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;
238 use core::array::LengthAtMost32;
240 use core::cell::Cell;
241 use core::cmp::Ordering;
242 use core::convert::{From, TryFrom};
244 use core::hash::{Hash, Hasher};
245 use core::intrinsics::abort;
247 use core::marker::{self, PhantomData, Unpin, Unsize};
248 use core::mem::{self, align_of, align_of_val, forget, size_of_val};
249 use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
251 use core::ptr::{self, NonNull};
252 use core::slice::{self, from_raw_parts_mut};
255 use crate::alloc::{box_free, handle_alloc_error, AllocInit, AllocRef, Global, Layout};
256 use crate::string::String;
262 // This is repr(C) to future-proof against possible field-reordering, which
263 // would interfere with otherwise safe [into|from]_raw() of transmutable
266 struct RcBox<T: ?Sized> {
272 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
275 /// See the [module-level documentation](./index.html) for more details.
277 /// The inherent methods of `Rc` are all associated functions, which means
278 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
279 /// `value.get_mut()`. This avoids conflicts with methods of the inner
282 /// [get_mut]: #method.get_mut
283 #[cfg_attr(not(test), lang = "rc")]
284 #[stable(feature = "rust1", since = "1.0.0")]
285 pub struct Rc<T: ?Sized> {
286 ptr: NonNull<RcBox<T>>,
287 phantom: PhantomData<RcBox<T>>,
290 #[stable(feature = "rust1", since = "1.0.0")]
291 impl<T: ?Sized> !marker::Send for Rc<T> {}
292 #[stable(feature = "rust1", since = "1.0.0")]
293 impl<T: ?Sized> !marker::Sync for Rc<T> {}
295 #[unstable(feature = "coerce_unsized", issue = "27732")]
296 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
298 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
299 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
301 impl<T: ?Sized> Rc<T> {
302 fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
303 Self { ptr, phantom: PhantomData }
306 unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
307 Self::from_inner(NonNull::new_unchecked(ptr))
312 /// Constructs a new `Rc<T>`.
319 /// let five = Rc::new(5);
321 #[stable(feature = "rust1", since = "1.0.0")]
322 pub fn new(value: T) -> Rc<T> {
323 // There is an implicit weak pointer owned by all the strong
324 // pointers, which ensures that the weak destructor never frees
325 // the allocation while the strong destructor is running, even
326 // if the weak pointer is stored inside the strong one.
327 Self::from_inner(Box::into_raw_non_null(box RcBox {
328 strong: Cell::new(1),
334 /// Constructs a new `Rc` with uninitialized contents.
339 /// #![feature(new_uninit)]
340 /// #![feature(get_mut_unchecked)]
344 /// let mut five = Rc::<u32>::new_uninit();
346 /// let five = unsafe {
347 /// // Deferred initialization:
348 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
350 /// five.assume_init()
353 /// assert_eq!(*five, 5)
355 #[unstable(feature = "new_uninit", issue = "63291")]
356 pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
358 Rc::from_ptr(Rc::allocate_for_layout(Layout::new::<T>(), |mem| {
359 mem as *mut RcBox<mem::MaybeUninit<T>>
364 /// Constructs a new `Rc` with uninitialized contents, with the memory
365 /// being filled with `0` bytes.
367 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
368 /// incorrect usage of this method.
373 /// #![feature(new_uninit)]
377 /// let zero = Rc::<u32>::new_zeroed();
378 /// let zero = unsafe { zero.assume_init() };
380 /// assert_eq!(*zero, 0)
383 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
384 #[unstable(feature = "new_uninit", issue = "63291")]
385 pub fn new_zeroed() -> Rc<mem::MaybeUninit<T>> {
387 let mut uninit = Self::new_uninit();
388 ptr::write_bytes::<T>(Rc::get_mut_unchecked(&mut uninit).as_mut_ptr(), 0, 1);
393 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
394 /// `value` will be pinned in memory and unable to be moved.
395 #[stable(feature = "pin", since = "1.33.0")]
396 pub fn pin(value: T) -> Pin<Rc<T>> {
397 unsafe { Pin::new_unchecked(Rc::new(value)) }
400 /// Returns the inner value, if the `Rc` has exactly one strong reference.
402 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
405 /// This will succeed even if there are outstanding weak references.
407 /// [result]: ../../std/result/enum.Result.html
414 /// let x = Rc::new(3);
415 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
417 /// let x = Rc::new(4);
418 /// let _y = Rc::clone(&x);
419 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
422 #[stable(feature = "rc_unique", since = "1.4.0")]
423 pub fn try_unwrap(this: Self) -> Result<T, Self> {
424 if Rc::strong_count(&this) == 1 {
426 let val = ptr::read(&*this); // copy the contained object
428 // Indicate to Weaks that they can't be promoted by decrementing
429 // the strong count, and then remove the implicit "strong weak"
430 // pointer while also handling drop logic by just crafting a
433 let _weak = Weak { ptr: this.ptr };
444 /// Constructs a new reference-counted slice with uninitialized contents.
449 /// #![feature(new_uninit)]
450 /// #![feature(get_mut_unchecked)]
454 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
456 /// let values = unsafe {
457 /// // Deferred initialization:
458 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
459 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
460 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
462 /// values.assume_init()
465 /// assert_eq!(*values, [1, 2, 3])
467 #[unstable(feature = "new_uninit", issue = "63291")]
468 pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
469 unsafe { Rc::from_ptr(Rc::allocate_for_slice(len)) }
473 impl<T> Rc<mem::MaybeUninit<T>> {
474 /// Converts to `Rc<T>`.
478 /// As with [`MaybeUninit::assume_init`],
479 /// it is up to the caller to guarantee that the inner value
480 /// really is in an initialized state.
481 /// Calling this when the content is not yet fully initialized
482 /// causes immediate undefined behavior.
484 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
489 /// #![feature(new_uninit)]
490 /// #![feature(get_mut_unchecked)]
494 /// let mut five = Rc::<u32>::new_uninit();
496 /// let five = unsafe {
497 /// // Deferred initialization:
498 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
500 /// five.assume_init()
503 /// assert_eq!(*five, 5)
505 #[unstable(feature = "new_uninit", issue = "63291")]
507 pub unsafe fn assume_init(self) -> Rc<T> {
508 Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
512 impl<T> Rc<[mem::MaybeUninit<T>]> {
513 /// Converts to `Rc<[T]>`.
517 /// As with [`MaybeUninit::assume_init`],
518 /// it is up to the caller to guarantee that the inner value
519 /// really is in an initialized state.
520 /// Calling this when the content is not yet fully initialized
521 /// causes immediate undefined behavior.
523 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
528 /// #![feature(new_uninit)]
529 /// #![feature(get_mut_unchecked)]
533 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
535 /// let values = unsafe {
536 /// // Deferred initialization:
537 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
538 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
539 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
541 /// values.assume_init()
544 /// assert_eq!(*values, [1, 2, 3])
546 #[unstable(feature = "new_uninit", issue = "63291")]
548 pub unsafe fn assume_init(self) -> Rc<[T]> {
549 Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _)
553 impl<T: ?Sized> Rc<T> {
554 /// Consumes the `Rc`, returning the wrapped pointer.
556 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
557 /// [`Rc::from_raw`][from_raw].
559 /// [from_raw]: struct.Rc.html#method.from_raw
566 /// let x = Rc::new("hello".to_owned());
567 /// let x_ptr = Rc::into_raw(x);
568 /// assert_eq!(unsafe { &*x_ptr }, "hello");
570 #[stable(feature = "rc_raw", since = "1.17.0")]
571 pub fn into_raw(this: Self) -> *const T {
572 let ptr: *mut RcBox<T> = NonNull::as_ptr(this.ptr);
573 let fake_ptr = ptr as *mut T;
576 // SAFETY: This cannot go through Deref::deref.
577 // Instead, we manually offset the pointer rather than manifesting a reference.
578 // This is so that the returned pointer retains the same provenance as our pointer.
579 // This is required so that e.g. `get_mut` can write through the pointer
580 // after the Rc is recovered through `from_raw`.
582 let offset = data_offset(&(*ptr).value);
583 set_data_ptr(fake_ptr, (ptr as *mut u8).offset(offset))
587 /// Constructs an `Rc<T>` from a raw pointer.
589 /// The raw pointer must have been previously returned by a call to
590 /// [`Rc<U>::into_raw`][into_raw] where `U` must have the same size
591 /// and alignment as `T`. This is trivially true if `U` is `T`.
592 /// Note that if `U` is not `T` but has the same size and alignment, this is
593 /// basically like transmuting references of different types. See
594 /// [`mem::transmute`][transmute] for more information on what
595 /// restrictions apply in this case.
597 /// The user of `from_raw` has to make sure a specific value of `T` is only
600 /// This function is unsafe because improper use may lead to memory unsafety,
601 /// even if the returned `Rc<T>` is never accessed.
603 /// [into_raw]: struct.Rc.html#method.into_raw
604 /// [transmute]: ../../std/mem/fn.transmute.html
611 /// let x = Rc::new("hello".to_owned());
612 /// let x_ptr = Rc::into_raw(x);
615 /// // Convert back to an `Rc` to prevent leak.
616 /// let x = Rc::from_raw(x_ptr);
617 /// assert_eq!(&*x, "hello");
619 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
622 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
624 #[stable(feature = "rc_raw", since = "1.17.0")]
625 pub unsafe fn from_raw(ptr: *const T) -> Self {
626 let offset = data_offset(ptr);
628 // Reverse the offset to find the original RcBox.
629 let fake_ptr = ptr as *mut RcBox<T>;
630 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
632 Self::from_ptr(rc_ptr)
635 /// Consumes the `Rc`, returning the wrapped pointer as `NonNull<T>`.
640 /// #![feature(rc_into_raw_non_null)]
644 /// let x = Rc::new("hello".to_owned());
645 /// let ptr = Rc::into_raw_non_null(x);
646 /// let deref = unsafe { ptr.as_ref() };
647 /// assert_eq!(deref, "hello");
649 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
651 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
652 // safe because Rc guarantees its pointer is non-null
653 unsafe { NonNull::new_unchecked(Rc::into_raw(this) as *mut _) }
656 /// Creates a new [`Weak`][weak] pointer to this allocation.
658 /// [weak]: struct.Weak.html
665 /// let five = Rc::new(5);
667 /// let weak_five = Rc::downgrade(&five);
669 #[stable(feature = "rc_weak", since = "1.4.0")]
670 pub fn downgrade(this: &Self) -> Weak<T> {
672 // Make sure we do not create a dangling Weak
673 debug_assert!(!is_dangling(this.ptr));
674 Weak { ptr: this.ptr }
677 /// Gets the number of [`Weak`][weak] pointers to this allocation.
679 /// [weak]: struct.Weak.html
686 /// let five = Rc::new(5);
687 /// let _weak_five = Rc::downgrade(&five);
689 /// assert_eq!(1, Rc::weak_count(&five));
692 #[stable(feature = "rc_counts", since = "1.15.0")]
693 pub fn weak_count(this: &Self) -> usize {
697 /// Gets the number of strong (`Rc`) pointers to this allocation.
704 /// let five = Rc::new(5);
705 /// let _also_five = Rc::clone(&five);
707 /// assert_eq!(2, Rc::strong_count(&five));
710 #[stable(feature = "rc_counts", since = "1.15.0")]
711 pub fn strong_count(this: &Self) -> usize {
715 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
718 /// [weak]: struct.Weak.html
720 fn is_unique(this: &Self) -> bool {
721 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
724 /// Returns a mutable reference into the given `Rc`, if there are
725 /// no other `Rc` or [`Weak`][weak] pointers to the same allocation.
727 /// Returns [`None`] otherwise, because it is not safe to
728 /// mutate a shared value.
730 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
731 /// the inner value when there are other pointers.
733 /// [weak]: struct.Weak.html
734 /// [`None`]: ../../std/option/enum.Option.html#variant.None
735 /// [make_mut]: struct.Rc.html#method.make_mut
736 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
743 /// let mut x = Rc::new(3);
744 /// *Rc::get_mut(&mut x).unwrap() = 4;
745 /// assert_eq!(*x, 4);
747 /// let _y = Rc::clone(&x);
748 /// assert!(Rc::get_mut(&mut x).is_none());
751 #[stable(feature = "rc_unique", since = "1.4.0")]
752 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
753 if Rc::is_unique(this) { unsafe { Some(Rc::get_mut_unchecked(this)) } } else { None }
756 /// Returns a mutable reference into the given `Rc`,
757 /// without any check.
759 /// See also [`get_mut`], which is safe and does appropriate checks.
761 /// [`get_mut`]: struct.Rc.html#method.get_mut
765 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
766 /// for the duration of the returned borrow.
767 /// This is trivially the case if no such pointers exist,
768 /// for example immediately after `Rc::new`.
773 /// #![feature(get_mut_unchecked)]
777 /// let mut x = Rc::new(String::new());
779 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
781 /// assert_eq!(*x, "foo");
784 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
785 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
786 &mut this.ptr.as_mut().value
790 #[stable(feature = "ptr_eq", since = "1.17.0")]
791 /// Returns `true` if the two `Rc`s point to the same allocation
792 /// (in a vein similar to [`ptr::eq`]).
799 /// let five = Rc::new(5);
800 /// let same_five = Rc::clone(&five);
801 /// let other_five = Rc::new(5);
803 /// assert!(Rc::ptr_eq(&five, &same_five));
804 /// assert!(!Rc::ptr_eq(&five, &other_five));
807 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
808 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
809 this.ptr.as_ptr() == other.ptr.as_ptr()
813 impl<T: Clone> Rc<T> {
814 /// Makes a mutable reference into the given `Rc`.
816 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
817 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
818 /// referred to as clone-on-write.
820 /// If there are no other `Rc` pointers to this allocation, then [`Weak`]
821 /// pointers to this allocation will be disassociated.
823 /// See also [`get_mut`], which will fail rather than cloning.
825 /// [`Weak`]: struct.Weak.html
826 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
827 /// [`get_mut`]: struct.Rc.html#method.get_mut
834 /// let mut data = Rc::new(5);
836 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
837 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
838 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
839 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
840 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
842 /// // Now `data` and `other_data` point to different allocations.
843 /// assert_eq!(*data, 8);
844 /// assert_eq!(*other_data, 12);
847 /// [`Weak`] pointers will be disassociated:
852 /// let mut data = Rc::new(75);
853 /// let weak = Rc::downgrade(&data);
855 /// assert!(75 == *data);
856 /// assert!(75 == *weak.upgrade().unwrap());
858 /// *Rc::make_mut(&mut data) += 1;
860 /// assert!(76 == *data);
861 /// assert!(weak.upgrade().is_none());
864 #[stable(feature = "rc_unique", since = "1.4.0")]
865 pub fn make_mut(this: &mut Self) -> &mut T {
866 if Rc::strong_count(this) != 1 {
867 // Gotta clone the data, there are other Rcs
868 *this = Rc::new((**this).clone())
869 } else if Rc::weak_count(this) != 0 {
870 // Can just steal the data, all that's left is Weaks
872 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
873 mem::swap(this, &mut swap);
875 // Remove implicit strong-weak ref (no need to craft a fake
876 // Weak here -- we know other Weaks can clean up for us)
881 // This unsafety is ok because we're guaranteed that the pointer
882 // returned is the *only* pointer that will ever be returned to T. Our
883 // reference count is guaranteed to be 1 at this point, and we required
884 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
885 // reference to the allocation.
886 unsafe { &mut this.ptr.as_mut().value }
892 #[stable(feature = "rc_downcast", since = "1.29.0")]
893 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
898 /// use std::any::Any;
901 /// fn print_if_string(value: Rc<dyn Any>) {
902 /// if let Ok(string) = value.downcast::<String>() {
903 /// println!("String ({}): {}", string.len(), string);
907 /// let my_string = "Hello World".to_string();
908 /// print_if_string(Rc::new(my_string));
909 /// print_if_string(Rc::new(0i8));
911 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
912 if (*self).is::<T>() {
913 let ptr = self.ptr.cast::<RcBox<T>>();
915 Ok(Rc::from_inner(ptr))
922 impl<T: ?Sized> Rc<T> {
923 /// Allocates an `RcBox<T>` with sufficient space for
924 /// a possibly-unsized inner value where the value has the layout provided.
926 /// The function `mem_to_rcbox` is called with the data pointer
927 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
928 unsafe fn allocate_for_layout(
929 value_layout: Layout,
930 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
932 // Calculate layout using the given value layout.
933 // Previously, layout was calculated on the expression
934 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
935 // reference (see #54908).
936 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
938 // Allocate for the layout.
940 .alloc(layout, AllocInit::Uninitialized)
941 .unwrap_or_else(|_| handle_alloc_error(layout));
943 // Initialize the RcBox
944 let inner = mem_to_rcbox(mem.ptr().as_ptr());
945 debug_assert_eq!(Layout::for_value(&*inner), layout);
947 ptr::write(&mut (*inner).strong, Cell::new(1));
948 ptr::write(&mut (*inner).weak, Cell::new(1));
953 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
954 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
955 // Allocate for the `RcBox<T>` using the given value.
956 Self::allocate_for_layout(Layout::for_value(&*ptr), |mem| {
957 set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>
961 fn from_box(v: Box<T>) -> Rc<T> {
963 let box_unique = Box::into_unique(v);
964 let bptr = box_unique.as_ptr();
966 let value_size = size_of_val(&*bptr);
967 let ptr = Self::allocate_for_ptr(bptr);
969 // Copy value as bytes
970 ptr::copy_nonoverlapping(
971 bptr as *const T as *const u8,
972 &mut (*ptr).value as *mut _ as *mut u8,
976 // Free the allocation without dropping its contents
977 box_free(box_unique);
985 /// Allocates an `RcBox<[T]>` with the given length.
986 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
987 Self::allocate_for_layout(Layout::array::<T>(len).unwrap(), |mem| {
988 ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>
993 /// Sets the data pointer of a `?Sized` raw pointer.
995 /// For a slice/trait object, this sets the `data` field and leaves the rest
996 /// unchanged. For a sized raw pointer, this simply sets the pointer.
997 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
998 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
1003 /// Copy elements from slice into newly allocated Rc<[T]>
1005 /// Unsafe because the caller must either take ownership or bind `T: Copy`
1006 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
1007 let ptr = Self::allocate_for_slice(v.len());
1009 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).value as *mut [T] as *mut T, v.len());
1014 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
1016 /// Behavior is undefined should the size be wrong.
1017 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
1018 // Panic guard while cloning T elements.
1019 // In the event of a panic, elements that have been written
1020 // into the new RcBox will be dropped, then the memory freed.
1028 impl<T> Drop for Guard<T> {
1029 fn drop(&mut self) {
1031 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1032 ptr::drop_in_place(slice);
1034 Global.dealloc(self.mem, self.layout);
1039 let ptr = Self::allocate_for_slice(len);
1041 let mem = ptr as *mut _ as *mut u8;
1042 let layout = Layout::for_value(&*ptr);
1044 // Pointer to first element
1045 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1047 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1049 for (i, item) in iter.enumerate() {
1050 ptr::write(elems.add(i), item);
1054 // 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 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1202 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1203 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1204 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1205 /// the same value, than two `&T`s.
1207 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1208 #[stable(feature = "rust1", since = "1.0.0")]
1209 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
1211 fn eq(&self, other: &Rc<T>) -> bool {
1212 Rc::ptr_eq(self, other) || **self == **other
1216 fn ne(&self, other: &Rc<T>) -> bool {
1217 !Rc::ptr_eq(self, other) && **self != **other
1221 #[stable(feature = "rust1", since = "1.0.0")]
1222 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1223 /// Equality for two `Rc`s.
1225 /// Two `Rc`s are equal if their inner values are equal, even if they are
1226 /// stored in different allocation.
1228 /// If `T` also implements `Eq` (implying reflexivity of equality),
1229 /// two `Rc`s that point to the same allocation are
1235 /// use std::rc::Rc;
1237 /// let five = Rc::new(5);
1239 /// assert!(five == Rc::new(5));
1242 fn eq(&self, other: &Rc<T>) -> bool {
1243 RcEqIdent::eq(self, other)
1246 /// Inequality for two `Rc`s.
1248 /// Two `Rc`s are unequal if their inner values are unequal.
1250 /// If `T` also implements `Eq` (implying reflexivity of equality),
1251 /// two `Rc`s that point to the same allocation are
1257 /// use std::rc::Rc;
1259 /// let five = Rc::new(5);
1261 /// assert!(five != Rc::new(6));
1264 fn ne(&self, other: &Rc<T>) -> bool {
1265 RcEqIdent::ne(self, other)
1269 #[stable(feature = "rust1", since = "1.0.0")]
1270 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1272 #[stable(feature = "rust1", since = "1.0.0")]
1273 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1274 /// Partial comparison for two `Rc`s.
1276 /// The two are compared by calling `partial_cmp()` on their inner values.
1281 /// use std::rc::Rc;
1282 /// use std::cmp::Ordering;
1284 /// let five = Rc::new(5);
1286 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1289 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1290 (**self).partial_cmp(&**other)
1293 /// Less-than comparison for two `Rc`s.
1295 /// The two are compared by calling `<` on their inner values.
1300 /// use std::rc::Rc;
1302 /// let five = Rc::new(5);
1304 /// assert!(five < Rc::new(6));
1307 fn lt(&self, other: &Rc<T>) -> bool {
1311 /// 'Less than or equal to' comparison for two `Rc`s.
1313 /// The two are compared by calling `<=` on their inner values.
1318 /// use std::rc::Rc;
1320 /// let five = Rc::new(5);
1322 /// assert!(five <= Rc::new(5));
1325 fn le(&self, other: &Rc<T>) -> bool {
1329 /// Greater-than comparison for two `Rc`s.
1331 /// The two are compared by calling `>` on their inner values.
1336 /// use std::rc::Rc;
1338 /// let five = Rc::new(5);
1340 /// assert!(five > Rc::new(4));
1343 fn gt(&self, other: &Rc<T>) -> bool {
1347 /// 'Greater than or equal to' comparison for two `Rc`s.
1349 /// The two are compared by calling `>=` on their inner values.
1354 /// use std::rc::Rc;
1356 /// let five = Rc::new(5);
1358 /// assert!(five >= Rc::new(5));
1361 fn ge(&self, other: &Rc<T>) -> bool {
1366 #[stable(feature = "rust1", since = "1.0.0")]
1367 impl<T: ?Sized + Ord> Ord for Rc<T> {
1368 /// Comparison for two `Rc`s.
1370 /// The two are compared by calling `cmp()` on their inner values.
1375 /// use std::rc::Rc;
1376 /// use std::cmp::Ordering;
1378 /// let five = Rc::new(5);
1380 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1383 fn cmp(&self, other: &Rc<T>) -> Ordering {
1384 (**self).cmp(&**other)
1388 #[stable(feature = "rust1", since = "1.0.0")]
1389 impl<T: ?Sized + Hash> Hash for Rc<T> {
1390 fn hash<H: Hasher>(&self, state: &mut H) {
1391 (**self).hash(state);
1395 #[stable(feature = "rust1", since = "1.0.0")]
1396 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1397 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1398 fmt::Display::fmt(&**self, f)
1402 #[stable(feature = "rust1", since = "1.0.0")]
1403 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1404 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1405 fmt::Debug::fmt(&**self, f)
1409 #[stable(feature = "rust1", since = "1.0.0")]
1410 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1411 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1412 fmt::Pointer::fmt(&(&**self as *const T), f)
1416 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1417 impl<T> From<T> for Rc<T> {
1418 fn from(t: T) -> Self {
1423 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1424 impl<T: Clone> From<&[T]> for Rc<[T]> {
1426 fn from(v: &[T]) -> Rc<[T]> {
1427 <Self as RcFromSlice<T>>::from_slice(v)
1431 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1432 impl From<&str> for Rc<str> {
1434 fn from(v: &str) -> Rc<str> {
1435 let rc = Rc::<[u8]>::from(v.as_bytes());
1436 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1440 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1441 impl From<String> for Rc<str> {
1443 fn from(v: String) -> Rc<str> {
1448 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1449 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1451 fn from(v: Box<T>) -> Rc<T> {
1456 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1457 impl<T> From<Vec<T>> for Rc<[T]> {
1459 fn from(mut v: Vec<T>) -> Rc<[T]> {
1461 let rc = Rc::copy_from_slice(&v);
1463 // Allow the Vec to free its memory, but not destroy its contents
1471 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1472 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]>
1474 [T; N]: LengthAtMost32,
1476 type Error = Rc<[T]>;
1478 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1479 if boxed_slice.len() == N {
1480 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
1487 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1488 impl<T> iter::FromIterator<T> for Rc<[T]> {
1489 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1491 /// # Performance characteristics
1493 /// ## The general case
1495 /// In the general case, collecting into `Rc<[T]>` is done by first
1496 /// collecting into a `Vec<T>`. That is, when writing the following:
1499 /// # use std::rc::Rc;
1500 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1501 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1504 /// this behaves as if we wrote:
1507 /// # use std::rc::Rc;
1508 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1509 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1510 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1511 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1514 /// This will allocate as many times as needed for constructing the `Vec<T>`
1515 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1517 /// ## Iterators of known length
1519 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1520 /// a single allocation will be made for the `Rc<[T]>`. For example:
1523 /// # use std::rc::Rc;
1524 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1525 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1527 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1528 RcFromIter::from_iter(iter.into_iter())
1532 /// Specialization trait used for collecting into `Rc<[T]>`.
1533 trait RcFromIter<T, I> {
1534 fn from_iter(iter: I) -> Self;
1537 impl<T, I: Iterator<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1538 default fn from_iter(iter: I) -> Self {
1539 iter.collect::<Vec<T>>().into()
1543 impl<T, I: iter::TrustedLen<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1544 default fn from_iter(iter: I) -> Self {
1545 // This is the case for a `TrustedLen` iterator.
1546 let (low, high) = iter.size_hint();
1547 if let Some(high) = high {
1551 "TrustedLen iterator's size hint is not exact: {:?}",
1556 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1557 Rc::from_iter_exact(iter, low)
1560 // Fall back to normal implementation.
1561 iter.collect::<Vec<T>>().into()
1566 impl<'a, T: 'a + Clone> RcFromIter<&'a T, slice::Iter<'a, T>> for Rc<[T]> {
1567 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
1568 // Delegate to `impl<T: Clone> From<&[T]> for Rc<[T]>`.
1570 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
1571 // which is even more performant.
1573 // In the fall-back case we have `T: Clone`. This is still better
1574 // than the `TrustedLen` implementation as slices have a known length
1575 // and so we get to avoid calling `size_hint` and avoid the branching.
1576 iter.as_slice().into()
1580 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1581 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
1582 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1584 /// Since a `Weak` reference does not count towards ownership, it will not
1585 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
1586 /// guarantees about the value still being present. Thus it may return [`None`]
1587 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
1588 /// itself (the backing store) from being deallocated.
1590 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
1591 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
1592 /// prevent circular references between [`Rc`] pointers, since mutual owning references
1593 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1594 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1595 /// pointers from children back to their parents.
1597 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1599 /// [`Rc`]: struct.Rc.html
1600 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1601 /// [`upgrade`]: struct.Weak.html#method.upgrade
1602 /// [`Option`]: ../../std/option/enum.Option.html
1603 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1604 #[stable(feature = "rc_weak", since = "1.4.0")]
1605 pub struct Weak<T: ?Sized> {
1606 // This is a `NonNull` to allow optimizing the size of this type in enums,
1607 // but it is not necessarily a valid pointer.
1608 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1609 // to allocate space on the heap. That's not a value a real pointer
1610 // will ever have because RcBox has alignment at least 2.
1611 ptr: NonNull<RcBox<T>>,
1614 #[stable(feature = "rc_weak", since = "1.4.0")]
1615 impl<T: ?Sized> !marker::Send for Weak<T> {}
1616 #[stable(feature = "rc_weak", since = "1.4.0")]
1617 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1619 #[unstable(feature = "coerce_unsized", issue = "27732")]
1620 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1622 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1623 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1626 /// Constructs a new `Weak<T>`, without allocating any memory.
1627 /// Calling [`upgrade`] on the return value always gives [`None`].
1629 /// [`upgrade`]: #method.upgrade
1630 /// [`None`]: ../../std/option/enum.Option.html
1635 /// use std::rc::Weak;
1637 /// let empty: Weak<i64> = Weak::new();
1638 /// assert!(empty.upgrade().is_none());
1640 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1641 pub fn new() -> Weak<T> {
1642 Weak { ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0") }
1645 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1647 /// The pointer is valid only if there are some strong references. The pointer may be dangling
1648 /// or even [`null`] otherwise.
1653 /// #![feature(weak_into_raw)]
1655 /// use std::rc::Rc;
1658 /// let strong = Rc::new("hello".to_owned());
1659 /// let weak = Rc::downgrade(&strong);
1660 /// // Both point to the same object
1661 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1662 /// // The strong here keeps it alive, so we can still access the object.
1663 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1666 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1667 /// // undefined behaviour.
1668 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1671 /// [`null`]: ../../std/ptr/fn.null.html
1672 #[unstable(feature = "weak_into_raw", issue = "60728")]
1673 pub fn as_raw(&self) -> *const T {
1674 match self.inner() {
1675 None => ptr::null(),
1677 let offset = data_offset_sized::<T>();
1678 let ptr = inner as *const RcBox<T>;
1679 // Note: while the pointer we create may already point to dropped value, the
1680 // allocation still lives (it must hold the weak point as long as we are alive).
1681 // Therefore, the offset is OK to do, it won't get out of the allocation.
1682 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1688 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1690 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1691 /// can be turned back into the `Weak<T>` with [`from_raw`].
1693 /// The same restrictions of accessing the target of the pointer as with
1694 /// [`as_raw`] apply.
1699 /// #![feature(weak_into_raw)]
1701 /// use std::rc::{Rc, Weak};
1703 /// let strong = Rc::new("hello".to_owned());
1704 /// let weak = Rc::downgrade(&strong);
1705 /// let raw = weak.into_raw();
1707 /// assert_eq!(1, Rc::weak_count(&strong));
1708 /// assert_eq!("hello", unsafe { &*raw });
1710 /// drop(unsafe { Weak::from_raw(raw) });
1711 /// assert_eq!(0, Rc::weak_count(&strong));
1714 /// [`from_raw`]: struct.Weak.html#method.from_raw
1715 /// [`as_raw`]: struct.Weak.html#method.as_raw
1716 #[unstable(feature = "weak_into_raw", issue = "60728")]
1717 pub fn into_raw(self) -> *const T {
1718 let result = self.as_raw();
1723 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1725 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1726 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1728 /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
1729 /// as these don't have any corresponding weak count).
1733 /// The pointer must have originated from the [`into_raw`] (or [`as_raw`], provided there was
1734 /// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
1737 /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
1738 /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
1744 /// #![feature(weak_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 /// [`as_raw`]: struct.Weak.html#method.as_raw
1769 /// [`new`]: struct.Weak.html#method.new
1770 /// [`forget`]: ../../std/mem/fn.forget.html
1771 #[unstable(feature = "weak_into_raw", issue = "60728")]
1772 pub unsafe fn from_raw(ptr: *const T) -> Self {
1776 // 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") }
1785 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1786 let address = ptr.as_ptr() as *mut () as usize;
1787 address == usize::MAX
1790 impl<T: ?Sized> Weak<T> {
1791 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
1792 /// dropping of the inner value if successful.
1794 /// Returns [`None`] if the inner value has since been dropped.
1796 /// [`Rc`]: struct.Rc.html
1797 /// [`None`]: ../../std/option/enum.Option.html
1802 /// use std::rc::Rc;
1804 /// let five = Rc::new(5);
1806 /// let weak_five = Rc::downgrade(&five);
1808 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1809 /// assert!(strong_five.is_some());
1811 /// // Destroy all strong pointers.
1812 /// drop(strong_five);
1815 /// assert!(weak_five.upgrade().is_none());
1817 #[stable(feature = "rc_weak", since = "1.4.0")]
1818 pub fn upgrade(&self) -> Option<Rc<T>> {
1819 let inner = self.inner()?;
1820 if inner.strong() == 0 {
1824 Some(Rc::from_inner(self.ptr))
1828 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
1830 /// If `self` was created using [`Weak::new`], this will return 0.
1832 /// [`Weak::new`]: #method.new
1833 #[stable(feature = "weak_counts", since = "1.41.0")]
1834 pub fn strong_count(&self) -> usize {
1835 if let Some(inner) = self.inner() { inner.strong() } else { 0 }
1838 /// Gets the number of `Weak` pointers pointing to this allocation.
1840 /// If no strong pointers remain, this will return zero.
1841 #[stable(feature = "weak_counts", since = "1.41.0")]
1842 pub fn weak_count(&self) -> usize {
1845 if inner.strong() > 0 {
1846 inner.weak() - 1 // subtract the implicit weak ptr
1854 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1855 /// (i.e., when this `Weak` was created by `Weak::new`).
1857 fn inner(&self) -> Option<&RcBox<T>> {
1858 if is_dangling(self.ptr) { None } else { Some(unsafe { self.ptr.as_ref() }) }
1861 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1862 /// [`ptr::eq`]), or if both don't point to any allocation
1863 /// (because they were created with `Weak::new()`).
1867 /// Since this compares pointers it means that `Weak::new()` will equal each
1868 /// other, even though they don't point to any allocation.
1873 /// use std::rc::Rc;
1875 /// let first_rc = Rc::new(5);
1876 /// let first = Rc::downgrade(&first_rc);
1877 /// let second = Rc::downgrade(&first_rc);
1879 /// assert!(first.ptr_eq(&second));
1881 /// let third_rc = Rc::new(5);
1882 /// let third = Rc::downgrade(&third_rc);
1884 /// assert!(!first.ptr_eq(&third));
1887 /// Comparing `Weak::new`.
1890 /// use std::rc::{Rc, Weak};
1892 /// let first = Weak::new();
1893 /// let second = Weak::new();
1894 /// assert!(first.ptr_eq(&second));
1896 /// let third_rc = Rc::new(());
1897 /// let third = Rc::downgrade(&third_rc);
1898 /// assert!(!first.ptr_eq(&third));
1901 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1903 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1904 pub fn ptr_eq(&self, other: &Self) -> bool {
1905 self.ptr.as_ptr() == other.ptr.as_ptr()
1909 #[stable(feature = "rc_weak", since = "1.4.0")]
1910 impl<T: ?Sized> Drop for Weak<T> {
1911 /// Drops the `Weak` pointer.
1916 /// use std::rc::{Rc, Weak};
1920 /// impl Drop for Foo {
1921 /// fn drop(&mut self) {
1922 /// println!("dropped!");
1926 /// let foo = Rc::new(Foo);
1927 /// let weak_foo = Rc::downgrade(&foo);
1928 /// let other_weak_foo = Weak::clone(&weak_foo);
1930 /// drop(weak_foo); // Doesn't print anything
1931 /// drop(foo); // Prints "dropped!"
1933 /// assert!(other_weak_foo.upgrade().is_none());
1935 fn drop(&mut self) {
1936 if let Some(inner) = self.inner() {
1938 // the weak count starts at 1, and will only go to zero if all
1939 // the strong pointers have disappeared.
1940 if inner.weak() == 0 {
1942 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1949 #[stable(feature = "rc_weak", since = "1.4.0")]
1950 impl<T: ?Sized> Clone for Weak<T> {
1951 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1956 /// use std::rc::{Rc, Weak};
1958 /// let weak_five = Rc::downgrade(&Rc::new(5));
1960 /// let _ = Weak::clone(&weak_five);
1963 fn clone(&self) -> Weak<T> {
1964 if let Some(inner) = self.inner() {
1967 Weak { ptr: self.ptr }
1971 #[stable(feature = "rc_weak", since = "1.4.0")]
1972 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1973 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1978 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1979 impl<T> Default for Weak<T> {
1980 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1981 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1983 /// [`None`]: ../../std/option/enum.Option.html
1984 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
1989 /// use std::rc::Weak;
1991 /// let empty: Weak<i64> = Default::default();
1992 /// assert!(empty.upgrade().is_none());
1994 fn default() -> Weak<T> {
1999 // NOTE: We checked_add here to deal with mem::forget safely. In particular
2000 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
2001 // you can free the allocation while outstanding Rcs (or Weaks) exist.
2002 // We abort because this is such a degenerate scenario that we don't care about
2003 // what happens -- no real program should ever experience this.
2005 // This should have negligible overhead since you don't actually need to
2006 // clone these much in Rust thanks to ownership and move-semantics.
2009 trait RcBoxPtr<T: ?Sized> {
2010 fn inner(&self) -> &RcBox<T>;
2013 fn strong(&self) -> usize {
2014 self.inner().strong.get()
2018 fn inc_strong(&self) {
2019 let strong = self.strong();
2021 // We want to abort on overflow instead of dropping the value.
2022 // The reference count will never be zero when this is called;
2023 // nevertheless, we insert an abort here to hint LLVM at
2024 // an otherwise missed optimization.
2025 if strong == 0 || strong == usize::max_value() {
2030 self.inner().strong.set(strong + 1);
2034 fn dec_strong(&self) {
2035 self.inner().strong.set(self.strong() - 1);
2039 fn weak(&self) -> usize {
2040 self.inner().weak.get()
2044 fn inc_weak(&self) {
2045 let weak = self.weak();
2047 // We want to abort on overflow instead of dropping the value.
2048 // The reference count will never be zero when this is called;
2049 // nevertheless, we insert an abort here to hint LLVM at
2050 // an otherwise missed optimization.
2051 if weak == 0 || weak == usize::max_value() {
2056 self.inner().weak.set(weak + 1);
2060 fn dec_weak(&self) {
2061 self.inner().weak.set(self.weak() - 1);
2065 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
2067 fn inner(&self) -> &RcBox<T> {
2068 unsafe { self.ptr.as_ref() }
2072 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
2074 fn inner(&self) -> &RcBox<T> {
2079 #[stable(feature = "rust1", since = "1.0.0")]
2080 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2081 fn borrow(&self) -> &T {
2086 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2087 impl<T: ?Sized> AsRef<T> for Rc<T> {
2088 fn as_ref(&self) -> &T {
2093 #[stable(feature = "pin", since = "1.33.0")]
2094 impl<T: ?Sized> Unpin for Rc<T> {}
2096 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2097 // Align the unsized value to the end of the `RcBox`.
2098 // Because it is ?Sized, it will always be the last field in memory.
2099 // Note: This is a detail of the current implementation of the compiler,
2100 // and is not a guaranteed language detail. Do not rely on it outside of std.
2101 data_offset_align(align_of_val(&*ptr))
2104 /// Computes the offset of the data field within `RcBox`.
2106 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2107 fn data_offset_sized<T>() -> isize {
2108 data_offset_align(align_of::<T>())
2112 fn data_offset_align(align: usize) -> isize {
2113 let layout = Layout::new::<RcBox<()>>();
2114 (layout.size() + layout.padding_needed_for(align)) as isize