1 //! Single-threaded reference-counting pointers. 'Rc' stands for 'Reference
4 //! The type [`Rc<T>`][`Rc`] provides shared ownership of a value of type `T`,
5 //! allocated in the heap. Invoking [`clone`][clone] on [`Rc`] produces a new
6 //! pointer to the same allocation in the heap. When the last [`Rc`] pointer to a
7 //! given allocation is destroyed, the value stored in that allocation (often
8 //! referred to as "inner value") is also dropped.
10 //! Shared references in Rust disallow mutation by default, and [`Rc`]
11 //! is no exception: you cannot generally obtain a mutable reference to
12 //! something inside an [`Rc`]. If you need mutability, put a [`Cell`]
13 //! or [`RefCell`] inside the [`Rc`]; see [an example of mutability
14 //! inside an `Rc`][mutability].
16 //! [`Rc`] uses non-atomic reference counting. This means that overhead is very
17 //! low, but an [`Rc`] cannot be sent between threads, and consequently [`Rc`]
18 //! does not implement [`Send`][send]. As a result, the Rust compiler
19 //! will check *at compile time* that you are not sending [`Rc`]s between
20 //! threads. If you need multi-threaded, atomic reference counting, use
21 //! [`sync::Arc`][arc].
23 //! The [`downgrade`][downgrade] method can be used to create a non-owning
24 //! [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
25 //! to an [`Rc`], but this will return [`None`] if the value stored in the allocation has
26 //! already been dropped. In other words, `Weak` pointers do not keep the value
27 //! inside the allocation alive; however, they *do* keep the allocation
28 //! (the backing store for the inner value) alive.
30 //! A cycle between [`Rc`] pointers will never be deallocated. For this reason,
31 //! [`Weak`] is used to break cycles. For example, a tree could have strong
32 //! [`Rc`] pointers from parent nodes to children, and [`Weak`] pointers from
33 //! children back to their parents.
35 //! `Rc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
36 //! so you can call `T`'s methods on a value of type [`Rc<T>`][`Rc`]. To avoid name
37 //! clashes with `T`'s methods, the methods of [`Rc<T>`][`Rc`] itself are associated
38 //! functions, called using [fully qualified syntax]:
43 //! let my_rc = Rc::new(());
44 //! Rc::downgrade(&my_rc);
47 //! `Rc<T>`'s implementations of traits like `Clone` may also be called using
48 //! fully qualified syntax. Some people prefer to use fully qualified syntax,
49 //! while others prefer using method-call syntax.
54 //! let rc = Rc::new(());
55 //! // Method-call syntax
56 //! let rc2 = rc.clone();
57 //! // Fully qualified syntax
58 //! let rc3 = Rc::clone(&rc);
61 //! [`Weak<T>`][`Weak`] does not auto-dereference to `T`, because the inner value may have
62 //! already been dropped.
64 //! # Cloning references
66 //! Creating a new reference to the same allocation as an existing reference counted pointer
67 //! is done using the `Clone` trait implemented for [`Rc<T>`][`Rc`] and [`Weak<T>`][`Weak`].
72 //! let foo = Rc::new(vec![1.0, 2.0, 3.0]);
73 //! // The two syntaxes below are equivalent.
74 //! let a = foo.clone();
75 //! let b = Rc::clone(&foo);
76 //! // a and b both point to the same memory location as foo.
79 //! The `Rc::clone(&from)` syntax is the most idiomatic because it conveys more explicitly
80 //! the meaning of the code. In the example above, this syntax makes it easier to see that
81 //! this code is creating a new reference rather than copying the whole content of foo.
85 //! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`.
86 //! We want to have our `Gadget`s point to their `Owner`. We can't do this with
87 //! unique ownership, because more than one gadget may belong to the same
88 //! `Owner`. [`Rc`] allows us to share an `Owner` between multiple `Gadget`s,
89 //! and have the `Owner` remain allocated as long as any `Gadget` points at it.
96 //! // ...other fields
101 //! owner: Rc<Owner>,
102 //! // ...other fields
106 //! // Create a reference-counted `Owner`.
107 //! let gadget_owner: Rc<Owner> = Rc::new(
109 //! name: "Gadget Man".to_string(),
113 //! // Create `Gadget`s belonging to `gadget_owner`. Cloning the `Rc<Owner>`
114 //! // gives us a new pointer to the same `Owner` allocation, incrementing
115 //! // the reference count in the process.
116 //! let gadget1 = Gadget {
118 //! owner: Rc::clone(&gadget_owner),
120 //! let gadget2 = Gadget {
122 //! owner: Rc::clone(&gadget_owner),
125 //! // Dispose of our local variable `gadget_owner`.
126 //! drop(gadget_owner);
128 //! // Despite dropping `gadget_owner`, we're still able to print out the name
129 //! // of the `Owner` of the `Gadget`s. This is because we've only dropped a
130 //! // single `Rc<Owner>`, not the `Owner` it points to. As long as there are
131 //! // other `Rc<Owner>` pointing at the same `Owner` allocation, it will remain
132 //! // live. The field projection `gadget1.owner.name` works because
133 //! // `Rc<Owner>` automatically dereferences to `Owner`.
134 //! println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name);
135 //! println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name);
137 //! // At the end of the function, `gadget1` and `gadget2` are destroyed, and
138 //! // with them the last counted references to our `Owner`. Gadget Man now
139 //! // gets destroyed as well.
143 //! If our requirements change, and we also need to be able to traverse from
144 //! `Owner` to `Gadget`, we will run into problems. An [`Rc`] pointer from `Owner`
145 //! to `Gadget` introduces a cycle. This means that their
146 //! reference counts can never reach 0, and the allocation will never be destroyed:
147 //! a memory leak. In order to get around this, we can use [`Weak`]
150 //! Rust actually makes it somewhat difficult to produce this loop in the first
151 //! place. In order to end up with two values that point at each other, one of
152 //! them needs to be mutable. This is difficult because [`Rc`] enforces
153 //! memory safety by only giving out shared references to the value it wraps,
154 //! and these don't allow direct mutation. We need to wrap the part of the
155 //! value we wish to mutate in a [`RefCell`], which provides *interior
156 //! mutability*: a method to achieve mutability through a shared reference.
157 //! [`RefCell`] enforces Rust's borrowing rules at runtime.
161 //! use std::rc::Weak;
162 //! use std::cell::RefCell;
166 //! gadgets: RefCell<Vec<Weak<Gadget>>>,
167 //! // ...other fields
172 //! owner: Rc<Owner>,
173 //! // ...other fields
177 //! // Create a reference-counted `Owner`. Note that we've put the `Owner`'s
178 //! // vector of `Gadget`s inside a `RefCell` so that we can mutate it through
179 //! // a shared reference.
180 //! let gadget_owner: Rc<Owner> = Rc::new(
182 //! name: "Gadget Man".to_string(),
183 //! gadgets: RefCell::new(vec![]),
187 //! // Create `Gadget`s belonging to `gadget_owner`, as before.
188 //! let gadget1 = Rc::new(
191 //! owner: Rc::clone(&gadget_owner),
194 //! let gadget2 = Rc::new(
197 //! owner: Rc::clone(&gadget_owner),
201 //! // Add the `Gadget`s to their `Owner`.
203 //! let mut gadgets = gadget_owner.gadgets.borrow_mut();
204 //! gadgets.push(Rc::downgrade(&gadget1));
205 //! gadgets.push(Rc::downgrade(&gadget2));
207 //! // `RefCell` dynamic borrow ends here.
210 //! // Iterate over our `Gadget`s, printing their details out.
211 //! for gadget_weak in gadget_owner.gadgets.borrow().iter() {
213 //! // `gadget_weak` is a `Weak<Gadget>`. Since `Weak` pointers can't
214 //! // guarantee the allocation still exists, we need to call
215 //! // `upgrade`, which returns an `Option<Rc<Gadget>>`.
217 //! // In this case we know the allocation still exists, so we simply
218 //! // `unwrap` the `Option`. In a more complicated program, you might
219 //! // need graceful error handling for a `None` result.
221 //! let gadget = gadget_weak.upgrade().unwrap();
222 //! println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
225 //! // At the end of the function, `gadget_owner`, `gadget1`, and `gadget2`
226 //! // are destroyed. There are now no strong (`Rc`) pointers to the
227 //! // gadgets, so they are destroyed. This zeroes the reference count on
228 //! // Gadget Man, so he gets destroyed as well.
232 //! [clone]: Clone::clone
233 //! [`Cell`]: core::cell::Cell
234 //! [`RefCell`]: core::cell::RefCell
235 //! [send]: core::marker::Send
236 //! [arc]: crate::sync::Arc
237 //! [`Deref`]: core::ops::Deref
238 //! [downgrade]: Rc::downgrade
239 //! [upgrade]: Weak::upgrade
240 //! [mutability]: core::cell#introducing-mutability-inside-of-something-immutable
241 //! [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
243 #![stable(feature = "rust1", since = "1.0.0")]
246 use crate::boxed
::Box
;
252 use core
::cell
::Cell
;
253 use core
::cmp
::Ordering
;
254 use core
::convert
::{From, TryFrom}
;
256 use core
::hash
::{Hash, Hasher}
;
257 use core
::intrinsics
::abort
;
258 #[cfg(not(no_global_oom_handling))]
260 use core
::marker
::{self, PhantomData, Unpin, Unsize}
;
261 #[cfg(not(no_global_oom_handling))]
262 use core
::mem
::size_of_val
;
263 use core
::mem
::{self, align_of_val_raw, forget}
;
264 use core
::ops
::{CoerceUnsized, Deref, DispatchFromDyn, Receiver}
;
265 #[cfg(not(no_global_oom_handling))]
267 use core
::ptr
::{self, NonNull}
;
268 #[cfg(not(no_global_oom_handling))]
269 use core
::slice
::from_raw_parts_mut
;
271 #[cfg(not(no_global_oom_handling))]
272 use crate::alloc
::handle_alloc_error
;
273 #[cfg(not(no_global_oom_handling))]
274 use crate::alloc
::{box_free, WriteCloneIntoRaw}
;
275 use crate::alloc
::{AllocError, Allocator, Global, Layout}
;
276 use crate::borrow
::{Cow, ToOwned}
;
277 #[cfg(not(no_global_oom_handling))]
278 use crate::string
::String
;
279 #[cfg(not(no_global_oom_handling))]
285 // This is repr(C) to future-proof against possible field-reordering, which
286 // would interfere with otherwise safe [into|from]_raw() of transmutable
289 struct RcBox
<T
: ?Sized
> {
295 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
298 /// See the [module-level documentation](./index.html) for more details.
300 /// The inherent methods of `Rc` are all associated functions, which means
301 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
302 /// `value.get_mut()`. This avoids conflicts with methods of the inner type `T`.
304 /// [get_mut]: Rc::get_mut
305 #[cfg_attr(not(test), rustc_diagnostic_item = "Rc")]
306 #[stable(feature = "rust1", since = "1.0.0")]
307 pub struct Rc
<T
: ?Sized
> {
308 ptr
: NonNull
<RcBox
<T
>>,
309 phantom
: PhantomData
<RcBox
<T
>>,
312 #[stable(feature = "rust1", since = "1.0.0")]
313 impl<T
: ?Sized
> !marker
::Send
for Rc
<T
> {}
314 #[stable(feature = "rust1", since = "1.0.0")]
315 impl<T
: ?Sized
> !marker
::Sync
for Rc
<T
> {}
317 #[unstable(feature = "coerce_unsized", issue = "27732")]
318 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Rc
<U
>> for Rc
<T
> {}
320 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
321 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> DispatchFromDyn
<Rc
<U
>> for Rc
<T
> {}
323 impl<T
: ?Sized
> Rc
<T
> {
325 fn inner(&self) -> &RcBox
<T
> {
326 // This unsafety is ok because while this Rc is alive we're guaranteed
327 // that the inner pointer is valid.
328 unsafe { self.ptr.as_ref() }
331 fn from_inner(ptr
: NonNull
<RcBox
<T
>>) -> Self {
332 Self { ptr, phantom: PhantomData }
335 unsafe fn from_ptr(ptr
: *mut RcBox
<T
>) -> Self {
336 Self::from_inner(unsafe { NonNull::new_unchecked(ptr) }
)
341 /// Constructs a new `Rc<T>`.
348 /// let five = Rc::new(5);
350 #[cfg(not(no_global_oom_handling))]
351 #[stable(feature = "rust1", since = "1.0.0")]
352 pub fn new(value
: T
) -> Rc
<T
> {
353 // There is an implicit weak pointer owned by all the strong
354 // pointers, which ensures that the weak destructor never frees
355 // the allocation while the strong destructor is running, even
356 // if the weak pointer is stored inside the strong one.
358 Box
::leak(box RcBox { strong: Cell::new(1), weak: Cell::new(1), value }
).into(),
362 /// Constructs a new `Rc<T>` using a weak reference to itself. Attempting
363 /// to upgrade the weak reference before this function returns will result
364 /// in a `None` value. However, the weak reference may be cloned freely and
365 /// stored for use at a later time.
370 /// #![feature(arc_new_cyclic)]
371 /// #![allow(dead_code)]
372 /// use std::rc::{Rc, Weak};
375 /// self_weak: Weak<Self>,
376 /// // ... more fields
379 /// pub fn new() -> Rc<Self> {
380 /// Rc::new_cyclic(|self_weak| {
381 /// Gadget { self_weak: self_weak.clone(), /* ... */ }
386 #[cfg(not(no_global_oom_handling))]
387 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
388 pub fn new_cyclic(data_fn
: impl FnOnce(&Weak
<T
>) -> T
) -> Rc
<T
> {
389 // Construct the inner in the "uninitialized" state with a single
391 let uninit_ptr
: NonNull
<_
> = Box
::leak(box RcBox
{
392 strong
: Cell
::new(0),
394 value
: mem
::MaybeUninit
::<T
>::uninit(),
398 let init_ptr
: NonNull
<RcBox
<T
>> = uninit_ptr
.cast();
400 let weak
= Weak { ptr: init_ptr }
;
402 // It's important we don't give up ownership of the weak pointer, or
403 // else the memory might be freed by the time `data_fn` returns. If
404 // we really wanted to pass ownership, we could create an additional
405 // weak pointer for ourselves, but this would result in additional
406 // updates to the weak reference count which might not be necessary
408 let data
= data_fn(&weak
);
411 let inner
= init_ptr
.as_ptr();
412 ptr
::write(ptr
::addr_of_mut
!((*inner
).value
), data
);
414 let prev_value
= (*inner
).strong
.get();
415 debug_assert_eq
!(prev_value
, 0, "No prior strong references should exist");
416 (*inner
).strong
.set(1);
419 let strong
= Rc
::from_inner(init_ptr
);
421 // Strong references should collectively own a shared weak reference,
422 // so don't run the destructor for our old weak reference.
427 /// Constructs a new `Rc` with uninitialized contents.
432 /// #![feature(new_uninit)]
433 /// #![feature(get_mut_unchecked)]
437 /// let mut five = Rc::<u32>::new_uninit();
439 /// let five = unsafe {
440 /// // Deferred initialization:
441 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
443 /// five.assume_init()
446 /// assert_eq!(*five, 5)
448 #[cfg(not(no_global_oom_handling))]
449 #[unstable(feature = "new_uninit", issue = "63291")]
450 pub fn new_uninit() -> Rc
<mem
::MaybeUninit
<T
>> {
452 Rc
::from_ptr(Rc
::allocate_for_layout(
454 |layout
| Global
.allocate(layout
),
455 |mem
| mem
as *mut RcBox
<mem
::MaybeUninit
<T
>>,
460 /// Constructs a new `Rc` with uninitialized contents, with the memory
461 /// being filled with `0` bytes.
463 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
464 /// incorrect usage of this method.
469 /// #![feature(new_uninit)]
473 /// let zero = Rc::<u32>::new_zeroed();
474 /// let zero = unsafe { zero.assume_init() };
476 /// assert_eq!(*zero, 0)
479 /// [zeroed]: mem::MaybeUninit::zeroed
480 #[cfg(not(no_global_oom_handling))]
481 #[unstable(feature = "new_uninit", issue = "63291")]
482 pub fn new_zeroed() -> Rc
<mem
::MaybeUninit
<T
>> {
484 Rc
::from_ptr(Rc
::allocate_for_layout(
486 |layout
| Global
.allocate_zeroed(layout
),
487 |mem
| mem
as *mut RcBox
<mem
::MaybeUninit
<T
>>,
492 /// Constructs a new `Rc<T>`, returning an error if the allocation fails
497 /// #![feature(allocator_api)]
500 /// let five = Rc::try_new(5);
501 /// # Ok::<(), std::alloc::AllocError>(())
503 #[unstable(feature = "allocator_api", issue = "32838")]
504 pub fn try_new(value
: T
) -> Result
<Rc
<T
>, AllocError
> {
505 // There is an implicit weak pointer owned by all the strong
506 // pointers, which ensures that the weak destructor never frees
507 // the allocation while the strong destructor is running, even
508 // if the weak pointer is stored inside the strong one.
510 Box
::leak(Box
::try_new(RcBox { strong: Cell::new(1), weak: Cell::new(1), value }
)?
)
515 /// Constructs a new `Rc` with uninitialized contents, returning an error if the allocation fails
520 /// #![feature(allocator_api, new_uninit)]
521 /// #![feature(get_mut_unchecked)]
525 /// let mut five = Rc::<u32>::try_new_uninit()?;
527 /// let five = unsafe {
528 /// // Deferred initialization:
529 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
531 /// five.assume_init()
534 /// assert_eq!(*five, 5);
535 /// # Ok::<(), std::alloc::AllocError>(())
537 #[unstable(feature = "allocator_api", issue = "32838")]
538 // #[unstable(feature = "new_uninit", issue = "63291")]
539 pub fn try_new_uninit() -> Result
<Rc
<mem
::MaybeUninit
<T
>>, AllocError
> {
541 Ok(Rc
::from_ptr(Rc
::try_allocate_for_layout(
543 |layout
| Global
.allocate(layout
),
544 |mem
| mem
as *mut RcBox
<mem
::MaybeUninit
<T
>>,
549 /// Constructs a new `Rc` with uninitialized contents, with the memory
550 /// being filled with `0` bytes, returning an error if the allocation fails
552 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
553 /// incorrect usage of this method.
558 /// #![feature(allocator_api, new_uninit)]
562 /// let zero = Rc::<u32>::try_new_zeroed()?;
563 /// let zero = unsafe { zero.assume_init() };
565 /// assert_eq!(*zero, 0);
566 /// # Ok::<(), std::alloc::AllocError>(())
569 /// [zeroed]: mem::MaybeUninit::zeroed
570 #[unstable(feature = "allocator_api", issue = "32838")]
571 //#[unstable(feature = "new_uninit", issue = "63291")]
572 pub fn try_new_zeroed() -> Result
<Rc
<mem
::MaybeUninit
<T
>>, AllocError
> {
574 Ok(Rc
::from_ptr(Rc
::try_allocate_for_layout(
576 |layout
| Global
.allocate_zeroed(layout
),
577 |mem
| mem
as *mut RcBox
<mem
::MaybeUninit
<T
>>,
581 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
582 /// `value` will be pinned in memory and unable to be moved.
583 #[cfg(not(no_global_oom_handling))]
584 #[stable(feature = "pin", since = "1.33.0")]
585 pub fn pin(value
: T
) -> Pin
<Rc
<T
>> {
586 unsafe { Pin::new_unchecked(Rc::new(value)) }
589 /// Returns the inner value, if the `Rc` has exactly one strong reference.
591 /// Otherwise, an [`Err`] is returned with the same `Rc` that was
594 /// This will succeed even if there are outstanding weak references.
601 /// let x = Rc::new(3);
602 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
604 /// let x = Rc::new(4);
605 /// let _y = Rc::clone(&x);
606 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
609 #[stable(feature = "rc_unique", since = "1.4.0")]
610 pub fn try_unwrap(this
: Self) -> Result
<T
, Self> {
611 if Rc
::strong_count(&this
) == 1 {
613 let val
= ptr
::read(&*this
); // copy the contained object
615 // Indicate to Weaks that they can't be promoted by decrementing
616 // the strong count, and then remove the implicit "strong weak"
617 // pointer while also handling drop logic by just crafting a
619 this
.inner().dec_strong();
620 let _weak
= Weak { ptr: this.ptr }
;
631 /// Constructs a new reference-counted slice with uninitialized contents.
636 /// #![feature(new_uninit)]
637 /// #![feature(get_mut_unchecked)]
641 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
643 /// let values = unsafe {
644 /// // Deferred initialization:
645 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
646 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
647 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
649 /// values.assume_init()
652 /// assert_eq!(*values, [1, 2, 3])
654 #[cfg(not(no_global_oom_handling))]
655 #[unstable(feature = "new_uninit", issue = "63291")]
656 pub fn new_uninit_slice(len
: usize) -> Rc
<[mem
::MaybeUninit
<T
>]> {
657 unsafe { Rc::from_ptr(Rc::allocate_for_slice(len)) }
660 /// Constructs a new reference-counted slice with uninitialized contents, with the memory being
661 /// filled with `0` bytes.
663 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
664 /// incorrect usage of this method.
669 /// #![feature(new_uninit)]
673 /// let values = Rc::<[u32]>::new_zeroed_slice(3);
674 /// let values = unsafe { values.assume_init() };
676 /// assert_eq!(*values, [0, 0, 0])
679 /// [zeroed]: mem::MaybeUninit::zeroed
680 #[cfg(not(no_global_oom_handling))]
681 #[unstable(feature = "new_uninit", issue = "63291")]
682 pub fn new_zeroed_slice(len
: usize) -> Rc
<[mem
::MaybeUninit
<T
>]> {
684 Rc
::from_ptr(Rc
::allocate_for_layout(
685 Layout
::array
::<T
>(len
).unwrap(),
686 |layout
| Global
.allocate_zeroed(layout
),
688 ptr
::slice_from_raw_parts_mut(mem
as *mut T
, len
)
689 as *mut RcBox
<[mem
::MaybeUninit
<T
>]>
696 impl<T
> Rc
<mem
::MaybeUninit
<T
>> {
697 /// Converts to `Rc<T>`.
701 /// As with [`MaybeUninit::assume_init`],
702 /// it is up to the caller to guarantee that the inner value
703 /// really is in an initialized state.
704 /// Calling this when the content is not yet fully initialized
705 /// causes immediate undefined behavior.
707 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
712 /// #![feature(new_uninit)]
713 /// #![feature(get_mut_unchecked)]
717 /// let mut five = Rc::<u32>::new_uninit();
719 /// let five = unsafe {
720 /// // Deferred initialization:
721 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
723 /// five.assume_init()
726 /// assert_eq!(*five, 5)
728 #[unstable(feature = "new_uninit", issue = "63291")]
730 pub unsafe fn assume_init(self) -> Rc
<T
> {
731 Rc
::from_inner(mem
::ManuallyDrop
::new(self).ptr
.cast())
735 impl<T
> Rc
<[mem
::MaybeUninit
<T
>]> {
736 /// Converts to `Rc<[T]>`.
740 /// As with [`MaybeUninit::assume_init`],
741 /// it is up to the caller to guarantee that the inner value
742 /// really is in an initialized state.
743 /// Calling this when the content is not yet fully initialized
744 /// causes immediate undefined behavior.
746 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
751 /// #![feature(new_uninit)]
752 /// #![feature(get_mut_unchecked)]
756 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
758 /// let values = unsafe {
759 /// // Deferred initialization:
760 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
761 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
762 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
764 /// values.assume_init()
767 /// assert_eq!(*values, [1, 2, 3])
769 #[unstable(feature = "new_uninit", issue = "63291")]
771 pub unsafe fn assume_init(self) -> Rc
<[T
]> {
772 unsafe { Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
776 impl<T
: ?Sized
> Rc
<T
> {
777 /// Consumes the `Rc`, returning the wrapped pointer.
779 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
780 /// [`Rc::from_raw`][from_raw].
782 /// [from_raw]: Rc::from_raw
789 /// let x = Rc::new("hello".to_owned());
790 /// let x_ptr = Rc::into_raw(x);
791 /// assert_eq!(unsafe { &*x_ptr }, "hello");
793 #[stable(feature = "rc_raw", since = "1.17.0")]
794 pub fn into_raw(this
: Self) -> *const T
{
795 let ptr
= Self::as_ptr(&this
);
800 /// Provides a raw pointer to the data.
802 /// The counts are not affected in any way and the `Rc` is not consumed. The pointer is valid
803 /// for as long there are strong counts in the `Rc`.
810 /// let x = Rc::new("hello".to_owned());
811 /// let y = Rc::clone(&x);
812 /// let x_ptr = Rc::as_ptr(&x);
813 /// assert_eq!(x_ptr, Rc::as_ptr(&y));
814 /// assert_eq!(unsafe { &*x_ptr }, "hello");
816 #[stable(feature = "weak_into_raw", since = "1.45.0")]
817 pub fn as_ptr(this
: &Self) -> *const T
{
818 let ptr
: *mut RcBox
<T
> = NonNull
::as_ptr(this
.ptr
);
820 // SAFETY: This cannot go through Deref::deref or Rc::inner because
821 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
822 // write through the pointer after the Rc is recovered through `from_raw`.
823 unsafe { ptr::addr_of_mut!((*ptr).value) }
826 /// Constructs an `Rc<T>` from a raw pointer.
828 /// The raw pointer must have been previously returned by a call to
829 /// [`Rc<U>::into_raw`][into_raw] where `U` must have the same size
830 /// and alignment as `T`. This is trivially true if `U` is `T`.
831 /// Note that if `U` is not `T` but has the same size and alignment, this is
832 /// basically like transmuting references of different types. See
833 /// [`mem::transmute`][transmute] for more information on what
834 /// restrictions apply in this case.
836 /// The user of `from_raw` has to make sure a specific value of `T` is only
839 /// This function is unsafe because improper use may lead to memory unsafety,
840 /// even if the returned `Rc<T>` is never accessed.
842 /// [into_raw]: Rc::into_raw
843 /// [transmute]: core::mem::transmute
850 /// let x = Rc::new("hello".to_owned());
851 /// let x_ptr = Rc::into_raw(x);
854 /// // Convert back to an `Rc` to prevent leak.
855 /// let x = Rc::from_raw(x_ptr);
856 /// assert_eq!(&*x, "hello");
858 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
861 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
863 #[stable(feature = "rc_raw", since = "1.17.0")]
864 pub unsafe fn from_raw(ptr
: *const T
) -> Self {
865 let offset
= unsafe { data_offset(ptr) }
;
867 // Reverse the offset to find the original RcBox.
869 unsafe { (ptr as *mut RcBox<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
;
871 unsafe { Self::from_ptr(rc_ptr) }
874 /// Creates a new [`Weak`] pointer to this allocation.
881 /// let five = Rc::new(5);
883 /// let weak_five = Rc::downgrade(&five);
885 #[stable(feature = "rc_weak", since = "1.4.0")]
886 pub fn downgrade(this
: &Self) -> Weak
<T
> {
887 this
.inner().inc_weak();
888 // Make sure we do not create a dangling Weak
889 debug_assert
!(!is_dangling(this
.ptr
.as_ptr()));
890 Weak { ptr: this.ptr }
893 /// Gets the number of [`Weak`] pointers to this allocation.
900 /// let five = Rc::new(5);
901 /// let _weak_five = Rc::downgrade(&five);
903 /// assert_eq!(1, Rc::weak_count(&five));
906 #[stable(feature = "rc_counts", since = "1.15.0")]
907 pub fn weak_count(this
: &Self) -> usize {
908 this
.inner().weak() - 1
911 /// Gets the number of strong (`Rc`) pointers to this allocation.
918 /// let five = Rc::new(5);
919 /// let _also_five = Rc::clone(&five);
921 /// assert_eq!(2, Rc::strong_count(&five));
924 #[stable(feature = "rc_counts", since = "1.15.0")]
925 pub fn strong_count(this
: &Self) -> usize {
926 this
.inner().strong()
929 /// Increments the strong reference count on the `Rc<T>` associated with the
930 /// provided pointer by one.
934 /// The pointer must have been obtained through `Rc::into_raw`, and the
935 /// associated `Rc` instance must be valid (i.e. the strong count must be at
936 /// least 1) for the duration of this method.
943 /// let five = Rc::new(5);
946 /// let ptr = Rc::into_raw(five);
947 /// Rc::increment_strong_count(ptr);
949 /// let five = Rc::from_raw(ptr);
950 /// assert_eq!(2, Rc::strong_count(&five));
954 #[stable(feature = "rc_mutate_strong_count", since = "1.53.0")]
955 pub unsafe fn increment_strong_count(ptr
: *const T
) {
956 // Retain Rc, but don't touch refcount by wrapping in ManuallyDrop
957 let rc
= unsafe { mem::ManuallyDrop::new(Rc::<T>::from_raw(ptr)) }
;
958 // Now increase refcount, but don't drop new refcount either
959 let _rc_clone
: mem
::ManuallyDrop
<_
> = rc
.clone();
962 /// Decrements the strong reference count on the `Rc<T>` associated with the
963 /// provided pointer by one.
967 /// The pointer must have been obtained through `Rc::into_raw`, and the
968 /// associated `Rc` instance must be valid (i.e. the strong count must be at
969 /// least 1) when invoking this method. This method can be used to release
970 /// the final `Rc` and backing storage, but **should not** be called after
971 /// the final `Rc` has been released.
978 /// let five = Rc::new(5);
981 /// let ptr = Rc::into_raw(five);
982 /// Rc::increment_strong_count(ptr);
984 /// let five = Rc::from_raw(ptr);
985 /// assert_eq!(2, Rc::strong_count(&five));
986 /// Rc::decrement_strong_count(ptr);
987 /// assert_eq!(1, Rc::strong_count(&five));
991 #[stable(feature = "rc_mutate_strong_count", since = "1.53.0")]
992 pub unsafe fn decrement_strong_count(ptr
: *const T
) {
993 unsafe { mem::drop(Rc::from_raw(ptr)) }
;
996 /// Returns `true` if there are no other `Rc` or [`Weak`] pointers to
999 fn is_unique(this
: &Self) -> bool
{
1000 Rc
::weak_count(this
) == 0 && Rc
::strong_count(this
) == 1
1003 /// Returns a mutable reference into the given `Rc`, if there are
1004 /// no other `Rc` or [`Weak`] pointers to the same allocation.
1006 /// Returns [`None`] otherwise, because it is not safe to
1007 /// mutate a shared value.
1009 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1010 /// the inner value when there are other pointers.
1012 /// [make_mut]: Rc::make_mut
1013 /// [clone]: Clone::clone
1018 /// use std::rc::Rc;
1020 /// let mut x = Rc::new(3);
1021 /// *Rc::get_mut(&mut x).unwrap() = 4;
1022 /// assert_eq!(*x, 4);
1024 /// let _y = Rc::clone(&x);
1025 /// assert!(Rc::get_mut(&mut x).is_none());
1028 #[stable(feature = "rc_unique", since = "1.4.0")]
1029 pub fn get_mut(this
: &mut Self) -> Option
<&mut T
> {
1030 if Rc
::is_unique(this
) { unsafe { Some(Rc::get_mut_unchecked(this)) }
} else { None }
1033 /// Returns a mutable reference into the given `Rc`,
1034 /// without any check.
1036 /// See also [`get_mut`], which is safe and does appropriate checks.
1038 /// [`get_mut`]: Rc::get_mut
1042 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
1043 /// for the duration of the returned borrow.
1044 /// This is trivially the case if no such pointers exist,
1045 /// for example immediately after `Rc::new`.
1050 /// #![feature(get_mut_unchecked)]
1052 /// use std::rc::Rc;
1054 /// let mut x = Rc::new(String::new());
1056 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
1058 /// assert_eq!(*x, "foo");
1061 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1062 pub unsafe fn get_mut_unchecked(this
: &mut Self) -> &mut T
{
1063 // We are careful to *not* create a reference covering the "count" fields, as
1064 // this would conflict with accesses to the reference counts (e.g. by `Weak`).
1065 unsafe { &mut (*this.ptr.as_ptr()).value }
1069 #[stable(feature = "ptr_eq", since = "1.17.0")]
1070 /// Returns `true` if the two `Rc`s point to the same allocation
1071 /// (in a vein similar to [`ptr::eq`]).
1076 /// use std::rc::Rc;
1078 /// let five = Rc::new(5);
1079 /// let same_five = Rc::clone(&five);
1080 /// let other_five = Rc::new(5);
1082 /// assert!(Rc::ptr_eq(&five, &same_five));
1083 /// assert!(!Rc::ptr_eq(&five, &other_five));
1086 /// [`ptr::eq`]: core::ptr::eq
1087 pub fn ptr_eq(this
: &Self, other
: &Self) -> bool
{
1088 this
.ptr
.as_ptr() == other
.ptr
.as_ptr()
1092 impl<T
: Clone
> Rc
<T
> {
1093 /// Makes a mutable reference into the given `Rc`.
1095 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
1096 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1097 /// referred to as clone-on-write.
1099 /// If there are no other `Rc` pointers to this allocation, then [`Weak`]
1100 /// pointers to this allocation will be disassociated.
1102 /// See also [`get_mut`], which will fail rather than cloning.
1104 /// [`clone`]: Clone::clone
1105 /// [`get_mut`]: Rc::get_mut
1110 /// use std::rc::Rc;
1112 /// let mut data = Rc::new(5);
1114 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
1115 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
1116 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
1117 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
1118 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
1120 /// // Now `data` and `other_data` point to different allocations.
1121 /// assert_eq!(*data, 8);
1122 /// assert_eq!(*other_data, 12);
1125 /// [`Weak`] pointers will be disassociated:
1128 /// use std::rc::Rc;
1130 /// let mut data = Rc::new(75);
1131 /// let weak = Rc::downgrade(&data);
1133 /// assert!(75 == *data);
1134 /// assert!(75 == *weak.upgrade().unwrap());
1136 /// *Rc::make_mut(&mut data) += 1;
1138 /// assert!(76 == *data);
1139 /// assert!(weak.upgrade().is_none());
1141 #[cfg(not(no_global_oom_handling))]
1143 #[stable(feature = "rc_unique", since = "1.4.0")]
1144 pub fn make_mut(this
: &mut Self) -> &mut T
{
1145 if Rc
::strong_count(this
) != 1 {
1146 // Gotta clone the data, there are other Rcs.
1147 // Pre-allocate memory to allow writing the cloned value directly.
1148 let mut rc
= Self::new_uninit();
1150 let data
= Rc
::get_mut_unchecked(&mut rc
);
1151 (**this
).write_clone_into_raw(data
.as_mut_ptr());
1152 *this
= rc
.assume_init();
1154 } else if Rc
::weak_count(this
) != 0 {
1155 // Can just steal the data, all that's left is Weaks
1156 let mut rc
= Self::new_uninit();
1158 let data
= Rc
::get_mut_unchecked(&mut rc
);
1159 data
.as_mut_ptr().copy_from_nonoverlapping(&**this
, 1);
1161 this
.inner().dec_strong();
1162 // Remove implicit strong-weak ref (no need to craft a fake
1163 // Weak here -- we know other Weaks can clean up for us)
1164 this
.inner().dec_weak();
1165 ptr
::write(this
, rc
.assume_init());
1168 // This unsafety is ok because we're guaranteed that the pointer
1169 // returned is the *only* pointer that will ever be returned to T. Our
1170 // reference count is guaranteed to be 1 at this point, and we required
1171 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
1172 // reference to the allocation.
1173 unsafe { &mut this.ptr.as_mut().value }
1179 #[stable(feature = "rc_downcast", since = "1.29.0")]
1180 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
1185 /// use std::any::Any;
1186 /// use std::rc::Rc;
1188 /// fn print_if_string(value: Rc<dyn Any>) {
1189 /// if let Ok(string) = value.downcast::<String>() {
1190 /// println!("String ({}): {}", string.len(), string);
1194 /// let my_string = "Hello World".to_string();
1195 /// print_if_string(Rc::new(my_string));
1196 /// print_if_string(Rc::new(0i8));
1198 pub fn downcast
<T
: Any
>(self) -> Result
<Rc
<T
>, Rc
<dyn Any
>> {
1199 if (*self).is
::<T
>() {
1200 let ptr
= self.ptr
.cast
::<RcBox
<T
>>();
1202 Ok(Rc
::from_inner(ptr
))
1209 impl<T
: ?Sized
> Rc
<T
> {
1210 /// Allocates an `RcBox<T>` with sufficient space for
1211 /// a possibly-unsized inner value where the value has the layout provided.
1213 /// The function `mem_to_rcbox` is called with the data pointer
1214 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
1215 #[cfg(not(no_global_oom_handling))]
1216 unsafe fn allocate_for_layout(
1217 value_layout
: Layout
,
1218 allocate
: impl FnOnce(Layout
) -> Result
<NonNull
<[u8]>, AllocError
>,
1219 mem_to_rcbox
: impl FnOnce(*mut u8) -> *mut RcBox
<T
>,
1220 ) -> *mut RcBox
<T
> {
1221 // Calculate layout using the given value layout.
1222 // Previously, layout was calculated on the expression
1223 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
1224 // reference (see #54908).
1225 let layout
= Layout
::new
::<RcBox
<()>>().extend(value_layout
).unwrap().0.pad_to_align();
1227 Rc
::try_allocate_for_layout(value_layout
, allocate
, mem_to_rcbox
)
1228 .unwrap_or_else(|_
| handle_alloc_error(layout
))
1232 /// Allocates an `RcBox<T>` with sufficient space for
1233 /// a possibly-unsized inner value where the value has the layout provided,
1234 /// returning an error if allocation fails.
1236 /// The function `mem_to_rcbox` is called with the data pointer
1237 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
1239 unsafe fn try_allocate_for_layout(
1240 value_layout
: Layout
,
1241 allocate
: impl FnOnce(Layout
) -> Result
<NonNull
<[u8]>, AllocError
>,
1242 mem_to_rcbox
: impl FnOnce(*mut u8) -> *mut RcBox
<T
>,
1243 ) -> Result
<*mut RcBox
<T
>, AllocError
> {
1244 // Calculate layout using the given value layout.
1245 // Previously, layout was calculated on the expression
1246 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
1247 // reference (see #54908).
1248 let layout
= Layout
::new
::<RcBox
<()>>().extend(value_layout
).unwrap().0.pad_to_align();
1250 // Allocate for the layout.
1251 let ptr
= allocate(layout
)?
;
1253 // Initialize the RcBox
1254 let inner
= mem_to_rcbox(ptr
.as_non_null_ptr().as_ptr());
1256 debug_assert_eq
!(Layout
::for_value(&*inner
), layout
);
1258 ptr
::write(&mut (*inner
).strong
, Cell
::new(1));
1259 ptr
::write(&mut (*inner
).weak
, Cell
::new(1));
1265 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
1266 #[cfg(not(no_global_oom_handling))]
1267 unsafe fn allocate_for_ptr(ptr
: *const T
) -> *mut RcBox
<T
> {
1268 // Allocate for the `RcBox<T>` using the given value.
1270 Self::allocate_for_layout(
1271 Layout
::for_value(&*ptr
),
1272 |layout
| Global
.allocate(layout
),
1273 |mem
| (ptr
as *mut RcBox
<T
>).set_ptr_value(mem
),
1278 #[cfg(not(no_global_oom_handling))]
1279 fn from_box(v
: Box
<T
>) -> Rc
<T
> {
1281 let (box_unique
, alloc
) = Box
::into_unique(v
);
1282 let bptr
= box_unique
.as_ptr();
1284 let value_size
= size_of_val(&*bptr
);
1285 let ptr
= Self::allocate_for_ptr(bptr
);
1287 // Copy value as bytes
1288 ptr
::copy_nonoverlapping(
1289 bptr
as *const T
as *const u8,
1290 &mut (*ptr
).value
as *mut _
as *mut u8,
1294 // Free the allocation without dropping its contents
1295 box_free(box_unique
, alloc
);
1303 /// Allocates an `RcBox<[T]>` with the given length.
1304 #[cfg(not(no_global_oom_handling))]
1305 unsafe fn allocate_for_slice(len
: usize) -> *mut RcBox
<[T
]> {
1307 Self::allocate_for_layout(
1308 Layout
::array
::<T
>(len
).unwrap(),
1309 |layout
| Global
.allocate(layout
),
1310 |mem
| ptr
::slice_from_raw_parts_mut(mem
as *mut T
, len
) as *mut RcBox
<[T
]>,
1315 /// Copy elements from slice into newly allocated Rc<\[T\]>
1317 /// Unsafe because the caller must either take ownership or bind `T: Copy`
1318 #[cfg(not(no_global_oom_handling))]
1319 unsafe fn copy_from_slice(v
: &[T
]) -> Rc
<[T
]> {
1321 let ptr
= Self::allocate_for_slice(v
.len());
1322 ptr
::copy_nonoverlapping(v
.as_ptr(), &mut (*ptr
).value
as *mut [T
] as *mut T
, v
.len());
1327 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
1329 /// Behavior is undefined should the size be wrong.
1330 #[cfg(not(no_global_oom_handling))]
1331 unsafe fn from_iter_exact(iter
: impl iter
::Iterator
<Item
= T
>, len
: usize) -> Rc
<[T
]> {
1332 // Panic guard while cloning T elements.
1333 // In the event of a panic, elements that have been written
1334 // into the new RcBox will be dropped, then the memory freed.
1342 impl<T
> Drop
for Guard
<T
> {
1343 fn drop(&mut self) {
1345 let slice
= from_raw_parts_mut(self.elems
, self.n_elems
);
1346 ptr
::drop_in_place(slice
);
1348 Global
.deallocate(self.mem
, self.layout
);
1354 let ptr
= Self::allocate_for_slice(len
);
1356 let mem
= ptr
as *mut _
as *mut u8;
1357 let layout
= Layout
::for_value(&*ptr
);
1359 // Pointer to first element
1360 let elems
= &mut (*ptr
).value
as *mut [T
] as *mut T
;
1362 let mut guard
= Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 }
;
1364 for (i
, item
) in iter
.enumerate() {
1365 ptr
::write(elems
.add(i
), item
);
1369 // All clear. Forget the guard so it doesn't free the new RcBox.
1377 /// Specialization trait used for `From<&[T]>`.
1378 trait RcFromSlice
<T
> {
1379 fn from_slice(slice
: &[T
]) -> Self;
1382 #[cfg(not(no_global_oom_handling))]
1383 impl<T
: Clone
> RcFromSlice
<T
> for Rc
<[T
]> {
1385 default fn from_slice(v
: &[T
]) -> Self {
1386 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1390 #[cfg(not(no_global_oom_handling))]
1391 impl<T
: Copy
> RcFromSlice
<T
> for Rc
<[T
]> {
1393 fn from_slice(v
: &[T
]) -> Self {
1394 unsafe { Rc::copy_from_slice(v) }
1398 #[stable(feature = "rust1", since = "1.0.0")]
1399 impl<T
: ?Sized
> Deref
for Rc
<T
> {
1403 fn deref(&self) -> &T
{
1408 #[unstable(feature = "receiver_trait", issue = "none")]
1409 impl<T
: ?Sized
> Receiver
for Rc
<T
> {}
1411 #[stable(feature = "rust1", since = "1.0.0")]
1412 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1415 /// This will decrement the strong reference count. If the strong reference
1416 /// count reaches zero then the only other references (if any) are
1417 /// [`Weak`], so we `drop` the inner value.
1422 /// use std::rc::Rc;
1426 /// impl Drop for Foo {
1427 /// fn drop(&mut self) {
1428 /// println!("dropped!");
1432 /// let foo = Rc::new(Foo);
1433 /// let foo2 = Rc::clone(&foo);
1435 /// drop(foo); // Doesn't print anything
1436 /// drop(foo2); // Prints "dropped!"
1438 fn drop(&mut self) {
1440 self.inner().dec_strong();
1441 if self.inner().strong() == 0 {
1442 // destroy the contained object
1443 ptr
::drop_in_place(Self::get_mut_unchecked(self));
1445 // remove the implicit "strong weak" pointer now that we've
1446 // destroyed the contents.
1447 self.inner().dec_weak();
1449 if self.inner().weak() == 0 {
1450 Global
.deallocate(self.ptr
.cast(), Layout
::for_value(self.ptr
.as_ref()));
1457 #[stable(feature = "rust1", since = "1.0.0")]
1458 impl<T
: ?Sized
> Clone
for Rc
<T
> {
1459 /// Makes a clone of the `Rc` pointer.
1461 /// This creates another pointer to the same allocation, increasing the
1462 /// strong reference count.
1467 /// use std::rc::Rc;
1469 /// let five = Rc::new(5);
1471 /// let _ = Rc::clone(&five);
1474 fn clone(&self) -> Rc
<T
> {
1475 self.inner().inc_strong();
1476 Self::from_inner(self.ptr
)
1480 #[cfg(not(no_global_oom_handling))]
1481 #[stable(feature = "rust1", since = "1.0.0")]
1482 impl<T
: Default
> Default
for Rc
<T
> {
1483 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1488 /// use std::rc::Rc;
1490 /// let x: Rc<i32> = Default::default();
1491 /// assert_eq!(*x, 0);
1494 fn default() -> Rc
<T
> {
1495 Rc
::new(Default
::default())
1499 #[stable(feature = "rust1", since = "1.0.0")]
1500 trait RcEqIdent
<T
: ?Sized
+ PartialEq
> {
1501 fn eq(&self, other
: &Rc
<T
>) -> bool
;
1502 fn ne(&self, other
: &Rc
<T
>) -> bool
;
1505 #[stable(feature = "rust1", since = "1.0.0")]
1506 impl<T
: ?Sized
+ PartialEq
> RcEqIdent
<T
> for Rc
<T
> {
1508 default fn eq(&self, other
: &Rc
<T
>) -> bool
{
1513 default fn ne(&self, other
: &Rc
<T
>) -> bool
{
1518 // Hack to allow specializing on `Eq` even though `Eq` has a method.
1519 #[rustc_unsafe_specialization_marker]
1520 pub(crate) trait MarkerEq
: PartialEq
<Self> {}
1522 impl<T
: Eq
> MarkerEq
for T {}
1524 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1525 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1526 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1527 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1528 /// the same value, than two `&T`s.
1530 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1531 #[stable(feature = "rust1", since = "1.0.0")]
1532 impl<T
: ?Sized
+ MarkerEq
> RcEqIdent
<T
> for Rc
<T
> {
1534 fn eq(&self, other
: &Rc
<T
>) -> bool
{
1535 Rc
::ptr_eq(self, other
) || **self == **other
1539 fn ne(&self, other
: &Rc
<T
>) -> bool
{
1540 !Rc
::ptr_eq(self, other
) && **self != **other
1544 #[stable(feature = "rust1", since = "1.0.0")]
1545 impl<T
: ?Sized
+ PartialEq
> PartialEq
for Rc
<T
> {
1546 /// Equality for two `Rc`s.
1548 /// Two `Rc`s are equal if their inner values are equal, even if they are
1549 /// stored in different allocation.
1551 /// If `T` also implements `Eq` (implying reflexivity of equality),
1552 /// two `Rc`s that point to the same allocation are
1558 /// use std::rc::Rc;
1560 /// let five = Rc::new(5);
1562 /// assert!(five == Rc::new(5));
1565 fn eq(&self, other
: &Rc
<T
>) -> bool
{
1566 RcEqIdent
::eq(self, other
)
1569 /// Inequality for two `Rc`s.
1571 /// Two `Rc`s are unequal if their inner values are unequal.
1573 /// If `T` also implements `Eq` (implying reflexivity of equality),
1574 /// two `Rc`s that point to the same allocation are
1580 /// use std::rc::Rc;
1582 /// let five = Rc::new(5);
1584 /// assert!(five != Rc::new(6));
1587 fn ne(&self, other
: &Rc
<T
>) -> bool
{
1588 RcEqIdent
::ne(self, other
)
1592 #[stable(feature = "rust1", since = "1.0.0")]
1593 impl<T
: ?Sized
+ Eq
> Eq
for Rc
<T
> {}
1595 #[stable(feature = "rust1", since = "1.0.0")]
1596 impl<T
: ?Sized
+ PartialOrd
> PartialOrd
for Rc
<T
> {
1597 /// Partial comparison for two `Rc`s.
1599 /// The two are compared by calling `partial_cmp()` on their inner values.
1604 /// use std::rc::Rc;
1605 /// use std::cmp::Ordering;
1607 /// let five = Rc::new(5);
1609 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1612 fn partial_cmp(&self, other
: &Rc
<T
>) -> Option
<Ordering
> {
1613 (**self).partial_cmp(&**other
)
1616 /// Less-than comparison for two `Rc`s.
1618 /// The two are compared by calling `<` on their inner values.
1623 /// use std::rc::Rc;
1625 /// let five = Rc::new(5);
1627 /// assert!(five < Rc::new(6));
1630 fn lt(&self, other
: &Rc
<T
>) -> bool
{
1634 /// 'Less than or equal to' comparison for two `Rc`s.
1636 /// The two are compared by calling `<=` on their inner values.
1641 /// use std::rc::Rc;
1643 /// let five = Rc::new(5);
1645 /// assert!(five <= Rc::new(5));
1648 fn le(&self, other
: &Rc
<T
>) -> bool
{
1652 /// Greater-than comparison for two `Rc`s.
1654 /// The two are compared by calling `>` on their inner values.
1659 /// use std::rc::Rc;
1661 /// let five = Rc::new(5);
1663 /// assert!(five > Rc::new(4));
1666 fn gt(&self, other
: &Rc
<T
>) -> bool
{
1670 /// 'Greater than or equal to' comparison for two `Rc`s.
1672 /// The two are compared by calling `>=` on their inner values.
1677 /// use std::rc::Rc;
1679 /// let five = Rc::new(5);
1681 /// assert!(five >= Rc::new(5));
1684 fn ge(&self, other
: &Rc
<T
>) -> bool
{
1689 #[stable(feature = "rust1", since = "1.0.0")]
1690 impl<T
: ?Sized
+ Ord
> Ord
for Rc
<T
> {
1691 /// Comparison for two `Rc`s.
1693 /// The two are compared by calling `cmp()` on their inner values.
1698 /// use std::rc::Rc;
1699 /// use std::cmp::Ordering;
1701 /// let five = Rc::new(5);
1703 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1706 fn cmp(&self, other
: &Rc
<T
>) -> Ordering
{
1707 (**self).cmp(&**other
)
1711 #[stable(feature = "rust1", since = "1.0.0")]
1712 impl<T
: ?Sized
+ Hash
> Hash
for Rc
<T
> {
1713 fn hash
<H
: Hasher
>(&self, state
: &mut H
) {
1714 (**self).hash(state
);
1718 #[stable(feature = "rust1", since = "1.0.0")]
1719 impl<T
: ?Sized
+ fmt
::Display
> fmt
::Display
for Rc
<T
> {
1720 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1721 fmt
::Display
::fmt(&**self, f
)
1725 #[stable(feature = "rust1", since = "1.0.0")]
1726 impl<T
: ?Sized
+ fmt
::Debug
> fmt
::Debug
for Rc
<T
> {
1727 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1728 fmt
::Debug
::fmt(&**self, f
)
1732 #[stable(feature = "rust1", since = "1.0.0")]
1733 impl<T
: ?Sized
> fmt
::Pointer
for Rc
<T
> {
1734 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1735 fmt
::Pointer
::fmt(&(&**self as *const T
), f
)
1739 #[cfg(not(no_global_oom_handling))]
1740 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1741 impl<T
> From
<T
> for Rc
<T
> {
1742 /// Converts a generic type `T` into a `Rc<T>`
1744 /// The conversion allocates on the heap and moves `t`
1745 /// from the stack into it.
1749 /// # use std::rc::Rc;
1751 /// let rc = Rc::new(5);
1753 /// assert_eq!(Rc::from(x), rc);
1755 fn from(t
: T
) -> Self {
1760 #[cfg(not(no_global_oom_handling))]
1761 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1762 impl<T
: Clone
> From
<&[T
]> for Rc
<[T
]> {
1763 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
1768 /// # use std::rc::Rc;
1769 /// let original: &[i32] = &[1, 2, 3];
1770 /// let shared: Rc<[i32]> = Rc::from(original);
1771 /// assert_eq!(&[1, 2, 3], &shared[..]);
1774 fn from(v
: &[T
]) -> Rc
<[T
]> {
1775 <Self as RcFromSlice
<T
>>::from_slice(v
)
1779 #[cfg(not(no_global_oom_handling))]
1780 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1781 impl From
<&str> for Rc
<str> {
1782 /// Allocate a reference-counted string slice and copy `v` into it.
1787 /// # use std::rc::Rc;
1788 /// let shared: Rc<str> = Rc::from("statue");
1789 /// assert_eq!("statue", &shared[..]);
1792 fn from(v
: &str) -> Rc
<str> {
1793 let rc
= Rc
::<[u8]>::from(v
.as_bytes());
1794 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1798 #[cfg(not(no_global_oom_handling))]
1799 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1800 impl From
<String
> for Rc
<str> {
1801 /// Allocate a reference-counted string slice and copy `v` into it.
1806 /// # use std::rc::Rc;
1807 /// let original: String = "statue".to_owned();
1808 /// let shared: Rc<str> = Rc::from(original);
1809 /// assert_eq!("statue", &shared[..]);
1812 fn from(v
: String
) -> Rc
<str> {
1817 #[cfg(not(no_global_oom_handling))]
1818 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1819 impl<T
: ?Sized
> From
<Box
<T
>> for Rc
<T
> {
1820 /// Move a boxed object to a new, reference counted, allocation.
1825 /// # use std::rc::Rc;
1826 /// let original: Box<i32> = Box::new(1);
1827 /// let shared: Rc<i32> = Rc::from(original);
1828 /// assert_eq!(1, *shared);
1831 fn from(v
: Box
<T
>) -> Rc
<T
> {
1836 #[cfg(not(no_global_oom_handling))]
1837 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1838 impl<T
> From
<Vec
<T
>> for Rc
<[T
]> {
1839 /// Allocate a reference-counted slice and move `v`'s items into it.
1844 /// # use std::rc::Rc;
1845 /// let original: Box<Vec<i32>> = Box::new(vec![1, 2, 3]);
1846 /// let shared: Rc<Vec<i32>> = Rc::from(original);
1847 /// assert_eq!(vec![1, 2, 3], *shared);
1850 fn from(mut v
: Vec
<T
>) -> Rc
<[T
]> {
1852 let rc
= Rc
::copy_from_slice(&v
);
1854 // Allow the Vec to free its memory, but not destroy its contents
1862 #[stable(feature = "shared_from_cow", since = "1.45.0")]
1863 impl<'a
, B
> From
<Cow
<'a
, B
>> for Rc
<B
>
1865 B
: ToOwned
+ ?Sized
,
1866 Rc
<B
>: From
<&'a B
> + From
<B
::Owned
>,
1868 /// Create a reference-counted pointer from
1869 /// a clone-on-write pointer by copying its content.
1874 /// # use std::rc::Rc;
1875 /// # use std::borrow::Cow;
1876 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
1877 /// let shared: Rc<str> = Rc::from(cow);
1878 /// assert_eq!("eggplant", &shared[..]);
1881 fn from(cow
: Cow
<'a
, B
>) -> Rc
<B
> {
1883 Cow
::Borrowed(s
) => Rc
::from(s
),
1884 Cow
::Owned(s
) => Rc
::from(s
),
1889 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1890 impl<T
, const N
: usize> TryFrom
<Rc
<[T
]>> for Rc
<[T
; N
]> {
1891 type Error
= Rc
<[T
]>;
1893 fn try_from(boxed_slice
: Rc
<[T
]>) -> Result
<Self, Self::Error
> {
1894 if boxed_slice
.len() == N
{
1895 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) }
)
1902 #[cfg(not(no_global_oom_handling))]
1903 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1904 impl<T
> iter
::FromIterator
<T
> for Rc
<[T
]> {
1905 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1907 /// # Performance characteristics
1909 /// ## The general case
1911 /// In the general case, collecting into `Rc<[T]>` is done by first
1912 /// collecting into a `Vec<T>`. That is, when writing the following:
1915 /// # use std::rc::Rc;
1916 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1917 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1920 /// this behaves as if we wrote:
1923 /// # use std::rc::Rc;
1924 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1925 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1926 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1927 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1930 /// This will allocate as many times as needed for constructing the `Vec<T>`
1931 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1933 /// ## Iterators of known length
1935 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1936 /// a single allocation will be made for the `Rc<[T]>`. For example:
1939 /// # use std::rc::Rc;
1940 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1941 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1943 fn from_iter
<I
: iter
::IntoIterator
<Item
= T
>>(iter
: I
) -> Self {
1944 ToRcSlice
::to_rc_slice(iter
.into_iter())
1948 /// Specialization trait used for collecting into `Rc<[T]>`.
1949 #[cfg(not(no_global_oom_handling))]
1950 trait ToRcSlice
<T
>: Iterator
<Item
= T
> + Sized
{
1951 fn to_rc_slice(self) -> Rc
<[T
]>;
1954 #[cfg(not(no_global_oom_handling))]
1955 impl<T
, I
: Iterator
<Item
= T
>> ToRcSlice
<T
> for I
{
1956 default fn to_rc_slice(self) -> Rc
<[T
]> {
1957 self.collect
::<Vec
<T
>>().into()
1961 #[cfg(not(no_global_oom_handling))]
1962 impl<T
, I
: iter
::TrustedLen
<Item
= T
>> ToRcSlice
<T
> for I
{
1963 fn to_rc_slice(self) -> Rc
<[T
]> {
1964 // This is the case for a `TrustedLen` iterator.
1965 let (low
, high
) = self.size_hint();
1966 if let Some(high
) = high
{
1970 "TrustedLen iterator's size hint is not exact: {:?}",
1975 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1976 Rc
::from_iter_exact(self, low
)
1979 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
1980 // length exceeding `usize::MAX`.
1981 // The default implementation would collect into a vec which would panic.
1982 // Thus we panic here immediately without invoking `Vec` code.
1983 panic
!("capacity overflow");
1988 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1989 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
1990 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1992 /// Since a `Weak` reference does not count towards ownership, it will not
1993 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
1994 /// guarantees about the value still being present. Thus it may return [`None`]
1995 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
1996 /// itself (the backing store) from being deallocated.
1998 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
1999 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
2000 /// prevent circular references between [`Rc`] pointers, since mutual owning references
2001 /// would never allow either [`Rc`] to be dropped. For example, a tree could
2002 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
2003 /// pointers from children back to their parents.
2005 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
2007 /// [`upgrade`]: Weak::upgrade
2008 #[stable(feature = "rc_weak", since = "1.4.0")]
2009 pub struct Weak
<T
: ?Sized
> {
2010 // This is a `NonNull` to allow optimizing the size of this type in enums,
2011 // but it is not necessarily a valid pointer.
2012 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
2013 // to allocate space on the heap. That's not a value a real pointer
2014 // will ever have because RcBox has alignment at least 2.
2015 // This is only possible when `T: Sized`; unsized `T` never dangle.
2016 ptr
: NonNull
<RcBox
<T
>>,
2019 #[stable(feature = "rc_weak", since = "1.4.0")]
2020 impl<T
: ?Sized
> !marker
::Send
for Weak
<T
> {}
2021 #[stable(feature = "rc_weak", since = "1.4.0")]
2022 impl<T
: ?Sized
> !marker
::Sync
for Weak
<T
> {}
2024 #[unstable(feature = "coerce_unsized", issue = "27732")]
2025 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Weak
<U
>> for Weak
<T
> {}
2027 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
2028 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> DispatchFromDyn
<Weak
<U
>> for Weak
<T
> {}
2031 /// Constructs a new `Weak<T>`, without allocating any memory.
2032 /// Calling [`upgrade`] on the return value always gives [`None`].
2034 /// [`upgrade`]: Weak::upgrade
2039 /// use std::rc::Weak;
2041 /// let empty: Weak<i64> = Weak::new();
2042 /// assert!(empty.upgrade().is_none());
2044 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2045 pub fn new() -> Weak
<T
> {
2046 Weak { ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0") }
2050 pub(crate) fn is_dangling
<T
: ?Sized
>(ptr
: *mut T
) -> bool
{
2051 let address
= ptr
as *mut () as usize;
2052 address
== usize::MAX
2055 /// Helper type to allow accessing the reference counts without
2056 /// making any assertions about the data field.
2057 struct WeakInner
<'a
> {
2058 weak
: &'a Cell
<usize>,
2059 strong
: &'a Cell
<usize>,
2062 impl<T
: ?Sized
> Weak
<T
> {
2063 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
2065 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
2066 /// unaligned or even [`null`] otherwise.
2071 /// use std::rc::Rc;
2074 /// let strong = Rc::new("hello".to_owned());
2075 /// let weak = Rc::downgrade(&strong);
2076 /// // Both point to the same object
2077 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
2078 /// // The strong here keeps it alive, so we can still access the object.
2079 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
2082 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
2083 /// // undefined behaviour.
2084 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
2087 /// [`null`]: core::ptr::null
2088 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
2089 pub fn as_ptr(&self) -> *const T
{
2090 let ptr
: *mut RcBox
<T
> = NonNull
::as_ptr(self.ptr
);
2092 if is_dangling(ptr
) {
2093 // If the pointer is dangling, we return the sentinel directly. This cannot be
2094 // a valid payload address, as the payload is at least as aligned as RcBox (usize).
2097 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
2098 // The payload may be dropped at this point, and we have to maintain provenance,
2099 // so use raw pointer manipulation.
2100 unsafe { ptr::addr_of_mut!((*ptr).value) }
2104 /// Consumes the `Weak<T>` and turns it into a raw pointer.
2106 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2107 /// one weak reference (the weak count is not modified by this operation). It can be turned
2108 /// back into the `Weak<T>` with [`from_raw`].
2110 /// The same restrictions of accessing the target of the pointer as with
2111 /// [`as_ptr`] apply.
2116 /// use std::rc::{Rc, Weak};
2118 /// let strong = Rc::new("hello".to_owned());
2119 /// let weak = Rc::downgrade(&strong);
2120 /// let raw = weak.into_raw();
2122 /// assert_eq!(1, Rc::weak_count(&strong));
2123 /// assert_eq!("hello", unsafe { &*raw });
2125 /// drop(unsafe { Weak::from_raw(raw) });
2126 /// assert_eq!(0, Rc::weak_count(&strong));
2129 /// [`from_raw`]: Weak::from_raw
2130 /// [`as_ptr`]: Weak::as_ptr
2131 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2132 pub fn into_raw(self) -> *const T
{
2133 let result
= self.as_ptr();
2138 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
2140 /// This can be used to safely get a strong reference (by calling [`upgrade`]
2141 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2143 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2144 /// as these don't own anything; the method still works on them).
2148 /// The pointer must have originated from the [`into_raw`] and must still own its potential
2151 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2152 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2153 /// count is not modified by this operation) and therefore it must be paired with a previous
2154 /// call to [`into_raw`].
2159 /// use std::rc::{Rc, Weak};
2161 /// let strong = Rc::new("hello".to_owned());
2163 /// let raw_1 = Rc::downgrade(&strong).into_raw();
2164 /// let raw_2 = Rc::downgrade(&strong).into_raw();
2166 /// assert_eq!(2, Rc::weak_count(&strong));
2168 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2169 /// assert_eq!(1, Rc::weak_count(&strong));
2173 /// // Decrement the last weak count.
2174 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2177 /// [`into_raw`]: Weak::into_raw
2178 /// [`upgrade`]: Weak::upgrade
2179 /// [`new`]: Weak::new
2180 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2181 pub unsafe fn from_raw(ptr
: *const T
) -> Self {
2182 // See Weak::as_ptr for context on how the input pointer is derived.
2184 let ptr
= if is_dangling(ptr
as *mut T
) {
2185 // This is a dangling Weak.
2186 ptr
as *mut RcBox
<T
>
2188 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
2189 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
2190 let offset
= unsafe { data_offset(ptr) }
;
2191 // Thus, we reverse the offset to get the whole RcBox.
2192 // SAFETY: the pointer originated from a Weak, so this offset is safe.
2193 unsafe { (ptr as *mut RcBox<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
2196 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
2197 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) }
}
2200 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
2201 /// dropping of the inner value if successful.
2203 /// Returns [`None`] if the inner value has since been dropped.
2208 /// use std::rc::Rc;
2210 /// let five = Rc::new(5);
2212 /// let weak_five = Rc::downgrade(&five);
2214 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
2215 /// assert!(strong_five.is_some());
2217 /// // Destroy all strong pointers.
2218 /// drop(strong_five);
2221 /// assert!(weak_five.upgrade().is_none());
2223 #[stable(feature = "rc_weak", since = "1.4.0")]
2224 pub fn upgrade(&self) -> Option
<Rc
<T
>> {
2225 let inner
= self.inner()?
;
2226 if inner
.strong() == 0 {
2230 Some(Rc
::from_inner(self.ptr
))
2234 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
2236 /// If `self` was created using [`Weak::new`], this will return 0.
2237 #[stable(feature = "weak_counts", since = "1.41.0")]
2238 pub fn strong_count(&self) -> usize {
2239 if let Some(inner
) = self.inner() { inner.strong() }
else { 0 }
2242 /// Gets the number of `Weak` pointers pointing to this allocation.
2244 /// If no strong pointers remain, this will return zero.
2245 #[stable(feature = "weak_counts", since = "1.41.0")]
2246 pub fn weak_count(&self) -> usize {
2249 if inner
.strong() > 0 {
2250 inner
.weak() - 1 // subtract the implicit weak ptr
2258 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`,
2259 /// (i.e., when this `Weak` was created by `Weak::new`).
2261 fn inner(&self) -> Option
<WeakInner
<'_
>> {
2262 if is_dangling(self.ptr
.as_ptr()) {
2265 // We are careful to *not* create a reference covering the "data" field, as
2266 // the field may be mutated concurrently (for example, if the last `Rc`
2267 // is dropped, the data field will be dropped in-place).
2269 let ptr
= self.ptr
.as_ptr();
2270 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
2275 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
2276 /// [`ptr::eq`]), or if both don't point to any allocation
2277 /// (because they were created with `Weak::new()`).
2281 /// Since this compares pointers it means that `Weak::new()` will equal each
2282 /// other, even though they don't point to any allocation.
2287 /// use std::rc::Rc;
2289 /// let first_rc = Rc::new(5);
2290 /// let first = Rc::downgrade(&first_rc);
2291 /// let second = Rc::downgrade(&first_rc);
2293 /// assert!(first.ptr_eq(&second));
2295 /// let third_rc = Rc::new(5);
2296 /// let third = Rc::downgrade(&third_rc);
2298 /// assert!(!first.ptr_eq(&third));
2301 /// Comparing `Weak::new`.
2304 /// use std::rc::{Rc, Weak};
2306 /// let first = Weak::new();
2307 /// let second = Weak::new();
2308 /// assert!(first.ptr_eq(&second));
2310 /// let third_rc = Rc::new(());
2311 /// let third = Rc::downgrade(&third_rc);
2312 /// assert!(!first.ptr_eq(&third));
2315 /// [`ptr::eq`]: core::ptr::eq
2317 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
2318 pub fn ptr_eq(&self, other
: &Self) -> bool
{
2319 self.ptr
.as_ptr() == other
.ptr
.as_ptr()
2323 #[stable(feature = "rc_weak", since = "1.4.0")]
2324 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2325 /// Drops the `Weak` pointer.
2330 /// use std::rc::{Rc, Weak};
2334 /// impl Drop for Foo {
2335 /// fn drop(&mut self) {
2336 /// println!("dropped!");
2340 /// let foo = Rc::new(Foo);
2341 /// let weak_foo = Rc::downgrade(&foo);
2342 /// let other_weak_foo = Weak::clone(&weak_foo);
2344 /// drop(weak_foo); // Doesn't print anything
2345 /// drop(foo); // Prints "dropped!"
2347 /// assert!(other_weak_foo.upgrade().is_none());
2349 fn drop(&mut self) {
2350 let inner
= if let Some(inner
) = self.inner() { inner }
else { return }
;
2353 // the weak count starts at 1, and will only go to zero if all
2354 // the strong pointers have disappeared.
2355 if inner
.weak() == 0 {
2357 Global
.deallocate(self.ptr
.cast(), Layout
::for_value_raw(self.ptr
.as_ptr()));
2363 #[stable(feature = "rc_weak", since = "1.4.0")]
2364 impl<T
: ?Sized
> Clone
for Weak
<T
> {
2365 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2370 /// use std::rc::{Rc, Weak};
2372 /// let weak_five = Rc::downgrade(&Rc::new(5));
2374 /// let _ = Weak::clone(&weak_five);
2377 fn clone(&self) -> Weak
<T
> {
2378 if let Some(inner
) = self.inner() {
2381 Weak { ptr: self.ptr }
2385 #[stable(feature = "rc_weak", since = "1.4.0")]
2386 impl<T
: ?Sized
+ fmt
::Debug
> fmt
::Debug
for Weak
<T
> {
2387 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
2392 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2393 impl<T
> Default
for Weak
<T
> {
2394 /// Constructs a new `Weak<T>`, without allocating any memory.
2395 /// Calling [`upgrade`] on the return value always gives [`None`].
2397 /// [`None`]: Option
2398 /// [`upgrade`]: Weak::upgrade
2403 /// use std::rc::Weak;
2405 /// let empty: Weak<i64> = Default::default();
2406 /// assert!(empty.upgrade().is_none());
2408 fn default() -> Weak
<T
> {
2413 // NOTE: We checked_add here to deal with mem::forget safely. In particular
2414 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
2415 // you can free the allocation while outstanding Rcs (or Weaks) exist.
2416 // We abort because this is such a degenerate scenario that we don't care about
2417 // what happens -- no real program should ever experience this.
2419 // This should have negligible overhead since you don't actually need to
2420 // clone these much in Rust thanks to ownership and move-semantics.
2424 fn weak_ref(&self) -> &Cell
<usize>;
2425 fn strong_ref(&self) -> &Cell
<usize>;
2428 fn strong(&self) -> usize {
2429 self.strong_ref().get()
2433 fn inc_strong(&self) {
2434 let strong
= self.strong();
2436 // We want to abort on overflow instead of dropping the value.
2437 // The reference count will never be zero when this is called;
2438 // nevertheless, we insert an abort here to hint LLVM at
2439 // an otherwise missed optimization.
2440 if strong
== 0 || strong
== usize::MAX
{
2443 self.strong_ref().set(strong
+ 1);
2447 fn dec_strong(&self) {
2448 self.strong_ref().set(self.strong() - 1);
2452 fn weak(&self) -> usize {
2453 self.weak_ref().get()
2457 fn inc_weak(&self) {
2458 let weak
= self.weak();
2460 // We want to abort on overflow instead of dropping the value.
2461 // The reference count will never be zero when this is called;
2462 // nevertheless, we insert an abort here to hint LLVM at
2463 // an otherwise missed optimization.
2464 if weak
== 0 || weak
== usize::MAX
{
2467 self.weak_ref().set(weak
+ 1);
2471 fn dec_weak(&self) {
2472 self.weak_ref().set(self.weak() - 1);
2476 impl<T
: ?Sized
> RcInnerPtr
for RcBox
<T
> {
2478 fn weak_ref(&self) -> &Cell
<usize> {
2483 fn strong_ref(&self) -> &Cell
<usize> {
2488 impl<'a
> RcInnerPtr
for WeakInner
<'a
> {
2490 fn weak_ref(&self) -> &Cell
<usize> {
2495 fn strong_ref(&self) -> &Cell
<usize> {
2500 #[stable(feature = "rust1", since = "1.0.0")]
2501 impl<T
: ?Sized
> borrow
::Borrow
<T
> for Rc
<T
> {
2502 fn borrow(&self) -> &T
{
2507 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2508 impl<T
: ?Sized
> AsRef
<T
> for Rc
<T
> {
2509 fn as_ref(&self) -> &T
{
2514 #[stable(feature = "pin", since = "1.33.0")]
2515 impl<T
: ?Sized
> Unpin
for Rc
<T
> {}
2517 /// Get the offset within an `RcBox` for the payload behind a pointer.
2521 /// The pointer must point to (and have valid metadata for) a previously
2522 /// valid instance of T, but the T is allowed to be dropped.
2523 unsafe fn data_offset
<T
: ?Sized
>(ptr
: *const T
) -> isize {
2524 // Align the unsized value to the end of the RcBox.
2525 // Because RcBox is repr(C), it will always be the last field in memory.
2526 // SAFETY: since the only unsized types possible are slices, trait objects,
2527 // and extern types, the input safety requirement is currently enough to
2528 // satisfy the requirements of align_of_val_raw; this is an implementation
2529 // detail of the language that may not be relied upon outside of std.
2530 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2534 fn data_offset_align(align
: usize) -> isize {
2535 let layout
= Layout
::new
::<RcBox
<()>>();
2536 (layout
.size() + layout
.padding_needed_for(align
)) as isize