies a value based on its `size_of_val()`.
2365 ptr::copy_nonoverlapping(
2366 ptr::from_ref(&**this).cast::<u8>(),
2367 in_progress.data_ptr().cast::<u8>(),
2368 size_of_val,
2369 );
2370
2371 ptr::write(this, in_progress.into_arc());
2372 }
2373 } else {
2374 // We were the sole reference of either kind; bump back up the
2375 // strong ref count.
2376 this.inner().strong.store(1, Release);
2377 }
2378
2379 // As with `get_mut()`, the unsafety is ok because our reference was
2380 // either unique to begin with, or became one upon cloning the contents.
2381 unsafe { Self::get_mut_unchecked(this) }
2382 }
2383}
2384
2385impl<T: Clone, A: Allocator> Arc<T, A> {
2386 /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
2387 /// clone.
2388 ///
2389 /// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
2390 /// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
2391 ///
2392 /// # Examples
2393 ///
2394 /// ```
2395 /// # use std::{ptr, sync::Arc};
2396 /// let inner = String::from("test");
2397 /// let ptr = inner.as_ptr();
2398 ///
2399 /// let arc = Arc::new(inner);
2400 /// let inner = Arc::unwrap_or_clone(arc);
2401 /// // The inner value was not cloned
2402 /// assert!(ptr::eq(ptr, inner.as_ptr()));
2403 ///
2404 /// let arc = Arc::new(inner);
2405 /// let arc2 = arc.clone();
2406 /// let inner = Arc::unwrap_or_clone(arc);
2407 /// // Because there were 2 references, we had to clone the inner value.
2408 /// assert!(!ptr::eq(ptr, inner.as_ptr()));
2409 /// // `arc2` is the last reference, so when we unwrap it we get back
2410 /// // the original `String`.
2411 /// let inner = Arc::unwrap_or_clone(arc2);
2412 /// assert!(ptr::eq(ptr, inner.as_ptr()));
2413 /// ```
2414 #[inline]
2415 #[stable(feature = "arc_unwrap_or_clone", since = "1.76.0")]
2416 pub fn unwrap_or_clone(this: Self) -> T {
2417 Arc::try_unwrap(this).unwrap_or_else(|arc| (*arc).clone())
2418 }
2419}
2420
2421impl<T: ?Sized, A: Allocator> Arc<T, A> {
2422 /// Returns a mutable reference into the given `Arc`, if there are
2423 /// no other `Arc` or [`Weak`] pointers to the same allocation.
2424 ///
2425 /// Returns [`None`] otherwise, because it is not safe to
2426 /// mutate a shared value.
2427 ///
2428 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
2429 /// the inner value when there are other `Arc` pointers.
2430 ///
2431 /// [make_mut]: Arc::make_mut
2432 /// [clone]: Clone::clone
2433 ///
2434 /// # Examples
2435 ///
2436 /// ```
2437 /// use std::sync::Arc;
2438 ///
2439 /// let mut x = Arc::new(3);
2440 /// *Arc::get_mut(&mut x).unwrap() = 4;
2441 /// assert_eq!(*x, 4);
2442 ///
2443 /// let _y = Arc::clone(&x);
2444 /// assert!(Arc::get_mut(&mut x).is_none());
2445 /// ```
2446 #[inline]
2447 #[stable(feature = "arc_unique", since = "1.4.0")]
2448 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
2449 if this.is_unique() {
2450 // This unsafety is ok because we're guaranteed that the pointer
2451 // returned is the *only* pointer that will ever be returned to T. Our
2452 // reference count is guaranteed to be 1 at this point, and we required
2453 // the Arc itself to be `mut`, so we're returning the only possible
2454 // reference to the inner data.
2455 unsafe { Some(Arc::get_mut_unchecked(this)) }
2456 } else {
2457 None
2458 }
2459 }
2460
2461 /// Returns a mutable reference into the given `Arc`,
2462 /// without any check.
2463 ///
2464 /// See also [`get_mut`], which is safe and does appropriate checks.
2465 ///
2466 /// [`get_mut`]: Arc::get_mut
2467 ///
2468 /// # Safety
2469 ///
2470 /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
2471 /// they must not be dereferenced or have active borrows for the duration
2472 /// of the returned borrow, and their inner type must be exactly the same as the
2473 /// inner type of this Rc (including lifetimes). This is trivially the case if no
2474 /// such pointers exist, for example immediately after `Arc::new`.
2475 ///
2476 /// # Examples
2477 ///
2478 /// ```
2479 /// #![feature(get_mut_unchecked)]
2480 ///
2481 /// use std::sync::Arc;
2482 ///
2483 /// let mut x = Arc::new(String::new());
2484 /// unsafe {
2485 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
2486 /// }
2487 /// assert_eq!(*x, "foo");
2488 /// ```
2489 /// Other `Arc` pointers to the same allocation must be to the same type.
2490 /// ```no_run
2491 /// #![feature(get_mut_unchecked)]
2492 ///
2493 /// use std::sync::Arc;
2494 ///
2495 /// let x: Arc<str> = Arc::from("Hello, world!");
2496 /// let mut y: Arc<[u8]> = x.clone().into();
2497 /// unsafe {
2498 /// // this is Undefined Behavior, because x's inner type is str, not [u8]
2499 /// Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
2500 /// }
2501 /// println!("{}", &*x); // Invalid UTF-8 in a str
2502 /// ```
2503 /// Other `Arc` pointers to the same allocation must be to the exact same type, including lifetimes.
2504 /// ```no_run
2505 /// #![feature(get_mut_unchecked)]
2506 ///
2507 /// use std::sync::Arc;
2508 ///
2509 /// let x: Arc<&str> = Arc::new("Hello, world!");
2510 /// {
2511 /// let s = String::from("Oh, no!");
2512 /// let mut y: Arc<&str> = x.clone();
2513 /// unsafe {
2514 /// // this is Undefined Behavior, because x's inner type
2515 /// // is &'long str, not &'short str
2516 /// *Arc::get_mut_unchecked(&mut y) = &s;
2517 /// }
2518 /// }
2519 /// println!("{}", &*x); // Use-after-free
2520 /// ```
2521 #[inline]
2522 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
2523 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
2524 // We are careful to *not* create a reference covering the "count" fields, as
2525 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
2526 unsafe { &mut (*this.ptr.as_ptr()).data }
2527 }
2528
2529 /// Determine whether this is the unique reference (including weak refs) to
2530 /// the underlying data.
2531 ///
2532 /// Note that this requires locking the weak ref count.
2533 fn is_unique(&mut self) -> bool {
2534 // lock the weak pointer count if we appear to be the sole weak pointer
2535 // holder.
2536 //
2537 // The acquire label here ensures a happens-before relationship with any
2538 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
2539 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
2540 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
2541 if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
2542 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
2543 // counter in `drop` -- the only access that happens when any but the last reference
2544 // is being dropped.
2545 let unique = self.inner().strong.load(Acquire) == 1;
2546
2547 // The release write here synchronizes with a read in `downgrade`,
2548 // effectively preventing the above read of `strong` from happening
2549 // after the write.
2550 self.inner().weak.store(1, Release); // release the lock
2551 unique
2552 } else {
2553 false
2554 }
2555 }
2556}
2557
2558#[stable(feature = "rust1", since = "1.0.0")]
2559unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Arc<T, A> {
2560 /// Drops the `Arc`.
2561 ///
2562 /// This will decrement the strong reference count. If the strong reference
2563 /// count reaches zero then the only other references (if any) are
2564 /// [`Weak`], so we `drop` the inner value.
2565 ///
2566 /// # Examples
2567 ///
2568 /// ```
2569 /// use std::sync::Arc;
2570 ///
2571 /// struct Foo;
2572 ///
2573 /// impl Drop for Foo {
2574 /// fn drop(&mut self) {
2575 /// println!("dropped!");
2576 /// }
2577 /// }
2578 ///
2579 /// let foo = Arc::new(Foo);
2580 /// let foo2 = Arc::clone(&foo);
2581 ///
2582 /// drop(foo); // Doesn't print anything
2583 /// drop(foo2); // Prints "dropped!"
2584 /// ```
2585 #[inline]
2586 fn drop(&mut self) {
2587 // Because `fetch_sub` is already atomic, we do not need to synchronize
2588 // with other threads unless we are going to delete the object. This
2589 // same logic applies to the below `fetch_sub` to the `weak` count.
2590 if self.inner().strong.fetch_sub(1, Release) != 1 {
2591 return;
2592 }
2593
2594 // This fence is needed to prevent reordering of use of the data and
2595 // deletion of the data. Because it is marked `Release`, the decreasing
2596 // of the reference count synchronizes with this `Acquire` fence. This
2597 // means that use of the data happens before decreasing the reference
2598 // count, which happens before this fence, which happens before the
2599 // deletion of the data.
2600 //
2601 // As explained in the [Boost documentation][1],
2602 //
2603 // > It is important to enforce any possible access to the object in one
2604 // > thread (through an existing reference) to *happen before* deleting
2605 // > the object in a different thread. This is achieved by a "release"
2606 // > operation after dropping a reference (any access to the object
2607 // > through this reference must obviously happened before), and an
2608 // > "acquire" operation before deleting the object.
2609 //
2610 // In particular, while the contents of an Arc are usually immutable, it's
2611 // possible to have interior writes to something like a Mutex<T>. Since a
2612 // Mutex is not acquired when it is deleted, we can't rely on its
2613 // synchronization logic to make writes in thread A visible to a destructor
2614 // running in thread B.
2615 //
2616 // Also note that the Acquire fence here could probably be replaced with an
2617 // Acquire load, which could improve performance in highly-contended
2618 // situations. See [2].
2619 //
2620 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
2621 // [2]: (https://github.com/rust-lang/rust/pull/41714)
2622 acquire!(self.inner().strong);
2623
2624 // Make sure we aren't trying to "drop" the shared static for empty slices
2625 // used by Default::default.
2626 debug_assert!(
2627 !ptr::addr_eq(self.ptr.as_ptr(), &STATIC_INNER_SLICE.inner),
2628 "Arcs backed by a static should never reach a strong count of 0. \
2629 Likely decrement_strong_count or from_raw were called too many times.",
2630 );
2631
2632 unsafe {
2633 self.drop_slow();
2634 }
2635 }
2636}
2637
2638impl<A: Allocator> Arc<dyn Any + Send + Sync, A> {
2639 /// Attempts to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
2640 ///
2641 /// # Examples
2642 ///
2643 /// ```
2644 /// use std::any::Any;
2645 /// use std::sync::Arc;
2646 ///
2647 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
2648 /// if let Ok(string) = value.downcast::<String>() {
2649 /// println!("String ({}): {}", string.len(), string);
2650 /// }
2651 /// }
2652 ///
2653 /// let my_string = "Hello World".to_string();
2654 /// print_if_string(Arc::new(my_string));
2655 /// print_if_string(Arc::new(0i8));
2656 /// ```
2657 #[inline]
2658 #[stable(feature = "rc_downcast", since = "1.29.0")]
2659 pub fn downcast<T>(self) -> Result<Arc<T, A>, Self>
2660 where
2661 T: Any + Send + Sync,
2662 {
2663 if (*self).is::<T>() {
2664 unsafe {
2665 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
2666 Ok(Arc::from_inner_in(ptr.cast(), alloc))
2667 }
2668 } else {
2669 Err(self)
2670 }
2671 }
2672
2673 /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
2674 ///
2675 /// For a safe alternative see [`downcast`].
2676 ///
2677 /// # Examples
2678 ///
2679 /// ```
2680 /// #![feature(downcast_unchecked)]
2681 ///
2682 /// use std::any::Any;
2683 /// use std::sync::Arc;
2684 ///
2685 /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
2686 ///
2687 /// unsafe {
2688 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
2689 /// }
2690 /// ```
2691 ///
2692 /// # Safety
2693 ///
2694 /// The contained value must be of type `T`. Calling this method
2695 /// with the incorrect type is *undefined behavior*.
2696 ///
2697 ///
2698 /// [`downcast`]: Self::downcast
2699 #[inline]
2700 #[unstable(feature = "downcast_unchecked", issue = "90850")]
2701 pub unsafe fn downcast_unchecked<T>(self) -> Arc<T, A>
2702 where
2703 T: Any + Send + Sync,
2704 {
2705 unsafe {
2706 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
2707 Arc::from_inner_in(ptr.cast(), alloc)
2708 }
2709 }
2710}
2711
2712impl<T> Weak<T> {
2713 /// Constructs a new `Weak<T>`, without allocating any memory.
2714 /// Calling [`upgrade`] on the return value always gives [`None`].
2715 ///
2716 /// [`upgrade`]: Weak::upgrade
2717 ///
2718 /// # Examples
2719 ///
2720 /// ```
2721 /// use std::sync::Weak;
2722 ///
2723 /// let empty: Weak<i64> = Weak::new();
2724 /// assert!(empty.upgrade().is_none());
2725 /// ```
2726 #[inline]
2727 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2728 #[rustc_const_stable(feature = "const_weak_new", since = "1.73.0")]
2729 #[must_use]
2730 pub const fn new() -> Weak<T> {
2731 Weak { ptr: NonNull::without_provenance(addr:NonZeroUsize::MAX), alloc: Global }
2732 }
2733}
2734
2735impl<T, A: Allocator> Weak<T, A> {
2736 /// Constructs a new `Weak<T, A>`, without allocating any memory, technically in the provided
2737 /// allocator.
2738 /// Calling [`upgrade`] on the return value always gives [`None`].
2739 ///
2740 /// [`upgrade`]: Weak::upgrade
2741 ///
2742 /// # Examples
2743 ///
2744 /// ```
2745 /// #![feature(allocator_api)]
2746 ///
2747 /// use std::sync::Weak;
2748 /// use std::alloc::System;
2749 ///
2750 /// let empty: Weak<i64, _> = Weak::new_in(System);
2751 /// assert!(empty.upgrade().is_none());
2752 /// ```
2753 #[inline]
2754 #[unstable(feature = "allocator_api", issue = "32838")]
2755 pub fn new_in(alloc: A) -> Weak<T, A> {
2756 Weak { ptr: NonNull::without_provenance(addr:NonZeroUsize::MAX), alloc }
2757 }
2758}
2759
2760/// Helper type to allow accessing the reference counts without
2761/// making any assertions about the data field.
2762struct WeakInner<'a> {
2763 weak: &'a atomic::AtomicUsize,
2764 strong: &'a atomic::AtomicUsize,
2765}
2766
2767impl<T: ?Sized> Weak<T> {
2768 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
2769 ///
2770 /// This can be used to safely get a strong reference (by calling [`upgrade`]
2771 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2772 ///
2773 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2774 /// as these don't own anything; the method still works on them).
2775 ///
2776 /// # Safety
2777 ///
2778 /// The pointer must have originated from the [`into_raw`] and must still own its potential
2779 /// weak reference, and must point to a block of memory allocated by global allocator.
2780 ///
2781 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2782 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2783 /// count is not modified by this operation) and therefore it must be paired with a previous
2784 /// call to [`into_raw`].
2785 /// # Examples
2786 ///
2787 /// ```
2788 /// use std::sync::{Arc, Weak};
2789 ///
2790 /// let strong = Arc::new("hello".to_owned());
2791 ///
2792 /// let raw_1 = Arc::downgrade(&strong).into_raw();
2793 /// let raw_2 = Arc::downgrade(&strong).into_raw();
2794 ///
2795 /// assert_eq!(2, Arc::weak_count(&strong));
2796 ///
2797 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2798 /// assert_eq!(1, Arc::weak_count(&strong));
2799 ///
2800 /// drop(strong);
2801 ///
2802 /// // Decrement the last weak count.
2803 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2804 /// ```
2805 ///
2806 /// [`new`]: Weak::new
2807 /// [`into_raw`]: Weak::into_raw
2808 /// [`upgrade`]: Weak::upgrade
2809 #[inline]
2810 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2811 pub unsafe fn from_raw(ptr: *const T) -> Self {
2812 unsafe { Weak::from_raw_in(ptr, Global) }
2813 }
2814}
2815
2816impl<T: ?Sized, A: Allocator> Weak<T, A> {
2817 /// Returns a reference to the underlying allocator.
2818 #[inline]
2819 #[unstable(feature = "allocator_api", issue = "32838")]
2820 pub fn allocator(&self) -> &A {
2821 &self.alloc
2822 }
2823
2824 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
2825 ///
2826 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
2827 /// unaligned or even [`null`] otherwise.
2828 ///
2829 /// # Examples
2830 ///
2831 /// ```
2832 /// use std::sync::Arc;
2833 /// use std::ptr;
2834 ///
2835 /// let strong = Arc::new("hello".to_owned());
2836 /// let weak = Arc::downgrade(&strong);
2837 /// // Both point to the same object
2838 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
2839 /// // The strong here keeps it alive, so we can still access the object.
2840 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
2841 ///
2842 /// drop(strong);
2843 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
2844 /// // undefined behavior.
2845 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
2846 /// ```
2847 ///
2848 /// [`null`]: core::ptr::null "ptr::null"
2849 #[must_use]
2850 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2851 pub fn as_ptr(&self) -> *const T {
2852 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
2853
2854 if is_dangling(ptr) {
2855 // If the pointer is dangling, we return the sentinel directly. This cannot be
2856 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
2857 ptr as *const T
2858 } else {
2859 // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
2860 // The payload may be dropped at this point, and we have to maintain provenance,
2861 // so use raw pointer manipulation.
2862 unsafe { &raw mut (*ptr).data }
2863 }
2864 }
2865
2866 /// Consumes the `Weak<T>` and turns it into a raw pointer.
2867 ///
2868 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2869 /// one weak reference (the weak count is not modified by this operation). It can be turned
2870 /// back into the `Weak<T>` with [`from_raw`].
2871 ///
2872 /// The same restrictions of accessing the target of the pointer as with
2873 /// [`as_ptr`] apply.
2874 ///
2875 /// # Examples
2876 ///
2877 /// ```
2878 /// use std::sync::{Arc, Weak};
2879 ///
2880 /// let strong = Arc::new("hello".to_owned());
2881 /// let weak = Arc::downgrade(&strong);
2882 /// let raw = weak.into_raw();
2883 ///
2884 /// assert_eq!(1, Arc::weak_count(&strong));
2885 /// assert_eq!("hello", unsafe { &*raw });
2886 ///
2887 /// drop(unsafe { Weak::from_raw(raw) });
2888 /// assert_eq!(0, Arc::weak_count(&strong));
2889 /// ```
2890 ///
2891 /// [`from_raw`]: Weak::from_raw
2892 /// [`as_ptr`]: Weak::as_ptr
2893 #[must_use = "losing the pointer will leak memory"]
2894 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2895 pub fn into_raw(self) -> *const T {
2896 ManuallyDrop::new(self).as_ptr()
2897 }
2898
2899 /// Consumes the `Weak<T>`, returning the wrapped pointer and allocator.
2900 ///
2901 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2902 /// one weak reference (the weak count is not modified by this operation). It can be turned
2903 /// back into the `Weak<T>` with [`from_raw_in`].
2904 ///
2905 /// The same restrictions of accessing the target of the pointer as with
2906 /// [`as_ptr`] apply.
2907 ///
2908 /// # Examples
2909 ///
2910 /// ```
2911 /// #![feature(allocator_api)]
2912 /// use std::sync::{Arc, Weak};
2913 /// use std::alloc::System;
2914 ///
2915 /// let strong = Arc::new_in("hello".to_owned(), System);
2916 /// let weak = Arc::downgrade(&strong);
2917 /// let (raw, alloc) = weak.into_raw_with_allocator();
2918 ///
2919 /// assert_eq!(1, Arc::weak_count(&strong));
2920 /// assert_eq!("hello", unsafe { &*raw });
2921 ///
2922 /// drop(unsafe { Weak::from_raw_in(raw, alloc) });
2923 /// assert_eq!(0, Arc::weak_count(&strong));
2924 /// ```
2925 ///
2926 /// [`from_raw_in`]: Weak::from_raw_in
2927 /// [`as_ptr`]: Weak::as_ptr
2928 #[must_use = "losing the pointer will leak memory"]
2929 #[unstable(feature = "allocator_api", issue = "32838")]
2930 pub fn into_raw_with_allocator(self) -> (*const T, A) {
2931 let this = mem::ManuallyDrop::new(self);
2932 let result = this.as_ptr();
2933 // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
2934 let alloc = unsafe { ptr::read(&this.alloc) };
2935 (result, alloc)
2936 }
2937
2938 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>` in the provided
2939 /// allocator.
2940 ///
2941 /// This can be used to safely get a strong reference (by calling [`upgrade`]
2942 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2943 ///
2944 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2945 /// as these don't own anything; the method still works on them).
2946 ///
2947 /// # Safety
2948 ///
2949 /// The pointer must have originated from the [`into_raw`] and must still own its potential
2950 /// weak reference, and must point to a block of memory allocated by `alloc`.
2951 ///
2952 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2953 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2954 /// count is not modified by this operation) and therefore it must be paired with a previous
2955 /// call to [`into_raw`].
2956 /// # Examples
2957 ///
2958 /// ```
2959 /// use std::sync::{Arc, Weak};
2960 ///
2961 /// let strong = Arc::new("hello".to_owned());
2962 ///
2963 /// let raw_1 = Arc::downgrade(&strong).into_raw();
2964 /// let raw_2 = Arc::downgrade(&strong).into_raw();
2965 ///
2966 /// assert_eq!(2, Arc::weak_count(&strong));
2967 ///
2968 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2969 /// assert_eq!(1, Arc::weak_count(&strong));
2970 ///
2971 /// drop(strong);
2972 ///
2973 /// // Decrement the last weak count.
2974 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2975 /// ```
2976 ///
2977 /// [`new`]: Weak::new
2978 /// [`into_raw`]: Weak::into_raw
2979 /// [`upgrade`]: Weak::upgrade
2980 #[inline]
2981 #[unstable(feature = "allocator_api", issue = "32838")]
2982 pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self {
2983 // See Weak::as_ptr for context on how the input pointer is derived.
2984
2985 let ptr = if is_dangling(ptr) {
2986 // This is a dangling Weak.
2987 ptr as *mut ArcInner<T>
2988 } else {
2989 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
2990 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
2991 let offset = unsafe { data_offset(ptr) };
2992 // Thus, we reverse the offset to get the whole RcInner.
2993 // SAFETY: the pointer originated from a Weak, so this offset is safe.
2994 unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
2995 };
2996
2997 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
2998 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) }, alloc }
2999 }
3000}
3001
3002impl<T: ?Sized, A: Allocator> Weak<T, A> {
3003 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
3004 /// dropping of the inner value if successful.
3005 ///
3006 /// Returns [`None`] if the inner value has since been dropped.
3007 ///
3008 /// # Examples
3009 ///
3010 /// ```
3011 /// use std::sync::Arc;
3012 ///
3013 /// let five = Arc::new(5);
3014 ///
3015 /// let weak_five = Arc::downgrade(&five);
3016 ///
3017 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
3018 /// assert!(strong_five.is_some());
3019 ///
3020 /// // Destroy all strong pointers.
3021 /// drop(strong_five);
3022 /// drop(five);
3023 ///
3024 /// assert!(weak_five.upgrade().is_none());
3025 /// ```
3026 #[must_use = "this returns a new `Arc`, \
3027 without modifying the original weak pointer"]
3028 #[stable(feature = "arc_weak", since = "1.4.0")]
3029 pub fn upgrade(&self) -> Option<Arc<T, A>>
3030 where
3031 A: Clone,
3032 {
3033 #[inline]
3034 fn checked_increment(n: usize) -> Option<usize> {
3035 // Any write of 0 we can observe leaves the field in permanently zero state.
3036 if n == 0 {
3037 return None;
3038 }
3039 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
3040 assert!(n <= MAX_REFCOUNT, "{}", INTERNAL_OVERFLOW_ERROR);
3041 Some(n + 1)
3042 }
3043
3044 // We use a CAS loop to increment the strong count instead of a
3045 // fetch_add as this function should never take the reference count
3046 // from zero to one.
3047 //
3048 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
3049 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
3050 // value can be initialized after `Weak` references have already been created. In that case, we
3051 // expect to observe the fully initialized value.
3052 if self.inner()?.strong.fetch_update(Acquire, Relaxed, checked_increment).is_ok() {
3053 // SAFETY: pointer is not null, verified in checked_increment
3054 unsafe { Some(Arc::from_inner_in(self.ptr, self.alloc.clone())) }
3055 } else {
3056 None
3057 }
3058 }
3059
3060 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
3061 ///
3062 /// If `self` was created using [`Weak::new`], this will return 0.
3063 #[must_use]
3064 #[stable(feature = "weak_counts", since = "1.41.0")]
3065 pub fn strong_count(&self) -> usize {
3066 if let Some(inner) = self.inner() { inner.strong.load(Relaxed) } else { 0 }
3067 }
3068
3069 /// Gets an approximation of the number of `Weak` pointers pointing to this
3070 /// allocation.
3071 ///
3072 /// If `self` was created using [`Weak::new`], or if there are no remaining
3073 /// strong pointers, this will return 0.
3074 ///
3075 /// # Accuracy
3076 ///
3077 /// Due to implementation details, the returned value can be off by 1 in
3078 /// either direction when other threads are manipulating any `Arc`s or
3079 /// `Weak`s pointing to the same allocation.
3080 #[must_use]
3081 #[stable(feature = "weak_counts", since = "1.41.0")]
3082 pub fn weak_count(&self) -> usize {
3083 if let Some(inner) = self.inner() {
3084 let weak = inner.weak.load(Acquire);
3085 let strong = inner.strong.load(Relaxed);
3086 if strong == 0 {
3087 0
3088 } else {
3089 // Since we observed that there was at least one strong pointer
3090 // after reading the weak count, we know that the implicit weak
3091 // reference (present whenever any strong references are alive)
3092 // was still around when we observed the weak count, and can
3093 // therefore safely subtract it.
3094 weak - 1
3095 }
3096 } else {
3097 0
3098 }
3099 }
3100
3101 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
3102 /// (i.e., when this `Weak` was created by `Weak::new`).
3103 #[inline]
3104 fn inner(&self) -> Option<WeakInner<'_>> {
3105 let ptr = self.ptr.as_ptr();
3106 if is_dangling(ptr) {
3107 None
3108 } else {
3109 // We are careful to *not* create a reference covering the "data" field, as
3110 // the field may be mutated concurrently (for example, if the last `Arc`
3111 // is dropped, the data field will be dropped in-place).
3112 Some(unsafe { WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak } })
3113 }
3114 }
3115
3116 /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
3117 /// both don't point to any allocation (because they were created with `Weak::new()`). However,
3118 /// this function ignores the metadata of `dyn Trait` pointers.
3119 ///
3120 /// # Notes
3121 ///
3122 /// Since this compares pointers it means that `Weak::new()` will equal each
3123 /// other, even though they don't point to any allocation.
3124 ///
3125 /// # Examples
3126 ///
3127 /// ```
3128 /// use std::sync::Arc;
3129 ///
3130 /// let first_rc = Arc::new(5);
3131 /// let first = Arc::downgrade(&first_rc);
3132 /// let second = Arc::downgrade(&first_rc);
3133 ///
3134 /// assert!(first.ptr_eq(&second));
3135 ///
3136 /// let third_rc = Arc::new(5);
3137 /// let third = Arc::downgrade(&third_rc);
3138 ///
3139 /// assert!(!first.ptr_eq(&third));
3140 /// ```
3141 ///
3142 /// Comparing `Weak::new`.
3143 ///
3144 /// ```
3145 /// use std::sync::{Arc, Weak};
3146 ///
3147 /// let first = Weak::new();
3148 /// let second = Weak::new();
3149 /// assert!(first.ptr_eq(&second));
3150 ///
3151 /// let third_rc = Arc::new(());
3152 /// let third = Arc::downgrade(&third_rc);
3153 /// assert!(!first.ptr_eq(&third));
3154 /// ```
3155 ///
3156 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
3157 #[inline]
3158 #[must_use]
3159 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
3160 pub fn ptr_eq(&self, other: &Self) -> bool {
3161 ptr::addr_eq(self.ptr.as_ptr(), other.ptr.as_ptr())
3162 }
3163}
3164
3165#[stable(feature = "arc_weak", since = "1.4.0")]
3166impl<T: ?Sized, A: Allocator + Clone> Clone for Weak<T, A> {
3167 /// Makes a clone of the `Weak` pointer that points to the same allocation.
3168 ///
3169 /// # Examples
3170 ///
3171 /// ```
3172 /// use std::sync::{Arc, Weak};
3173 ///
3174 /// let weak_five = Arc::downgrade(&Arc::new(5));
3175 ///
3176 /// let _ = Weak::clone(&weak_five);
3177 /// ```
3178 #[inline]
3179 fn clone(&self) -> Weak<T, A> {
3180 if let Some(inner) = self.inner() {
3181 // See comments in Arc::clone() for why this is relaxed. This can use a
3182 // fetch_add (ignoring the lock) because the weak count is only locked
3183 // where are *no other* weak pointers in existence. (So we can't be
3184 // running this code in that case).
3185 let old_size = inner.weak.fetch_add(1, Relaxed);
3186
3187 // See comments in Arc::clone() for why we do this (for mem::forget).
3188 if old_size > MAX_REFCOUNT {
3189 abort();
3190 }
3191 }
3192
3193 Weak { ptr: self.ptr, alloc: self.alloc.clone() }
3194 }
3195}
3196
3197#[unstable(feature = "ergonomic_clones", issue = "132290")]
3198impl<T: ?Sized, A: Allocator + Clone> UseCloned for Weak<T, A> {}
3199
3200#[stable(feature = "downgraded_weak", since = "1.10.0")]
3201impl<T> Default for Weak<T> {
3202 /// Constructs a new `Weak<T>`, without allocating memory.
3203 /// Calling [`upgrade`] on the return value always
3204 /// gives [`None`].
3205 ///
3206 /// [`upgrade`]: Weak::upgrade
3207 ///
3208 /// # Examples
3209 ///
3210 /// ```
3211 /// use std::sync::Weak;
3212 ///
3213 /// let empty: Weak<i64> = Default::default();
3214 /// assert!(empty.upgrade().is_none());
3215 /// ```
3216 fn default() -> Weak<T> {
3217 Weak::new()
3218 }
3219}
3220
3221#[stable(feature = "arc_weak", since = "1.4.0")]
3222unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Weak<T, A> {
3223 /// Drops the `Weak` pointer.
3224 ///
3225 /// # Examples
3226 ///
3227 /// ```
3228 /// use std::sync::{Arc, Weak};
3229 ///
3230 /// struct Foo;
3231 ///
3232 /// impl Drop for Foo {
3233 /// fn drop(&mut self) {
3234 /// println!("dropped!");
3235 /// }
3236 /// }
3237 ///
3238 /// let foo = Arc::new(Foo);
3239 /// let weak_foo = Arc::downgrade(&foo);
3240 /// let other_weak_foo = Weak::clone(&weak_foo);
3241 ///
3242 /// drop(weak_foo); // Doesn't print anything
3243 /// drop(foo); // Prints "dropped!"
3244 ///
3245 /// assert!(other_weak_foo.upgrade().is_none());
3246 /// ```
3247 fn drop(&mut self) {
3248 // If we find out that we were the last weak pointer, then its time to
3249 // deallocate the data entirely. See the discussion in Arc::drop() about
3250 // the memory orderings
3251 //
3252 // It's not necessary to check for the locked state here, because the
3253 // weak count can only be locked if there was precisely one weak ref,
3254 // meaning that drop could only subsequently run ON that remaining weak
3255 // ref, which can only happen after the lock is released.
3256 let inner = if let Some(inner) = self.inner() { inner } else { return };
3257
3258 if inner.weak.fetch_sub(1, Release) == 1 {
3259 acquire!(inner.weak);
3260
3261 // Make sure we aren't trying to "deallocate" the shared static for empty slices
3262 // used by Default::default.
3263 debug_assert!(
3264 !ptr::addr_eq(self.ptr.as_ptr(), &STATIC_INNER_SLICE.inner),
3265 "Arc/Weaks backed by a static should never be deallocated. \
3266 Likely decrement_strong_count or from_raw were called too many times.",
3267 );
3268
3269 unsafe {
3270 self.alloc.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()))
3271 }
3272 }
3273 }
3274}
3275
3276#[stable(feature = "rust1", since = "1.0.0")]
3277trait ArcEqIdent<T: ?Sized + PartialEq, A: Allocator> {
3278 fn eq(&self, other: &Arc<T, A>) -> bool;
3279 fn ne(&self, other: &Arc<T, A>) -> bool;
3280}
3281
3282#[stable(feature = "rust1", since = "1.0.0")]
3283impl<T: ?Sized + PartialEq, A: Allocator> ArcEqIdent<T, A> for Arc<T, A> {
3284 #[inline]
3285 default fn eq(&self, other: &Arc<T, A>) -> bool {
3286 **self == **other
3287 }
3288 #[inline]
3289 default fn ne(&self, other: &Arc<T, A>) -> bool {
3290 **self != **other
3291 }
3292}
3293
3294/// We're doing this specialization here, and not as a more general optimization on `&T`, because it
3295/// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
3296/// store large values, that are slow to clone, but also heavy to check for equality, causing this
3297/// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
3298/// the same value, than two `&T`s.
3299///
3300/// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
3301#[stable(feature = "rust1", since = "1.0.0")]
3302impl<T: ?Sized + crate::rc::MarkerEq, A: Allocator> ArcEqIdent<T, A> for Arc<T, A> {
3303 #[inline]
3304 fn eq(&self, other: &Arc<T, A>) -> bool {
3305 Arc::ptr_eq(self, other) || **self == **other
3306 }
3307
3308 #[inline]
3309 fn ne(&self, other: &Arc<T, A>) -> bool {
3310 !Arc::ptr_eq(self, other) && **self != **other
3311 }
3312}
3313
3314#[stable(feature = "rust1", since = "1.0.0")]
3315impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Arc<T, A> {
3316 /// Equality for two `Arc`s.
3317 ///
3318 /// Two `Arc`s are equal if their inner values are equal, even if they are
3319 /// stored in different allocation.
3320 ///
3321 /// If `T` also implements `Eq` (implying reflexivity of equality),
3322 /// two `Arc`s that point to the same allocation are always equal.
3323 ///
3324 /// # Examples
3325 ///
3326 /// ```
3327 /// use std::sync::Arc;
3328 ///
3329 /// let five = Arc::new(5);
3330 ///
3331 /// assert!(five == Arc::new(5));
3332 /// ```
3333 #[inline]
3334 fn eq(&self, other: &Arc<T, A>) -> bool {
3335 ArcEqIdent::eq(self, other)
3336 }
3337
3338 /// Inequality for two `Arc`s.
3339 ///
3340 /// Two `Arc`s are not equal if their inner values are not equal.
3341 ///
3342 /// If `T` also implements `Eq` (implying reflexivity of equality),
3343 /// two `Arc`s that point to the same value are always equal.
3344 ///
3345 /// # Examples
3346 ///
3347 /// ```
3348 /// use std::sync::Arc;
3349 ///
3350 /// let five = Arc::new(5);
3351 ///
3352 /// assert!(five != Arc::new(6));
3353 /// ```
3354 #[inline]
3355 fn ne(&self, other: &Arc<T, A>) -> bool {
3356 ArcEqIdent::ne(self, other)
3357 }
3358}
3359
3360#[stable(feature = "rust1", since = "1.0.0")]
3361impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Arc<T, A> {
3362 /// Partial comparison for two `Arc`s.
3363 ///
3364 /// The two are compared by calling `partial_cmp()` on their inner values.
3365 ///
3366 /// # Examples
3367 ///
3368 /// ```
3369 /// use std::sync::Arc;
3370 /// use std::cmp::Ordering;
3371 ///
3372 /// let five = Arc::new(5);
3373 ///
3374 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
3375 /// ```
3376 fn partial_cmp(&self, other: &Arc<T, A>) -> Option<Ordering> {
3377 (**self).partial_cmp(&**other)
3378 }
3379
3380 /// Less-than comparison for two `Arc`s.
3381 ///
3382 /// The two are compared by calling `<` on their inner values.
3383 ///
3384 /// # Examples
3385 ///
3386 /// ```
3387 /// use std::sync::Arc;
3388 ///
3389 /// let five = Arc::new(5);
3390 ///
3391 /// assert!(five < Arc::new(6));
3392 /// ```
3393 fn lt(&self, other: &Arc<T, A>) -> bool {
3394 *(*self) < *(*other)
3395 }
3396
3397 /// 'Less than or equal to' comparison for two `Arc`s.
3398 ///
3399 /// The two are compared by calling `<=` on their inner values.
3400 ///
3401 /// # Examples
3402 ///
3403 /// ```
3404 /// use std::sync::Arc;
3405 ///
3406 /// let five = Arc::new(5);
3407 ///
3408 /// assert!(five <= Arc::new(5));
3409 /// ```
3410 fn le(&self, other: &Arc<T, A>) -> bool {
3411 *(*self) <= *(*other)
3412 }
3413
3414 /// Greater-than comparison for two `Arc`s.
3415 ///
3416 /// The two are compared by calling `>` on their inner values.
3417 ///
3418 /// # Examples
3419 ///
3420 /// ```
3421 /// use std::sync::Arc;
3422 ///
3423 /// let five = Arc::new(5);
3424 ///
3425 /// assert!(five > Arc::new(4));
3426 /// ```
3427 fn gt(&self, other: &Arc<T, A>) -> bool {
3428 *(*self) > *(*other)
3429 }
3430
3431 /// 'Greater than or equal to' comparison for two `Arc`s.
3432 ///
3433 /// The two are compared by calling `>=` on their inner values.
3434 ///
3435 /// # Examples
3436 ///
3437 /// ```
3438 /// use std::sync::Arc;
3439 ///
3440 /// let five = Arc::new(5);
3441 ///
3442 /// assert!(five >= Arc::new(5));
3443 /// ```
3444 fn ge(&self, other: &Arc<T, A>) -> bool {
3445 *(*self) >= *(*other)
3446 }
3447}
3448#[stable(feature = "rust1", since = "1.0.0")]
3449impl<T: ?Sized + Ord, A: Allocator> Ord for Arc<T, A> {
3450 /// Comparison for two `Arc`s.
3451 ///
3452 /// The two are compared by calling `cmp()` on their inner values.
3453 ///
3454 /// # Examples
3455 ///
3456 /// ```
3457 /// use std::sync::Arc;
3458 /// use std::cmp::Ordering;
3459 ///
3460 /// let five = Arc::new(5);
3461 ///
3462 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
3463 /// ```
3464 fn cmp(&self, other: &Arc<T, A>) -> Ordering {
3465 (**self).cmp(&**other)
3466 }
3467}
3468#[stable(feature = "rust1", since = "1.0.0")]
3469impl<T: ?Sized + Eq, A: Allocator> Eq for Arc<T, A> {}
3470
3471#[stable(feature = "rust1", since = "1.0.0")]
3472impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for Arc<T, A> {
3473 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3474 fmt::Display::fmt(&**self, f)
3475 }
3476}
3477
3478#[stable(feature = "rust1", since = "1.0.0")]
3479impl<T: ?Sized + fmt::Debug, A: Allocator> fmt::Debug for Arc<T, A> {
3480 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3481 fmt::Debug::fmt(&**self, f)
3482 }
3483}
3484
3485#[stable(feature = "rust1", since = "1.0.0")]
3486impl<T: ?Sized, A: Allocator> fmt::Pointer for Arc<T, A> {
3487 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3488 fmt::Pointer::fmt(&(&raw const **self), f)
3489 }
3490}
3491
3492#[cfg(not(no_global_oom_handling))]
3493#[stable(feature = "rust1", since = "1.0.0")]
3494impl<T: Default> Default for Arc<T> {
3495 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
3496 ///
3497 /// # Examples
3498 ///
3499 /// ```
3500 /// use std::sync::Arc;
3501 ///
3502 /// let x: Arc<i32> = Default::default();
3503 /// assert_eq!(*x, 0);
3504 /// ```
3505 fn default() -> Arc<T> {
3506 unsafe {
3507 Self::from_inner(
3508 Box::leak(Box::write(
3509 Box::new_uninit(),
3510 ArcInner {
3511 strong: atomic::AtomicUsize::new(1),
3512 weak: atomic::AtomicUsize::new(1),
3513 data: T::default(),
3514 },
3515 ))
3516 .into(),
3517 )
3518 }
3519 }
3520}
3521
3522/// Struct to hold the static `ArcInner` used for empty `Arc<str/CStr/[T]>` as
3523/// returned by `Default::default`.
3524///
3525/// Layout notes:
3526/// * `repr(align(16))` so we can use it for `[T]` with `align_of::<T>() <= 16`.
3527/// * `repr(C)` so `inner` is at offset 0 (and thus guaranteed to actually be aligned to 16).
3528/// * `[u8; 1]` (to be initialized with 0) so it can be used for `Arc<CStr>`.
3529#[repr(C, align(16))]
3530struct SliceArcInnerForStatic {
3531 inner: ArcInner<[u8; 1]>,
3532}
3533#[cfg(not(no_global_oom_handling))]
3534const MAX_STATIC_INNER_SLICE_ALIGNMENT: usize = 16;
3535
3536static STATIC_INNER_SLICE: SliceArcInnerForStatic = SliceArcInnerForStatic {
3537 inner: ArcInner {
3538 strong: atomic::AtomicUsize::new(1),
3539 weak: atomic::AtomicUsize::new(1),
3540 data: [0],
3541 },
3542};
3543
3544#[cfg(not(no_global_oom_handling))]
3545#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3546impl Default for Arc<str> {
3547 /// Creates an empty str inside an Arc
3548 ///
3549 /// This may or may not share an allocation with other Arcs.
3550 #[inline]
3551 fn default() -> Self {
3552 let arc: Arc<[u8]> = Default::default();
3553 debug_assert!(core::str::from_utf8(&*arc).is_ok());
3554 let (ptr: NonNull>, alloc: Global) = Arc::into_inner_with_allocator(this:arc);
3555 unsafe { Arc::from_ptr_in(ptr.as_ptr() as *mut ArcInner<str>, alloc) }
3556 }
3557}
3558
3559#[cfg(not(no_global_oom_handling))]
3560#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3561impl Default for Arc<core::ffi::CStr> {
3562 /// Creates an empty CStr inside an Arc
3563 ///
3564 /// This may or may not share an allocation with other Arcs.
3565 #[inline]
3566 fn default() -> Self {
3567 use core::ffi::CStr;
3568 let inner: NonNull<ArcInner<[u8]>> = NonNull::from(&STATIC_INNER_SLICE.inner);
3569 let inner: NonNull<ArcInner<CStr>> =
3570 NonNull::new(inner.as_ptr() as *mut ArcInner<CStr>).unwrap();
3571 // `this` semantically is the Arc "owned" by the static, so make sure not to drop it.
3572 let this: mem::ManuallyDrop<Arc<CStr>> =
3573 unsafe { mem::ManuallyDrop::new(Arc::from_inner(ptr:inner)) };
3574 (*this).clone()
3575 }
3576}
3577
3578#[cfg(not(no_global_oom_handling))]
3579#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3580impl<T> Default for Arc<[T]> {
3581 /// Creates an empty `[T]` inside an Arc
3582 ///
3583 /// This may or may not share an allocation with other Arcs.
3584 #[inline]
3585 fn default() -> Self {
3586 if align_of::<T>() <= MAX_STATIC_INNER_SLICE_ALIGNMENT {
3587 // We take a reference to the whole struct instead of the ArcInner<[u8; 1]> inside it so
3588 // we don't shrink the range of bytes the ptr is allowed to access under Stacked Borrows.
3589 // (Miri complains on 32-bit targets with Arc<[Align16]> otherwise.)
3590 // (Note that NonNull::from(&STATIC_INNER_SLICE.inner) is fine under Tree Borrows.)
3591 let inner: NonNull<SliceArcInnerForStatic> = NonNull::from(&STATIC_INNER_SLICE);
3592 let inner: NonNull<ArcInner<[T; 0]>> = inner.cast();
3593 // `this` semantically is the Arc "owned" by the static, so make sure not to drop it.
3594 let this: mem::ManuallyDrop<Arc<[T; 0]>> =
3595 unsafe { mem::ManuallyDrop::new(Arc::from_inner(ptr:inner)) };
3596 return (*this).clone();
3597 }
3598
3599 // If T's alignment is too large for the static, make a new unique allocation.
3600 let arr: [T; 0] = [];
3601 Arc::from(arr)
3602 }
3603}
3604
3605#[stable(feature = "rust1", since = "1.0.0")]
3606impl<T: ?Sized + Hash, A: Allocator> Hash for Arc<T, A> {
3607 fn hash<H: Hasher>(&self, state: &mut H) {
3608 (**self).hash(state)
3609 }
3610}
3611
3612#[cfg(not(no_global_oom_handling))]
3613#[stable(feature = "from_for_ptrs", since = "1.6.0")]
3614impl<T> From<T> for Arc<T> {
3615 /// Converts a `T` into an `Arc<T>`
3616 ///
3617 /// The conversion moves the value into a
3618 /// newly allocated `Arc`. It is equivalent to
3619 /// calling `Arc::new(t)`.
3620 ///
3621 /// # Example
3622 /// ```rust
3623 /// # use std::sync::Arc;
3624 /// let x = 5;
3625 /// let arc = Arc::new(5);
3626 ///
3627 /// assert_eq!(Arc::from(x), arc);
3628 /// ```
3629 fn from(t: T) -> Self {
3630 Arc::new(data:t)
3631 }
3632}
3633
3634#[cfg(not(no_global_oom_handling))]
3635#[stable(feature = "shared_from_array", since = "1.74.0")]
3636impl<T, const N: usize> From<[T; N]> for Arc<[T]> {
3637 /// Converts a [`[T; N]`](prim@array) into an `Arc<[T]>`.
3638 ///
3639 /// The conversion moves the array into a newly allocated `Arc`.
3640 ///
3641 /// # Example
3642 ///
3643 /// ```
3644 /// # use std::sync::Arc;
3645 /// let original: [i32; 3] = [1, 2, 3];
3646 /// let shared: Arc<[i32]> = Arc::from(original);
3647 /// assert_eq!(&[1, 2, 3], &shared[..]);
3648 /// ```
3649 #[inline]
3650 fn from(v: [T; N]) -> Arc<[T]> {
3651 Arc::<[T; N]>::from(v)
3652 }
3653}
3654
3655#[cfg(not(no_global_oom_handling))]
3656#[stable(feature = "shared_from_slice", since = "1.21.0")]
3657impl<T: Clone> From<&[T]> for Arc<[T]> {
3658 /// Allocates a reference-counted slice and fills it by cloning `v`'s items.
3659 ///
3660 /// # Example
3661 ///
3662 /// ```
3663 /// # use std::sync::Arc;
3664 /// let original: &[i32] = &[1, 2, 3];
3665 /// let shared: Arc<[i32]> = Arc::from(original);
3666 /// assert_eq!(&[1, 2, 3], &shared[..]);
3667 /// ```
3668 #[inline]
3669 fn from(v: &[T]) -> Arc<[T]> {
3670 <Self as ArcFromSlice<T>>::from_slice(v)
3671 }
3672}
3673
3674#[cfg(not(no_global_oom_handling))]
3675#[stable(feature = "shared_from_mut_slice", since = "1.84.0")]
3676impl<T: Clone> From<&mut [T]> for Arc<[T]> {
3677 /// Allocates a reference-counted slice and fills it by cloning `v`'s items.
3678 ///
3679 /// # Example
3680 ///
3681 /// ```
3682 /// # use std::sync::Arc;
3683 /// let mut original = [1, 2, 3];
3684 /// let original: &mut [i32] = &mut original;
3685 /// let shared: Arc<[i32]> = Arc::from(original);
3686 /// assert_eq!(&[1, 2, 3], &shared[..]);
3687 /// ```
3688 #[inline]
3689 fn from(v: &mut [T]) -> Arc<[T]> {
3690 Arc::from(&*v)
3691 }
3692}
3693
3694#[cfg(not(no_global_oom_handling))]
3695#[stable(feature = "shared_from_slice", since = "1.21.0")]
3696impl From<&str> for Arc<str> {
3697 /// Allocates a reference-counted `str` and copies `v` into it.
3698 ///
3699 /// # Example
3700 ///
3701 /// ```
3702 /// # use std::sync::Arc;
3703 /// let shared: Arc<str> = Arc::from("eggplant");
3704 /// assert_eq!("eggplant", &shared[..]);
3705 /// ```
3706 #[inline]
3707 fn from(v: &str) -> Arc<str> {
3708 let arc: Arc<[u8]> = Arc::<[u8]>::from(v.as_bytes());
3709 unsafe { Arc::from_raw(ptr:Arc::into_raw(this:arc) as *const str) }
3710 }
3711}
3712
3713#[cfg(not(no_global_oom_handling))]
3714#[stable(feature = "shared_from_mut_slice", since = "1.84.0")]
3715impl From<&mut str> for Arc<str> {
3716 /// Allocates a reference-counted `str` and copies `v` into it.
3717 ///
3718 /// # Example
3719 ///
3720 /// ```
3721 /// # use std::sync::Arc;
3722 /// let mut original = String::from("eggplant");
3723 /// let original: &mut str = &mut original;
3724 /// let shared: Arc<str> = Arc::from(original);
3725 /// assert_eq!("eggplant", &shared[..]);
3726 /// ```
3727 #[inline]
3728 fn from(v: &mut str) -> Arc<str> {
3729 Arc::from(&*v)
3730 }
3731}
3732
3733#[cfg(not(no_global_oom_handling))]
3734#[stable(feature = "shared_from_slice", since = "1.21.0")]
3735impl From<String> for Arc<str> {
3736 /// Allocates a reference-counted `str` and copies `v` into it.
3737 ///
3738 /// # Example
3739 ///
3740 /// ```
3741 /// # use std::sync::Arc;
3742 /// let unique: String = "eggplant".to_owned();
3743 /// let shared: Arc<str> = Arc::from(unique);
3744 /// assert_eq!("eggplant", &shared[..]);
3745 /// ```
3746 #[inline]
3747 fn from(v: String) -> Arc<str> {
3748 Arc::from(&v[..])
3749 }
3750}
3751
3752#[cfg(not(no_global_oom_handling))]
3753#[stable(feature = "shared_from_slice", since = "1.21.0")]
3754impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Arc<T, A> {
3755 /// Move a boxed object to a new, reference-counted allocation.
3756 ///
3757 /// # Example
3758 ///
3759 /// ```
3760 /// # use std::sync::Arc;
3761 /// let unique: Box<str> = Box::from("eggplant");
3762 /// let shared: Arc<str> = Arc::from(unique);
3763 /// assert_eq!("eggplant", &shared[..]);
3764 /// ```
3765 #[inline]
3766 fn from(v: Box<T, A>) -> Arc<T, A> {
3767 Arc::from_box_in(src:v)
3768 }
3769}
3770
3771#[cfg(not(no_global_oom_handling))]
3772#[stable(feature = "shared_from_slice", since = "1.21.0")]
3773impl<T, A: Allocator + Clone> From<Vec<T, A>> for Arc<[T], A> {
3774 /// Allocates a reference-counted slice and moves `v`'s items into it.
3775 ///
3776 /// # Example
3777 ///
3778 /// ```
3779 /// # use std::sync::Arc;
3780 /// let unique: Vec<i32> = vec![1, 2, 3];
3781 /// let shared: Arc<[i32]> = Arc::from(unique);
3782 /// assert_eq!(&[1, 2, 3], &shared[..]);
3783 /// ```
3784 #[inline]
3785 fn from(v: Vec<T, A>) -> Arc<[T], A> {
3786 unsafe {
3787 let (vec_ptr, len, cap, alloc) = v.into_raw_parts_with_alloc();
3788
3789 let rc_ptr = Self::allocate_for_slice_in(len, &alloc);
3790 ptr::copy_nonoverlapping(vec_ptr, (&raw mut (*rc_ptr).data) as *mut T, len);
3791
3792 // Create a `Vec<T, &A>` with length 0, to deallocate the buffer
3793 // without dropping its contents or the allocator
3794 let _ = Vec::from_raw_parts_in(vec_ptr, 0, cap, &alloc);
3795
3796 Self::from_ptr_in(rc_ptr, alloc)
3797 }
3798 }
3799}
3800
3801#[stable(feature = "shared_from_cow", since = "1.45.0")]
3802impl<'a, B> From<Cow<'a, B>> for Arc<B>
3803where
3804 B: ToOwned + ?Sized,
3805 Arc<B>: From<&'a B> + From<B::Owned>,
3806{
3807 /// Creates an atomically reference-counted pointer from a clone-on-write
3808 /// pointer by copying its content.
3809 ///
3810 /// # Example
3811 ///
3812 /// ```rust
3813 /// # use std::sync::Arc;
3814 /// # use std::borrow::Cow;
3815 /// let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
3816 /// let shared: Arc<str> = Arc::from(cow);
3817 /// assert_eq!("eggplant", &shared[..]);
3818 /// ```
3819 #[inline]
3820 fn from(cow: Cow<'a, B>) -> Arc<B> {
3821 match cow {
3822 Cow::Borrowed(s: &B) => Arc::from(s),
3823 Cow::Owned(s: ::Owned) => Arc::from(s),
3824 }
3825 }
3826}
3827
3828#[stable(feature = "shared_from_str", since = "1.62.0")]
3829impl From<Arc<str>> for Arc<[u8]> {
3830 /// Converts an atomically reference-counted string slice into a byte slice.
3831 ///
3832 /// # Example
3833 ///
3834 /// ```
3835 /// # use std::sync::Arc;
3836 /// let string: Arc<str> = Arc::from("eggplant");
3837 /// let bytes: Arc<[u8]> = Arc::from(string);
3838 /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
3839 /// ```
3840 #[inline]
3841 fn from(rc: Arc<str>) -> Self {
3842 // SAFETY: `str` has the same layout as `[u8]`.
3843 unsafe { Arc::from_raw(ptr:Arc::into_raw(this:rc) as *const [u8]) }
3844 }
3845}
3846
3847#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
3848impl<T, A: Allocator, const N: usize> TryFrom<Arc<[T], A>> for Arc<[T; N], A> {
3849 type Error = Arc<[T], A>;
3850
3851 fn try_from(boxed_slice: Arc<[T], A>) -> Result<Self, Self::Error> {
3852 if boxed_slice.len() == N {
3853 let (ptr: NonNull>, alloc: A) = Arc::into_inner_with_allocator(this:boxed_slice);
3854 Ok(unsafe { Arc::from_inner_in(ptr.cast(), alloc) })
3855 } else {
3856 Err(boxed_slice)
3857 }
3858 }
3859}
3860
3861#[cfg(not(no_global_oom_handling))]
3862#[stable(feature = "shared_from_iter", since = "1.37.0")]
3863impl<T> FromIterator<T> for Arc<[T]> {
3864 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
3865 ///
3866 /// # Performance characteristics
3867 ///
3868 /// ## The general case
3869 ///
3870 /// In the general case, collecting into `Arc<[T]>` is done by first
3871 /// collecting into a `Vec<T>`. That is, when writing the following:
3872 ///
3873 /// ```rust
3874 /// # use std::sync::Arc;
3875 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
3876 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
3877 /// ```
3878 ///
3879 /// this behaves as if we wrote:
3880 ///
3881 /// ```rust
3882 /// # use std::sync::Arc;
3883 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
3884 /// .collect::<Vec<_>>() // The first set of allocations happens here.
3885 /// .into(); // A second allocation for `Arc<[T]>` happens here.
3886 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
3887 /// ```
3888 ///
3889 /// This will allocate as many times as needed for constructing the `Vec<T>`
3890 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
3891 ///
3892 /// ## Iterators of known length
3893 ///
3894 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
3895 /// a single allocation will be made for the `Arc<[T]>`. For example:
3896 ///
3897 /// ```rust
3898 /// # use std::sync::Arc;
3899 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
3900 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
3901 /// ```
3902 fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
3903 ToArcSlice::to_arc_slice(iter.into_iter())
3904 }
3905}
3906
3907#[cfg(not(no_global_oom_handling))]
3908/// Specialization trait used for collecting into `Arc<[T]>`.
3909trait ToArcSlice<T>: Iterator<Item = T> + Sized {
3910 fn to_arc_slice(self) -> Arc<[T]>;
3911}
3912
3913#[cfg(not(no_global_oom_handling))]
3914impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
3915 default fn to_arc_slice(self) -> Arc<[T]> {
3916 self.collect::<Vec<T>>().into()
3917 }
3918}
3919
3920#[cfg(not(no_global_oom_handling))]
3921impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
3922 fn to_arc_slice(self) -> Arc<[T]> {
3923 // This is the case for a `TrustedLen` iterator.
3924 let (low, high) = self.size_hint();
3925 if let Some(high) = high {
3926 debug_assert_eq!(
3927 low,
3928 high,
3929 "TrustedLen iterator's size hint is not exact: {:?}",
3930 (low, high)
3931 );
3932
3933 unsafe {
3934 // SAFETY: We need to ensure that the iterator has an exact length and we have.
3935 Arc::from_iter_exact(self, low)
3936 }
3937 } else {
3938 // TrustedLen contract guarantees that `upper_bound == None` implies an iterator
3939 // length exceeding `usize::MAX`.
3940 // The default implementation would collect into a vec which would panic.
3941 // Thus we panic here immediately without invoking `Vec` code.
3942 panic!("capacity overflow");
3943 }
3944 }
3945}
3946
3947#[stable(feature = "rust1", since = "1.0.0")]
3948impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Arc<T, A> {
3949 fn borrow(&self) -> &T {
3950 &**self
3951 }
3952}
3953
3954#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
3955impl<T: ?Sized, A: Allocator> AsRef<T> for Arc<T, A> {
3956 fn as_ref(&self) -> &T {
3957 &**self
3958 }
3959}
3960
3961#[stable(feature = "pin", since = "1.33.0")]
3962impl<T: ?Sized, A: Allocator> Unpin for Arc<T, A> {}
3963
3964/// Gets the offset within an `ArcInner` for the payload behind a pointer.
3965///
3966/// # Safety
3967///
3968/// The pointer must point to (and have valid metadata for) a previously
3969/// valid instance of T, but the T is allowed to be dropped.
3970unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
3971 // Align the unsized value to the end of the ArcInner.
3972 // Because RcInner is repr(C), it will always be the last field in memory.
3973 // SAFETY: since the only unsized types possible are slices, trait objects,
3974 // and extern types, the input safety requirement is currently enough to
3975 // satisfy the requirements of align_of_val_raw; this is an implementation
3976 // detail of the language that must not be relied upon outside of std.
3977 unsafe { data_offset_align(align_of_val_raw(val:ptr)) }
3978}
3979
3980#[inline]
3981fn data_offset_align(align: usize) -> usize {
3982 let layout: Layout = Layout::new::<ArcInner<()>>();
3983 layout.size() + layout.padding_needed_for(align)
3984}
3985
3986/// A unique owning pointer to an [`ArcInner`] **that does not imply the contents are initialized,**
3987/// but will deallocate it (without dropping the value) when dropped.
3988///
3989/// This is a helper for [`Arc::make_mut()`] to ensure correct cleanup on panic.
3990#[cfg(not(no_global_oom_handling))]
3991struct UniqueArcUninit<T: ?Sized, A: Allocator> {
3992 ptr: NonNull<ArcInner<T>>,
3993 layout_for_value: Layout,
3994 alloc: Option<A>,
3995}
3996
3997#[cfg(not(no_global_oom_handling))]
3998impl<T: ?Sized, A: Allocator> UniqueArcUninit<T, A> {
3999 /// Allocates an ArcInner with layout suitable to contain `for_value` or a clone of it.
4000 fn new(for_value: &T, alloc: A) -> UniqueArcUninit<T, A> {
4001 let layout = Layout::for_value(for_value);
4002 let ptr = unsafe {
4003 Arc::allocate_for_layout(
4004 layout,
4005 |layout_for_arcinner| alloc.allocate(layout_for_arcinner),
4006 |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const ArcInner<T>),
4007 )
4008 };
4009 Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) }
4010 }
4011
4012 /// Returns the pointer to be written into to initialize the [`Arc`].
4013 fn data_ptr(&mut self) -> *mut T {
4014 let offset = data_offset_align(self.layout_for_value.align());
4015 unsafe { self.ptr.as_ptr().byte_add(offset) as *mut T }
4016 }
4017
4018 /// Upgrade this into a normal [`Arc`].
4019 ///
4020 /// # Safety
4021 ///
4022 /// The data must have been initialized (by writing to [`Self::data_ptr()`]).
4023 unsafe fn into_arc(self) -> Arc<T, A> {
4024 let mut this = ManuallyDrop::new(self);
4025 let ptr = this.ptr.as_ptr();
4026 let alloc = this.alloc.take().unwrap();
4027
4028 // SAFETY: The pointer is valid as per `UniqueArcUninit::new`, and the caller is responsible
4029 // for having initialized the data.
4030 unsafe { Arc::from_ptr_in(ptr, alloc) }
4031 }
4032}
4033
4034#[cfg(not(no_global_oom_handling))]
4035impl<T: ?Sized, A: Allocator> Drop for UniqueArcUninit<T, A> {
4036 fn drop(&mut self) {
4037 // SAFETY:
4038 // * new() produced a pointer safe to deallocate.
4039 // * We own the pointer unless into_arc() was called, which forgets us.
4040 unsafe {
4041 self.alloc.take().unwrap().deallocate(
4042 self.ptr.cast(),
4043 arcinner_layout_for_value_layout(self.layout_for_value),
4044 );
4045 }
4046 }
4047}
4048
4049#[stable(feature = "arc_error", since = "1.52.0")]
4050impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
4051 #[allow(deprecated, deprecated_in_future)]
4052 fn description(&self) -> &str {
4053 core::error::Error::description(&**self)
4054 }
4055
4056 #[allow(deprecated)]
4057 fn cause(&self) -> Option<&dyn core::error::Error> {
4058 core::error::Error::cause(&**self)
4059 }
4060
4061 fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
4062 core::error::Error::source(&**self)
4063 }
4064
4065 fn provide<'a>(&'a self, req: &mut core::error::Request<'a>) {
4066 core::error::Error::provide(&**self, request:req);
4067 }
4068}
4069