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 //! let my_weak = 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 use core::panic::{RefUnwindSafe, UnwindSafe};
266 #[cfg(not(no_global_oom_handling))]
268 use core::ptr::{self, NonNull};
269 #[cfg(not(no_global_oom_handling))]
270 use core::slice::from_raw_parts_mut;
272 #[cfg(not(no_global_oom_handling))]
273 use crate::alloc::handle_alloc_error;
274 #[cfg(not(no_global_oom_handling))]
275 use crate::alloc::{box_free, WriteCloneIntoRaw};
276 use crate::alloc::{AllocError, Allocator, Global, Layout};
277 use crate::borrow::{Cow, ToOwned};
278 #[cfg(not(no_global_oom_handling))]
279 use crate::string::String;
280 #[cfg(not(no_global_oom_handling))]
286 // This is repr(C) to future-proof against possible field-reordering, which
287 // would interfere with otherwise safe [into|from]_raw() of transmutable
290 struct RcBox<T: ?Sized> {
296 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
299 /// See the [module-level documentation](./index.html) for more details.
301 /// The inherent methods of `Rc` are all associated functions, which means
302 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
303 /// `value.get_mut()`. This avoids conflicts with methods of the inner type `T`.
305 /// [get_mut]: Rc::get_mut
306 #[cfg_attr(not(test), rustc_diagnostic_item = "Rc")]
307 #[stable(feature = "rust1", since = "1.0.0")]
308 #[rustc_insignificant_dtor]
309 pub struct Rc<T: ?Sized> {
310 ptr: NonNull<RcBox<T>>,
311 phantom: PhantomData<RcBox<T>>,
314 #[stable(feature = "rust1", since = "1.0.0")]
315 impl<T: ?Sized> !marker::Send for Rc<T> {}
317 // Note that this negative impl isn't strictly necessary for correctness,
318 // as `Rc` transitively contains a `Cell`, which is itself `!Sync`.
319 // However, given how important `Rc`'s `!Sync`-ness is,
320 // having an explicit negative impl is nice for documentation purposes
321 // and results in nicer error messages.
322 #[stable(feature = "rust1", since = "1.0.0")]
323 impl<T: ?Sized> !marker::Sync for Rc<T> {}
325 #[stable(feature = "catch_unwind", since = "1.9.0")]
326 impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Rc<T> {}
327 #[stable(feature = "rc_ref_unwind_safe", since = "1.58.0")]
328 impl<T: RefUnwindSafe + ?Sized> RefUnwindSafe for Rc<T> {}
330 #[unstable(feature = "coerce_unsized", issue = "27732")]
331 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
333 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
334 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
336 impl<T: ?Sized> Rc<T> {
338 fn inner(&self) -> &RcBox<T> {
339 // This unsafety is ok because while this Rc is alive we're guaranteed
340 // that the inner pointer is valid.
341 unsafe { self.ptr.as_ref() }
344 unsafe fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
345 Self { ptr, phantom: PhantomData }
348 unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
349 unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
354 /// Constructs a new `Rc<T>`.
361 /// let five = Rc::new(5);
363 #[cfg(not(no_global_oom_handling))]
364 #[stable(feature = "rust1", since = "1.0.0")]
365 pub fn new(value: T) -> Rc<T> {
366 // There is an implicit weak pointer owned by all the strong
367 // pointers, which ensures that the weak destructor never frees
368 // the allocation while the strong destructor is running, even
369 // if the weak pointer is stored inside the strong one.
372 Box::leak(Box::new(RcBox { strong: Cell::new(1), weak: Cell::new(1), value }))
378 /// Constructs a new `Rc<T>` while giving you a `Weak<T>` to the allocation,
379 /// to allow you to construct a `T` which holds a weak pointer to itself.
381 /// Generally, a structure circularly referencing itself, either directly or
382 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
383 /// Using this function, you get access to the weak pointer during the
384 /// initialization of `T`, before the `Rc<T>` is created, such that you can
385 /// clone and store it inside the `T`.
387 /// `new_cyclic` first allocates the managed allocation for the `Rc<T>`,
388 /// then calls your closure, giving it a `Weak<T>` to this allocation,
389 /// and only afterwards completes the construction of the `Rc<T>` by placing
390 /// the `T` returned from your closure into the allocation.
392 /// Since the new `Rc<T>` is not fully-constructed until `Rc<T>::new_cyclic`
393 /// returns, calling [`upgrade`] on the weak reference inside your closure will
394 /// fail and result in a `None` value.
398 /// If `data_fn` panics, the panic is propagated to the caller, and the
399 /// temporary [`Weak<T>`] is dropped normally.
404 /// # #![allow(dead_code)]
405 /// use std::rc::{Rc, Weak};
408 /// me: Weak<Gadget>,
412 /// /// Construct a reference counted Gadget.
413 /// fn new() -> Rc<Self> {
414 /// // `me` is a `Weak<Gadget>` pointing at the new allocation of the
415 /// // `Rc` we're constructing.
416 /// Rc::new_cyclic(|me| {
417 /// // Create the actual struct here.
418 /// Gadget { me: me.clone() }
422 /// /// Return a reference counted pointer to Self.
423 /// fn me(&self) -> Rc<Self> {
424 /// self.me.upgrade().unwrap()
428 /// [`upgrade`]: Weak::upgrade
429 #[cfg(not(no_global_oom_handling))]
430 #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
431 pub fn new_cyclic<F>(data_fn: F) -> Rc<T>
433 F: FnOnce(&Weak<T>) -> T,
435 // Construct the inner in the "uninitialized" state with a single
437 let uninit_ptr: NonNull<_> = Box::leak(Box::new(RcBox {
438 strong: Cell::new(0),
440 value: mem::MaybeUninit::<T>::uninit(),
444 let init_ptr: NonNull<RcBox<T>> = uninit_ptr.cast();
446 let weak = Weak { ptr: init_ptr };
448 // It's important we don't give up ownership of the weak pointer, or
449 // else the memory might be freed by the time `data_fn` returns. If
450 // we really wanted to pass ownership, we could create an additional
451 // weak pointer for ourselves, but this would result in additional
452 // updates to the weak reference count which might not be necessary
454 let data = data_fn(&weak);
456 let strong = unsafe {
457 let inner = init_ptr.as_ptr();
458 ptr::write(ptr::addr_of_mut!((*inner).value), data);
460 let prev_value = (*inner).strong.get();
461 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
462 (*inner).strong.set(1);
464 Rc::from_inner(init_ptr)
467 // Strong references should collectively own a shared weak reference,
468 // so don't run the destructor for our old weak reference.
473 /// Constructs a new `Rc` with uninitialized contents.
478 /// #![feature(new_uninit)]
479 /// #![feature(get_mut_unchecked)]
483 /// let mut five = Rc::<u32>::new_uninit();
485 /// // Deferred initialization:
486 /// Rc::get_mut(&mut five).unwrap().write(5);
488 /// let five = unsafe { five.assume_init() };
490 /// assert_eq!(*five, 5)
492 #[cfg(not(no_global_oom_handling))]
493 #[unstable(feature = "new_uninit", issue = "63291")]
495 pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
497 Rc::from_ptr(Rc::allocate_for_layout(
499 |layout| Global.allocate(layout),
500 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
505 /// Constructs a new `Rc` with uninitialized contents, with the memory
506 /// being filled with `0` bytes.
508 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
509 /// incorrect usage of this method.
514 /// #![feature(new_uninit)]
518 /// let zero = Rc::<u32>::new_zeroed();
519 /// let zero = unsafe { zero.assume_init() };
521 /// assert_eq!(*zero, 0)
524 /// [zeroed]: mem::MaybeUninit::zeroed
525 #[cfg(not(no_global_oom_handling))]
526 #[unstable(feature = "new_uninit", issue = "63291")]
528 pub fn new_zeroed() -> Rc<mem::MaybeUninit<T>> {
530 Rc::from_ptr(Rc::allocate_for_layout(
532 |layout| Global.allocate_zeroed(layout),
533 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
538 /// Constructs a new `Rc<T>`, returning an error if the allocation fails
543 /// #![feature(allocator_api)]
546 /// let five = Rc::try_new(5);
547 /// # Ok::<(), std::alloc::AllocError>(())
549 #[unstable(feature = "allocator_api", issue = "32838")]
550 pub fn try_new(value: T) -> Result<Rc<T>, AllocError> {
551 // There is an implicit weak pointer owned by all the strong
552 // pointers, which ensures that the weak destructor never frees
553 // the allocation while the strong destructor is running, even
554 // if the weak pointer is stored inside the strong one.
557 Box::leak(Box::try_new(RcBox { strong: Cell::new(1), weak: Cell::new(1), value })?)
563 /// Constructs a new `Rc` with uninitialized contents, returning an error if the allocation fails
568 /// #![feature(allocator_api, new_uninit)]
569 /// #![feature(get_mut_unchecked)]
573 /// let mut five = Rc::<u32>::try_new_uninit()?;
575 /// // Deferred initialization:
576 /// Rc::get_mut(&mut five).unwrap().write(5);
578 /// let five = unsafe { five.assume_init() };
580 /// assert_eq!(*five, 5);
581 /// # Ok::<(), std::alloc::AllocError>(())
583 #[unstable(feature = "allocator_api", issue = "32838")]
584 // #[unstable(feature = "new_uninit", issue = "63291")]
585 pub fn try_new_uninit() -> Result<Rc<mem::MaybeUninit<T>>, AllocError> {
587 Ok(Rc::from_ptr(Rc::try_allocate_for_layout(
589 |layout| Global.allocate(layout),
590 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
595 /// Constructs a new `Rc` with uninitialized contents, with the memory
596 /// being filled with `0` bytes, returning an error if the allocation fails
598 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
599 /// incorrect usage of this method.
604 /// #![feature(allocator_api, new_uninit)]
608 /// let zero = Rc::<u32>::try_new_zeroed()?;
609 /// let zero = unsafe { zero.assume_init() };
611 /// assert_eq!(*zero, 0);
612 /// # Ok::<(), std::alloc::AllocError>(())
615 /// [zeroed]: mem::MaybeUninit::zeroed
616 #[unstable(feature = "allocator_api", issue = "32838")]
617 //#[unstable(feature = "new_uninit", issue = "63291")]
618 pub fn try_new_zeroed() -> Result<Rc<mem::MaybeUninit<T>>, AllocError> {
620 Ok(Rc::from_ptr(Rc::try_allocate_for_layout(
622 |layout| Global.allocate_zeroed(layout),
623 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
627 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
628 /// `value` will be pinned in memory and unable to be moved.
629 #[cfg(not(no_global_oom_handling))]
630 #[stable(feature = "pin", since = "1.33.0")]
632 pub fn pin(value: T) -> Pin<Rc<T>> {
633 unsafe { Pin::new_unchecked(Rc::new(value)) }
636 /// Returns the inner value, if the `Rc` has exactly one strong reference.
638 /// Otherwise, an [`Err`] is returned with the same `Rc` that was
641 /// This will succeed even if there are outstanding weak references.
648 /// let x = Rc::new(3);
649 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
651 /// let x = Rc::new(4);
652 /// let _y = Rc::clone(&x);
653 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
656 #[stable(feature = "rc_unique", since = "1.4.0")]
657 pub fn try_unwrap(this: Self) -> Result<T, Self> {
658 if Rc::strong_count(&this) == 1 {
660 let val = ptr::read(&*this); // copy the contained object
662 // Indicate to Weaks that they can't be promoted by decrementing
663 // the strong count, and then remove the implicit "strong weak"
664 // pointer while also handling drop logic by just crafting a
666 this.inner().dec_strong();
667 let _weak = Weak { ptr: this.ptr };
678 /// Constructs a new reference-counted slice with uninitialized contents.
683 /// #![feature(new_uninit)]
684 /// #![feature(get_mut_unchecked)]
688 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
690 /// // Deferred initialization:
691 /// let data = Rc::get_mut(&mut values).unwrap();
692 /// data[0].write(1);
693 /// data[1].write(2);
694 /// data[2].write(3);
696 /// let values = unsafe { values.assume_init() };
698 /// assert_eq!(*values, [1, 2, 3])
700 #[cfg(not(no_global_oom_handling))]
701 #[unstable(feature = "new_uninit", issue = "63291")]
703 pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
704 unsafe { Rc::from_ptr(Rc::allocate_for_slice(len)) }
707 /// Constructs a new reference-counted slice with uninitialized contents, with the memory being
708 /// filled with `0` bytes.
710 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
711 /// incorrect usage of this method.
716 /// #![feature(new_uninit)]
720 /// let values = Rc::<[u32]>::new_zeroed_slice(3);
721 /// let values = unsafe { values.assume_init() };
723 /// assert_eq!(*values, [0, 0, 0])
726 /// [zeroed]: mem::MaybeUninit::zeroed
727 #[cfg(not(no_global_oom_handling))]
728 #[unstable(feature = "new_uninit", issue = "63291")]
730 pub fn new_zeroed_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
732 Rc::from_ptr(Rc::allocate_for_layout(
733 Layout::array::<T>(len).unwrap(),
734 |layout| Global.allocate_zeroed(layout),
736 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
737 as *mut RcBox<[mem::MaybeUninit<T>]>
744 impl<T> Rc<mem::MaybeUninit<T>> {
745 /// Converts to `Rc<T>`.
749 /// As with [`MaybeUninit::assume_init`],
750 /// it is up to the caller to guarantee that the inner value
751 /// really is in an initialized state.
752 /// Calling this when the content is not yet fully initialized
753 /// causes immediate undefined behavior.
755 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
760 /// #![feature(new_uninit)]
761 /// #![feature(get_mut_unchecked)]
765 /// let mut five = Rc::<u32>::new_uninit();
767 /// // Deferred initialization:
768 /// Rc::get_mut(&mut five).unwrap().write(5);
770 /// let five = unsafe { five.assume_init() };
772 /// assert_eq!(*five, 5)
774 #[unstable(feature = "new_uninit", issue = "63291")]
776 pub unsafe fn assume_init(self) -> Rc<T> {
777 unsafe { Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast()) }
781 impl<T> Rc<[mem::MaybeUninit<T>]> {
782 /// Converts to `Rc<[T]>`.
786 /// As with [`MaybeUninit::assume_init`],
787 /// it is up to the caller to guarantee that the inner value
788 /// really is in an initialized state.
789 /// Calling this when the content is not yet fully initialized
790 /// causes immediate undefined behavior.
792 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
797 /// #![feature(new_uninit)]
798 /// #![feature(get_mut_unchecked)]
802 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
804 /// // Deferred initialization:
805 /// let data = Rc::get_mut(&mut values).unwrap();
806 /// data[0].write(1);
807 /// data[1].write(2);
808 /// data[2].write(3);
810 /// let values = unsafe { values.assume_init() };
812 /// assert_eq!(*values, [1, 2, 3])
814 #[unstable(feature = "new_uninit", issue = "63291")]
816 pub unsafe fn assume_init(self) -> Rc<[T]> {
817 unsafe { Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
821 impl<T: ?Sized> Rc<T> {
822 /// Consumes the `Rc`, returning the wrapped pointer.
824 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
825 /// [`Rc::from_raw`].
832 /// let x = Rc::new("hello".to_owned());
833 /// let x_ptr = Rc::into_raw(x);
834 /// assert_eq!(unsafe { &*x_ptr }, "hello");
836 #[stable(feature = "rc_raw", since = "1.17.0")]
837 pub fn into_raw(this: Self) -> *const T {
838 let ptr = Self::as_ptr(&this);
843 /// Provides a raw pointer to the data.
845 /// The counts are not affected in any way and the `Rc` is not consumed. The pointer is valid
846 /// for as long there are strong counts in the `Rc`.
853 /// let x = Rc::new("hello".to_owned());
854 /// let y = Rc::clone(&x);
855 /// let x_ptr = Rc::as_ptr(&x);
856 /// assert_eq!(x_ptr, Rc::as_ptr(&y));
857 /// assert_eq!(unsafe { &*x_ptr }, "hello");
859 #[stable(feature = "weak_into_raw", since = "1.45.0")]
860 pub fn as_ptr(this: &Self) -> *const T {
861 let ptr: *mut RcBox<T> = NonNull::as_ptr(this.ptr);
863 // SAFETY: This cannot go through Deref::deref or Rc::inner because
864 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
865 // write through the pointer after the Rc is recovered through `from_raw`.
866 unsafe { ptr::addr_of_mut!((*ptr).value) }
869 /// Constructs an `Rc<T>` from a raw pointer.
871 /// The raw pointer must have been previously returned by a call to
872 /// [`Rc<U>::into_raw`][into_raw] where `U` must have the same size
873 /// and alignment as `T`. This is trivially true if `U` is `T`.
874 /// Note that if `U` is not `T` but has the same size and alignment, this is
875 /// basically like transmuting references of different types. See
876 /// [`mem::transmute`] for more information on what
877 /// restrictions apply in this case.
879 /// The user of `from_raw` has to make sure a specific value of `T` is only
882 /// This function is unsafe because improper use may lead to memory unsafety,
883 /// even if the returned `Rc<T>` is never accessed.
885 /// [into_raw]: Rc::into_raw
892 /// let x = Rc::new("hello".to_owned());
893 /// let x_ptr = Rc::into_raw(x);
896 /// // Convert back to an `Rc` to prevent leak.
897 /// let x = Rc::from_raw(x_ptr);
898 /// assert_eq!(&*x, "hello");
900 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
903 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
905 #[stable(feature = "rc_raw", since = "1.17.0")]
906 pub unsafe fn from_raw(ptr: *const T) -> Self {
907 let offset = unsafe { data_offset(ptr) };
909 // Reverse the offset to find the original RcBox.
911 unsafe { (ptr as *mut u8).offset(-offset).with_metadata_of(ptr as *mut RcBox<T>) };
913 unsafe { Self::from_ptr(rc_ptr) }
916 /// Creates a new [`Weak`] pointer to this allocation.
923 /// let five = Rc::new(5);
925 /// let weak_five = Rc::downgrade(&five);
927 #[must_use = "this returns a new `Weak` pointer, \
928 without modifying the original `Rc`"]
929 #[stable(feature = "rc_weak", since = "1.4.0")]
930 pub fn downgrade(this: &Self) -> Weak<T> {
931 this.inner().inc_weak();
932 // Make sure we do not create a dangling Weak
933 debug_assert!(!is_dangling(this.ptr.as_ptr()));
934 Weak { ptr: this.ptr }
937 /// Gets the number of [`Weak`] pointers to this allocation.
944 /// let five = Rc::new(5);
945 /// let _weak_five = Rc::downgrade(&five);
947 /// assert_eq!(1, Rc::weak_count(&five));
950 #[stable(feature = "rc_counts", since = "1.15.0")]
951 pub fn weak_count(this: &Self) -> usize {
952 this.inner().weak() - 1
955 /// Gets the number of strong (`Rc`) pointers to this allocation.
962 /// let five = Rc::new(5);
963 /// let _also_five = Rc::clone(&five);
965 /// assert_eq!(2, Rc::strong_count(&five));
968 #[stable(feature = "rc_counts", since = "1.15.0")]
969 pub fn strong_count(this: &Self) -> usize {
970 this.inner().strong()
973 /// Increments the strong reference count on the `Rc<T>` associated with the
974 /// provided pointer by one.
978 /// The pointer must have been obtained through `Rc::into_raw`, and the
979 /// associated `Rc` instance must be valid (i.e. the strong count must be at
980 /// least 1) for the duration of this method.
987 /// let five = Rc::new(5);
990 /// let ptr = Rc::into_raw(five);
991 /// Rc::increment_strong_count(ptr);
993 /// let five = Rc::from_raw(ptr);
994 /// assert_eq!(2, Rc::strong_count(&five));
998 #[stable(feature = "rc_mutate_strong_count", since = "1.53.0")]
999 pub unsafe fn increment_strong_count(ptr: *const T) {
1000 // Retain Rc, but don't touch refcount by wrapping in ManuallyDrop
1001 let rc = unsafe { mem::ManuallyDrop::new(Rc::<T>::from_raw(ptr)) };
1002 // Now increase refcount, but don't drop new refcount either
1003 let _rc_clone: mem::ManuallyDrop<_> = rc.clone();
1006 /// Decrements the strong reference count on the `Rc<T>` associated with the
1007 /// provided pointer by one.
1011 /// The pointer must have been obtained through `Rc::into_raw`, and the
1012 /// associated `Rc` instance must be valid (i.e. the strong count must be at
1013 /// least 1) when invoking this method. This method can be used to release
1014 /// the final `Rc` and backing storage, but **should not** be called after
1015 /// the final `Rc` has been released.
1020 /// use std::rc::Rc;
1022 /// let five = Rc::new(5);
1025 /// let ptr = Rc::into_raw(five);
1026 /// Rc::increment_strong_count(ptr);
1028 /// let five = Rc::from_raw(ptr);
1029 /// assert_eq!(2, Rc::strong_count(&five));
1030 /// Rc::decrement_strong_count(ptr);
1031 /// assert_eq!(1, Rc::strong_count(&five));
1035 #[stable(feature = "rc_mutate_strong_count", since = "1.53.0")]
1036 pub unsafe fn decrement_strong_count(ptr: *const T) {
1037 unsafe { mem::drop(Rc::from_raw(ptr)) };
1040 /// Returns `true` if there are no other `Rc` or [`Weak`] pointers to
1041 /// this allocation.
1043 fn is_unique(this: &Self) -> bool {
1044 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
1047 /// Returns a mutable reference into the given `Rc`, if there are
1048 /// no other `Rc` or [`Weak`] pointers to the same allocation.
1050 /// Returns [`None`] otherwise, because it is not safe to
1051 /// mutate a shared value.
1053 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1054 /// the inner value when there are other `Rc` pointers.
1056 /// [make_mut]: Rc::make_mut
1057 /// [clone]: Clone::clone
1062 /// use std::rc::Rc;
1064 /// let mut x = Rc::new(3);
1065 /// *Rc::get_mut(&mut x).unwrap() = 4;
1066 /// assert_eq!(*x, 4);
1068 /// let _y = Rc::clone(&x);
1069 /// assert!(Rc::get_mut(&mut x).is_none());
1072 #[stable(feature = "rc_unique", since = "1.4.0")]
1073 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1074 if Rc::is_unique(this) { unsafe { Some(Rc::get_mut_unchecked(this)) } } else { None }
1077 /// Returns a mutable reference into the given `Rc`,
1078 /// without any check.
1080 /// See also [`get_mut`], which is safe and does appropriate checks.
1082 /// [`get_mut`]: Rc::get_mut
1086 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
1087 /// for the duration of the returned borrow.
1088 /// This is trivially the case if no such pointers exist,
1089 /// for example immediately after `Rc::new`.
1094 /// #![feature(get_mut_unchecked)]
1096 /// use std::rc::Rc;
1098 /// let mut x = Rc::new(String::new());
1100 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
1102 /// assert_eq!(*x, "foo");
1105 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1106 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1107 // We are careful to *not* create a reference covering the "count" fields, as
1108 // this would conflict with accesses to the reference counts (e.g. by `Weak`).
1109 unsafe { &mut (*this.ptr.as_ptr()).value }
1113 #[stable(feature = "ptr_eq", since = "1.17.0")]
1114 /// Returns `true` if the two `Rc`s point to the same allocation
1115 /// (in a vein similar to [`ptr::eq`]).
1120 /// use std::rc::Rc;
1122 /// let five = Rc::new(5);
1123 /// let same_five = Rc::clone(&five);
1124 /// let other_five = Rc::new(5);
1126 /// assert!(Rc::ptr_eq(&five, &same_five));
1127 /// assert!(!Rc::ptr_eq(&five, &other_five));
1129 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1130 this.ptr.as_ptr() == other.ptr.as_ptr()
1134 impl<T: Clone> Rc<T> {
1135 /// Makes a mutable reference into the given `Rc`.
1137 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
1138 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1139 /// referred to as clone-on-write.
1141 /// However, if there are no other `Rc` pointers to this allocation, but some [`Weak`]
1142 /// pointers, then the [`Weak`] pointers will be disassociated and the inner value will not
1145 /// See also [`get_mut`], which will fail rather than cloning the inner value
1146 /// or diassociating [`Weak`] pointers.
1148 /// [`clone`]: Clone::clone
1149 /// [`get_mut`]: Rc::get_mut
1154 /// use std::rc::Rc;
1156 /// let mut data = Rc::new(5);
1158 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
1159 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
1160 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
1161 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
1162 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
1164 /// // Now `data` and `other_data` point to different allocations.
1165 /// assert_eq!(*data, 8);
1166 /// assert_eq!(*other_data, 12);
1169 /// [`Weak`] pointers will be disassociated:
1172 /// use std::rc::Rc;
1174 /// let mut data = Rc::new(75);
1175 /// let weak = Rc::downgrade(&data);
1177 /// assert!(75 == *data);
1178 /// assert!(75 == *weak.upgrade().unwrap());
1180 /// *Rc::make_mut(&mut data) += 1;
1182 /// assert!(76 == *data);
1183 /// assert!(weak.upgrade().is_none());
1185 #[cfg(not(no_global_oom_handling))]
1187 #[stable(feature = "rc_unique", since = "1.4.0")]
1188 pub fn make_mut(this: &mut Self) -> &mut T {
1189 if Rc::strong_count(this) != 1 {
1190 // Gotta clone the data, there are other Rcs.
1191 // Pre-allocate memory to allow writing the cloned value directly.
1192 let mut rc = Self::new_uninit();
1194 let data = Rc::get_mut_unchecked(&mut rc);
1195 (**this).write_clone_into_raw(data.as_mut_ptr());
1196 *this = rc.assume_init();
1198 } else if Rc::weak_count(this) != 0 {
1199 // Can just steal the data, all that's left is Weaks
1200 let mut rc = Self::new_uninit();
1202 let data = Rc::get_mut_unchecked(&mut rc);
1203 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1205 this.inner().dec_strong();
1206 // Remove implicit strong-weak ref (no need to craft a fake
1207 // Weak here -- we know other Weaks can clean up for us)
1208 this.inner().dec_weak();
1209 ptr::write(this, rc.assume_init());
1212 // This unsafety is ok because we're guaranteed that the pointer
1213 // returned is the *only* pointer that will ever be returned to T. Our
1214 // reference count is guaranteed to be 1 at this point, and we required
1215 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
1216 // reference to the allocation.
1217 unsafe { &mut this.ptr.as_mut().value }
1220 /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
1223 /// Assuming `rc_t` is of type `Rc<T>`, this function is functionally equivalent to
1224 /// `(*rc_t).clone()`, but will avoid cloning the inner value where possible.
1229 /// #![feature(arc_unwrap_or_clone)]
1230 /// # use std::{ptr, rc::Rc};
1231 /// let inner = String::from("test");
1232 /// let ptr = inner.as_ptr();
1234 /// let rc = Rc::new(inner);
1235 /// let inner = Rc::unwrap_or_clone(rc);
1236 /// // The inner value was not cloned
1237 /// assert!(ptr::eq(ptr, inner.as_ptr()));
1239 /// let rc = Rc::new(inner);
1240 /// let rc2 = rc.clone();
1241 /// let inner = Rc::unwrap_or_clone(rc);
1242 /// // Because there were 2 references, we had to clone the inner value.
1243 /// assert!(!ptr::eq(ptr, inner.as_ptr()));
1244 /// // `rc2` is the last reference, so when we unwrap it we get back
1245 /// // the original `String`.
1246 /// let inner = Rc::unwrap_or_clone(rc2);
1247 /// assert!(ptr::eq(ptr, inner.as_ptr()));
1250 #[unstable(feature = "arc_unwrap_or_clone", issue = "93610")]
1251 pub fn unwrap_or_clone(this: Self) -> T {
1252 Rc::try_unwrap(this).unwrap_or_else(|rc| (*rc).clone())
1258 #[stable(feature = "rc_downcast", since = "1.29.0")]
1259 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
1264 /// use std::any::Any;
1265 /// use std::rc::Rc;
1267 /// fn print_if_string(value: Rc<dyn Any>) {
1268 /// if let Ok(string) = value.downcast::<String>() {
1269 /// println!("String ({}): {}", string.len(), string);
1273 /// let my_string = "Hello World".to_string();
1274 /// print_if_string(Rc::new(my_string));
1275 /// print_if_string(Rc::new(0i8));
1277 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
1278 if (*self).is::<T>() {
1280 let ptr = self.ptr.cast::<RcBox<T>>();
1282 Ok(Rc::from_inner(ptr))
1290 impl<T: ?Sized> Rc<T> {
1291 /// Allocates an `RcBox<T>` with sufficient space for
1292 /// a possibly-unsized inner value where the value has the layout provided.
1294 /// The function `mem_to_rcbox` is called with the data pointer
1295 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
1296 #[cfg(not(no_global_oom_handling))]
1297 unsafe fn allocate_for_layout(
1298 value_layout: Layout,
1299 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1300 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
1301 ) -> *mut RcBox<T> {
1302 // Calculate layout using the given value layout.
1303 // Previously, layout was calculated on the expression
1304 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
1305 // reference (see #54908).
1306 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
1308 Rc::try_allocate_for_layout(value_layout, allocate, mem_to_rcbox)
1309 .unwrap_or_else(|_| handle_alloc_error(layout))
1313 /// Allocates an `RcBox<T>` with sufficient space for
1314 /// a possibly-unsized inner value where the value has the layout provided,
1315 /// returning an error if allocation fails.
1317 /// The function `mem_to_rcbox` is called with the data pointer
1318 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
1320 unsafe fn try_allocate_for_layout(
1321 value_layout: Layout,
1322 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1323 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
1324 ) -> Result<*mut RcBox<T>, AllocError> {
1325 // Calculate layout using the given value layout.
1326 // Previously, layout was calculated on the expression
1327 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
1328 // reference (see #54908).
1329 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
1331 // Allocate for the layout.
1332 let ptr = allocate(layout)?;
1334 // Initialize the RcBox
1335 let inner = mem_to_rcbox(ptr.as_non_null_ptr().as_ptr());
1337 debug_assert_eq!(Layout::for_value(&*inner), layout);
1339 ptr::write(&mut (*inner).strong, Cell::new(1));
1340 ptr::write(&mut (*inner).weak, Cell::new(1));
1346 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
1347 #[cfg(not(no_global_oom_handling))]
1348 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
1349 // Allocate for the `RcBox<T>` using the given value.
1351 Self::allocate_for_layout(
1352 Layout::for_value(&*ptr),
1353 |layout| Global.allocate(layout),
1354 |mem| mem.with_metadata_of(ptr as *mut RcBox<T>),
1359 #[cfg(not(no_global_oom_handling))]
1360 fn from_box(v: Box<T>) -> Rc<T> {
1362 let (box_unique, alloc) = Box::into_unique(v);
1363 let bptr = box_unique.as_ptr();
1365 let value_size = size_of_val(&*bptr);
1366 let ptr = Self::allocate_for_ptr(bptr);
1368 // Copy value as bytes
1369 ptr::copy_nonoverlapping(
1370 bptr as *const T as *const u8,
1371 &mut (*ptr).value as *mut _ as *mut u8,
1375 // Free the allocation without dropping its contents
1376 box_free(box_unique, alloc);
1384 /// Allocates an `RcBox<[T]>` with the given length.
1385 #[cfg(not(no_global_oom_handling))]
1386 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
1388 Self::allocate_for_layout(
1389 Layout::array::<T>(len).unwrap(),
1390 |layout| Global.allocate(layout),
1391 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>,
1396 /// Copy elements from slice into newly allocated Rc<\[T\]>
1398 /// Unsafe because the caller must either take ownership or bind `T: Copy`
1399 #[cfg(not(no_global_oom_handling))]
1400 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
1402 let ptr = Self::allocate_for_slice(v.len());
1403 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).value as *mut [T] as *mut T, v.len());
1408 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
1410 /// Behavior is undefined should the size be wrong.
1411 #[cfg(not(no_global_oom_handling))]
1412 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
1413 // Panic guard while cloning T elements.
1414 // In the event of a panic, elements that have been written
1415 // into the new RcBox will be dropped, then the memory freed.
1423 impl<T> Drop for Guard<T> {
1424 fn drop(&mut self) {
1426 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1427 ptr::drop_in_place(slice);
1429 Global.deallocate(self.mem, self.layout);
1435 let ptr = Self::allocate_for_slice(len);
1437 let mem = ptr as *mut _ as *mut u8;
1438 let layout = Layout::for_value(&*ptr);
1440 // Pointer to first element
1441 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1443 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1445 for (i, item) in iter.enumerate() {
1446 ptr::write(elems.add(i), item);
1450 // All clear. Forget the guard so it doesn't free the new RcBox.
1458 /// Specialization trait used for `From<&[T]>`.
1459 trait RcFromSlice<T> {
1460 fn from_slice(slice: &[T]) -> Self;
1463 #[cfg(not(no_global_oom_handling))]
1464 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
1466 default fn from_slice(v: &[T]) -> Self {
1467 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1471 #[cfg(not(no_global_oom_handling))]
1472 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
1474 fn from_slice(v: &[T]) -> Self {
1475 unsafe { Rc::copy_from_slice(v) }
1479 #[stable(feature = "rust1", since = "1.0.0")]
1480 impl<T: ?Sized> Deref for Rc<T> {
1484 fn deref(&self) -> &T {
1489 #[unstable(feature = "receiver_trait", issue = "none")]
1490 impl<T: ?Sized> Receiver for Rc<T> {}
1492 #[stable(feature = "rust1", since = "1.0.0")]
1493 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1496 /// This will decrement the strong reference count. If the strong reference
1497 /// count reaches zero then the only other references (if any) are
1498 /// [`Weak`], so we `drop` the inner value.
1503 /// use std::rc::Rc;
1507 /// impl Drop for Foo {
1508 /// fn drop(&mut self) {
1509 /// println!("dropped!");
1513 /// let foo = Rc::new(Foo);
1514 /// let foo2 = Rc::clone(&foo);
1516 /// drop(foo); // Doesn't print anything
1517 /// drop(foo2); // Prints "dropped!"
1519 fn drop(&mut self) {
1521 self.inner().dec_strong();
1522 if self.inner().strong() == 0 {
1523 // destroy the contained object
1524 ptr::drop_in_place(Self::get_mut_unchecked(self));
1526 // remove the implicit "strong weak" pointer now that we've
1527 // destroyed the contents.
1528 self.inner().dec_weak();
1530 if self.inner().weak() == 0 {
1531 Global.deallocate(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1538 #[stable(feature = "rust1", since = "1.0.0")]
1539 impl<T: ?Sized> Clone for Rc<T> {
1540 /// Makes a clone of the `Rc` pointer.
1542 /// This creates another pointer to the same allocation, increasing the
1543 /// strong reference count.
1548 /// use std::rc::Rc;
1550 /// let five = Rc::new(5);
1552 /// let _ = Rc::clone(&five);
1555 fn clone(&self) -> Rc<T> {
1557 self.inner().inc_strong();
1558 Self::from_inner(self.ptr)
1563 #[cfg(not(no_global_oom_handling))]
1564 #[stable(feature = "rust1", since = "1.0.0")]
1565 impl<T: Default> Default for Rc<T> {
1566 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1571 /// use std::rc::Rc;
1573 /// let x: Rc<i32> = Default::default();
1574 /// assert_eq!(*x, 0);
1577 fn default() -> Rc<T> {
1578 Rc::new(Default::default())
1582 #[stable(feature = "rust1", since = "1.0.0")]
1583 trait RcEqIdent<T: ?Sized + PartialEq> {
1584 fn eq(&self, other: &Rc<T>) -> bool;
1585 fn ne(&self, other: &Rc<T>) -> bool;
1588 #[stable(feature = "rust1", since = "1.0.0")]
1589 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
1591 default fn eq(&self, other: &Rc<T>) -> bool {
1596 default fn ne(&self, other: &Rc<T>) -> bool {
1601 // Hack to allow specializing on `Eq` even though `Eq` has a method.
1602 #[rustc_unsafe_specialization_marker]
1603 pub(crate) trait MarkerEq: PartialEq<Self> {}
1605 impl<T: Eq> MarkerEq for T {}
1607 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1608 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1609 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1610 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1611 /// the same value, than two `&T`s.
1613 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1614 #[stable(feature = "rust1", since = "1.0.0")]
1615 impl<T: ?Sized + MarkerEq> RcEqIdent<T> for Rc<T> {
1617 fn eq(&self, other: &Rc<T>) -> bool {
1618 Rc::ptr_eq(self, other) || **self == **other
1622 fn ne(&self, other: &Rc<T>) -> bool {
1623 !Rc::ptr_eq(self, other) && **self != **other
1627 #[stable(feature = "rust1", since = "1.0.0")]
1628 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1629 /// Equality for two `Rc`s.
1631 /// Two `Rc`s are equal if their inner values are equal, even if they are
1632 /// stored in different allocation.
1634 /// If `T` also implements `Eq` (implying reflexivity of equality),
1635 /// two `Rc`s that point to the same allocation are
1641 /// use std::rc::Rc;
1643 /// let five = Rc::new(5);
1645 /// assert!(five == Rc::new(5));
1648 fn eq(&self, other: &Rc<T>) -> bool {
1649 RcEqIdent::eq(self, other)
1652 /// Inequality for two `Rc`s.
1654 /// Two `Rc`s are unequal if their inner values are unequal.
1656 /// If `T` also implements `Eq` (implying reflexivity of equality),
1657 /// two `Rc`s that point to the same allocation are
1663 /// use std::rc::Rc;
1665 /// let five = Rc::new(5);
1667 /// assert!(five != Rc::new(6));
1670 fn ne(&self, other: &Rc<T>) -> bool {
1671 RcEqIdent::ne(self, other)
1675 #[stable(feature = "rust1", since = "1.0.0")]
1676 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1678 #[stable(feature = "rust1", since = "1.0.0")]
1679 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1680 /// Partial comparison for two `Rc`s.
1682 /// The two are compared by calling `partial_cmp()` on their inner values.
1687 /// use std::rc::Rc;
1688 /// use std::cmp::Ordering;
1690 /// let five = Rc::new(5);
1692 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1695 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1696 (**self).partial_cmp(&**other)
1699 /// Less-than comparison for two `Rc`s.
1701 /// The two are compared by calling `<` on their inner values.
1706 /// use std::rc::Rc;
1708 /// let five = Rc::new(5);
1710 /// assert!(five < Rc::new(6));
1713 fn lt(&self, other: &Rc<T>) -> bool {
1717 /// 'Less than or equal to' comparison for two `Rc`s.
1719 /// The two are compared by calling `<=` on their inner values.
1724 /// use std::rc::Rc;
1726 /// let five = Rc::new(5);
1728 /// assert!(five <= Rc::new(5));
1731 fn le(&self, other: &Rc<T>) -> bool {
1735 /// Greater-than comparison for two `Rc`s.
1737 /// The two are compared by calling `>` on their inner values.
1742 /// use std::rc::Rc;
1744 /// let five = Rc::new(5);
1746 /// assert!(five > Rc::new(4));
1749 fn gt(&self, other: &Rc<T>) -> bool {
1753 /// 'Greater than or equal to' comparison for two `Rc`s.
1755 /// The two are compared by calling `>=` on their inner values.
1760 /// use std::rc::Rc;
1762 /// let five = Rc::new(5);
1764 /// assert!(five >= Rc::new(5));
1767 fn ge(&self, other: &Rc<T>) -> bool {
1772 #[stable(feature = "rust1", since = "1.0.0")]
1773 impl<T: ?Sized + Ord> Ord for Rc<T> {
1774 /// Comparison for two `Rc`s.
1776 /// The two are compared by calling `cmp()` on their inner values.
1781 /// use std::rc::Rc;
1782 /// use std::cmp::Ordering;
1784 /// let five = Rc::new(5);
1786 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1789 fn cmp(&self, other: &Rc<T>) -> Ordering {
1790 (**self).cmp(&**other)
1794 #[stable(feature = "rust1", since = "1.0.0")]
1795 impl<T: ?Sized + Hash> Hash for Rc<T> {
1796 fn hash<H: Hasher>(&self, state: &mut H) {
1797 (**self).hash(state);
1801 #[stable(feature = "rust1", since = "1.0.0")]
1802 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1803 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1804 fmt::Display::fmt(&**self, f)
1808 #[stable(feature = "rust1", since = "1.0.0")]
1809 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1810 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1811 fmt::Debug::fmt(&**self, f)
1815 #[stable(feature = "rust1", since = "1.0.0")]
1816 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1817 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1818 fmt::Pointer::fmt(&(&**self as *const T), f)
1822 #[cfg(not(no_global_oom_handling))]
1823 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1824 impl<T> From<T> for Rc<T> {
1825 /// Converts a generic type `T` into an `Rc<T>`
1827 /// The conversion allocates on the heap and moves `t`
1828 /// from the stack into it.
1832 /// # use std::rc::Rc;
1834 /// let rc = Rc::new(5);
1836 /// assert_eq!(Rc::from(x), rc);
1838 fn from(t: T) -> Self {
1843 #[cfg(not(no_global_oom_handling))]
1844 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1845 impl<T: Clone> From<&[T]> for Rc<[T]> {
1846 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
1851 /// # use std::rc::Rc;
1852 /// let original: &[i32] = &[1, 2, 3];
1853 /// let shared: Rc<[i32]> = Rc::from(original);
1854 /// assert_eq!(&[1, 2, 3], &shared[..]);
1857 fn from(v: &[T]) -> Rc<[T]> {
1858 <Self as RcFromSlice<T>>::from_slice(v)
1862 #[cfg(not(no_global_oom_handling))]
1863 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1864 impl From<&str> for Rc<str> {
1865 /// Allocate a reference-counted string slice and copy `v` into it.
1870 /// # use std::rc::Rc;
1871 /// let shared: Rc<str> = Rc::from("statue");
1872 /// assert_eq!("statue", &shared[..]);
1875 fn from(v: &str) -> Rc<str> {
1876 let rc = Rc::<[u8]>::from(v.as_bytes());
1877 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1881 #[cfg(not(no_global_oom_handling))]
1882 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1883 impl From<String> for Rc<str> {
1884 /// Allocate a reference-counted string slice and copy `v` into it.
1889 /// # use std::rc::Rc;
1890 /// let original: String = "statue".to_owned();
1891 /// let shared: Rc<str> = Rc::from(original);
1892 /// assert_eq!("statue", &shared[..]);
1895 fn from(v: String) -> Rc<str> {
1900 #[cfg(not(no_global_oom_handling))]
1901 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1902 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1903 /// Move a boxed object to a new, reference counted, allocation.
1908 /// # use std::rc::Rc;
1909 /// let original: Box<i32> = Box::new(1);
1910 /// let shared: Rc<i32> = Rc::from(original);
1911 /// assert_eq!(1, *shared);
1914 fn from(v: Box<T>) -> Rc<T> {
1919 #[cfg(not(no_global_oom_handling))]
1920 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1921 impl<T> From<Vec<T>> for Rc<[T]> {
1922 /// Allocate a reference-counted slice and move `v`'s items into it.
1927 /// # use std::rc::Rc;
1928 /// let original: Box<Vec<i32>> = Box::new(vec![1, 2, 3]);
1929 /// let shared: Rc<Vec<i32>> = Rc::from(original);
1930 /// assert_eq!(vec![1, 2, 3], *shared);
1933 fn from(mut v: Vec<T>) -> Rc<[T]> {
1935 let rc = Rc::copy_from_slice(&v);
1937 // Allow the Vec to free its memory, but not destroy its contents
1945 #[stable(feature = "shared_from_cow", since = "1.45.0")]
1946 impl<'a, B> From<Cow<'a, B>> for Rc<B>
1948 B: ToOwned + ?Sized,
1949 Rc<B>: From<&'a B> + From<B::Owned>,
1951 /// Create a reference-counted pointer from
1952 /// a clone-on-write pointer by copying its content.
1957 /// # use std::rc::Rc;
1958 /// # use std::borrow::Cow;
1959 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
1960 /// let shared: Rc<str> = Rc::from(cow);
1961 /// assert_eq!("eggplant", &shared[..]);
1964 fn from(cow: Cow<'a, B>) -> Rc<B> {
1966 Cow::Borrowed(s) => Rc::from(s),
1967 Cow::Owned(s) => Rc::from(s),
1972 #[stable(feature = "shared_from_str", since = "1.62.0")]
1973 impl From<Rc<str>> for Rc<[u8]> {
1974 /// Converts a reference-counted string slice into a byte slice.
1979 /// # use std::rc::Rc;
1980 /// let string: Rc<str> = Rc::from("eggplant");
1981 /// let bytes: Rc<[u8]> = Rc::from(string);
1982 /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
1985 fn from(rc: Rc<str>) -> Self {
1986 // SAFETY: `str` has the same layout as `[u8]`.
1987 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const [u8]) }
1991 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1992 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]> {
1993 type Error = Rc<[T]>;
1995 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1996 if boxed_slice.len() == N {
1997 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
2004 #[cfg(not(no_global_oom_handling))]
2005 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2006 impl<T> iter::FromIterator<T> for Rc<[T]> {
2007 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
2009 /// # Performance characteristics
2011 /// ## The general case
2013 /// In the general case, collecting into `Rc<[T]>` is done by first
2014 /// collecting into a `Vec<T>`. That is, when writing the following:
2017 /// # use std::rc::Rc;
2018 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2019 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2022 /// this behaves as if we wrote:
2025 /// # use std::rc::Rc;
2026 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2027 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2028 /// .into(); // A second allocation for `Rc<[T]>` happens here.
2029 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2032 /// This will allocate as many times as needed for constructing the `Vec<T>`
2033 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
2035 /// ## Iterators of known length
2037 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2038 /// a single allocation will be made for the `Rc<[T]>`. For example:
2041 /// # use std::rc::Rc;
2042 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2043 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2045 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
2046 ToRcSlice::to_rc_slice(iter.into_iter())
2050 /// Specialization trait used for collecting into `Rc<[T]>`.
2051 #[cfg(not(no_global_oom_handling))]
2052 trait ToRcSlice<T>: Iterator<Item = T> + Sized {
2053 fn to_rc_slice(self) -> Rc<[T]>;
2056 #[cfg(not(no_global_oom_handling))]
2057 impl<T, I: Iterator<Item = T>> ToRcSlice<T> for I {
2058 default fn to_rc_slice(self) -> Rc<[T]> {
2059 self.collect::<Vec<T>>().into()
2063 #[cfg(not(no_global_oom_handling))]
2064 impl<T, I: iter::TrustedLen<Item = T>> ToRcSlice<T> for I {
2065 fn to_rc_slice(self) -> Rc<[T]> {
2066 // This is the case for a `TrustedLen` iterator.
2067 let (low, high) = self.size_hint();
2068 if let Some(high) = high {
2072 "TrustedLen iterator's size hint is not exact: {:?}",
2077 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2078 Rc::from_iter_exact(self, low)
2081 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2082 // length exceeding `usize::MAX`.
2083 // The default implementation would collect into a vec which would panic.
2084 // Thus we panic here immediately without invoking `Vec` code.
2085 panic!("capacity overflow");
2090 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
2091 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
2092 /// pointer, which returns an <code>[Option]<[Rc]\<T>></code>.
2094 /// Since a `Weak` reference does not count towards ownership, it will not
2095 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
2096 /// guarantees about the value still being present. Thus it may return [`None`]
2097 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
2098 /// itself (the backing store) from being deallocated.
2100 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
2101 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
2102 /// prevent circular references between [`Rc`] pointers, since mutual owning references
2103 /// would never allow either [`Rc`] to be dropped. For example, a tree could
2104 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
2105 /// pointers from children back to their parents.
2107 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
2109 /// [`upgrade`]: Weak::upgrade
2110 #[stable(feature = "rc_weak", since = "1.4.0")]
2111 pub struct Weak<T: ?Sized> {
2112 // This is a `NonNull` to allow optimizing the size of this type in enums,
2113 // but it is not necessarily a valid pointer.
2114 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
2115 // to allocate space on the heap. That's not a value a real pointer
2116 // will ever have because RcBox has alignment at least 2.
2117 // This is only possible when `T: Sized`; unsized `T` never dangle.
2118 ptr: NonNull<RcBox<T>>,
2121 #[stable(feature = "rc_weak", since = "1.4.0")]
2122 impl<T: ?Sized> !marker::Send for Weak<T> {}
2123 #[stable(feature = "rc_weak", since = "1.4.0")]
2124 impl<T: ?Sized> !marker::Sync for Weak<T> {}
2126 #[unstable(feature = "coerce_unsized", issue = "27732")]
2127 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
2129 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
2130 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
2133 /// Constructs a new `Weak<T>`, without allocating any memory.
2134 /// Calling [`upgrade`] on the return value always gives [`None`].
2136 /// [`upgrade`]: Weak::upgrade
2141 /// use std::rc::Weak;
2143 /// let empty: Weak<i64> = Weak::new();
2144 /// assert!(empty.upgrade().is_none());
2146 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2147 #[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
2149 pub const fn new() -> Weak<T> {
2150 Weak { ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<RcBox<T>>(usize::MAX)) } }
2154 pub(crate) fn is_dangling<T: ?Sized>(ptr: *mut T) -> bool {
2155 (ptr as *mut ()).addr() == usize::MAX
2158 /// Helper type to allow accessing the reference counts without
2159 /// making any assertions about the data field.
2160 struct WeakInner<'a> {
2161 weak: &'a Cell<usize>,
2162 strong: &'a Cell<usize>,
2165 impl<T: ?Sized> Weak<T> {
2166 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
2168 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
2169 /// unaligned or even [`null`] otherwise.
2174 /// use std::rc::Rc;
2177 /// let strong = Rc::new("hello".to_owned());
2178 /// let weak = Rc::downgrade(&strong);
2179 /// // Both point to the same object
2180 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
2181 /// // The strong here keeps it alive, so we can still access the object.
2182 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
2185 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
2186 /// // undefined behaviour.
2187 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
2190 /// [`null`]: ptr::null
2192 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
2193 pub fn as_ptr(&self) -> *const T {
2194 let ptr: *mut RcBox<T> = NonNull::as_ptr(self.ptr);
2196 if is_dangling(ptr) {
2197 // If the pointer is dangling, we return the sentinel directly. This cannot be
2198 // a valid payload address, as the payload is at least as aligned as RcBox (usize).
2201 // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
2202 // The payload may be dropped at this point, and we have to maintain provenance,
2203 // so use raw pointer manipulation.
2204 unsafe { ptr::addr_of_mut!((*ptr).value) }
2208 /// Consumes the `Weak<T>` and turns it into a raw pointer.
2210 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2211 /// one weak reference (the weak count is not modified by this operation). It can be turned
2212 /// back into the `Weak<T>` with [`from_raw`].
2214 /// The same restrictions of accessing the target of the pointer as with
2215 /// [`as_ptr`] apply.
2220 /// use std::rc::{Rc, Weak};
2222 /// let strong = Rc::new("hello".to_owned());
2223 /// let weak = Rc::downgrade(&strong);
2224 /// let raw = weak.into_raw();
2226 /// assert_eq!(1, Rc::weak_count(&strong));
2227 /// assert_eq!("hello", unsafe { &*raw });
2229 /// drop(unsafe { Weak::from_raw(raw) });
2230 /// assert_eq!(0, Rc::weak_count(&strong));
2233 /// [`from_raw`]: Weak::from_raw
2234 /// [`as_ptr`]: Weak::as_ptr
2235 #[must_use = "`self` will be dropped if the result is not used"]
2236 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2237 pub fn into_raw(self) -> *const T {
2238 let result = self.as_ptr();
2243 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
2245 /// This can be used to safely get a strong reference (by calling [`upgrade`]
2246 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2248 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2249 /// as these don't own anything; the method still works on them).
2253 /// The pointer must have originated from the [`into_raw`] and must still own its potential
2256 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2257 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2258 /// count is not modified by this operation) and therefore it must be paired with a previous
2259 /// call to [`into_raw`].
2264 /// use std::rc::{Rc, Weak};
2266 /// let strong = Rc::new("hello".to_owned());
2268 /// let raw_1 = Rc::downgrade(&strong).into_raw();
2269 /// let raw_2 = Rc::downgrade(&strong).into_raw();
2271 /// assert_eq!(2, Rc::weak_count(&strong));
2273 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2274 /// assert_eq!(1, Rc::weak_count(&strong));
2278 /// // Decrement the last weak count.
2279 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2282 /// [`into_raw`]: Weak::into_raw
2283 /// [`upgrade`]: Weak::upgrade
2284 /// [`new`]: Weak::new
2285 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2286 pub unsafe fn from_raw(ptr: *const T) -> Self {
2287 // See Weak::as_ptr for context on how the input pointer is derived.
2289 let ptr = if is_dangling(ptr as *mut T) {
2290 // This is a dangling Weak.
2291 ptr as *mut RcBox<T>
2293 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
2294 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
2295 let offset = unsafe { data_offset(ptr) };
2296 // Thus, we reverse the offset to get the whole RcBox.
2297 // SAFETY: the pointer originated from a Weak, so this offset is safe.
2298 unsafe { (ptr as *mut u8).offset(-offset).with_metadata_of(ptr as *mut RcBox<T>) }
2301 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
2302 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
2305 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
2306 /// dropping of the inner value if successful.
2308 /// Returns [`None`] if the inner value has since been dropped.
2313 /// use std::rc::Rc;
2315 /// let five = Rc::new(5);
2317 /// let weak_five = Rc::downgrade(&five);
2319 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
2320 /// assert!(strong_five.is_some());
2322 /// // Destroy all strong pointers.
2323 /// drop(strong_five);
2326 /// assert!(weak_five.upgrade().is_none());
2328 #[must_use = "this returns a new `Rc`, \
2329 without modifying the original weak pointer"]
2330 #[stable(feature = "rc_weak", since = "1.4.0")]
2331 pub fn upgrade(&self) -> Option<Rc<T>> {
2332 let inner = self.inner()?;
2334 if inner.strong() == 0 {
2339 Some(Rc::from_inner(self.ptr))
2344 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
2346 /// If `self` was created using [`Weak::new`], this will return 0.
2348 #[stable(feature = "weak_counts", since = "1.41.0")]
2349 pub fn strong_count(&self) -> usize {
2350 if let Some(inner) = self.inner() { inner.strong() } else { 0 }
2353 /// Gets the number of `Weak` pointers pointing to this allocation.
2355 /// If no strong pointers remain, this will return zero.
2357 #[stable(feature = "weak_counts", since = "1.41.0")]
2358 pub fn weak_count(&self) -> usize {
2361 if inner.strong() > 0 {
2362 inner.weak() - 1 // subtract the implicit weak ptr
2370 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`,
2371 /// (i.e., when this `Weak` was created by `Weak::new`).
2373 fn inner(&self) -> Option<WeakInner<'_>> {
2374 if is_dangling(self.ptr.as_ptr()) {
2377 // We are careful to *not* create a reference covering the "data" field, as
2378 // the field may be mutated concurrently (for example, if the last `Rc`
2379 // is dropped, the data field will be dropped in-place).
2381 let ptr = self.ptr.as_ptr();
2382 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
2387 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
2388 /// [`ptr::eq`]), or if both don't point to any allocation
2389 /// (because they were created with `Weak::new()`).
2393 /// Since this compares pointers it means that `Weak::new()` will equal each
2394 /// other, even though they don't point to any allocation.
2399 /// use std::rc::Rc;
2401 /// let first_rc = Rc::new(5);
2402 /// let first = Rc::downgrade(&first_rc);
2403 /// let second = Rc::downgrade(&first_rc);
2405 /// assert!(first.ptr_eq(&second));
2407 /// let third_rc = Rc::new(5);
2408 /// let third = Rc::downgrade(&third_rc);
2410 /// assert!(!first.ptr_eq(&third));
2413 /// Comparing `Weak::new`.
2416 /// use std::rc::{Rc, Weak};
2418 /// let first = Weak::new();
2419 /// let second = Weak::new();
2420 /// assert!(first.ptr_eq(&second));
2422 /// let third_rc = Rc::new(());
2423 /// let third = Rc::downgrade(&third_rc);
2424 /// assert!(!first.ptr_eq(&third));
2428 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
2429 pub fn ptr_eq(&self, other: &Self) -> bool {
2430 self.ptr.as_ptr() == other.ptr.as_ptr()
2434 #[stable(feature = "rc_weak", since = "1.4.0")]
2435 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2436 /// Drops the `Weak` pointer.
2441 /// use std::rc::{Rc, Weak};
2445 /// impl Drop for Foo {
2446 /// fn drop(&mut self) {
2447 /// println!("dropped!");
2451 /// let foo = Rc::new(Foo);
2452 /// let weak_foo = Rc::downgrade(&foo);
2453 /// let other_weak_foo = Weak::clone(&weak_foo);
2455 /// drop(weak_foo); // Doesn't print anything
2456 /// drop(foo); // Prints "dropped!"
2458 /// assert!(other_weak_foo.upgrade().is_none());
2460 fn drop(&mut self) {
2461 let inner = if let Some(inner) = self.inner() { inner } else { return };
2464 // the weak count starts at 1, and will only go to zero if all
2465 // the strong pointers have disappeared.
2466 if inner.weak() == 0 {
2468 Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()));
2474 #[stable(feature = "rc_weak", since = "1.4.0")]
2475 impl<T: ?Sized> Clone for Weak<T> {
2476 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2481 /// use std::rc::{Rc, Weak};
2483 /// let weak_five = Rc::downgrade(&Rc::new(5));
2485 /// let _ = Weak::clone(&weak_five);
2488 fn clone(&self) -> Weak<T> {
2489 if let Some(inner) = self.inner() {
2492 Weak { ptr: self.ptr }
2496 #[stable(feature = "rc_weak", since = "1.4.0")]
2497 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
2498 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2503 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2504 impl<T> Default for Weak<T> {
2505 /// Constructs a new `Weak<T>`, without allocating any memory.
2506 /// Calling [`upgrade`] on the return value always gives [`None`].
2508 /// [`upgrade`]: Weak::upgrade
2513 /// use std::rc::Weak;
2515 /// let empty: Weak<i64> = Default::default();
2516 /// assert!(empty.upgrade().is_none());
2518 fn default() -> Weak<T> {
2523 // NOTE: We checked_add here to deal with mem::forget safely. In particular
2524 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
2525 // you can free the allocation while outstanding Rcs (or Weaks) exist.
2526 // We abort because this is such a degenerate scenario that we don't care about
2527 // what happens -- no real program should ever experience this.
2529 // This should have negligible overhead since you don't actually need to
2530 // clone these much in Rust thanks to ownership and move-semantics.
2534 fn weak_ref(&self) -> &Cell<usize>;
2535 fn strong_ref(&self) -> &Cell<usize>;
2538 fn strong(&self) -> usize {
2539 self.strong_ref().get()
2543 fn inc_strong(&self) {
2544 let strong = self.strong();
2546 // We insert an `assume` here to hint LLVM at an otherwise
2547 // missed optimization.
2548 // SAFETY: The reference count will never be zero when this is
2551 core::intrinsics::assume(strong != 0);
2554 let strong = strong.wrapping_add(1);
2555 self.strong_ref().set(strong);
2557 // We want to abort on overflow instead of dropping the value.
2558 // Checking for overflow after the store instead of before
2559 // allows for slightly better code generation.
2560 if core::intrinsics::unlikely(strong == 0) {
2566 fn dec_strong(&self) {
2567 self.strong_ref().set(self.strong() - 1);
2571 fn weak(&self) -> usize {
2572 self.weak_ref().get()
2576 fn inc_weak(&self) {
2577 let weak = self.weak();
2579 // We insert an `assume` here to hint LLVM at an otherwise
2580 // missed optimization.
2581 // SAFETY: The reference count will never be zero when this is
2584 core::intrinsics::assume(weak != 0);
2587 let weak = weak.wrapping_add(1);
2588 self.weak_ref().set(weak);
2590 // We want to abort on overflow instead of dropping the value.
2591 // Checking for overflow after the store instead of before
2592 // allows for slightly better code generation.
2593 if core::intrinsics::unlikely(weak == 0) {
2599 fn dec_weak(&self) {
2600 self.weak_ref().set(self.weak() - 1);
2604 impl<T: ?Sized> RcInnerPtr for RcBox<T> {
2606 fn weak_ref(&self) -> &Cell<usize> {
2611 fn strong_ref(&self) -> &Cell<usize> {
2616 impl<'a> RcInnerPtr for WeakInner<'a> {
2618 fn weak_ref(&self) -> &Cell<usize> {
2623 fn strong_ref(&self) -> &Cell<usize> {
2628 #[stable(feature = "rust1", since = "1.0.0")]
2629 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2630 fn borrow(&self) -> &T {
2635 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2636 impl<T: ?Sized> AsRef<T> for Rc<T> {
2637 fn as_ref(&self) -> &T {
2642 #[stable(feature = "pin", since = "1.33.0")]
2643 impl<T: ?Sized> Unpin for Rc<T> {}
2645 /// Get the offset within an `RcBox` for the payload behind a pointer.
2649 /// The pointer must point to (and have valid metadata for) a previously
2650 /// valid instance of T, but the T is allowed to be dropped.
2651 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2652 // Align the unsized value to the end of the RcBox.
2653 // Because RcBox is repr(C), it will always be the last field in memory.
2654 // SAFETY: since the only unsized types possible are slices, trait objects,
2655 // and extern types, the input safety requirement is currently enough to
2656 // satisfy the requirements of align_of_val_raw; this is an implementation
2657 // detail of the language that must not be relied upon outside of std.
2658 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2662 fn data_offset_align(align: usize) -> isize {
2663 let layout = Layout::new::<RcBox<()>>();
2664 (layout.size() + layout.padding_needed_for(align)) as isize