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: *mut RcBox<T> = NonNull::as_ptr(this.ptr);
569 let fake_ptr = ptr as *mut T;
572 // SAFETY: This cannot go through Deref::deref.
573 // Instead, we manually offset the pointer rather than manifesting a reference.
574 // This is so that the returned pointer retains the same provenance as our pointer.
575 // This is required so that e.g. `get_mut` can write through the pointer
576 // after the Rc is recovered through `from_raw`.
578 let offset = data_offset(&(*ptr).value);
579 set_data_ptr(fake_ptr, (ptr as *mut u8).offset(offset))
583 /// Constructs an `Rc` from a raw pointer.
585 /// The raw pointer must have been previously returned by a call to a
586 /// [`Rc::into_raw`][into_raw].
588 /// This function is unsafe because improper use may lead to memory problems. For example, a
589 /// double-free may occur if the function is called twice on the same raw pointer.
591 /// [into_raw]: struct.Rc.html#method.into_raw
598 /// let x = Rc::new("hello".to_owned());
599 /// let x_ptr = Rc::into_raw(x);
602 /// // Convert back to an `Rc` to prevent leak.
603 /// let x = Rc::from_raw(x_ptr);
604 /// assert_eq!(&*x, "hello");
606 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
609 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
611 #[stable(feature = "rc_raw", since = "1.17.0")]
612 pub unsafe fn from_raw(ptr: *const T) -> Self {
613 let offset = data_offset(ptr);
615 // Reverse the offset to find the original RcBox.
616 let fake_ptr = ptr as *mut RcBox<T>;
617 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
619 Self::from_ptr(rc_ptr)
622 /// Consumes the `Rc`, returning the wrapped pointer as `NonNull<T>`.
627 /// #![feature(rc_into_raw_non_null)]
631 /// let x = Rc::new("hello".to_owned());
632 /// let ptr = Rc::into_raw_non_null(x);
633 /// let deref = unsafe { ptr.as_ref() };
634 /// assert_eq!(deref, "hello");
636 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
638 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
639 // safe because Rc guarantees its pointer is non-null
640 unsafe { NonNull::new_unchecked(Rc::into_raw(this) as *mut _) }
643 /// Creates a new [`Weak`][weak] pointer to this allocation.
645 /// [weak]: struct.Weak.html
652 /// let five = Rc::new(5);
654 /// let weak_five = Rc::downgrade(&five);
656 #[stable(feature = "rc_weak", since = "1.4.0")]
657 pub fn downgrade(this: &Self) -> Weak<T> {
659 // Make sure we do not create a dangling Weak
660 debug_assert!(!is_dangling(this.ptr));
661 Weak { ptr: this.ptr }
664 /// Gets the number of [`Weak`][weak] pointers to this allocation.
666 /// [weak]: struct.Weak.html
673 /// let five = Rc::new(5);
674 /// let _weak_five = Rc::downgrade(&five);
676 /// assert_eq!(1, Rc::weak_count(&five));
679 #[stable(feature = "rc_counts", since = "1.15.0")]
680 pub fn weak_count(this: &Self) -> usize {
684 /// Gets the number of strong (`Rc`) pointers to this allocation.
691 /// let five = Rc::new(5);
692 /// let _also_five = Rc::clone(&five);
694 /// assert_eq!(2, Rc::strong_count(&five));
697 #[stable(feature = "rc_counts", since = "1.15.0")]
698 pub fn strong_count(this: &Self) -> usize {
702 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
705 /// [weak]: struct.Weak.html
707 fn is_unique(this: &Self) -> bool {
708 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
711 /// Returns a mutable reference into the given `Rc`, if there are
712 /// no other `Rc` or [`Weak`][weak] pointers to the same allocation.
714 /// Returns [`None`] otherwise, because it is not safe to
715 /// mutate a shared value.
717 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
718 /// the inner value when there are other pointers.
720 /// [weak]: struct.Weak.html
721 /// [`None`]: ../../std/option/enum.Option.html#variant.None
722 /// [make_mut]: struct.Rc.html#method.make_mut
723 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
730 /// let mut x = Rc::new(3);
731 /// *Rc::get_mut(&mut x).unwrap() = 4;
732 /// assert_eq!(*x, 4);
734 /// let _y = Rc::clone(&x);
735 /// assert!(Rc::get_mut(&mut x).is_none());
738 #[stable(feature = "rc_unique", since = "1.4.0")]
739 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
740 if Rc::is_unique(this) { unsafe { Some(Rc::get_mut_unchecked(this)) } } else { None }
743 /// Returns a mutable reference into the given `Rc`,
744 /// without any check.
746 /// See also [`get_mut`], which is safe and does appropriate checks.
748 /// [`get_mut`]: struct.Rc.html#method.get_mut
752 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
753 /// for the duration of the returned borrow.
754 /// This is trivially the case if no such pointers exist,
755 /// for example immediately after `Rc::new`.
760 /// #![feature(get_mut_unchecked)]
764 /// let mut x = Rc::new(String::new());
766 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
768 /// assert_eq!(*x, "foo");
771 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
772 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
773 &mut this.ptr.as_mut().value
777 #[stable(feature = "ptr_eq", since = "1.17.0")]
778 /// Returns `true` if the two `Rc`s point to the same allocation
779 /// (in a vein similar to [`ptr::eq`]).
786 /// let five = Rc::new(5);
787 /// let same_five = Rc::clone(&five);
788 /// let other_five = Rc::new(5);
790 /// assert!(Rc::ptr_eq(&five, &same_five));
791 /// assert!(!Rc::ptr_eq(&five, &other_five));
794 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
795 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
796 this.ptr.as_ptr() == other.ptr.as_ptr()
800 impl<T: Clone> Rc<T> {
801 /// Makes a mutable reference into the given `Rc`.
803 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
804 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
805 /// referred to as clone-on-write.
807 /// If there are no other `Rc` pointers to this allocation, then [`Weak`]
808 /// pointers to this allocation will be disassociated.
810 /// See also [`get_mut`], which will fail rather than cloning.
812 /// [`Weak`]: struct.Weak.html
813 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
814 /// [`get_mut`]: struct.Rc.html#method.get_mut
821 /// let mut data = Rc::new(5);
823 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
824 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
825 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
826 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
827 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
829 /// // Now `data` and `other_data` point to different allocations.
830 /// assert_eq!(*data, 8);
831 /// assert_eq!(*other_data, 12);
834 /// [`Weak`] pointers will be disassociated:
839 /// let mut data = Rc::new(75);
840 /// let weak = Rc::downgrade(&data);
842 /// assert!(75 == *data);
843 /// assert!(75 == *weak.upgrade().unwrap());
845 /// *Rc::make_mut(&mut data) += 1;
847 /// assert!(76 == *data);
848 /// assert!(weak.upgrade().is_none());
851 #[stable(feature = "rc_unique", since = "1.4.0")]
852 pub fn make_mut(this: &mut Self) -> &mut T {
853 if Rc::strong_count(this) != 1 {
854 // Gotta clone the data, there are other Rcs
855 *this = Rc::new((**this).clone())
856 } else if Rc::weak_count(this) != 0 {
857 // Can just steal the data, all that's left is Weaks
859 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
860 mem::swap(this, &mut swap);
862 // Remove implicit strong-weak ref (no need to craft a fake
863 // Weak here -- we know other Weaks can clean up for us)
868 // This unsafety is ok because we're guaranteed that the pointer
869 // returned is the *only* pointer that will ever be returned to T. Our
870 // reference count is guaranteed to be 1 at this point, and we required
871 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
872 // reference to the allocation.
873 unsafe { &mut this.ptr.as_mut().value }
879 #[stable(feature = "rc_downcast", since = "1.29.0")]
880 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
885 /// use std::any::Any;
888 /// fn print_if_string(value: Rc<dyn Any>) {
889 /// if let Ok(string) = value.downcast::<String>() {
890 /// println!("String ({}): {}", string.len(), string);
894 /// let my_string = "Hello World".to_string();
895 /// print_if_string(Rc::new(my_string));
896 /// print_if_string(Rc::new(0i8));
898 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
899 if (*self).is::<T>() {
900 let ptr = self.ptr.cast::<RcBox<T>>();
902 Ok(Rc::from_inner(ptr))
909 impl<T: ?Sized> Rc<T> {
910 /// Allocates an `RcBox<T>` with sufficient space for
911 /// a possibly-unsized inner value where the value has the layout provided.
913 /// The function `mem_to_rcbox` is called with the data pointer
914 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
915 unsafe fn allocate_for_layout(
916 value_layout: Layout,
917 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
919 // Calculate layout using the given value layout.
920 // Previously, layout was calculated on the expression
921 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
922 // reference (see #54908).
923 let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align();
925 // Allocate for the layout.
926 let mem = Global.alloc(layout).unwrap_or_else(|_| handle_alloc_error(layout));
928 // Initialize the RcBox
929 let inner = mem_to_rcbox(mem.as_ptr());
930 debug_assert_eq!(Layout::for_value(&*inner), layout);
932 ptr::write(&mut (*inner).strong, Cell::new(1));
933 ptr::write(&mut (*inner).weak, Cell::new(1));
938 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
939 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
940 // Allocate for the `RcBox<T>` using the given value.
941 Self::allocate_for_layout(Layout::for_value(&*ptr), |mem| {
942 set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>
946 fn from_box(v: Box<T>) -> Rc<T> {
948 let box_unique = Box::into_unique(v);
949 let bptr = box_unique.as_ptr();
951 let value_size = size_of_val(&*bptr);
952 let ptr = Self::allocate_for_ptr(bptr);
954 // Copy value as bytes
955 ptr::copy_nonoverlapping(
956 bptr as *const T as *const u8,
957 &mut (*ptr).value as *mut _ as *mut u8,
961 // Free the allocation without dropping its contents
962 box_free(box_unique);
970 /// Allocates an `RcBox<[T]>` with the given length.
971 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
972 Self::allocate_for_layout(Layout::array::<T>(len).unwrap(), |mem| {
973 ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>
978 /// Sets the data pointer of a `?Sized` raw pointer.
980 /// For a slice/trait object, this sets the `data` field and leaves the rest
981 /// unchanged. For a sized raw pointer, this simply sets the pointer.
982 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
983 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
988 /// Copy elements from slice into newly allocated Rc<[T]>
990 /// Unsafe because the caller must either take ownership or bind `T: Copy`
991 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
992 let ptr = Self::allocate_for_slice(v.len());
994 ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).value as *mut [T] as *mut T, v.len());
999 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
1001 /// Behavior is undefined should the size be wrong.
1002 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
1003 // Panic guard while cloning T elements.
1004 // In the event of a panic, elements that have been written
1005 // into the new RcBox will be dropped, then the memory freed.
1013 impl<T> Drop for Guard<T> {
1014 fn drop(&mut self) {
1016 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1017 ptr::drop_in_place(slice);
1019 Global.dealloc(self.mem, self.layout);
1024 let ptr = Self::allocate_for_slice(len);
1026 let mem = ptr as *mut _ as *mut u8;
1027 let layout = Layout::for_value(&*ptr);
1029 // Pointer to first element
1030 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1032 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
1034 for (i, item) in iter.enumerate() {
1035 ptr::write(elems.add(i), item);
1039 // All clear. Forget the guard so it doesn't free the new RcBox.
1046 /// Specialization trait used for `From<&[T]>`.
1047 trait RcFromSlice<T> {
1048 fn from_slice(slice: &[T]) -> Self;
1051 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
1053 default fn from_slice(v: &[T]) -> Self {
1054 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
1058 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
1060 fn from_slice(v: &[T]) -> Self {
1061 unsafe { Rc::copy_from_slice(v) }
1065 #[stable(feature = "rust1", since = "1.0.0")]
1066 impl<T: ?Sized> Deref for Rc<T> {
1070 fn deref(&self) -> &T {
1075 #[unstable(feature = "receiver_trait", issue = "none")]
1076 impl<T: ?Sized> Receiver for Rc<T> {}
1078 #[stable(feature = "rust1", since = "1.0.0")]
1079 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1082 /// This will decrement the strong reference count. If the strong reference
1083 /// count reaches zero then the only other references (if any) are
1084 /// [`Weak`], so we `drop` the inner value.
1089 /// use std::rc::Rc;
1093 /// impl Drop for Foo {
1094 /// fn drop(&mut self) {
1095 /// println!("dropped!");
1099 /// let foo = Rc::new(Foo);
1100 /// let foo2 = Rc::clone(&foo);
1102 /// drop(foo); // Doesn't print anything
1103 /// drop(foo2); // Prints "dropped!"
1106 /// [`Weak`]: ../../std/rc/struct.Weak.html
1107 fn drop(&mut self) {
1110 if self.strong() == 0 {
1111 // destroy the contained object
1112 ptr::drop_in_place(self.ptr.as_mut());
1114 // remove the implicit "strong weak" pointer now that we've
1115 // destroyed the contents.
1118 if self.weak() == 0 {
1119 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1126 #[stable(feature = "rust1", since = "1.0.0")]
1127 impl<T: ?Sized> Clone for Rc<T> {
1128 /// Makes a clone of the `Rc` pointer.
1130 /// This creates another pointer to the same allocation, increasing the
1131 /// strong reference count.
1136 /// use std::rc::Rc;
1138 /// let five = Rc::new(5);
1140 /// let _ = Rc::clone(&five);
1143 fn clone(&self) -> Rc<T> {
1145 Self::from_inner(self.ptr)
1149 #[stable(feature = "rust1", since = "1.0.0")]
1150 impl<T: Default> Default for Rc<T> {
1151 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1156 /// use std::rc::Rc;
1158 /// let x: Rc<i32> = Default::default();
1159 /// assert_eq!(*x, 0);
1162 fn default() -> Rc<T> {
1163 Rc::new(Default::default())
1167 #[stable(feature = "rust1", since = "1.0.0")]
1168 trait RcEqIdent<T: ?Sized + PartialEq> {
1169 fn eq(&self, other: &Rc<T>) -> bool;
1170 fn ne(&self, other: &Rc<T>) -> bool;
1173 #[stable(feature = "rust1", since = "1.0.0")]
1174 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
1176 default fn eq(&self, other: &Rc<T>) -> bool {
1181 default fn ne(&self, other: &Rc<T>) -> bool {
1186 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1187 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1188 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1189 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1190 /// the same value, than two `&T`s.
1192 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1193 #[stable(feature = "rust1", since = "1.0.0")]
1194 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
1196 fn eq(&self, other: &Rc<T>) -> bool {
1197 Rc::ptr_eq(self, other) || **self == **other
1201 fn ne(&self, other: &Rc<T>) -> bool {
1202 !Rc::ptr_eq(self, other) && **self != **other
1206 #[stable(feature = "rust1", since = "1.0.0")]
1207 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1208 /// Equality for two `Rc`s.
1210 /// Two `Rc`s are equal if their inner values are equal, even if they are
1211 /// stored in different allocation.
1213 /// If `T` also implements `Eq` (implying reflexivity of equality),
1214 /// two `Rc`s that point to the same allocation are
1220 /// use std::rc::Rc;
1222 /// let five = Rc::new(5);
1224 /// assert!(five == Rc::new(5));
1227 fn eq(&self, other: &Rc<T>) -> bool {
1228 RcEqIdent::eq(self, other)
1231 /// Inequality for two `Rc`s.
1233 /// Two `Rc`s are unequal if their inner values are unequal.
1235 /// If `T` also implements `Eq` (implying reflexivity of equality),
1236 /// two `Rc`s that point to the same allocation are
1242 /// use std::rc::Rc;
1244 /// let five = Rc::new(5);
1246 /// assert!(five != Rc::new(6));
1249 fn ne(&self, other: &Rc<T>) -> bool {
1250 RcEqIdent::ne(self, other)
1254 #[stable(feature = "rust1", since = "1.0.0")]
1255 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1257 #[stable(feature = "rust1", since = "1.0.0")]
1258 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1259 /// Partial comparison for two `Rc`s.
1261 /// The two are compared by calling `partial_cmp()` on their inner values.
1266 /// use std::rc::Rc;
1267 /// use std::cmp::Ordering;
1269 /// let five = Rc::new(5);
1271 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1274 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1275 (**self).partial_cmp(&**other)
1278 /// Less-than comparison for two `Rc`s.
1280 /// The two are compared by calling `<` on their inner values.
1285 /// use std::rc::Rc;
1287 /// let five = Rc::new(5);
1289 /// assert!(five < Rc::new(6));
1292 fn lt(&self, other: &Rc<T>) -> bool {
1296 /// 'Less than or equal to' comparison for two `Rc`s.
1298 /// The two are compared by calling `<=` on their inner values.
1303 /// use std::rc::Rc;
1305 /// let five = Rc::new(5);
1307 /// assert!(five <= Rc::new(5));
1310 fn le(&self, other: &Rc<T>) -> bool {
1314 /// Greater-than comparison for two `Rc`s.
1316 /// The two are compared by calling `>` on their inner values.
1321 /// use std::rc::Rc;
1323 /// let five = Rc::new(5);
1325 /// assert!(five > Rc::new(4));
1328 fn gt(&self, other: &Rc<T>) -> bool {
1332 /// 'Greater than or equal to' comparison for two `Rc`s.
1334 /// The two are compared by calling `>=` on their inner values.
1339 /// use std::rc::Rc;
1341 /// let five = Rc::new(5);
1343 /// assert!(five >= Rc::new(5));
1346 fn ge(&self, other: &Rc<T>) -> bool {
1351 #[stable(feature = "rust1", since = "1.0.0")]
1352 impl<T: ?Sized + Ord> Ord for Rc<T> {
1353 /// Comparison for two `Rc`s.
1355 /// The two are compared by calling `cmp()` on their inner values.
1360 /// use std::rc::Rc;
1361 /// use std::cmp::Ordering;
1363 /// let five = Rc::new(5);
1365 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1368 fn cmp(&self, other: &Rc<T>) -> Ordering {
1369 (**self).cmp(&**other)
1373 #[stable(feature = "rust1", since = "1.0.0")]
1374 impl<T: ?Sized + Hash> Hash for Rc<T> {
1375 fn hash<H: Hasher>(&self, state: &mut H) {
1376 (**self).hash(state);
1380 #[stable(feature = "rust1", since = "1.0.0")]
1381 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1382 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1383 fmt::Display::fmt(&**self, f)
1387 #[stable(feature = "rust1", since = "1.0.0")]
1388 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1389 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1390 fmt::Debug::fmt(&**self, f)
1394 #[stable(feature = "rust1", since = "1.0.0")]
1395 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1396 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1397 fmt::Pointer::fmt(&(&**self as *const T), f)
1401 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1402 impl<T> From<T> for Rc<T> {
1403 fn from(t: T) -> Self {
1408 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1409 impl<T: Clone> From<&[T]> for Rc<[T]> {
1411 fn from(v: &[T]) -> Rc<[T]> {
1412 <Self as RcFromSlice<T>>::from_slice(v)
1416 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1417 impl From<&str> for Rc<str> {
1419 fn from(v: &str) -> Rc<str> {
1420 let rc = Rc::<[u8]>::from(v.as_bytes());
1421 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1425 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1426 impl From<String> for Rc<str> {
1428 fn from(v: String) -> Rc<str> {
1433 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1434 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1436 fn from(v: Box<T>) -> Rc<T> {
1441 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1442 impl<T> From<Vec<T>> for Rc<[T]> {
1444 fn from(mut v: Vec<T>) -> Rc<[T]> {
1446 let rc = Rc::copy_from_slice(&v);
1448 // Allow the Vec to free its memory, but not destroy its contents
1456 #[unstable(feature = "boxed_slice_try_from", issue = "none")]
1457 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]>
1459 [T; N]: LengthAtMost32,
1461 type Error = Rc<[T]>;
1463 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1464 if boxed_slice.len() == N {
1465 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
1472 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1473 impl<T> iter::FromIterator<T> for Rc<[T]> {
1474 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1476 /// # Performance characteristics
1478 /// ## The general case
1480 /// In the general case, collecting into `Rc<[T]>` is done by first
1481 /// collecting into a `Vec<T>`. That is, when writing the following:
1484 /// # use std::rc::Rc;
1485 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1486 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1489 /// this behaves as if we wrote:
1492 /// # use std::rc::Rc;
1493 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1494 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1495 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1496 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1499 /// This will allocate as many times as needed for constructing the `Vec<T>`
1500 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1502 /// ## Iterators of known length
1504 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1505 /// a single allocation will be made for the `Rc<[T]>`. For example:
1508 /// # use std::rc::Rc;
1509 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1510 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1512 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1513 RcFromIter::from_iter(iter.into_iter())
1517 /// Specialization trait used for collecting into `Rc<[T]>`.
1518 trait RcFromIter<T, I> {
1519 fn from_iter(iter: I) -> Self;
1522 impl<T, I: Iterator<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1523 default fn from_iter(iter: I) -> Self {
1524 iter.collect::<Vec<T>>().into()
1528 impl<T, I: iter::TrustedLen<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1529 default fn from_iter(iter: I) -> Self {
1530 // This is the case for a `TrustedLen` iterator.
1531 let (low, high) = iter.size_hint();
1532 if let Some(high) = high {
1536 "TrustedLen iterator's size hint is not exact: {:?}",
1541 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1542 Rc::from_iter_exact(iter, low)
1545 // Fall back to normal implementation.
1546 iter.collect::<Vec<T>>().into()
1551 impl<'a, T: 'a + Clone> RcFromIter<&'a T, slice::Iter<'a, T>> for Rc<[T]> {
1552 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
1553 // Delegate to `impl<T: Clone> From<&[T]> for Rc<[T]>`.
1555 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
1556 // which is even more performant.
1558 // In the fall-back case we have `T: Clone`. This is still better
1559 // than the `TrustedLen` implementation as slices have a known length
1560 // and so we get to avoid calling `size_hint` and avoid the branching.
1561 iter.as_slice().into()
1565 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1566 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
1567 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1569 /// Since a `Weak` reference does not count towards ownership, it will not
1570 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
1571 /// guarantees about the value still being present. Thus it may return [`None`]
1572 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
1573 /// itself (the backing store) from being deallocated.
1575 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
1576 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
1577 /// prevent circular references between [`Rc`] pointers, since mutual owning references
1578 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1579 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1580 /// pointers from children back to their parents.
1582 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1584 /// [`Rc`]: struct.Rc.html
1585 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1586 /// [`upgrade`]: struct.Weak.html#method.upgrade
1587 /// [`Option`]: ../../std/option/enum.Option.html
1588 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1589 #[stable(feature = "rc_weak", since = "1.4.0")]
1590 pub struct Weak<T: ?Sized> {
1591 // This is a `NonNull` to allow optimizing the size of this type in enums,
1592 // but it is not necessarily a valid pointer.
1593 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1594 // to allocate space on the heap. That's not a value a real pointer
1595 // will ever have because RcBox has alignment at least 2.
1596 ptr: NonNull<RcBox<T>>,
1599 #[stable(feature = "rc_weak", since = "1.4.0")]
1600 impl<T: ?Sized> !marker::Send for Weak<T> {}
1601 #[stable(feature = "rc_weak", since = "1.4.0")]
1602 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1604 #[unstable(feature = "coerce_unsized", issue = "27732")]
1605 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1607 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1608 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1611 /// Constructs a new `Weak<T>`, without allocating any memory.
1612 /// Calling [`upgrade`] on the return value always gives [`None`].
1614 /// [`upgrade`]: #method.upgrade
1615 /// [`None`]: ../../std/option/enum.Option.html
1620 /// use std::rc::Weak;
1622 /// let empty: Weak<i64> = Weak::new();
1623 /// assert!(empty.upgrade().is_none());
1625 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1626 pub fn new() -> Weak<T> {
1627 Weak { ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0") }
1630 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1632 /// The pointer is valid only if there are some strong references. The pointer may be dangling
1633 /// or even [`null`] otherwise.
1638 /// #![feature(weak_into_raw)]
1640 /// use std::rc::Rc;
1643 /// let strong = Rc::new("hello".to_owned());
1644 /// let weak = Rc::downgrade(&strong);
1645 /// // Both point to the same object
1646 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1647 /// // The strong here keeps it alive, so we can still access the object.
1648 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1651 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1652 /// // undefined behaviour.
1653 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1656 /// [`null`]: ../../std/ptr/fn.null.html
1657 #[unstable(feature = "weak_into_raw", issue = "60728")]
1658 pub fn as_raw(&self) -> *const T {
1659 match self.inner() {
1660 None => ptr::null(),
1662 let offset = data_offset_sized::<T>();
1663 let ptr = inner as *const RcBox<T>;
1664 // Note: while the pointer we create may already point to dropped value, the
1665 // allocation still lives (it must hold the weak point as long as we are alive).
1666 // Therefore, the offset is OK to do, it won't get out of the allocation.
1667 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1673 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1675 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1676 /// can be turned back into the `Weak<T>` with [`from_raw`].
1678 /// The same restrictions of accessing the target of the pointer as with
1679 /// [`as_raw`] apply.
1684 /// #![feature(weak_into_raw)]
1686 /// use std::rc::{Rc, Weak};
1688 /// let strong = Rc::new("hello".to_owned());
1689 /// let weak = Rc::downgrade(&strong);
1690 /// let raw = weak.into_raw();
1692 /// assert_eq!(1, Rc::weak_count(&strong));
1693 /// assert_eq!("hello", unsafe { &*raw });
1695 /// drop(unsafe { Weak::from_raw(raw) });
1696 /// assert_eq!(0, Rc::weak_count(&strong));
1699 /// [`from_raw`]: struct.Weak.html#method.from_raw
1700 /// [`as_raw`]: struct.Weak.html#method.as_raw
1701 #[unstable(feature = "weak_into_raw", issue = "60728")]
1702 pub fn into_raw(self) -> *const T {
1703 let result = self.as_raw();
1708 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1710 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1711 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1713 /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
1714 /// as these don't have any corresponding weak count).
1718 /// The pointer must have originated from the [`into_raw`] (or [`as_raw`], provided there was
1719 /// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
1722 /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
1723 /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
1729 /// #![feature(weak_into_raw)]
1731 /// use std::rc::{Rc, Weak};
1733 /// let strong = Rc::new("hello".to_owned());
1735 /// let raw_1 = Rc::downgrade(&strong).into_raw();
1736 /// let raw_2 = Rc::downgrade(&strong).into_raw();
1738 /// assert_eq!(2, Rc::weak_count(&strong));
1740 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1741 /// assert_eq!(1, Rc::weak_count(&strong));
1745 /// // Decrement the last weak count.
1746 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1749 /// [`into_raw`]: struct.Weak.html#method.into_raw
1750 /// [`upgrade`]: struct.Weak.html#method.upgrade
1751 /// [`Rc`]: struct.Rc.html
1752 /// [`Weak`]: struct.Weak.html
1753 /// [`as_raw`]: struct.Weak.html#method.as_raw
1754 /// [`new`]: struct.Weak.html#method.new
1755 /// [`forget`]: ../../std/mem/fn.forget.html
1756 #[unstable(feature = "weak_into_raw", issue = "60728")]
1757 pub unsafe fn from_raw(ptr: *const T) -> Self {
1761 // See Rc::from_raw for details
1762 let offset = data_offset(ptr);
1763 let fake_ptr = ptr as *mut RcBox<T>;
1764 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1765 Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
1770 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1771 let address = ptr.as_ptr() as *mut () as usize;
1772 address == usize::MAX
1775 impl<T: ?Sized> Weak<T> {
1776 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
1777 /// dropping of the inner value if successful.
1779 /// Returns [`None`] if the inner value has since been dropped.
1781 /// [`Rc`]: struct.Rc.html
1782 /// [`None`]: ../../std/option/enum.Option.html
1787 /// use std::rc::Rc;
1789 /// let five = Rc::new(5);
1791 /// let weak_five = Rc::downgrade(&five);
1793 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1794 /// assert!(strong_five.is_some());
1796 /// // Destroy all strong pointers.
1797 /// drop(strong_five);
1800 /// assert!(weak_five.upgrade().is_none());
1802 #[stable(feature = "rc_weak", since = "1.4.0")]
1803 pub fn upgrade(&self) -> Option<Rc<T>> {
1804 let inner = self.inner()?;
1805 if inner.strong() == 0 {
1809 Some(Rc::from_inner(self.ptr))
1813 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
1815 /// If `self` was created using [`Weak::new`], this will return 0.
1817 /// [`Weak::new`]: #method.new
1818 #[stable(feature = "weak_counts", since = "1.41.0")]
1819 pub fn strong_count(&self) -> usize {
1820 if let Some(inner) = self.inner() { inner.strong() } else { 0 }
1823 /// Gets the number of `Weak` pointers pointing to this allocation.
1825 /// If no strong pointers remain, this will return zero.
1826 #[stable(feature = "weak_counts", since = "1.41.0")]
1827 pub fn weak_count(&self) -> usize {
1830 if inner.strong() > 0 {
1831 inner.weak() - 1 // subtract the implicit weak ptr
1839 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1840 /// (i.e., when this `Weak` was created by `Weak::new`).
1842 fn inner(&self) -> Option<&RcBox<T>> {
1843 if is_dangling(self.ptr) { None } else { Some(unsafe { self.ptr.as_ref() }) }
1846 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1847 /// [`ptr::eq`]), or if both don't point to any allocation
1848 /// (because they were created with `Weak::new()`).
1852 /// Since this compares pointers it means that `Weak::new()` will equal each
1853 /// other, even though they don't point to any allocation.
1858 /// use std::rc::Rc;
1860 /// let first_rc = Rc::new(5);
1861 /// let first = Rc::downgrade(&first_rc);
1862 /// let second = Rc::downgrade(&first_rc);
1864 /// assert!(first.ptr_eq(&second));
1866 /// let third_rc = Rc::new(5);
1867 /// let third = Rc::downgrade(&third_rc);
1869 /// assert!(!first.ptr_eq(&third));
1872 /// Comparing `Weak::new`.
1875 /// use std::rc::{Rc, Weak};
1877 /// let first = Weak::new();
1878 /// let second = Weak::new();
1879 /// assert!(first.ptr_eq(&second));
1881 /// let third_rc = Rc::new(());
1882 /// let third = Rc::downgrade(&third_rc);
1883 /// assert!(!first.ptr_eq(&third));
1886 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1888 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1889 pub fn ptr_eq(&self, other: &Self) -> bool {
1890 self.ptr.as_ptr() == other.ptr.as_ptr()
1894 #[stable(feature = "rc_weak", since = "1.4.0")]
1895 impl<T: ?Sized> Drop for Weak<T> {
1896 /// Drops the `Weak` pointer.
1901 /// use std::rc::{Rc, Weak};
1905 /// impl Drop for Foo {
1906 /// fn drop(&mut self) {
1907 /// println!("dropped!");
1911 /// let foo = Rc::new(Foo);
1912 /// let weak_foo = Rc::downgrade(&foo);
1913 /// let other_weak_foo = Weak::clone(&weak_foo);
1915 /// drop(weak_foo); // Doesn't print anything
1916 /// drop(foo); // Prints "dropped!"
1918 /// assert!(other_weak_foo.upgrade().is_none());
1920 fn drop(&mut self) {
1921 if let Some(inner) = self.inner() {
1923 // the weak count starts at 1, and will only go to zero if all
1924 // the strong pointers have disappeared.
1925 if inner.weak() == 0 {
1927 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1934 #[stable(feature = "rc_weak", since = "1.4.0")]
1935 impl<T: ?Sized> Clone for Weak<T> {
1936 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1941 /// use std::rc::{Rc, Weak};
1943 /// let weak_five = Rc::downgrade(&Rc::new(5));
1945 /// let _ = Weak::clone(&weak_five);
1948 fn clone(&self) -> Weak<T> {
1949 if let Some(inner) = self.inner() {
1952 Weak { ptr: self.ptr }
1956 #[stable(feature = "rc_weak", since = "1.4.0")]
1957 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1958 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1963 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1964 impl<T> Default for Weak<T> {
1965 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1966 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1968 /// [`None`]: ../../std/option/enum.Option.html
1969 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
1974 /// use std::rc::Weak;
1976 /// let empty: Weak<i64> = Default::default();
1977 /// assert!(empty.upgrade().is_none());
1979 fn default() -> Weak<T> {
1984 // NOTE: We checked_add here to deal with mem::forget safely. In particular
1985 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1986 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1987 // We abort because this is such a degenerate scenario that we don't care about
1988 // what happens -- no real program should ever experience this.
1990 // This should have negligible overhead since you don't actually need to
1991 // clone these much in Rust thanks to ownership and move-semantics.
1994 trait RcBoxPtr<T: ?Sized> {
1995 fn inner(&self) -> &RcBox<T>;
1998 fn strong(&self) -> usize {
1999 self.inner().strong.get()
2003 fn inc_strong(&self) {
2004 let strong = self.strong();
2006 // We want to abort on overflow instead of dropping the value.
2007 // The reference count will never be zero when this is called;
2008 // nevertheless, we insert an abort here to hint LLVM at
2009 // an otherwise missed optimization.
2010 if strong == 0 || strong == usize::max_value() {
2015 self.inner().strong.set(strong + 1);
2019 fn dec_strong(&self) {
2020 self.inner().strong.set(self.strong() - 1);
2024 fn weak(&self) -> usize {
2025 self.inner().weak.get()
2029 fn inc_weak(&self) {
2030 let weak = self.weak();
2032 // We want to abort on overflow instead of dropping the value.
2033 // The reference count will never be zero when this is called;
2034 // nevertheless, we insert an abort here to hint LLVM at
2035 // an otherwise missed optimization.
2036 if weak == 0 || weak == usize::max_value() {
2041 self.inner().weak.set(weak + 1);
2045 fn dec_weak(&self) {
2046 self.inner().weak.set(self.weak() - 1);
2050 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
2052 fn inner(&self) -> &RcBox<T> {
2053 unsafe { self.ptr.as_ref() }
2057 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
2059 fn inner(&self) -> &RcBox<T> {
2064 #[stable(feature = "rust1", since = "1.0.0")]
2065 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2066 fn borrow(&self) -> &T {
2071 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2072 impl<T: ?Sized> AsRef<T> for Rc<T> {
2073 fn as_ref(&self) -> &T {
2078 #[stable(feature = "pin", since = "1.33.0")]
2079 impl<T: ?Sized> Unpin for Rc<T> {}
2081 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2082 // Align the unsized value to the end of the `RcBox`.
2083 // Because it is ?Sized, it will always be the last field in memory.
2084 // Note: This is a detail of the current implementation of the compiler,
2085 // and is not a guaranteed language detail. Do not rely on it outside of std.
2086 data_offset_align(align_of_val(&*ptr))
2089 /// Computes the offset of the data field within `RcBox`.
2091 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2092 fn data_offset_sized<T>() -> isize {
2093 data_offset_align(align_of::<T>())
2097 fn data_offset_align(align: usize) -> isize {
2098 let layout = Layout::new::<RcBox<()>>();
2099 (layout.size() + layout.padding_needed_for(align)) as isize