1 //! Single-threaded reference-counting pointers. 'Rc' stands for 'Reference
4 //! The type [`Rc<T>`][`Rc`] provides shared ownership of a value of type `T`,
5 //! allocated in the heap. Invoking [`clone`][clone] on [`Rc`] produces a new
6 //! pointer to the same allocation in the heap. When the last [`Rc`] pointer to a
7 //! given allocation is destroyed, the value stored in that allocation (often
8 //! referred to as "inner value") is also dropped.
10 //! Shared references in Rust disallow mutation by default, and [`Rc`]
11 //! is no exception: you cannot generally obtain a mutable reference to
12 //! something inside an [`Rc`]. If you need mutability, put a [`Cell`]
13 //! or [`RefCell`] inside the [`Rc`]; see [an example of mutability
14 //! inside an Rc][mutability].
16 //! [`Rc`] uses non-atomic reference counting. This means that overhead is very
17 //! low, but an [`Rc`] cannot be sent between threads, and consequently [`Rc`]
18 //! does not implement [`Send`][send]. As a result, the Rust compiler
19 //! will check *at compile time* that you are not sending [`Rc`]s between
20 //! threads. If you need multi-threaded, atomic reference counting, use
21 //! [`sync::Arc`][arc].
23 //! The [`downgrade`][downgrade] method can be used to create a non-owning
24 //! [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
25 //! to an [`Rc`], but this will return [`None`] if the value stored in the allocation has
26 //! already been dropped. In other words, `Weak` pointers do not keep the value
27 //! inside the allocation alive; however, they *do* keep the allocation
28 //! (the backing store for the inner value) alive.
30 //! A cycle between [`Rc`] pointers will never be deallocated. For this reason,
31 //! [`Weak`] is used to break cycles. For example, a tree could have strong
32 //! [`Rc`] pointers from parent nodes to children, and [`Weak`] pointers from
33 //! children back to their parents.
35 //! `Rc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
36 //! so you can call `T`'s methods on a value of type [`Rc<T>`][`Rc`]. To avoid name
37 //! clashes with `T`'s methods, the methods of [`Rc<T>`][`Rc`] itself are associated
38 //! functions, called using function-like syntax:
42 //! let my_rc = Rc::new(());
44 //! Rc::downgrade(&my_rc);
47 //! [`Weak<T>`][`Weak`] does not auto-dereference to `T`, because the inner value may have
48 //! already been dropped.
50 //! # Cloning references
52 //! Creating a new reference to the same allocation as an existing reference counted pointer
53 //! is done using the `Clone` trait implemented for [`Rc<T>`][`Rc`] and [`Weak<T>`][`Weak`].
57 //! let foo = Rc::new(vec![1.0, 2.0, 3.0]);
58 //! // The two syntaxes below are equivalent.
59 //! let a = foo.clone();
60 //! let b = Rc::clone(&foo);
61 //! // a and b both point to the same memory location as foo.
64 //! The `Rc::clone(&from)` syntax is the most idiomatic because it conveys more explicitly
65 //! the meaning of the code. In the example above, this syntax makes it easier to see that
66 //! this code is creating a new reference rather than copying the whole content of foo.
70 //! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`.
71 //! We want to have our `Gadget`s point to their `Owner`. We can't do this with
72 //! unique ownership, because more than one gadget may belong to the same
73 //! `Owner`. [`Rc`] allows us to share an `Owner` between multiple `Gadget`s,
74 //! and have the `Owner` remain allocated as long as any `Gadget` points at it.
81 //! // ...other fields
87 //! // ...other fields
91 //! // Create a reference-counted `Owner`.
92 //! let gadget_owner: Rc<Owner> = Rc::new(
94 //! name: "Gadget Man".to_string(),
98 //! // Create `Gadget`s belonging to `gadget_owner`. Cloning the `Rc<Owner>`
99 //! // gives us a new pointer to the same `Owner` allocation, incrementing
100 //! // the reference count in the process.
101 //! let gadget1 = Gadget {
103 //! owner: Rc::clone(&gadget_owner),
105 //! let gadget2 = Gadget {
107 //! owner: Rc::clone(&gadget_owner),
110 //! // Dispose of our local variable `gadget_owner`.
111 //! drop(gadget_owner);
113 //! // Despite dropping `gadget_owner`, we're still able to print out the name
114 //! // of the `Owner` of the `Gadget`s. This is because we've only dropped a
115 //! // single `Rc<Owner>`, not the `Owner` it points to. As long as there are
116 //! // other `Rc<Owner>` pointing at the same `Owner` allocation, it will remain
117 //! // live. The field projection `gadget1.owner.name` works because
118 //! // `Rc<Owner>` automatically dereferences to `Owner`.
119 //! println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name);
120 //! println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name);
122 //! // At the end of the function, `gadget1` and `gadget2` are destroyed, and
123 //! // with them the last counted references to our `Owner`. Gadget Man now
124 //! // gets destroyed as well.
128 //! If our requirements change, and we also need to be able to traverse from
129 //! `Owner` to `Gadget`, we will run into problems. An [`Rc`] pointer from `Owner`
130 //! to `Gadget` introduces a cycle. This means that their
131 //! reference counts can never reach 0, and the allocation will never be destroyed:
132 //! a memory leak. In order to get around this, we can use [`Weak`]
135 //! Rust actually makes it somewhat difficult to produce this loop in the first
136 //! place. In order to end up with two values that point at each other, one of
137 //! them needs to be mutable. This is difficult because [`Rc`] enforces
138 //! memory safety by only giving out shared references to the value it wraps,
139 //! and these don't allow direct mutation. We need to wrap the part of the
140 //! value we wish to mutate in a [`RefCell`], which provides *interior
141 //! mutability*: a method to achieve mutability through a shared reference.
142 //! [`RefCell`] enforces Rust's borrowing rules at runtime.
146 //! use std::rc::Weak;
147 //! use std::cell::RefCell;
151 //! gadgets: RefCell<Vec<Weak<Gadget>>>,
152 //! // ...other fields
157 //! owner: Rc<Owner>,
158 //! // ...other fields
162 //! // Create a reference-counted `Owner`. Note that we've put the `Owner`'s
163 //! // vector of `Gadget`s inside a `RefCell` so that we can mutate it through
164 //! // a shared reference.
165 //! let gadget_owner: Rc<Owner> = Rc::new(
167 //! name: "Gadget Man".to_string(),
168 //! gadgets: RefCell::new(vec![]),
172 //! // Create `Gadget`s belonging to `gadget_owner`, as before.
173 //! let gadget1 = Rc::new(
176 //! owner: Rc::clone(&gadget_owner),
179 //! let gadget2 = Rc::new(
182 //! owner: Rc::clone(&gadget_owner),
186 //! // Add the `Gadget`s to their `Owner`.
188 //! let mut gadgets = gadget_owner.gadgets.borrow_mut();
189 //! gadgets.push(Rc::downgrade(&gadget1));
190 //! gadgets.push(Rc::downgrade(&gadget2));
192 //! // `RefCell` dynamic borrow ends here.
195 //! // Iterate over our `Gadget`s, printing their details out.
196 //! for gadget_weak in gadget_owner.gadgets.borrow().iter() {
198 //! // `gadget_weak` is a `Weak<Gadget>`. Since `Weak` pointers can't
199 //! // guarantee the allocation still exists, we need to call
200 //! // `upgrade`, which returns an `Option<Rc<Gadget>>`.
202 //! // In this case we know the allocation still exists, so we simply
203 //! // `unwrap` the `Option`. In a more complicated program, you might
204 //! // need graceful error handling for a `None` result.
206 //! let gadget = gadget_weak.upgrade().unwrap();
207 //! println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
210 //! // At the end of the function, `gadget_owner`, `gadget1`, and `gadget2`
211 //! // are destroyed. There are now no strong (`Rc`) pointers to the
212 //! // gadgets, so they are destroyed. This zeroes the reference count on
213 //! // Gadget Man, so he gets destroyed as well.
217 //! [`Rc`]: struct.Rc.html
218 //! [`Weak`]: struct.Weak.html
219 //! [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
220 //! [`Cell`]: ../../std/cell/struct.Cell.html
221 //! [`RefCell`]: ../../std/cell/struct.RefCell.html
222 //! [send]: ../../std/marker/trait.Send.html
223 //! [arc]: ../../std/sync/struct.Arc.html
224 //! [`Deref`]: ../../std/ops/trait.Deref.html
225 //! [downgrade]: struct.Rc.html#method.downgrade
226 //! [upgrade]: struct.Weak.html#method.upgrade
227 //! [`None`]: ../../std/option/enum.Option.html#variant.None
228 //! [mutability]: ../../std/cell/index.html#introducing-mutability-inside-of-something-immutable
230 #![stable(feature = "rust1", since = "1.0.0")]
233 use crate::boxed::Box;
238 use core::array::LengthAtMost32;
240 use core::cell::Cell;
241 use core::cmp::Ordering;
243 use core::hash::{Hash, Hasher};
244 use core::intrinsics::abort;
246 use core::marker::{self, Unpin, Unsize, PhantomData};
247 use core::mem::{self, align_of, align_of_val, forget, size_of_val};
248 use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn};
250 use core::ptr::{self, NonNull};
251 use core::slice::{self, from_raw_parts_mut};
252 use core::convert::{From, TryFrom};
255 use crate::alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
256 use crate::string::String;
262 struct RcBox<T: ?Sized> {
268 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
271 /// See the [module-level documentation](./index.html) for more details.
273 /// The inherent methods of `Rc` are all associated functions, which means
274 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
275 /// `value.get_mut()`. This avoids conflicts with methods of the inner
278 /// [get_mut]: #method.get_mut
279 #[cfg_attr(not(test), lang = "rc")]
280 #[stable(feature = "rust1", since = "1.0.0")]
281 pub struct Rc<T: ?Sized> {
282 ptr: NonNull<RcBox<T>>,
283 phantom: PhantomData<RcBox<T>>,
286 #[stable(feature = "rust1", since = "1.0.0")]
287 impl<T: ?Sized> !marker::Send for Rc<T> {}
288 #[stable(feature = "rust1", since = "1.0.0")]
289 impl<T: ?Sized> !marker::Sync for Rc<T> {}
291 #[unstable(feature = "coerce_unsized", issue = "27732")]
292 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
294 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
295 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
297 impl<T: ?Sized> Rc<T> {
298 fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
301 phantom: PhantomData,
305 unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
306 Self::from_inner(NonNull::new_unchecked(ptr))
311 /// Constructs a new `Rc<T>`.
318 /// let five = Rc::new(5);
320 #[stable(feature = "rust1", since = "1.0.0")]
321 pub fn new(value: T) -> Rc<T> {
322 // There is an implicit weak pointer owned by all the strong
323 // pointers, which ensures that the weak destructor never frees
324 // the allocation while the strong destructor is running, even
325 // if the weak pointer is stored inside the strong one.
326 Self::from_inner(Box::into_raw_non_null(box RcBox {
327 strong: Cell::new(1),
333 /// Constructs a new `Rc` with uninitialized contents.
338 /// #![feature(new_uninit)]
339 /// #![feature(get_mut_unchecked)]
343 /// let mut five = Rc::<u32>::new_uninit();
345 /// let five = unsafe {
346 /// // Deferred initialization:
347 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
349 /// five.assume_init()
352 /// assert_eq!(*five, 5)
354 #[unstable(feature = "new_uninit", issue = "63291")]
355 pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
357 Rc::from_ptr(Rc::allocate_for_layout(
359 |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
364 /// Constructs a new `Rc` with uninitialized contents, with the memory
365 /// being filled with `0` bytes.
367 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
368 /// incorrect usage of this method.
373 /// #![feature(new_uninit)]
377 /// let zero = Rc::<u32>::new_zeroed();
378 /// let zero = unsafe { zero.assume_init() };
380 /// assert_eq!(*zero, 0)
383 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
384 #[unstable(feature = "new_uninit", issue = "63291")]
385 pub fn new_zeroed() -> Rc<mem::MaybeUninit<T>> {
387 let mut uninit = Self::new_uninit();
388 ptr::write_bytes::<T>(Rc::get_mut_unchecked(&mut uninit).as_mut_ptr(), 0, 1);
393 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
394 /// `value` will be pinned in memory and unable to be moved.
395 #[stable(feature = "pin", since = "1.33.0")]
396 pub fn pin(value: T) -> Pin<Rc<T>> {
397 unsafe { Pin::new_unchecked(Rc::new(value)) }
400 /// Returns the inner value, if the `Rc` has exactly one strong reference.
402 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
405 /// This will succeed even if there are outstanding weak references.
407 /// [result]: ../../std/result/enum.Result.html
414 /// let x = Rc::new(3);
415 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
417 /// let x = Rc::new(4);
418 /// let _y = Rc::clone(&x);
419 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
422 #[stable(feature = "rc_unique", since = "1.4.0")]
423 pub fn try_unwrap(this: Self) -> Result<T, Self> {
424 if Rc::strong_count(&this) == 1 {
426 let val = ptr::read(&*this); // copy the contained object
428 // Indicate to Weaks that they can't be promoted by decrementing
429 // the strong count, and then remove the implicit "strong weak"
430 // pointer while also handling drop logic by just crafting a
433 let _weak = Weak { ptr: this.ptr };
444 /// Constructs a new reference-counted slice with uninitialized contents.
449 /// #![feature(new_uninit)]
450 /// #![feature(get_mut_unchecked)]
454 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
456 /// let values = unsafe {
457 /// // Deferred initialization:
458 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
459 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
460 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
462 /// values.assume_init()
465 /// assert_eq!(*values, [1, 2, 3])
467 #[unstable(feature = "new_uninit", issue = "63291")]
468 pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
470 Rc::from_ptr(Rc::allocate_for_slice(len))
475 impl<T> Rc<mem::MaybeUninit<T>> {
476 /// Converts to `Rc<T>`.
480 /// As with [`MaybeUninit::assume_init`],
481 /// it is up to the caller to guarantee that the inner value
482 /// really is in an initialized state.
483 /// Calling this when the content is not yet fully initialized
484 /// causes immediate undefined behavior.
486 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
491 /// #![feature(new_uninit)]
492 /// #![feature(get_mut_unchecked)]
496 /// let mut five = Rc::<u32>::new_uninit();
498 /// let five = unsafe {
499 /// // Deferred initialization:
500 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
502 /// five.assume_init()
505 /// assert_eq!(*five, 5)
507 #[unstable(feature = "new_uninit", issue = "63291")]
509 pub unsafe fn assume_init(self) -> Rc<T> {
510 Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
514 impl<T> Rc<[mem::MaybeUninit<T>]> {
515 /// Converts to `Rc<[T]>`.
519 /// As with [`MaybeUninit::assume_init`],
520 /// it is up to the caller to guarantee that the inner value
521 /// really is in an initialized state.
522 /// Calling this when the content is not yet fully initialized
523 /// causes immediate undefined behavior.
525 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
530 /// #![feature(new_uninit)]
531 /// #![feature(get_mut_unchecked)]
535 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
537 /// let values = unsafe {
538 /// // Deferred initialization:
539 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
540 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
541 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
543 /// values.assume_init()
546 /// assert_eq!(*values, [1, 2, 3])
548 #[unstable(feature = "new_uninit", issue = "63291")]
550 pub unsafe fn assume_init(self) -> Rc<[T]> {
551 Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _)
555 impl<T: ?Sized> Rc<T> {
556 /// Consumes the `Rc`, returning the wrapped pointer.
558 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
559 /// [`Rc::from_raw`][from_raw].
561 /// [from_raw]: struct.Rc.html#method.from_raw
568 /// let x = Rc::new("hello".to_owned());
569 /// let x_ptr = Rc::into_raw(x);
570 /// assert_eq!(unsafe { &*x_ptr }, "hello");
572 #[stable(feature = "rc_raw", since = "1.17.0")]
573 pub fn into_raw(this: Self) -> *const T {
574 let ptr: *mut RcBox<T> = NonNull::as_ptr(this.ptr);
575 let fake_ptr = ptr as *mut T;
579 let offset = data_offset(&(*ptr).value);
580 set_data_ptr(fake_ptr, (ptr as *mut u8).offset(offset))
584 /// Constructs an `Rc` from a raw pointer.
586 /// The raw pointer must have been previously returned by a call to a
587 /// [`Rc::into_raw`][into_raw].
589 /// This function is unsafe because improper use may lead to memory problems. For example, a
590 /// double-free may occur if the function is called twice on the same raw pointer.
592 /// [into_raw]: struct.Rc.html#method.into_raw
599 /// let x = Rc::new("hello".to_owned());
600 /// let x_ptr = Rc::into_raw(x);
603 /// // Convert back to an `Rc` to prevent leak.
604 /// let x = Rc::from_raw(x_ptr);
605 /// assert_eq!(&*x, "hello");
607 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
610 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
612 #[stable(feature = "rc_raw", since = "1.17.0")]
613 pub unsafe fn from_raw(ptr: *const T) -> Self {
614 let offset = data_offset(ptr);
616 // Reverse the offset to find the original RcBox.
617 let fake_ptr = ptr as *mut RcBox<T>;
618 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
620 Self::from_ptr(rc_ptr)
623 /// Consumes the `Rc`, returning the wrapped pointer as `NonNull<T>`.
628 /// #![feature(rc_into_raw_non_null)]
632 /// let x = Rc::new("hello".to_owned());
633 /// let ptr = Rc::into_raw_non_null(x);
634 /// let deref = unsafe { ptr.as_ref() };
635 /// assert_eq!(deref, "hello");
637 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
639 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
640 // safe because Rc guarantees its pointer is non-null
641 unsafe { NonNull::new_unchecked(Rc::into_raw(this) as *mut _) }
644 /// Creates a new [`Weak`][weak] pointer to this allocation.
646 /// [weak]: struct.Weak.html
653 /// let five = Rc::new(5);
655 /// let weak_five = Rc::downgrade(&five);
657 #[stable(feature = "rc_weak", since = "1.4.0")]
658 pub fn downgrade(this: &Self) -> Weak<T> {
660 // Make sure we do not create a dangling Weak
661 debug_assert!(!is_dangling(this.ptr));
662 Weak { ptr: this.ptr }
665 /// Gets the number of [`Weak`][weak] pointers to this allocation.
667 /// [weak]: struct.Weak.html
674 /// let five = Rc::new(5);
675 /// let _weak_five = Rc::downgrade(&five);
677 /// assert_eq!(1, Rc::weak_count(&five));
680 #[stable(feature = "rc_counts", since = "1.15.0")]
681 pub fn weak_count(this: &Self) -> usize {
685 /// Gets the number of strong (`Rc`) pointers to this allocation.
692 /// let five = Rc::new(5);
693 /// let _also_five = Rc::clone(&five);
695 /// assert_eq!(2, Rc::strong_count(&five));
698 #[stable(feature = "rc_counts", since = "1.15.0")]
699 pub fn strong_count(this: &Self) -> usize {
703 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
706 /// [weak]: struct.Weak.html
708 fn is_unique(this: &Self) -> bool {
709 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
712 /// Returns a mutable reference into the given `Rc`, if there are
713 /// no other `Rc` or [`Weak`][weak] pointers to the same allocation.
715 /// Returns [`None`] otherwise, because it is not safe to
716 /// mutate a shared value.
718 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
719 /// the inner value when there are other pointers.
721 /// [weak]: struct.Weak.html
722 /// [`None`]: ../../std/option/enum.Option.html#variant.None
723 /// [make_mut]: struct.Rc.html#method.make_mut
724 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
731 /// let mut x = Rc::new(3);
732 /// *Rc::get_mut(&mut x).unwrap() = 4;
733 /// assert_eq!(*x, 4);
735 /// let _y = Rc::clone(&x);
736 /// assert!(Rc::get_mut(&mut x).is_none());
739 #[stable(feature = "rc_unique", since = "1.4.0")]
740 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
741 if Rc::is_unique(this) {
743 Some(Rc::get_mut_unchecked(this))
750 /// Returns a mutable reference into the given `Rc`,
751 /// without any check.
753 /// See also [`get_mut`], which is safe and does appropriate checks.
755 /// [`get_mut`]: struct.Rc.html#method.get_mut
759 /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
760 /// for the duration of the returned borrow.
761 /// This is trivially the case if no such pointers exist,
762 /// for example immediately after `Rc::new`.
767 /// #![feature(get_mut_unchecked)]
771 /// let mut x = Rc::new(String::new());
773 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
775 /// assert_eq!(*x, "foo");
778 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
779 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
780 &mut this.ptr.as_mut().value
784 #[stable(feature = "ptr_eq", since = "1.17.0")]
785 /// Returns `true` if the two `Rc`s point to the same allocation
786 /// (in a vein similar to [`ptr::eq`]).
793 /// let five = Rc::new(5);
794 /// let same_five = Rc::clone(&five);
795 /// let other_five = Rc::new(5);
797 /// assert!(Rc::ptr_eq(&five, &same_five));
798 /// assert!(!Rc::ptr_eq(&five, &other_five));
801 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
802 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
803 this.ptr.as_ptr() == other.ptr.as_ptr()
807 impl<T: Clone> Rc<T> {
808 /// Makes a mutable reference into the given `Rc`.
810 /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
811 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
812 /// referred to as clone-on-write.
814 /// If there are no other `Rc` pointers to this allocation, then [`Weak`]
815 /// pointers to this allocation will be disassociated.
817 /// See also [`get_mut`], which will fail rather than cloning.
819 /// [`Weak`]: struct.Weak.html
820 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
821 /// [`get_mut`]: struct.Rc.html#method.get_mut
828 /// let mut data = Rc::new(5);
830 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
831 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
832 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
833 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
834 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
836 /// // Now `data` and `other_data` point to different allocations.
837 /// assert_eq!(*data, 8);
838 /// assert_eq!(*other_data, 12);
841 /// [`Weak`] pointers will be disassociated:
846 /// let mut data = Rc::new(75);
847 /// let weak = Rc::downgrade(&data);
849 /// assert!(75 == *data);
850 /// assert!(75 == *weak.upgrade().unwrap());
852 /// *Rc::make_mut(&mut data) += 1;
854 /// assert!(76 == *data);
855 /// assert!(weak.upgrade().is_none());
858 #[stable(feature = "rc_unique", since = "1.4.0")]
859 pub fn make_mut(this: &mut Self) -> &mut T {
860 if Rc::strong_count(this) != 1 {
861 // Gotta clone the data, there are other Rcs
862 *this = Rc::new((**this).clone())
863 } else if Rc::weak_count(this) != 0 {
864 // Can just steal the data, all that's left is Weaks
866 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
867 mem::swap(this, &mut swap);
869 // Remove implicit strong-weak ref (no need to craft a fake
870 // Weak here -- we know other Weaks can clean up for us)
875 // This unsafety is ok because we're guaranteed that the pointer
876 // returned is the *only* pointer that will ever be returned to T. Our
877 // reference count is guaranteed to be 1 at this point, and we required
878 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
879 // reference to the allocation.
881 &mut this.ptr.as_mut().value
888 #[stable(feature = "rc_downcast", since = "1.29.0")]
889 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
894 /// use std::any::Any;
897 /// fn print_if_string(value: Rc<dyn Any>) {
898 /// if let Ok(string) = value.downcast::<String>() {
899 /// println!("String ({}): {}", string.len(), string);
903 /// let my_string = "Hello World".to_string();
904 /// print_if_string(Rc::new(my_string));
905 /// print_if_string(Rc::new(0i8));
907 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
908 if (*self).is::<T>() {
909 let ptr = self.ptr.cast::<RcBox<T>>();
911 Ok(Rc::from_inner(ptr))
918 impl<T: ?Sized> Rc<T> {
919 /// Allocates an `RcBox<T>` with sufficient space for
920 /// a possibly-unsized inner value where the value has the layout provided.
922 /// The function `mem_to_rcbox` is called with the data pointer
923 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
924 unsafe fn allocate_for_layout(
925 value_layout: Layout,
926 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>
928 // Calculate layout using the given value layout.
929 // Previously, layout was calculated on the expression
930 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
931 // reference (see #54908).
932 let layout = Layout::new::<RcBox<()>>()
933 .extend(value_layout).unwrap().0
936 // Allocate for the layout.
937 let mem = Global.alloc(layout)
938 .unwrap_or_else(|_| handle_alloc_error(layout));
940 // Initialize the RcBox
941 let inner = mem_to_rcbox(mem.as_ptr());
942 debug_assert_eq!(Layout::for_value(&*inner), layout);
944 ptr::write(&mut (*inner).strong, Cell::new(1));
945 ptr::write(&mut (*inner).weak, Cell::new(1));
950 /// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
951 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
952 // Allocate for the `RcBox<T>` using the given value.
953 Self::allocate_for_layout(
954 Layout::for_value(&*ptr),
955 |mem| set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>,
959 fn from_box(v: Box<T>) -> Rc<T> {
961 let box_unique = Box::into_unique(v);
962 let bptr = box_unique.as_ptr();
964 let value_size = size_of_val(&*bptr);
965 let ptr = Self::allocate_for_ptr(bptr);
967 // Copy value as bytes
968 ptr::copy_nonoverlapping(
969 bptr as *const T as *const u8,
970 &mut (*ptr).value as *mut _ as *mut u8,
973 // Free the allocation without dropping its contents
974 box_free(box_unique);
982 /// Allocates an `RcBox<[T]>` with the given length.
983 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
984 Self::allocate_for_layout(
985 Layout::array::<T>(len).unwrap(),
986 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>,
991 /// Sets the data pointer of a `?Sized` raw pointer.
993 /// For a slice/trait object, this sets the `data` field and leaves the rest
994 /// unchanged. For a sized raw pointer, this simply sets the pointer.
995 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
996 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
1001 /// Copy elements from slice into newly allocated Rc<[T]>
1003 /// Unsafe because the caller must either take ownership or bind `T: Copy`
1004 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
1005 let ptr = Self::allocate_for_slice(v.len());
1007 ptr::copy_nonoverlapping(
1009 &mut (*ptr).value as *mut [T] as *mut T,
1015 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
1017 /// Behavior is undefined should the size be wrong.
1018 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
1019 // Panic guard while cloning T elements.
1020 // In the event of a panic, elements that have been written
1021 // into the new RcBox will be dropped, then the memory freed.
1029 impl<T> Drop for Guard<T> {
1030 fn drop(&mut self) {
1032 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1033 ptr::drop_in_place(slice);
1035 Global.dealloc(self.mem, self.layout);
1040 let ptr = Self::allocate_for_slice(len);
1042 let mem = ptr as *mut _ as *mut u8;
1043 let layout = Layout::for_value(&*ptr);
1045 // Pointer to first element
1046 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1048 let mut guard = Guard {
1049 mem: NonNull::new_unchecked(mem),
1055 for (i, item) in iter.enumerate() {
1056 ptr::write(elems.add(i), item);
1060 // All clear. Forget the guard so it doesn't free the new RcBox.
1067 /// Specialization trait used for `From<&[T]>`.
1068 trait RcFromSlice<T> {
1069 fn from_slice(slice: &[T]) -> Self;
1072 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
1074 default fn from_slice(v: &[T]) -> Self {
1076 Self::from_iter_exact(v.iter().cloned(), v.len())
1081 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
1083 fn from_slice(v: &[T]) -> Self {
1084 unsafe { Rc::copy_from_slice(v) }
1088 #[stable(feature = "rust1", since = "1.0.0")]
1089 impl<T: ?Sized> Deref for Rc<T> {
1093 fn deref(&self) -> &T {
1098 #[unstable(feature = "receiver_trait", issue = "0")]
1099 impl<T: ?Sized> Receiver for Rc<T> {}
1101 #[stable(feature = "rust1", since = "1.0.0")]
1102 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1105 /// This will decrement the strong reference count. If the strong reference
1106 /// count reaches zero then the only other references (if any) are
1107 /// [`Weak`], so we `drop` the inner value.
1112 /// use std::rc::Rc;
1116 /// impl Drop for Foo {
1117 /// fn drop(&mut self) {
1118 /// println!("dropped!");
1122 /// let foo = Rc::new(Foo);
1123 /// let foo2 = Rc::clone(&foo);
1125 /// drop(foo); // Doesn't print anything
1126 /// drop(foo2); // Prints "dropped!"
1129 /// [`Weak`]: ../../std/rc/struct.Weak.html
1130 fn drop(&mut self) {
1133 if self.strong() == 0 {
1134 // destroy the contained object
1135 ptr::drop_in_place(self.ptr.as_mut());
1137 // remove the implicit "strong weak" pointer now that we've
1138 // destroyed the contents.
1141 if self.weak() == 0 {
1142 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1149 #[stable(feature = "rust1", since = "1.0.0")]
1150 impl<T: ?Sized> Clone for Rc<T> {
1151 /// Makes a clone of the `Rc` pointer.
1153 /// This creates another pointer to the same allocation, increasing the
1154 /// strong reference count.
1159 /// use std::rc::Rc;
1161 /// let five = Rc::new(5);
1163 /// let _ = Rc::clone(&five);
1166 fn clone(&self) -> Rc<T> {
1168 Self::from_inner(self.ptr)
1172 #[stable(feature = "rust1", since = "1.0.0")]
1173 impl<T: Default> Default for Rc<T> {
1174 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1179 /// use std::rc::Rc;
1181 /// let x: Rc<i32> = Default::default();
1182 /// assert_eq!(*x, 0);
1185 fn default() -> Rc<T> {
1186 Rc::new(Default::default())
1190 #[stable(feature = "rust1", since = "1.0.0")]
1191 trait RcEqIdent<T: ?Sized + PartialEq> {
1192 fn eq(&self, other: &Rc<T>) -> bool;
1193 fn ne(&self, other: &Rc<T>) -> bool;
1196 #[stable(feature = "rust1", since = "1.0.0")]
1197 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
1199 default fn eq(&self, other: &Rc<T>) -> bool {
1204 default fn ne(&self, other: &Rc<T>) -> bool {
1209 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1210 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1211 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1212 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1213 /// the same value, than two `&T`s.
1215 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1216 #[stable(feature = "rust1", since = "1.0.0")]
1217 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
1219 fn eq(&self, other: &Rc<T>) -> bool {
1220 Rc::ptr_eq(self, other) || **self == **other
1224 fn ne(&self, other: &Rc<T>) -> bool {
1225 !Rc::ptr_eq(self, other) && **self != **other
1229 #[stable(feature = "rust1", since = "1.0.0")]
1230 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1231 /// Equality for two `Rc`s.
1233 /// Two `Rc`s are equal if their inner values are equal, even if they are
1234 /// stored in different allocation.
1236 /// If `T` also implements `Eq` (implying reflexivity of equality),
1237 /// two `Rc`s that point to the same allocation are
1243 /// use std::rc::Rc;
1245 /// let five = Rc::new(5);
1247 /// assert!(five == Rc::new(5));
1250 fn eq(&self, other: &Rc<T>) -> bool {
1251 RcEqIdent::eq(self, other)
1254 /// Inequality for two `Rc`s.
1256 /// Two `Rc`s are unequal if their inner values are unequal.
1258 /// If `T` also implements `Eq` (implying reflexivity of equality),
1259 /// two `Rc`s that point to the same allocation are
1265 /// use std::rc::Rc;
1267 /// let five = Rc::new(5);
1269 /// assert!(five != Rc::new(6));
1272 fn ne(&self, other: &Rc<T>) -> bool {
1273 RcEqIdent::ne(self, other)
1277 #[stable(feature = "rust1", since = "1.0.0")]
1278 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1280 #[stable(feature = "rust1", since = "1.0.0")]
1281 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1282 /// Partial comparison for two `Rc`s.
1284 /// The two are compared by calling `partial_cmp()` on their inner values.
1289 /// use std::rc::Rc;
1290 /// use std::cmp::Ordering;
1292 /// let five = Rc::new(5);
1294 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1297 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1298 (**self).partial_cmp(&**other)
1301 /// Less-than comparison for two `Rc`s.
1303 /// The two are compared by calling `<` on their inner values.
1308 /// use std::rc::Rc;
1310 /// let five = Rc::new(5);
1312 /// assert!(five < Rc::new(6));
1315 fn lt(&self, other: &Rc<T>) -> bool {
1319 /// 'Less than or equal to' comparison for two `Rc`s.
1321 /// The two are compared by calling `<=` on their inner values.
1326 /// use std::rc::Rc;
1328 /// let five = Rc::new(5);
1330 /// assert!(five <= Rc::new(5));
1333 fn le(&self, other: &Rc<T>) -> bool {
1337 /// Greater-than comparison for two `Rc`s.
1339 /// The two are compared by calling `>` on their inner values.
1344 /// use std::rc::Rc;
1346 /// let five = Rc::new(5);
1348 /// assert!(five > Rc::new(4));
1351 fn gt(&self, other: &Rc<T>) -> bool {
1355 /// 'Greater than or equal to' comparison for two `Rc`s.
1357 /// The two are compared by calling `>=` on their inner values.
1362 /// use std::rc::Rc;
1364 /// let five = Rc::new(5);
1366 /// assert!(five >= Rc::new(5));
1369 fn ge(&self, other: &Rc<T>) -> bool {
1374 #[stable(feature = "rust1", since = "1.0.0")]
1375 impl<T: ?Sized + Ord> Ord for Rc<T> {
1376 /// Comparison for two `Rc`s.
1378 /// The two are compared by calling `cmp()` on their inner values.
1383 /// use std::rc::Rc;
1384 /// use std::cmp::Ordering;
1386 /// let five = Rc::new(5);
1388 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1391 fn cmp(&self, other: &Rc<T>) -> Ordering {
1392 (**self).cmp(&**other)
1396 #[stable(feature = "rust1", since = "1.0.0")]
1397 impl<T: ?Sized + Hash> Hash for Rc<T> {
1398 fn hash<H: Hasher>(&self, state: &mut H) {
1399 (**self).hash(state);
1403 #[stable(feature = "rust1", since = "1.0.0")]
1404 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1405 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1406 fmt::Display::fmt(&**self, f)
1410 #[stable(feature = "rust1", since = "1.0.0")]
1411 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1412 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1413 fmt::Debug::fmt(&**self, f)
1417 #[stable(feature = "rust1", since = "1.0.0")]
1418 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1419 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1420 fmt::Pointer::fmt(&(&**self as *const T), f)
1424 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1425 impl<T> From<T> for Rc<T> {
1426 fn from(t: T) -> Self {
1431 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1432 impl<T: Clone> From<&[T]> for Rc<[T]> {
1434 fn from(v: &[T]) -> Rc<[T]> {
1435 <Self as RcFromSlice<T>>::from_slice(v)
1439 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1440 impl From<&str> for Rc<str> {
1442 fn from(v: &str) -> Rc<str> {
1443 let rc = Rc::<[u8]>::from(v.as_bytes());
1444 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1448 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1449 impl From<String> for Rc<str> {
1451 fn from(v: String) -> Rc<str> {
1456 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1457 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1459 fn from(v: Box<T>) -> Rc<T> {
1464 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1465 impl<T> From<Vec<T>> for Rc<[T]> {
1467 fn from(mut v: Vec<T>) -> Rc<[T]> {
1469 let rc = Rc::copy_from_slice(&v);
1471 // Allow the Vec to free its memory, but not destroy its contents
1479 #[unstable(feature = "boxed_slice_try_from", issue = "0")]
1480 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]>
1482 [T; N]: LengthAtMost32,
1484 type Error = Rc<[T]>;
1486 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1487 if boxed_slice.len() == N {
1488 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
1495 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1496 impl<T> iter::FromIterator<T> for Rc<[T]> {
1497 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1499 /// # Performance characteristics
1501 /// ## The general case
1503 /// In the general case, collecting into `Rc<[T]>` is done by first
1504 /// collecting into a `Vec<T>`. That is, when writing the following:
1507 /// # use std::rc::Rc;
1508 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1509 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1512 /// this behaves as if we wrote:
1515 /// # use std::rc::Rc;
1516 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1517 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1518 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1519 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1522 /// This will allocate as many times as needed for constructing the `Vec<T>`
1523 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1525 /// ## Iterators of known length
1527 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1528 /// a single allocation will be made for the `Rc<[T]>`. For example:
1531 /// # use std::rc::Rc;
1532 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1533 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1535 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1536 RcFromIter::from_iter(iter.into_iter())
1540 /// Specialization trait used for collecting into `Rc<[T]>`.
1541 trait RcFromIter<T, I> {
1542 fn from_iter(iter: I) -> Self;
1545 impl<T, I: Iterator<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1546 default fn from_iter(iter: I) -> Self {
1547 iter.collect::<Vec<T>>().into()
1551 impl<T, I: iter::TrustedLen<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1552 default fn from_iter(iter: I) -> Self {
1553 // This is the case for a `TrustedLen` iterator.
1554 let (low, high) = iter.size_hint();
1555 if let Some(high) = high {
1558 "TrustedLen iterator's size hint is not exact: {:?}",
1563 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1564 Rc::from_iter_exact(iter, low)
1567 // Fall back to normal implementation.
1568 iter.collect::<Vec<T>>().into()
1573 impl<'a, T: 'a + Clone> RcFromIter<&'a T, slice::Iter<'a, T>> for Rc<[T]> {
1574 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
1575 // Delegate to `impl<T: Clone> From<&[T]> for Rc<[T]>`.
1577 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
1578 // which is even more performant.
1580 // In the fall-back case we have `T: Clone`. This is still better
1581 // than the `TrustedLen` implementation as slices have a known length
1582 // and so we get to avoid calling `size_hint` and avoid the branching.
1583 iter.as_slice().into()
1587 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1588 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
1589 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1591 /// Since a `Weak` reference does not count towards ownership, it will not
1592 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
1593 /// guarantees about the value still being present. Thus it may return [`None`]
1594 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
1595 /// itself (the backing store) from being deallocated.
1597 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
1598 /// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
1599 /// prevent circular references between [`Rc`] pointers, since mutual owning references
1600 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1601 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1602 /// pointers from children back to their parents.
1604 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1606 /// [`Rc`]: struct.Rc.html
1607 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1608 /// [`upgrade`]: struct.Weak.html#method.upgrade
1609 /// [`Option`]: ../../std/option/enum.Option.html
1610 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1611 #[stable(feature = "rc_weak", since = "1.4.0")]
1612 pub struct Weak<T: ?Sized> {
1613 // This is a `NonNull` to allow optimizing the size of this type in enums,
1614 // but it is not necessarily a valid pointer.
1615 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1616 // to allocate space on the heap. That's not a value a real pointer
1617 // will ever have because RcBox has alignment at least 2.
1618 ptr: NonNull<RcBox<T>>,
1621 #[stable(feature = "rc_weak", since = "1.4.0")]
1622 impl<T: ?Sized> !marker::Send for Weak<T> {}
1623 #[stable(feature = "rc_weak", since = "1.4.0")]
1624 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1626 #[unstable(feature = "coerce_unsized", issue = "27732")]
1627 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1629 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
1630 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1633 /// Constructs a new `Weak<T>`, without allocating any memory.
1634 /// Calling [`upgrade`] on the return value always gives [`None`].
1636 /// [`upgrade`]: #method.upgrade
1637 /// [`None`]: ../../std/option/enum.Option.html
1642 /// use std::rc::Weak;
1644 /// let empty: Weak<i64> = Weak::new();
1645 /// assert!(empty.upgrade().is_none());
1647 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1648 pub fn new() -> Weak<T> {
1650 ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"),
1654 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1656 /// The pointer is valid only if there are some strong references. The pointer may be dangling
1657 /// or even [`null`] otherwise.
1662 /// #![feature(weak_into_raw)]
1664 /// use std::rc::Rc;
1667 /// let strong = Rc::new("hello".to_owned());
1668 /// let weak = Rc::downgrade(&strong);
1669 /// // Both point to the same object
1670 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1671 /// // The strong here keeps it alive, so we can still access the object.
1672 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1675 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1676 /// // undefined behaviour.
1677 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1680 /// [`null`]: ../../std/ptr/fn.null.html
1681 #[unstable(feature = "weak_into_raw", issue = "60728")]
1682 pub fn as_raw(&self) -> *const T {
1683 match self.inner() {
1684 None => ptr::null(),
1686 let offset = data_offset_sized::<T>();
1687 let ptr = inner as *const RcBox<T>;
1688 // Note: while the pointer we create may already point to dropped value, the
1689 // allocation still lives (it must hold the weak point as long as we are alive).
1690 // Therefore, the offset is OK to do, it won't get out of the allocation.
1691 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1697 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1699 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1700 /// can be turned back into the `Weak<T>` with [`from_raw`].
1702 /// The same restrictions of accessing the target of the pointer as with
1703 /// [`as_raw`] apply.
1708 /// #![feature(weak_into_raw)]
1710 /// use std::rc::{Rc, Weak};
1712 /// let strong = Rc::new("hello".to_owned());
1713 /// let weak = Rc::downgrade(&strong);
1714 /// let raw = weak.into_raw();
1716 /// assert_eq!(1, Rc::weak_count(&strong));
1717 /// assert_eq!("hello", unsafe { &*raw });
1719 /// drop(unsafe { Weak::from_raw(raw) });
1720 /// assert_eq!(0, Rc::weak_count(&strong));
1723 /// [`from_raw`]: struct.Weak.html#method.from_raw
1724 /// [`as_raw`]: struct.Weak.html#method.as_raw
1725 #[unstable(feature = "weak_into_raw", issue = "60728")]
1726 pub fn into_raw(self) -> *const T {
1727 let result = self.as_raw();
1732 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1734 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1735 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1737 /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
1738 /// as these don't have any corresponding weak count).
1742 /// The pointer must have originated from the [`into_raw`] (or [`as_raw`], provided there was
1743 /// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
1746 /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
1747 /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
1753 /// #![feature(weak_into_raw)]
1755 /// use std::rc::{Rc, Weak};
1757 /// let strong = Rc::new("hello".to_owned());
1759 /// let raw_1 = Rc::downgrade(&strong).into_raw();
1760 /// let raw_2 = Rc::downgrade(&strong).into_raw();
1762 /// assert_eq!(2, Rc::weak_count(&strong));
1764 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1765 /// assert_eq!(1, Rc::weak_count(&strong));
1769 /// // Decrement the last weak count.
1770 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1773 /// [`into_raw`]: struct.Weak.html#method.into_raw
1774 /// [`upgrade`]: struct.Weak.html#method.upgrade
1775 /// [`Rc`]: struct.Rc.html
1776 /// [`Weak`]: struct.Weak.html
1777 /// [`as_raw`]: struct.Weak.html#method.as_raw
1778 /// [`new`]: struct.Weak.html#method.new
1779 /// [`forget`]: ../../std/mem/fn.forget.html
1780 #[unstable(feature = "weak_into_raw", issue = "60728")]
1781 pub unsafe fn from_raw(ptr: *const T) -> Self {
1785 // See Rc::from_raw for details
1786 let offset = data_offset(ptr);
1787 let fake_ptr = ptr as *mut RcBox<T>;
1788 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1790 ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
1796 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1797 let address = ptr.as_ptr() as *mut () as usize;
1798 address == usize::MAX
1801 impl<T: ?Sized> Weak<T> {
1802 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
1803 /// dropping of the inner value if successful.
1805 /// Returns [`None`] if the inner value has since been dropped.
1807 /// [`Rc`]: struct.Rc.html
1808 /// [`None`]: ../../std/option/enum.Option.html
1813 /// use std::rc::Rc;
1815 /// let five = Rc::new(5);
1817 /// let weak_five = Rc::downgrade(&five);
1819 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1820 /// assert!(strong_five.is_some());
1822 /// // Destroy all strong pointers.
1823 /// drop(strong_five);
1826 /// assert!(weak_five.upgrade().is_none());
1828 #[stable(feature = "rc_weak", since = "1.4.0")]
1829 pub fn upgrade(&self) -> Option<Rc<T>> {
1830 let inner = self.inner()?;
1831 if inner.strong() == 0 {
1835 Some(Rc::from_inner(self.ptr))
1839 /// Gets the number of strong (`Rc`) pointers pointing to this allocation.
1841 /// If `self` was created using [`Weak::new`], this will return 0.
1843 /// [`Weak::new`]: #method.new
1844 #[unstable(feature = "weak_counts", issue = "57977")]
1845 pub fn strong_count(&self) -> usize {
1846 if let Some(inner) = self.inner() {
1853 /// Gets the number of `Weak` pointers pointing to this allocation.
1855 /// If `self` was created using [`Weak::new`], this will return `None`. If
1856 /// not, the returned value is at least 1, since `self` still points to the
1859 /// [`Weak::new`]: #method.new
1860 #[unstable(feature = "weak_counts", issue = "57977")]
1861 pub fn weak_count(&self) -> Option<usize> {
1862 self.inner().map(|inner| {
1863 if inner.strong() > 0 {
1864 inner.weak() - 1 // subtract the implicit weak ptr
1871 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1872 /// (i.e., when this `Weak` was created by `Weak::new`).
1874 fn inner(&self) -> Option<&RcBox<T>> {
1875 if is_dangling(self.ptr) {
1878 Some(unsafe { self.ptr.as_ref() })
1882 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1883 /// [`ptr::eq`]), or if both don't point to any allocation
1884 /// (because they were created with `Weak::new()`).
1888 /// Since this compares pointers it means that `Weak::new()` will equal each
1889 /// other, even though they don't point to any allocation.
1894 /// use std::rc::Rc;
1896 /// let first_rc = Rc::new(5);
1897 /// let first = Rc::downgrade(&first_rc);
1898 /// let second = Rc::downgrade(&first_rc);
1900 /// assert!(first.ptr_eq(&second));
1902 /// let third_rc = Rc::new(5);
1903 /// let third = Rc::downgrade(&third_rc);
1905 /// assert!(!first.ptr_eq(&third));
1908 /// Comparing `Weak::new`.
1911 /// use std::rc::{Rc, Weak};
1913 /// let first = Weak::new();
1914 /// let second = Weak::new();
1915 /// assert!(first.ptr_eq(&second));
1917 /// let third_rc = Rc::new(());
1918 /// let third = Rc::downgrade(&third_rc);
1919 /// assert!(!first.ptr_eq(&third));
1922 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1924 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1925 pub fn ptr_eq(&self, other: &Self) -> bool {
1926 self.ptr.as_ptr() == other.ptr.as_ptr()
1930 #[stable(feature = "rc_weak", since = "1.4.0")]
1931 impl<T: ?Sized> Drop for Weak<T> {
1932 /// Drops the `Weak` pointer.
1937 /// use std::rc::{Rc, Weak};
1941 /// impl Drop for Foo {
1942 /// fn drop(&mut self) {
1943 /// println!("dropped!");
1947 /// let foo = Rc::new(Foo);
1948 /// let weak_foo = Rc::downgrade(&foo);
1949 /// let other_weak_foo = Weak::clone(&weak_foo);
1951 /// drop(weak_foo); // Doesn't print anything
1952 /// drop(foo); // Prints "dropped!"
1954 /// assert!(other_weak_foo.upgrade().is_none());
1956 fn drop(&mut self) {
1957 if let Some(inner) = self.inner() {
1959 // the weak count starts at 1, and will only go to zero if all
1960 // the strong pointers have disappeared.
1961 if inner.weak() == 0 {
1963 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1970 #[stable(feature = "rc_weak", since = "1.4.0")]
1971 impl<T: ?Sized> Clone for Weak<T> {
1972 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1977 /// use std::rc::{Rc, Weak};
1979 /// let weak_five = Rc::downgrade(&Rc::new(5));
1981 /// let _ = Weak::clone(&weak_five);
1984 fn clone(&self) -> Weak<T> {
1985 if let Some(inner) = self.inner() {
1988 Weak { ptr: self.ptr }
1992 #[stable(feature = "rc_weak", since = "1.4.0")]
1993 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1994 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1999 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2000 impl<T> Default for Weak<T> {
2001 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
2002 /// it. Calling [`upgrade`] on the return value always gives [`None`].
2004 /// [`None`]: ../../std/option/enum.Option.html
2005 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
2010 /// use std::rc::Weak;
2012 /// let empty: Weak<i64> = Default::default();
2013 /// assert!(empty.upgrade().is_none());
2015 fn default() -> Weak<T> {
2020 // NOTE: We checked_add here to deal with mem::forget safely. In particular
2021 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
2022 // you can free the allocation while outstanding Rcs (or Weaks) exist.
2023 // We abort because this is such a degenerate scenario that we don't care about
2024 // what happens -- no real program should ever experience this.
2026 // This should have negligible overhead since you don't actually need to
2027 // clone these much in Rust thanks to ownership and move-semantics.
2030 trait RcBoxPtr<T: ?Sized> {
2031 fn inner(&self) -> &RcBox<T>;
2034 fn strong(&self) -> usize {
2035 self.inner().strong.get()
2039 fn inc_strong(&self) {
2040 let strong = self.strong();
2042 // We want to abort on overflow instead of dropping the value.
2043 // The reference count will never be zero when this is called;
2044 // nevertheless, we insert an abort here to hint LLVM at
2045 // an otherwise missed optimization.
2046 if strong == 0 || strong == usize::max_value() {
2049 self.inner().strong.set(strong + 1);
2053 fn dec_strong(&self) {
2054 self.inner().strong.set(self.strong() - 1);
2058 fn weak(&self) -> usize {
2059 self.inner().weak.get()
2063 fn inc_weak(&self) {
2064 let weak = self.weak();
2066 // We want to abort on overflow instead of dropping the value.
2067 // The reference count will never be zero when this is called;
2068 // nevertheless, we insert an abort here to hint LLVM at
2069 // an otherwise missed optimization.
2070 if weak == 0 || weak == usize::max_value() {
2073 self.inner().weak.set(weak + 1);
2077 fn dec_weak(&self) {
2078 self.inner().weak.set(self.weak() - 1);
2082 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
2084 fn inner(&self) -> &RcBox<T> {
2091 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
2093 fn inner(&self) -> &RcBox<T> {
2098 #[stable(feature = "rust1", since = "1.0.0")]
2099 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2100 fn borrow(&self) -> &T {
2105 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2106 impl<T: ?Sized> AsRef<T> for Rc<T> {
2107 fn as_ref(&self) -> &T {
2112 #[stable(feature = "pin", since = "1.33.0")]
2113 impl<T: ?Sized> Unpin for Rc<T> { }
2115 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2116 // Align the unsized value to the end of the `RcBox`.
2117 // Because it is ?Sized, it will always be the last field in memory.
2118 data_offset_align(align_of_val(&*ptr))
2121 /// Computes the offset of the data field within `RcBox`.
2123 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2124 fn data_offset_sized<T>() -> isize {
2125 data_offset_align(align_of::<T>())
2129 fn data_offset_align(align: usize) -> isize {
2130 let layout = Layout::new::<RcBox<()>>();
2131 (layout.size() + layout.padding_needed_for(align)) as isize