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 RcBox { strong: Cell::new(1), weak: Cell::new(1), value }).into(),
377 /// Constructs a new `Rc<T>` using a closure `data_fn` that has access to a
378 /// weak reference to the constructing `Rc<T>`.
380 /// Generally, a structure circularly referencing itself, either directly or
381 /// indirectly, should not hold a strong reference to prevent a memory leak.
382 /// In `data_fn`, initialization of `T` can make use of the weak reference
383 /// by cloning and storing it inside `T` for use at a later time.
385 /// Since the new `Rc<T>` is not fully-constructed until `Rc<T>::new_cyclic`
386 /// returns, calling [`upgrade`] on the weak reference inside `data_fn` will
387 /// fail and result in a `None` value.
390 /// If `data_fn` panics, the panic is propagated to the caller, and the
391 /// temporary [`Weak<T>`] is dropped normally.
396 /// #![allow(dead_code)]
397 /// use std::rc::{Rc, Weak};
400 /// me: Weak<Gadget>,
404 /// /// Construct a reference counted Gadget.
405 /// fn new() -> Rc<Self> {
406 /// Rc::new_cyclic(|me| Gadget { me: me.clone() })
409 /// /// Return a reference counted pointer to Self.
410 /// fn me(&self) -> Rc<Self> {
411 /// self.me.upgrade().unwrap()
415 /// [`upgrade`]: Weak::upgrade
416 #[cfg(not(no_global_oom_handling))]
417 #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
418 pub fn new_cyclic<F>(data_fn: F) -> Rc<T>
420 F: FnOnce(&Weak<T>) -> T,
422 // Construct the inner in the "uninitialized" state with a single
424 let uninit_ptr: NonNull<_> = Box::leak(box RcBox {
425 strong: Cell::new(0),
427 value: mem::MaybeUninit::<T>::uninit(),
431 let init_ptr: NonNull<RcBox<T>> = uninit_ptr.cast();
433 let weak = Weak { ptr: init_ptr };
435 // It's important we don't give up ownership of the weak pointer, or
436 // else the memory might be freed by the time `data_fn` returns. If
437 // we really wanted to pass ownership, we could create an additional
438 // weak pointer for ourselves, but this would result in additional
439 // updates to the weak reference count which might not be necessary
441 let data = data_fn(&weak);
443 let strong = unsafe {
444 let inner = init_ptr.as_ptr();
445 ptr::write(ptr::addr_of_mut!((*inner).value), data);
447 let prev_value = (*inner).strong.get();
448 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
449 (*inner).strong.set(1);
451 Rc::from_inner(init_ptr)
454 // Strong references should collectively own a shared weak reference,
455 // so don't run the destructor for our old weak reference.
460 /// Constructs a new `Rc` with uninitialized contents.
465 /// #![feature(new_uninit)]
466 /// #![feature(get_mut_unchecked)]
470 /// let mut five = Rc::<u32>::new_uninit();
472 /// // Deferred initialization:
473 /// Rc::get_mut(&mut five).unwrap().write(5);
475 /// let five = unsafe { five.assume_init() };
477 /// assert_eq!(*five, 5)
479 #[cfg(not(no_global_oom_handling))]
480 #[unstable(feature = "new_uninit", issue = "63291")]
482 pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
484 Rc::from_ptr(Rc::allocate_for_layout(
486 |layout| Global.allocate(layout),
487 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
492 /// Constructs a new `Rc` with uninitialized contents, with the memory
493 /// being filled with `0` bytes.
495 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
496 /// incorrect usage of this method.
501 /// #![feature(new_uninit)]
505 /// let zero = Rc::<u32>::new_zeroed();
506 /// let zero = unsafe { zero.assume_init() };
508 /// assert_eq!(*zero, 0)
511 /// [zeroed]: mem::MaybeUninit::zeroed
512 #[cfg(not(no_global_oom_handling))]
513 #[unstable(feature = "new_uninit", issue = "63291")]
515 pub fn new_zeroed() -> Rc<mem::MaybeUninit<T>> {
517 Rc::from_ptr(Rc::allocate_for_layout(
519 |layout| Global.allocate_zeroed(layout),
520 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
525 /// Constructs a new `Rc<T>`, returning an error if the allocation fails
530 /// #![feature(allocator_api)]
533 /// let five = Rc::try_new(5);
534 /// # Ok::<(), std::alloc::AllocError>(())
536 #[unstable(feature = "allocator_api", issue = "32838")]
537 pub fn try_new(value: T) -> Result<Rc<T>, AllocError> {
538 // There is an implicit weak pointer owned by all the strong
539 // pointers, which ensures that the weak destructor never frees
540 // the allocation while the strong destructor is running, even
541 // if the weak pointer is stored inside the strong one.
544 Box::leak(Box::try_new(RcBox { strong: Cell::new(1), weak: Cell::new(1), value })?)
550 /// Constructs a new `Rc` with uninitialized contents, returning an error if the allocation fails
555 /// #![feature(allocator_api, new_uninit)]
556 /// #![feature(get_mut_unchecked)]
560 /// let mut five = Rc::<u32>::try_new_uninit()?;
562 /// // Deferred initialization:
563 /// Rc::get_mut(&mut five).unwrap().write(5);
565 /// let five = unsafe { five.assume_init() };
567 /// assert_eq!(*five, 5);
568 /// # Ok::<(), std::alloc::AllocError>(())
570 #[unstable(feature = "allocator_api", issue = "32838")]
571 // #[unstable(feature = "new_uninit", issue = "63291")]
572 pub fn try_new_uninit() -> Result<Rc<mem::MaybeUninit<T>>, AllocError> {
574 Ok(Rc::from_ptr(Rc::try_allocate_for_layout(
576 |layout| Global.allocate(layout),
577 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
582 /// Constructs a new `Rc` with uninitialized contents, with the memory
583 /// being filled with `0` bytes, returning an error if the allocation fails
585 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
586 /// incorrect usage of this method.
591 /// #![feature(allocator_api, new_uninit)]
595 /// let zero = Rc::<u32>::try_new_zeroed()?;
596 /// let zero = unsafe { zero.assume_init() };
598 /// assert_eq!(*zero, 0);
599 /// # Ok::<(), std::alloc::AllocError>(())
602 /// [zeroed]: mem::MaybeUninit::zeroed
603 #[unstable(feature = "allocator_api", issue = "32838")]
604 //#[unstable(feature = "new_uninit", issue = "63291")]
605 pub fn try_new_zeroed() -> Result<Rc<mem::MaybeUninit<T>>, AllocError> {
607 Ok(Rc::from_ptr(Rc::try_allocate_for_layout(
609 |layout| Global.allocate_zeroed(layout),
610 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
614 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
615 /// `value` will be pinned in memory and unable to be moved.
616 #[cfg(not(no_global_oom_handling))]
617 #[stable(feature = "pin", since = "1.33.0")]
619 pub fn pin(value: T) -> Pin<Rc<T>> {
620 unsafe { Pin::new_unchecked(Rc::new(value)) }
623 /// Returns the inner value, if the `Rc` has exactly one strong reference.
625 /// Otherwise, an [`Err`] is returned with the same `Rc` that was
628 /// This will succeed even if there are outstanding weak references.
635 /// let x = Rc::new(3);
636 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
638 /// let x = Rc::new(4);
639 /// let _y = Rc::clone(&x);
640 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
643 #[stable(feature = "rc_unique", since = "1.4.0")]
644 pub fn try_unwrap(this: Self) -> Result<T, Self> {
645 if Rc::strong_count(&this) == 1 {
647 let val = ptr::read(&*this); // copy the contained object
649 // Indicate to Weaks that they can't be promoted by decrementing
650 // the strong count, and then remove the implicit "strong weak"
651 // pointer while also handling drop logic by just crafting a
653 this.inner().dec_strong();
654 let _weak = Weak { ptr: this.ptr };
665 /// Constructs a new reference-counted slice with uninitialized contents.
670 /// #![feature(new_uninit)]
671 /// #![feature(get_mut_unchecked)]
675 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
677 /// // Deferred initialization:
678 /// let data = Rc::get_mut(&mut values).unwrap();
679 /// data[0].write(1);
680 /// data[1].write(2);
681 /// data[2].write(3);
683 /// let values = unsafe { values.assume_init() };
685 /// assert_eq!(*values, [1, 2, 3])
687 #[cfg(not(no_global_oom_handling))]
688 #[unstable(feature = "new_uninit", issue = "63291")]
690 pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
691 unsafe { Rc::from_ptr(Rc::allocate_for_slice(len)) }
694 /// Constructs a new reference-counted slice with uninitialized contents, with the memory being
695 /// filled with `0` bytes.
697 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
698 /// incorrect usage of this method.
703 /// #![feature(new_uninit)]
707 /// let values = Rc::<[u32]>::new_zeroed_slice(3);
708 /// let values = unsafe { values.assume_init() };
710 /// assert_eq!(*values, [0, 0, 0])
713 /// [zeroed]: mem::MaybeUninit::zeroed
714 #[cfg(not(no_global_oom_handling))]
715 #[unstable(feature = "new_uninit", issue = "63291")]
717 pub fn new_zeroed_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
719 Rc::from_ptr(Rc::allocate_for_layout(
720 Layout::array::<T>(len).unwrap(),
721 |layout| Global.allocate_zeroed(layout),
723 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
724 as *mut RcBox<[mem::MaybeUninit<T>]>
731 impl<T> Rc<mem::MaybeUninit<T>> {
732 /// Converts to `Rc<T>`.
736 /// As with [`MaybeUninit::assume_init`],
737 /// it is up to the caller to guarantee that the inner value
738 /// really is in an initialized state.
739 /// Calling this when the content is not yet fully initialized
740 /// causes immediate undefined behavior.
742 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
747 /// #![feature(new_uninit)]
748 /// #![feature(get_mut_unchecked)]
752 /// let mut five = Rc::<u32>::new_uninit();
754 /// // Deferred initialization:
755 /// Rc::get_mut(&mut five).unwrap().write(5);
757 /// let five = unsafe { five.assume_init() };
759 /// assert_eq!(*five, 5)
761 #[unstable(feature = "new_uninit", issue = "63291")]
763 pub unsafe fn assume_init(self) -> Rc<T> {
764 unsafe { Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast()) }
768 impl<T> Rc<[mem::MaybeUninit<T>]> {
769 /// Converts to `Rc<[T]>`.
773 /// As with [`MaybeUninit::assume_init`],
774 /// it is up to the caller to guarantee that the inner value
775 /// really is in an initialized state.
776 /// Calling this when the content is not yet fully initialized
777 /// causes immediate undefined behavior.
779 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
784 /// #![feature(new_uninit)]
785 /// #![feature(get_mut_unchecked)]
789 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
791 /// // Deferred initialization:
792 /// let data = Rc::get_mut(&mut values).unwrap();
793 /// data[0].write(1);
794 /// data[1].write(2);
795 /// data[2].write(3);
797 /// let values = unsafe { values.assume_init() };
799 /// assert_eq!(*values, [1, 2, 3])
801 #[unstable(feature = "new_uninit", issue = "63291")]
803 pub unsafe fn assume_init(self) -> Rc<[T]> {
804 unsafe { Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
808 impl<T: ?Sized> Rc<T> {
809 /// Consumes the `Rc`, returning the wrapped pointer.
811 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
812 /// [`Rc::from_raw`].
819 /// let x = Rc::new("hello".to_owned());
820 /// let x_ptr = Rc::into_raw(x);
821 /// assert_eq!(unsafe { &*x_ptr }, "hello");
823 #[stable(feature = "rc_raw", since = "1.17.0")]
824 pub fn into_raw(this: Self) -> *const T {
825 let ptr = Self::as_ptr(&this);
830 /// Provides a raw pointer to the data.
832 /// The counts are not affected in any way and the `Rc` is not consumed. The pointer is valid
833 /// for as long there are strong counts in the `Rc`.
840 /// let x = Rc::new("hello".to_owned());
841 /// let y = Rc::clone(&x);
842 /// let x_ptr = Rc::as_ptr(&x);
843 /// assert_eq!(x_ptr, Rc::as_ptr(&y));
844 /// assert_eq!(unsafe { &*x_ptr }, "hello");
846 #[stable(feature = "weak_into_raw", since = "1.45.0")]
847 pub fn as_ptr(this: &Self) -> *const T {
848 let ptr: *mut RcBox<T> = NonNull::as_ptr(this.ptr);
850 // SAFETY: This cannot go through Deref::deref or Rc::inner because
851 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
852 // write through the pointer after the Rc is recovered through `from_raw`.
853 unsafe { ptr::addr_of_mut!((*ptr).value) }
856 /// Constructs an `Rc<T>` from a raw pointer.
858 /// The raw pointer must have been previously returned by a call to
859 /// [`Rc<U>::into_raw`][into_raw] where `U` must have the same size
860 /// and alignment as `T`. This is trivially true if `U` is `T`.
861 /// Note that if `U` is not `T` but has the same size and alignment, this is
862 /// basically like transmuting references of different types. See
863 /// [`mem::transmute`] for more information on what
864 /// restrictions apply in this case.
866 /// The user of `from_raw` has to make sure a specific value of `T` is only
869 /// This function is unsafe because improper use may lead to memory unsafety,
870 /// even if the returned `Rc<T>` is never accessed.
872 /// [into_raw]: Rc::into_raw
879 /// let x = Rc::new("hello".to_owned());
880 /// let x_ptr = Rc::into_raw(x);
883 /// // Convert back to an `Rc` to prevent leak.
884 /// let x = Rc::from_raw(x_ptr);
885 /// assert_eq!(&*x, "hello");
887 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
890 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
892 #[stable(feature = "rc_raw", since = "1.17.0")]
893 pub unsafe fn from_raw(ptr: *const T) -> Self {
894 let offset = unsafe { data_offset(ptr) };
896 // Reverse the offset to find the original RcBox.
898 unsafe { (ptr as *mut RcBox<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) };
900 unsafe { Self::from_ptr(rc_ptr) }
903 /// Creates a new [`Weak`] pointer to this allocation.
910 /// let five = Rc::new(5);
912 /// let weak_five = Rc::downgrade(&five);
914 #[must_use = "this returns a new `Weak` pointer, \
915 without modifying the original `Rc`"]
916 #[stable(feature = "rc_weak", since = "1.4.0")]
917 pub fn downgrade(this: &Self) -> Weak<T> {
918 this.inner().inc_weak();
919 // Make sure we do not create a dangling Weak
920 debug_assert!(!is_dangling(this.ptr.as_ptr()));
921 Weak { ptr: this.ptr }
924 /// Gets the number of [`Weak`] pointers to this allocation.
931 /// let five = Rc::new(5);
932 /// let _weak_five = Rc::downgrade(&five);
934 /// assert_eq!(1, Rc::weak_count(&five));
937 #[stable(feature = "rc_counts", since = "1.15.0")]
938 pub fn weak_count(this: &Self) -> usize {
939 this.inner().weak() - 1
942 /// Gets the number of strong (`Rc`) pointers to this allocation.
949 /// let five = Rc::new(5);
950 /// let _also_five = Rc::clone(&five);
952 /// assert_eq!(2, Rc::strong_count(&five));
955 #[stable(feature = "rc_counts", since = "1.15.0")]
956 pub fn strong_count(this: &Self) -> usize {
957 this.inner().strong()
960 /// Increments the strong reference count on the `Rc<T>` associated with the
961 /// provided pointer by one.
965 /// The pointer must have been obtained through `Rc::into_raw`, and the
966 /// associated `Rc` instance must be valid (i.e. the strong count must be at
967 /// least 1) for the duration of this method.
974 /// let five = Rc::new(5);
977 /// let ptr = Rc::into_raw(five);
978 /// Rc::increment_strong_count(ptr);
980 /// let five = Rc::from_raw(ptr);
981 /// assert_eq!(2, Rc::strong_count(&five));
985 #[stable(feature = "rc_mutate_strong_count", since = "1.53.0")]
986 pub unsafe fn increment_strong_count(ptr: *const T) {
987 // Retain Rc, but don't touch refcount by wrapping in ManuallyDrop
988 let rc = unsafe { mem::ManuallyDrop::new(Rc::<T>::from_raw(ptr)) };
989 // Now increase refcount, but don't drop new refcount either
990 let _rc_clone: mem::ManuallyDrop<_> = rc.clone();
993 /// Decrements the strong reference count on the `Rc<T>` associated with the
994 /// provided pointer by one.
998 /// The pointer must have been obtained through `Rc::into_raw`, and the
999 /// associated `Rc` instance must be valid (i.e. the strong count must be at
1000 /// least 1) when invoking this method. This method can be used to release
1001 /// the final `Rc` and backing storage, but **should not** be called after
1002 /// the final `Rc` has been released.
1007 /// use std::rc::Rc;
1009 /// let five = Rc::new(5);
1012 /// let ptr = Rc::into_raw(five);
1013 /// Rc::increment_strong_count(ptr);
1015 /// let five = Rc::from_raw(ptr);
1016 /// assert_eq!(2, Rc::strong_count(&five));
1017 /// Rc::decrement_strong_count(ptr);
1018 /// assert_eq!(1, Rc::strong_count(&five));
1022 #[stable(feature = "rc_mutate_strong_count", since = "1.53.0")]
1023 pub unsafe fn decrement_strong_count(ptr: *const T) {
1024 unsafe { mem::drop(Rc::from_raw(ptr)) };
1027 /// Returns `true` if there are no other `Rc` or [`Weak`] pointers to
1028 /// this allocation.
1030 fn is_unique(this: &Self) -> bool {
1031 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
1034 /// Returns a mutable reference into the given `Rc`, if there are
1035 /// no other `Rc` or [`Weak`] pointers to the same allocation.
1037 /// Returns [`None`] otherwise, because it is not safe to
1038 /// mutate a shared value.
1040 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1041 /// the inner value when there are other `Rc` pointers.
1043 /// [make_mut]: Rc::make_mut
1044 /// [clone]: Clone::clone
1049 /// use std::rc::Rc;
1051 /// let mut x = Rc::new(3);
1052 /// *Rc::get_mut(&mut x).unwrap() = 4;
1053 /// assert_eq!(*x, 4);
1055 /// let _y = Rc::clone(&x);
1056 /// assert!(Rc::get_mut(&mut x).is_none());
1059 #[stable(feature = "rc_unique", since = "1.4.0")]
1060 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
1061 if Rc::is_unique(this) { unsafe { Some(Rc::get_mut_unchecked(this)) } } else { None }
1064 /// Returns a mutable reference into the given `Rc`,
1065 /// without any check.
1067 /// See also [`get_mut`], which is safe and does appropriate checks.
1069 /// [`get_mut`]: Rc::get_mut
1073 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
1074 /// for the duration of the returned borrow.
1075 /// This is trivially the case if no such pointers exist,
1076 /// for example immediately after `Rc::new`.
1081 /// #![feature(get_mut_unchecked)]
1083 /// use std::rc::Rc;
1085 /// let mut x = Rc::new(String::new());
1087 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
1089 /// assert_eq!(*x, "foo");
1092 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1093 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
1094 // We are careful to *not* create a reference covering the "count" fields, as
1095 // this would conflict with accesses to the reference counts (e.g. by `Weak`).
1096 unsafe { &mut (*this.ptr.as_ptr()).value }
1100 #[stable(feature = "ptr_eq", since = "1.17.0")]
1101 /// Returns `true` if the two `Rc`s point to the same allocation
1102 /// (in a vein similar to [`ptr::eq`]).
1107 /// use std::rc::Rc;
1109 /// let five = Rc::new(5);
1110 /// let same_five = Rc::clone(&five);
1111 /// let other_five = Rc::new(5);
1113 /// assert!(Rc::ptr_eq(&five, &same_five));
1114 /// assert!(!Rc::ptr_eq(&five, &other_five));
1116 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1117 this.ptr.as_ptr() == other.ptr.as_ptr()
1121 impl<T: Clone> Rc<T> {
1122 /// Makes a mutable reference into the given `Rc`.
1124 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
1125 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
1126 /// referred to as clone-on-write.
1128 /// However, if there are no other `Rc` pointers to this allocation, but some [`Weak`]
1129 /// pointers, then the [`Weak`] pointers will be disassociated and the inner value will not
1132 /// See also [`get_mut`], which will fail rather than cloning the inner value
1133 /// or diassociating [`Weak`] pointers.
1135 /// [`clone`]: Clone::clone
1136 /// [`get_mut`]: Rc::get_mut
1141 /// use std::rc::Rc;
1143 /// let mut data = Rc::new(5);
1145 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
1146 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
1147 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
1148 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
1149 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
1151 /// // Now `data` and `other_data` point to different allocations.
1152 /// assert_eq!(*data, 8);
1153 /// assert_eq!(*other_data, 12);
1156 /// [`Weak`] pointers will be disassociated:
1159 /// use std::rc::Rc;
1161 /// let mut data = Rc::new(75);
1162 /// let weak = Rc::downgrade(&data);
1164 /// assert!(75 == *data);
1165 /// assert!(75 == *weak.upgrade().unwrap());
1167 /// *Rc::make_mut(&mut data) += 1;
1169 /// assert!(76 == *data);
1170 /// assert!(weak.upgrade().is_none());
1172 #[cfg(not(no_global_oom_handling))]
1174 #[stable(feature = "rc_unique", since = "1.4.0")]
1175 pub fn make_mut(this: &mut Self) -> &mut T {
1176 if Rc::strong_count(this) != 1 {
1177 // Gotta clone the data, there are other Rcs.
1178 // Pre-allocate memory to allow writing the cloned value directly.
1179 let mut rc = Self::new_uninit();
1181 let data = Rc::get_mut_unchecked(&mut rc);
1182 (**this).write_clone_into_raw(data.as_mut_ptr());
1183 *this = rc.assume_init();
1185 } else if Rc::weak_count(this) != 0 {
1186 // Can just steal the data, all that's left is Weaks
1187 let mut rc = Self::new_uninit();
1189 let data = Rc::get_mut_unchecked(&mut rc);
1190 data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
1192 this.inner().dec_strong();
1193 // Remove implicit strong-weak ref (no need to craft a fake
1194 // Weak here -- we know other Weaks can clean up for us)
1195 this.inner().dec_weak();
1196 ptr::write(this, rc.assume_init());
1199 // This unsafety is ok because we're guaranteed that the pointer
1200 // returned is the *only* pointer that will ever be returned to T. Our
1201 // reference count is guaranteed to be 1 at this point, and we required
1202 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
1203 // reference to the allocation.
1204 unsafe { &mut this.ptr.as_mut().value }
1210 #[stable(feature = "rc_downcast", since = "1.29.0")]
1211 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
1216 /// use std::any::Any;
1217 /// use std::rc::Rc;
1219 /// fn print_if_string(value: Rc<dyn Any>) {
1220 /// if let Ok(string) = value.downcast::<String>() {
1221 /// println!("String ({}): {}", string.len(), string);
1225 /// let my_string = "Hello World".to_string();
1226 /// print_if_string(Rc::new(my_string));
1227 /// print_if_string(Rc::new(0i8));
1229 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
1230 if (*self).is::<T>() {
1232 let ptr = self.ptr.cast::<RcBox<T>>();
1234 Ok(Rc::from_inner(ptr))
1242 impl<T: ?Sized> Rc<T> {
1243 /// Allocates an `RcBox<T>` with sufficient space for
1244 /// a possibly-unsized inner value where the value has the layout provided.
1246 /// The function `mem_to_rcbox` is called with the data pointer
1247 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
1248 #[cfg(not(no_global_oom_handling))]
1249 unsafe fn allocate_for_layout(
1250 value_layout: Layout,
1251 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1252 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
1253 ) -> *mut RcBox<T> {
1254 // Calculate layout using the given value layout.
1255 // Previously, layout was calculated on the expression
1256 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
1257 // reference (see #54908).
1258 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
1260 Rc::try_allocate_for_layout(value_layout, allocate, mem_to_rcbox)
1261 .unwrap_or_else(|_| handle_alloc_error(layout))
1265 /// Allocates an `RcBox<T>` with sufficient space for
1266 /// a possibly-unsized inner value where the value has the layout provided,
1267 /// returning an error if allocation fails.
1269 /// The function `mem_to_rcbox` is called with the data pointer
1270 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
1272 unsafe fn try_allocate_for_layout(
1273 value_layout: Layout,
1274 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1275 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
1276 ) -> Result<*mut RcBox<T>, AllocError> {
1277 // Calculate layout using the given value layout.
1278 // Previously, layout was calculated on the expression
1279 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
1280 // reference (see #54908).
1281 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
1283 // Allocate for the layout.
1284 let ptr = allocate(layout)?;
1286 // Initialize the RcBox
1287 let inner = mem_to_rcbox(ptr.as_non_null_ptr().as_ptr());
1289 debug_assert_eq!(Layout::for_value(&*inner), layout);
1291 ptr::write(&mut (*inner).strong, Cell::new(1));
1292 ptr::write(&mut (*inner).weak, Cell::new(1));
1298 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
1299 #[cfg(not(no_global_oom_handling))]
1300 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
1301 // Allocate for the `RcBox<T>` using the given value.
1303 Self::allocate_for_layout(
1304 Layout::for_value(&*ptr),
1305 |layout| Global.allocate(layout),
1306 |mem| (ptr as *mut RcBox<T>).set_ptr_value(mem),
1311 #[cfg(not(no_global_oom_handling))]
1312 fn from_box(v: Box<T>) -> Rc<T> {
1314 let (box_unique, alloc) = Box::into_unique(v);
1315 let bptr = box_unique.as_ptr();
1317 let value_size = size_of_val(&*bptr);
1318 let ptr = Self::allocate_for_ptr(bptr);
1320 // Copy value as bytes
1321 ptr::copy_nonoverlapping(
1322 bptr as *const T as *const u8,
1323 &mut (*ptr).value as *mut _ as *mut u8,
1327 // Free the allocation without dropping its contents
1328 box_free(box_unique, alloc);
1336 /// Allocates an `RcBox<[T]>` with the given length.
1337 #[cfg(not(no_global_oom_handling))]
1338 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
1340 Self::allocate_for_layout(
1341 Layout::array::<T>(len).unwrap(),
1342 |layout| Global.allocate(layout),
1343 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>,
1348 /// Copy elements from slice into newly allocated Rc<\[T\]>
1350 /// Unsafe because the caller must either take ownership or bind `T: Copy`
1351 #[cfg(not(no_global_oom_handling))]
1352 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
1354 let ptr = Self::allocate_for_slice(v.len());
1355 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).value as *mut [T] as *mut T, v.len());
1360 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
1362 /// Behavior is undefined should the size be wrong.
1363 #[cfg(not(no_global_oom_handling))]
1364 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
1365 // Panic guard while cloning T elements.
1366 // In the event of a panic, elements that have been written
1367 // into the new RcBox will be dropped, then the memory freed.
1375 impl<T> Drop for Guard<T> {
1376 fn drop(&mut self) {
1378 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1379 ptr::drop_in_place(slice);
1381 Global.deallocate(self.mem, self.layout);
1387 let ptr = Self::allocate_for_slice(len);
1389 let mem = ptr as *mut _ as *mut u8;
1390 let layout = Layout::for_value(&*ptr);
1392 // Pointer to first element
1393 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1395 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1397 for (i, item) in iter.enumerate() {
1398 ptr::write(elems.add(i), item);
1402 // All clear. Forget the guard so it doesn't free the new RcBox.
1410 /// Specialization trait used for `From<&[T]>`.
1411 trait RcFromSlice<T> {
1412 fn from_slice(slice: &[T]) -> Self;
1415 #[cfg(not(no_global_oom_handling))]
1416 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
1418 default fn from_slice(v: &[T]) -> Self {
1419 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1423 #[cfg(not(no_global_oom_handling))]
1424 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
1426 fn from_slice(v: &[T]) -> Self {
1427 unsafe { Rc::copy_from_slice(v) }
1431 #[stable(feature = "rust1", since = "1.0.0")]
1432 impl<T: ?Sized> Deref for Rc<T> {
1436 fn deref(&self) -> &T {
1441 #[unstable(feature = "receiver_trait", issue = "none")]
1442 impl<T: ?Sized> Receiver for Rc<T> {}
1444 #[stable(feature = "rust1", since = "1.0.0")]
1445 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1448 /// This will decrement the strong reference count. If the strong reference
1449 /// count reaches zero then the only other references (if any) are
1450 /// [`Weak`], so we `drop` the inner value.
1455 /// use std::rc::Rc;
1459 /// impl Drop for Foo {
1460 /// fn drop(&mut self) {
1461 /// println!("dropped!");
1465 /// let foo = Rc::new(Foo);
1466 /// let foo2 = Rc::clone(&foo);
1468 /// drop(foo); // Doesn't print anything
1469 /// drop(foo2); // Prints "dropped!"
1471 fn drop(&mut self) {
1473 self.inner().dec_strong();
1474 if self.inner().strong() == 0 {
1475 // destroy the contained object
1476 ptr::drop_in_place(Self::get_mut_unchecked(self));
1478 // remove the implicit "strong weak" pointer now that we've
1479 // destroyed the contents.
1480 self.inner().dec_weak();
1482 if self.inner().weak() == 0 {
1483 Global.deallocate(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1490 #[stable(feature = "rust1", since = "1.0.0")]
1491 impl<T: ?Sized> Clone for Rc<T> {
1492 /// Makes a clone of the `Rc` pointer.
1494 /// This creates another pointer to the same allocation, increasing the
1495 /// strong reference count.
1500 /// use std::rc::Rc;
1502 /// let five = Rc::new(5);
1504 /// let _ = Rc::clone(&five);
1507 fn clone(&self) -> Rc<T> {
1509 self.inner().inc_strong();
1510 Self::from_inner(self.ptr)
1515 #[cfg(not(no_global_oom_handling))]
1516 #[stable(feature = "rust1", since = "1.0.0")]
1517 impl<T: Default> Default for Rc<T> {
1518 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1523 /// use std::rc::Rc;
1525 /// let x: Rc<i32> = Default::default();
1526 /// assert_eq!(*x, 0);
1529 fn default() -> Rc<T> {
1530 Rc::new(Default::default())
1534 #[stable(feature = "rust1", since = "1.0.0")]
1535 trait RcEqIdent<T: ?Sized + PartialEq> {
1536 fn eq(&self, other: &Rc<T>) -> bool;
1537 fn ne(&self, other: &Rc<T>) -> bool;
1540 #[stable(feature = "rust1", since = "1.0.0")]
1541 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
1543 default fn eq(&self, other: &Rc<T>) -> bool {
1548 default fn ne(&self, other: &Rc<T>) -> bool {
1553 // Hack to allow specializing on `Eq` even though `Eq` has a method.
1554 #[rustc_unsafe_specialization_marker]
1555 pub(crate) trait MarkerEq: PartialEq<Self> {}
1557 impl<T: Eq> MarkerEq for T {}
1559 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1560 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1561 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1562 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1563 /// the same value, than two `&T`s.
1565 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1566 #[stable(feature = "rust1", since = "1.0.0")]
1567 impl<T: ?Sized + MarkerEq> RcEqIdent<T> for Rc<T> {
1569 fn eq(&self, other: &Rc<T>) -> bool {
1570 Rc::ptr_eq(self, other) || **self == **other
1574 fn ne(&self, other: &Rc<T>) -> bool {
1575 !Rc::ptr_eq(self, other) && **self != **other
1579 #[stable(feature = "rust1", since = "1.0.0")]
1580 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1581 /// Equality for two `Rc`s.
1583 /// Two `Rc`s are equal if their inner values are equal, even if they are
1584 /// stored in different allocation.
1586 /// If `T` also implements `Eq` (implying reflexivity of equality),
1587 /// two `Rc`s that point to the same allocation are
1593 /// use std::rc::Rc;
1595 /// let five = Rc::new(5);
1597 /// assert!(five == Rc::new(5));
1600 fn eq(&self, other: &Rc<T>) -> bool {
1601 RcEqIdent::eq(self, other)
1604 /// Inequality for two `Rc`s.
1606 /// Two `Rc`s are unequal if their inner values are unequal.
1608 /// If `T` also implements `Eq` (implying reflexivity of equality),
1609 /// two `Rc`s that point to the same allocation are
1615 /// use std::rc::Rc;
1617 /// let five = Rc::new(5);
1619 /// assert!(five != Rc::new(6));
1622 fn ne(&self, other: &Rc<T>) -> bool {
1623 RcEqIdent::ne(self, other)
1627 #[stable(feature = "rust1", since = "1.0.0")]
1628 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1630 #[stable(feature = "rust1", since = "1.0.0")]
1631 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1632 /// Partial comparison for two `Rc`s.
1634 /// The two are compared by calling `partial_cmp()` on their inner values.
1639 /// use std::rc::Rc;
1640 /// use std::cmp::Ordering;
1642 /// let five = Rc::new(5);
1644 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1647 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1648 (**self).partial_cmp(&**other)
1651 /// Less-than comparison for two `Rc`s.
1653 /// The two are compared by calling `<` on their inner values.
1658 /// use std::rc::Rc;
1660 /// let five = Rc::new(5);
1662 /// assert!(five < Rc::new(6));
1665 fn lt(&self, other: &Rc<T>) -> bool {
1669 /// 'Less than or equal to' comparison for two `Rc`s.
1671 /// The two are compared by calling `<=` on their inner values.
1676 /// use std::rc::Rc;
1678 /// let five = Rc::new(5);
1680 /// assert!(five <= Rc::new(5));
1683 fn le(&self, other: &Rc<T>) -> bool {
1687 /// Greater-than comparison for two `Rc`s.
1689 /// The two are compared by calling `>` on their inner values.
1694 /// use std::rc::Rc;
1696 /// let five = Rc::new(5);
1698 /// assert!(five > Rc::new(4));
1701 fn gt(&self, other: &Rc<T>) -> bool {
1705 /// 'Greater than or equal to' comparison for two `Rc`s.
1707 /// The two are compared by calling `>=` on their inner values.
1712 /// use std::rc::Rc;
1714 /// let five = Rc::new(5);
1716 /// assert!(five >= Rc::new(5));
1719 fn ge(&self, other: &Rc<T>) -> bool {
1724 #[stable(feature = "rust1", since = "1.0.0")]
1725 impl<T: ?Sized + Ord> Ord for Rc<T> {
1726 /// Comparison for two `Rc`s.
1728 /// The two are compared by calling `cmp()` on their inner values.
1733 /// use std::rc::Rc;
1734 /// use std::cmp::Ordering;
1736 /// let five = Rc::new(5);
1738 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1741 fn cmp(&self, other: &Rc<T>) -> Ordering {
1742 (**self).cmp(&**other)
1746 #[stable(feature = "rust1", since = "1.0.0")]
1747 impl<T: ?Sized + Hash> Hash for Rc<T> {
1748 fn hash<H: Hasher>(&self, state: &mut H) {
1749 (**self).hash(state);
1753 #[stable(feature = "rust1", since = "1.0.0")]
1754 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1755 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1756 fmt::Display::fmt(&**self, f)
1760 #[stable(feature = "rust1", since = "1.0.0")]
1761 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1762 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1763 fmt::Debug::fmt(&**self, f)
1767 #[stable(feature = "rust1", since = "1.0.0")]
1768 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1769 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1770 fmt::Pointer::fmt(&(&**self as *const T), f)
1774 #[cfg(not(no_global_oom_handling))]
1775 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1776 impl<T> From<T> for Rc<T> {
1777 /// Converts a generic type `T` into an `Rc<T>`
1779 /// The conversion allocates on the heap and moves `t`
1780 /// from the stack into it.
1784 /// # use std::rc::Rc;
1786 /// let rc = Rc::new(5);
1788 /// assert_eq!(Rc::from(x), rc);
1790 fn from(t: T) -> Self {
1795 #[cfg(not(no_global_oom_handling))]
1796 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1797 impl<T: Clone> From<&[T]> for Rc<[T]> {
1798 /// Allocate a reference-counted slice and fill it by cloning `v`'s items.
1803 /// # use std::rc::Rc;
1804 /// let original: &[i32] = &[1, 2, 3];
1805 /// let shared: Rc<[i32]> = Rc::from(original);
1806 /// assert_eq!(&[1, 2, 3], &shared[..]);
1809 fn from(v: &[T]) -> Rc<[T]> {
1810 <Self as RcFromSlice<T>>::from_slice(v)
1814 #[cfg(not(no_global_oom_handling))]
1815 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1816 impl From<&str> for Rc<str> {
1817 /// Allocate a reference-counted string slice and copy `v` into it.
1822 /// # use std::rc::Rc;
1823 /// let shared: Rc<str> = Rc::from("statue");
1824 /// assert_eq!("statue", &shared[..]);
1827 fn from(v: &str) -> Rc<str> {
1828 let rc = Rc::<[u8]>::from(v.as_bytes());
1829 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1833 #[cfg(not(no_global_oom_handling))]
1834 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1835 impl From<String> for Rc<str> {
1836 /// Allocate a reference-counted string slice and copy `v` into it.
1841 /// # use std::rc::Rc;
1842 /// let original: String = "statue".to_owned();
1843 /// let shared: Rc<str> = Rc::from(original);
1844 /// assert_eq!("statue", &shared[..]);
1847 fn from(v: String) -> Rc<str> {
1852 #[cfg(not(no_global_oom_handling))]
1853 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1854 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1855 /// Move a boxed object to a new, reference counted, allocation.
1860 /// # use std::rc::Rc;
1861 /// let original: Box<i32> = Box::new(1);
1862 /// let shared: Rc<i32> = Rc::from(original);
1863 /// assert_eq!(1, *shared);
1866 fn from(v: Box<T>) -> Rc<T> {
1871 #[cfg(not(no_global_oom_handling))]
1872 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1873 impl<T> From<Vec<T>> for Rc<[T]> {
1874 /// Allocate a reference-counted slice and move `v`'s items into it.
1879 /// # use std::rc::Rc;
1880 /// let original: Box<Vec<i32>> = Box::new(vec![1, 2, 3]);
1881 /// let shared: Rc<Vec<i32>> = Rc::from(original);
1882 /// assert_eq!(vec![1, 2, 3], *shared);
1885 fn from(mut v: Vec<T>) -> Rc<[T]> {
1887 let rc = Rc::copy_from_slice(&v);
1889 // Allow the Vec to free its memory, but not destroy its contents
1897 #[stable(feature = "shared_from_cow", since = "1.45.0")]
1898 impl<'a, B> From<Cow<'a, B>> for Rc<B>
1900 B: ToOwned + ?Sized,
1901 Rc<B>: From<&'a B> + From<B::Owned>,
1903 /// Create a reference-counted pointer from
1904 /// a clone-on-write pointer by copying its content.
1909 /// # use std::rc::Rc;
1910 /// # use std::borrow::Cow;
1911 /// let cow: Cow<str> = Cow::Borrowed("eggplant");
1912 /// let shared: Rc<str> = Rc::from(cow);
1913 /// assert_eq!("eggplant", &shared[..]);
1916 fn from(cow: Cow<'a, B>) -> Rc<B> {
1918 Cow::Borrowed(s) => Rc::from(s),
1919 Cow::Owned(s) => Rc::from(s),
1924 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1925 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]> {
1926 type Error = Rc<[T]>;
1928 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1929 if boxed_slice.len() == N {
1930 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
1937 #[cfg(not(no_global_oom_handling))]
1938 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1939 impl<T> iter::FromIterator<T> for Rc<[T]> {
1940 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1942 /// # Performance characteristics
1944 /// ## The general case
1946 /// In the general case, collecting into `Rc<[T]>` is done by first
1947 /// collecting into a `Vec<T>`. That is, when writing the following:
1950 /// # use std::rc::Rc;
1951 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1952 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1955 /// this behaves as if we wrote:
1958 /// # use std::rc::Rc;
1959 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1960 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1961 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1962 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1965 /// This will allocate as many times as needed for constructing the `Vec<T>`
1966 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1968 /// ## Iterators of known length
1970 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1971 /// a single allocation will be made for the `Rc<[T]>`. For example:
1974 /// # use std::rc::Rc;
1975 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1976 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1978 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1979 ToRcSlice::to_rc_slice(iter.into_iter())
1983 /// Specialization trait used for collecting into `Rc<[T]>`.
1984 #[cfg(not(no_global_oom_handling))]
1985 trait ToRcSlice<T>: Iterator<Item = T> + Sized {
1986 fn to_rc_slice(self) -> Rc<[T]>;
1989 #[cfg(not(no_global_oom_handling))]
1990 impl<T, I: Iterator<Item = T>> ToRcSlice<T> for I {
1991 default fn to_rc_slice(self) -> Rc<[T]> {
1992 self.collect::<Vec<T>>().into()
1996 #[cfg(not(no_global_oom_handling))]
1997 impl<T, I: iter::TrustedLen<Item = T>> ToRcSlice<T> for I {
1998 fn to_rc_slice(self) -> Rc<[T]> {
1999 // This is the case for a `TrustedLen` iterator.
2000 let (low, high) = self.size_hint();
2001 if let Some(high) = high {
2005 "TrustedLen iterator's size hint is not exact: {:?}",
2010 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2011 Rc::from_iter_exact(self, low)
2014 // TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
2015 // length exceeding `usize::MAX`.
2016 // The default implementation would collect into a vec which would panic.
2017 // Thus we panic here immediately without invoking `Vec` code.
2018 panic!("capacity overflow");
2023 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
2024 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
2025 /// pointer, which returns an <code>[Option]<[Rc]\<T>></code>.
2027 /// Since a `Weak` reference does not count towards ownership, it will not
2028 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
2029 /// guarantees about the value still being present. Thus it may return [`None`]
2030 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
2031 /// itself (the backing store) from being deallocated.
2033 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
2034 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
2035 /// prevent circular references between [`Rc`] pointers, since mutual owning references
2036 /// would never allow either [`Rc`] to be dropped. For example, a tree could
2037 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
2038 /// pointers from children back to their parents.
2040 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
2042 /// [`upgrade`]: Weak::upgrade
2043 #[stable(feature = "rc_weak", since = "1.4.0")]
2044 pub struct Weak<T: ?Sized> {
2045 // This is a `NonNull` to allow optimizing the size of this type in enums,
2046 // but it is not necessarily a valid pointer.
2047 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
2048 // to allocate space on the heap. That's not a value a real pointer
2049 // will ever have because RcBox has alignment at least 2.
2050 // This is only possible when `T: Sized`; unsized `T` never dangle.
2051 ptr: NonNull<RcBox<T>>,
2054 #[stable(feature = "rc_weak", since = "1.4.0")]
2055 impl<T: ?Sized> !marker::Send for Weak<T> {}
2056 #[stable(feature = "rc_weak", since = "1.4.0")]
2057 impl<T: ?Sized> !marker::Sync for Weak<T> {}
2059 #[unstable(feature = "coerce_unsized", issue = "27732")]
2060 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
2062 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
2063 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
2066 /// Constructs a new `Weak<T>`, without allocating any memory.
2067 /// Calling [`upgrade`] on the return value always gives [`None`].
2069 /// [`upgrade`]: Weak::upgrade
2074 /// use std::rc::Weak;
2076 /// let empty: Weak<i64> = Weak::new();
2077 /// assert!(empty.upgrade().is_none());
2079 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2081 pub fn new() -> Weak<T> {
2082 Weak { ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0") }
2086 pub(crate) fn is_dangling<T: ?Sized>(ptr: *mut T) -> bool {
2087 let address = ptr as *mut () as usize;
2088 address == usize::MAX
2091 /// Helper type to allow accessing the reference counts without
2092 /// making any assertions about the data field.
2093 struct WeakInner<'a> {
2094 weak: &'a Cell<usize>,
2095 strong: &'a Cell<usize>,
2098 impl<T: ?Sized> Weak<T> {
2099 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
2101 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
2102 /// unaligned or even [`null`] otherwise.
2107 /// use std::rc::Rc;
2110 /// let strong = Rc::new("hello".to_owned());
2111 /// let weak = Rc::downgrade(&strong);
2112 /// // Both point to the same object
2113 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
2114 /// // The strong here keeps it alive, so we can still access the object.
2115 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
2118 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
2119 /// // undefined behaviour.
2120 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
2123 /// [`null`]: ptr::null
2125 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
2126 pub fn as_ptr(&self) -> *const T {
2127 let ptr: *mut RcBox<T> = NonNull::as_ptr(self.ptr);
2129 if is_dangling(ptr) {
2130 // If the pointer is dangling, we return the sentinel directly. This cannot be
2131 // a valid payload address, as the payload is at least as aligned as RcBox (usize).
2134 // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
2135 // The payload may be dropped at this point, and we have to maintain provenance,
2136 // so use raw pointer manipulation.
2137 unsafe { ptr::addr_of_mut!((*ptr).value) }
2141 /// Consumes the `Weak<T>` and turns it into a raw pointer.
2143 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2144 /// one weak reference (the weak count is not modified by this operation). It can be turned
2145 /// back into the `Weak<T>` with [`from_raw`].
2147 /// The same restrictions of accessing the target of the pointer as with
2148 /// [`as_ptr`] apply.
2153 /// use std::rc::{Rc, Weak};
2155 /// let strong = Rc::new("hello".to_owned());
2156 /// let weak = Rc::downgrade(&strong);
2157 /// let raw = weak.into_raw();
2159 /// assert_eq!(1, Rc::weak_count(&strong));
2160 /// assert_eq!("hello", unsafe { &*raw });
2162 /// drop(unsafe { Weak::from_raw(raw) });
2163 /// assert_eq!(0, Rc::weak_count(&strong));
2166 /// [`from_raw`]: Weak::from_raw
2167 /// [`as_ptr`]: Weak::as_ptr
2168 #[must_use = "`self` will be dropped if the result is not used"]
2169 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2170 pub fn into_raw(self) -> *const T {
2171 let result = self.as_ptr();
2176 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
2178 /// This can be used to safely get a strong reference (by calling [`upgrade`]
2179 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2181 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2182 /// as these don't own anything; the method still works on them).
2186 /// The pointer must have originated from the [`into_raw`] and must still own its potential
2189 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2190 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2191 /// count is not modified by this operation) and therefore it must be paired with a previous
2192 /// call to [`into_raw`].
2197 /// use std::rc::{Rc, Weak};
2199 /// let strong = Rc::new("hello".to_owned());
2201 /// let raw_1 = Rc::downgrade(&strong).into_raw();
2202 /// let raw_2 = Rc::downgrade(&strong).into_raw();
2204 /// assert_eq!(2, Rc::weak_count(&strong));
2206 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2207 /// assert_eq!(1, Rc::weak_count(&strong));
2211 /// // Decrement the last weak count.
2212 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2215 /// [`into_raw`]: Weak::into_raw
2216 /// [`upgrade`]: Weak::upgrade
2217 /// [`new`]: Weak::new
2218 #[stable(feature = "weak_into_raw", since = "1.45.0")]
2219 pub unsafe fn from_raw(ptr: *const T) -> Self {
2220 // See Weak::as_ptr for context on how the input pointer is derived.
2222 let ptr = if is_dangling(ptr as *mut T) {
2223 // This is a dangling Weak.
2224 ptr as *mut RcBox<T>
2226 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
2227 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
2228 let offset = unsafe { data_offset(ptr) };
2229 // Thus, we reverse the offset to get the whole RcBox.
2230 // SAFETY: the pointer originated from a Weak, so this offset is safe.
2231 unsafe { (ptr as *mut RcBox<T>).set_ptr_value((ptr as *mut u8).offset(-offset)) }
2234 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
2235 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
2238 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
2239 /// dropping of the inner value if successful.
2241 /// Returns [`None`] if the inner value has since been dropped.
2246 /// use std::rc::Rc;
2248 /// let five = Rc::new(5);
2250 /// let weak_five = Rc::downgrade(&five);
2252 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
2253 /// assert!(strong_five.is_some());
2255 /// // Destroy all strong pointers.
2256 /// drop(strong_five);
2259 /// assert!(weak_five.upgrade().is_none());
2261 #[must_use = "this returns a new `Rc`, \
2262 without modifying the original weak pointer"]
2263 #[stable(feature = "rc_weak", since = "1.4.0")]
2264 pub fn upgrade(&self) -> Option<Rc<T>> {
2265 let inner = self.inner()?;
2267 if inner.strong() == 0 {
2272 Some(Rc::from_inner(self.ptr))
2277 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
2279 /// If `self` was created using [`Weak::new`], this will return 0.
2281 #[stable(feature = "weak_counts", since = "1.41.0")]
2282 pub fn strong_count(&self) -> usize {
2283 if let Some(inner) = self.inner() { inner.strong() } else { 0 }
2286 /// Gets the number of `Weak` pointers pointing to this allocation.
2288 /// If no strong pointers remain, this will return zero.
2290 #[stable(feature = "weak_counts", since = "1.41.0")]
2291 pub fn weak_count(&self) -> usize {
2294 if inner.strong() > 0 {
2295 inner.weak() - 1 // subtract the implicit weak ptr
2303 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`,
2304 /// (i.e., when this `Weak` was created by `Weak::new`).
2306 fn inner(&self) -> Option<WeakInner<'_>> {
2307 if is_dangling(self.ptr.as_ptr()) {
2310 // We are careful to *not* create a reference covering the "data" field, as
2311 // the field may be mutated concurrently (for example, if the last `Rc`
2312 // is dropped, the data field will be dropped in-place).
2314 let ptr = self.ptr.as_ptr();
2315 WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
2320 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
2321 /// [`ptr::eq`]), or if both don't point to any allocation
2322 /// (because they were created with `Weak::new()`).
2326 /// Since this compares pointers it means that `Weak::new()` will equal each
2327 /// other, even though they don't point to any allocation.
2332 /// use std::rc::Rc;
2334 /// let first_rc = Rc::new(5);
2335 /// let first = Rc::downgrade(&first_rc);
2336 /// let second = Rc::downgrade(&first_rc);
2338 /// assert!(first.ptr_eq(&second));
2340 /// let third_rc = Rc::new(5);
2341 /// let third = Rc::downgrade(&third_rc);
2343 /// assert!(!first.ptr_eq(&third));
2346 /// Comparing `Weak::new`.
2349 /// use std::rc::{Rc, Weak};
2351 /// let first = Weak::new();
2352 /// let second = Weak::new();
2353 /// assert!(first.ptr_eq(&second));
2355 /// let third_rc = Rc::new(());
2356 /// let third = Rc::downgrade(&third_rc);
2357 /// assert!(!first.ptr_eq(&third));
2361 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
2362 pub fn ptr_eq(&self, other: &Self) -> bool {
2363 self.ptr.as_ptr() == other.ptr.as_ptr()
2367 #[stable(feature = "rc_weak", since = "1.4.0")]
2368 unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
2369 /// Drops the `Weak` pointer.
2374 /// use std::rc::{Rc, Weak};
2378 /// impl Drop for Foo {
2379 /// fn drop(&mut self) {
2380 /// println!("dropped!");
2384 /// let foo = Rc::new(Foo);
2385 /// let weak_foo = Rc::downgrade(&foo);
2386 /// let other_weak_foo = Weak::clone(&weak_foo);
2388 /// drop(weak_foo); // Doesn't print anything
2389 /// drop(foo); // Prints "dropped!"
2391 /// assert!(other_weak_foo.upgrade().is_none());
2393 fn drop(&mut self) {
2394 let inner = if let Some(inner) = self.inner() { inner } else { return };
2397 // the weak count starts at 1, and will only go to zero if all
2398 // the strong pointers have disappeared.
2399 if inner.weak() == 0 {
2401 Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()));
2407 #[stable(feature = "rc_weak", since = "1.4.0")]
2408 impl<T: ?Sized> Clone for Weak<T> {
2409 /// Makes a clone of the `Weak` pointer that points to the same allocation.
2414 /// use std::rc::{Rc, Weak};
2416 /// let weak_five = Rc::downgrade(&Rc::new(5));
2418 /// let _ = Weak::clone(&weak_five);
2421 fn clone(&self) -> Weak<T> {
2422 if let Some(inner) = self.inner() {
2425 Weak { ptr: self.ptr }
2429 #[stable(feature = "rc_weak", since = "1.4.0")]
2430 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
2431 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2436 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2437 impl<T> Default for Weak<T> {
2438 /// Constructs a new `Weak<T>`, without allocating any memory.
2439 /// Calling [`upgrade`] on the return value always gives [`None`].
2441 /// [`upgrade`]: Weak::upgrade
2446 /// use std::rc::Weak;
2448 /// let empty: Weak<i64> = Default::default();
2449 /// assert!(empty.upgrade().is_none());
2451 fn default() -> Weak<T> {
2456 // NOTE: We checked_add here to deal with mem::forget safely. In particular
2457 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
2458 // you can free the allocation while outstanding Rcs (or Weaks) exist.
2459 // We abort because this is such a degenerate scenario that we don't care about
2460 // what happens -- no real program should ever experience this.
2462 // This should have negligible overhead since you don't actually need to
2463 // clone these much in Rust thanks to ownership and move-semantics.
2467 fn weak_ref(&self) -> &Cell<usize>;
2468 fn strong_ref(&self) -> &Cell<usize>;
2471 fn strong(&self) -> usize {
2472 self.strong_ref().get()
2476 fn inc_strong(&self) {
2477 let strong = self.strong();
2479 // We want to abort on overflow instead of dropping the value.
2480 // The reference count will never be zero when this is called;
2481 // nevertheless, we insert an abort here to hint LLVM at
2482 // an otherwise missed optimization.
2483 if strong == 0 || strong == usize::MAX {
2486 self.strong_ref().set(strong + 1);
2490 fn dec_strong(&self) {
2491 self.strong_ref().set(self.strong() - 1);
2495 fn weak(&self) -> usize {
2496 self.weak_ref().get()
2500 fn inc_weak(&self) {
2501 let weak = self.weak();
2503 // We want to abort on overflow instead of dropping the value.
2504 // The reference count will never be zero when this is called;
2505 // nevertheless, we insert an abort here to hint LLVM at
2506 // an otherwise missed optimization.
2507 if weak == 0 || weak == usize::MAX {
2510 self.weak_ref().set(weak + 1);
2514 fn dec_weak(&self) {
2515 self.weak_ref().set(self.weak() - 1);
2519 impl<T: ?Sized> RcInnerPtr for RcBox<T> {
2521 fn weak_ref(&self) -> &Cell<usize> {
2526 fn strong_ref(&self) -> &Cell<usize> {
2531 impl<'a> RcInnerPtr for WeakInner<'a> {
2533 fn weak_ref(&self) -> &Cell<usize> {
2538 fn strong_ref(&self) -> &Cell<usize> {
2543 #[stable(feature = "rust1", since = "1.0.0")]
2544 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2545 fn borrow(&self) -> &T {
2550 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2551 impl<T: ?Sized> AsRef<T> for Rc<T> {
2552 fn as_ref(&self) -> &T {
2557 #[stable(feature = "pin", since = "1.33.0")]
2558 impl<T: ?Sized> Unpin for Rc<T> {}
2560 /// Get the offset within an `RcBox` for the payload behind a pointer.
2564 /// The pointer must point to (and have valid metadata for) a previously
2565 /// valid instance of T, but the T is allowed to be dropped.
2566 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2567 // Align the unsized value to the end of the RcBox.
2568 // Because RcBox is repr(C), it will always be the last field in memory.
2569 // SAFETY: since the only unsized types possible are slices, trait objects,
2570 // and extern types, the input safety requirement is currently enough to
2571 // satisfy the requirements of align_of_val_raw; this is an implementation
2572 // detail of the language that must not be relied upon outside of std.
2573 unsafe { data_offset_align(align_of_val_raw(ptr)) }
2577 fn data_offset_align(align: usize) -> isize {
2578 let layout = Layout::new::<RcBox<()>>();
2579 (layout.size() + layout.padding_needed_for(align)) as isize