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` from a raw pointer.
575 /// The raw pointer must have been previously returned by a call to a
576 /// [`Rc::into_raw`][into_raw].
578 /// This function is unsafe because improper use may lead to memory problems. For example, a
579 /// double-free may occur if the function is called twice on the same raw pointer.
581 /// [into_raw]: struct.Rc.html#method.into_raw
588 /// let x = Rc::new("hello".to_owned());
589 /// let x_ptr = Rc::into_raw(x);
592 /// // Convert back to an `Rc` to prevent leak.
593 /// let x = Rc::from_raw(x_ptr);
594 /// assert_eq!(&*x, "hello");
596 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
599 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
601 #[stable(feature = "rc_raw", since = "1.17.0")]
602 pub unsafe fn from_raw(ptr: *const T) -> Self {
603 let offset = data_offset(ptr);
605 // Reverse the offset to find the original RcBox.
606 let fake_ptr = ptr as *mut RcBox<T>;
607 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
609 Self::from_ptr(rc_ptr)
612 /// Consumes the `Rc`, returning the wrapped pointer as `NonNull<T>`.
617 /// #![feature(rc_into_raw_non_null)]
621 /// let x = Rc::new("hello".to_owned());
622 /// let ptr = Rc::into_raw_non_null(x);
623 /// let deref = unsafe { ptr.as_ref() };
624 /// assert_eq!(deref, "hello");
626 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
628 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
629 // safe because Rc guarantees its pointer is non-null
630 unsafe { NonNull::new_unchecked(Rc::into_raw(this) as *mut _) }
633 /// Creates a new [`Weak`][weak] pointer to this allocation.
635 /// [weak]: struct.Weak.html
642 /// let five = Rc::new(5);
644 /// let weak_five = Rc::downgrade(&five);
646 #[stable(feature = "rc_weak", since = "1.4.0")]
647 pub fn downgrade(this: &Self) -> Weak<T> {
649 // Make sure we do not create a dangling Weak
650 debug_assert!(!is_dangling(this.ptr));
651 Weak { ptr: this.ptr }
654 /// Gets the number of [`Weak`][weak] pointers to this allocation.
656 /// [weak]: struct.Weak.html
663 /// let five = Rc::new(5);
664 /// let _weak_five = Rc::downgrade(&five);
666 /// assert_eq!(1, Rc::weak_count(&five));
669 #[stable(feature = "rc_counts", since = "1.15.0")]
670 pub fn weak_count(this: &Self) -> usize {
674 /// Gets the number of strong (`Rc`) pointers to this allocation.
681 /// let five = Rc::new(5);
682 /// let _also_five = Rc::clone(&five);
684 /// assert_eq!(2, Rc::strong_count(&five));
687 #[stable(feature = "rc_counts", since = "1.15.0")]
688 pub fn strong_count(this: &Self) -> usize {
692 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
695 /// [weak]: struct.Weak.html
697 fn is_unique(this: &Self) -> bool {
698 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
701 /// Returns a mutable reference into the given `Rc`, if there are
702 /// no other `Rc` or [`Weak`][weak] pointers to the same allocation.
704 /// Returns [`None`] otherwise, because it is not safe to
705 /// mutate a shared value.
707 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
708 /// the inner value when there are other pointers.
710 /// [weak]: struct.Weak.html
711 /// [`None`]: ../../std/option/enum.Option.html#variant.None
712 /// [make_mut]: struct.Rc.html#method.make_mut
713 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
720 /// let mut x = Rc::new(3);
721 /// *Rc::get_mut(&mut x).unwrap() = 4;
722 /// assert_eq!(*x, 4);
724 /// let _y = Rc::clone(&x);
725 /// assert!(Rc::get_mut(&mut x).is_none());
728 #[stable(feature = "rc_unique", since = "1.4.0")]
729 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
730 if Rc::is_unique(this) { unsafe { Some(Rc::get_mut_unchecked(this)) } } else { None }
733 /// Returns a mutable reference into the given `Rc`,
734 /// without any check.
736 /// See also [`get_mut`], which is safe and does appropriate checks.
738 /// [`get_mut`]: struct.Rc.html#method.get_mut
742 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
743 /// for the duration of the returned borrow.
744 /// This is trivially the case if no such pointers exist,
745 /// for example immediately after `Rc::new`.
750 /// #![feature(get_mut_unchecked)]
754 /// let mut x = Rc::new(String::new());
756 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
758 /// assert_eq!(*x, "foo");
761 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
762 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
763 &mut this.ptr.as_mut().value
767 #[stable(feature = "ptr_eq", since = "1.17.0")]
768 /// Returns `true` if the two `Rc`s point to the same allocation
769 /// (in a vein similar to [`ptr::eq`]).
776 /// let five = Rc::new(5);
777 /// let same_five = Rc::clone(&five);
778 /// let other_five = Rc::new(5);
780 /// assert!(Rc::ptr_eq(&five, &same_five));
781 /// assert!(!Rc::ptr_eq(&five, &other_five));
784 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
785 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
786 this.ptr.as_ptr() == other.ptr.as_ptr()
790 impl<T: Clone> Rc<T> {
791 /// Makes a mutable reference into the given `Rc`.
793 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
794 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
795 /// referred to as clone-on-write.
797 /// If there are no other `Rc` pointers to this allocation, then [`Weak`]
798 /// pointers to this allocation will be disassociated.
800 /// See also [`get_mut`], which will fail rather than cloning.
802 /// [`Weak`]: struct.Weak.html
803 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
804 /// [`get_mut`]: struct.Rc.html#method.get_mut
811 /// let mut data = Rc::new(5);
813 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
814 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
815 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
816 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
817 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
819 /// // Now `data` and `other_data` point to different allocations.
820 /// assert_eq!(*data, 8);
821 /// assert_eq!(*other_data, 12);
824 /// [`Weak`] pointers will be disassociated:
829 /// let mut data = Rc::new(75);
830 /// let weak = Rc::downgrade(&data);
832 /// assert!(75 == *data);
833 /// assert!(75 == *weak.upgrade().unwrap());
835 /// *Rc::make_mut(&mut data) += 1;
837 /// assert!(76 == *data);
838 /// assert!(weak.upgrade().is_none());
841 #[stable(feature = "rc_unique", since = "1.4.0")]
842 pub fn make_mut(this: &mut Self) -> &mut T {
843 if Rc::strong_count(this) != 1 {
844 // Gotta clone the data, there are other Rcs
845 *this = Rc::new((**this).clone())
846 } else if Rc::weak_count(this) != 0 {
847 // Can just steal the data, all that's left is Weaks
849 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
850 mem::swap(this, &mut swap);
852 // Remove implicit strong-weak ref (no need to craft a fake
853 // Weak here -- we know other Weaks can clean up for us)
858 // This unsafety is ok because we're guaranteed that the pointer
859 // returned is the *only* pointer that will ever be returned to T. Our
860 // reference count is guaranteed to be 1 at this point, and we required
861 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
862 // reference to the allocation.
863 unsafe { &mut this.ptr.as_mut().value }
869 #[stable(feature = "rc_downcast", since = "1.29.0")]
870 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
875 /// use std::any::Any;
878 /// fn print_if_string(value: Rc<dyn Any>) {
879 /// if let Ok(string) = value.downcast::<String>() {
880 /// println!("String ({}): {}", string.len(), string);
884 /// let my_string = "Hello World".to_string();
885 /// print_if_string(Rc::new(my_string));
886 /// print_if_string(Rc::new(0i8));
888 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
889 if (*self).is::<T>() {
890 let ptr = self.ptr.cast::<RcBox<T>>();
892 Ok(Rc::from_inner(ptr))
899 impl<T: ?Sized> Rc<T> {
900 /// Allocates an `RcBox<T>` with sufficient space for
901 /// a possibly-unsized inner value where the value has the layout provided.
903 /// The function `mem_to_rcbox` is called with the data pointer
904 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
905 unsafe fn allocate_for_layout(
906 value_layout: Layout,
907 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
909 // Calculate layout using the given value layout.
910 // Previously, layout was calculated on the expression
911 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
912 // reference (see #54908).
913 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
915 // Allocate for the layout.
916 let mem = Global.alloc(layout).unwrap_or_else(|_| handle_alloc_error(layout));
918 // Initialize the RcBox
919 let inner = mem_to_rcbox(mem.as_ptr());
920 debug_assert_eq!(Layout::for_value(&*inner), layout);
922 ptr::write(&mut (*inner).strong, Cell::new(1));
923 ptr::write(&mut (*inner).weak, Cell::new(1));
928 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
929 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
930 // Allocate for the `RcBox<T>` using the given value.
931 Self::allocate_for_layout(Layout::for_value(&*ptr), |mem| {
932 set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>
936 fn from_box(v: Box<T>) -> Rc<T> {
938 let box_unique = Box::into_unique(v);
939 let bptr = box_unique.as_ptr();
941 let value_size = size_of_val(&*bptr);
942 let ptr = Self::allocate_for_ptr(bptr);
944 // Copy value as bytes
945 ptr::copy_nonoverlapping(
946 bptr as *const T as *const u8,
947 &mut (*ptr).value as *mut _ as *mut u8,
951 // Free the allocation without dropping its contents
952 box_free(box_unique);
960 /// Allocates an `RcBox<[T]>` with the given length.
961 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
962 Self::allocate_for_layout(Layout::array::<T>(len).unwrap(), |mem| {
963 ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>
968 /// Sets the data pointer of a `?Sized` raw pointer.
970 /// For a slice/trait object, this sets the `data` field and leaves the rest
971 /// unchanged. For a sized raw pointer, this simply sets the pointer.
972 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
973 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
978 /// Copy elements from slice into newly allocated Rc<[T]>
980 /// Unsafe because the caller must either take ownership or bind `T: Copy`
981 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
982 let ptr = Self::allocate_for_slice(v.len());
984 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).value as *mut [T] as *mut T, v.len());
989 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
991 /// Behavior is undefined should the size be wrong.
992 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
993 // Panic guard while cloning T elements.
994 // In the event of a panic, elements that have been written
995 // into the new RcBox will be dropped, then the memory freed.
1003 impl<T> Drop for Guard<T> {
1004 fn drop(&mut self) {
1006 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1007 ptr::drop_in_place(slice);
1009 Global.dealloc(self.mem, self.layout);
1014 let ptr = Self::allocate_for_slice(len);
1016 let mem = ptr as *mut _ as *mut u8;
1017 let layout = Layout::for_value(&*ptr);
1019 // Pointer to first element
1020 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1022 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1024 for (i, item) in iter.enumerate() {
1025 ptr::write(elems.add(i), item);
1029 // All clear. Forget the guard so it doesn't free the new RcBox.
1036 /// Specialization trait used for `From<&[T]>`.
1037 trait RcFromSlice<T> {
1038 fn from_slice(slice: &[T]) -> Self;
1041 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
1043 default fn from_slice(v: &[T]) -> Self {
1044 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1048 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
1050 fn from_slice(v: &[T]) -> Self {
1051 unsafe { Rc::copy_from_slice(v) }
1055 #[stable(feature = "rust1", since = "1.0.0")]
1056 impl<T: ?Sized> Deref for Rc<T> {
1060 fn deref(&self) -> &T {
1065 #[unstable(feature = "receiver_trait", issue = "none")]
1066 impl<T: ?Sized> Receiver for Rc<T> {}
1068 #[stable(feature = "rust1", since = "1.0.0")]
1069 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1072 /// This will decrement the strong reference count. If the strong reference
1073 /// count reaches zero then the only other references (if any) are
1074 /// [`Weak`], so we `drop` the inner value.
1079 /// use std::rc::Rc;
1083 /// impl Drop for Foo {
1084 /// fn drop(&mut self) {
1085 /// println!("dropped!");
1089 /// let foo = Rc::new(Foo);
1090 /// let foo2 = Rc::clone(&foo);
1092 /// drop(foo); // Doesn't print anything
1093 /// drop(foo2); // Prints "dropped!"
1096 /// [`Weak`]: ../../std/rc/struct.Weak.html
1097 fn drop(&mut self) {
1100 if self.strong() == 0 {
1101 // destroy the contained object
1102 ptr::drop_in_place(self.ptr.as_mut());
1104 // remove the implicit "strong weak" pointer now that we've
1105 // destroyed the contents.
1108 if self.weak() == 0 {
1109 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1116 #[stable(feature = "rust1", since = "1.0.0")]
1117 impl<T: ?Sized> Clone for Rc<T> {
1118 /// Makes a clone of the `Rc` pointer.
1120 /// This creates another pointer to the same allocation, increasing the
1121 /// strong reference count.
1126 /// use std::rc::Rc;
1128 /// let five = Rc::new(5);
1130 /// let _ = Rc::clone(&five);
1133 fn clone(&self) -> Rc<T> {
1135 Self::from_inner(self.ptr)
1139 #[stable(feature = "rust1", since = "1.0.0")]
1140 impl<T: Default> Default for Rc<T> {
1141 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1146 /// use std::rc::Rc;
1148 /// let x: Rc<i32> = Default::default();
1149 /// assert_eq!(*x, 0);
1152 fn default() -> Rc<T> {
1153 Rc::new(Default::default())
1157 #[stable(feature = "rust1", since = "1.0.0")]
1158 trait RcEqIdent<T: ?Sized + PartialEq> {
1159 fn eq(&self, other: &Rc<T>) -> bool;
1160 fn ne(&self, other: &Rc<T>) -> bool;
1163 #[stable(feature = "rust1", since = "1.0.0")]
1164 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
1166 default fn eq(&self, other: &Rc<T>) -> bool {
1171 default fn ne(&self, other: &Rc<T>) -> bool {
1176 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1177 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1178 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1179 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1180 /// the same value, than two `&T`s.
1182 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1183 #[stable(feature = "rust1", since = "1.0.0")]
1184 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
1186 fn eq(&self, other: &Rc<T>) -> bool {
1187 Rc::ptr_eq(self, other) || **self == **other
1191 fn ne(&self, other: &Rc<T>) -> bool {
1192 !Rc::ptr_eq(self, other) && **self != **other
1196 #[stable(feature = "rust1", since = "1.0.0")]
1197 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1198 /// Equality for two `Rc`s.
1200 /// Two `Rc`s are equal if their inner values are equal, even if they are
1201 /// stored in different allocation.
1203 /// If `T` also implements `Eq` (implying reflexivity of equality),
1204 /// two `Rc`s that point to the same allocation are
1210 /// use std::rc::Rc;
1212 /// let five = Rc::new(5);
1214 /// assert!(five == Rc::new(5));
1217 fn eq(&self, other: &Rc<T>) -> bool {
1218 RcEqIdent::eq(self, other)
1221 /// Inequality for two `Rc`s.
1223 /// Two `Rc`s are unequal if their inner values are unequal.
1225 /// If `T` also implements `Eq` (implying reflexivity of equality),
1226 /// two `Rc`s that point to the same allocation are
1232 /// use std::rc::Rc;
1234 /// let five = Rc::new(5);
1236 /// assert!(five != Rc::new(6));
1239 fn ne(&self, other: &Rc<T>) -> bool {
1240 RcEqIdent::ne(self, other)
1244 #[stable(feature = "rust1", since = "1.0.0")]
1245 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1247 #[stable(feature = "rust1", since = "1.0.0")]
1248 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1249 /// Partial comparison for two `Rc`s.
1251 /// The two are compared by calling `partial_cmp()` on their inner values.
1256 /// use std::rc::Rc;
1257 /// use std::cmp::Ordering;
1259 /// let five = Rc::new(5);
1261 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1264 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1265 (**self).partial_cmp(&**other)
1268 /// Less-than comparison for two `Rc`s.
1270 /// The two are compared by calling `<` on their inner values.
1275 /// use std::rc::Rc;
1277 /// let five = Rc::new(5);
1279 /// assert!(five < Rc::new(6));
1282 fn lt(&self, other: &Rc<T>) -> bool {
1286 /// 'Less than or equal to' comparison for two `Rc`s.
1288 /// The two are compared by calling `<=` on their inner values.
1293 /// use std::rc::Rc;
1295 /// let five = Rc::new(5);
1297 /// assert!(five <= Rc::new(5));
1300 fn le(&self, other: &Rc<T>) -> bool {
1304 /// Greater-than comparison for two `Rc`s.
1306 /// The two are compared by calling `>` on their inner values.
1311 /// use std::rc::Rc;
1313 /// let five = Rc::new(5);
1315 /// assert!(five > Rc::new(4));
1318 fn gt(&self, other: &Rc<T>) -> bool {
1322 /// 'Greater than or equal to' comparison for two `Rc`s.
1324 /// The two are compared by calling `>=` on their inner values.
1329 /// use std::rc::Rc;
1331 /// let five = Rc::new(5);
1333 /// assert!(five >= Rc::new(5));
1336 fn ge(&self, other: &Rc<T>) -> bool {
1341 #[stable(feature = "rust1", since = "1.0.0")]
1342 impl<T: ?Sized + Ord> Ord for Rc<T> {
1343 /// Comparison for two `Rc`s.
1345 /// The two are compared by calling `cmp()` on their inner values.
1350 /// use std::rc::Rc;
1351 /// use std::cmp::Ordering;
1353 /// let five = Rc::new(5);
1355 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1358 fn cmp(&self, other: &Rc<T>) -> Ordering {
1359 (**self).cmp(&**other)
1363 #[stable(feature = "rust1", since = "1.0.0")]
1364 impl<T: ?Sized + Hash> Hash for Rc<T> {
1365 fn hash<H: Hasher>(&self, state: &mut H) {
1366 (**self).hash(state);
1370 #[stable(feature = "rust1", since = "1.0.0")]
1371 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1372 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1373 fmt::Display::fmt(&**self, f)
1377 #[stable(feature = "rust1", since = "1.0.0")]
1378 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1379 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1380 fmt::Debug::fmt(&**self, f)
1384 #[stable(feature = "rust1", since = "1.0.0")]
1385 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1386 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1387 fmt::Pointer::fmt(&(&**self as *const T), f)
1391 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1392 impl<T> From<T> for Rc<T> {
1393 fn from(t: T) -> Self {
1398 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1399 impl<T: Clone> From<&[T]> for Rc<[T]> {
1401 fn from(v: &[T]) -> Rc<[T]> {
1402 <Self as RcFromSlice<T>>::from_slice(v)
1406 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1407 impl From<&str> for Rc<str> {
1409 fn from(v: &str) -> Rc<str> {
1410 let rc = Rc::<[u8]>::from(v.as_bytes());
1411 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1415 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1416 impl From<String> for Rc<str> {
1418 fn from(v: String) -> Rc<str> {
1423 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1424 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1426 fn from(v: Box<T>) -> Rc<T> {
1431 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1432 impl<T> From<Vec<T>> for Rc<[T]> {
1434 fn from(mut v: Vec<T>) -> Rc<[T]> {
1436 let rc = Rc::copy_from_slice(&v);
1438 // Allow the Vec to free its memory, but not destroy its contents
1446 #[unstable(feature = "boxed_slice_try_from", issue = "none")]
1447 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]>
1449 [T; N]: LengthAtMost32,
1451 type Error = Rc<[T]>;
1453 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1454 if boxed_slice.len() == N {
1455 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
1462 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1463 impl<T> iter::FromIterator<T> for Rc<[T]> {
1464 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1466 /// # Performance characteristics
1468 /// ## The general case
1470 /// In the general case, collecting into `Rc<[T]>` is done by first
1471 /// collecting into a `Vec<T>`. That is, when writing the following:
1474 /// # use std::rc::Rc;
1475 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1476 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1479 /// this behaves as if we wrote:
1482 /// # use std::rc::Rc;
1483 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1484 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1485 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1486 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1489 /// This will allocate as many times as needed for constructing the `Vec<T>`
1490 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1492 /// ## Iterators of known length
1494 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1495 /// a single allocation will be made for the `Rc<[T]>`. For example:
1498 /// # use std::rc::Rc;
1499 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1500 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1502 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1503 RcFromIter::from_iter(iter.into_iter())
1507 /// Specialization trait used for collecting into `Rc<[T]>`.
1508 trait RcFromIter<T, I> {
1509 fn from_iter(iter: I) -> Self;
1512 impl<T, I: Iterator<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1513 default fn from_iter(iter: I) -> Self {
1514 iter.collect::<Vec<T>>().into()
1518 impl<T, I: iter::TrustedLen<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1519 default fn from_iter(iter: I) -> Self {
1520 // This is the case for a `TrustedLen` iterator.
1521 let (low, high) = iter.size_hint();
1522 if let Some(high) = high {
1526 "TrustedLen iterator's size hint is not exact: {:?}",
1531 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1532 Rc::from_iter_exact(iter, low)
1535 // Fall back to normal implementation.
1536 iter.collect::<Vec<T>>().into()
1541 impl<'a, T: 'a + Clone> RcFromIter<&'a T, slice::Iter<'a, T>> for Rc<[T]> {
1542 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
1543 // Delegate to `impl<T: Clone> From<&[T]> for Rc<[T]>`.
1545 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
1546 // which is even more performant.
1548 // In the fall-back case we have `T: Clone`. This is still better
1549 // than the `TrustedLen` implementation as slices have a known length
1550 // and so we get to avoid calling `size_hint` and avoid the branching.
1551 iter.as_slice().into()
1555 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1556 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
1557 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1559 /// Since a `Weak` reference does not count towards ownership, it will not
1560 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
1561 /// guarantees about the value still being present. Thus it may return [`None`]
1562 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
1563 /// itself (the backing store) from being deallocated.
1565 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
1566 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
1567 /// prevent circular references between [`Rc`] pointers, since mutual owning references
1568 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1569 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1570 /// pointers from children back to their parents.
1572 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1574 /// [`Rc`]: struct.Rc.html
1575 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1576 /// [`upgrade`]: struct.Weak.html#method.upgrade
1577 /// [`Option`]: ../../std/option/enum.Option.html
1578 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1579 #[stable(feature = "rc_weak", since = "1.4.0")]
1580 pub struct Weak<T: ?Sized> {
1581 // This is a `NonNull` to allow optimizing the size of this type in enums,
1582 // but it is not necessarily a valid pointer.
1583 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1584 // to allocate space on the heap. That's not a value a real pointer
1585 // will ever have because RcBox has alignment at least 2.
1586 ptr: NonNull<RcBox<T>>,
1589 #[stable(feature = "rc_weak", since = "1.4.0")]
1590 impl<T: ?Sized> !marker::Send for Weak<T> {}
1591 #[stable(feature = "rc_weak", since = "1.4.0")]
1592 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1594 #[unstable(feature = "coerce_unsized", issue = "27732")]
1595 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1597 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1598 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1601 /// Constructs a new `Weak<T>`, without allocating any memory.
1602 /// Calling [`upgrade`] on the return value always gives [`None`].
1604 /// [`upgrade`]: #method.upgrade
1605 /// [`None`]: ../../std/option/enum.Option.html
1610 /// use std::rc::Weak;
1612 /// let empty: Weak<i64> = Weak::new();
1613 /// assert!(empty.upgrade().is_none());
1615 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1616 pub fn new() -> Weak<T> {
1617 Weak { 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 /// The pointer is valid only if there are some strong references. The pointer may be dangling
1623 /// or even [`null`] otherwise.
1628 /// #![feature(weak_into_raw)]
1630 /// use std::rc::Rc;
1633 /// let strong = Rc::new("hello".to_owned());
1634 /// let weak = Rc::downgrade(&strong);
1635 /// // Both point to the same object
1636 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1637 /// // The strong here keeps it alive, so we can still access the object.
1638 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1641 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1642 /// // undefined behaviour.
1643 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1646 /// [`null`]: ../../std/ptr/fn.null.html
1647 #[unstable(feature = "weak_into_raw", issue = "60728")]
1648 pub fn as_raw(&self) -> *const T {
1649 match self.inner() {
1650 None => ptr::null(),
1652 let offset = data_offset_sized::<T>();
1653 let ptr = inner as *const RcBox<T>;
1654 // Note: while the pointer we create may already point to dropped value, the
1655 // allocation still lives (it must hold the weak point as long as we are alive).
1656 // Therefore, the offset is OK to do, it won't get out of the allocation.
1657 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1663 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1665 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1666 /// can be turned back into the `Weak<T>` with [`from_raw`].
1668 /// The same restrictions of accessing the target of the pointer as with
1669 /// [`as_raw`] apply.
1674 /// #![feature(weak_into_raw)]
1676 /// use std::rc::{Rc, Weak};
1678 /// let strong = Rc::new("hello".to_owned());
1679 /// let weak = Rc::downgrade(&strong);
1680 /// let raw = weak.into_raw();
1682 /// assert_eq!(1, Rc::weak_count(&strong));
1683 /// assert_eq!("hello", unsafe { &*raw });
1685 /// drop(unsafe { Weak::from_raw(raw) });
1686 /// assert_eq!(0, Rc::weak_count(&strong));
1689 /// [`from_raw`]: struct.Weak.html#method.from_raw
1690 /// [`as_raw`]: struct.Weak.html#method.as_raw
1691 #[unstable(feature = "weak_into_raw", issue = "60728")]
1692 pub fn into_raw(self) -> *const T {
1693 let result = self.as_raw();
1698 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1700 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1701 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1703 /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
1704 /// as these don't have any corresponding weak count).
1708 /// The pointer must have originated from the [`into_raw`] (or [`as_raw`], provided there was
1709 /// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
1712 /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
1713 /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
1719 /// #![feature(weak_into_raw)]
1721 /// use std::rc::{Rc, Weak};
1723 /// let strong = Rc::new("hello".to_owned());
1725 /// let raw_1 = Rc::downgrade(&strong).into_raw();
1726 /// let raw_2 = Rc::downgrade(&strong).into_raw();
1728 /// assert_eq!(2, Rc::weak_count(&strong));
1730 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1731 /// assert_eq!(1, Rc::weak_count(&strong));
1735 /// // Decrement the last weak count.
1736 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1739 /// [`into_raw`]: struct.Weak.html#method.into_raw
1740 /// [`upgrade`]: struct.Weak.html#method.upgrade
1741 /// [`Rc`]: struct.Rc.html
1742 /// [`Weak`]: struct.Weak.html
1743 /// [`as_raw`]: struct.Weak.html#method.as_raw
1744 /// [`new`]: struct.Weak.html#method.new
1745 /// [`forget`]: ../../std/mem/fn.forget.html
1746 #[unstable(feature = "weak_into_raw", issue = "60728")]
1747 pub unsafe fn from_raw(ptr: *const T) -> Self {
1751 // See Rc::from_raw for details
1752 let offset = data_offset(ptr);
1753 let fake_ptr = ptr as *mut RcBox<T>;
1754 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1755 Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
1760 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1761 let address = ptr.as_ptr() as *mut () as usize;
1762 address == usize::MAX
1765 impl<T: ?Sized> Weak<T> {
1766 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
1767 /// dropping of the inner value if successful.
1769 /// Returns [`None`] if the inner value has since been dropped.
1771 /// [`Rc`]: struct.Rc.html
1772 /// [`None`]: ../../std/option/enum.Option.html
1777 /// use std::rc::Rc;
1779 /// let five = Rc::new(5);
1781 /// let weak_five = Rc::downgrade(&five);
1783 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1784 /// assert!(strong_five.is_some());
1786 /// // Destroy all strong pointers.
1787 /// drop(strong_five);
1790 /// assert!(weak_five.upgrade().is_none());
1792 #[stable(feature = "rc_weak", since = "1.4.0")]
1793 pub fn upgrade(&self) -> Option<Rc<T>> {
1794 let inner = self.inner()?;
1795 if inner.strong() == 0 {
1799 Some(Rc::from_inner(self.ptr))
1803 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
1805 /// If `self` was created using [`Weak::new`], this will return 0.
1807 /// [`Weak::new`]: #method.new
1808 #[stable(feature = "weak_counts", since = "1.41.0")]
1809 pub fn strong_count(&self) -> usize {
1810 if let Some(inner) = self.inner() { inner.strong() } else { 0 }
1813 /// Gets the number of `Weak` pointers pointing to this allocation.
1815 /// If no strong pointers remain, this will return zero.
1816 #[stable(feature = "weak_counts", since = "1.41.0")]
1817 pub fn weak_count(&self) -> usize {
1820 if inner.strong() > 0 {
1821 inner.weak() - 1 // subtract the implicit weak ptr
1829 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1830 /// (i.e., when this `Weak` was created by `Weak::new`).
1832 fn inner(&self) -> Option<&RcBox<T>> {
1833 if is_dangling(self.ptr) { None } else { Some(unsafe { self.ptr.as_ref() }) }
1836 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1837 /// [`ptr::eq`]), or if both don't point to any allocation
1838 /// (because they were created with `Weak::new()`).
1842 /// Since this compares pointers it means that `Weak::new()` will equal each
1843 /// other, even though they don't point to any allocation.
1848 /// use std::rc::Rc;
1850 /// let first_rc = Rc::new(5);
1851 /// let first = Rc::downgrade(&first_rc);
1852 /// let second = Rc::downgrade(&first_rc);
1854 /// assert!(first.ptr_eq(&second));
1856 /// let third_rc = Rc::new(5);
1857 /// let third = Rc::downgrade(&third_rc);
1859 /// assert!(!first.ptr_eq(&third));
1862 /// Comparing `Weak::new`.
1865 /// use std::rc::{Rc, Weak};
1867 /// let first = Weak::new();
1868 /// let second = Weak::new();
1869 /// assert!(first.ptr_eq(&second));
1871 /// let third_rc = Rc::new(());
1872 /// let third = Rc::downgrade(&third_rc);
1873 /// assert!(!first.ptr_eq(&third));
1876 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1878 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1879 pub fn ptr_eq(&self, other: &Self) -> bool {
1880 self.ptr.as_ptr() == other.ptr.as_ptr()
1884 #[stable(feature = "rc_weak", since = "1.4.0")]
1885 impl<T: ?Sized> Drop for Weak<T> {
1886 /// Drops the `Weak` pointer.
1891 /// use std::rc::{Rc, Weak};
1895 /// impl Drop for Foo {
1896 /// fn drop(&mut self) {
1897 /// println!("dropped!");
1901 /// let foo = Rc::new(Foo);
1902 /// let weak_foo = Rc::downgrade(&foo);
1903 /// let other_weak_foo = Weak::clone(&weak_foo);
1905 /// drop(weak_foo); // Doesn't print anything
1906 /// drop(foo); // Prints "dropped!"
1908 /// assert!(other_weak_foo.upgrade().is_none());
1910 fn drop(&mut self) {
1911 if let Some(inner) = self.inner() {
1913 // the weak count starts at 1, and will only go to zero if all
1914 // the strong pointers have disappeared.
1915 if inner.weak() == 0 {
1917 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1924 #[stable(feature = "rc_weak", since = "1.4.0")]
1925 impl<T: ?Sized> Clone for Weak<T> {
1926 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1931 /// use std::rc::{Rc, Weak};
1933 /// let weak_five = Rc::downgrade(&Rc::new(5));
1935 /// let _ = Weak::clone(&weak_five);
1938 fn clone(&self) -> Weak<T> {
1939 if let Some(inner) = self.inner() {
1942 Weak { ptr: self.ptr }
1946 #[stable(feature = "rc_weak", since = "1.4.0")]
1947 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1948 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1953 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1954 impl<T> Default for Weak<T> {
1955 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1956 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1958 /// [`None`]: ../../std/option/enum.Option.html
1959 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
1964 /// use std::rc::Weak;
1966 /// let empty: Weak<i64> = Default::default();
1967 /// assert!(empty.upgrade().is_none());
1969 fn default() -> Weak<T> {
1974 // NOTE: We checked_add here to deal with mem::forget safely. In particular
1975 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1976 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1977 // We abort because this is such a degenerate scenario that we don't care about
1978 // what happens -- no real program should ever experience this.
1980 // This should have negligible overhead since you don't actually need to
1981 // clone these much in Rust thanks to ownership and move-semantics.
1984 trait RcBoxPtr<T: ?Sized> {
1985 fn inner(&self) -> &RcBox<T>;
1988 fn strong(&self) -> usize {
1989 self.inner().strong.get()
1993 fn inc_strong(&self) {
1994 let strong = self.strong();
1996 // We want to abort on overflow instead of dropping the value.
1997 // The reference count will never be zero when this is called;
1998 // nevertheless, we insert an abort here to hint LLVM at
1999 // an otherwise missed optimization.
2000 if strong == 0 || strong == usize::max_value() {
2005 self.inner().strong.set(strong + 1);
2009 fn dec_strong(&self) {
2010 self.inner().strong.set(self.strong() - 1);
2014 fn weak(&self) -> usize {
2015 self.inner().weak.get()
2019 fn inc_weak(&self) {
2020 let weak = self.weak();
2022 // We want to abort on overflow instead of dropping the value.
2023 // The reference count will never be zero when this is called;
2024 // nevertheless, we insert an abort here to hint LLVM at
2025 // an otherwise missed optimization.
2026 if weak == 0 || weak == usize::max_value() {
2031 self.inner().weak.set(weak + 1);
2035 fn dec_weak(&self) {
2036 self.inner().weak.set(self.weak() - 1);
2040 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
2042 fn inner(&self) -> &RcBox<T> {
2043 unsafe { self.ptr.as_ref() }
2047 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
2049 fn inner(&self) -> &RcBox<T> {
2054 #[stable(feature = "rust1", since = "1.0.0")]
2055 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2056 fn borrow(&self) -> &T {
2061 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2062 impl<T: ?Sized> AsRef<T> for Rc<T> {
2063 fn as_ref(&self) -> &T {
2068 #[stable(feature = "pin", since = "1.33.0")]
2069 impl<T: ?Sized> Unpin for Rc<T> {}
2071 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2072 // Align the unsized value to the end of the `RcBox`.
2073 // Because it is ?Sized, it will always be the last field in memory.
2074 // Note: This is a detail of the current implementation of the compiler,
2075 // and is not a guaranteed language detail. Do not rely on it outside of std.
2076 data_offset_align(align_of_val(&*ptr))
2079 /// Computes the offset of the data field within `RcBox`.
2081 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2082 fn data_offset_sized<T>() -> isize {
2083 data_offset_align(align_of::<T>())
2087 fn data_offset_align(align: usize) -> isize {
2088 let layout = Layout::new::<RcBox<()>>();
2089 (layout.size() + layout.padding_needed_for(align)) as isize