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};
266 use core::ptr::{self, NonNull};
267 #[cfg(not(no_global_oom_handling))]
268 use core::slice::from_raw_parts_mut;
270 #[cfg(not(no_global_oom_handling))]
271 use crate::alloc::handle_alloc_error;
272 #[cfg(not(no_global_oom_handling))]
273 use crate::alloc::{box_free, WriteCloneIntoRaw};
274 use crate::alloc::{AllocError, Allocator, Global, Layout};
275 use crate::borrow::{Cow, ToOwned};
276 #[cfg(not(no_global_oom_handling))]
277 use crate::string::String;
278 #[cfg(not(no_global_oom_handling))]
284 // This is repr(C) to future-proof against possible field-reordering, which
285 // would interfere with otherwise safe [into|from]_raw() of transmutable
288 struct RcBox<T: ?Sized> {
294 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
297 /// See the [module-level documentation](./index.html) for more details.
299 /// The inherent methods of `Rc` are all associated functions, which means
300 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
301 /// `value.get_mut()`. This avoids conflicts with methods of the inner type `T`.
303 /// [get_mut]: Rc::get_mut
304 #[cfg_attr(not(test), rustc_diagnostic_item = "Rc")]
305 #[stable(feature = "rust1", since = "1.0.0")]
306 pub struct Rc<T: ?Sized> {
307 ptr: NonNull<RcBox<T>>,
308 phantom: PhantomData<RcBox<T>>,
311 #[stable(feature = "rust1", since = "1.0.0")]
312 impl<T: ?Sized> !marker::Send for Rc<T> {}
313 #[stable(feature = "rust1", since = "1.0.0")]
314 impl<T: ?Sized> !marker::Sync for Rc<T> {}
316 #[unstable(feature = "coerce_unsized", issue = "27732")]
317 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
319 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
320 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
322 impl<T: ?Sized> Rc<T> {
324 fn inner(&self) -> &RcBox<T> {
325 // This unsafety is ok because while this Rc is alive we're guaranteed
326 // that the inner pointer is valid.
327 unsafe { self.ptr.as_ref() }
330 fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
331 Self { ptr, phantom: PhantomData }
334 unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
335 Self::from_inner(unsafe { NonNull::new_unchecked(ptr) })
340 /// Constructs a new `Rc<T>`.
347 /// let five = Rc::new(5);
349 #[stable(feature = "rust1", since = "1.0.0")]
350 pub fn new(value: T) -> Rc<T> {
351 // There is an implicit weak pointer owned by all the strong
352 // pointers, which ensures that the weak destructor never frees
353 // the allocation while the strong destructor is running, even
354 // if the weak pointer is stored inside the strong one.
356 Box::leak(box RcBox { strong: Cell::new(1), weak: Cell::new(1), value }).into(),
360 /// Constructs a new `Rc<T>` using a weak reference to itself. Attempting
361 /// to upgrade the weak reference before this function returns will result
362 /// in a `None` value. However, the weak reference may be cloned freely and
363 /// stored for use at a later time.
368 /// #![feature(arc_new_cyclic)]
369 /// #![allow(dead_code)]
370 /// use std::rc::{Rc, Weak};
373 /// self_weak: Weak<Self>,
374 /// // ... more fields
377 /// pub fn new() -> Rc<Self> {
378 /// Rc::new_cyclic(|self_weak| {
379 /// Gadget { self_weak: self_weak.clone(), /* ... */ }
384 #[unstable(feature = "arc_new_cyclic", issue = "75861")]
385 pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Rc<T> {
386 // Construct the inner in the "uninitialized" state with a single
388 let uninit_ptr: NonNull<_> = Box::leak(box RcBox {
389 strong: Cell::new(0),
391 value: mem::MaybeUninit::<T>::uninit(),
395 let init_ptr: NonNull<RcBox<T>> = uninit_ptr.cast();
397 let weak = Weak { ptr: init_ptr };
399 // It's important we don't give up ownership of the weak pointer, or
400 // else the memory might be freed by the time `data_fn` returns. If
401 // we really wanted to pass ownership, we could create an additional
402 // weak pointer for ourselves, but this would result in additional
403 // updates to the weak reference count which might not be necessary
405 let data = data_fn(&weak);
408 let inner = init_ptr.as_ptr();
409 ptr::write(ptr::addr_of_mut!((*inner).value), data);
411 let prev_value = (*inner).strong.get();
412 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
413 (*inner).strong.set(1);
416 let strong = Rc::from_inner(init_ptr);
418 // Strong references should collectively own a shared weak reference,
419 // so don't run the destructor for our old weak reference.
424 /// Constructs a new `Rc` with uninitialized contents.
429 /// #![feature(new_uninit)]
430 /// #![feature(get_mut_unchecked)]
434 /// let mut five = Rc::<u32>::new_uninit();
436 /// let five = unsafe {
437 /// // Deferred initialization:
438 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
440 /// five.assume_init()
443 /// assert_eq!(*five, 5)
445 #[cfg(not(no_global_oom_handling))]
446 #[unstable(feature = "new_uninit", issue = "63291")]
447 pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
449 Rc::from_ptr(Rc::allocate_for_layout(
451 |layout| Global.allocate(layout),
452 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
457 /// Constructs a new `Rc` with uninitialized contents, with the memory
458 /// being filled with `0` bytes.
460 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
461 /// incorrect usage of this method.
466 /// #![feature(new_uninit)]
470 /// let zero = Rc::<u32>::new_zeroed();
471 /// let zero = unsafe { zero.assume_init() };
473 /// assert_eq!(*zero, 0)
476 /// [zeroed]: mem::MaybeUninit::zeroed
477 #[cfg(not(no_global_oom_handling))]
478 #[unstable(feature = "new_uninit", issue = "63291")]
479 pub fn new_zeroed() -> Rc<mem::MaybeUninit<T>> {
481 Rc::from_ptr(Rc::allocate_for_layout(
483 |layout| Global.allocate_zeroed(layout),
484 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
489 /// Constructs a new `Rc<T>`, returning an error if the allocation fails
494 /// #![feature(allocator_api)]
497 /// let five = Rc::try_new(5);
498 /// # Ok::<(), std::alloc::AllocError>(())
500 #[unstable(feature = "allocator_api", issue = "32838")]
501 pub fn try_new(value: T) -> Result<Rc<T>, AllocError> {
502 // There is an implicit weak pointer owned by all the strong
503 // pointers, which ensures that the weak destructor never frees
504 // the allocation while the strong destructor is running, even
505 // if the weak pointer is stored inside the strong one.
507 Box::leak(Box::try_new(RcBox { strong: Cell::new(1), weak: Cell::new(1), value })?)
512 /// Constructs a new `Rc` with uninitialized contents, returning an error if the allocation fails
517 /// #![feature(allocator_api, new_uninit)]
518 /// #![feature(get_mut_unchecked)]
522 /// let mut five = Rc::<u32>::try_new_uninit()?;
524 /// let five = unsafe {
525 /// // Deferred initialization:
526 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
528 /// five.assume_init()
531 /// assert_eq!(*five, 5);
532 /// # Ok::<(), std::alloc::AllocError>(())
534 #[unstable(feature = "allocator_api", issue = "32838")]
535 // #[unstable(feature = "new_uninit", issue = "63291")]
536 pub fn try_new_uninit() -> Result<Rc<mem::MaybeUninit<T>>, AllocError> {
538 Ok(Rc::from_ptr(Rc::try_allocate_for_layout(
540 |layout| Global.allocate(layout),
541 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
546 /// Constructs a new `Rc` with uninitialized contents, with the memory
547 /// being filled with `0` bytes, returning an error if the allocation fails
549 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
550 /// incorrect usage of this method.
555 /// #![feature(allocator_api, new_uninit)]
559 /// let zero = Rc::<u32>::try_new_zeroed()?;
560 /// let zero = unsafe { zero.assume_init() };
562 /// assert_eq!(*zero, 0);
563 /// # Ok::<(), std::alloc::AllocError>(())
566 /// [zeroed]: mem::MaybeUninit::zeroed
567 #[unstable(feature = "allocator_api", issue = "32838")]
568 //#[unstable(feature = "new_uninit", issue = "63291")]
569 pub fn try_new_zeroed() -> Result<Rc<mem::MaybeUninit<T>>, AllocError> {
571 Ok(Rc::from_ptr(Rc::try_allocate_for_layout(
573 |layout| Global.allocate_zeroed(layout),
574 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
578 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
579 /// `value` will be pinned in memory and unable to be moved.
580 #[stable(feature = "pin", since = "1.33.0")]
581 pub fn pin(value: T) -> Pin<Rc<T>> {
582 unsafe { Pin::new_unchecked(Rc::new(value)) }
585 /// Returns the inner value, if the `Rc` has exactly one strong reference.
587 /// Otherwise, an [`Err`] is returned with the same `Rc` that was
590 /// This will succeed even if there are outstanding weak references.
597 /// let x = Rc::new(3);
598 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
600 /// let x = Rc::new(4);
601 /// let _y = Rc::clone(&x);
602 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
605 #[stable(feature = "rc_unique", since = "1.4.0")]
606 pub fn try_unwrap(this: Self) -> Result<T, Self> {
607 if Rc::strong_count(&this) == 1 {
609 let val = ptr::read(&*this); // copy the contained object
611 // Indicate to Weaks that they can't be promoted by decrementing
612 // the strong count, and then remove the implicit "strong weak"
613 // pointer while also handling drop logic by just crafting a
615 this.inner().dec_strong();
616 let _weak = Weak { ptr: this.ptr };
627 /// Constructs a new reference-counted slice with uninitialized contents.
632 /// #![feature(new_uninit)]
633 /// #![feature(get_mut_unchecked)]
637 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
639 /// let values = unsafe {
640 /// // Deferred initialization:
641 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
642 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
643 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
645 /// values.assume_init()
648 /// assert_eq!(*values, [1, 2, 3])
650 #[cfg(not(no_global_oom_handling))]
651 #[unstable(feature = "new_uninit", issue = "63291")]
652 pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
653 unsafe { Rc::from_ptr(Rc::allocate_for_slice(len)) }
656 /// Constructs a new reference-counted slice with uninitialized contents, with the memory being
657 /// filled with `0` bytes.
659 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
660 /// incorrect usage of this method.
665 /// #![feature(new_uninit)]
669 /// let values = Rc::<[u32]>::new_zeroed_slice(3);
670 /// let values = unsafe { values.assume_init() };
672 /// assert_eq!(*values, [0, 0, 0])
675 /// [zeroed]: mem::MaybeUninit::zeroed
676 #[cfg(not(no_global_oom_handling))]
677 #[unstable(feature = "new_uninit", issue = "63291")]
678 pub fn new_zeroed_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
680 Rc::from_ptr(Rc::allocate_for_layout(
681 Layout::array::<T>(len).unwrap(),
682 |layout| Global.allocate_zeroed(layout),
684 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
685 as *mut RcBox<[mem::MaybeUninit<T>]>
692 impl<T> Rc<mem::MaybeUninit<T>> {
693 /// Converts to `Rc<T>`.
697 /// As with [`MaybeUninit::assume_init`],
698 /// it is up to the caller to guarantee that the inner value
699 /// really is in an initialized state.
700 /// Calling this when the content is not yet fully initialized
701 /// causes immediate undefined behavior.
703 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
708 /// #![feature(new_uninit)]
709 /// #![feature(get_mut_unchecked)]
713 /// let mut five = Rc::<u32>::new_uninit();
715 /// let five = unsafe {
716 /// // Deferred initialization:
717 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
719 /// five.assume_init()
722 /// assert_eq!(*five, 5)
724 #[unstable(feature = "new_uninit", issue = "63291")]
726 pub unsafe fn assume_init(self) -> Rc<T> {
727 Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
731 impl<T> Rc<[mem::MaybeUninit<T>]> {
732 /// Converts to `Rc<[T]>`.
736 /// As with [`MaybeUninit::assume_init`],
737 /// it is up to the caller to guarantee that the inner value
738 /// really is in an initialized state.
739 /// Calling this when the content is not yet fully initialized
740 /// causes immediate undefined behavior.
742 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
747 /// #![feature(new_uninit)]
748 /// #![feature(get_mut_unchecked)]
752 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
754 /// let values = unsafe {
755 /// // Deferred initialization:
756 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
757 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
758 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
760 /// values.assume_init()
763 /// assert_eq!(*values, [1, 2, 3])
765 #[unstable(feature = "new_uninit", issue = "63291")]
767 pub unsafe fn assume_init(self) -> Rc<[T]> {
768 unsafe { Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
772 impl<T: ?Sized> Rc<T> {
773 /// Consumes the `Rc`, returning the wrapped pointer.
775 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
776 /// [`Rc::from_raw`][from_raw].
778 /// [from_raw]: Rc::from_raw
785 /// let x = Rc::new("hello".to_owned());
786 /// let x_ptr = Rc::into_raw(x);
787 /// assert_eq!(unsafe { &*x_ptr }, "hello");
789 #[stable(feature = "rc_raw", since = "1.17.0")]
790 pub fn into_raw(this: Self) -> *const T {
791 let ptr = Self::as_ptr(&this);
796 /// Provides a raw pointer to the data.
798 /// The counts are not affected in any way and the `Rc` is not consumed. The pointer is valid
799 /// for as long there are strong counts in the `Rc`.
806 /// let x = Rc::new("hello".to_owned());
807 /// let y = Rc::clone(&x);
808 /// let x_ptr = Rc::as_ptr(&x);
809 /// assert_eq!(x_ptr, Rc::as_ptr(&y));
810 /// assert_eq!(unsafe { &*x_ptr }, "hello");
812 #[stable(feature = "weak_into_raw", since = "1.45.0")]
813 pub fn as_ptr(this: &Self) -> *const T {
814 let ptr: *mut RcBox<T> = NonNull::as_ptr(this.ptr);
816 // SAFETY: This cannot go through Deref::deref or Rc::inner because
817 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
818 // write through the pointer after the Rc is recovered through `from_raw`.
819 unsafe { ptr::addr_of_mut!((*ptr).value) }
822 /// Constructs an `Rc<T>` from a raw pointer.
824 /// The raw pointer must have been previously returned by a call to
825 /// [`Rc<U>::into_raw`][into_raw] where `U` must have the same size
826 /// and alignment as `T`. This is trivially true if `U` is `T`.
827 /// Note that if `U` is not `T` but has the same size and alignment, this is
828 /// basically like transmuting references of different types. See
829 /// [`mem::transmute`][transmute] for more information on what
830 /// restrictions apply in this case.
832 /// The user of `from_raw` has to make sure a specific value of `T` is only
835 /// This function is unsafe because improper use may lead to memory unsafety,
836 /// even if the returned `Rc<T>` is never accessed.
838 /// [into_raw]: Rc::into_raw
839 /// [transmute]: core::mem::transmute
846 /// let x = Rc::new("hello".to_owned());
847 /// let x_ptr = Rc::into_raw(x);
850 /// // Convert back to an `Rc` to prevent leak.
851 /// let x = Rc::from_raw(x_ptr);
852 /// assert_eq!(&*x, "hello");
854 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
857 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
859 #[stable(feature = "rc_raw", since = "1.17.0")]
860 pub unsafe fn from_raw(ptr: *const T) -> Self {
861 let offset = unsafe { data_offset(ptr) };
863 // Reverse the offset to find the original RcBox.
865 unsafe { (ptr as *mut RcBox<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) };
867 unsafe { Self::from_ptr(rc_ptr) }
870 /// Creates a new [`Weak`] pointer to this allocation.
877 /// let five = Rc::new(5);
879 /// let weak_five = Rc::downgrade(&five);
881 #[stable(feature = "rc_weak", since = "1.4.0")]
882 pub fn downgrade(this: &Self) -> Weak<T> {
883 this.inner().inc_weak();
884 // Make sure we do not create a dangling Weak
885 debug_assert!(!is_dangling(this.ptr.as_ptr()));
886 Weak { ptr: this.ptr }
889 /// Gets the number of [`Weak`] pointers to this allocation.
896 /// let five = Rc::new(5);
897 /// let _weak_five = Rc::downgrade(&five);
899 /// assert_eq!(1, Rc::weak_count(&five));
902 #[stable(feature = "rc_counts", since = "1.15.0")]
903 pub fn weak_count(this: &Self) -> usize {
904 this.inner().weak() - 1
907 /// Gets the number of strong (`Rc`) pointers to this allocation.
914 /// let five = Rc::new(5);
915 /// let _also_five = Rc::clone(&five);
917 /// assert_eq!(2, Rc::strong_count(&five));
920 #[stable(feature = "rc_counts", since = "1.15.0")]
921 pub fn strong_count(this: &Self) -> usize {
922 this.inner().strong()
925 /// Increments the strong reference count on the `Rc<T>` associated with the
926 /// provided pointer by one.
930 /// The pointer must have been obtained through `Rc::into_raw`, and the
931 /// associated `Rc` instance must be valid (i.e. the strong count must be at
932 /// least 1) for the duration of this method.
939 /// let five = Rc::new(5);
942 /// let ptr = Rc::into_raw(five);
943 /// Rc::increment_strong_count(ptr);
945 /// let five = Rc::from_raw(ptr);
946 /// assert_eq!(2, Rc::strong_count(&five));
950 #[stable(feature = "rc_mutate_strong_count", since = "1.53.0")]
951 pub unsafe fn increment_strong_count(ptr: *const T) {
952 // Retain Rc, but don't touch refcount by wrapping in ManuallyDrop
953 let rc = unsafe { mem::ManuallyDrop::new(Rc::<T>::from_raw(ptr)) };
954 // Now increase refcount, but don't drop new refcount either
955 let _rc_clone: mem::ManuallyDrop<_> = rc.clone();
958 /// Decrements the strong reference count on the `Rc<T>` associated with the
959 /// provided pointer by one.
963 /// The pointer must have been obtained through `Rc::into_raw`, and the
964 /// associated `Rc` instance must be valid (i.e. the strong count must be at
965 /// least 1) when invoking this method. This method can be used to release
966 /// the final `Rc` and backing storage, but **should not** be called after
967 /// the final `Rc` has been released.
974 /// let five = Rc::new(5);
977 /// let ptr = Rc::into_raw(five);
978 /// Rc::increment_strong_count(ptr);
980 /// let five = Rc::from_raw(ptr);
981 /// assert_eq!(2, Rc::strong_count(&five));
982 /// Rc::decrement_strong_count(ptr);
983 /// assert_eq!(1, Rc::strong_count(&five));
987 #[stable(feature = "rc_mutate_strong_count", since = "1.53.0")]
988 pub unsafe fn decrement_strong_count(ptr: *const T) {
989 unsafe { mem::drop(Rc::from_raw(ptr)) };
992 /// Returns `true` if there are no other `Rc` or [`Weak`] pointers to
995 fn is_unique(this: &Self) -> bool {
996 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
999 /// Returns a mutable reference into the given `Rc`, if there are
1000 /// no other `Rc` or [`Weak`] pointers to the same allocation.
1002 /// Returns [`None`] otherwise, because it is not safe to
1003 /// mutate a shared value.
1005 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1006 /// the inner value when there are other pointers.
1008 /// [make_mut]: Rc::make_mut
1009 /// [clone]: Clone::clone
1014 /// use std::rc::Rc;
1016 /// let mut x = Rc::new(3);
1017 /// *Rc::get_mut(&mut x).unwrap() = 4;
1018 /// assert_eq!(*x, 4);
1020 /// let _y = Rc::clone(&x);
1021 /// assert!(Rc::get_mut(&mut x).is_none());
1024 #[stable(feature = "rc_unique", since = "1.4.0")]
1025 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1026 if Rc::is_unique(this) { unsafe { Some(Rc::get_mut_unchecked(this)) } } else { None }
1029 /// Returns a mutable reference into the given `Rc`,
1030 /// without any check.
1032 /// See also [`get_mut`], which is safe and does appropriate checks.
1034 /// [`get_mut`]: Rc::get_mut
1038 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
1039 /// for the duration of the returned borrow.
1040 /// This is trivially the case if no such pointers exist,
1041 /// for example immediately after `Rc::new`.
1046 /// #![feature(get_mut_unchecked)]
1048 /// use std::rc::Rc;
1050 /// let mut x = Rc::new(String::new());
1052 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
1054 /// assert_eq!(*x, "foo");
1057 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1058 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1059 // We are careful to *not* create a reference covering the "count" fields, as
1060 // this would conflict with accesses to the reference counts (e.g. by `Weak`).
1061 unsafe { &mut (*this.ptr.as_ptr()).value }
1065 #[stable(feature = "ptr_eq", since = "1.17.0")]
1066 /// Returns `true` if the two `Rc`s point to the same allocation
1067 /// (in a vein similar to [`ptr::eq`]).
1072 /// use std::rc::Rc;
1074 /// let five = Rc::new(5);
1075 /// let same_five = Rc::clone(&five);
1076 /// let other_five = Rc::new(5);
1078 /// assert!(Rc::ptr_eq(&five, &same_five));
1079 /// assert!(!Rc::ptr_eq(&five, &other_five));
1082 /// [`ptr::eq`]: core::ptr::eq
1083 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1084 this.ptr.as_ptr() == other.ptr.as_ptr()
1088 impl<T: Clone> Rc<T> {
1089 /// Makes a mutable reference into the given `Rc`.
1091 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
1092 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1093 /// referred to as clone-on-write.
1095 /// If there are no other `Rc` pointers to this allocation, then [`Weak`]
1096 /// pointers to this allocation will be disassociated.
1098 /// See also [`get_mut`], which will fail rather than cloning.
1100 /// [`clone`]: Clone::clone
1101 /// [`get_mut`]: Rc::get_mut
1106 /// use std::rc::Rc;
1108 /// let mut data = Rc::new(5);
1110 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
1111 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
1112 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
1113 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
1114 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
1116 /// // Now `data` and `other_data` point to different allocations.
1117 /// assert_eq!(*data, 8);
1118 /// assert_eq!(*other_data, 12);
1121 /// [`Weak`] pointers will be disassociated:
1124 /// use std::rc::Rc;
1126 /// let mut data = Rc::new(75);
1127 /// let weak = Rc::downgrade(&data);
1129 /// assert!(75 == *data);
1130 /// assert!(75 == *weak.upgrade().unwrap());
1132 /// *Rc::make_mut(&mut data) += 1;
1134 /// assert!(76 == *data);
1135 /// assert!(weak.upgrade().is_none());
1137 #[cfg(not(no_global_oom_handling))]
1139 #[stable(feature = "rc_unique", since = "1.4.0")]
1140 pub fn make_mut(this: &mut Self) -> &mut T {
1141 if Rc::strong_count(this) != 1 {
1142 // Gotta clone the data, there are other Rcs.
1143 // Pre-allocate memory to allow writing the cloned value directly.
1144 let mut rc = Self::new_uninit();
1146 let data = Rc::get_mut_unchecked(&mut rc);
1147 (**this).write_clone_into_raw(data.as_mut_ptr());
1148 *this = rc.assume_init();
1150 } else if Rc::weak_count(this) != 0 {
1151 // Can just steal the data, all that's left is Weaks
1152 let mut rc = Self::new_uninit();
1154 let data = Rc::get_mut_unchecked(&mut rc);
1155 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1157 this.inner().dec_strong();
1158 // Remove implicit strong-weak ref (no need to craft a fake
1159 // Weak here -- we know other Weaks can clean up for us)
1160 this.inner().dec_weak();
1161 ptr::write(this, rc.assume_init());
1164 // This unsafety is ok because we're guaranteed that the pointer
1165 // returned is the *only* pointer that will ever be returned to T. Our
1166 // reference count is guaranteed to be 1 at this point, and we required
1167 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
1168 // reference to the allocation.
1169 unsafe { &mut this.ptr.as_mut().value }
1175 #[stable(feature = "rc_downcast", since = "1.29.0")]
1176 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
1181 /// use std::any::Any;
1182 /// use std::rc::Rc;
1184 /// fn print_if_string(value: Rc<dyn Any>) {
1185 /// if let Ok(string) = value.downcast::<String>() {
1186 /// println!("String ({}): {}", string.len(), string);
1190 /// let my_string = "Hello World".to_string();
1191 /// print_if_string(Rc::new(my_string));
1192 /// print_if_string(Rc::new(0i8));
1194 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
1195 if (*self).is::<T>() {
1196 let ptr = self.ptr.cast::<RcBox<T>>();
1198 Ok(Rc::from_inner(ptr))
1205 impl<T: ?Sized> Rc<T> {
1206 /// Allocates an `RcBox<T>` with sufficient space for
1207 /// a possibly-unsized inner value where the value has the layout provided.
1209 /// The function `mem_to_rcbox` is called with the data pointer
1210 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
1211 #[cfg(not(no_global_oom_handling))]
1212 unsafe fn allocate_for_layout(
1213 value_layout: Layout,
1214 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1215 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
1216 ) -> *mut RcBox<T> {
1217 // Calculate layout using the given value layout.
1218 // Previously, layout was calculated on the expression
1219 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
1220 // reference (see #54908).
1221 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
1223 Rc::try_allocate_for_layout(value_layout, allocate, mem_to_rcbox)
1224 .unwrap_or_else(|_| handle_alloc_error(layout))
1228 /// Allocates an `RcBox<T>` with sufficient space for
1229 /// a possibly-unsized inner value where the value has the layout provided,
1230 /// returning an error if allocation fails.
1232 /// The function `mem_to_rcbox` is called with the data pointer
1233 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
1235 unsafe fn try_allocate_for_layout(
1236 value_layout: Layout,
1237 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1238 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
1239 ) -> Result<*mut RcBox<T>, AllocError> {
1240 // Calculate layout using the given value layout.
1241 // Previously, layout was calculated on the expression
1242 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
1243 // reference (see #54908).
1244 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
1246 // Allocate for the layout.
1247 let ptr = allocate(layout)?;
1249 // Initialize the RcBox
1250 let inner = mem_to_rcbox(ptr.as_non_null_ptr().as_ptr());
1252 debug_assert_eq!(Layout::for_value(&*inner), layout);
1254 ptr::write(&mut (*inner).strong, Cell::new(1));
1255 ptr::write(&mut (*inner).weak, Cell::new(1));
1261 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
1262 #[cfg(not(no_global_oom_handling))]
1263 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
1264 // Allocate for the `RcBox<T>` using the given value.
1266 Self::allocate_for_layout(
1267 Layout::for_value(&*ptr),
1268 |layout| Global.allocate(layout),
1269 |mem| (ptr as *mut RcBox<T>).set_ptr_value(mem),
1274 #[cfg(not(no_global_oom_handling))]
1275 fn from_box(v: Box<T>) -> Rc<T> {
1277 let (box_unique, alloc) = Box::into_unique(v);
1278 let bptr = box_unique.as_ptr();
1280 let value_size = size_of_val(&*bptr);
1281 let ptr = Self::allocate_for_ptr(bptr);
1283 // Copy value as bytes
1284 ptr::copy_nonoverlapping(
1285 bptr as *const T as *const u8,
1286 &mut (*ptr).value as *mut _ as *mut u8,
1290 // Free the allocation without dropping its contents
1291 box_free(box_unique, alloc);
1299 /// Allocates an `RcBox<[T]>` with the given length.
1300 #[cfg(not(no_global_oom_handling))]
1301 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
1303 Self::allocate_for_layout(
1304 Layout::array::<T>(len).unwrap(),
1305 |layout| Global.allocate(layout),
1306 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>,
1311 /// Copy elements from slice into newly allocated Rc<\[T\]>
1313 /// Unsafe because the caller must either take ownership or bind `T: Copy`
1314 #[cfg(not(no_global_oom_handling))]
1315 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
1317 let ptr = Self::allocate_for_slice(v.len());
1318 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).value as *mut [T] as *mut T, v.len());
1323 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
1325 /// Behavior is undefined should the size be wrong.
1326 #[cfg(not(no_global_oom_handling))]
1327 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
1328 // Panic guard while cloning T elements.
1329 // In the event of a panic, elements that have been written
1330 // into the new RcBox will be dropped, then the memory freed.
1338 impl<T> Drop for Guard<T> {
1339 fn drop(&mut self) {
1341 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1342 ptr::drop_in_place(slice);
1344 Global.deallocate(self.mem, self.layout);
1350 let ptr = Self::allocate_for_slice(len);
1352 let mem = ptr as *mut _ as *mut u8;
1353 let layout = Layout::for_value(&*ptr);
1355 // Pointer to first element
1356 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1358 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1360 for (i, item) in iter.enumerate() {
1361 ptr::write(elems.add(i), item);
1365 // All clear. Forget the guard so it doesn't free the new RcBox.
1373 /// Specialization trait used for `From<&[T]>`.
1374 trait RcFromSlice<T> {
1375 fn from_slice(slice: &[T]) -> Self;
1378 #[cfg(not(no_global_oom_handling))]
1379 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
1381 default fn from_slice(v: &[T]) -> Self {
1382 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1386 #[cfg(not(no_global_oom_handling))]
1387 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
1389 fn from_slice(v: &[T]) -> Self {
1390 unsafe { Rc::copy_from_slice(v) }
1394 #[stable(feature = "rust1", since = "1.0.0")]
1395 impl<T: ?Sized> Deref for Rc<T> {
1399 fn deref(&self) -> &T {
1404 #[unstable(feature = "receiver_trait", issue = "none")]
1405 impl<T: ?Sized> Receiver for Rc<T> {}
1407 #[stable(feature = "rust1", since = "1.0.0")]
1408 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1411 /// This will decrement the strong reference count. If the strong reference
1412 /// count reaches zero then the only other references (if any) are
1413 /// [`Weak`], so we `drop` the inner value.
1418 /// use std::rc::Rc;
1422 /// impl Drop for Foo {
1423 /// fn drop(&mut self) {
1424 /// println!("dropped!");
1428 /// let foo = Rc::new(Foo);
1429 /// let foo2 = Rc::clone(&foo);
1431 /// drop(foo); // Doesn't print anything
1432 /// drop(foo2); // Prints "dropped!"
1434 fn drop(&mut self) {
1436 self.inner().dec_strong();
1437 if self.inner().strong() == 0 {
1438 // destroy the contained object
1439 ptr::drop_in_place(Self::get_mut_unchecked(self));
1441 // remove the implicit "strong weak" pointer now that we've
1442 // destroyed the contents.
1443 self.inner().dec_weak();
1445 if self.inner().weak() == 0 {
1446 Global.deallocate(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1453 #[stable(feature = "rust1", since = "1.0.0")]
1454 impl<T: ?Sized> Clone for Rc<T> {
1455 /// Makes a clone of the `Rc` pointer.
1457 /// This creates another pointer to the same allocation, increasing the
1458 /// strong reference count.
1463 /// use std::rc::Rc;
1465 /// let five = Rc::new(5);
1467 /// let _ = Rc::clone(&five);
1470 fn clone(&self) -> Rc<T> {
1471 self.inner().inc_strong();
1472 Self::from_inner(self.ptr)
1476 #[stable(feature = "rust1", since = "1.0.0")]
1477 impl<T: Default> Default for Rc<T> {
1478 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1483 /// use std::rc::Rc;
1485 /// let x: Rc<i32> = Default::default();
1486 /// assert_eq!(*x, 0);
1489 fn default() -> Rc<T> {
1490 Rc::new(Default::default())
1494 #[stable(feature = "rust1", since = "1.0.0")]
1495 trait RcEqIdent<T: ?Sized + PartialEq> {
1496 fn eq(&self, other: &Rc<T>) -> bool;
1497 fn ne(&self, other: &Rc<T>) -> bool;
1500 #[stable(feature = "rust1", since = "1.0.0")]
1501 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
1503 default fn eq(&self, other: &Rc<T>) -> bool {
1508 default fn ne(&self, other: &Rc<T>) -> bool {
1513 // Hack to allow specializing on `Eq` even though `Eq` has a method.
1514 #[rustc_unsafe_specialization_marker]
1515 pub(crate) trait MarkerEq: PartialEq<Self> {}
1517 impl<T: Eq> MarkerEq for T {}
1519 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1520 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1521 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1522 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1523 /// the same value, than two `&T`s.
1525 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1526 #[stable(feature = "rust1", since = "1.0.0")]
1527 impl<T: ?Sized + MarkerEq> RcEqIdent<T> for Rc<T> {
1529 fn eq(&self, other: &Rc<T>) -> bool {
1530 Rc::ptr_eq(self, other) || **self == **other
1534 fn ne(&self, other: &Rc<T>) -> bool {
1535 !Rc::ptr_eq(self, other) && **self != **other
1539 #[stable(feature = "rust1", since = "1.0.0")]
1540 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1541 /// Equality for two `Rc`s.
1543 /// Two `Rc`s are equal if their inner values are equal, even if they are
1544 /// stored in different allocation.
1546 /// If `T` also implements `Eq` (implying reflexivity of equality),
1547 /// two `Rc`s that point to the same allocation are
1553 /// use std::rc::Rc;
1555 /// let five = Rc::new(5);
1557 /// assert!(five == Rc::new(5));
1560 fn eq(&self, other: &Rc<T>) -> bool {
1561 RcEqIdent::eq(self, other)
1564 /// Inequality for two `Rc`s.
1566 /// Two `Rc`s are unequal if their inner values are unequal.
1568 /// If `T` also implements `Eq` (implying reflexivity of equality),
1569 /// two `Rc`s that point to the same allocation are
1575 /// use std::rc::Rc;
1577 /// let five = Rc::new(5);
1579 /// assert!(five != Rc::new(6));
1582 fn ne(&self, other: &Rc<T>) -> bool {
1583 RcEqIdent::ne(self, other)
1587 #[stable(feature = "rust1", since = "1.0.0")]
1588 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1590 #[stable(feature = "rust1", since = "1.0.0")]
1591 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1592 /// Partial comparison for two `Rc`s.
1594 /// The two are compared by calling `partial_cmp()` on their inner values.
1599 /// use std::rc::Rc;
1600 /// use std::cmp::Ordering;
1602 /// let five = Rc::new(5);
1604 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1607 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1608 (**self).partial_cmp(&**other)
1611 /// Less-than comparison for two `Rc`s.
1613 /// The two are compared by calling `<` on their inner values.
1618 /// use std::rc::Rc;
1620 /// let five = Rc::new(5);
1622 /// assert!(five < Rc::new(6));
1625 fn lt(&self, other: &Rc<T>) -> bool {
1629 /// 'Less than or equal to' comparison for two `Rc`s.
1631 /// The two are compared by calling `<=` on their inner values.
1636 /// use std::rc::Rc;
1638 /// let five = Rc::new(5);
1640 /// assert!(five <= Rc::new(5));
1643 fn le(&self, other: &Rc<T>) -> bool {
1647 /// Greater-than comparison for two `Rc`s.
1649 /// The two are compared by calling `>` on their inner values.
1654 /// use std::rc::Rc;
1656 /// let five = Rc::new(5);
1658 /// assert!(five > Rc::new(4));
1661 fn gt(&self, other: &Rc<T>) -> bool {
1665 /// 'Greater than or equal to' comparison for two `Rc`s.
1667 /// The two are compared by calling `>=` on their inner values.
1672 /// use std::rc::Rc;
1674 /// let five = Rc::new(5);
1676 /// assert!(five >= Rc::new(5));
1679 fn ge(&self, other: &Rc<T>) -> bool {
1684 #[stable(feature = "rust1", since = "1.0.0")]
1685 impl<T: ?Sized + Ord> Ord for Rc<T> {
1686 /// Comparison for two `Rc`s.
1688 /// The two are compared by calling `cmp()` on their inner values.
1693 /// use std::rc::Rc;
1694 /// use std::cmp::Ordering;
1696 /// let five = Rc::new(5);
1698 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1701 fn cmp(&self, other: &Rc<T>) -> Ordering {
1702 (**self).cmp(&**other)
1706 #[stable(feature = "rust1", since = "1.0.0")]
1707 impl<T: ?Sized + Hash> Hash for Rc<T> {
1708 fn hash<H: Hasher>(&self, state: &mut H) {
1709 (**self).hash(state);
1713 #[stable(feature = "rust1", since = "1.0.0")]
1714 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1715 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1716 fmt::Display::fmt(&**self, f)
1720 #[stable(feature = "rust1", since = "1.0.0")]
1721 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1722 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1723 fmt::Debug::fmt(&**self, f)
1727 #[stable(feature = "rust1", since = "1.0.0")]
1728 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1729 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1730 fmt::Pointer::fmt(&(&**self as *const T), f)
1734 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1735 impl<T> From<T> for Rc<T> {
1736 /// Converts a generic type `T` into a `Rc<T>`
1738 /// The conversion allocates on the heap and moves `t`
1739 /// from the stack into it.
1743 /// # use std::rc::Rc;
1745 /// let rc = Rc::new(5);
1747 /// assert_eq!(Rc::from(x), rc);
1749 fn from(t: T) -> Self {
1754 #[cfg(not(no_global_oom_handling))]
1755 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1756 impl<T: Clone> From<&[T]> for Rc<[T]> {
1757 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
1762 /// # use std::rc::Rc;
1763 /// let original: &[i32] = &[1, 2, 3];
1764 /// let shared: Rc<[i32]> = Rc::from(original);
1765 /// assert_eq!(&[1, 2, 3], &shared[..]);
1768 fn from(v: &[T]) -> Rc<[T]> {
1769 <Self as RcFromSlice<T>>::from_slice(v)
1773 #[cfg(not(no_global_oom_handling))]
1774 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1775 impl From<&str> for Rc<str> {
1776 /// Allocate a reference-counted string slice and copy `v` into it.
1781 /// # use std::rc::Rc;
1782 /// let shared: Rc<str> = Rc::from("statue");
1783 /// assert_eq!("statue", &shared[..]);
1786 fn from(v: &str) -> Rc<str> {
1787 let rc = Rc::<[u8]>::from(v.as_bytes());
1788 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1792 #[cfg(not(no_global_oom_handling))]
1793 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1794 impl From<String> for Rc<str> {
1795 /// Allocate a reference-counted string slice and copy `v` into it.
1800 /// # use std::rc::Rc;
1801 /// let original: String = "statue".to_owned();
1802 /// let shared: Rc<str> = Rc::from(original);
1803 /// assert_eq!("statue", &shared[..]);
1806 fn from(v: String) -> Rc<str> {
1811 #[cfg(not(no_global_oom_handling))]
1812 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1813 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1814 /// Move a boxed object to a new, reference counted, allocation.
1819 /// # use std::rc::Rc;
1820 /// let original: Box<i32> = Box::new(1);
1821 /// let shared: Rc<i32> = Rc::from(original);
1822 /// assert_eq!(1, *shared);
1825 fn from(v: Box<T>) -> Rc<T> {
1830 #[cfg(not(no_global_oom_handling))]
1831 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1832 impl<T> From<Vec<T>> for Rc<[T]> {
1833 /// Allocate a reference-counted slice and move `v`'s items into it.
1838 /// # use std::rc::Rc;
1839 /// let original: Box<Vec<i32>> = Box::new(vec![1, 2, 3]);
1840 /// let shared: Rc<Vec<i32>> = Rc::from(original);
1841 /// assert_eq!(vec![1, 2, 3], *shared);
1844 fn from(mut v: Vec<T>) -> Rc<[T]> {
1846 let rc = Rc::copy_from_slice(&v);
1848 // Allow the Vec to free its memory, but not destroy its contents
1856 #[stable(feature = "shared_from_cow", since = "1.45.0")]
1857 impl<'a, B> From<Cow<'a, B>> for Rc<B>
1859 B: ToOwned + ?Sized,
1860 Rc<B>: From<&'a B> + From<B::Owned>,
1862 /// Create a reference-counted pointer from
1863 /// a clone-on-write pointer by copying its content.
1868 /// # use std::rc::Rc;
1869 /// # use std::borrow::Cow;
1870 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
1871 /// let shared: Rc<str> = Rc::from(cow);
1872 /// assert_eq!("eggplant", &shared[..]);
1875 fn from(cow: Cow<'a, B>) -> Rc<B> {
1877 Cow::Borrowed(s) => Rc::from(s),
1878 Cow::Owned(s) => Rc::from(s),
1883 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1884 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]> {
1885 type Error = Rc<[T]>;
1887 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1888 if boxed_slice.len() == N {
1889 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
1896 #[cfg(not(no_global_oom_handling))]
1897 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1898 impl<T> iter::FromIterator<T> for Rc<[T]> {
1899 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1901 /// # Performance characteristics
1903 /// ## The general case
1905 /// In the general case, collecting into `Rc<[T]>` is done by first
1906 /// collecting into a `Vec<T>`. That is, when writing the following:
1909 /// # use std::rc::Rc;
1910 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1911 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1914 /// this behaves as if we wrote:
1917 /// # use std::rc::Rc;
1918 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1919 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1920 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1921 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1924 /// This will allocate as many times as needed for constructing the `Vec<T>`
1925 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1927 /// ## Iterators of known length
1929 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1930 /// a single allocation will be made for the `Rc<[T]>`. For example:
1933 /// # use std::rc::Rc;
1934 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1935 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1937 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1938 ToRcSlice::to_rc_slice(iter.into_iter())
1942 /// Specialization trait used for collecting into `Rc<[T]>`.
1943 #[cfg(not(no_global_oom_handling))]
1944 trait ToRcSlice<T>: Iterator<Item = T> + Sized {
1945 fn to_rc_slice(self) -> Rc<[T]>;
1948 #[cfg(not(no_global_oom_handling))]
1949 impl<T, I: Iterator<Item = T>> ToRcSlice<T> for I {
1950 default fn to_rc_slice(self) -> Rc<[T]> {
1951 self.collect::<Vec<T>>().into()
1955 #[cfg(not(no_global_oom_handling))]
1956 impl<T, I: iter::TrustedLen<Item = T>> ToRcSlice<T> for I {
1957 fn to_rc_slice(self) -> Rc<[T]> {
1958 // This is the case for a `TrustedLen` iterator.
1959 let (low, high) = self.size_hint();
1960 if let Some(high) = high {
1964 "TrustedLen iterator's size hint is not exact: {:?}",
1969 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1970 Rc::from_iter_exact(self, low)
1973 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
1974 // length exceeding `usize::MAX`.
1975 // The default implementation would collect into a vec which would panic.
1976 // Thus we panic here immediately without invoking `Vec` code.
1977 panic!("capacity overflow");
1982 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1983 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
1984 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1986 /// Since a `Weak` reference does not count towards ownership, it will not
1987 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
1988 /// guarantees about the value still being present. Thus it may return [`None`]
1989 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
1990 /// itself (the backing store) from being deallocated.
1992 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
1993 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
1994 /// prevent circular references between [`Rc`] pointers, since mutual owning references
1995 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1996 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1997 /// pointers from children back to their parents.
1999 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
2001 /// [`upgrade`]: Weak::upgrade
2002 #[stable(feature = "rc_weak", since = "1.4.0")]
2003 pub struct Weak<T: ?Sized> {
2004 // This is a `NonNull` to allow optimizing the size of this type in enums,
2005 // but it is not necessarily a valid pointer.
2006 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
2007 // to allocate space on the heap. That's not a value a real pointer
2008 // will ever have because RcBox has alignment at least 2.
2009 // This is only possible when `T: Sized`; unsized `T` never dangle.
2010 ptr: NonNull<RcBox<T>>,
2013 #[stable(feature = "rc_weak", since = "1.4.0")]
2014 impl<T: ?Sized> !marker::Send for Weak<T> {}
2015 #[stable(feature = "rc_weak", since = "1.4.0")]
2016 impl<T: ?Sized> !marker::Sync for Weak<T> {}
2018 #[unstable(feature = "coerce_unsized", issue = "27732")]
2019 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
2021 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
2022 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
2025 /// Constructs a new `Weak<T>`, without allocating any memory.
2026 /// Calling [`upgrade`] on the return value always gives [`None`].
2028 /// [`upgrade`]: Weak::upgrade
2033 /// use std::rc::Weak;
2035 /// let empty: Weak<i64> = Weak::new();
2036 /// assert!(empty.upgrade().is_none());
2038 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2039 pub fn new() -> Weak<T> {
2040 Weak { ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0") }
2044 pub(crate) fn is_dangling<T: ?Sized>(ptr: *mut T) -> bool {
2045 let address = ptr as *mut () as usize;
2046 address == usize::MAX
2049 /// Helper type to allow accessing the reference counts without
2050 /// making any assertions about the data field.
2051 struct WeakInner<'a> {
2052 weak: &'a Cell<usize>,
2053 strong: &'a Cell<usize>,
2056 impl<T: ?Sized> Weak<T> {
2057 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
2059 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
2060 /// unaligned or even [`null`] otherwise.
2065 /// use std::rc::Rc;
2068 /// let strong = Rc::new("hello".to_owned());
2069 /// let weak = Rc::downgrade(&strong);
2070 /// // Both point to the same object
2071 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
2072 /// // The strong here keeps it alive, so we can still access the object.
2073 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
2076 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
2077 /// // undefined behaviour.
2078 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
2081 /// [`null`]: core::ptr::null
2082 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
2083 pub fn as_ptr(&self) -> *const T {
2084 let ptr: *mut RcBox<T> = NonNull::as_ptr(self.ptr);
2086 if is_dangling(ptr) {
2087 // If the pointer is dangling, we return the sentinel directly. This cannot be
2088 // a valid payload address, as the payload is at least as aligned as RcBox (usize).
2091 // SAFETY: if is_dangling returns false, then the pointer is dereferencable.
2092 // The payload may be dropped at this point, and we have to maintain provenance,
2093 // so use raw pointer manipulation.
2094 unsafe { ptr::addr_of_mut!((*ptr).value) }
2098 /// Consumes the `Weak<T>` and turns it into a raw pointer.
2100 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2101 /// one weak reference (the weak count is not modified by this operation). It can be turned
2102 /// back into the `Weak<T>` with [`from_raw`].
2104 /// The same restrictions of accessing the target of the pointer as with
2105 /// [`as_ptr`] apply.
2110 /// use std::rc::{Rc, Weak};
2112 /// let strong = Rc::new("hello".to_owned());
2113 /// let weak = Rc::downgrade(&strong);
2114 /// let raw = weak.into_raw();
2116 /// assert_eq!(1, Rc::weak_count(&strong));
2117 /// assert_eq!("hello", unsafe { &*raw });
2119 /// drop(unsafe { Weak::from_raw(raw) });
2120 /// assert_eq!(0, Rc::weak_count(&strong));
2123 /// [`from_raw`]: Weak::from_raw
2124 /// [`as_ptr`]: Weak::as_ptr
2125 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2126 pub fn into_raw(self) -> *const T {
2127 let result = self.as_ptr();
2132 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
2134 /// This can be used to safely get a strong reference (by calling [`upgrade`]
2135 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2137 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2138 /// as these don't own anything; the method still works on them).
2142 /// The pointer must have originated from the [`into_raw`] and must still own its potential
2145 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2146 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2147 /// count is not modified by this operation) and therefore it must be paired with a previous
2148 /// call to [`into_raw`].
2153 /// use std::rc::{Rc, Weak};
2155 /// let strong = Rc::new("hello".to_owned());
2157 /// let raw_1 = Rc::downgrade(&strong).into_raw();
2158 /// let raw_2 = Rc::downgrade(&strong).into_raw();
2160 /// assert_eq!(2, Rc::weak_count(&strong));
2162 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2163 /// assert_eq!(1, Rc::weak_count(&strong));
2167 /// // Decrement the last weak count.
2168 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2171 /// [`into_raw`]: Weak::into_raw
2172 /// [`upgrade`]: Weak::upgrade
2173 /// [`new`]: Weak::new
2174 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2175 pub unsafe fn from_raw(ptr: *const T) -> Self {
2176 // See Weak::as_ptr for context on how the input pointer is derived.
2178 let ptr = if is_dangling(ptr as *mut T) {
2179 // This is a dangling Weak.
2180 ptr as *mut RcBox<T>
2182 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
2183 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
2184 let offset = unsafe { data_offset(ptr) };
2185 // Thus, we reverse the offset to get the whole RcBox.
2186 // SAFETY: the pointer originated from a Weak, so this offset is safe.
2187 unsafe { (ptr as *mut RcBox<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
2190 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
2191 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
2194 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
2195 /// dropping of the inner value if successful.
2197 /// Returns [`None`] if the inner value has since been dropped.
2202 /// use std::rc::Rc;
2204 /// let five = Rc::new(5);
2206 /// let weak_five = Rc::downgrade(&five);
2208 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
2209 /// assert!(strong_five.is_some());
2211 /// // Destroy all strong pointers.
2212 /// drop(strong_five);
2215 /// assert!(weak_five.upgrade().is_none());
2217 #[stable(feature = "rc_weak", since = "1.4.0")]
2218 pub fn upgrade(&self) -> Option<Rc<T>> {
2219 let inner = self.inner()?;
2220 if inner.strong() == 0 {
2224 Some(Rc::from_inner(self.ptr))
2228 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
2230 /// If `self` was created using [`Weak::new`], this will return 0.
2231 #[stable(feature = "weak_counts", since = "1.41.0")]
2232 pub fn strong_count(&self) -> usize {
2233 if let Some(inner) = self.inner() { inner.strong() } else { 0 }
2236 /// Gets the number of `Weak` pointers pointing to this allocation.
2238 /// If no strong pointers remain, this will return zero.
2239 #[stable(feature = "weak_counts", since = "1.41.0")]
2240 pub fn weak_count(&self) -> usize {
2243 if inner.strong() > 0 {
2244 inner.weak() - 1 // subtract the implicit weak ptr
2252 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`,
2253 /// (i.e., when this `Weak` was created by `Weak::new`).
2255 fn inner(&self) -> Option<WeakInner<'_>> {
2256 if is_dangling(self.ptr.as_ptr()) {
2259 // We are careful to *not* create a reference covering the "data" field, as
2260 // the field may be mutated concurrently (for example, if the last `Rc`
2261 // is dropped, the data field will be dropped in-place).
2263 let ptr = self.ptr.as_ptr();
2264 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
2269 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
2270 /// [`ptr::eq`]), or if both don't point to any allocation
2271 /// (because they were created with `Weak::new()`).
2275 /// Since this compares pointers it means that `Weak::new()` will equal each
2276 /// other, even though they don't point to any allocation.
2281 /// use std::rc::Rc;
2283 /// let first_rc = Rc::new(5);
2284 /// let first = Rc::downgrade(&first_rc);
2285 /// let second = Rc::downgrade(&first_rc);
2287 /// assert!(first.ptr_eq(&second));
2289 /// let third_rc = Rc::new(5);
2290 /// let third = Rc::downgrade(&third_rc);
2292 /// assert!(!first.ptr_eq(&third));
2295 /// Comparing `Weak::new`.
2298 /// use std::rc::{Rc, Weak};
2300 /// let first = Weak::new();
2301 /// let second = Weak::new();
2302 /// assert!(first.ptr_eq(&second));
2304 /// let third_rc = Rc::new(());
2305 /// let third = Rc::downgrade(&third_rc);
2306 /// assert!(!first.ptr_eq(&third));
2309 /// [`ptr::eq`]: core::ptr::eq
2311 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
2312 pub fn ptr_eq(&self, other: &Self) -> bool {
2313 self.ptr.as_ptr() == other.ptr.as_ptr()
2317 #[stable(feature = "rc_weak", since = "1.4.0")]
2318 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2319 /// Drops the `Weak` pointer.
2324 /// use std::rc::{Rc, Weak};
2328 /// impl Drop for Foo {
2329 /// fn drop(&mut self) {
2330 /// println!("dropped!");
2334 /// let foo = Rc::new(Foo);
2335 /// let weak_foo = Rc::downgrade(&foo);
2336 /// let other_weak_foo = Weak::clone(&weak_foo);
2338 /// drop(weak_foo); // Doesn't print anything
2339 /// drop(foo); // Prints "dropped!"
2341 /// assert!(other_weak_foo.upgrade().is_none());
2343 fn drop(&mut self) {
2344 let inner = if let Some(inner) = self.inner() { inner } else { return };
2347 // the weak count starts at 1, and will only go to zero if all
2348 // the strong pointers have disappeared.
2349 if inner.weak() == 0 {
2351 Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()));
2357 #[stable(feature = "rc_weak", since = "1.4.0")]
2358 impl<T: ?Sized> Clone for Weak<T> {
2359 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2364 /// use std::rc::{Rc, Weak};
2366 /// let weak_five = Rc::downgrade(&Rc::new(5));
2368 /// let _ = Weak::clone(&weak_five);
2371 fn clone(&self) -> Weak<T> {
2372 if let Some(inner) = self.inner() {
2375 Weak { ptr: self.ptr }
2379 #[stable(feature = "rc_weak", since = "1.4.0")]
2380 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
2381 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2386 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2387 impl<T> Default for Weak<T> {
2388 /// Constructs a new `Weak<T>`, without allocating any memory.
2389 /// Calling [`upgrade`] on the return value always gives [`None`].
2391 /// [`None`]: Option
2392 /// [`upgrade`]: Weak::upgrade
2397 /// use std::rc::Weak;
2399 /// let empty: Weak<i64> = Default::default();
2400 /// assert!(empty.upgrade().is_none());
2402 fn default() -> Weak<T> {
2407 // NOTE: We checked_add here to deal with mem::forget safely. In particular
2408 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
2409 // you can free the allocation while outstanding Rcs (or Weaks) exist.
2410 // We abort because this is such a degenerate scenario that we don't care about
2411 // what happens -- no real program should ever experience this.
2413 // This should have negligible overhead since you don't actually need to
2414 // clone these much in Rust thanks to ownership and move-semantics.
2418 fn weak_ref(&self) -> &Cell<usize>;
2419 fn strong_ref(&self) -> &Cell<usize>;
2422 fn strong(&self) -> usize {
2423 self.strong_ref().get()
2427 fn inc_strong(&self) {
2428 let strong = self.strong();
2430 // We want to abort on overflow instead of dropping the value.
2431 // The reference count will never be zero when this is called;
2432 // nevertheless, we insert an abort here to hint LLVM at
2433 // an otherwise missed optimization.
2434 if strong == 0 || strong == usize::MAX {
2437 self.strong_ref().set(strong + 1);
2441 fn dec_strong(&self) {
2442 self.strong_ref().set(self.strong() - 1);
2446 fn weak(&self) -> usize {
2447 self.weak_ref().get()
2451 fn inc_weak(&self) {
2452 let weak = self.weak();
2454 // We want to abort on overflow instead of dropping the value.
2455 // The reference count will never be zero when this is called;
2456 // nevertheless, we insert an abort here to hint LLVM at
2457 // an otherwise missed optimization.
2458 if weak == 0 || weak == usize::MAX {
2461 self.weak_ref().set(weak + 1);
2465 fn dec_weak(&self) {
2466 self.weak_ref().set(self.weak() - 1);
2470 impl<T: ?Sized> RcInnerPtr for RcBox<T> {
2472 fn weak_ref(&self) -> &Cell<usize> {
2477 fn strong_ref(&self) -> &Cell<usize> {
2482 impl<'a> RcInnerPtr for WeakInner<'a> {
2484 fn weak_ref(&self) -> &Cell<usize> {
2489 fn strong_ref(&self) -> &Cell<usize> {
2494 #[stable(feature = "rust1", since = "1.0.0")]
2495 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2496 fn borrow(&self) -> &T {
2501 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2502 impl<T: ?Sized> AsRef<T> for Rc<T> {
2503 fn as_ref(&self) -> &T {
2508 #[stable(feature = "pin", since = "1.33.0")]
2509 impl<T: ?Sized> Unpin for Rc<T> {}
2511 /// Get the offset within an `RcBox` for the payload behind a pointer.
2515 /// The pointer must point to (and have valid metadata for) a previously
2516 /// valid instance of T, but the T is allowed to be dropped.
2517 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2518 // Align the unsized value to the end of the RcBox.
2519 // Because RcBox is repr(C), it will always be the last field in memory.
2520 // SAFETY: since the only unsized types possible are slices, trait objects,
2521 // and extern types, the input safety requirement is currently enough to
2522 // satisfy the requirements of align_of_val_raw; this is an implementation
2523 // detail of the language that may not be relied upon outside of std.
2524 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2528 fn data_offset_align(align: usize) -> isize {
2529 let layout = Layout::new::<RcBox<()>>();
2530 (layout.size() + layout.padding_needed_for(align)) as isize