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, Alloc, Global, Layout};
256 use crate::string::String;
262 struct RcBox<T: ?Sized> {
268 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
271 /// See the [module-level documentation](./index.html) for more details.
273 /// The inherent methods of `Rc` are all associated functions, which means
274 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
275 /// `value.get_mut()`. This avoids conflicts with methods of the inner
278 /// [get_mut]: #method.get_mut
279 #[cfg_attr(not(test), lang = "rc")]
280 #[stable(feature = "rust1", since = "1.0.0")]
281 pub struct Rc<T: ?Sized> {
282 ptr: NonNull<RcBox<T>>,
283 phantom: PhantomData<RcBox<T>>,
286 #[stable(feature = "rust1", since = "1.0.0")]
287 impl<T: ?Sized> !marker::Send for Rc<T> {}
288 #[stable(feature = "rust1", since = "1.0.0")]
289 impl<T: ?Sized> !marker::Sync for Rc<T> {}
291 #[unstable(feature = "coerce_unsized", issue = "27732")]
292 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
294 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
295 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
297 impl<T: ?Sized> Rc<T> {
298 fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
299 Self { ptr, phantom: PhantomData }
302 unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
303 Self::from_inner(NonNull::new_unchecked(ptr))
308 /// Constructs a new `Rc<T>`.
315 /// let five = Rc::new(5);
317 #[stable(feature = "rust1", since = "1.0.0")]
318 pub fn new(value: T) -> Rc<T> {
319 // There is an implicit weak pointer owned by all the strong
320 // pointers, which ensures that the weak destructor never frees
321 // the allocation while the strong destructor is running, even
322 // if the weak pointer is stored inside the strong one.
323 Self::from_inner(Box::into_raw_non_null(box RcBox {
324 strong: Cell::new(1),
330 /// Constructs a new `Rc` with uninitialized contents.
335 /// #![feature(new_uninit)]
336 /// #![feature(get_mut_unchecked)]
340 /// let mut five = Rc::<u32>::new_uninit();
342 /// let five = unsafe {
343 /// // Deferred initialization:
344 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
346 /// five.assume_init()
349 /// assert_eq!(*five, 5)
351 #[unstable(feature = "new_uninit", issue = "63291")]
352 pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
354 Rc::from_ptr(Rc::allocate_for_layout(Layout::new::<T>(), |mem| {
355 mem as *mut RcBox<mem::MaybeUninit<T>>
360 /// Constructs a new `Rc` with uninitialized contents, with the memory
361 /// being filled with `0` bytes.
363 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
364 /// incorrect usage of this method.
369 /// #![feature(new_uninit)]
373 /// let zero = Rc::<u32>::new_zeroed();
374 /// let zero = unsafe { zero.assume_init() };
376 /// assert_eq!(*zero, 0)
379 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
380 #[unstable(feature = "new_uninit", issue = "63291")]
381 pub fn new_zeroed() -> Rc<mem::MaybeUninit<T>> {
383 let mut uninit = Self::new_uninit();
384 ptr::write_bytes::<T>(Rc::get_mut_unchecked(&mut uninit).as_mut_ptr(), 0, 1);
389 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
390 /// `value` will be pinned in memory and unable to be moved.
391 #[stable(feature = "pin", since = "1.33.0")]
392 pub fn pin(value: T) -> Pin<Rc<T>> {
393 unsafe { Pin::new_unchecked(Rc::new(value)) }
396 /// Returns the inner value, if the `Rc` has exactly one strong reference.
398 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
401 /// This will succeed even if there are outstanding weak references.
403 /// [result]: ../../std/result/enum.Result.html
410 /// let x = Rc::new(3);
411 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
413 /// let x = Rc::new(4);
414 /// let _y = Rc::clone(&x);
415 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
418 #[stable(feature = "rc_unique", since = "1.4.0")]
419 pub fn try_unwrap(this: Self) -> Result<T, Self> {
420 if Rc::strong_count(&this) == 1 {
422 let val = ptr::read(&*this); // copy the contained object
424 // Indicate to Weaks that they can't be promoted by decrementing
425 // the strong count, and then remove the implicit "strong weak"
426 // pointer while also handling drop logic by just crafting a
429 let _weak = Weak { ptr: this.ptr };
440 /// Constructs a new reference-counted slice with uninitialized contents.
445 /// #![feature(new_uninit)]
446 /// #![feature(get_mut_unchecked)]
450 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
452 /// let values = unsafe {
453 /// // Deferred initialization:
454 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
455 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
456 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
458 /// values.assume_init()
461 /// assert_eq!(*values, [1, 2, 3])
463 #[unstable(feature = "new_uninit", issue = "63291")]
464 pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
465 unsafe { Rc::from_ptr(Rc::allocate_for_slice(len)) }
469 impl<T> Rc<mem::MaybeUninit<T>> {
470 /// Converts to `Rc<T>`.
474 /// As with [`MaybeUninit::assume_init`],
475 /// it is up to the caller to guarantee that the inner value
476 /// really is in an initialized state.
477 /// Calling this when the content is not yet fully initialized
478 /// causes immediate undefined behavior.
480 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
485 /// #![feature(new_uninit)]
486 /// #![feature(get_mut_unchecked)]
490 /// let mut five = Rc::<u32>::new_uninit();
492 /// let five = unsafe {
493 /// // Deferred initialization:
494 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
496 /// five.assume_init()
499 /// assert_eq!(*five, 5)
501 #[unstable(feature = "new_uninit", issue = "63291")]
503 pub unsafe fn assume_init(self) -> Rc<T> {
504 Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
508 impl<T> Rc<[mem::MaybeUninit<T>]> {
509 /// Converts to `Rc<[T]>`.
513 /// As with [`MaybeUninit::assume_init`],
514 /// it is up to the caller to guarantee that the inner value
515 /// really is in an initialized state.
516 /// Calling this when the content is not yet fully initialized
517 /// causes immediate undefined behavior.
519 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
524 /// #![feature(new_uninit)]
525 /// #![feature(get_mut_unchecked)]
529 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
531 /// let values = unsafe {
532 /// // Deferred initialization:
533 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
534 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
535 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
537 /// values.assume_init()
540 /// assert_eq!(*values, [1, 2, 3])
542 #[unstable(feature = "new_uninit", issue = "63291")]
544 pub unsafe fn assume_init(self) -> Rc<[T]> {
545 Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _)
549 impl<T: ?Sized> Rc<T> {
550 /// Consumes the `Rc`, returning the wrapped pointer.
552 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
553 /// [`Rc::from_raw`][from_raw].
555 /// [from_raw]: struct.Rc.html#method.from_raw
562 /// let x = Rc::new("hello".to_owned());
563 /// let x_ptr = Rc::into_raw(x);
564 /// assert_eq!(unsafe { &*x_ptr }, "hello");
566 #[stable(feature = "rc_raw", since = "1.17.0")]
567 pub fn into_raw(this: Self) -> *const T {
568 let ptr: *const T = &*this;
573 /// Constructs an `Rc<T>` from a raw pointer.
575 /// The raw pointer must have been previously returned by a call to
576 /// [`Rc<U>::into_raw`][into_raw] where `U` must have the same size
577 /// and alignment as `T`. This is trivially true if `U` is `T`.
578 /// Note that if `U` is not `T` but has the same size and alignment, this is
579 /// basically like transmuting references of different types. See
580 /// [`mem::transmute`][transmute] for more information on what
581 /// restrictions apply in this case.
583 /// The user of `from_raw` has to make sure a specific value of `T` is only
586 /// This function is unsafe because improper use may lead to memory unsafety,
587 /// even if the returned `Rc<T>` is never accessed.
589 /// [into_raw]: struct.Rc.html#method.into_raw
590 /// [transmute]: ../../std/mem/fn.transmute.html
597 /// let x = Rc::new("hello".to_owned());
598 /// let x_ptr = Rc::into_raw(x);
601 /// // Convert back to an `Rc` to prevent leak.
602 /// let x = Rc::from_raw(x_ptr);
603 /// assert_eq!(&*x, "hello");
605 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
608 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
610 #[stable(feature = "rc_raw", since = "1.17.0")]
611 pub unsafe fn from_raw(ptr: *const T) -> Self {
612 let offset = data_offset(ptr);
614 // Reverse the offset to find the original RcBox.
615 let fake_ptr = ptr as *mut RcBox<T>;
616 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
618 Self::from_ptr(rc_ptr)
621 /// Consumes the `Rc`, returning the wrapped pointer as `NonNull<T>`.
626 /// #![feature(rc_into_raw_non_null)]
630 /// let x = Rc::new("hello".to_owned());
631 /// let ptr = Rc::into_raw_non_null(x);
632 /// let deref = unsafe { ptr.as_ref() };
633 /// assert_eq!(deref, "hello");
635 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
637 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
638 // safe because Rc guarantees its pointer is non-null
639 unsafe { NonNull::new_unchecked(Rc::into_raw(this) as *mut _) }
642 /// Creates a new [`Weak`][weak] pointer to this allocation.
644 /// [weak]: struct.Weak.html
651 /// let five = Rc::new(5);
653 /// let weak_five = Rc::downgrade(&five);
655 #[stable(feature = "rc_weak", since = "1.4.0")]
656 pub fn downgrade(this: &Self) -> Weak<T> {
658 // Make sure we do not create a dangling Weak
659 debug_assert!(!is_dangling(this.ptr));
660 Weak { ptr: this.ptr }
663 /// Gets the number of [`Weak`][weak] pointers to this allocation.
665 /// [weak]: struct.Weak.html
672 /// let five = Rc::new(5);
673 /// let _weak_five = Rc::downgrade(&five);
675 /// assert_eq!(1, Rc::weak_count(&five));
678 #[stable(feature = "rc_counts", since = "1.15.0")]
679 pub fn weak_count(this: &Self) -> usize {
683 /// Gets the number of strong (`Rc`) pointers to this allocation.
690 /// let five = Rc::new(5);
691 /// let _also_five = Rc::clone(&five);
693 /// assert_eq!(2, Rc::strong_count(&five));
696 #[stable(feature = "rc_counts", since = "1.15.0")]
697 pub fn strong_count(this: &Self) -> usize {
701 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
704 /// [weak]: struct.Weak.html
706 fn is_unique(this: &Self) -> bool {
707 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
710 /// Returns a mutable reference into the given `Rc`, if there are
711 /// no other `Rc` or [`Weak`][weak] pointers to the same allocation.
713 /// Returns [`None`] otherwise, because it is not safe to
714 /// mutate a shared value.
716 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
717 /// the inner value when there are other pointers.
719 /// [weak]: struct.Weak.html
720 /// [`None`]: ../../std/option/enum.Option.html#variant.None
721 /// [make_mut]: struct.Rc.html#method.make_mut
722 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
729 /// let mut x = Rc::new(3);
730 /// *Rc::get_mut(&mut x).unwrap() = 4;
731 /// assert_eq!(*x, 4);
733 /// let _y = Rc::clone(&x);
734 /// assert!(Rc::get_mut(&mut x).is_none());
737 #[stable(feature = "rc_unique", since = "1.4.0")]
738 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
739 if Rc::is_unique(this) { unsafe { Some(Rc::get_mut_unchecked(this)) } } else { None }
742 /// Returns a mutable reference into the given `Rc`,
743 /// without any check.
745 /// See also [`get_mut`], which is safe and does appropriate checks.
747 /// [`get_mut`]: struct.Rc.html#method.get_mut
751 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
752 /// for the duration of the returned borrow.
753 /// This is trivially the case if no such pointers exist,
754 /// for example immediately after `Rc::new`.
759 /// #![feature(get_mut_unchecked)]
763 /// let mut x = Rc::new(String::new());
765 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
767 /// assert_eq!(*x, "foo");
770 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
771 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
772 &mut this.ptr.as_mut().value
776 #[stable(feature = "ptr_eq", since = "1.17.0")]
777 /// Returns `true` if the two `Rc`s point to the same allocation
778 /// (in a vein similar to [`ptr::eq`]).
785 /// let five = Rc::new(5);
786 /// let same_five = Rc::clone(&five);
787 /// let other_five = Rc::new(5);
789 /// assert!(Rc::ptr_eq(&five, &same_five));
790 /// assert!(!Rc::ptr_eq(&five, &other_five));
793 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
794 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
795 this.ptr.as_ptr() == other.ptr.as_ptr()
799 impl<T: Clone> Rc<T> {
800 /// Makes a mutable reference into the given `Rc`.
802 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
803 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
804 /// referred to as clone-on-write.
806 /// If there are no other `Rc` pointers to this allocation, then [`Weak`]
807 /// pointers to this allocation will be disassociated.
809 /// See also [`get_mut`], which will fail rather than cloning.
811 /// [`Weak`]: struct.Weak.html
812 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
813 /// [`get_mut`]: struct.Rc.html#method.get_mut
820 /// let mut data = Rc::new(5);
822 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
823 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
824 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
825 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
826 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
828 /// // Now `data` and `other_data` point to different allocations.
829 /// assert_eq!(*data, 8);
830 /// assert_eq!(*other_data, 12);
833 /// [`Weak`] pointers will be disassociated:
838 /// let mut data = Rc::new(75);
839 /// let weak = Rc::downgrade(&data);
841 /// assert!(75 == *data);
842 /// assert!(75 == *weak.upgrade().unwrap());
844 /// *Rc::make_mut(&mut data) += 1;
846 /// assert!(76 == *data);
847 /// assert!(weak.upgrade().is_none());
850 #[stable(feature = "rc_unique", since = "1.4.0")]
851 pub fn make_mut(this: &mut Self) -> &mut T {
852 if Rc::strong_count(this) != 1 {
853 // Gotta clone the data, there are other Rcs
854 *this = Rc::new((**this).clone())
855 } else if Rc::weak_count(this) != 0 {
856 // Can just steal the data, all that's left is Weaks
858 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
859 mem::swap(this, &mut swap);
861 // Remove implicit strong-weak ref (no need to craft a fake
862 // Weak here -- we know other Weaks can clean up for us)
867 // This unsafety is ok because we're guaranteed that the pointer
868 // returned is the *only* pointer that will ever be returned to T. Our
869 // reference count is guaranteed to be 1 at this point, and we required
870 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
871 // reference to the allocation.
872 unsafe { &mut this.ptr.as_mut().value }
878 #[stable(feature = "rc_downcast", since = "1.29.0")]
879 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
884 /// use std::any::Any;
887 /// fn print_if_string(value: Rc<dyn Any>) {
888 /// if let Ok(string) = value.downcast::<String>() {
889 /// println!("String ({}): {}", string.len(), string);
893 /// let my_string = "Hello World".to_string();
894 /// print_if_string(Rc::new(my_string));
895 /// print_if_string(Rc::new(0i8));
897 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
898 if (*self).is::<T>() {
899 let ptr = self.ptr.cast::<RcBox<T>>();
901 Ok(Rc::from_inner(ptr))
908 impl<T: ?Sized> Rc<T> {
909 /// Allocates an `RcBox<T>` with sufficient space for
910 /// a possibly-unsized inner value where the value has the layout provided.
912 /// The function `mem_to_rcbox` is called with the data pointer
913 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
914 unsafe fn allocate_for_layout(
915 value_layout: Layout,
916 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
918 // Calculate layout using the given value layout.
919 // Previously, layout was calculated on the expression
920 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
921 // reference (see #54908).
922 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
924 // Allocate for the layout.
925 let mem = Global.alloc(layout).unwrap_or_else(|_| handle_alloc_error(layout));
927 // Initialize the RcBox
928 let inner = mem_to_rcbox(mem.as_ptr());
929 debug_assert_eq!(Layout::for_value(&*inner), layout);
931 ptr::write(&mut (*inner).strong, Cell::new(1));
932 ptr::write(&mut (*inner).weak, Cell::new(1));
937 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
938 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
939 // Allocate for the `RcBox<T>` using the given value.
940 Self::allocate_for_layout(Layout::for_value(&*ptr), |mem| {
941 set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>
945 fn from_box(v: Box<T>) -> Rc<T> {
947 let box_unique = Box::into_unique(v);
948 let bptr = box_unique.as_ptr();
950 let value_size = size_of_val(&*bptr);
951 let ptr = Self::allocate_for_ptr(bptr);
953 // Copy value as bytes
954 ptr::copy_nonoverlapping(
955 bptr as *const T as *const u8,
956 &mut (*ptr).value as *mut _ as *mut u8,
960 // Free the allocation without dropping its contents
961 box_free(box_unique);
969 /// Allocates an `RcBox<[T]>` with the given length.
970 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
971 Self::allocate_for_layout(Layout::array::<T>(len).unwrap(), |mem| {
972 ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>
977 /// Sets the data pointer of a `?Sized` raw pointer.
979 /// For a slice/trait object, this sets the `data` field and leaves the rest
980 /// unchanged. For a sized raw pointer, this simply sets the pointer.
981 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
982 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
987 /// Copy elements from slice into newly allocated Rc<[T]>
989 /// Unsafe because the caller must either take ownership or bind `T: Copy`
990 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
991 let ptr = Self::allocate_for_slice(v.len());
993 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).value as *mut [T] as *mut T, v.len());
998 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
1000 /// Behavior is undefined should the size be wrong.
1001 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
1002 // Panic guard while cloning T elements.
1003 // In the event of a panic, elements that have been written
1004 // into the new RcBox will be dropped, then the memory freed.
1012 impl<T> Drop for Guard<T> {
1013 fn drop(&mut self) {
1015 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1016 ptr::drop_in_place(slice);
1018 Global.dealloc(self.mem, self.layout);
1023 let ptr = Self::allocate_for_slice(len);
1025 let mem = ptr as *mut _ as *mut u8;
1026 let layout = Layout::for_value(&*ptr);
1028 // Pointer to first element
1029 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1031 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1033 for (i, item) in iter.enumerate() {
1034 ptr::write(elems.add(i), item);
1038 // All clear. Forget the guard so it doesn't free the new RcBox.
1045 /// Specialization trait used for `From<&[T]>`.
1046 trait RcFromSlice<T> {
1047 fn from_slice(slice: &[T]) -> Self;
1050 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
1052 default fn from_slice(v: &[T]) -> Self {
1053 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1057 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
1059 fn from_slice(v: &[T]) -> Self {
1060 unsafe { Rc::copy_from_slice(v) }
1064 #[stable(feature = "rust1", since = "1.0.0")]
1065 impl<T: ?Sized> Deref for Rc<T> {
1069 fn deref(&self) -> &T {
1074 #[unstable(feature = "receiver_trait", issue = "none")]
1075 impl<T: ?Sized> Receiver for Rc<T> {}
1077 #[stable(feature = "rust1", since = "1.0.0")]
1078 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1081 /// This will decrement the strong reference count. If the strong reference
1082 /// count reaches zero then the only other references (if any) are
1083 /// [`Weak`], so we `drop` the inner value.
1088 /// use std::rc::Rc;
1092 /// impl Drop for Foo {
1093 /// fn drop(&mut self) {
1094 /// println!("dropped!");
1098 /// let foo = Rc::new(Foo);
1099 /// let foo2 = Rc::clone(&foo);
1101 /// drop(foo); // Doesn't print anything
1102 /// drop(foo2); // Prints "dropped!"
1105 /// [`Weak`]: ../../std/rc/struct.Weak.html
1106 fn drop(&mut self) {
1109 if self.strong() == 0 {
1110 // destroy the contained object
1111 ptr::drop_in_place(self.ptr.as_mut());
1113 // remove the implicit "strong weak" pointer now that we've
1114 // destroyed the contents.
1117 if self.weak() == 0 {
1118 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1125 #[stable(feature = "rust1", since = "1.0.0")]
1126 impl<T: ?Sized> Clone for Rc<T> {
1127 /// Makes a clone of the `Rc` pointer.
1129 /// This creates another pointer to the same allocation, increasing the
1130 /// strong reference count.
1135 /// use std::rc::Rc;
1137 /// let five = Rc::new(5);
1139 /// let _ = Rc::clone(&five);
1142 fn clone(&self) -> Rc<T> {
1144 Self::from_inner(self.ptr)
1148 #[stable(feature = "rust1", since = "1.0.0")]
1149 impl<T: Default> Default for Rc<T> {
1150 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1155 /// use std::rc::Rc;
1157 /// let x: Rc<i32> = Default::default();
1158 /// assert_eq!(*x, 0);
1161 fn default() -> Rc<T> {
1162 Rc::new(Default::default())
1166 #[stable(feature = "rust1", since = "1.0.0")]
1167 trait RcEqIdent<T: ?Sized + PartialEq> {
1168 fn eq(&self, other: &Rc<T>) -> bool;
1169 fn ne(&self, other: &Rc<T>) -> bool;
1172 #[stable(feature = "rust1", since = "1.0.0")]
1173 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
1175 default fn eq(&self, other: &Rc<T>) -> bool {
1180 default fn ne(&self, other: &Rc<T>) -> bool {
1185 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1186 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1187 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1188 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1189 /// the same value, than two `&T`s.
1191 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1192 #[stable(feature = "rust1", since = "1.0.0")]
1193 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
1195 fn eq(&self, other: &Rc<T>) -> bool {
1196 Rc::ptr_eq(self, other) || **self == **other
1200 fn ne(&self, other: &Rc<T>) -> bool {
1201 !Rc::ptr_eq(self, other) && **self != **other
1205 #[stable(feature = "rust1", since = "1.0.0")]
1206 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1207 /// Equality for two `Rc`s.
1209 /// Two `Rc`s are equal if their inner values are equal, even if they are
1210 /// stored in different allocation.
1212 /// If `T` also implements `Eq` (implying reflexivity of equality),
1213 /// two `Rc`s that point to the same allocation are
1219 /// use std::rc::Rc;
1221 /// let five = Rc::new(5);
1223 /// assert!(five == Rc::new(5));
1226 fn eq(&self, other: &Rc<T>) -> bool {
1227 RcEqIdent::eq(self, other)
1230 /// Inequality for two `Rc`s.
1232 /// Two `Rc`s are unequal if their inner values are unequal.
1234 /// If `T` also implements `Eq` (implying reflexivity of equality),
1235 /// two `Rc`s that point to the same allocation are
1241 /// use std::rc::Rc;
1243 /// let five = Rc::new(5);
1245 /// assert!(five != Rc::new(6));
1248 fn ne(&self, other: &Rc<T>) -> bool {
1249 RcEqIdent::ne(self, other)
1253 #[stable(feature = "rust1", since = "1.0.0")]
1254 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1256 #[stable(feature = "rust1", since = "1.0.0")]
1257 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1258 /// Partial comparison for two `Rc`s.
1260 /// The two are compared by calling `partial_cmp()` on their inner values.
1265 /// use std::rc::Rc;
1266 /// use std::cmp::Ordering;
1268 /// let five = Rc::new(5);
1270 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1273 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1274 (**self).partial_cmp(&**other)
1277 /// Less-than comparison for two `Rc`s.
1279 /// The two are compared by calling `<` on their inner values.
1284 /// use std::rc::Rc;
1286 /// let five = Rc::new(5);
1288 /// assert!(five < Rc::new(6));
1291 fn lt(&self, other: &Rc<T>) -> bool {
1295 /// 'Less than or equal to' comparison for two `Rc`s.
1297 /// The two are compared by calling `<=` on their inner values.
1302 /// use std::rc::Rc;
1304 /// let five = Rc::new(5);
1306 /// assert!(five <= Rc::new(5));
1309 fn le(&self, other: &Rc<T>) -> bool {
1313 /// Greater-than comparison for two `Rc`s.
1315 /// The two are compared by calling `>` on their inner values.
1320 /// use std::rc::Rc;
1322 /// let five = Rc::new(5);
1324 /// assert!(five > Rc::new(4));
1327 fn gt(&self, other: &Rc<T>) -> bool {
1331 /// 'Greater than or equal to' comparison for two `Rc`s.
1333 /// The two are compared by calling `>=` on their inner values.
1338 /// use std::rc::Rc;
1340 /// let five = Rc::new(5);
1342 /// assert!(five >= Rc::new(5));
1345 fn ge(&self, other: &Rc<T>) -> bool {
1350 #[stable(feature = "rust1", since = "1.0.0")]
1351 impl<T: ?Sized + Ord> Ord for Rc<T> {
1352 /// Comparison for two `Rc`s.
1354 /// The two are compared by calling `cmp()` on their inner values.
1359 /// use std::rc::Rc;
1360 /// use std::cmp::Ordering;
1362 /// let five = Rc::new(5);
1364 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1367 fn cmp(&self, other: &Rc<T>) -> Ordering {
1368 (**self).cmp(&**other)
1372 #[stable(feature = "rust1", since = "1.0.0")]
1373 impl<T: ?Sized + Hash> Hash for Rc<T> {
1374 fn hash<H: Hasher>(&self, state: &mut H) {
1375 (**self).hash(state);
1379 #[stable(feature = "rust1", since = "1.0.0")]
1380 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1381 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1382 fmt::Display::fmt(&**self, f)
1386 #[stable(feature = "rust1", since = "1.0.0")]
1387 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1388 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1389 fmt::Debug::fmt(&**self, f)
1393 #[stable(feature = "rust1", since = "1.0.0")]
1394 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1395 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1396 fmt::Pointer::fmt(&(&**self as *const T), f)
1400 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1401 impl<T> From<T> for Rc<T> {
1402 fn from(t: T) -> Self {
1407 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1408 impl<T: Clone> From<&[T]> for Rc<[T]> {
1410 fn from(v: &[T]) -> Rc<[T]> {
1411 <Self as RcFromSlice<T>>::from_slice(v)
1415 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1416 impl From<&str> for Rc<str> {
1418 fn from(v: &str) -> Rc<str> {
1419 let rc = Rc::<[u8]>::from(v.as_bytes());
1420 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1424 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1425 impl From<String> for Rc<str> {
1427 fn from(v: String) -> Rc<str> {
1432 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1433 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1435 fn from(v: Box<T>) -> Rc<T> {
1440 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1441 impl<T> From<Vec<T>> for Rc<[T]> {
1443 fn from(mut v: Vec<T>) -> Rc<[T]> {
1445 let rc = Rc::copy_from_slice(&v);
1447 // Allow the Vec to free its memory, but not destroy its contents
1455 #[unstable(feature = "boxed_slice_try_from", issue = "none")]
1456 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]>
1458 [T; N]: LengthAtMost32,
1460 type Error = Rc<[T]>;
1462 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1463 if boxed_slice.len() == N {
1464 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
1471 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1472 impl<T> iter::FromIterator<T> for Rc<[T]> {
1473 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1475 /// # Performance characteristics
1477 /// ## The general case
1479 /// In the general case, collecting into `Rc<[T]>` is done by first
1480 /// collecting into a `Vec<T>`. That is, when writing the following:
1483 /// # use std::rc::Rc;
1484 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1485 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1488 /// this behaves as if we wrote:
1491 /// # use std::rc::Rc;
1492 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1493 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1494 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1495 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1498 /// This will allocate as many times as needed for constructing the `Vec<T>`
1499 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1501 /// ## Iterators of known length
1503 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1504 /// a single allocation will be made for the `Rc<[T]>`. For example:
1507 /// # use std::rc::Rc;
1508 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1509 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1511 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1512 RcFromIter::from_iter(iter.into_iter())
1516 /// Specialization trait used for collecting into `Rc<[T]>`.
1517 trait RcFromIter<T, I> {
1518 fn from_iter(iter: I) -> Self;
1521 impl<T, I: Iterator<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1522 default fn from_iter(iter: I) -> Self {
1523 iter.collect::<Vec<T>>().into()
1527 impl<T, I: iter::TrustedLen<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1528 default fn from_iter(iter: I) -> Self {
1529 // This is the case for a `TrustedLen` iterator.
1530 let (low, high) = iter.size_hint();
1531 if let Some(high) = high {
1535 "TrustedLen iterator's size hint is not exact: {:?}",
1540 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1541 Rc::from_iter_exact(iter, low)
1544 // Fall back to normal implementation.
1545 iter.collect::<Vec<T>>().into()
1550 impl<'a, T: 'a + Clone> RcFromIter<&'a T, slice::Iter<'a, T>> for Rc<[T]> {
1551 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
1552 // Delegate to `impl<T: Clone> From<&[T]> for Rc<[T]>`.
1554 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
1555 // which is even more performant.
1557 // In the fall-back case we have `T: Clone`. This is still better
1558 // than the `TrustedLen` implementation as slices have a known length
1559 // and so we get to avoid calling `size_hint` and avoid the branching.
1560 iter.as_slice().into()
1564 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1565 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
1566 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1568 /// Since a `Weak` reference does not count towards ownership, it will not
1569 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
1570 /// guarantees about the value still being present. Thus it may return [`None`]
1571 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
1572 /// itself (the backing store) from being deallocated.
1574 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
1575 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
1576 /// prevent circular references between [`Rc`] pointers, since mutual owning references
1577 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1578 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1579 /// pointers from children back to their parents.
1581 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1583 /// [`Rc`]: struct.Rc.html
1584 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1585 /// [`upgrade`]: struct.Weak.html#method.upgrade
1586 /// [`Option`]: ../../std/option/enum.Option.html
1587 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1588 #[stable(feature = "rc_weak", since = "1.4.0")]
1589 pub struct Weak<T: ?Sized> {
1590 // This is a `NonNull` to allow optimizing the size of this type in enums,
1591 // but it is not necessarily a valid pointer.
1592 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1593 // to allocate space on the heap. That's not a value a real pointer
1594 // will ever have because RcBox has alignment at least 2.
1595 ptr: NonNull<RcBox<T>>,
1598 #[stable(feature = "rc_weak", since = "1.4.0")]
1599 impl<T: ?Sized> !marker::Send for Weak<T> {}
1600 #[stable(feature = "rc_weak", since = "1.4.0")]
1601 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1603 #[unstable(feature = "coerce_unsized", issue = "27732")]
1604 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1606 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1607 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1610 /// Constructs a new `Weak<T>`, without allocating any memory.
1611 /// Calling [`upgrade`] on the return value always gives [`None`].
1613 /// [`upgrade`]: #method.upgrade
1614 /// [`None`]: ../../std/option/enum.Option.html
1619 /// use std::rc::Weak;
1621 /// let empty: Weak<i64> = Weak::new();
1622 /// assert!(empty.upgrade().is_none());
1624 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1625 pub fn new() -> Weak<T> {
1626 Weak { ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0") }
1629 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1631 /// The pointer is valid only if there are some strong references. The pointer may be dangling
1632 /// or even [`null`] otherwise.
1637 /// #![feature(weak_into_raw)]
1639 /// use std::rc::Rc;
1642 /// let strong = Rc::new("hello".to_owned());
1643 /// let weak = Rc::downgrade(&strong);
1644 /// // Both point to the same object
1645 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1646 /// // The strong here keeps it alive, so we can still access the object.
1647 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1650 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1651 /// // undefined behaviour.
1652 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1655 /// [`null`]: ../../std/ptr/fn.null.html
1656 #[unstable(feature = "weak_into_raw", issue = "60728")]
1657 pub fn as_raw(&self) -> *const T {
1658 match self.inner() {
1659 None => ptr::null(),
1661 let offset = data_offset_sized::<T>();
1662 let ptr = inner as *const RcBox<T>;
1663 // Note: while the pointer we create may already point to dropped value, the
1664 // allocation still lives (it must hold the weak point as long as we are alive).
1665 // Therefore, the offset is OK to do, it won't get out of the allocation.
1666 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1672 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1674 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1675 /// can be turned back into the `Weak<T>` with [`from_raw`].
1677 /// The same restrictions of accessing the target of the pointer as with
1678 /// [`as_raw`] apply.
1683 /// #![feature(weak_into_raw)]
1685 /// use std::rc::{Rc, Weak};
1687 /// let strong = Rc::new("hello".to_owned());
1688 /// let weak = Rc::downgrade(&strong);
1689 /// let raw = weak.into_raw();
1691 /// assert_eq!(1, Rc::weak_count(&strong));
1692 /// assert_eq!("hello", unsafe { &*raw });
1694 /// drop(unsafe { Weak::from_raw(raw) });
1695 /// assert_eq!(0, Rc::weak_count(&strong));
1698 /// [`from_raw`]: struct.Weak.html#method.from_raw
1699 /// [`as_raw`]: struct.Weak.html#method.as_raw
1700 #[unstable(feature = "weak_into_raw", issue = "60728")]
1701 pub fn into_raw(self) -> *const T {
1702 let result = self.as_raw();
1707 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1709 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1710 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1712 /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
1713 /// as these don't have any corresponding weak count).
1717 /// The pointer must have originated from the [`into_raw`] (or [`as_raw`], provided there was
1718 /// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
1721 /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
1722 /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
1728 /// #![feature(weak_into_raw)]
1730 /// use std::rc::{Rc, Weak};
1732 /// let strong = Rc::new("hello".to_owned());
1734 /// let raw_1 = Rc::downgrade(&strong).into_raw();
1735 /// let raw_2 = Rc::downgrade(&strong).into_raw();
1737 /// assert_eq!(2, Rc::weak_count(&strong));
1739 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1740 /// assert_eq!(1, Rc::weak_count(&strong));
1744 /// // Decrement the last weak count.
1745 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1748 /// [`into_raw`]: struct.Weak.html#method.into_raw
1749 /// [`upgrade`]: struct.Weak.html#method.upgrade
1750 /// [`Rc`]: struct.Rc.html
1751 /// [`Weak`]: struct.Weak.html
1752 /// [`as_raw`]: struct.Weak.html#method.as_raw
1753 /// [`new`]: struct.Weak.html#method.new
1754 /// [`forget`]: ../../std/mem/fn.forget.html
1755 #[unstable(feature = "weak_into_raw", issue = "60728")]
1756 pub unsafe fn from_raw(ptr: *const T) -> Self {
1760 // See Rc::from_raw for details
1761 let offset = data_offset(ptr);
1762 let fake_ptr = ptr as *mut RcBox<T>;
1763 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1764 Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
1769 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1770 let address = ptr.as_ptr() as *mut () as usize;
1771 address == usize::MAX
1774 impl<T: ?Sized> Weak<T> {
1775 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
1776 /// dropping of the inner value if successful.
1778 /// Returns [`None`] if the inner value has since been dropped.
1780 /// [`Rc`]: struct.Rc.html
1781 /// [`None`]: ../../std/option/enum.Option.html
1786 /// use std::rc::Rc;
1788 /// let five = Rc::new(5);
1790 /// let weak_five = Rc::downgrade(&five);
1792 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1793 /// assert!(strong_five.is_some());
1795 /// // Destroy all strong pointers.
1796 /// drop(strong_five);
1799 /// assert!(weak_five.upgrade().is_none());
1801 #[stable(feature = "rc_weak", since = "1.4.0")]
1802 pub fn upgrade(&self) -> Option<Rc<T>> {
1803 let inner = self.inner()?;
1804 if inner.strong() == 0 {
1808 Some(Rc::from_inner(self.ptr))
1812 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
1814 /// If `self` was created using [`Weak::new`], this will return 0.
1816 /// [`Weak::new`]: #method.new
1817 #[stable(feature = "weak_counts", since = "1.41.0")]
1818 pub fn strong_count(&self) -> usize {
1819 if let Some(inner) = self.inner() { inner.strong() } else { 0 }
1822 /// Gets the number of `Weak` pointers pointing to this allocation.
1824 /// If no strong pointers remain, this will return zero.
1825 #[stable(feature = "weak_counts", since = "1.41.0")]
1826 pub fn weak_count(&self) -> usize {
1829 if inner.strong() > 0 {
1830 inner.weak() - 1 // subtract the implicit weak ptr
1838 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1839 /// (i.e., when this `Weak` was created by `Weak::new`).
1841 fn inner(&self) -> Option<&RcBox<T>> {
1842 if is_dangling(self.ptr) { None } else { Some(unsafe { self.ptr.as_ref() }) }
1845 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1846 /// [`ptr::eq`]), or if both don't point to any allocation
1847 /// (because they were created with `Weak::new()`).
1851 /// Since this compares pointers it means that `Weak::new()` will equal each
1852 /// other, even though they don't point to any allocation.
1857 /// use std::rc::Rc;
1859 /// let first_rc = Rc::new(5);
1860 /// let first = Rc::downgrade(&first_rc);
1861 /// let second = Rc::downgrade(&first_rc);
1863 /// assert!(first.ptr_eq(&second));
1865 /// let third_rc = Rc::new(5);
1866 /// let third = Rc::downgrade(&third_rc);
1868 /// assert!(!first.ptr_eq(&third));
1871 /// Comparing `Weak::new`.
1874 /// use std::rc::{Rc, Weak};
1876 /// let first = Weak::new();
1877 /// let second = Weak::new();
1878 /// assert!(first.ptr_eq(&second));
1880 /// let third_rc = Rc::new(());
1881 /// let third = Rc::downgrade(&third_rc);
1882 /// assert!(!first.ptr_eq(&third));
1885 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1887 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1888 pub fn ptr_eq(&self, other: &Self) -> bool {
1889 self.ptr.as_ptr() == other.ptr.as_ptr()
1893 #[stable(feature = "rc_weak", since = "1.4.0")]
1894 impl<T: ?Sized> Drop for Weak<T> {
1895 /// Drops the `Weak` pointer.
1900 /// use std::rc::{Rc, Weak};
1904 /// impl Drop for Foo {
1905 /// fn drop(&mut self) {
1906 /// println!("dropped!");
1910 /// let foo = Rc::new(Foo);
1911 /// let weak_foo = Rc::downgrade(&foo);
1912 /// let other_weak_foo = Weak::clone(&weak_foo);
1914 /// drop(weak_foo); // Doesn't print anything
1915 /// drop(foo); // Prints "dropped!"
1917 /// assert!(other_weak_foo.upgrade().is_none());
1919 fn drop(&mut self) {
1920 if let Some(inner) = self.inner() {
1922 // the weak count starts at 1, and will only go to zero if all
1923 // the strong pointers have disappeared.
1924 if inner.weak() == 0 {
1926 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1933 #[stable(feature = "rc_weak", since = "1.4.0")]
1934 impl<T: ?Sized> Clone for Weak<T> {
1935 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1940 /// use std::rc::{Rc, Weak};
1942 /// let weak_five = Rc::downgrade(&Rc::new(5));
1944 /// let _ = Weak::clone(&weak_five);
1947 fn clone(&self) -> Weak<T> {
1948 if let Some(inner) = self.inner() {
1951 Weak { ptr: self.ptr }
1955 #[stable(feature = "rc_weak", since = "1.4.0")]
1956 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1957 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1962 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1963 impl<T> Default for Weak<T> {
1964 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1965 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1967 /// [`None`]: ../../std/option/enum.Option.html
1968 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
1973 /// use std::rc::Weak;
1975 /// let empty: Weak<i64> = Default::default();
1976 /// assert!(empty.upgrade().is_none());
1978 fn default() -> Weak<T> {
1983 // NOTE: We checked_add here to deal with mem::forget safely. In particular
1984 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1985 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1986 // We abort because this is such a degenerate scenario that we don't care about
1987 // what happens -- no real program should ever experience this.
1989 // This should have negligible overhead since you don't actually need to
1990 // clone these much in Rust thanks to ownership and move-semantics.
1993 trait RcBoxPtr<T: ?Sized> {
1994 fn inner(&self) -> &RcBox<T>;
1997 fn strong(&self) -> usize {
1998 self.inner().strong.get()
2002 fn inc_strong(&self) {
2003 let strong = self.strong();
2005 // We want to abort on overflow instead of dropping the value.
2006 // The reference count will never be zero when this is called;
2007 // nevertheless, we insert an abort here to hint LLVM at
2008 // an otherwise missed optimization.
2009 if strong == 0 || strong == usize::max_value() {
2014 self.inner().strong.set(strong + 1);
2018 fn dec_strong(&self) {
2019 self.inner().strong.set(self.strong() - 1);
2023 fn weak(&self) -> usize {
2024 self.inner().weak.get()
2028 fn inc_weak(&self) {
2029 let weak = self.weak();
2031 // We want to abort on overflow instead of dropping the value.
2032 // The reference count will never be zero when this is called;
2033 // nevertheless, we insert an abort here to hint LLVM at
2034 // an otherwise missed optimization.
2035 if weak == 0 || weak == usize::max_value() {
2040 self.inner().weak.set(weak + 1);
2044 fn dec_weak(&self) {
2045 self.inner().weak.set(self.weak() - 1);
2049 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
2051 fn inner(&self) -> &RcBox<T> {
2052 unsafe { self.ptr.as_ref() }
2056 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
2058 fn inner(&self) -> &RcBox<T> {
2063 #[stable(feature = "rust1", since = "1.0.0")]
2064 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2065 fn borrow(&self) -> &T {
2070 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2071 impl<T: ?Sized> AsRef<T> for Rc<T> {
2072 fn as_ref(&self) -> &T {
2077 #[stable(feature = "pin", since = "1.33.0")]
2078 impl<T: ?Sized> Unpin for Rc<T> {}
2080 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2081 // Align the unsized value to the end of the `RcBox`.
2082 // Because it is ?Sized, it will always be the last field in memory.
2083 // Note: This is a detail of the current implementation of the compiler,
2084 // and is not a guaranteed language detail. Do not rely on it outside of std.
2085 data_offset_align(align_of_val(&*ptr))
2088 /// Computes the offset of the data field within `RcBox`.
2090 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2091 fn data_offset_sized<T>() -> isize {
2092 data_offset_align(align_of::<T>())
2096 fn data_offset_align(align: usize) -> isize {
2097 let layout = Layout::new::<RcBox<()>>();
2098 (layout.size() + layout.padding_needed_for(align)) as isize