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;
243 use core::hash::{Hash, Hasher};
244 use core::intrinsics::abort;
246 use core::marker::{self, Unpin, Unsize, PhantomData};
247 use core::mem::{self, align_of, align_of_val, forget, size_of_val};
248 use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn};
250 use core::ptr::{self, NonNull};
251 use core::slice::{self, from_raw_parts_mut};
252 use core::convert::{From, TryFrom};
255 use crate::alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
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 = "0")]
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 {
301 phantom: PhantomData,
305 unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
306 Self::from_inner(NonNull::new_unchecked(ptr))
311 /// Constructs a new `Rc<T>`.
318 /// let five = Rc::new(5);
320 #[stable(feature = "rust1", since = "1.0.0")]
321 pub fn new(value: T) -> Rc<T> {
322 // There is an implicit weak pointer owned by all the strong
323 // pointers, which ensures that the weak destructor never frees
324 // the allocation while the strong destructor is running, even
325 // if the weak pointer is stored inside the strong one.
326 Self::from_inner(Box::into_raw_non_null(box RcBox {
327 strong: Cell::new(1),
333 /// Constructs a new `Rc` with uninitialized contents.
338 /// #![feature(new_uninit)]
339 /// #![feature(get_mut_unchecked)]
343 /// let mut five = Rc::<u32>::new_uninit();
345 /// let five = unsafe {
346 /// // Deferred initialization:
347 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
349 /// five.assume_init()
352 /// assert_eq!(*five, 5)
354 #[unstable(feature = "new_uninit", issue = "63291")]
355 pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
357 Rc::from_ptr(Rc::allocate_for_layout(
359 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
364 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
365 /// `value` will be pinned in memory and unable to be moved.
366 #[stable(feature = "pin", since = "1.33.0")]
367 pub fn pin(value: T) -> Pin<Rc<T>> {
368 unsafe { Pin::new_unchecked(Rc::new(value)) }
371 /// Returns the inner value, if the `Rc` has exactly one strong reference.
373 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
376 /// This will succeed even if there are outstanding weak references.
378 /// [result]: ../../std/result/enum.Result.html
385 /// let x = Rc::new(3);
386 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
388 /// let x = Rc::new(4);
389 /// let _y = Rc::clone(&x);
390 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
393 #[stable(feature = "rc_unique", since = "1.4.0")]
394 pub fn try_unwrap(this: Self) -> Result<T, Self> {
395 if Rc::strong_count(&this) == 1 {
397 let val = ptr::read(&*this); // copy the contained object
399 // Indicate to Weaks that they can't be promoted by decrementing
400 // the strong count, and then remove the implicit "strong weak"
401 // pointer while also handling drop logic by just crafting a
404 let _weak = Weak { ptr: this.ptr };
415 /// Constructs a new reference-counted slice with uninitialized contents.
420 /// #![feature(new_uninit)]
421 /// #![feature(get_mut_unchecked)]
425 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
427 /// let values = unsafe {
428 /// // Deferred initialization:
429 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
430 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
431 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
433 /// values.assume_init()
436 /// assert_eq!(*values, [1, 2, 3])
438 #[unstable(feature = "new_uninit", issue = "63291")]
439 pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
441 Rc::from_ptr(Rc::allocate_for_slice(len))
446 impl<T> Rc<mem::MaybeUninit<T>> {
447 /// Converts to `Rc<T>`.
451 /// As with [`MaybeUninit::assume_init`],
452 /// it is up to the caller to guarantee that the inner value
453 /// really is in an initialized state.
454 /// Calling this when the content is not yet fully initialized
455 /// causes immediate undefined behavior.
457 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
462 /// #![feature(new_uninit)]
463 /// #![feature(get_mut_unchecked)]
467 /// let mut five = Rc::<u32>::new_uninit();
469 /// let five = unsafe {
470 /// // Deferred initialization:
471 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
473 /// five.assume_init()
476 /// assert_eq!(*five, 5)
478 #[unstable(feature = "new_uninit", issue = "63291")]
480 pub unsafe fn assume_init(self) -> Rc<T> {
481 Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
485 impl<T> Rc<[mem::MaybeUninit<T>]> {
486 /// Converts to `Rc<[T]>`.
490 /// As with [`MaybeUninit::assume_init`],
491 /// it is up to the caller to guarantee that the inner value
492 /// really is in an initialized state.
493 /// Calling this when the content is not yet fully initialized
494 /// causes immediate undefined behavior.
496 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
501 /// #![feature(new_uninit)]
502 /// #![feature(get_mut_unchecked)]
506 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
508 /// let values = unsafe {
509 /// // Deferred initialization:
510 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
511 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
512 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
514 /// values.assume_init()
517 /// assert_eq!(*values, [1, 2, 3])
519 #[unstable(feature = "new_uninit", issue = "63291")]
521 pub unsafe fn assume_init(self) -> Rc<[T]> {
522 Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _)
526 impl<T: ?Sized> Rc<T> {
527 /// Consumes the `Rc`, returning the wrapped pointer.
529 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
530 /// [`Rc::from_raw`][from_raw].
532 /// [from_raw]: struct.Rc.html#method.from_raw
539 /// let x = Rc::new("hello".to_owned());
540 /// let x_ptr = Rc::into_raw(x);
541 /// assert_eq!(unsafe { &*x_ptr }, "hello");
543 #[stable(feature = "rc_raw", since = "1.17.0")]
544 pub fn into_raw(this: Self) -> *const T {
545 let ptr: *const T = &*this;
550 /// Constructs an `Rc` from a raw pointer.
552 /// The raw pointer must have been previously returned by a call to a
553 /// [`Rc::into_raw`][into_raw].
555 /// This function is unsafe because improper use may lead to memory problems. For example, a
556 /// double-free may occur if the function is called twice on the same raw pointer.
558 /// [into_raw]: struct.Rc.html#method.into_raw
565 /// let x = Rc::new("hello".to_owned());
566 /// let x_ptr = Rc::into_raw(x);
569 /// // Convert back to an `Rc` to prevent leak.
570 /// let x = Rc::from_raw(x_ptr);
571 /// assert_eq!(&*x, "hello");
573 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
576 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
578 #[stable(feature = "rc_raw", since = "1.17.0")]
579 pub unsafe fn from_raw(ptr: *const T) -> Self {
580 let offset = data_offset(ptr);
582 // Reverse the offset to find the original RcBox.
583 let fake_ptr = ptr as *mut RcBox<T>;
584 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
586 Self::from_ptr(rc_ptr)
589 /// Consumes the `Rc`, returning the wrapped pointer as `NonNull<T>`.
594 /// #![feature(rc_into_raw_non_null)]
598 /// let x = Rc::new("hello".to_owned());
599 /// let ptr = Rc::into_raw_non_null(x);
600 /// let deref = unsafe { ptr.as_ref() };
601 /// assert_eq!(deref, "hello");
603 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
605 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
606 // safe because Rc guarantees its pointer is non-null
607 unsafe { NonNull::new_unchecked(Rc::into_raw(this) as *mut _) }
610 /// Creates a new [`Weak`][weak] pointer to this allocation.
612 /// [weak]: struct.Weak.html
619 /// let five = Rc::new(5);
621 /// let weak_five = Rc::downgrade(&five);
623 #[stable(feature = "rc_weak", since = "1.4.0")]
624 pub fn downgrade(this: &Self) -> Weak<T> {
626 // Make sure we do not create a dangling Weak
627 debug_assert!(!is_dangling(this.ptr));
628 Weak { ptr: this.ptr }
631 /// Gets the number of [`Weak`][weak] pointers to this allocation.
633 /// [weak]: struct.Weak.html
640 /// let five = Rc::new(5);
641 /// let _weak_five = Rc::downgrade(&five);
643 /// assert_eq!(1, Rc::weak_count(&five));
646 #[stable(feature = "rc_counts", since = "1.15.0")]
647 pub fn weak_count(this: &Self) -> usize {
651 /// Gets the number of strong (`Rc`) pointers to this allocation.
658 /// let five = Rc::new(5);
659 /// let _also_five = Rc::clone(&five);
661 /// assert_eq!(2, Rc::strong_count(&five));
664 #[stable(feature = "rc_counts", since = "1.15.0")]
665 pub fn strong_count(this: &Self) -> usize {
669 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
672 /// [weak]: struct.Weak.html
674 fn is_unique(this: &Self) -> bool {
675 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
678 /// Returns a mutable reference into the given `Rc`, if there are
679 /// no other `Rc` or [`Weak`][weak] pointers to the same allocation.
681 /// Returns [`None`] otherwise, because it is not safe to
682 /// mutate a shared value.
684 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
685 /// the inner value when there are other pointers.
687 /// [weak]: struct.Weak.html
688 /// [`None`]: ../../std/option/enum.Option.html#variant.None
689 /// [make_mut]: struct.Rc.html#method.make_mut
690 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
697 /// let mut x = Rc::new(3);
698 /// *Rc::get_mut(&mut x).unwrap() = 4;
699 /// assert_eq!(*x, 4);
701 /// let _y = Rc::clone(&x);
702 /// assert!(Rc::get_mut(&mut x).is_none());
705 #[stable(feature = "rc_unique", since = "1.4.0")]
706 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
707 if Rc::is_unique(this) {
709 Some(Rc::get_mut_unchecked(this))
716 /// Returns a mutable reference into the given `Rc`,
717 /// without any check.
719 /// See also [`get_mut`], which is safe and does appropriate checks.
721 /// [`get_mut`]: struct.Rc.html#method.get_mut
725 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
726 /// for the duration of the returned borrow.
727 /// This is trivially the case if no such pointers exist,
728 /// for example immediately after `Rc::new`.
733 /// #![feature(get_mut_unchecked)]
737 /// let mut x = Rc::new(String::new());
739 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
741 /// assert_eq!(*x, "foo");
744 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
745 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
746 &mut this.ptr.as_mut().value
750 #[stable(feature = "ptr_eq", since = "1.17.0")]
751 /// Returns `true` if the two `Rc`s point to the same allocation
752 /// (in a vein similar to [`ptr::eq`]).
759 /// let five = Rc::new(5);
760 /// let same_five = Rc::clone(&five);
761 /// let other_five = Rc::new(5);
763 /// assert!(Rc::ptr_eq(&five, &same_five));
764 /// assert!(!Rc::ptr_eq(&five, &other_five));
767 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
768 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
769 this.ptr.as_ptr() == other.ptr.as_ptr()
773 impl<T: Clone> Rc<T> {
774 /// Makes a mutable reference into the given `Rc`.
776 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
777 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
778 /// referred to as clone-on-write.
780 /// If there are no other `Rc` pointers to this allocation, then [`Weak`]
781 /// pointers to this allocation will be disassociated.
783 /// See also [`get_mut`], which will fail rather than cloning.
785 /// [`Weak`]: struct.Weak.html
786 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
787 /// [`get_mut`]: struct.Rc.html#method.get_mut
794 /// let mut data = Rc::new(5);
796 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
797 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
798 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
799 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
800 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
802 /// // Now `data` and `other_data` point to different allocations.
803 /// assert_eq!(*data, 8);
804 /// assert_eq!(*other_data, 12);
807 /// [`Weak`] pointers will be disassociated:
812 /// let mut data = Rc::new(75);
813 /// let weak = Rc::downgrade(&data);
815 /// assert!(75 == *data);
816 /// assert!(75 == *weak.upgrade().unwrap());
818 /// *Rc::make_mut(&mut data) += 1;
820 /// assert!(76 == *data);
821 /// assert!(weak.upgrade().is_none());
824 #[stable(feature = "rc_unique", since = "1.4.0")]
825 pub fn make_mut(this: &mut Self) -> &mut T {
826 if Rc::strong_count(this) != 1 {
827 // Gotta clone the data, there are other Rcs
828 *this = Rc::new((**this).clone())
829 } else if Rc::weak_count(this) != 0 {
830 // Can just steal the data, all that's left is Weaks
832 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
833 mem::swap(this, &mut swap);
835 // Remove implicit strong-weak ref (no need to craft a fake
836 // Weak here -- we know other Weaks can clean up for us)
841 // This unsafety is ok because we're guaranteed that the pointer
842 // returned is the *only* pointer that will ever be returned to T. Our
843 // reference count is guaranteed to be 1 at this point, and we required
844 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
845 // reference to the allocation.
847 &mut this.ptr.as_mut().value
854 #[stable(feature = "rc_downcast", since = "1.29.0")]
855 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
860 /// use std::any::Any;
863 /// fn print_if_string(value: Rc<dyn Any>) {
864 /// if let Ok(string) = value.downcast::<String>() {
865 /// println!("String ({}): {}", string.len(), string);
869 /// let my_string = "Hello World".to_string();
870 /// print_if_string(Rc::new(my_string));
871 /// print_if_string(Rc::new(0i8));
873 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
874 if (*self).is::<T>() {
875 let ptr = self.ptr.cast::<RcBox<T>>();
877 Ok(Rc::from_inner(ptr))
884 impl<T: ?Sized> Rc<T> {
885 /// Allocates an `RcBox<T>` with sufficient space for
886 /// a possibly-unsized inner value where the value has the layout provided.
888 /// The function `mem_to_rcbox` is called with the data pointer
889 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
890 unsafe fn allocate_for_layout(
891 value_layout: Layout,
892 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>
894 // Calculate layout using the given value layout.
895 // Previously, layout was calculated on the expression
896 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
897 // reference (see #54908).
898 let layout = Layout::new::<RcBox<()>>()
899 .extend(value_layout).unwrap().0
900 .pad_to_align().unwrap();
902 // Allocate for the layout.
903 let mem = Global.alloc(layout)
904 .unwrap_or_else(|_| handle_alloc_error(layout));
906 // Initialize the RcBox
907 let inner = mem_to_rcbox(mem.as_ptr());
908 debug_assert_eq!(Layout::for_value(&*inner), layout);
910 ptr::write(&mut (*inner).strong, Cell::new(1));
911 ptr::write(&mut (*inner).weak, Cell::new(1));
916 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
917 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
918 // Allocate for the `RcBox<T>` using the given value.
919 Self::allocate_for_layout(
920 Layout::for_value(&*ptr),
921 |mem| set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>,
925 fn from_box(v: Box<T>) -> Rc<T> {
927 let box_unique = Box::into_unique(v);
928 let bptr = box_unique.as_ptr();
930 let value_size = size_of_val(&*bptr);
931 let ptr = Self::allocate_for_ptr(bptr);
933 // Copy value as bytes
934 ptr::copy_nonoverlapping(
935 bptr as *const T as *const u8,
936 &mut (*ptr).value as *mut _ as *mut u8,
939 // Free the allocation without dropping its contents
940 box_free(box_unique);
948 /// Allocates an `RcBox<[T]>` with the given length.
949 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
950 Self::allocate_for_layout(
951 Layout::array::<T>(len).unwrap(),
952 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>,
957 /// Sets the data pointer of a `?Sized` raw pointer.
959 /// For a slice/trait object, this sets the `data` field and leaves the rest
960 /// unchanged. For a sized raw pointer, this simply sets the pointer.
961 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
962 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
967 /// Copy elements from slice into newly allocated Rc<[T]>
969 /// Unsafe because the caller must either take ownership or bind `T: Copy`
970 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
971 let ptr = Self::allocate_for_slice(v.len());
973 ptr::copy_nonoverlapping(
975 &mut (*ptr).value as *mut [T] as *mut T,
981 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
983 /// Behavior is undefined should the size be wrong.
984 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
985 // Panic guard while cloning T elements.
986 // In the event of a panic, elements that have been written
987 // into the new RcBox will be dropped, then the memory freed.
995 impl<T> Drop for Guard<T> {
998 let slice = from_raw_parts_mut(self.elems, self.n_elems);
999 ptr::drop_in_place(slice);
1001 Global.dealloc(self.mem, self.layout);
1006 let ptr = Self::allocate_for_slice(len);
1008 let mem = ptr as *mut _ as *mut u8;
1009 let layout = Layout::for_value(&*ptr);
1011 // Pointer to first element
1012 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1014 let mut guard = Guard {
1015 mem: NonNull::new_unchecked(mem),
1021 for (i, item) in iter.enumerate() {
1022 ptr::write(elems.add(i), item);
1026 // All clear. Forget the guard so it doesn't free the new RcBox.
1033 /// Specialization trait used for `From<&[T]>`.
1034 trait RcFromSlice<T> {
1035 fn from_slice(slice: &[T]) -> Self;
1038 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
1040 default fn from_slice(v: &[T]) -> Self {
1042 Self::from_iter_exact(v.iter().cloned(), v.len())
1047 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
1049 fn from_slice(v: &[T]) -> Self {
1050 unsafe { Rc::copy_from_slice(v) }
1054 #[stable(feature = "rust1", since = "1.0.0")]
1055 impl<T: ?Sized> Deref for Rc<T> {
1059 fn deref(&self) -> &T {
1064 #[unstable(feature = "receiver_trait", issue = "0")]
1065 impl<T: ?Sized> Receiver for Rc<T> {}
1067 #[stable(feature = "rust1", since = "1.0.0")]
1068 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1071 /// This will decrement the strong reference count. If the strong reference
1072 /// count reaches zero then the only other references (if any) are
1073 /// [`Weak`], so we `drop` the inner value.
1078 /// use std::rc::Rc;
1082 /// impl Drop for Foo {
1083 /// fn drop(&mut self) {
1084 /// println!("dropped!");
1088 /// let foo = Rc::new(Foo);
1089 /// let foo2 = Rc::clone(&foo);
1091 /// drop(foo); // Doesn't print anything
1092 /// drop(foo2); // Prints "dropped!"
1095 /// [`Weak`]: ../../std/rc/struct.Weak.html
1096 fn drop(&mut self) {
1099 if self.strong() == 0 {
1100 // destroy the contained object
1101 ptr::drop_in_place(self.ptr.as_mut());
1103 // remove the implicit "strong weak" pointer now that we've
1104 // destroyed the contents.
1107 if self.weak() == 0 {
1108 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1115 #[stable(feature = "rust1", since = "1.0.0")]
1116 impl<T: ?Sized> Clone for Rc<T> {
1117 /// Makes a clone of the `Rc` pointer.
1119 /// This creates another pointer to the same allocation, increasing the
1120 /// strong reference count.
1125 /// use std::rc::Rc;
1127 /// let five = Rc::new(5);
1129 /// let _ = Rc::clone(&five);
1132 fn clone(&self) -> Rc<T> {
1134 Self::from_inner(self.ptr)
1138 #[stable(feature = "rust1", since = "1.0.0")]
1139 impl<T: Default> Default for Rc<T> {
1140 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1145 /// use std::rc::Rc;
1147 /// let x: Rc<i32> = Default::default();
1148 /// assert_eq!(*x, 0);
1151 fn default() -> Rc<T> {
1152 Rc::new(Default::default())
1156 #[stable(feature = "rust1", since = "1.0.0")]
1157 trait RcEqIdent<T: ?Sized + PartialEq> {
1158 fn eq(&self, other: &Rc<T>) -> bool;
1159 fn ne(&self, other: &Rc<T>) -> bool;
1162 #[stable(feature = "rust1", since = "1.0.0")]
1163 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
1165 default fn eq(&self, other: &Rc<T>) -> bool {
1170 default fn ne(&self, other: &Rc<T>) -> bool {
1175 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1176 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1177 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1178 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1179 /// the same value, than two `&T`s.
1181 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1182 #[stable(feature = "rust1", since = "1.0.0")]
1183 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
1185 fn eq(&self, other: &Rc<T>) -> bool {
1186 Rc::ptr_eq(self, other) || **self == **other
1190 fn ne(&self, other: &Rc<T>) -> bool {
1191 !Rc::ptr_eq(self, other) && **self != **other
1195 #[stable(feature = "rust1", since = "1.0.0")]
1196 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1197 /// Equality for two `Rc`s.
1199 /// Two `Rc`s are equal if their inner values are equal, even if they are
1200 /// stored in different allocation.
1202 /// If `T` also implements `Eq` (implying reflexivity of equality),
1203 /// two `Rc`s that point to the same allocation are
1209 /// use std::rc::Rc;
1211 /// let five = Rc::new(5);
1213 /// assert!(five == Rc::new(5));
1216 fn eq(&self, other: &Rc<T>) -> bool {
1217 RcEqIdent::eq(self, other)
1220 /// Inequality for two `Rc`s.
1222 /// Two `Rc`s are unequal if their inner values are unequal.
1224 /// If `T` also implements `Eq` (implying reflexivity of equality),
1225 /// two `Rc`s that point to the same allocation are
1231 /// use std::rc::Rc;
1233 /// let five = Rc::new(5);
1235 /// assert!(five != Rc::new(6));
1238 fn ne(&self, other: &Rc<T>) -> bool {
1239 RcEqIdent::ne(self, other)
1243 #[stable(feature = "rust1", since = "1.0.0")]
1244 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1246 #[stable(feature = "rust1", since = "1.0.0")]
1247 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1248 /// Partial comparison for two `Rc`s.
1250 /// The two are compared by calling `partial_cmp()` on their inner values.
1255 /// use std::rc::Rc;
1256 /// use std::cmp::Ordering;
1258 /// let five = Rc::new(5);
1260 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1263 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1264 (**self).partial_cmp(&**other)
1267 /// Less-than comparison for two `Rc`s.
1269 /// The two are compared by calling `<` on their inner values.
1274 /// use std::rc::Rc;
1276 /// let five = Rc::new(5);
1278 /// assert!(five < Rc::new(6));
1281 fn lt(&self, other: &Rc<T>) -> bool {
1285 /// 'Less than or equal to' comparison for two `Rc`s.
1287 /// The two are compared by calling `<=` on their inner values.
1292 /// use std::rc::Rc;
1294 /// let five = Rc::new(5);
1296 /// assert!(five <= Rc::new(5));
1299 fn le(&self, other: &Rc<T>) -> bool {
1303 /// Greater-than comparison for two `Rc`s.
1305 /// The two are compared by calling `>` on their inner values.
1310 /// use std::rc::Rc;
1312 /// let five = Rc::new(5);
1314 /// assert!(five > Rc::new(4));
1317 fn gt(&self, other: &Rc<T>) -> bool {
1321 /// 'Greater than or equal to' comparison for two `Rc`s.
1323 /// The two are compared by calling `>=` on their inner values.
1328 /// use std::rc::Rc;
1330 /// let five = Rc::new(5);
1332 /// assert!(five >= Rc::new(5));
1335 fn ge(&self, other: &Rc<T>) -> bool {
1340 #[stable(feature = "rust1", since = "1.0.0")]
1341 impl<T: ?Sized + Ord> Ord for Rc<T> {
1342 /// Comparison for two `Rc`s.
1344 /// The two are compared by calling `cmp()` on their inner values.
1349 /// use std::rc::Rc;
1350 /// use std::cmp::Ordering;
1352 /// let five = Rc::new(5);
1354 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1357 fn cmp(&self, other: &Rc<T>) -> Ordering {
1358 (**self).cmp(&**other)
1362 #[stable(feature = "rust1", since = "1.0.0")]
1363 impl<T: ?Sized + Hash> Hash for Rc<T> {
1364 fn hash<H: Hasher>(&self, state: &mut H) {
1365 (**self).hash(state);
1369 #[stable(feature = "rust1", since = "1.0.0")]
1370 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1371 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1372 fmt::Display::fmt(&**self, f)
1376 #[stable(feature = "rust1", since = "1.0.0")]
1377 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1378 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1379 fmt::Debug::fmt(&**self, f)
1383 #[stable(feature = "rust1", since = "1.0.0")]
1384 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1385 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1386 fmt::Pointer::fmt(&(&**self as *const T), f)
1390 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1391 impl<T> From<T> for Rc<T> {
1392 fn from(t: T) -> Self {
1397 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1398 impl<T: Clone> From<&[T]> for Rc<[T]> {
1400 fn from(v: &[T]) -> Rc<[T]> {
1401 <Self as RcFromSlice<T>>::from_slice(v)
1405 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1406 impl From<&str> for Rc<str> {
1408 fn from(v: &str) -> Rc<str> {
1409 let rc = Rc::<[u8]>::from(v.as_bytes());
1410 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1414 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1415 impl From<String> for Rc<str> {
1417 fn from(v: String) -> Rc<str> {
1422 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1423 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1425 fn from(v: Box<T>) -> Rc<T> {
1430 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1431 impl<T> From<Vec<T>> for Rc<[T]> {
1433 fn from(mut v: Vec<T>) -> Rc<[T]> {
1435 let rc = Rc::copy_from_slice(&v);
1437 // Allow the Vec to free its memory, but not destroy its contents
1445 #[unstable(feature = "boxed_slice_try_from", issue = "0")]
1446 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]>
1448 [T; N]: LengthAtMost32,
1450 type Error = Rc<[T]>;
1452 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1453 if boxed_slice.len() == N {
1454 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
1461 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1462 impl<T> iter::FromIterator<T> for Rc<[T]> {
1463 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1465 /// # Performance characteristics
1467 /// ## The general case
1469 /// In the general case, collecting into `Rc<[T]>` is done by first
1470 /// collecting into a `Vec<T>`. That is, when writing the following:
1473 /// # use std::rc::Rc;
1474 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1475 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1478 /// this behaves as if we wrote:
1481 /// # use std::rc::Rc;
1482 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1483 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1484 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1485 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1488 /// This will allocate as many times as needed for constructing the `Vec<T>`
1489 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1491 /// ## Iterators of known length
1493 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1494 /// a single allocation will be made for the `Rc<[T]>`. For example:
1497 /// # use std::rc::Rc;
1498 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1499 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1501 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1502 RcFromIter::from_iter(iter.into_iter())
1506 /// Specialization trait used for collecting into `Rc<[T]>`.
1507 trait RcFromIter<T, I> {
1508 fn from_iter(iter: I) -> Self;
1511 impl<T, I: Iterator<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1512 default fn from_iter(iter: I) -> Self {
1513 iter.collect::<Vec<T>>().into()
1517 impl<T, I: iter::TrustedLen<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1518 default fn from_iter(iter: I) -> Self {
1519 // This is the case for a `TrustedLen` iterator.
1520 let (low, high) = iter.size_hint();
1521 if let Some(high) = high {
1524 "TrustedLen iterator's size hint is not exact: {:?}",
1529 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1530 Rc::from_iter_exact(iter, low)
1533 // Fall back to normal implementation.
1534 iter.collect::<Vec<T>>().into()
1539 impl<'a, T: 'a + Clone> RcFromIter<&'a T, slice::Iter<'a, T>> for Rc<[T]> {
1540 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
1541 // Delegate to `impl<T: Clone> From<&[T]> for Rc<[T]>`.
1543 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
1544 // which is even more performant.
1546 // In the fall-back case we have `T: Clone`. This is still better
1547 // than the `TrustedLen` implementation as slices have a known length
1548 // and so we get to avoid calling `size_hint` and avoid the branching.
1549 iter.as_slice().into()
1553 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1554 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
1555 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1557 /// Since a `Weak` reference does not count towards ownership, it will not
1558 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
1559 /// guarantees about the value still being present. Thus it may return [`None`]
1560 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
1561 /// itself (the backing store) from being deallocated.
1563 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
1564 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
1565 /// prevent circular references between [`Rc`] pointers, since mutual owning references
1566 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1567 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1568 /// pointers from children back to their parents.
1570 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1572 /// [`Rc`]: struct.Rc.html
1573 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1574 /// [`upgrade`]: struct.Weak.html#method.upgrade
1575 /// [`Option`]: ../../std/option/enum.Option.html
1576 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1577 #[stable(feature = "rc_weak", since = "1.4.0")]
1578 pub struct Weak<T: ?Sized> {
1579 // This is a `NonNull` to allow optimizing the size of this type in enums,
1580 // but it is not necessarily a valid pointer.
1581 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1582 // to allocate space on the heap. That's not a value a real pointer
1583 // will ever have because RcBox has alignment at least 2.
1584 ptr: NonNull<RcBox<T>>,
1587 #[stable(feature = "rc_weak", since = "1.4.0")]
1588 impl<T: ?Sized> !marker::Send for Weak<T> {}
1589 #[stable(feature = "rc_weak", since = "1.4.0")]
1590 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1592 #[unstable(feature = "coerce_unsized", issue = "27732")]
1593 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1595 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
1596 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1599 /// Constructs a new `Weak<T>`, without allocating any memory.
1600 /// Calling [`upgrade`] on the return value always gives [`None`].
1602 /// [`upgrade`]: #method.upgrade
1603 /// [`None`]: ../../std/option/enum.Option.html
1608 /// use std::rc::Weak;
1610 /// let empty: Weak<i64> = Weak::new();
1611 /// assert!(empty.upgrade().is_none());
1613 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1614 pub fn new() -> Weak<T> {
1616 ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"),
1620 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1622 /// It is up to the caller to ensure that the object is still alive when accessing it through
1625 /// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
1630 /// #![feature(weak_into_raw)]
1632 /// use std::rc::Rc;
1635 /// let strong = Rc::new("hello".to_owned());
1636 /// let weak = Rc::downgrade(&strong);
1637 /// // Both point to the same object
1638 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1639 /// // The strong here keeps it alive, so we can still access the object.
1640 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1643 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1644 /// // undefined behaviour.
1645 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1648 /// [`null`]: ../../std/ptr/fn.null.html
1649 #[unstable(feature = "weak_into_raw", issue = "60728")]
1650 pub fn as_raw(&self) -> *const T {
1651 match self.inner() {
1652 None => ptr::null(),
1654 let offset = data_offset_sized::<T>();
1655 let ptr = inner as *const RcBox<T>;
1656 // Note: while the pointer we create may already point to dropped value, the
1657 // allocation still lives (it must hold the weak point as long as we are alive).
1658 // Therefore, the offset is OK to do, it won't get out of the allocation.
1659 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1665 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1667 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1668 /// can be turned back into the `Weak<T>` with [`from_raw`].
1670 /// The same restrictions of accessing the target of the pointer as with
1671 /// [`as_raw`] apply.
1676 /// #![feature(weak_into_raw)]
1678 /// use std::rc::{Rc, Weak};
1680 /// let strong = Rc::new("hello".to_owned());
1681 /// let weak = Rc::downgrade(&strong);
1682 /// let raw = weak.into_raw();
1684 /// assert_eq!(1, Rc::weak_count(&strong));
1685 /// assert_eq!("hello", unsafe { &*raw });
1687 /// drop(unsafe { Weak::from_raw(raw) });
1688 /// assert_eq!(0, Rc::weak_count(&strong));
1691 /// [`from_raw`]: struct.Weak.html#method.from_raw
1692 /// [`as_raw`]: struct.Weak.html#method.as_raw
1693 #[unstable(feature = "weak_into_raw", issue = "60728")]
1694 pub fn into_raw(self) -> *const T {
1695 let result = self.as_raw();
1700 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1702 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1703 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1705 /// It takes ownership of one weak count. In case a [`null`] is passed, a dangling [`Weak`] is
1710 /// The pointer must represent one valid weak count. In other words, it must point to `T` which
1711 /// is or *was* managed by an [`Rc`] and the weak count of that [`Rc`] must not have reached
1712 /// 0. It is allowed for the strong count to be 0.
1717 /// #![feature(weak_into_raw)]
1719 /// use std::rc::{Rc, Weak};
1721 /// let strong = Rc::new("hello".to_owned());
1723 /// let raw_1 = Rc::downgrade(&strong).into_raw();
1724 /// let raw_2 = Rc::downgrade(&strong).into_raw();
1726 /// assert_eq!(2, Rc::weak_count(&strong));
1728 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1729 /// assert_eq!(1, Rc::weak_count(&strong));
1733 /// // Decrement the last weak count.
1734 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1737 /// [`null`]: ../../std/ptr/fn.null.html
1738 /// [`into_raw`]: struct.Weak.html#method.into_raw
1739 /// [`upgrade`]: struct.Weak.html#method.upgrade
1740 /// [`Rc`]: struct.Rc.html
1741 /// [`Weak`]: struct.Weak.html
1742 #[unstable(feature = "weak_into_raw", issue = "60728")]
1743 pub unsafe fn from_raw(ptr: *const T) -> Self {
1747 // See Rc::from_raw for details
1748 let offset = data_offset(ptr);
1749 let fake_ptr = ptr as *mut RcBox<T>;
1750 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1752 ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
1758 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1759 let address = ptr.as_ptr() as *mut () as usize;
1760 address == usize::MAX
1763 impl<T: ?Sized> Weak<T> {
1764 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
1765 /// dropping of the inner value if successful.
1767 /// Returns [`None`] if the inner value has since been dropped.
1769 /// [`Rc`]: struct.Rc.html
1770 /// [`None`]: ../../std/option/enum.Option.html
1775 /// use std::rc::Rc;
1777 /// let five = Rc::new(5);
1779 /// let weak_five = Rc::downgrade(&five);
1781 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1782 /// assert!(strong_five.is_some());
1784 /// // Destroy all strong pointers.
1785 /// drop(strong_five);
1788 /// assert!(weak_five.upgrade().is_none());
1790 #[stable(feature = "rc_weak", since = "1.4.0")]
1791 pub fn upgrade(&self) -> Option<Rc<T>> {
1792 let inner = self.inner()?;
1793 if inner.strong() == 0 {
1797 Some(Rc::from_inner(self.ptr))
1801 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
1803 /// If `self` was created using [`Weak::new`], this will return 0.
1805 /// [`Weak::new`]: #method.new
1806 #[unstable(feature = "weak_counts", issue = "57977")]
1807 pub fn strong_count(&self) -> usize {
1808 if let Some(inner) = self.inner() {
1815 /// Gets the number of `Weak` pointers pointing to this allocation.
1817 /// If `self` was created using [`Weak::new`], this will return `None`. If
1818 /// not, the returned value is at least 1, since `self` still points to the
1821 /// [`Weak::new`]: #method.new
1822 #[unstable(feature = "weak_counts", issue = "57977")]
1823 pub fn weak_count(&self) -> Option<usize> {
1824 self.inner().map(|inner| {
1825 if inner.strong() > 0 {
1826 inner.weak() - 1 // subtract the implicit weak ptr
1833 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1834 /// (i.e., when this `Weak` was created by `Weak::new`).
1836 fn inner(&self) -> Option<&RcBox<T>> {
1837 if is_dangling(self.ptr) {
1840 Some(unsafe { self.ptr.as_ref() })
1844 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1845 /// [`ptr::eq`]), or if both don't point to any allocation
1846 /// (because they were created with `Weak::new()`).
1850 /// Since this compares pointers it means that `Weak::new()` will equal each
1851 /// other, even though they don't point to any allocation.
1856 /// use std::rc::Rc;
1858 /// let first_rc = Rc::new(5);
1859 /// let first = Rc::downgrade(&first_rc);
1860 /// let second = Rc::downgrade(&first_rc);
1862 /// assert!(first.ptr_eq(&second));
1864 /// let third_rc = Rc::new(5);
1865 /// let third = Rc::downgrade(&third_rc);
1867 /// assert!(!first.ptr_eq(&third));
1870 /// Comparing `Weak::new`.
1873 /// use std::rc::{Rc, Weak};
1875 /// let first = Weak::new();
1876 /// let second = Weak::new();
1877 /// assert!(first.ptr_eq(&second));
1879 /// let third_rc = Rc::new(());
1880 /// let third = Rc::downgrade(&third_rc);
1881 /// assert!(!first.ptr_eq(&third));
1884 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1886 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1887 pub fn ptr_eq(&self, other: &Self) -> bool {
1888 self.ptr.as_ptr() == other.ptr.as_ptr()
1892 #[stable(feature = "rc_weak", since = "1.4.0")]
1893 impl<T: ?Sized> Drop for Weak<T> {
1894 /// Drops the `Weak` pointer.
1899 /// use std::rc::{Rc, Weak};
1903 /// impl Drop for Foo {
1904 /// fn drop(&mut self) {
1905 /// println!("dropped!");
1909 /// let foo = Rc::new(Foo);
1910 /// let weak_foo = Rc::downgrade(&foo);
1911 /// let other_weak_foo = Weak::clone(&weak_foo);
1913 /// drop(weak_foo); // Doesn't print anything
1914 /// drop(foo); // Prints "dropped!"
1916 /// assert!(other_weak_foo.upgrade().is_none());
1918 fn drop(&mut self) {
1919 if let Some(inner) = self.inner() {
1921 // the weak count starts at 1, and will only go to zero if all
1922 // the strong pointers have disappeared.
1923 if inner.weak() == 0 {
1925 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1932 #[stable(feature = "rc_weak", since = "1.4.0")]
1933 impl<T: ?Sized> Clone for Weak<T> {
1934 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1939 /// use std::rc::{Rc, Weak};
1941 /// let weak_five = Rc::downgrade(&Rc::new(5));
1943 /// let _ = Weak::clone(&weak_five);
1946 fn clone(&self) -> Weak<T> {
1947 if let Some(inner) = self.inner() {
1950 Weak { ptr: self.ptr }
1954 #[stable(feature = "rc_weak", since = "1.4.0")]
1955 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1956 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1961 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1962 impl<T> Default for Weak<T> {
1963 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1964 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1966 /// [`None`]: ../../std/option/enum.Option.html
1967 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
1972 /// use std::rc::Weak;
1974 /// let empty: Weak<i64> = Default::default();
1975 /// assert!(empty.upgrade().is_none());
1977 fn default() -> Weak<T> {
1982 // NOTE: We checked_add here to deal with mem::forget safely. In particular
1983 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1984 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1985 // We abort because this is such a degenerate scenario that we don't care about
1986 // what happens -- no real program should ever experience this.
1988 // This should have negligible overhead since you don't actually need to
1989 // clone these much in Rust thanks to ownership and move-semantics.
1992 trait RcBoxPtr<T: ?Sized> {
1993 fn inner(&self) -> &RcBox<T>;
1996 fn strong(&self) -> usize {
1997 self.inner().strong.get()
2001 fn inc_strong(&self) {
2002 let strong = self.strong();
2004 // We want to abort on overflow instead of dropping the value.
2005 // The reference count will never be zero when this is called;
2006 // nevertheless, we insert an abort here to hint LLVM at
2007 // an otherwise missed optimization.
2008 if strong == 0 || strong == usize::max_value() {
2011 self.inner().strong.set(strong + 1);
2015 fn dec_strong(&self) {
2016 self.inner().strong.set(self.strong() - 1);
2020 fn weak(&self) -> usize {
2021 self.inner().weak.get()
2025 fn inc_weak(&self) {
2026 let weak = self.weak();
2028 // We want to abort on overflow instead of dropping the value.
2029 // The reference count will never be zero when this is called;
2030 // nevertheless, we insert an abort here to hint LLVM at
2031 // an otherwise missed optimization.
2032 if weak == 0 || weak == usize::max_value() {
2035 self.inner().weak.set(weak + 1);
2039 fn dec_weak(&self) {
2040 self.inner().weak.set(self.weak() - 1);
2044 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
2046 fn inner(&self) -> &RcBox<T> {
2053 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
2055 fn inner(&self) -> &RcBox<T> {
2060 #[stable(feature = "rust1", since = "1.0.0")]
2061 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2062 fn borrow(&self) -> &T {
2067 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2068 impl<T: ?Sized> AsRef<T> for Rc<T> {
2069 fn as_ref(&self) -> &T {
2074 #[stable(feature = "pin", since = "1.33.0")]
2075 impl<T: ?Sized> Unpin for Rc<T> { }
2077 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2078 // Align the unsized value to the end of the `RcBox`.
2079 // Because it is ?Sized, it will always be the last field in memory.
2080 data_offset_align(align_of_val(&*ptr))
2083 /// Computes the offset of the data field within `RcBox`.
2085 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2086 fn data_offset_sized<T>() -> isize {
2087 data_offset_align(align_of::<T>())
2091 fn data_offset_align(align: usize) -> isize {
2092 let layout = Layout::new::<RcBox<()>>();
2093 (layout.size() + layout.padding_needed_for(align)) as isize