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 function-like syntax:
42 //! let my_rc = Rc::new(());
44 //! Rc::downgrade(&my_rc);
47 //! [`Weak<T>`][`Weak`] does not auto-dereference to `T`, because the inner value may have
48 //! already been dropped.
50 //! # Cloning references
52 //! Creating a new reference to the same allocation as an existing reference counted pointer
53 //! is done using the `Clone` trait implemented for [`Rc<T>`][`Rc`] and [`Weak<T>`][`Weak`].
57 //! let foo = Rc::new(vec![1.0, 2.0, 3.0]);
58 //! // The two syntaxes below are equivalent.
59 //! let a = foo.clone();
60 //! let b = Rc::clone(&foo);
61 //! // a and b both point to the same memory location as foo.
64 //! The `Rc::clone(&from)` syntax is the most idiomatic because it conveys more explicitly
65 //! the meaning of the code. In the example above, this syntax makes it easier to see that
66 //! this code is creating a new reference rather than copying the whole content of foo.
70 //! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`.
71 //! We want to have our `Gadget`s point to their `Owner`. We can't do this with
72 //! unique ownership, because more than one gadget may belong to the same
73 //! `Owner`. [`Rc`] allows us to share an `Owner` between multiple `Gadget`s,
74 //! and have the `Owner` remain allocated as long as any `Gadget` points at it.
81 //! // ...other fields
87 //! // ...other fields
91 //! // Create a reference-counted `Owner`.
92 //! let gadget_owner: Rc<Owner> = Rc::new(
94 //! name: "Gadget Man".to_string(),
98 //! // Create `Gadget`s belonging to `gadget_owner`. Cloning the `Rc<Owner>`
99 //! // gives us a new pointer to the same `Owner` allocation, incrementing
100 //! // the reference count in the process.
101 //! let gadget1 = Gadget {
103 //! owner: Rc::clone(&gadget_owner),
105 //! let gadget2 = Gadget {
107 //! owner: Rc::clone(&gadget_owner),
110 //! // Dispose of our local variable `gadget_owner`.
111 //! drop(gadget_owner);
113 //! // Despite dropping `gadget_owner`, we're still able to print out the name
114 //! // of the `Owner` of the `Gadget`s. This is because we've only dropped a
115 //! // single `Rc<Owner>`, not the `Owner` it points to. As long as there are
116 //! // other `Rc<Owner>` pointing at the same `Owner` allocation, it will remain
117 //! // live. The field projection `gadget1.owner.name` works because
118 //! // `Rc<Owner>` automatically dereferences to `Owner`.
119 //! println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name);
120 //! println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name);
122 //! // At the end of the function, `gadget1` and `gadget2` are destroyed, and
123 //! // with them the last counted references to our `Owner`. Gadget Man now
124 //! // gets destroyed as well.
128 //! If our requirements change, and we also need to be able to traverse from
129 //! `Owner` to `Gadget`, we will run into problems. An [`Rc`] pointer from `Owner`
130 //! to `Gadget` introduces a cycle. This means that their
131 //! reference counts can never reach 0, and the allocation will never be destroyed:
132 //! a memory leak. In order to get around this, we can use [`Weak`]
135 //! Rust actually makes it somewhat difficult to produce this loop in the first
136 //! place. In order to end up with two values that point at each other, one of
137 //! them needs to be mutable. This is difficult because [`Rc`] enforces
138 //! memory safety by only giving out shared references to the value it wraps,
139 //! and these don't allow direct mutation. We need to wrap the part of the
140 //! value we wish to mutate in a [`RefCell`], which provides *interior
141 //! mutability*: a method to achieve mutability through a shared reference.
142 //! [`RefCell`] enforces Rust's borrowing rules at runtime.
146 //! use std::rc::Weak;
147 //! use std::cell::RefCell;
151 //! gadgets: RefCell<Vec<Weak<Gadget>>>,
152 //! // ...other fields
157 //! owner: Rc<Owner>,
158 //! // ...other fields
162 //! // Create a reference-counted `Owner`. Note that we've put the `Owner`'s
163 //! // vector of `Gadget`s inside a `RefCell` so that we can mutate it through
164 //! // a shared reference.
165 //! let gadget_owner: Rc<Owner> = Rc::new(
167 //! name: "Gadget Man".to_string(),
168 //! gadgets: RefCell::new(vec![]),
172 //! // Create `Gadget`s belonging to `gadget_owner`, as before.
173 //! let gadget1 = Rc::new(
176 //! owner: Rc::clone(&gadget_owner),
179 //! let gadget2 = Rc::new(
182 //! owner: Rc::clone(&gadget_owner),
186 //! // Add the `Gadget`s to their `Owner`.
188 //! let mut gadgets = gadget_owner.gadgets.borrow_mut();
189 //! gadgets.push(Rc::downgrade(&gadget1));
190 //! gadgets.push(Rc::downgrade(&gadget2));
192 //! // `RefCell` dynamic borrow ends here.
195 //! // Iterate over our `Gadget`s, printing their details out.
196 //! for gadget_weak in gadget_owner.gadgets.borrow().iter() {
198 //! // `gadget_weak` is a `Weak<Gadget>`. Since `Weak` pointers can't
199 //! // guarantee the allocation still exists, we need to call
200 //! // `upgrade`, which returns an `Option<Rc<Gadget>>`.
202 //! // In this case we know the allocation still exists, so we simply
203 //! // `unwrap` the `Option`. In a more complicated program, you might
204 //! // need graceful error handling for a `None` result.
206 //! let gadget = gadget_weak.upgrade().unwrap();
207 //! println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
210 //! // At the end of the function, `gadget_owner`, `gadget1`, and `gadget2`
211 //! // are destroyed. There are now no strong (`Rc`) pointers to the
212 //! // gadgets, so they are destroyed. This zeroes the reference count on
213 //! // Gadget Man, so he gets destroyed as well.
217 //! [`Rc`]: struct.Rc.html
218 //! [`Weak`]: struct.Weak.html
219 //! [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
220 //! [`Cell`]: ../../std/cell/struct.Cell.html
221 //! [`RefCell`]: ../../std/cell/struct.RefCell.html
222 //! [send]: ../../std/marker/trait.Send.html
223 //! [arc]: ../../std/sync/struct.Arc.html
224 //! [`Deref`]: ../../std/ops/trait.Deref.html
225 //! [downgrade]: struct.Rc.html#method.downgrade
226 //! [upgrade]: struct.Weak.html#method.upgrade
227 //! [`None`]: ../../std/option/enum.Option.html#variant.None
228 //! [mutability]: ../../std/cell/index.html#introducing-mutability-inside-of-something-immutable
230 #![stable(feature = "rust1", since = "1.0.0")]
233 use crate::boxed::Box;
239 use core::cell::Cell;
240 use core::cmp::Ordering;
241 use core::convert::{From, TryFrom};
243 use core::hash::{Hash, Hasher};
244 use core::intrinsics::abort;
246 use core::marker::{self, PhantomData, Unpin, Unsize};
247 use core::mem::{self, align_of_val_raw, forget, size_of_val};
248 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
250 use core::ptr::{self, NonNull};
251 use core::slice::from_raw_parts_mut;
253 use crate::alloc::{box_free, handle_alloc_error, AllocRef, Global, Layout};
254 use crate::borrow::{Cow, ToOwned};
255 use crate::string::String;
261 // This is repr(C) to future-proof against possible field-reordering, which
262 // would interfere with otherwise safe [into|from]_raw() of transmutable
265 struct RcBox<T: ?Sized> {
271 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
274 /// See the [module-level documentation](./index.html) for more details.
276 /// The inherent methods of `Rc` are all associated functions, which means
277 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
278 /// `value.get_mut()`. This avoids conflicts with methods of the inner
281 /// [get_mut]: #method.get_mut
282 #[cfg_attr(not(test), rustc_diagnostic_item = "Rc")]
283 #[stable(feature = "rust1", since = "1.0.0")]
284 pub struct Rc<T: ?Sized> {
285 ptr: NonNull<RcBox<T>>,
286 phantom: PhantomData<RcBox<T>>,
289 #[stable(feature = "rust1", since = "1.0.0")]
290 impl<T: ?Sized> !marker::Send for Rc<T> {}
291 #[stable(feature = "rust1", since = "1.0.0")]
292 impl<T: ?Sized> !marker::Sync for Rc<T> {}
294 #[unstable(feature = "coerce_unsized", issue = "27732")]
295 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
297 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
298 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
300 impl<T: ?Sized> Rc<T> {
301 fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
302 Self { ptr, phantom: PhantomData }
305 unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
306 Self::from_inner(unsafe { NonNull::new_unchecked(ptr) })
311 /// Constructs a new `Rc<T>`.
318 /// let five = Rc::new(5);
320 #[stable(feature = "rust1", since = "1.0.0")]
321 pub fn new(value: T) -> Rc<T> {
322 // There is an implicit weak pointer owned by all the strong
323 // pointers, which ensures that the weak destructor never frees
324 // the allocation while the strong destructor is running, even
325 // if the weak pointer is stored inside the strong one.
327 Box::leak(box RcBox { strong: Cell::new(1), weak: Cell::new(1), value }).into(),
331 /// Constructs a new `Rc` with uninitialized contents.
336 /// #![feature(new_uninit)]
337 /// #![feature(get_mut_unchecked)]
341 /// let mut five = Rc::<u32>::new_uninit();
343 /// let five = unsafe {
344 /// // Deferred initialization:
345 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
347 /// five.assume_init()
350 /// assert_eq!(*five, 5)
352 #[unstable(feature = "new_uninit", issue = "63291")]
353 pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
355 Rc::from_ptr(Rc::allocate_for_layout(Layout::new::<T>(), |mem| {
356 mem as *mut RcBox<mem::MaybeUninit<T>>
361 /// Constructs a new `Rc` with uninitialized contents, with the memory
362 /// being filled with `0` bytes.
364 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
365 /// incorrect usage of this method.
370 /// #![feature(new_uninit)]
374 /// let zero = Rc::<u32>::new_zeroed();
375 /// let zero = unsafe { zero.assume_init() };
377 /// assert_eq!(*zero, 0)
380 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
381 #[unstable(feature = "new_uninit", issue = "63291")]
382 pub fn new_zeroed() -> Rc<mem::MaybeUninit<T>> {
384 let mut uninit = Self::new_uninit();
385 ptr::write_bytes::<T>(Rc::get_mut_unchecked(&mut uninit).as_mut_ptr(), 0, 1);
390 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
391 /// `value` will be pinned in memory and unable to be moved.
392 #[stable(feature = "pin", since = "1.33.0")]
393 pub fn pin(value: T) -> Pin<Rc<T>> {
394 unsafe { Pin::new_unchecked(Rc::new(value)) }
397 /// Returns the inner value, if the `Rc` has exactly one strong reference.
399 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
402 /// This will succeed even if there are outstanding weak references.
404 /// [result]: ../../std/result/enum.Result.html
411 /// let x = Rc::new(3);
412 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
414 /// let x = Rc::new(4);
415 /// let _y = Rc::clone(&x);
416 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
419 #[stable(feature = "rc_unique", since = "1.4.0")]
420 pub fn try_unwrap(this: Self) -> Result<T, Self> {
421 if Rc::strong_count(&this) == 1 {
423 let val = ptr::read(&*this); // copy the contained object
425 // Indicate to Weaks that they can't be promoted by decrementing
426 // the strong count, and then remove the implicit "strong weak"
427 // pointer while also handling drop logic by just crafting a
430 let _weak = Weak { ptr: this.ptr };
441 /// Constructs a new reference-counted slice with uninitialized contents.
446 /// #![feature(new_uninit)]
447 /// #![feature(get_mut_unchecked)]
451 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
453 /// let values = unsafe {
454 /// // Deferred initialization:
455 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
456 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
457 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
459 /// values.assume_init()
462 /// assert_eq!(*values, [1, 2, 3])
464 #[unstable(feature = "new_uninit", issue = "63291")]
465 pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
466 unsafe { Rc::from_ptr(Rc::allocate_for_slice(len)) }
470 impl<T> Rc<mem::MaybeUninit<T>> {
471 /// Converts to `Rc<T>`.
475 /// As with [`MaybeUninit::assume_init`],
476 /// it is up to the caller to guarantee that the inner value
477 /// really is in an initialized state.
478 /// Calling this when the content is not yet fully initialized
479 /// causes immediate undefined behavior.
481 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
486 /// #![feature(new_uninit)]
487 /// #![feature(get_mut_unchecked)]
491 /// let mut five = Rc::<u32>::new_uninit();
493 /// let five = unsafe {
494 /// // Deferred initialization:
495 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
497 /// five.assume_init()
500 /// assert_eq!(*five, 5)
502 #[unstable(feature = "new_uninit", issue = "63291")]
504 pub unsafe fn assume_init(self) -> Rc<T> {
505 Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
509 impl<T> Rc<[mem::MaybeUninit<T>]> {
510 /// Converts to `Rc<[T]>`.
514 /// As with [`MaybeUninit::assume_init`],
515 /// it is up to the caller to guarantee that the inner value
516 /// really is in an initialized state.
517 /// Calling this when the content is not yet fully initialized
518 /// causes immediate undefined behavior.
520 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
525 /// #![feature(new_uninit)]
526 /// #![feature(get_mut_unchecked)]
530 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
532 /// let values = unsafe {
533 /// // Deferred initialization:
534 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
535 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
536 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
538 /// values.assume_init()
541 /// assert_eq!(*values, [1, 2, 3])
543 #[unstable(feature = "new_uninit", issue = "63291")]
545 pub unsafe fn assume_init(self) -> Rc<[T]> {
546 unsafe { Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
550 impl<T: ?Sized> Rc<T> {
551 /// Consumes the `Rc`, returning the wrapped pointer.
553 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
554 /// [`Rc::from_raw`][from_raw].
556 /// [from_raw]: struct.Rc.html#method.from_raw
563 /// let x = Rc::new("hello".to_owned());
564 /// let x_ptr = Rc::into_raw(x);
565 /// assert_eq!(unsafe { &*x_ptr }, "hello");
567 #[stable(feature = "rc_raw", since = "1.17.0")]
568 pub fn into_raw(this: Self) -> *const T {
569 let ptr = Self::as_ptr(&this);
574 /// Provides a raw pointer to the data.
576 /// The counts are not affected in any way and the `Rc` is not consumed. The pointer is valid
577 /// for as long there are strong counts in the `Rc`.
584 /// let x = Rc::new("hello".to_owned());
585 /// let y = Rc::clone(&x);
586 /// let x_ptr = Rc::as_ptr(&x);
587 /// assert_eq!(x_ptr, Rc::as_ptr(&y));
588 /// assert_eq!(unsafe { &*x_ptr }, "hello");
590 #[stable(feature = "weak_into_raw", since = "1.45.0")]
591 pub fn as_ptr(this: &Self) -> *const T {
592 let ptr: *mut RcBox<T> = NonNull::as_ptr(this.ptr);
594 // SAFETY: This cannot go through Deref::deref or Rc::inner because
595 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
596 // write through the pointer after the Rc is recovered through `from_raw`.
597 unsafe { &raw const (*ptr).value }
600 /// Constructs an `Rc<T>` from a raw pointer.
602 /// The raw pointer must have been previously returned by a call to
603 /// [`Rc<U>::into_raw`][into_raw] where `U` must have the same size
604 /// and alignment as `T`. This is trivially true if `U` is `T`.
605 /// Note that if `U` is not `T` but has the same size and alignment, this is
606 /// basically like transmuting references of different types. See
607 /// [`mem::transmute`][transmute] for more information on what
608 /// restrictions apply in this case.
610 /// The user of `from_raw` has to make sure a specific value of `T` is only
613 /// This function is unsafe because improper use may lead to memory unsafety,
614 /// even if the returned `Rc<T>` is never accessed.
616 /// [into_raw]: struct.Rc.html#method.into_raw
617 /// [transmute]: ../../std/mem/fn.transmute.html
624 /// let x = Rc::new("hello".to_owned());
625 /// let x_ptr = Rc::into_raw(x);
628 /// // Convert back to an `Rc` to prevent leak.
629 /// let x = Rc::from_raw(x_ptr);
630 /// assert_eq!(&*x, "hello");
632 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
635 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
637 #[stable(feature = "rc_raw", since = "1.17.0")]
638 pub unsafe fn from_raw(ptr: *const T) -> Self {
639 let offset = unsafe { data_offset(ptr) };
641 // Reverse the offset to find the original RcBox.
642 let fake_ptr = ptr as *mut RcBox<T>;
643 let rc_ptr = unsafe { set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset)) };
645 unsafe { Self::from_ptr(rc_ptr) }
648 /// Creates a new [`Weak`][weak] pointer to this allocation.
650 /// [weak]: struct.Weak.html
657 /// let five = Rc::new(5);
659 /// let weak_five = Rc::downgrade(&five);
661 #[stable(feature = "rc_weak", since = "1.4.0")]
662 pub fn downgrade(this: &Self) -> Weak<T> {
664 // Make sure we do not create a dangling Weak
665 debug_assert!(!is_dangling(this.ptr));
666 Weak { ptr: this.ptr }
669 /// Gets the number of [`Weak`][weak] pointers to this allocation.
671 /// [weak]: struct.Weak.html
678 /// let five = Rc::new(5);
679 /// let _weak_five = Rc::downgrade(&five);
681 /// assert_eq!(1, Rc::weak_count(&five));
684 #[stable(feature = "rc_counts", since = "1.15.0")]
685 pub fn weak_count(this: &Self) -> usize {
689 /// Gets the number of strong (`Rc`) pointers to this allocation.
696 /// let five = Rc::new(5);
697 /// let _also_five = Rc::clone(&five);
699 /// assert_eq!(2, Rc::strong_count(&five));
702 #[stable(feature = "rc_counts", since = "1.15.0")]
703 pub fn strong_count(this: &Self) -> usize {
707 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
710 /// [weak]: struct.Weak.html
712 fn is_unique(this: &Self) -> bool {
713 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
716 /// Returns a mutable reference into the given `Rc`, if there are
717 /// no other `Rc` or [`Weak`][weak] pointers to the same allocation.
719 /// Returns [`None`] otherwise, because it is not safe to
720 /// mutate a shared value.
722 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
723 /// the inner value when there are other pointers.
725 /// [weak]: struct.Weak.html
726 /// [`None`]: ../../std/option/enum.Option.html#variant.None
727 /// [make_mut]: struct.Rc.html#method.make_mut
728 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
735 /// let mut x = Rc::new(3);
736 /// *Rc::get_mut(&mut x).unwrap() = 4;
737 /// assert_eq!(*x, 4);
739 /// let _y = Rc::clone(&x);
740 /// assert!(Rc::get_mut(&mut x).is_none());
743 #[stable(feature = "rc_unique", since = "1.4.0")]
744 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
745 if Rc::is_unique(this) { unsafe { Some(Rc::get_mut_unchecked(this)) } } else { None }
748 /// Returns a mutable reference into the given `Rc`,
749 /// without any check.
751 /// See also [`get_mut`], which is safe and does appropriate checks.
753 /// [`get_mut`]: struct.Rc.html#method.get_mut
757 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
758 /// for the duration of the returned borrow.
759 /// This is trivially the case if no such pointers exist,
760 /// for example immediately after `Rc::new`.
765 /// #![feature(get_mut_unchecked)]
769 /// let mut x = Rc::new(String::new());
771 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
773 /// assert_eq!(*x, "foo");
776 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
777 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
778 unsafe { &mut this.ptr.as_mut().value }
782 #[stable(feature = "ptr_eq", since = "1.17.0")]
783 /// Returns `true` if the two `Rc`s point to the same allocation
784 /// (in a vein similar to [`ptr::eq`]).
791 /// let five = Rc::new(5);
792 /// let same_five = Rc::clone(&five);
793 /// let other_five = Rc::new(5);
795 /// assert!(Rc::ptr_eq(&five, &same_five));
796 /// assert!(!Rc::ptr_eq(&five, &other_five));
799 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
800 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
801 this.ptr.as_ptr() == other.ptr.as_ptr()
805 impl<T: Clone> Rc<T> {
806 /// Makes a mutable reference into the given `Rc`.
808 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
809 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
810 /// referred to as clone-on-write.
812 /// If there are no other `Rc` pointers to this allocation, then [`Weak`]
813 /// pointers to this allocation will be disassociated.
815 /// See also [`get_mut`], which will fail rather than cloning.
817 /// [`Weak`]: struct.Weak.html
818 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
819 /// [`get_mut`]: struct.Rc.html#method.get_mut
826 /// let mut data = Rc::new(5);
828 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
829 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
830 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
831 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
832 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
834 /// // Now `data` and `other_data` point to different allocations.
835 /// assert_eq!(*data, 8);
836 /// assert_eq!(*other_data, 12);
839 /// [`Weak`] pointers will be disassociated:
844 /// let mut data = Rc::new(75);
845 /// let weak = Rc::downgrade(&data);
847 /// assert!(75 == *data);
848 /// assert!(75 == *weak.upgrade().unwrap());
850 /// *Rc::make_mut(&mut data) += 1;
852 /// assert!(76 == *data);
853 /// assert!(weak.upgrade().is_none());
856 #[stable(feature = "rc_unique", since = "1.4.0")]
857 pub fn make_mut(this: &mut Self) -> &mut T {
858 if Rc::strong_count(this) != 1 {
859 // Gotta clone the data, there are other Rcs
860 *this = Rc::new((**this).clone())
861 } else if Rc::weak_count(this) != 0 {
862 // Can just steal the data, all that's left is Weaks
864 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
865 mem::swap(this, &mut swap);
867 // Remove implicit strong-weak ref (no need to craft a fake
868 // Weak here -- we know other Weaks can clean up for us)
873 // This unsafety is ok because we're guaranteed that the pointer
874 // returned is the *only* pointer that will ever be returned to T. Our
875 // reference count is guaranteed to be 1 at this point, and we required
876 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
877 // reference to the allocation.
878 unsafe { &mut this.ptr.as_mut().value }
884 #[stable(feature = "rc_downcast", since = "1.29.0")]
885 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
890 /// use std::any::Any;
893 /// fn print_if_string(value: Rc<dyn Any>) {
894 /// if let Ok(string) = value.downcast::<String>() {
895 /// println!("String ({}): {}", string.len(), string);
899 /// let my_string = "Hello World".to_string();
900 /// print_if_string(Rc::new(my_string));
901 /// print_if_string(Rc::new(0i8));
903 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
904 if (*self).is::<T>() {
905 let ptr = self.ptr.cast::<RcBox<T>>();
907 Ok(Rc::from_inner(ptr))
914 impl<T: ?Sized> Rc<T> {
915 /// Allocates an `RcBox<T>` with sufficient space for
916 /// a possibly-unsized inner value where the value has the layout provided.
918 /// The function `mem_to_rcbox` is called with the data pointer
919 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
920 unsafe fn allocate_for_layout(
921 value_layout: Layout,
922 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
924 // Calculate layout using the given value layout.
925 // Previously, layout was calculated on the expression
926 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
927 // reference (see #54908).
928 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
930 // Allocate for the layout.
931 let ptr = Global.alloc(layout).unwrap_or_else(|_| handle_alloc_error(layout));
933 // Initialize the RcBox
934 let inner = mem_to_rcbox(ptr.as_non_null_ptr().as_ptr());
936 debug_assert_eq!(Layout::for_value(&*inner), layout);
938 ptr::write(&mut (*inner).strong, Cell::new(1));
939 ptr::write(&mut (*inner).weak, Cell::new(1));
945 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
946 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
947 // Allocate for the `RcBox<T>` using the given value.
949 Self::allocate_for_layout(Layout::for_value(&*ptr), |mem| {
950 set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>
955 fn from_box(v: Box<T>) -> Rc<T> {
957 let box_unique = Box::into_unique(v);
958 let bptr = box_unique.as_ptr();
960 let value_size = size_of_val(&*bptr);
961 let ptr = Self::allocate_for_ptr(bptr);
963 // Copy value as bytes
964 ptr::copy_nonoverlapping(
965 bptr as *const T as *const u8,
966 &mut (*ptr).value as *mut _ as *mut u8,
970 // Free the allocation without dropping its contents
971 box_free(box_unique);
979 /// Allocates an `RcBox<[T]>` with the given length.
980 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
982 Self::allocate_for_layout(Layout::array::<T>(len).unwrap(), |mem| {
983 ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>
989 /// Sets the data pointer of a `?Sized` raw pointer.
991 /// For a slice/trait object, this sets the `data` field and leaves the rest
992 /// unchanged. For a sized raw pointer, this simply sets the pointer.
993 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
995 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
1001 /// Copy elements from slice into newly allocated Rc<\[T\]>
1003 /// Unsafe because the caller must either take ownership or bind `T: Copy`
1004 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
1006 let ptr = Self::allocate_for_slice(v.len());
1007 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).value as *mut [T] as *mut T, v.len());
1012 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
1014 /// Behavior is undefined should the size be wrong.
1015 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
1016 // Panic guard while cloning T elements.
1017 // In the event of a panic, elements that have been written
1018 // into the new RcBox will be dropped, then the memory freed.
1026 impl<T> Drop for Guard<T> {
1027 fn drop(&mut self) {
1029 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1030 ptr::drop_in_place(slice);
1032 Global.dealloc(self.mem, self.layout);
1038 let ptr = Self::allocate_for_slice(len);
1040 let mem = ptr as *mut _ as *mut u8;
1041 let layout = Layout::for_value(&*ptr);
1043 // Pointer to first element
1044 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1046 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1048 for (i, item) in iter.enumerate() {
1049 ptr::write(elems.add(i), item);
1053 // All clear. Forget the guard so it doesn't free the new RcBox.
1061 /// Specialization trait used for `From<&[T]>`.
1062 trait RcFromSlice<T> {
1063 fn from_slice(slice: &[T]) -> Self;
1066 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
1068 default fn from_slice(v: &[T]) -> Self {
1069 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1073 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
1075 fn from_slice(v: &[T]) -> Self {
1076 unsafe { Rc::copy_from_slice(v) }
1080 #[stable(feature = "rust1", since = "1.0.0")]
1081 impl<T: ?Sized> Deref for Rc<T> {
1085 fn deref(&self) -> &T {
1090 #[unstable(feature = "receiver_trait", issue = "none")]
1091 impl<T: ?Sized> Receiver for Rc<T> {}
1093 #[stable(feature = "rust1", since = "1.0.0")]
1094 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1097 /// This will decrement the strong reference count. If the strong reference
1098 /// count reaches zero then the only other references (if any) are
1099 /// [`Weak`], so we `drop` the inner value.
1104 /// use std::rc::Rc;
1108 /// impl Drop for Foo {
1109 /// fn drop(&mut self) {
1110 /// println!("dropped!");
1114 /// let foo = Rc::new(Foo);
1115 /// let foo2 = Rc::clone(&foo);
1117 /// drop(foo); // Doesn't print anything
1118 /// drop(foo2); // Prints "dropped!"
1121 /// [`Weak`]: ../../std/rc/struct.Weak.html
1122 fn drop(&mut self) {
1125 if self.strong() == 0 {
1126 // destroy the contained object
1127 ptr::drop_in_place(self.ptr.as_mut());
1129 // remove the implicit "strong weak" pointer now that we've
1130 // destroyed the contents.
1133 if self.weak() == 0 {
1134 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1141 #[stable(feature = "rust1", since = "1.0.0")]
1142 impl<T: ?Sized> Clone for Rc<T> {
1143 /// Makes a clone of the `Rc` pointer.
1145 /// This creates another pointer to the same allocation, increasing the
1146 /// strong reference count.
1151 /// use std::rc::Rc;
1153 /// let five = Rc::new(5);
1155 /// let _ = Rc::clone(&five);
1158 fn clone(&self) -> Rc<T> {
1160 Self::from_inner(self.ptr)
1164 #[stable(feature = "rust1", since = "1.0.0")]
1165 impl<T: Default> Default for Rc<T> {
1166 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1171 /// use std::rc::Rc;
1173 /// let x: Rc<i32> = Default::default();
1174 /// assert_eq!(*x, 0);
1177 fn default() -> Rc<T> {
1178 Rc::new(Default::default())
1182 #[stable(feature = "rust1", since = "1.0.0")]
1183 trait RcEqIdent<T: ?Sized + PartialEq> {
1184 fn eq(&self, other: &Rc<T>) -> bool;
1185 fn ne(&self, other: &Rc<T>) -> bool;
1188 #[stable(feature = "rust1", since = "1.0.0")]
1189 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
1191 default fn eq(&self, other: &Rc<T>) -> bool {
1196 default fn ne(&self, other: &Rc<T>) -> bool {
1201 // Hack to allow specializing on `Eq` even though `Eq` has a method.
1202 #[rustc_unsafe_specialization_marker]
1203 pub(crate) trait MarkerEq: PartialEq<Self> {}
1205 impl<T: Eq> MarkerEq for T {}
1207 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1208 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1209 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1210 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1211 /// the same value, than two `&T`s.
1213 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1214 #[stable(feature = "rust1", since = "1.0.0")]
1215 impl<T: ?Sized + MarkerEq> RcEqIdent<T> for Rc<T> {
1217 fn eq(&self, other: &Rc<T>) -> bool {
1218 Rc::ptr_eq(self, other) || **self == **other
1222 fn ne(&self, other: &Rc<T>) -> bool {
1223 !Rc::ptr_eq(self, other) && **self != **other
1227 #[stable(feature = "rust1", since = "1.0.0")]
1228 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1229 /// Equality for two `Rc`s.
1231 /// Two `Rc`s are equal if their inner values are equal, even if they are
1232 /// stored in different allocation.
1234 /// If `T` also implements `Eq` (implying reflexivity of equality),
1235 /// two `Rc`s that point to the same allocation are
1241 /// use std::rc::Rc;
1243 /// let five = Rc::new(5);
1245 /// assert!(five == Rc::new(5));
1248 fn eq(&self, other: &Rc<T>) -> bool {
1249 RcEqIdent::eq(self, other)
1252 /// Inequality for two `Rc`s.
1254 /// Two `Rc`s are unequal if their inner values are unequal.
1256 /// If `T` also implements `Eq` (implying reflexivity of equality),
1257 /// two `Rc`s that point to the same allocation are
1263 /// use std::rc::Rc;
1265 /// let five = Rc::new(5);
1267 /// assert!(five != Rc::new(6));
1270 fn ne(&self, other: &Rc<T>) -> bool {
1271 RcEqIdent::ne(self, other)
1275 #[stable(feature = "rust1", since = "1.0.0")]
1276 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1278 #[stable(feature = "rust1", since = "1.0.0")]
1279 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1280 /// Partial comparison for two `Rc`s.
1282 /// The two are compared by calling `partial_cmp()` on their inner values.
1287 /// use std::rc::Rc;
1288 /// use std::cmp::Ordering;
1290 /// let five = Rc::new(5);
1292 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1295 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1296 (**self).partial_cmp(&**other)
1299 /// Less-than comparison for two `Rc`s.
1301 /// The two are compared by calling `<` on their inner values.
1306 /// use std::rc::Rc;
1308 /// let five = Rc::new(5);
1310 /// assert!(five < Rc::new(6));
1313 fn lt(&self, other: &Rc<T>) -> bool {
1317 /// 'Less than or equal to' comparison for two `Rc`s.
1319 /// The two are compared by calling `<=` on their inner values.
1324 /// use std::rc::Rc;
1326 /// let five = Rc::new(5);
1328 /// assert!(five <= Rc::new(5));
1331 fn le(&self, other: &Rc<T>) -> bool {
1335 /// Greater-than comparison for two `Rc`s.
1337 /// The two are compared by calling `>` on their inner values.
1342 /// use std::rc::Rc;
1344 /// let five = Rc::new(5);
1346 /// assert!(five > Rc::new(4));
1349 fn gt(&self, other: &Rc<T>) -> bool {
1353 /// 'Greater than or equal to' comparison for two `Rc`s.
1355 /// The two are compared by calling `>=` on their inner values.
1360 /// use std::rc::Rc;
1362 /// let five = Rc::new(5);
1364 /// assert!(five >= Rc::new(5));
1367 fn ge(&self, other: &Rc<T>) -> bool {
1372 #[stable(feature = "rust1", since = "1.0.0")]
1373 impl<T: ?Sized + Ord> Ord for Rc<T> {
1374 /// Comparison for two `Rc`s.
1376 /// The two are compared by calling `cmp()` on their inner values.
1381 /// use std::rc::Rc;
1382 /// use std::cmp::Ordering;
1384 /// let five = Rc::new(5);
1386 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1389 fn cmp(&self, other: &Rc<T>) -> Ordering {
1390 (**self).cmp(&**other)
1394 #[stable(feature = "rust1", since = "1.0.0")]
1395 impl<T: ?Sized + Hash> Hash for Rc<T> {
1396 fn hash<H: Hasher>(&self, state: &mut H) {
1397 (**self).hash(state);
1401 #[stable(feature = "rust1", since = "1.0.0")]
1402 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1403 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1404 fmt::Display::fmt(&**self, f)
1408 #[stable(feature = "rust1", since = "1.0.0")]
1409 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1410 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1411 fmt::Debug::fmt(&**self, f)
1415 #[stable(feature = "rust1", since = "1.0.0")]
1416 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1417 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1418 fmt::Pointer::fmt(&(&**self as *const T), f)
1422 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1423 impl<T> From<T> for Rc<T> {
1424 fn from(t: T) -> Self {
1429 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1430 impl<T: Clone> From<&[T]> for Rc<[T]> {
1432 fn from(v: &[T]) -> Rc<[T]> {
1433 <Self as RcFromSlice<T>>::from_slice(v)
1437 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1438 impl From<&str> for Rc<str> {
1440 fn from(v: &str) -> Rc<str> {
1441 let rc = Rc::<[u8]>::from(v.as_bytes());
1442 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1446 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1447 impl From<String> for Rc<str> {
1449 fn from(v: String) -> Rc<str> {
1454 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1455 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1457 fn from(v: Box<T>) -> Rc<T> {
1462 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1463 impl<T> From<Vec<T>> for Rc<[T]> {
1465 fn from(mut v: Vec<T>) -> Rc<[T]> {
1467 let rc = Rc::copy_from_slice(&v);
1469 // Allow the Vec to free its memory, but not destroy its contents
1477 #[stable(feature = "shared_from_cow", since = "1.45.0")]
1478 impl<'a, B> From<Cow<'a, B>> for Rc<B>
1480 B: ToOwned + ?Sized,
1481 Rc<B>: From<&'a B> + From<B::Owned>,
1484 fn from(cow: Cow<'a, B>) -> Rc<B> {
1486 Cow::Borrowed(s) => Rc::from(s),
1487 Cow::Owned(s) => Rc::from(s),
1492 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1493 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]> {
1494 type Error = Rc<[T]>;
1496 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1497 if boxed_slice.len() == N {
1498 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
1505 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1506 impl<T> iter::FromIterator<T> for Rc<[T]> {
1507 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1509 /// # Performance characteristics
1511 /// ## The general case
1513 /// In the general case, collecting into `Rc<[T]>` is done by first
1514 /// collecting into a `Vec<T>`. That is, when writing the following:
1517 /// # use std::rc::Rc;
1518 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1519 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1522 /// this behaves as if we wrote:
1525 /// # use std::rc::Rc;
1526 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1527 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1528 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1529 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1532 /// This will allocate as many times as needed for constructing the `Vec<T>`
1533 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1535 /// ## Iterators of known length
1537 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1538 /// a single allocation will be made for the `Rc<[T]>`. For example:
1541 /// # use std::rc::Rc;
1542 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1543 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1545 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1546 ToRcSlice::to_rc_slice(iter.into_iter())
1550 /// Specialization trait used for collecting into `Rc<[T]>`.
1551 trait ToRcSlice<T>: Iterator<Item = T> + Sized {
1552 fn to_rc_slice(self) -> Rc<[T]>;
1555 impl<T, I: Iterator<Item = T>> ToRcSlice<T> for I {
1556 default fn to_rc_slice(self) -> Rc<[T]> {
1557 self.collect::<Vec<T>>().into()
1561 impl<T, I: iter::TrustedLen<Item = T>> ToRcSlice<T> for I {
1562 fn to_rc_slice(self) -> Rc<[T]> {
1563 // This is the case for a `TrustedLen` iterator.
1564 let (low, high) = self.size_hint();
1565 if let Some(high) = high {
1569 "TrustedLen iterator's size hint is not exact: {:?}",
1574 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1575 Rc::from_iter_exact(self, low)
1578 // Fall back to normal implementation.
1579 self.collect::<Vec<T>>().into()
1584 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1585 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
1586 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1588 /// Since a `Weak` reference does not count towards ownership, it will not
1589 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
1590 /// guarantees about the value still being present. Thus it may return [`None`]
1591 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
1592 /// itself (the backing store) from being deallocated.
1594 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
1595 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
1596 /// prevent circular references between [`Rc`] pointers, since mutual owning references
1597 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1598 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1599 /// pointers from children back to their parents.
1601 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1603 /// [`Rc`]: struct.Rc.html
1604 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1605 /// [`upgrade`]: struct.Weak.html#method.upgrade
1606 /// [`Option`]: ../../std/option/enum.Option.html
1607 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1608 #[stable(feature = "rc_weak", since = "1.4.0")]
1609 pub struct Weak<T: ?Sized> {
1610 // This is a `NonNull` to allow optimizing the size of this type in enums,
1611 // but it is not necessarily a valid pointer.
1612 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1613 // to allocate space on the heap. That's not a value a real pointer
1614 // will ever have because RcBox has alignment at least 2.
1615 // This is only possible when `T: Sized`; unsized `T` never dangle.
1616 ptr: NonNull<RcBox<T>>,
1619 #[stable(feature = "rc_weak", since = "1.4.0")]
1620 impl<T: ?Sized> !marker::Send for Weak<T> {}
1621 #[stable(feature = "rc_weak", since = "1.4.0")]
1622 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1624 #[unstable(feature = "coerce_unsized", issue = "27732")]
1625 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1627 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1628 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1631 /// Constructs a new `Weak<T>`, without allocating any memory.
1632 /// Calling [`upgrade`] on the return value always gives [`None`].
1634 /// [`upgrade`]: #method.upgrade
1635 /// [`None`]: ../../std/option/enum.Option.html
1640 /// use std::rc::Weak;
1642 /// let empty: Weak<i64> = Weak::new();
1643 /// assert!(empty.upgrade().is_none());
1645 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1646 pub fn new() -> Weak<T> {
1647 Weak { ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0") }
1650 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1652 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
1653 /// unaligned or even [`null`] otherwise.
1658 /// use std::rc::Rc;
1661 /// let strong = Rc::new("hello".to_owned());
1662 /// let weak = Rc::downgrade(&strong);
1663 /// // Both point to the same object
1664 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
1665 /// // The strong here keeps it alive, so we can still access the object.
1666 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
1669 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
1670 /// // undefined behaviour.
1671 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
1674 /// [`null`]: ../../std/ptr/fn.null.html
1675 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
1676 pub fn as_ptr(&self) -> *const T {
1677 let ptr: *mut RcBox<T> = NonNull::as_ptr(self.ptr);
1679 // SAFETY: we must offset the pointer manually, and said pointer may be
1680 // a dangling weak (usize::MAX) if T is sized. data_offset is safe to call,
1681 // because we know that a pointer to unsized T was derived from a real
1682 // unsized T, as dangling weaks are only created for sized T. wrapping_offset
1683 // is used so that we can use the same code path for the non-dangling
1684 // unsized case and the potentially dangling sized case.
1686 let offset = data_offset(ptr as *mut T);
1687 set_data_ptr(ptr as *mut T, (ptr as *mut u8).wrapping_offset(offset))
1691 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1693 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
1694 /// one weak reference (the weak count is not modified by this operation). It can be turned
1695 /// back into the `Weak<T>` with [`from_raw`].
1697 /// The same restrictions of accessing the target of the pointer as with
1698 /// [`as_ptr`] apply.
1703 /// use std::rc::{Rc, Weak};
1705 /// let strong = Rc::new("hello".to_owned());
1706 /// let weak = Rc::downgrade(&strong);
1707 /// let raw = weak.into_raw();
1709 /// assert_eq!(1, Rc::weak_count(&strong));
1710 /// assert_eq!("hello", unsafe { &*raw });
1712 /// drop(unsafe { Weak::from_raw(raw) });
1713 /// assert_eq!(0, Rc::weak_count(&strong));
1716 /// [`from_raw`]: struct.Weak.html#method.from_raw
1717 /// [`as_ptr`]: struct.Weak.html#method.as_ptr
1718 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1719 pub fn into_raw(self) -> *const T {
1720 let result = self.as_ptr();
1725 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1727 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1728 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1730 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
1731 /// as these don't own anything; the method still works on them).
1735 /// The pointer must have originated from the [`into_raw`] and must still own its potential
1738 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
1739 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
1740 /// count is not modified by this operation) and therefore it must be paired with a previous
1741 /// call to [`into_raw`].
1746 /// use std::rc::{Rc, Weak};
1748 /// let strong = Rc::new("hello".to_owned());
1750 /// let raw_1 = Rc::downgrade(&strong).into_raw();
1751 /// let raw_2 = Rc::downgrade(&strong).into_raw();
1753 /// assert_eq!(2, Rc::weak_count(&strong));
1755 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1756 /// assert_eq!(1, Rc::weak_count(&strong));
1760 /// // Decrement the last weak count.
1761 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1764 /// [`into_raw`]: struct.Weak.html#method.into_raw
1765 /// [`upgrade`]: struct.Weak.html#method.upgrade
1766 /// [`Rc`]: struct.Rc.html
1767 /// [`Weak`]: struct.Weak.html
1768 /// [`new`]: struct.Weak.html#method.new
1769 /// [`forget`]: ../../std/mem/fn.forget.html
1770 #[stable(feature = "weak_into_raw", since = "1.45.0")]
1771 pub unsafe fn from_raw(ptr: *const T) -> Self {
1775 // See Rc::from_raw for details
1777 let offset = data_offset(ptr);
1778 let fake_ptr = ptr as *mut RcBox<T>;
1779 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1780 Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
1786 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1787 let address = ptr.as_ptr() as *mut () as usize;
1788 address == usize::MAX
1791 impl<T: ?Sized> Weak<T> {
1792 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
1793 /// dropping of the inner value if successful.
1795 /// Returns [`None`] if the inner value has since been dropped.
1797 /// [`Rc`]: struct.Rc.html
1798 /// [`None`]: ../../std/option/enum.Option.html
1803 /// use std::rc::Rc;
1805 /// let five = Rc::new(5);
1807 /// let weak_five = Rc::downgrade(&five);
1809 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1810 /// assert!(strong_five.is_some());
1812 /// // Destroy all strong pointers.
1813 /// drop(strong_five);
1816 /// assert!(weak_five.upgrade().is_none());
1818 #[stable(feature = "rc_weak", since = "1.4.0")]
1819 pub fn upgrade(&self) -> Option<Rc<T>> {
1820 let inner = self.inner()?;
1821 if inner.strong() == 0 {
1825 Some(Rc::from_inner(self.ptr))
1829 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
1831 /// If `self` was created using [`Weak::new`], this will return 0.
1833 /// [`Weak::new`]: #method.new
1834 #[stable(feature = "weak_counts", since = "1.41.0")]
1835 pub fn strong_count(&self) -> usize {
1836 if let Some(inner) = self.inner() { inner.strong() } else { 0 }
1839 /// Gets the number of `Weak` pointers pointing to this allocation.
1841 /// If no strong pointers remain, this will return zero.
1842 #[stable(feature = "weak_counts", since = "1.41.0")]
1843 pub fn weak_count(&self) -> usize {
1846 if inner.strong() > 0 {
1847 inner.weak() - 1 // subtract the implicit weak ptr
1855 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1856 /// (i.e., when this `Weak` was created by `Weak::new`).
1858 fn inner(&self) -> Option<&RcBox<T>> {
1859 if is_dangling(self.ptr) { None } else { Some(unsafe { self.ptr.as_ref() }) }
1862 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1863 /// [`ptr::eq`]), or if both don't point to any allocation
1864 /// (because they were created with `Weak::new()`).
1868 /// Since this compares pointers it means that `Weak::new()` will equal each
1869 /// other, even though they don't point to any allocation.
1874 /// use std::rc::Rc;
1876 /// let first_rc = Rc::new(5);
1877 /// let first = Rc::downgrade(&first_rc);
1878 /// let second = Rc::downgrade(&first_rc);
1880 /// assert!(first.ptr_eq(&second));
1882 /// let third_rc = Rc::new(5);
1883 /// let third = Rc::downgrade(&third_rc);
1885 /// assert!(!first.ptr_eq(&third));
1888 /// Comparing `Weak::new`.
1891 /// use std::rc::{Rc, Weak};
1893 /// let first = Weak::new();
1894 /// let second = Weak::new();
1895 /// assert!(first.ptr_eq(&second));
1897 /// let third_rc = Rc::new(());
1898 /// let third = Rc::downgrade(&third_rc);
1899 /// assert!(!first.ptr_eq(&third));
1902 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1904 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1905 pub fn ptr_eq(&self, other: &Self) -> bool {
1906 self.ptr.as_ptr() == other.ptr.as_ptr()
1910 #[stable(feature = "rc_weak", since = "1.4.0")]
1911 impl<T: ?Sized> Drop for Weak<T> {
1912 /// Drops the `Weak` pointer.
1917 /// use std::rc::{Rc, Weak};
1921 /// impl Drop for Foo {
1922 /// fn drop(&mut self) {
1923 /// println!("dropped!");
1927 /// let foo = Rc::new(Foo);
1928 /// let weak_foo = Rc::downgrade(&foo);
1929 /// let other_weak_foo = Weak::clone(&weak_foo);
1931 /// drop(weak_foo); // Doesn't print anything
1932 /// drop(foo); // Prints "dropped!"
1934 /// assert!(other_weak_foo.upgrade().is_none());
1936 fn drop(&mut self) {
1937 if let Some(inner) = self.inner() {
1939 // the weak count starts at 1, and will only go to zero if all
1940 // the strong pointers have disappeared.
1941 if inner.weak() == 0 {
1943 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1950 #[stable(feature = "rc_weak", since = "1.4.0")]
1951 impl<T: ?Sized> Clone for Weak<T> {
1952 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1957 /// use std::rc::{Rc, Weak};
1959 /// let weak_five = Rc::downgrade(&Rc::new(5));
1961 /// let _ = Weak::clone(&weak_five);
1964 fn clone(&self) -> Weak<T> {
1965 if let Some(inner) = self.inner() {
1968 Weak { ptr: self.ptr }
1972 #[stable(feature = "rc_weak", since = "1.4.0")]
1973 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1974 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1979 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1980 impl<T> Default for Weak<T> {
1981 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1982 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1984 /// [`None`]: ../../std/option/enum.Option.html
1985 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
1990 /// use std::rc::Weak;
1992 /// let empty: Weak<i64> = Default::default();
1993 /// assert!(empty.upgrade().is_none());
1995 fn default() -> Weak<T> {
2000 // NOTE: We checked_add here to deal with mem::forget safely. In particular
2001 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
2002 // you can free the allocation while outstanding Rcs (or Weaks) exist.
2003 // We abort because this is such a degenerate scenario that we don't care about
2004 // what happens -- no real program should ever experience this.
2006 // This should have negligible overhead since you don't actually need to
2007 // clone these much in Rust thanks to ownership and move-semantics.
2010 trait RcBoxPtr<T: ?Sized> {
2011 fn inner(&self) -> &RcBox<T>;
2014 fn strong(&self) -> usize {
2015 self.inner().strong.get()
2019 fn inc_strong(&self) {
2020 let strong = self.strong();
2022 // We want to abort on overflow instead of dropping the value.
2023 // The reference count will never be zero when this is called;
2024 // nevertheless, we insert an abort here to hint LLVM at
2025 // an otherwise missed optimization.
2026 if strong == 0 || strong == usize::MAX {
2029 self.inner().strong.set(strong + 1);
2033 fn dec_strong(&self) {
2034 self.inner().strong.set(self.strong() - 1);
2038 fn weak(&self) -> usize {
2039 self.inner().weak.get()
2043 fn inc_weak(&self) {
2044 let weak = self.weak();
2046 // We want to abort on overflow instead of dropping the value.
2047 // The reference count will never be zero when this is called;
2048 // nevertheless, we insert an abort here to hint LLVM at
2049 // an otherwise missed optimization.
2050 if weak == 0 || weak == usize::MAX {
2053 self.inner().weak.set(weak + 1);
2057 fn dec_weak(&self) {
2058 self.inner().weak.set(self.weak() - 1);
2062 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
2064 fn inner(&self) -> &RcBox<T> {
2065 unsafe { self.ptr.as_ref() }
2069 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
2071 fn inner(&self) -> &RcBox<T> {
2076 #[stable(feature = "rust1", since = "1.0.0")]
2077 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2078 fn borrow(&self) -> &T {
2083 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2084 impl<T: ?Sized> AsRef<T> for Rc<T> {
2085 fn as_ref(&self) -> &T {
2090 #[stable(feature = "pin", since = "1.33.0")]
2091 impl<T: ?Sized> Unpin for Rc<T> {}
2093 /// Get the offset within an `ArcInner` for
2094 /// a payload of type described by a pointer.
2098 /// This has the same safety requirements as `align_of_val_raw`. In effect:
2100 /// - This function is safe for any argument if `T` is sized, and
2101 /// - if `T` is unsized, the pointer must have appropriate pointer metadata
2102 /// acquired from the real instance that you are getting this offset for.
2103 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2104 // Align the unsized value to the end of the `RcBox`.
2105 // Because it is ?Sized, it will always be the last field in memory.
2106 // Note: This is a detail of the current implementation of the compiler,
2107 // and is not a guaranteed language detail. Do not rely on it outside of std.
2108 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2112 fn data_offset_align(align: usize) -> isize {
2113 let layout = Layout::new::<RcBox<()>>();
2114 (layout.size() + layout.padding_needed_for(align)) as isize