1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][Arc] documentation for more details.
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")]`.
13 use core
::cmp
::Ordering
;
15 use core
::hash
::{Hash, Hasher}
;
17 use core
::intrinsics
::abort
;
18 #[cfg(not(no_global_oom_handling))]
20 use core
::marker
::{PhantomData, Unsize}
;
21 #[cfg(not(no_global_oom_handling))]
22 use core
::mem
::size_of_val
;
23 use core
::mem
::{self, align_of_val_raw}
;
24 use core
::ops
::{CoerceUnsized, Deref, DispatchFromDyn, Receiver}
;
25 use core
::panic
::{RefUnwindSafe, UnwindSafe}
;
27 use core
::ptr
::{self, NonNull}
;
28 #[cfg(not(no_global_oom_handling))]
29 use core
::slice
::from_raw_parts_mut
;
30 use core
::sync
::atomic
;
31 use core
::sync
::atomic
::Ordering
::{Acquire, Relaxed, Release}
;
33 #[cfg(not(no_global_oom_handling))]
34 use crate::alloc
::handle_alloc_error
;
35 #[cfg(not(no_global_oom_handling))]
36 use crate::alloc
::{box_free, WriteCloneIntoRaw}
;
37 use crate::alloc
::{AllocError, Allocator, Global, Layout}
;
38 use crate::borrow
::{Cow, ToOwned}
;
39 use crate::boxed
::Box
;
40 use crate::rc
::is_dangling
;
41 #[cfg(not(no_global_oom_handling))]
42 use crate::string
::String
;
43 #[cfg(not(no_global_oom_handling))]
49 /// A soft limit on the amount of references that may be made to an `Arc`.
51 /// Going above this limit will abort your program (although not
52 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
53 /// Trying to go above it might call a `panic` (if not actually going above it).
55 /// This is a global invariant, and also applies when using a compare-exchange loop.
57 /// See comment in `Arc::clone`.
58 const MAX_REFCOUNT
: usize = (isize::MAX
) as usize;
60 /// The error in case either counter reaches above `MAX_REFCOUNT`, and we can `panic` safely.
61 const INTERNAL_OVERFLOW_ERROR
: &str = "Arc counter overflow";
63 #[cfg(not(sanitize = "thread"))]
64 macro_rules
! acquire
{
66 atomic
::fence(Acquire
)
70 // ThreadSanitizer does not support memory fences. To avoid false positive
71 // reports in Arc / Weak implementation use atomic loads for synchronization
73 #[cfg(sanitize = "thread")]
74 macro_rules
! acquire
{
80 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
81 /// Reference Counted'.
83 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
84 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
85 /// a new `Arc` instance, which points to the same allocation on the heap as the
86 /// source `Arc`, while increasing a reference count. When the last `Arc`
87 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
88 /// referred to as "inner value") is also dropped.
90 /// Shared references in Rust disallow mutation by default, and `Arc` is no
91 /// exception: you cannot generally obtain a mutable reference to something
92 /// inside an `Arc`. If you need to mutate through an `Arc`, use
93 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
96 /// **Note**: This type is only available on platforms that support atomic
97 /// loads and stores of pointers, which includes all platforms that support
98 /// the `std` crate but not all those which only support [`alloc`](crate).
99 /// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
103 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
104 /// counting. This means that it is thread-safe. The disadvantage is that
105 /// atomic operations are more expensive than ordinary memory accesses. If you
106 /// are not sharing reference-counted allocations between threads, consider using
107 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
108 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
109 /// However, a library might choose `Arc<T>` in order to give library consumers
110 /// more flexibility.
112 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
113 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
114 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
115 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
116 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
117 /// data, but it doesn't add thread safety to its data. Consider
118 /// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
119 /// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
120 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
121 /// non-atomic operations.
123 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
124 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
126 /// ## Breaking cycles with `Weak`
128 /// The [`downgrade`][downgrade] method can be used to create a non-owning
129 /// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
130 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
131 /// already been dropped. In other words, `Weak` pointers do not keep the value
132 /// inside the allocation alive; however, they *do* keep the allocation
133 /// (the backing store for the value) alive.
135 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
136 /// [`Weak`] is used to break cycles. For example, a tree could have
137 /// strong `Arc` pointers from parent nodes to children, and [`Weak`]
138 /// pointers from children back to their parents.
140 /// # Cloning references
142 /// Creating a new reference from an existing reference-counted pointer is done using the
143 /// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
146 /// use std::sync::Arc;
147 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
148 /// // The two syntaxes below are equivalent.
149 /// let a = foo.clone();
150 /// let b = Arc::clone(&foo);
151 /// // a, b, and foo are all Arcs that point to the same memory location
154 /// ## `Deref` behavior
156 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
157 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
158 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
159 /// functions, called using [fully qualified syntax]:
162 /// use std::sync::Arc;
164 /// let my_arc = Arc::new(());
165 /// let my_weak = Arc::downgrade(&my_arc);
168 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
169 /// fully qualified syntax. Some people prefer to use fully qualified syntax,
170 /// while others prefer using method-call syntax.
173 /// use std::sync::Arc;
175 /// let arc = Arc::new(());
176 /// // Method-call syntax
177 /// let arc2 = arc.clone();
178 /// // Fully qualified syntax
179 /// let arc3 = Arc::clone(&arc);
182 /// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
183 /// already been dropped.
185 /// [`Rc<T>`]: crate::rc::Rc
186 /// [clone]: Clone::clone
187 /// [mutex]: ../../std/sync/struct.Mutex.html
188 /// [rwlock]: ../../std/sync/struct.RwLock.html
189 /// [atomic]: core::sync::atomic
190 /// [deref]: core::ops::Deref
191 /// [downgrade]: Arc::downgrade
192 /// [upgrade]: Weak::upgrade
193 /// [RefCell\<T>]: core::cell::RefCell
194 /// [`RefCell<T>`]: core::cell::RefCell
195 /// [`std::sync`]: ../../std/sync/index.html
196 /// [`Arc::clone(&from)`]: Arc::clone
197 /// [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
201 /// Sharing some immutable data between threads:
203 // Note that we **do not** run these tests here. The windows builders get super
204 // unhappy if a thread outlives the main thread and then exits at the same time
205 // (something deadlocks) so we just avoid this entirely by not running these
208 /// use std::sync::Arc;
211 /// let five = Arc::new(5);
214 /// let five = Arc::clone(&five);
216 /// thread::spawn(move || {
217 /// println!("{five:?}");
222 /// Sharing a mutable [`AtomicUsize`]:
224 /// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
227 /// use std::sync::Arc;
228 /// use std::sync::atomic::{AtomicUsize, Ordering};
231 /// let val = Arc::new(AtomicUsize::new(5));
234 /// let val = Arc::clone(&val);
236 /// thread::spawn(move || {
237 /// let v = val.fetch_add(1, Ordering::SeqCst);
238 /// println!("{v:?}");
243 /// See the [`rc` documentation][rc_examples] for more examples of reference
244 /// counting in general.
246 /// [rc_examples]: crate::rc#examples
247 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
248 #[stable(feature = "rust1", since = "1.0.0")]
249 pub struct Arc
<T
: ?Sized
> {
250 ptr
: NonNull
<ArcInner
<T
>>,
251 phantom
: PhantomData
<ArcInner
<T
>>,
254 #[stable(feature = "rust1", since = "1.0.0")]
255 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Send
for Arc
<T
> {}
256 #[stable(feature = "rust1", since = "1.0.0")]
257 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Sync
for Arc
<T
> {}
259 #[stable(feature = "catch_unwind", since = "1.9.0")]
260 impl<T
: RefUnwindSafe
+ ?Sized
> UnwindSafe
for Arc
<T
> {}
262 #[unstable(feature = "coerce_unsized", issue = "18598")]
263 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Arc
<U
>> for Arc
<T
> {}
265 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
266 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> DispatchFromDyn
<Arc
<U
>> for Arc
<T
> {}
268 impl<T
: ?Sized
> Arc
<T
> {
269 unsafe fn from_inner(ptr
: NonNull
<ArcInner
<T
>>) -> Self {
270 Self { ptr, phantom: PhantomData }
273 unsafe fn from_ptr(ptr
: *mut ArcInner
<T
>) -> Self {
274 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
278 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
279 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
280 /// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
282 /// Since a `Weak` reference does not count towards ownership, it will not
283 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
284 /// guarantees about the value still being present. Thus it may return [`None`]
285 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
286 /// itself (the backing store) from being deallocated.
288 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
289 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
290 /// prevent circular references between [`Arc`] pointers, since mutual owning references
291 /// would never allow either [`Arc`] to be dropped. For example, a tree could
292 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
293 /// pointers from children back to their parents.
295 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
297 /// [`upgrade`]: Weak::upgrade
298 #[stable(feature = "arc_weak", since = "1.4.0")]
299 pub struct Weak
<T
: ?Sized
> {
300 // This is a `NonNull` to allow optimizing the size of this type in enums,
301 // but it is not necessarily a valid pointer.
302 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
303 // to allocate space on the heap. That's not a value a real pointer
304 // will ever have because RcBox has alignment at least 2.
305 // This is only possible when `T: Sized`; unsized `T` never dangle.
306 ptr
: NonNull
<ArcInner
<T
>>,
309 #[stable(feature = "arc_weak", since = "1.4.0")]
310 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Send
for Weak
<T
> {}
311 #[stable(feature = "arc_weak", since = "1.4.0")]
312 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Sync
for Weak
<T
> {}
314 #[unstable(feature = "coerce_unsized", issue = "18598")]
315 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Weak
<U
>> for Weak
<T
> {}
316 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
317 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> DispatchFromDyn
<Weak
<U
>> for Weak
<T
> {}
319 #[stable(feature = "arc_weak", since = "1.4.0")]
320 impl<T
: ?Sized
> fmt
::Debug
for Weak
<T
> {
321 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
326 // This is repr(C) to future-proof against possible field-reordering, which
327 // would interfere with otherwise safe [into|from]_raw() of transmutable
330 struct ArcInner
<T
: ?Sized
> {
331 strong
: atomic
::AtomicUsize
,
333 // the value usize::MAX acts as a sentinel for temporarily "locking" the
334 // ability to upgrade weak pointers or downgrade strong ones; this is used
335 // to avoid races in `make_mut` and `get_mut`.
336 weak
: atomic
::AtomicUsize
,
341 /// Calculate layout for `ArcInner<T>` using the inner value's layout
342 fn arcinner_layout_for_value_layout(layout
: Layout
) -> Layout
{
343 // Calculate layout using the given value layout.
344 // Previously, layout was calculated on the expression
345 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
346 // reference (see #54908).
347 Layout
::new
::<ArcInner
<()>>().extend(layout
).unwrap().0.pad_to_align()
350 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Send
for ArcInner
<T
> {}
351 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Sync
for ArcInner
<T
> {}
354 /// Constructs a new `Arc<T>`.
359 /// use std::sync::Arc;
361 /// let five = Arc::new(5);
363 #[cfg(not(no_global_oom_handling))]
365 #[stable(feature = "rust1", since = "1.0.0")]
366 pub fn new(data
: T
) -> Arc
<T
> {
367 // Start the weak pointer count as 1 which is the weak pointer that's
368 // held by all the strong pointers (kinda), see std/rc.rs for more info
369 let x
: Box
<_
> = Box
::new(ArcInner
{
370 strong
: atomic
::AtomicUsize
::new(1),
371 weak
: atomic
::AtomicUsize
::new(1),
374 unsafe { Self::from_inner(Box::leak(x).into()) }
377 /// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
378 /// to allow you to construct a `T` which holds a weak pointer to itself.
380 /// Generally, a structure circularly referencing itself, either directly or
381 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
382 /// Using this function, you get access to the weak pointer during the
383 /// initialization of `T`, before the `Arc<T>` is created, such that you can
384 /// clone and store it inside the `T`.
386 /// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
387 /// then calls your closure, giving it a `Weak<T>` to this allocation,
388 /// and only afterwards completes the construction of the `Arc<T>` by placing
389 /// the `T` returned from your closure into the allocation.
391 /// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
392 /// returns, calling [`upgrade`] on the weak reference inside your closure will
393 /// fail and result in a `None` value.
397 /// If `data_fn` panics, the panic is propagated to the caller, and the
398 /// temporary [`Weak<T>`] is dropped normally.
403 /// # #![allow(dead_code)]
404 /// use std::sync::{Arc, Weak};
407 /// me: Weak<Gadget>,
411 /// /// Construct a reference counted Gadget.
412 /// fn new() -> Arc<Self> {
413 /// // `me` is a `Weak<Gadget>` pointing at the new allocation of the
414 /// // `Arc` we're constructing.
415 /// Arc::new_cyclic(|me| {
416 /// // Create the actual struct here.
417 /// Gadget { me: me.clone() }
421 /// /// Return a reference counted pointer to Self.
422 /// fn me(&self) -> Arc<Self> {
423 /// self.me.upgrade().unwrap()
427 /// [`upgrade`]: Weak::upgrade
428 #[cfg(not(no_global_oom_handling))]
430 #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
431 pub fn new_cyclic
<F
>(data_fn
: F
) -> Arc
<T
>
433 F
: FnOnce(&Weak
<T
>) -> T
,
435 // Construct the inner in the "uninitialized" state with a single
437 let uninit_ptr
: NonNull
<_
> = Box
::leak(Box
::new(ArcInner
{
438 strong
: atomic
::AtomicUsize
::new(0),
439 weak
: atomic
::AtomicUsize
::new(1),
440 data
: mem
::MaybeUninit
::<T
>::uninit(),
443 let init_ptr
: NonNull
<ArcInner
<T
>> = uninit_ptr
.cast();
445 let weak
= Weak { ptr: init_ptr }
;
447 // It's important we don't give up ownership of the weak pointer, or
448 // else the memory might be freed by the time `data_fn` returns. If
449 // we really wanted to pass ownership, we could create an additional
450 // weak pointer for ourselves, but this would result in additional
451 // updates to the weak reference count which might not be necessary
453 let data
= data_fn(&weak
);
455 // Now we can properly initialize the inner value and turn our weak
456 // reference into a strong reference.
457 let strong
= unsafe {
458 let inner
= init_ptr
.as_ptr();
459 ptr
::write(ptr
::addr_of_mut
!((*inner
).data
), data
);
461 // The above write to the data field must be visible to any threads which
462 // observe a non-zero strong count. Therefore we need at least "Release" ordering
463 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
465 // "Acquire" ordering is not required. When considering the possible behaviours
466 // of `data_fn` we only need to look at what it could do with a reference to a
467 // non-upgradeable `Weak`:
468 // - It can *clone* the `Weak`, increasing the weak reference count.
469 // - It can drop those clones, decreasing the weak reference count (but never to zero).
471 // These side effects do not impact us in any way, and no other side effects are
472 // possible with safe code alone.
473 let prev_value
= (*inner
).strong
.fetch_add(1, Release
);
474 debug_assert_eq
!(prev_value
, 0, "No prior strong references should exist");
476 Arc
::from_inner(init_ptr
)
479 // Strong references should collectively own a shared weak reference,
480 // so don't run the destructor for our old weak reference.
485 /// Constructs a new `Arc` with uninitialized contents.
490 /// #![feature(new_uninit)]
491 /// #![feature(get_mut_unchecked)]
493 /// use std::sync::Arc;
495 /// let mut five = Arc::<u32>::new_uninit();
497 /// // Deferred initialization:
498 /// Arc::get_mut(&mut five).unwrap().write(5);
500 /// let five = unsafe { five.assume_init() };
502 /// assert_eq!(*five, 5)
504 #[cfg(not(no_global_oom_handling))]
505 #[unstable(feature = "new_uninit", issue = "63291")]
507 pub fn new_uninit() -> Arc
<mem
::MaybeUninit
<T
>> {
509 Arc
::from_ptr(Arc
::allocate_for_layout(
511 |layout
| Global
.allocate(layout
),
512 |mem
| mem
as *mut ArcInner
<mem
::MaybeUninit
<T
>>,
517 /// Constructs a new `Arc` with uninitialized contents, with the memory
518 /// being filled with `0` bytes.
520 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
526 /// #![feature(new_uninit)]
528 /// use std::sync::Arc;
530 /// let zero = Arc::<u32>::new_zeroed();
531 /// let zero = unsafe { zero.assume_init() };
533 /// assert_eq!(*zero, 0)
536 /// [zeroed]: mem::MaybeUninit::zeroed
537 #[cfg(not(no_global_oom_handling))]
538 #[unstable(feature = "new_uninit", issue = "63291")]
540 pub fn new_zeroed() -> Arc
<mem
::MaybeUninit
<T
>> {
542 Arc
::from_ptr(Arc
::allocate_for_layout(
544 |layout
| Global
.allocate_zeroed(layout
),
545 |mem
| mem
as *mut ArcInner
<mem
::MaybeUninit
<T
>>,
550 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
551 /// `data` will be pinned in memory and unable to be moved.
552 #[cfg(not(no_global_oom_handling))]
553 #[stable(feature = "pin", since = "1.33.0")]
555 pub fn pin(data
: T
) -> Pin
<Arc
<T
>> {
556 unsafe { Pin::new_unchecked(Arc::new(data)) }
559 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
560 #[unstable(feature = "allocator_api", issue = "32838")]
562 pub fn try_pin(data
: T
) -> Result
<Pin
<Arc
<T
>>, AllocError
> {
563 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
566 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
571 /// #![feature(allocator_api)]
572 /// use std::sync::Arc;
574 /// let five = Arc::try_new(5)?;
575 /// # Ok::<(), std::alloc::AllocError>(())
577 #[unstable(feature = "allocator_api", issue = "32838")]
579 pub fn try_new(data
: T
) -> Result
<Arc
<T
>, AllocError
> {
580 // Start the weak pointer count as 1 which is the weak pointer that's
581 // held by all the strong pointers (kinda), see std/rc.rs for more info
582 let x
: Box
<_
> = Box
::try_new(ArcInner
{
583 strong
: atomic
::AtomicUsize
::new(1),
584 weak
: atomic
::AtomicUsize
::new(1),
587 unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
590 /// Constructs a new `Arc` with uninitialized contents, returning an error
591 /// if allocation fails.
596 /// #![feature(new_uninit, allocator_api)]
597 /// #![feature(get_mut_unchecked)]
599 /// use std::sync::Arc;
601 /// let mut five = Arc::<u32>::try_new_uninit()?;
603 /// // Deferred initialization:
604 /// Arc::get_mut(&mut five).unwrap().write(5);
606 /// let five = unsafe { five.assume_init() };
608 /// assert_eq!(*five, 5);
609 /// # Ok::<(), std::alloc::AllocError>(())
611 #[unstable(feature = "allocator_api", issue = "32838")]
612 // #[unstable(feature = "new_uninit", issue = "63291")]
613 pub fn try_new_uninit() -> Result
<Arc
<mem
::MaybeUninit
<T
>>, AllocError
> {
615 Ok(Arc
::from_ptr(Arc
::try_allocate_for_layout(
617 |layout
| Global
.allocate(layout
),
618 |mem
| mem
as *mut ArcInner
<mem
::MaybeUninit
<T
>>,
623 /// Constructs a new `Arc` with uninitialized contents, with the memory
624 /// being filled with `0` bytes, returning an error if allocation fails.
626 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
632 /// #![feature(new_uninit, allocator_api)]
634 /// use std::sync::Arc;
636 /// let zero = Arc::<u32>::try_new_zeroed()?;
637 /// let zero = unsafe { zero.assume_init() };
639 /// assert_eq!(*zero, 0);
640 /// # Ok::<(), std::alloc::AllocError>(())
643 /// [zeroed]: mem::MaybeUninit::zeroed
644 #[unstable(feature = "allocator_api", issue = "32838")]
645 // #[unstable(feature = "new_uninit", issue = "63291")]
646 pub fn try_new_zeroed() -> Result
<Arc
<mem
::MaybeUninit
<T
>>, AllocError
> {
648 Ok(Arc
::from_ptr(Arc
::try_allocate_for_layout(
650 |layout
| Global
.allocate_zeroed(layout
),
651 |mem
| mem
as *mut ArcInner
<mem
::MaybeUninit
<T
>>,
655 /// Returns the inner value, if the `Arc` has exactly one strong reference.
657 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
660 /// This will succeed even if there are outstanding weak references.
662 /// It is strongly recommended to use [`Arc::into_inner`] instead if you don't
663 /// want to keep the `Arc` in the [`Err`] case.
664 /// Immediately dropping the [`Err`] payload, like in the expression
665 /// `Arc::try_unwrap(this).ok()`, can still cause the strong count to
666 /// drop to zero and the inner value of the `Arc` to be dropped:
667 /// For instance if two threads each execute this expression in parallel, then
668 /// there is a race condition. The threads could first both check whether they
669 /// have the last clone of their `Arc` via `Arc::try_unwrap`, and then
670 /// both drop their `Arc` in the call to [`ok`][`Result::ok`],
671 /// taking the strong count from two down to zero.
676 /// use std::sync::Arc;
678 /// let x = Arc::new(3);
679 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
681 /// let x = Arc::new(4);
682 /// let _y = Arc::clone(&x);
683 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
686 #[stable(feature = "arc_unique", since = "1.4.0")]
687 pub fn try_unwrap(this
: Self) -> Result
<T
, Self> {
688 if this
.inner().strong
.compare_exchange(1, 0, Relaxed
, Relaxed
).is_err() {
692 acquire
!(this
.inner().strong
);
695 let elem
= ptr
::read(&this
.ptr
.as_ref().data
);
697 // Make a weak pointer to clean up the implicit strong-weak reference
698 let _weak
= Weak { ptr: this.ptr }
;
705 /// Returns the inner value, if the `Arc` has exactly one strong reference.
707 /// Otherwise, [`None`] is returned and the `Arc` is dropped.
709 /// This will succeed even if there are outstanding weak references.
711 /// If `Arc::into_inner` is called on every clone of this `Arc`,
712 /// it is guaranteed that exactly one of the calls returns the inner value.
713 /// This means in particular that the inner value is not dropped.
715 /// The similar expression `Arc::try_unwrap(this).ok()` does not
716 /// offer such a guarantee. See the last example below
717 /// and the documentation of [`Arc::try_unwrap`].
721 /// Minimal example demonstrating the guarantee that `Arc::into_inner` gives.
723 /// use std::sync::Arc;
725 /// let x = Arc::new(3);
726 /// let y = Arc::clone(&x);
728 /// // Two threads calling `Arc::into_inner` on both clones of an `Arc`:
729 /// let x_thread = std::thread::spawn(|| Arc::into_inner(x));
730 /// let y_thread = std::thread::spawn(|| Arc::into_inner(y));
732 /// let x_inner_value = x_thread.join().unwrap();
733 /// let y_inner_value = y_thread.join().unwrap();
735 /// // One of the threads is guaranteed to receive the inner value:
736 /// assert!(matches!(
737 /// (x_inner_value, y_inner_value),
738 /// (None, Some(3)) | (Some(3), None)
740 /// // The result could also be `(None, None)` if the threads called
741 /// // `Arc::try_unwrap(x).ok()` and `Arc::try_unwrap(y).ok()` instead.
744 /// A more practical example demonstrating the need for `Arc::into_inner`:
746 /// use std::sync::Arc;
748 /// // Definition of a simple singly linked list using `Arc`:
750 /// struct LinkedList<T>(Option<Arc<Node<T>>>);
751 /// struct Node<T>(T, Option<Arc<Node<T>>>);
753 /// // Dropping a long `LinkedList<T>` relying on the destructor of `Arc`
754 /// // can cause a stack overflow. To prevent this, we can provide a
755 /// // manual `Drop` implementation that does the destruction in a loop:
756 /// impl<T> Drop for LinkedList<T> {
757 /// fn drop(&mut self) {
758 /// let mut link = self.0.take();
759 /// while let Some(arc_node) = link.take() {
760 /// if let Some(Node(_value, next)) = Arc::into_inner(arc_node) {
767 /// // Implementation of `new` and `push` omitted
768 /// impl<T> LinkedList<T> {
770 /// # fn new() -> Self {
771 /// # LinkedList(None)
773 /// # fn push(&mut self, x: T) {
774 /// # self.0 = Some(Arc::new(Node(x, self.0.take())));
778 /// // The following code could have still caused a stack overflow
779 /// // despite the manual `Drop` impl if that `Drop` impl had used
780 /// // `Arc::try_unwrap(arc).ok()` instead of `Arc::into_inner(arc)`.
782 /// // Create a long list and clone it
783 /// let mut x = LinkedList::new();
784 /// for i in 0..100000 {
785 /// x.push(i); // Adds i to the front of x
787 /// let y = x.clone();
789 /// // Drop the clones in parallel
790 /// let x_thread = std::thread::spawn(|| drop(x));
791 /// let y_thread = std::thread::spawn(|| drop(y));
792 /// x_thread.join().unwrap();
793 /// y_thread.join().unwrap();
796 #[stable(feature = "arc_into_inner", since = "1.70.0")]
797 pub fn into_inner(this
: Self) -> Option
<T
> {
798 // Make sure that the ordinary `Drop` implementation isn’t called as well
799 let mut this
= mem
::ManuallyDrop
::new(this
);
801 // Following the implementation of `drop` and `drop_slow`
802 if this
.inner().strong
.fetch_sub(1, Release
) != 1 {
806 acquire
!(this
.inner().strong
);
808 // SAFETY: This mirrors the line
810 // unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
812 // in `drop_slow`. Instead of dropping the value behind the pointer,
813 // it is read and eventually returned; `ptr::read` has the same
814 // safety conditions as `ptr::drop_in_place`.
815 let inner
= unsafe { ptr::read(Self::get_mut_unchecked(&mut this)) }
;
817 drop(Weak { ptr: this.ptr }
);
824 /// Constructs a new atomically reference-counted slice with uninitialized contents.
829 /// #![feature(new_uninit)]
830 /// #![feature(get_mut_unchecked)]
832 /// use std::sync::Arc;
834 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
836 /// // Deferred initialization:
837 /// let data = Arc::get_mut(&mut values).unwrap();
838 /// data[0].write(1);
839 /// data[1].write(2);
840 /// data[2].write(3);
842 /// let values = unsafe { values.assume_init() };
844 /// assert_eq!(*values, [1, 2, 3])
846 #[cfg(not(no_global_oom_handling))]
847 #[unstable(feature = "new_uninit", issue = "63291")]
849 pub fn new_uninit_slice(len
: usize) -> Arc
<[mem
::MaybeUninit
<T
>]> {
850 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
853 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
854 /// filled with `0` bytes.
856 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
857 /// incorrect usage of this method.
862 /// #![feature(new_uninit)]
864 /// use std::sync::Arc;
866 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
867 /// let values = unsafe { values.assume_init() };
869 /// assert_eq!(*values, [0, 0, 0])
872 /// [zeroed]: mem::MaybeUninit::zeroed
873 #[cfg(not(no_global_oom_handling))]
874 #[unstable(feature = "new_uninit", issue = "63291")]
876 pub fn new_zeroed_slice(len
: usize) -> Arc
<[mem
::MaybeUninit
<T
>]> {
878 Arc
::from_ptr(Arc
::allocate_for_layout(
879 Layout
::array
::<T
>(len
).unwrap(),
880 |layout
| Global
.allocate_zeroed(layout
),
882 ptr
::slice_from_raw_parts_mut(mem
as *mut T
, len
)
883 as *mut ArcInner
<[mem
::MaybeUninit
<T
>]>
890 impl<T
> Arc
<mem
::MaybeUninit
<T
>> {
891 /// Converts to `Arc<T>`.
895 /// As with [`MaybeUninit::assume_init`],
896 /// it is up to the caller to guarantee that the inner value
897 /// really is in an initialized state.
898 /// Calling this when the content is not yet fully initialized
899 /// causes immediate undefined behavior.
901 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
906 /// #![feature(new_uninit)]
907 /// #![feature(get_mut_unchecked)]
909 /// use std::sync::Arc;
911 /// let mut five = Arc::<u32>::new_uninit();
913 /// // Deferred initialization:
914 /// Arc::get_mut(&mut five).unwrap().write(5);
916 /// let five = unsafe { five.assume_init() };
918 /// assert_eq!(*five, 5)
920 #[unstable(feature = "new_uninit", issue = "63291")]
921 #[must_use = "`self` will be dropped if the result is not used"]
923 pub unsafe fn assume_init(self) -> Arc
<T
> {
924 unsafe { Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast()) }
928 impl<T
> Arc
<[mem
::MaybeUninit
<T
>]> {
929 /// Converts to `Arc<[T]>`.
933 /// As with [`MaybeUninit::assume_init`],
934 /// it is up to the caller to guarantee that the inner value
935 /// really is in an initialized state.
936 /// Calling this when the content is not yet fully initialized
937 /// causes immediate undefined behavior.
939 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
944 /// #![feature(new_uninit)]
945 /// #![feature(get_mut_unchecked)]
947 /// use std::sync::Arc;
949 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
951 /// // Deferred initialization:
952 /// let data = Arc::get_mut(&mut values).unwrap();
953 /// data[0].write(1);
954 /// data[1].write(2);
955 /// data[2].write(3);
957 /// let values = unsafe { values.assume_init() };
959 /// assert_eq!(*values, [1, 2, 3])
961 #[unstable(feature = "new_uninit", issue = "63291")]
962 #[must_use = "`self` will be dropped if the result is not used"]
964 pub unsafe fn assume_init(self) -> Arc
<[T
]> {
965 unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
969 impl<T
: ?Sized
> Arc
<T
> {
970 /// Consumes the `Arc`, returning the wrapped pointer.
972 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
973 /// [`Arc::from_raw`].
978 /// use std::sync::Arc;
980 /// let x = Arc::new("hello".to_owned());
981 /// let x_ptr = Arc::into_raw(x);
982 /// assert_eq!(unsafe { &*x_ptr }, "hello");
984 #[must_use = "losing the pointer will leak memory"]
985 #[stable(feature = "rc_raw", since = "1.17.0")]
986 pub fn into_raw(this
: Self) -> *const T
{
987 let ptr
= Self::as_ptr(&this
);
992 /// Provides a raw pointer to the data.
994 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
995 /// as long as there are strong counts in the `Arc`.
1000 /// use std::sync::Arc;
1002 /// let x = Arc::new("hello".to_owned());
1003 /// let y = Arc::clone(&x);
1004 /// let x_ptr = Arc::as_ptr(&x);
1005 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
1006 /// assert_eq!(unsafe { &*x_ptr }, "hello");
1009 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
1010 pub fn as_ptr(this
: &Self) -> *const T
{
1011 let ptr
: *mut ArcInner
<T
> = NonNull
::as_ptr(this
.ptr
);
1013 // SAFETY: This cannot go through Deref::deref or RcBoxPtr::inner because
1014 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
1015 // write through the pointer after the Rc is recovered through `from_raw`.
1016 unsafe { ptr::addr_of_mut!((*ptr).data) }
1019 /// Constructs an `Arc<T>` from a raw pointer.
1021 /// The raw pointer must have been previously returned by a call to
1022 /// [`Arc<U>::into_raw`][into_raw] where `U` must have the same size and
1023 /// alignment as `T`. This is trivially true if `U` is `T`.
1024 /// Note that if `U` is not `T` but has the same size and alignment, this is
1025 /// basically like transmuting references of different types. See
1026 /// [`mem::transmute`][transmute] for more information on what
1027 /// restrictions apply in this case.
1029 /// The user of `from_raw` has to make sure a specific value of `T` is only
1032 /// This function is unsafe because improper use may lead to memory unsafety,
1033 /// even if the returned `Arc<T>` is never accessed.
1035 /// [into_raw]: Arc::into_raw
1036 /// [transmute]: core::mem::transmute
1041 /// use std::sync::Arc;
1043 /// let x = Arc::new("hello".to_owned());
1044 /// let x_ptr = Arc::into_raw(x);
1047 /// // Convert back to an `Arc` to prevent leak.
1048 /// let x = Arc::from_raw(x_ptr);
1049 /// assert_eq!(&*x, "hello");
1051 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1054 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1056 #[stable(feature = "rc_raw", since = "1.17.0")]
1057 pub unsafe fn from_raw(ptr
: *const T
) -> Self {
1059 let offset
= data_offset(ptr
);
1061 // Reverse the offset to find the original ArcInner.
1062 let arc_ptr
= ptr
.byte_sub(offset
) as *mut ArcInner
<T
>;
1064 Self::from_ptr(arc_ptr
)
1068 /// Creates a new [`Weak`] pointer to this allocation.
1073 /// use std::sync::Arc;
1075 /// let five = Arc::new(5);
1077 /// let weak_five = Arc::downgrade(&five);
1079 #[must_use = "this returns a new `Weak` pointer, \
1080 without modifying the original `Arc`"]
1081 #[stable(feature = "arc_weak", since = "1.4.0")]
1082 pub fn downgrade(this
: &Self) -> Weak
<T
> {
1083 // This Relaxed is OK because we're checking the value in the CAS
1085 let mut cur
= this
.inner().weak
.load(Relaxed
);
1088 // check if the weak counter is currently "locked"; if so, spin.
1089 if cur
== usize::MAX
{
1091 cur
= this
.inner().weak
.load(Relaxed
);
1095 // We can't allow the refcount to increase much past `MAX_REFCOUNT`.
1096 assert
!(cur
<= MAX_REFCOUNT
, "{}", INTERNAL_OVERFLOW_ERROR
);
1098 // NOTE: this code currently ignores the possibility of overflow
1099 // into usize::MAX; in general both Rc and Arc need to be adjusted
1100 // to deal with overflow.
1102 // Unlike with Clone(), we need this to be an Acquire read to
1103 // synchronize with the write coming from `is_unique`, so that the
1104 // events prior to that write happen before this read.
1105 match this
.inner().weak
.compare_exchange_weak(cur
, cur
+ 1, Acquire
, Relaxed
) {
1107 // Make sure we do not create a dangling Weak
1108 debug_assert
!(!is_dangling(this
.ptr
.as_ptr()));
1109 return Weak { ptr: this.ptr }
;
1111 Err(old
) => cur
= old
,
1116 /// Gets the number of [`Weak`] pointers to this allocation.
1120 /// This method by itself is safe, but using it correctly requires extra care.
1121 /// Another thread can change the weak count at any time,
1122 /// including potentially between calling this method and acting on the result.
1127 /// use std::sync::Arc;
1129 /// let five = Arc::new(5);
1130 /// let _weak_five = Arc::downgrade(&five);
1132 /// // This assertion is deterministic because we haven't shared
1133 /// // the `Arc` or `Weak` between threads.
1134 /// assert_eq!(1, Arc::weak_count(&five));
1138 #[stable(feature = "arc_counts", since = "1.15.0")]
1139 pub fn weak_count(this
: &Self) -> usize {
1140 let cnt
= this
.inner().weak
.load(Acquire
);
1141 // If the weak count is currently locked, the value of the
1142 // count was 0 just before taking the lock.
1143 if cnt
== usize::MAX { 0 }
else { cnt - 1 }
1146 /// Gets the number of strong (`Arc`) pointers to this allocation.
1150 /// This method by itself is safe, but using it correctly requires extra care.
1151 /// Another thread can change the strong count at any time,
1152 /// including potentially between calling this method and acting on the result.
1157 /// use std::sync::Arc;
1159 /// let five = Arc::new(5);
1160 /// let _also_five = Arc::clone(&five);
1162 /// // This assertion is deterministic because we haven't shared
1163 /// // the `Arc` between threads.
1164 /// assert_eq!(2, Arc::strong_count(&five));
1168 #[stable(feature = "arc_counts", since = "1.15.0")]
1169 pub fn strong_count(this
: &Self) -> usize {
1170 this
.inner().strong
.load(Acquire
)
1173 /// Increments the strong reference count on the `Arc<T>` associated with the
1174 /// provided pointer by one.
1178 /// The pointer must have been obtained through `Arc::into_raw`, and the
1179 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1180 /// least 1) for the duration of this method.
1185 /// use std::sync::Arc;
1187 /// let five = Arc::new(5);
1190 /// let ptr = Arc::into_raw(five);
1191 /// Arc::increment_strong_count(ptr);
1193 /// // This assertion is deterministic because we haven't shared
1194 /// // the `Arc` between threads.
1195 /// let five = Arc::from_raw(ptr);
1196 /// assert_eq!(2, Arc::strong_count(&five));
1200 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1201 pub unsafe fn increment_strong_count(ptr
: *const T
) {
1202 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1203 let arc
= unsafe { mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr)) }
;
1204 // Now increase refcount, but don't drop new refcount either
1205 let _arc_clone
: mem
::ManuallyDrop
<_
> = arc
.clone();
1208 /// Decrements the strong reference count on the `Arc<T>` associated with the
1209 /// provided pointer by one.
1213 /// The pointer must have been obtained through `Arc::into_raw`, and the
1214 /// associated `Arc` instance must be valid (i.e. the strong count must be at
1215 /// least 1) when invoking this method. This method can be used to release the final
1216 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1222 /// use std::sync::Arc;
1224 /// let five = Arc::new(5);
1227 /// let ptr = Arc::into_raw(five);
1228 /// Arc::increment_strong_count(ptr);
1230 /// // Those assertions are deterministic because we haven't shared
1231 /// // the `Arc` between threads.
1232 /// let five = Arc::from_raw(ptr);
1233 /// assert_eq!(2, Arc::strong_count(&five));
1234 /// Arc::decrement_strong_count(ptr);
1235 /// assert_eq!(1, Arc::strong_count(&five));
1239 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1240 pub unsafe fn decrement_strong_count(ptr
: *const T
) {
1241 unsafe { drop(Arc::from_raw(ptr)) }
;
1245 fn inner(&self) -> &ArcInner
<T
> {
1246 // This unsafety is ok because while this arc is alive we're guaranteed
1247 // that the inner pointer is valid. Furthermore, we know that the
1248 // `ArcInner` structure itself is `Sync` because the inner data is
1249 // `Sync` as well, so we're ok loaning out an immutable pointer to these
1251 unsafe { self.ptr.as_ref() }
1254 // Non-inlined part of `drop`.
1256 unsafe fn drop_slow(&mut self) {
1257 // Destroy the data at this time, even though we must not free the box
1258 // allocation itself (there might still be weak pointers lying around).
1259 unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) }
;
1261 // Drop the weak ref collectively held by all strong references
1262 drop(Weak { ptr: self.ptr }
);
1265 /// Returns `true` if the two `Arc`s point to the same allocation in a vein similar to
1266 /// [`ptr::eq`]. See [that function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
1271 /// use std::sync::Arc;
1273 /// let five = Arc::new(5);
1274 /// let same_five = Arc::clone(&five);
1275 /// let other_five = Arc::new(5);
1277 /// assert!(Arc::ptr_eq(&five, &same_five));
1278 /// assert!(!Arc::ptr_eq(&five, &other_five));
1281 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1284 #[stable(feature = "ptr_eq", since = "1.17.0")]
1285 pub fn ptr_eq(this
: &Self, other
: &Self) -> bool
{
1286 this
.ptr
.as_ptr() == other
.ptr
.as_ptr()
1290 impl<T
: ?Sized
> Arc
<T
> {
1291 /// Allocates an `ArcInner<T>` with sufficient space for
1292 /// a possibly-unsized inner value where the value has the layout provided.
1294 /// The function `mem_to_arcinner` is called with the data pointer
1295 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1296 #[cfg(not(no_global_oom_handling))]
1297 unsafe fn allocate_for_layout(
1298 value_layout
: Layout
,
1299 allocate
: impl FnOnce(Layout
) -> Result
<NonNull
<[u8]>, AllocError
>,
1300 mem_to_arcinner
: impl FnOnce(*mut u8) -> *mut ArcInner
<T
>,
1301 ) -> *mut ArcInner
<T
> {
1302 let layout
= arcinner_layout_for_value_layout(value_layout
);
1304 Arc
::try_allocate_for_layout(value_layout
, allocate
, mem_to_arcinner
)
1305 .unwrap_or_else(|_
| handle_alloc_error(layout
))
1309 /// Allocates an `ArcInner<T>` with sufficient space for
1310 /// a possibly-unsized inner value where the value has the layout provided,
1311 /// returning an error if allocation fails.
1313 /// The function `mem_to_arcinner` is called with the data pointer
1314 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1315 unsafe fn try_allocate_for_layout(
1316 value_layout
: Layout
,
1317 allocate
: impl FnOnce(Layout
) -> Result
<NonNull
<[u8]>, AllocError
>,
1318 mem_to_arcinner
: impl FnOnce(*mut u8) -> *mut ArcInner
<T
>,
1319 ) -> Result
<*mut ArcInner
<T
>, AllocError
> {
1320 let layout
= arcinner_layout_for_value_layout(value_layout
);
1322 let ptr
= allocate(layout
)?
;
1324 // Initialize the ArcInner
1325 let inner
= mem_to_arcinner(ptr
.as_non_null_ptr().as_ptr());
1326 debug_assert_eq
!(unsafe { Layout::for_value(&*inner) }
, layout
);
1329 ptr
::write(&mut (*inner
).strong
, atomic
::AtomicUsize
::new(1));
1330 ptr
::write(&mut (*inner
).weak
, atomic
::AtomicUsize
::new(1));
1336 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1337 #[cfg(not(no_global_oom_handling))]
1338 unsafe fn allocate_for_ptr(ptr
: *const T
) -> *mut ArcInner
<T
> {
1339 // Allocate for the `ArcInner<T>` using the given value.
1341 Self::allocate_for_layout(
1342 Layout
::for_value(&*ptr
),
1343 |layout
| Global
.allocate(layout
),
1344 |mem
| mem
.with_metadata_of(ptr
as *const ArcInner
<T
>),
1349 #[cfg(not(no_global_oom_handling))]
1350 fn from_box(v
: Box
<T
>) -> Arc
<T
> {
1352 let (box_unique
, alloc
) = Box
::into_unique(v
);
1353 let bptr
= box_unique
.as_ptr();
1355 let value_size
= size_of_val(&*bptr
);
1356 let ptr
= Self::allocate_for_ptr(bptr
);
1358 // Copy value as bytes
1359 ptr
::copy_nonoverlapping(
1360 bptr
as *const T
as *const u8,
1361 &mut (*ptr
).data
as *mut _
as *mut u8,
1365 // Free the allocation without dropping its contents
1366 box_free(box_unique
, alloc
);
1374 /// Allocates an `ArcInner<[T]>` with the given length.
1375 #[cfg(not(no_global_oom_handling))]
1376 unsafe fn allocate_for_slice(len
: usize) -> *mut ArcInner
<[T
]> {
1378 Self::allocate_for_layout(
1379 Layout
::array
::<T
>(len
).unwrap(),
1380 |layout
| Global
.allocate(layout
),
1381 |mem
| ptr
::slice_from_raw_parts_mut(mem
as *mut T
, len
) as *mut ArcInner
<[T
]>,
1386 /// Copy elements from slice into newly allocated `Arc<[T]>`
1388 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
1389 #[cfg(not(no_global_oom_handling))]
1390 unsafe fn copy_from_slice(v
: &[T
]) -> Arc
<[T
]> {
1392 let ptr
= Self::allocate_for_slice(v
.len());
1394 ptr
::copy_nonoverlapping(v
.as_ptr(), &mut (*ptr
).data
as *mut [T
] as *mut T
, v
.len());
1400 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
1402 /// Behavior is undefined should the size be wrong.
1403 #[cfg(not(no_global_oom_handling))]
1404 unsafe fn from_iter_exact(iter
: impl Iterator
<Item
= T
>, len
: usize) -> Arc
<[T
]> {
1405 // Panic guard while cloning T elements.
1406 // In the event of a panic, elements that have been written
1407 // into the new ArcInner will be dropped, then the memory freed.
1415 impl<T
> Drop
for Guard
<T
> {
1416 fn drop(&mut self) {
1418 let slice
= from_raw_parts_mut(self.elems
, self.n_elems
);
1419 ptr
::drop_in_place(slice
);
1421 Global
.deallocate(self.mem
, self.layout
);
1427 let ptr
= Self::allocate_for_slice(len
);
1429 let mem
= ptr
as *mut _
as *mut u8;
1430 let layout
= Layout
::for_value(&*ptr
);
1432 // Pointer to first element
1433 let elems
= &mut (*ptr
).data
as *mut [T
] as *mut T
;
1435 let mut guard
= Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 }
;
1437 for (i
, item
) in iter
.enumerate() {
1438 ptr
::write(elems
.add(i
), item
);
1442 // All clear. Forget the guard so it doesn't free the new ArcInner.
1450 /// Specialization trait used for `From<&[T]>`.
1451 #[cfg(not(no_global_oom_handling))]
1452 trait ArcFromSlice
<T
> {
1453 fn from_slice(slice
: &[T
]) -> Self;
1456 #[cfg(not(no_global_oom_handling))]
1457 impl<T
: Clone
> ArcFromSlice
<T
> for Arc
<[T
]> {
1459 default fn from_slice(v
: &[T
]) -> Self {
1460 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1464 #[cfg(not(no_global_oom_handling))]
1465 impl<T
: Copy
> ArcFromSlice
<T
> for Arc
<[T
]> {
1467 fn from_slice(v
: &[T
]) -> Self {
1468 unsafe { Arc::copy_from_slice(v) }
1472 #[stable(feature = "rust1", since = "1.0.0")]
1473 impl<T
: ?Sized
> Clone
for Arc
<T
> {
1474 /// Makes a clone of the `Arc` pointer.
1476 /// This creates another pointer to the same allocation, increasing the
1477 /// strong reference count.
1482 /// use std::sync::Arc;
1484 /// let five = Arc::new(5);
1486 /// let _ = Arc::clone(&five);
1489 fn clone(&self) -> Arc
<T
> {
1490 // Using a relaxed ordering is alright here, as knowledge of the
1491 // original reference prevents other threads from erroneously deleting
1494 // As explained in the [Boost documentation][1], Increasing the
1495 // reference counter can always be done with memory_order_relaxed: New
1496 // references to an object can only be formed from an existing
1497 // reference, and passing an existing reference from one thread to
1498 // another must already provide any required synchronization.
1500 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1501 let old_size
= self.inner().strong
.fetch_add(1, Relaxed
);
1503 // However we need to guard against massive refcounts in case someone is `mem::forget`ing
1504 // Arcs. If we don't do this the count can overflow and users will use-after free. This
1505 // branch will never be taken in any realistic program. We abort because such a program is
1506 // incredibly degenerate, and we don't care to support it.
1508 // This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
1509 // But we do that check *after* having done the increment, so there is a chance here that
1510 // the worst already happened and we actually do overflow the `usize` counter. However, that
1511 // requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
1512 // above and the `abort` below, which seems exceedingly unlikely.
1514 // This is a global invariant, and also applies when using a compare-exchange loop to increment
1515 // counters in other methods.
1516 // Otherwise, the counter could be brought to an almost-overflow using a compare-exchange loop,
1517 // and then overflow using a few `fetch_add`s.
1518 if old_size
> MAX_REFCOUNT
{
1522 unsafe { Self::from_inner(self.ptr) }
1526 #[stable(feature = "rust1", since = "1.0.0")]
1527 impl<T
: ?Sized
> Deref
for Arc
<T
> {
1531 fn deref(&self) -> &T
{
1536 #[unstable(feature = "receiver_trait", issue = "none")]
1537 impl<T
: ?Sized
> Receiver
for Arc
<T
> {}
1539 impl<T
: Clone
> Arc
<T
> {
1540 /// Makes a mutable reference into the given `Arc`.
1542 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
1543 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1544 /// referred to as clone-on-write.
1546 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
1547 /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
1550 /// See also [`get_mut`], which will fail rather than cloning the inner value
1551 /// or dissociating [`Weak`] pointers.
1553 /// [`clone`]: Clone::clone
1554 /// [`get_mut`]: Arc::get_mut
1559 /// use std::sync::Arc;
1561 /// let mut data = Arc::new(5);
1563 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1564 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1565 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1566 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1567 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1569 /// // Now `data` and `other_data` point to different allocations.
1570 /// assert_eq!(*data, 8);
1571 /// assert_eq!(*other_data, 12);
1574 /// [`Weak`] pointers will be dissociated:
1577 /// use std::sync::Arc;
1579 /// let mut data = Arc::new(75);
1580 /// let weak = Arc::downgrade(&data);
1582 /// assert!(75 == *data);
1583 /// assert!(75 == *weak.upgrade().unwrap());
1585 /// *Arc::make_mut(&mut data) += 1;
1587 /// assert!(76 == *data);
1588 /// assert!(weak.upgrade().is_none());
1590 #[cfg(not(no_global_oom_handling))]
1592 #[stable(feature = "arc_unique", since = "1.4.0")]
1593 pub fn make_mut(this
: &mut Self) -> &mut T
{
1594 // Note that we hold both a strong reference and a weak reference.
1595 // Thus, releasing our strong reference only will not, by itself, cause
1596 // the memory to be deallocated.
1598 // Use Acquire to ensure that we see any writes to `weak` that happen
1599 // before release writes (i.e., decrements) to `strong`. Since we hold a
1600 // weak count, there's no chance the ArcInner itself could be
1602 if this
.inner().strong
.compare_exchange(1, 0, Acquire
, Relaxed
).is_err() {
1603 // Another strong pointer exists, so we must clone.
1604 // Pre-allocate memory to allow writing the cloned value directly.
1605 let mut arc
= Self::new_uninit();
1607 let data
= Arc
::get_mut_unchecked(&mut arc
);
1608 (**this
).write_clone_into_raw(data
.as_mut_ptr());
1609 *this
= arc
.assume_init();
1611 } else if this
.inner().weak
.load(Relaxed
) != 1 {
1612 // Relaxed suffices in the above because this is fundamentally an
1613 // optimization: we are always racing with weak pointers being
1614 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1616 // We removed the last strong ref, but there are additional weak
1617 // refs remaining. We'll move the contents to a new Arc, and
1618 // invalidate the other weak refs.
1620 // Note that it is not possible for the read of `weak` to yield
1621 // usize::MAX (i.e., locked), since the weak count can only be
1622 // locked by a thread with a strong reference.
1624 // Materialize our own implicit weak pointer, so that it can clean
1625 // up the ArcInner as needed.
1626 let _weak
= Weak { ptr: this.ptr }
;
1628 // Can just steal the data, all that's left is Weaks
1629 let mut arc
= Self::new_uninit();
1631 let data
= Arc
::get_mut_unchecked(&mut arc
);
1632 data
.as_mut_ptr().copy_from_nonoverlapping(&**this
, 1);
1633 ptr
::write(this
, arc
.assume_init());
1636 // We were the sole reference of either kind; bump back up the
1637 // strong ref count.
1638 this
.inner().strong
.store(1, Release
);
1641 // As with `get_mut()`, the unsafety is ok because our reference was
1642 // either unique to begin with, or became one upon cloning the contents.
1643 unsafe { Self::get_mut_unchecked(this) }
1646 /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
1649 /// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
1650 /// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
1655 /// #![feature(arc_unwrap_or_clone)]
1656 /// # use std::{ptr, sync::Arc};
1657 /// let inner = String::from("test");
1658 /// let ptr = inner.as_ptr();
1660 /// let arc = Arc::new(inner);
1661 /// let inner = Arc::unwrap_or_clone(arc);
1662 /// // The inner value was not cloned
1663 /// assert!(ptr::eq(ptr, inner.as_ptr()));
1665 /// let arc = Arc::new(inner);
1666 /// let arc2 = arc.clone();
1667 /// let inner = Arc::unwrap_or_clone(arc);
1668 /// // Because there were 2 references, we had to clone the inner value.
1669 /// assert!(!ptr::eq(ptr, inner.as_ptr()));
1670 /// // `arc2` is the last reference, so when we unwrap it we get back
1671 /// // the original `String`.
1672 /// let inner = Arc::unwrap_or_clone(arc2);
1673 /// assert!(ptr::eq(ptr, inner.as_ptr()));
1676 #[unstable(feature = "arc_unwrap_or_clone", issue = "93610")]
1677 pub fn unwrap_or_clone(this
: Self) -> T
{
1678 Arc
::try_unwrap(this
).unwrap_or_else(|arc
| (*arc
).clone())
1682 impl<T
: ?Sized
> Arc
<T
> {
1683 /// Returns a mutable reference into the given `Arc`, if there are
1684 /// no other `Arc` or [`Weak`] pointers to the same allocation.
1686 /// Returns [`None`] otherwise, because it is not safe to
1687 /// mutate a shared value.
1689 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1690 /// the inner value when there are other `Arc` pointers.
1692 /// [make_mut]: Arc::make_mut
1693 /// [clone]: Clone::clone
1698 /// use std::sync::Arc;
1700 /// let mut x = Arc::new(3);
1701 /// *Arc::get_mut(&mut x).unwrap() = 4;
1702 /// assert_eq!(*x, 4);
1704 /// let _y = Arc::clone(&x);
1705 /// assert!(Arc::get_mut(&mut x).is_none());
1708 #[stable(feature = "arc_unique", since = "1.4.0")]
1709 pub fn get_mut(this
: &mut Self) -> Option
<&mut T
> {
1710 if this
.is_unique() {
1711 // This unsafety is ok because we're guaranteed that the pointer
1712 // returned is the *only* pointer that will ever be returned to T. Our
1713 // reference count is guaranteed to be 1 at this point, and we required
1714 // the Arc itself to be `mut`, so we're returning the only possible
1715 // reference to the inner data.
1716 unsafe { Some(Arc::get_mut_unchecked(this)) }
1722 /// Returns a mutable reference into the given `Arc`,
1723 /// without any check.
1725 /// See also [`get_mut`], which is safe and does appropriate checks.
1727 /// [`get_mut`]: Arc::get_mut
1731 /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
1732 /// they must not be dereferenced or have active borrows for the duration
1733 /// of the returned borrow, and their inner type must be exactly the same as the
1734 /// inner type of this Rc (including lifetimes). This is trivially the case if no
1735 /// such pointers exist, for example immediately after `Arc::new`.
1740 /// #![feature(get_mut_unchecked)]
1742 /// use std::sync::Arc;
1744 /// let mut x = Arc::new(String::new());
1746 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1748 /// assert_eq!(*x, "foo");
1750 /// Other `Arc` pointers to the same allocation must be to the same type.
1752 /// #![feature(get_mut_unchecked)]
1754 /// use std::sync::Arc;
1756 /// let x: Arc<str> = Arc::from("Hello, world!");
1757 /// let mut y: Arc<[u8]> = x.clone().into();
1759 /// // this is Undefined Behavior, because x's inner type is str, not [u8]
1760 /// Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
1762 /// println!("{}", &*x); // Invalid UTF-8 in a str
1764 /// Other `Arc` pointers to the same allocation must be to the exact same type, including lifetimes.
1766 /// #![feature(get_mut_unchecked)]
1768 /// use std::sync::Arc;
1770 /// let x: Arc<&str> = Arc::new("Hello, world!");
1772 /// let s = String::from("Oh, no!");
1773 /// let mut y: Arc<&str> = x.clone().into();
1775 /// // this is Undefined Behavior, because x's inner type
1776 /// // is &'long str, not &'short str
1777 /// *Arc::get_mut_unchecked(&mut y) = &s;
1780 /// println!("{}", &*x); // Use-after-free
1783 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1784 pub unsafe fn get_mut_unchecked(this
: &mut Self) -> &mut T
{
1785 // We are careful to *not* create a reference covering the "count" fields, as
1786 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
1787 unsafe { &mut (*this.ptr.as_ptr()).data }
1790 /// Determine whether this is the unique reference (including weak refs) to
1791 /// the underlying data.
1793 /// Note that this requires locking the weak ref count.
1794 fn is_unique(&mut self) -> bool
{
1795 // lock the weak pointer count if we appear to be the sole weak pointer
1798 // The acquire label here ensures a happens-before relationship with any
1799 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1800 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1801 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1802 if self.inner().weak
.compare_exchange(1, usize::MAX
, Acquire
, Relaxed
).is_ok() {
1803 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1804 // counter in `drop` -- the only access that happens when any but the last reference
1805 // is being dropped.
1806 let unique
= self.inner().strong
.load(Acquire
) == 1;
1808 // The release write here synchronizes with a read in `downgrade`,
1809 // effectively preventing the above read of `strong` from happening
1811 self.inner().weak
.store(1, Release
); // release the lock
1819 #[stable(feature = "rust1", since = "1.0.0")]
1820 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1821 /// Drops the `Arc`.
1823 /// This will decrement the strong reference count. If the strong reference
1824 /// count reaches zero then the only other references (if any) are
1825 /// [`Weak`], so we `drop` the inner value.
1830 /// use std::sync::Arc;
1834 /// impl Drop for Foo {
1835 /// fn drop(&mut self) {
1836 /// println!("dropped!");
1840 /// let foo = Arc::new(Foo);
1841 /// let foo2 = Arc::clone(&foo);
1843 /// drop(foo); // Doesn't print anything
1844 /// drop(foo2); // Prints "dropped!"
1847 fn drop(&mut self) {
1848 // Because `fetch_sub` is already atomic, we do not need to synchronize
1849 // with other threads unless we are going to delete the object. This
1850 // same logic applies to the below `fetch_sub` to the `weak` count.
1851 if self.inner().strong
.fetch_sub(1, Release
) != 1 {
1855 // This fence is needed to prevent reordering of use of the data and
1856 // deletion of the data. Because it is marked `Release`, the decreasing
1857 // of the reference count synchronizes with this `Acquire` fence. This
1858 // means that use of the data happens before decreasing the reference
1859 // count, which happens before this fence, which happens before the
1860 // deletion of the data.
1862 // As explained in the [Boost documentation][1],
1864 // > It is important to enforce any possible access to the object in one
1865 // > thread (through an existing reference) to *happen before* deleting
1866 // > the object in a different thread. This is achieved by a "release"
1867 // > operation after dropping a reference (any access to the object
1868 // > through this reference must obviously happened before), and an
1869 // > "acquire" operation before deleting the object.
1871 // In particular, while the contents of an Arc are usually immutable, it's
1872 // possible to have interior writes to something like a Mutex<T>. Since a
1873 // Mutex is not acquired when it is deleted, we can't rely on its
1874 // synchronization logic to make writes in thread A visible to a destructor
1875 // running in thread B.
1877 // Also note that the Acquire fence here could probably be replaced with an
1878 // Acquire load, which could improve performance in highly-contended
1879 // situations. See [2].
1881 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1882 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1883 acquire
!(self.inner().strong
);
1891 impl Arc
<dyn Any
+ Send
+ Sync
> {
1892 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1897 /// use std::any::Any;
1898 /// use std::sync::Arc;
1900 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1901 /// if let Ok(string) = value.downcast::<String>() {
1902 /// println!("String ({}): {}", string.len(), string);
1906 /// let my_string = "Hello World".to_string();
1907 /// print_if_string(Arc::new(my_string));
1908 /// print_if_string(Arc::new(0i8));
1911 #[stable(feature = "rc_downcast", since = "1.29.0")]
1912 pub fn downcast
<T
>(self) -> Result
<Arc
<T
>, Self>
1914 T
: Any
+ Send
+ Sync
,
1916 if (*self).is
::<T
>() {
1918 let ptr
= self.ptr
.cast
::<ArcInner
<T
>>();
1920 Ok(Arc
::from_inner(ptr
))
1927 /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
1929 /// For a safe alternative see [`downcast`].
1934 /// #![feature(downcast_unchecked)]
1936 /// use std::any::Any;
1937 /// use std::sync::Arc;
1939 /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
1942 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
1948 /// The contained value must be of type `T`. Calling this method
1949 /// with the incorrect type is *undefined behavior*.
1952 /// [`downcast`]: Self::downcast
1954 #[unstable(feature = "downcast_unchecked", issue = "90850")]
1955 pub unsafe fn downcast_unchecked
<T
>(self) -> Arc
<T
>
1957 T
: Any
+ Send
+ Sync
,
1960 let ptr
= self.ptr
.cast
::<ArcInner
<T
>>();
1962 Arc
::from_inner(ptr
)
1968 /// Constructs a new `Weak<T>`, without allocating any memory.
1969 /// Calling [`upgrade`] on the return value always gives [`None`].
1971 /// [`upgrade`]: Weak::upgrade
1976 /// use std::sync::Weak;
1978 /// let empty: Weak<i64> = Weak::new();
1979 /// assert!(empty.upgrade().is_none());
1981 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1982 #[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
1984 pub const fn new() -> Weak
<T
> {
1985 Weak { ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<ArcInner<T>>(usize::MAX)) }
}
1989 /// Helper type to allow accessing the reference counts without
1990 /// making any assertions about the data field.
1991 struct WeakInner
<'a
> {
1992 weak
: &'a atomic
::AtomicUsize
,
1993 strong
: &'a atomic
::AtomicUsize
,
1996 impl<T
: ?Sized
> Weak
<T
> {
1997 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1999 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
2000 /// unaligned or even [`null`] otherwise.
2005 /// use std::sync::Arc;
2008 /// let strong = Arc::new("hello".to_owned());
2009 /// let weak = Arc::downgrade(&strong);
2010 /// // Both point to the same object
2011 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
2012 /// // The strong here keeps it alive, so we can still access the object.
2013 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
2016 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
2017 /// // undefined behaviour.
2018 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
2021 /// [`null`]: core::ptr::null "ptr::null"
2023 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2024 pub fn as_ptr(&self) -> *const T
{
2025 let ptr
: *mut ArcInner
<T
> = NonNull
::as_ptr(self.ptr
);
2027 if is_dangling(ptr
) {
2028 // If the pointer is dangling, we return the sentinel directly. This cannot be
2029 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
2032 // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
2033 // The payload may be dropped at this point, and we have to maintain provenance,
2034 // so use raw pointer manipulation.
2035 unsafe { ptr::addr_of_mut!((*ptr).data) }
2039 /// Consumes the `Weak<T>` and turns it into a raw pointer.
2041 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2042 /// one weak reference (the weak count is not modified by this operation). It can be turned
2043 /// back into the `Weak<T>` with [`from_raw`].
2045 /// The same restrictions of accessing the target of the pointer as with
2046 /// [`as_ptr`] apply.
2051 /// use std::sync::{Arc, Weak};
2053 /// let strong = Arc::new("hello".to_owned());
2054 /// let weak = Arc::downgrade(&strong);
2055 /// let raw = weak.into_raw();
2057 /// assert_eq!(1, Arc::weak_count(&strong));
2058 /// assert_eq!("hello", unsafe { &*raw });
2060 /// drop(unsafe { Weak::from_raw(raw) });
2061 /// assert_eq!(0, Arc::weak_count(&strong));
2064 /// [`from_raw`]: Weak::from_raw
2065 /// [`as_ptr`]: Weak::as_ptr
2066 #[must_use = "`self` will be dropped if the result is not used"]
2067 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2068 pub fn into_raw(self) -> *const T
{
2069 let result
= self.as_ptr();
2074 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
2076 /// This can be used to safely get a strong reference (by calling [`upgrade`]
2077 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2079 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2080 /// as these don't own anything; the method still works on them).
2084 /// The pointer must have originated from the [`into_raw`] and must still own its potential
2087 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2088 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2089 /// count is not modified by this operation) and therefore it must be paired with a previous
2090 /// call to [`into_raw`].
2094 /// use std::sync::{Arc, Weak};
2096 /// let strong = Arc::new("hello".to_owned());
2098 /// let raw_1 = Arc::downgrade(&strong).into_raw();
2099 /// let raw_2 = Arc::downgrade(&strong).into_raw();
2101 /// assert_eq!(2, Arc::weak_count(&strong));
2103 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2104 /// assert_eq!(1, Arc::weak_count(&strong));
2108 /// // Decrement the last weak count.
2109 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2112 /// [`new`]: Weak::new
2113 /// [`into_raw`]: Weak::into_raw
2114 /// [`upgrade`]: Weak::upgrade
2115 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2116 pub unsafe fn from_raw(ptr
: *const T
) -> Self {
2117 // See Weak::as_ptr for context on how the input pointer is derived.
2119 let ptr
= if is_dangling(ptr
as *mut T
) {
2120 // This is a dangling Weak.
2121 ptr
as *mut ArcInner
<T
>
2123 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
2124 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
2125 let offset
= unsafe { data_offset(ptr) }
;
2126 // Thus, we reverse the offset to get the whole RcBox.
2127 // SAFETY: the pointer originated from a Weak, so this offset is safe.
2128 unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
2131 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
2132 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) }
}
2136 impl<T
: ?Sized
> Weak
<T
> {
2137 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
2138 /// dropping of the inner value if successful.
2140 /// Returns [`None`] if the inner value has since been dropped.
2145 /// use std::sync::Arc;
2147 /// let five = Arc::new(5);
2149 /// let weak_five = Arc::downgrade(&five);
2151 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
2152 /// assert!(strong_five.is_some());
2154 /// // Destroy all strong pointers.
2155 /// drop(strong_five);
2158 /// assert!(weak_five.upgrade().is_none());
2160 #[must_use = "this returns a new `Arc`, \
2161 without modifying the original weak pointer"]
2162 #[stable(feature = "arc_weak", since = "1.4.0")]
2163 pub fn upgrade(&self) -> Option
<Arc
<T
>> {
2164 // We use a CAS loop to increment the strong count instead of a
2165 // fetch_add as this function should never take the reference count
2166 // from zero to one.
2169 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
2170 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
2171 // value can be initialized after `Weak` references have already been created. In that case, we
2172 // expect to observe the fully initialized value.
2173 .fetch_update(Acquire
, Relaxed
, |n
| {
2174 // Any write of 0 we can observe leaves the field in permanently zero state.
2178 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
2179 assert
!(n
<= MAX_REFCOUNT
, "{}", INTERNAL_OVERFLOW_ERROR
);
2183 // null checked above
2184 .map(|_
| unsafe { Arc::from_inner(self.ptr) }
)
2187 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
2189 /// If `self` was created using [`Weak::new`], this will return 0.
2191 #[stable(feature = "weak_counts", since = "1.41.0")]
2192 pub fn strong_count(&self) -> usize {
2193 if let Some(inner
) = self.inner() { inner.strong.load(Acquire) }
else { 0 }
2196 /// Gets an approximation of the number of `Weak` pointers pointing to this
2199 /// If `self` was created using [`Weak::new`], or if there are no remaining
2200 /// strong pointers, this will return 0.
2204 /// Due to implementation details, the returned value can be off by 1 in
2205 /// either direction when other threads are manipulating any `Arc`s or
2206 /// `Weak`s pointing to the same allocation.
2208 #[stable(feature = "weak_counts", since = "1.41.0")]
2209 pub fn weak_count(&self) -> usize {
2212 let weak
= inner
.weak
.load(Acquire
);
2213 let strong
= inner
.strong
.load(Acquire
);
2217 // Since we observed that there was at least one strong pointer
2218 // after reading the weak count, we know that the implicit weak
2219 // reference (present whenever any strong references are alive)
2220 // was still around when we observed the weak count, and can
2221 // therefore safely subtract it.
2228 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
2229 /// (i.e., when this `Weak` was created by `Weak::new`).
2231 fn inner(&self) -> Option
<WeakInner
<'_
>> {
2232 if is_dangling(self.ptr
.as_ptr()) {
2235 // We are careful to *not* create a reference covering the "data" field, as
2236 // the field may be mutated concurrently (for example, if the last `Arc`
2237 // is dropped, the data field will be dropped in-place).
2239 let ptr
= self.ptr
.as_ptr();
2240 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
2245 /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
2246 /// both don't point to any allocation (because they were created with `Weak::new()`). See [that
2247 /// function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
2251 /// Since this compares pointers it means that `Weak::new()` will equal each
2252 /// other, even though they don't point to any allocation.
2257 /// use std::sync::Arc;
2259 /// let first_rc = Arc::new(5);
2260 /// let first = Arc::downgrade(&first_rc);
2261 /// let second = Arc::downgrade(&first_rc);
2263 /// assert!(first.ptr_eq(&second));
2265 /// let third_rc = Arc::new(5);
2266 /// let third = Arc::downgrade(&third_rc);
2268 /// assert!(!first.ptr_eq(&third));
2271 /// Comparing `Weak::new`.
2274 /// use std::sync::{Arc, Weak};
2276 /// let first = Weak::new();
2277 /// let second = Weak::new();
2278 /// assert!(first.ptr_eq(&second));
2280 /// let third_rc = Arc::new(());
2281 /// let third = Arc::downgrade(&third_rc);
2282 /// assert!(!first.ptr_eq(&third));
2285 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
2288 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
2289 pub fn ptr_eq(&self, other
: &Self) -> bool
{
2290 self.ptr
.as_ptr() == other
.ptr
.as_ptr()
2294 #[stable(feature = "arc_weak", since = "1.4.0")]
2295 impl<T
: ?Sized
> Clone
for Weak
<T
> {
2296 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2301 /// use std::sync::{Arc, Weak};
2303 /// let weak_five = Arc::downgrade(&Arc::new(5));
2305 /// let _ = Weak::clone(&weak_five);
2308 fn clone(&self) -> Weak
<T
> {
2309 let inner
= if let Some(inner
) = self.inner() {
2312 return Weak { ptr: self.ptr }
;
2314 // See comments in Arc::clone() for why this is relaxed. This can use a
2315 // fetch_add (ignoring the lock) because the weak count is only locked
2316 // where are *no other* weak pointers in existence. (So we can't be
2317 // running this code in that case).
2318 let old_size
= inner
.weak
.fetch_add(1, Relaxed
);
2320 // See comments in Arc::clone() for why we do this (for mem::forget).
2321 if old_size
> MAX_REFCOUNT
{
2325 Weak { ptr: self.ptr }
2329 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2330 impl<T
> Default
for Weak
<T
> {
2331 /// Constructs a new `Weak<T>`, without allocating memory.
2332 /// Calling [`upgrade`] on the return value always
2335 /// [`upgrade`]: Weak::upgrade
2340 /// use std::sync::Weak;
2342 /// let empty: Weak<i64> = Default::default();
2343 /// assert!(empty.upgrade().is_none());
2345 fn default() -> Weak
<T
> {
2350 #[stable(feature = "arc_weak", since = "1.4.0")]
2351 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2352 /// Drops the `Weak` pointer.
2357 /// use std::sync::{Arc, Weak};
2361 /// impl Drop for Foo {
2362 /// fn drop(&mut self) {
2363 /// println!("dropped!");
2367 /// let foo = Arc::new(Foo);
2368 /// let weak_foo = Arc::downgrade(&foo);
2369 /// let other_weak_foo = Weak::clone(&weak_foo);
2371 /// drop(weak_foo); // Doesn't print anything
2372 /// drop(foo); // Prints "dropped!"
2374 /// assert!(other_weak_foo.upgrade().is_none());
2376 fn drop(&mut self) {
2377 // If we find out that we were the last weak pointer, then its time to
2378 // deallocate the data entirely. See the discussion in Arc::drop() about
2379 // the memory orderings
2381 // It's not necessary to check for the locked state here, because the
2382 // weak count can only be locked if there was precisely one weak ref,
2383 // meaning that drop could only subsequently run ON that remaining weak
2384 // ref, which can only happen after the lock is released.
2385 let inner
= if let Some(inner
) = self.inner() { inner }
else { return }
;
2387 if inner
.weak
.fetch_sub(1, Release
) == 1 {
2388 acquire
!(inner
.weak
);
2389 unsafe { Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())) }
2394 #[stable(feature = "rust1", since = "1.0.0")]
2395 trait ArcEqIdent
<T
: ?Sized
+ PartialEq
> {
2396 fn eq(&self, other
: &Arc
<T
>) -> bool
;
2397 fn ne(&self, other
: &Arc
<T
>) -> bool
;
2400 #[stable(feature = "rust1", since = "1.0.0")]
2401 impl<T
: ?Sized
+ PartialEq
> ArcEqIdent
<T
> for Arc
<T
> {
2403 default fn eq(&self, other
: &Arc
<T
>) -> bool
{
2407 default fn ne(&self, other
: &Arc
<T
>) -> bool
{
2412 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
2413 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
2414 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
2415 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
2416 /// the same value, than two `&T`s.
2418 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
2419 #[stable(feature = "rust1", since = "1.0.0")]
2420 impl<T
: ?Sized
+ crate::rc
::MarkerEq
> ArcEqIdent
<T
> for Arc
<T
> {
2422 fn eq(&self, other
: &Arc
<T
>) -> bool
{
2423 Arc
::ptr_eq(self, other
) || **self == **other
2427 fn ne(&self, other
: &Arc
<T
>) -> bool
{
2428 !Arc
::ptr_eq(self, other
) && **self != **other
2432 #[stable(feature = "rust1", since = "1.0.0")]
2433 impl<T
: ?Sized
+ PartialEq
> PartialEq
for Arc
<T
> {
2434 /// Equality for two `Arc`s.
2436 /// Two `Arc`s are equal if their inner values are equal, even if they are
2437 /// stored in different allocation.
2439 /// If `T` also implements `Eq` (implying reflexivity of equality),
2440 /// two `Arc`s that point to the same allocation are always equal.
2445 /// use std::sync::Arc;
2447 /// let five = Arc::new(5);
2449 /// assert!(five == Arc::new(5));
2452 fn eq(&self, other
: &Arc
<T
>) -> bool
{
2453 ArcEqIdent
::eq(self, other
)
2456 /// Inequality for two `Arc`s.
2458 /// Two `Arc`s are not equal if their inner values are not equal.
2460 /// If `T` also implements `Eq` (implying reflexivity of equality),
2461 /// two `Arc`s that point to the same value are always equal.
2466 /// use std::sync::Arc;
2468 /// let five = Arc::new(5);
2470 /// assert!(five != Arc::new(6));
2473 fn ne(&self, other
: &Arc
<T
>) -> bool
{
2474 ArcEqIdent
::ne(self, other
)
2478 #[stable(feature = "rust1", since = "1.0.0")]
2479 impl<T
: ?Sized
+ PartialOrd
> PartialOrd
for Arc
<T
> {
2480 /// Partial comparison for two `Arc`s.
2482 /// The two are compared by calling `partial_cmp()` on their inner values.
2487 /// use std::sync::Arc;
2488 /// use std::cmp::Ordering;
2490 /// let five = Arc::new(5);
2492 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
2494 fn partial_cmp(&self, other
: &Arc
<T
>) -> Option
<Ordering
> {
2495 (**self).partial_cmp(&**other
)
2498 /// Less-than comparison for two `Arc`s.
2500 /// The two are compared by calling `<` on their inner values.
2505 /// use std::sync::Arc;
2507 /// let five = Arc::new(5);
2509 /// assert!(five < Arc::new(6));
2511 fn lt(&self, other
: &Arc
<T
>) -> bool
{
2512 *(*self) < *(*other
)
2515 /// 'Less than or equal to' comparison for two `Arc`s.
2517 /// The two are compared by calling `<=` on their inner values.
2522 /// use std::sync::Arc;
2524 /// let five = Arc::new(5);
2526 /// assert!(five <= Arc::new(5));
2528 fn le(&self, other
: &Arc
<T
>) -> bool
{
2529 *(*self) <= *(*other
)
2532 /// Greater-than comparison for two `Arc`s.
2534 /// The two are compared by calling `>` on their inner values.
2539 /// use std::sync::Arc;
2541 /// let five = Arc::new(5);
2543 /// assert!(five > Arc::new(4));
2545 fn gt(&self, other
: &Arc
<T
>) -> bool
{
2546 *(*self) > *(*other
)
2549 /// 'Greater than or equal to' comparison for two `Arc`s.
2551 /// The two are compared by calling `>=` on their inner values.
2556 /// use std::sync::Arc;
2558 /// let five = Arc::new(5);
2560 /// assert!(five >= Arc::new(5));
2562 fn ge(&self, other
: &Arc
<T
>) -> bool
{
2563 *(*self) >= *(*other
)
2566 #[stable(feature = "rust1", since = "1.0.0")]
2567 impl<T
: ?Sized
+ Ord
> Ord
for Arc
<T
> {
2568 /// Comparison for two `Arc`s.
2570 /// The two are compared by calling `cmp()` on their inner values.
2575 /// use std::sync::Arc;
2576 /// use std::cmp::Ordering;
2578 /// let five = Arc::new(5);
2580 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
2582 fn cmp(&self, other
: &Arc
<T
>) -> Ordering
{
2583 (**self).cmp(&**other
)
2586 #[stable(feature = "rust1", since = "1.0.0")]
2587 impl<T
: ?Sized
+ Eq
> Eq
for Arc
<T
> {}
2589 #[stable(feature = "rust1", since = "1.0.0")]
2590 impl<T
: ?Sized
+ fmt
::Display
> fmt
::Display
for Arc
<T
> {
2591 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
2592 fmt
::Display
::fmt(&**self, f
)
2596 #[stable(feature = "rust1", since = "1.0.0")]
2597 impl<T
: ?Sized
+ fmt
::Debug
> fmt
::Debug
for Arc
<T
> {
2598 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
2599 fmt
::Debug
::fmt(&**self, f
)
2603 #[stable(feature = "rust1", since = "1.0.0")]
2604 impl<T
: ?Sized
> fmt
::Pointer
for Arc
<T
> {
2605 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
2606 fmt
::Pointer
::fmt(&(&**self as *const T
), f
)
2610 #[cfg(not(no_global_oom_handling))]
2611 #[stable(feature = "rust1", since = "1.0.0")]
2612 impl<T
: Default
> Default
for Arc
<T
> {
2613 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
2618 /// use std::sync::Arc;
2620 /// let x: Arc<i32> = Default::default();
2621 /// assert_eq!(*x, 0);
2623 fn default() -> Arc
<T
> {
2624 Arc
::new(Default
::default())
2628 #[stable(feature = "rust1", since = "1.0.0")]
2629 impl<T
: ?Sized
+ Hash
> Hash
for Arc
<T
> {
2630 fn hash
<H
: Hasher
>(&self, state
: &mut H
) {
2631 (**self).hash(state
)
2635 #[cfg(not(no_global_oom_handling))]
2636 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
2637 impl<T
> From
<T
> for Arc
<T
> {
2638 /// Converts a `T` into an `Arc<T>`
2640 /// The conversion moves the value into a
2641 /// newly allocated `Arc`. It is equivalent to
2642 /// calling `Arc::new(t)`.
2646 /// # use std::sync::Arc;
2648 /// let arc = Arc::new(5);
2650 /// assert_eq!(Arc::from(x), arc);
2652 fn from(t
: T
) -> Self {
2657 #[cfg(not(no_global_oom_handling))]
2658 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2659 impl<T
: Clone
> From
<&[T
]> for Arc
<[T
]> {
2660 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
2665 /// # use std::sync::Arc;
2666 /// let original: &[i32] = &[1, 2, 3];
2667 /// let shared: Arc<[i32]> = Arc::from(original);
2668 /// assert_eq!(&[1, 2, 3], &shared[..]);
2671 fn from(v
: &[T
]) -> Arc
<[T
]> {
2672 <Self as ArcFromSlice
<T
>>::from_slice(v
)
2676 #[cfg(not(no_global_oom_handling))]
2677 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2678 impl From
<&str> for Arc
<str> {
2679 /// Allocate a reference-counted `str` and copy `v` into it.
2684 /// # use std::sync::Arc;
2685 /// let shared: Arc<str> = Arc::from("eggplant");
2686 /// assert_eq!("eggplant", &shared[..]);
2689 fn from(v
: &str) -> Arc
<str> {
2690 let arc
= Arc
::<[u8]>::from(v
.as_bytes());
2691 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
2695 #[cfg(not(no_global_oom_handling))]
2696 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2697 impl From
<String
> for Arc
<str> {
2698 /// Allocate a reference-counted `str` and copy `v` into it.
2703 /// # use std::sync::Arc;
2704 /// let unique: String = "eggplant".to_owned();
2705 /// let shared: Arc<str> = Arc::from(unique);
2706 /// assert_eq!("eggplant", &shared[..]);
2709 fn from(v
: String
) -> Arc
<str> {
2714 #[cfg(not(no_global_oom_handling))]
2715 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2716 impl<T
: ?Sized
> From
<Box
<T
>> for Arc
<T
> {
2717 /// Move a boxed object to a new, reference-counted allocation.
2722 /// # use std::sync::Arc;
2723 /// let unique: Box<str> = Box::from("eggplant");
2724 /// let shared: Arc<str> = Arc::from(unique);
2725 /// assert_eq!("eggplant", &shared[..]);
2728 fn from(v
: Box
<T
>) -> Arc
<T
> {
2733 #[cfg(not(no_global_oom_handling))]
2734 #[stable(feature = "shared_from_slice", since = "1.21.0")]
2735 impl<T
> From
<Vec
<T
>> for Arc
<[T
]> {
2736 /// Allocate a reference-counted slice and move `v`'s items into it.
2741 /// # use std::sync::Arc;
2742 /// let unique: Vec<i32> = vec![1, 2, 3];
2743 /// let shared: Arc<[i32]> = Arc::from(unique);
2744 /// assert_eq!(&[1, 2, 3], &shared[..]);
2747 fn from(mut v
: Vec
<T
>) -> Arc
<[T
]> {
2749 let rc
= Arc
::copy_from_slice(&v
);
2750 // Allow the Vec to free its memory, but not destroy its contents
2757 #[stable(feature = "shared_from_cow", since = "1.45.0")]
2758 impl<'a
, B
> From
<Cow
<'a
, B
>> for Arc
<B
>
2760 B
: ToOwned
+ ?Sized
,
2761 Arc
<B
>: From
<&'a B
> + From
<B
::Owned
>,
2763 /// Create an atomically reference-counted pointer from
2764 /// a clone-on-write pointer by copying its content.
2769 /// # use std::sync::Arc;
2770 /// # use std::borrow::Cow;
2771 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
2772 /// let shared: Arc<str> = Arc::from(cow);
2773 /// assert_eq!("eggplant", &shared[..]);
2776 fn from(cow
: Cow
<'a
, B
>) -> Arc
<B
> {
2778 Cow
::Borrowed(s
) => Arc
::from(s
),
2779 Cow
::Owned(s
) => Arc
::from(s
),
2784 #[stable(feature = "shared_from_str", since = "1.62.0")]
2785 impl From
<Arc
<str>> for Arc
<[u8]> {
2786 /// Converts an atomically reference-counted string slice into a byte slice.
2791 /// # use std::sync::Arc;
2792 /// let string: Arc<str> = Arc::from("eggplant");
2793 /// let bytes: Arc<[u8]> = Arc::from(string);
2794 /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
2797 fn from(rc
: Arc
<str>) -> Self {
2798 // SAFETY: `str` has the same layout as `[u8]`.
2799 unsafe { Arc::from_raw(Arc::into_raw(rc) as *const [u8]) }
2803 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2804 impl<T
, const N
: usize> TryFrom
<Arc
<[T
]>> for Arc
<[T
; N
]> {
2805 type Error
= Arc
<[T
]>;
2807 fn try_from(boxed_slice
: Arc
<[T
]>) -> Result
<Self, Self::Error
> {
2808 if boxed_slice
.len() == N
{
2809 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) }
)
2816 #[cfg(not(no_global_oom_handling))]
2817 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2818 impl<T
> FromIterator
<T
> for Arc
<[T
]> {
2819 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2821 /// # Performance characteristics
2823 /// ## The general case
2825 /// In the general case, collecting into `Arc<[T]>` is done by first
2826 /// collecting into a `Vec<T>`. That is, when writing the following:
2829 /// # use std::sync::Arc;
2830 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2831 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2834 /// this behaves as if we wrote:
2837 /// # use std::sync::Arc;
2838 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2839 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2840 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2841 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2844 /// This will allocate as many times as needed for constructing the `Vec<T>`
2845 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2847 /// ## Iterators of known length
2849 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2850 /// a single allocation will be made for the `Arc<[T]>`. For example:
2853 /// # use std::sync::Arc;
2854 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2855 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2857 fn from_iter
<I
: IntoIterator
<Item
= T
>>(iter
: I
) -> Self {
2858 ToArcSlice
::to_arc_slice(iter
.into_iter())
2862 /// Specialization trait used for collecting into `Arc<[T]>`.
2863 trait ToArcSlice
<T
>: Iterator
<Item
= T
> + Sized
{
2864 fn to_arc_slice(self) -> Arc
<[T
]>;
2867 #[cfg(not(no_global_oom_handling))]
2868 impl<T
, I
: Iterator
<Item
= T
>> ToArcSlice
<T
> for I
{
2869 default fn to_arc_slice(self) -> Arc
<[T
]> {
2870 self.collect
::<Vec
<T
>>().into()
2874 #[cfg(not(no_global_oom_handling))]
2875 impl<T
, I
: iter
::TrustedLen
<Item
= T
>> ToArcSlice
<T
> for I
{
2876 fn to_arc_slice(self) -> Arc
<[T
]> {
2877 // This is the case for a `TrustedLen` iterator.
2878 let (low
, high
) = self.size_hint();
2879 if let Some(high
) = high
{
2883 "TrustedLen iterator's size hint is not exact: {:?}",
2888 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2889 Arc
::from_iter_exact(self, low
)
2892 // TrustedLen contract guarantees that `upper_bound == None` implies an iterator
2893 // length exceeding `usize::MAX`.
2894 // The default implementation would collect into a vec which would panic.
2895 // Thus we panic here immediately without invoking `Vec` code.
2896 panic
!("capacity overflow");
2901 #[stable(feature = "rust1", since = "1.0.0")]
2902 impl<T
: ?Sized
> borrow
::Borrow
<T
> for Arc
<T
> {
2903 fn borrow(&self) -> &T
{
2908 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2909 impl<T
: ?Sized
> AsRef
<T
> for Arc
<T
> {
2910 fn as_ref(&self) -> &T
{
2915 #[stable(feature = "pin", since = "1.33.0")]
2916 impl<T
: ?Sized
> Unpin
for Arc
<T
> {}
2918 /// Get the offset within an `ArcInner` for the payload behind a pointer.
2922 /// The pointer must point to (and have valid metadata for) a previously
2923 /// valid instance of T, but the T is allowed to be dropped.
2924 unsafe fn data_offset
<T
: ?Sized
>(ptr
: *const T
) -> usize {
2925 // Align the unsized value to the end of the ArcInner.
2926 // Because RcBox is repr(C), it will always be the last field in memory.
2927 // SAFETY: since the only unsized types possible are slices, trait objects,
2928 // and extern types, the input safety requirement is currently enough to
2929 // satisfy the requirements of align_of_val_raw; this is an implementation
2930 // detail of the language that must not be relied upon outside of std.
2931 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2935 fn data_offset_align(align
: usize) -> usize {
2936 let layout
= Layout
::new
::<ArcInner
<()>>();
2937 layout
.size() + layout
.padding_needed_for(align
)
2940 #[stable(feature = "arc_error", since = "1.52.0")]
2941 impl<T
: core
::error
::Error
+ ?Sized
> core
::error
::Error
for Arc
<T
> {
2942 #[allow(deprecated, deprecated_in_future)]
2943 fn description(&self) -> &str {
2944 core
::error
::Error
::description(&**self)
2947 #[allow(deprecated)]
2948 fn cause(&self) -> Option
<&dyn core
::error
::Error
> {
2949 core
::error
::Error
::cause(&**self)
2952 fn source(&self) -> Option
<&(dyn core
::error
::Error
+ '
static)> {
2953 core
::error
::Error
::source(&**self)
2956 fn provide
<'a
>(&'a
self, req
: &mut core
::any
::Demand
<'a
>) {
2957 core
::error
::Error
::provide(&**self, req
);