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 value in the heap. When the last [`Rc`] pointer to a
7 //! given value is destroyed, the pointed-to value is also destroyed.
9 //! Shared references in Rust disallow mutation by default, and [`Rc`]
10 //! is no exception: you cannot generally obtain a mutable reference to
11 //! something inside an [`Rc`]. If you need mutability, put a [`Cell`]
12 //! or [`RefCell`] inside the [`Rc`]; see [an example of mutability
13 //! inside an Rc][mutability].
15 //! [`Rc`] uses non-atomic reference counting. This means that overhead is very
16 //! low, but an [`Rc`] cannot be sent between threads, and consequently [`Rc`]
17 //! does not implement [`Send`][send]. As a result, the Rust compiler
18 //! will check *at compile time* that you are not sending [`Rc`]s between
19 //! threads. If you need multi-threaded, atomic reference counting, use
20 //! [`sync::Arc`][arc].
22 //! The [`downgrade`][downgrade] method can be used to create a non-owning
23 //! [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
24 //! to an [`Rc`], but this will return [`None`] if the value has
25 //! already been dropped.
27 //! A cycle between [`Rc`] pointers will never be deallocated. For this reason,
28 //! [`Weak`] is used to break cycles. For example, a tree could have strong
29 //! [`Rc`] pointers from parent nodes to children, and [`Weak`] pointers from
30 //! children back to their parents.
32 //! `Rc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
33 //! so you can call `T`'s methods on a value of type [`Rc<T>`][`Rc`]. To avoid name
34 //! clashes with `T`'s methods, the methods of [`Rc<T>`][`Rc`] itself are associated
35 //! functions, called using function-like syntax:
39 //! let my_rc = Rc::new(());
41 //! Rc::downgrade(&my_rc);
44 //! [`Weak<T>`][`Weak`] does not auto-dereference to `T`, because the value may have
45 //! already been destroyed.
47 //! # Cloning references
49 //! Creating a new reference from an existing reference counted pointer is done using the
50 //! `Clone` trait implemented for [`Rc<T>`][`Rc`] and [`Weak<T>`][`Weak`].
54 //! let foo = Rc::new(vec![1.0, 2.0, 3.0]);
55 //! // The two syntaxes below are equivalent.
56 //! let a = foo.clone();
57 //! let b = Rc::clone(&foo);
58 //! // a and b both point to the same memory location as foo.
61 //! The `Rc::clone(&from)` syntax is the most idiomatic because it conveys more explicitly
62 //! the meaning of the code. In the example above, this syntax makes it easier to see that
63 //! this code is creating a new reference rather than copying the whole content of foo.
67 //! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`.
68 //! We want to have our `Gadget`s point to their `Owner`. We can't do this with
69 //! unique ownership, because more than one gadget may belong to the same
70 //! `Owner`. [`Rc`] allows us to share an `Owner` between multiple `Gadget`s,
71 //! and have the `Owner` remain allocated as long as any `Gadget` points at it.
78 //! // ...other fields
84 //! // ...other fields
88 //! // Create a reference-counted `Owner`.
89 //! let gadget_owner: Rc<Owner> = Rc::new(
91 //! name: "Gadget Man".to_string(),
95 //! // Create `Gadget`s belonging to `gadget_owner`. Cloning the `Rc<Owner>`
96 //! // value gives us a new pointer to the same `Owner` value, incrementing
97 //! // the reference count in the process.
98 //! let gadget1 = Gadget {
100 //! owner: Rc::clone(&gadget_owner),
102 //! let gadget2 = Gadget {
104 //! owner: Rc::clone(&gadget_owner),
107 //! // Dispose of our local variable `gadget_owner`.
108 //! drop(gadget_owner);
110 //! // Despite dropping `gadget_owner`, we're still able to print out the name
111 //! // of the `Owner` of the `Gadget`s. This is because we've only dropped a
112 //! // single `Rc<Owner>`, not the `Owner` it points to. As long as there are
113 //! // other `Rc<Owner>` values pointing at the same `Owner`, it will remain
114 //! // allocated. The field projection `gadget1.owner.name` works because
115 //! // `Rc<Owner>` automatically dereferences to `Owner`.
116 //! println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name);
117 //! println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name);
119 //! // At the end of the function, `gadget1` and `gadget2` are destroyed, and
120 //! // with them the last counted references to our `Owner`. Gadget Man now
121 //! // gets destroyed as well.
125 //! If our requirements change, and we also need to be able to traverse from
126 //! `Owner` to `Gadget`, we will run into problems. An [`Rc`] pointer from `Owner`
127 //! to `Gadget` introduces a cycle between the values. This means that their
128 //! reference counts can never reach 0, and the values will remain allocated
129 //! forever: a memory leak. In order to get around this, we can use [`Weak`]
132 //! Rust actually makes it somewhat difficult to produce this loop in the first
133 //! place. In order to end up with two values that point at each other, one of
134 //! them needs to be mutable. This is difficult because [`Rc`] enforces
135 //! memory safety by only giving out shared references to the value it wraps,
136 //! and these don't allow direct mutation. We need to wrap the part of the
137 //! value we wish to mutate in a [`RefCell`], which provides *interior
138 //! mutability*: a method to achieve mutability through a shared reference.
139 //! [`RefCell`] enforces Rust's borrowing rules at runtime.
143 //! use std::rc::Weak;
144 //! use std::cell::RefCell;
148 //! gadgets: RefCell<Vec<Weak<Gadget>>>,
149 //! // ...other fields
154 //! owner: Rc<Owner>,
155 //! // ...other fields
159 //! // Create a reference-counted `Owner`. Note that we've put the `Owner`'s
160 //! // vector of `Gadget`s inside a `RefCell` so that we can mutate it through
161 //! // a shared reference.
162 //! let gadget_owner: Rc<Owner> = Rc::new(
164 //! name: "Gadget Man".to_string(),
165 //! gadgets: RefCell::new(vec![]),
169 //! // Create `Gadget`s belonging to `gadget_owner`, as before.
170 //! let gadget1 = Rc::new(
173 //! owner: Rc::clone(&gadget_owner),
176 //! let gadget2 = Rc::new(
179 //! owner: Rc::clone(&gadget_owner),
183 //! // Add the `Gadget`s to their `Owner`.
185 //! let mut gadgets = gadget_owner.gadgets.borrow_mut();
186 //! gadgets.push(Rc::downgrade(&gadget1));
187 //! gadgets.push(Rc::downgrade(&gadget2));
189 //! // `RefCell` dynamic borrow ends here.
192 //! // Iterate over our `Gadget`s, printing their details out.
193 //! for gadget_weak in gadget_owner.gadgets.borrow().iter() {
195 //! // `gadget_weak` is a `Weak<Gadget>`. Since `Weak` pointers can't
196 //! // guarantee the value is still allocated, we need to call
197 //! // `upgrade`, which returns an `Option<Rc<Gadget>>`.
199 //! // In this case we know the value still exists, so we simply
200 //! // `unwrap` the `Option`. In a more complicated program, you might
201 //! // need graceful error handling for a `None` result.
203 //! let gadget = gadget_weak.upgrade().unwrap();
204 //! println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
207 //! // At the end of the function, `gadget_owner`, `gadget1`, and `gadget2`
208 //! // are destroyed. There are now no strong (`Rc`) pointers to the
209 //! // gadgets, so they are destroyed. This zeroes the reference count on
210 //! // Gadget Man, so he gets destroyed as well.
214 //! [`Rc`]: struct.Rc.html
215 //! [`Weak`]: struct.Weak.html
216 //! [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
217 //! [`Cell`]: ../../std/cell/struct.Cell.html
218 //! [`RefCell`]: ../../std/cell/struct.RefCell.html
219 //! [send]: ../../std/marker/trait.Send.html
220 //! [arc]: ../../std/sync/struct.Arc.html
221 //! [`Deref`]: ../../std/ops/trait.Deref.html
222 //! [downgrade]: struct.Rc.html#method.downgrade
223 //! [upgrade]: struct.Weak.html#method.upgrade
224 //! [`None`]: ../../std/option/enum.Option.html#variant.None
225 //! [mutability]: ../../std/cell/index.html#introducing-mutability-inside-of-something-immutable
227 #![stable(feature = "rust1", since = "1.0.0")]
230 use crate::boxed::Box;
235 use core::array::LengthAtMost32;
237 use core::cell::Cell;
238 use core::cmp::Ordering;
240 use core::hash::{Hash, Hasher};
241 use core::intrinsics::abort;
243 use core::marker::{self, Unpin, Unsize, PhantomData};
244 use core::mem::{self, align_of, align_of_val, forget, size_of_val};
245 use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn};
247 use core::ptr::{self, NonNull};
248 use core::slice::{self, from_raw_parts_mut};
249 use core::convert::{From, TryFrom};
252 use crate::alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
253 use crate::string::String;
259 struct RcBox<T: ?Sized> {
265 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
268 /// See the [module-level documentation](./index.html) for more details.
270 /// The inherent methods of `Rc` are all associated functions, which means
271 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
272 /// `value.get_mut()`. This avoids conflicts with methods of the inner
275 /// [get_mut]: #method.get_mut
276 #[cfg_attr(not(test), lang = "rc")]
277 #[stable(feature = "rust1", since = "1.0.0")]
278 pub struct Rc<T: ?Sized> {
279 ptr: NonNull<RcBox<T>>,
280 phantom: PhantomData<T>,
283 #[stable(feature = "rust1", since = "1.0.0")]
284 impl<T: ?Sized> !marker::Send for Rc<T> {}
285 #[stable(feature = "rust1", since = "1.0.0")]
286 impl<T: ?Sized> !marker::Sync for Rc<T> {}
288 #[unstable(feature = "coerce_unsized", issue = "27732")]
289 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
291 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
292 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
294 impl<T: ?Sized> Rc<T> {
295 fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
298 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 /// Construct 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>> {
353 let layout = Layout::new::<RcBox<mem::MaybeUninit<T>>>();
355 let mut ptr = Global.alloc(layout)
356 .unwrap_or_else(|_| handle_alloc_error(layout))
357 .cast::<RcBox<mem::MaybeUninit<T>>>();
358 ptr::write(&mut ptr.as_mut().strong, Cell::new(1));
359 ptr::write(&mut ptr.as_mut().weak, Cell::new(1));
362 phantom: PhantomData,
367 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
368 /// `value` will be pinned in memory and unable to be moved.
369 #[stable(feature = "pin", since = "1.33.0")]
370 pub fn pin(value: T) -> Pin<Rc<T>> {
371 unsafe { Pin::new_unchecked(Rc::new(value)) }
374 /// Returns the contained value, if the `Rc` has exactly one strong reference.
376 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
379 /// This will succeed even if there are outstanding weak references.
381 /// [result]: ../../std/result/enum.Result.html
388 /// let x = Rc::new(3);
389 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
391 /// let x = Rc::new(4);
392 /// let _y = Rc::clone(&x);
393 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
396 #[stable(feature = "rc_unique", since = "1.4.0")]
397 pub fn try_unwrap(this: Self) -> Result<T, Self> {
398 if Rc::strong_count(&this) == 1 {
400 let val = ptr::read(&*this); // copy the contained object
402 // Indicate to Weaks that they can't be promoted by decrementing
403 // the strong count, and then remove the implicit "strong weak"
404 // pointer while also handling drop logic by just crafting a
407 let _weak = Weak { ptr: this.ptr };
418 /// Construct a new reference-counted slice with uninitialized contents.
423 /// #![feature(new_uninit)]
424 /// #![feature(get_mut_unchecked)]
428 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
430 /// let values = unsafe {
431 /// // Deferred initialization:
432 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
433 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
434 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
436 /// values.assume_init()
439 /// assert_eq!(*values, [1, 2, 3])
441 #[unstable(feature = "new_uninit", issue = "63291")]
442 pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
443 let data_layout = Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
444 // This relies on `value` being the last field of `RcBox` in memory,
445 // so that the layout of `RcBox<T>` is the same as that of `RcBox<()>` followed by `T`.
446 let (layout, offset) = Layout::new::<RcBox<()>>().extend(data_layout).unwrap();
448 let allocated_ptr = Global.alloc(layout)
449 .unwrap_or_else(|_| handle_alloc_error(layout))
451 let data_ptr = allocated_ptr.add(offset) as *mut mem::MaybeUninit<T>;
452 let slice: *mut [mem::MaybeUninit<T>] = from_raw_parts_mut(data_ptr, len);
453 let wide_ptr = slice as *mut RcBox<[mem::MaybeUninit<T>]>;
454 let wide_ptr = set_data_ptr(wide_ptr, allocated_ptr);
455 ptr::write(&mut (*wide_ptr).strong, Cell::new(1));
456 ptr::write(&mut (*wide_ptr).weak, Cell::new(1));
458 ptr: NonNull::new_unchecked(wide_ptr),
459 phantom: PhantomData,
465 impl<T> Rc<mem::MaybeUninit<T>> {
466 /// Convert to `Rc<T>`.
470 /// As with [`MaybeUninit::assume_init`],
471 /// it is up to the caller to guarantee that the value
472 /// really is in an initialized state.
473 /// Calling this when the content is not yet fully initialized
474 /// causes immediate undefined behavior.
476 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
481 /// #![feature(new_uninit)]
482 /// #![feature(get_mut_unchecked)]
486 /// let mut five = Rc::<u32>::new_uninit();
488 /// let five = unsafe {
489 /// // Deferred initialization:
490 /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
492 /// five.assume_init()
495 /// assert_eq!(*five, 5)
497 #[unstable(feature = "new_uninit", issue = "63291")]
499 pub unsafe fn assume_init(self) -> Rc<T> {
501 ptr: mem::ManuallyDrop::new(self).ptr.cast(),
502 phantom: PhantomData,
507 impl<T> Rc<[mem::MaybeUninit<T>]> {
508 /// Convert to `Rc<[T]>`.
512 /// As with [`MaybeUninit::assume_init`],
513 /// it is up to the caller to guarantee that the value
514 /// really is in an initialized state.
515 /// Calling this when the content is not yet fully initialized
516 /// causes immediate undefined behavior.
518 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
523 /// #![feature(new_uninit)]
524 /// #![feature(get_mut_unchecked)]
528 /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
530 /// let values = unsafe {
531 /// // Deferred initialization:
532 /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
533 /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
534 /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
536 /// values.assume_init()
539 /// assert_eq!(*values, [1, 2, 3])
541 #[unstable(feature = "new_uninit", issue = "63291")]
543 pub unsafe fn assume_init(self) -> Rc<[T]> {
545 ptr: NonNull::new_unchecked(mem::ManuallyDrop::new(self).ptr.as_ptr() as _),
546 phantom: PhantomData,
551 impl<T: ?Sized> Rc<T> {
552 /// Consumes the `Rc`, returning the wrapped pointer.
554 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
555 /// [`Rc::from_raw`][from_raw].
557 /// [from_raw]: struct.Rc.html#method.from_raw
564 /// let x = Rc::new("hello".to_owned());
565 /// let x_ptr = Rc::into_raw(x);
566 /// assert_eq!(unsafe { &*x_ptr }, "hello");
568 #[stable(feature = "rc_raw", since = "1.17.0")]
569 pub fn into_raw(this: Self) -> *const T {
570 let ptr: *const T = &*this;
575 /// Constructs an `Rc` from a raw pointer.
577 /// The raw pointer must have been previously returned by a call to a
578 /// [`Rc::into_raw`][into_raw].
580 /// This function is unsafe because improper use may lead to memory problems. For example, a
581 /// double-free may occur if the function is called twice on the same raw pointer.
583 /// [into_raw]: struct.Rc.html#method.into_raw
590 /// let x = Rc::new("hello".to_owned());
591 /// let x_ptr = Rc::into_raw(x);
594 /// // Convert back to an `Rc` to prevent leak.
595 /// let x = Rc::from_raw(x_ptr);
596 /// assert_eq!(&*x, "hello");
598 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
601 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
603 #[stable(feature = "rc_raw", since = "1.17.0")]
604 pub unsafe fn from_raw(ptr: *const T) -> Self {
605 let offset = data_offset(ptr);
607 // Reverse the offset to find the original RcBox.
608 let fake_ptr = ptr as *mut RcBox<T>;
609 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
611 Self::from_ptr(rc_ptr)
614 /// Consumes the `Rc`, returning the wrapped pointer as `NonNull<T>`.
619 /// #![feature(rc_into_raw_non_null)]
623 /// let x = Rc::new("hello".to_owned());
624 /// let ptr = Rc::into_raw_non_null(x);
625 /// let deref = unsafe { ptr.as_ref() };
626 /// assert_eq!(deref, "hello");
628 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
630 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
631 // safe because Rc guarantees its pointer is non-null
632 unsafe { NonNull::new_unchecked(Rc::into_raw(this) as *mut _) }
635 /// Creates a new [`Weak`][weak] pointer to this value.
637 /// [weak]: struct.Weak.html
644 /// let five = Rc::new(5);
646 /// let weak_five = Rc::downgrade(&five);
648 #[stable(feature = "rc_weak", since = "1.4.0")]
649 pub fn downgrade(this: &Self) -> Weak<T> {
651 // Make sure we do not create a dangling Weak
652 debug_assert!(!is_dangling(this.ptr));
653 Weak { ptr: this.ptr }
656 /// Gets the number of [`Weak`][weak] pointers to this value.
658 /// [weak]: struct.Weak.html
665 /// let five = Rc::new(5);
666 /// let _weak_five = Rc::downgrade(&five);
668 /// assert_eq!(1, Rc::weak_count(&five));
671 #[stable(feature = "rc_counts", since = "1.15.0")]
672 pub fn weak_count(this: &Self) -> usize {
676 /// Gets the number of strong (`Rc`) pointers to this value.
683 /// let five = Rc::new(5);
684 /// let _also_five = Rc::clone(&five);
686 /// assert_eq!(2, Rc::strong_count(&five));
689 #[stable(feature = "rc_counts", since = "1.15.0")]
690 pub fn strong_count(this: &Self) -> usize {
694 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
695 /// this inner value.
697 /// [weak]: struct.Weak.html
699 fn is_unique(this: &Self) -> bool {
700 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
703 /// Returns a mutable reference to the inner value, if there are
704 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
706 /// Returns [`None`] otherwise, because it is not safe to
707 /// mutate a shared value.
709 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
710 /// the inner value when it's shared.
712 /// [weak]: struct.Weak.html
713 /// [`None`]: ../../std/option/enum.Option.html#variant.None
714 /// [make_mut]: struct.Rc.html#method.make_mut
715 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
722 /// let mut x = Rc::new(3);
723 /// *Rc::get_mut(&mut x).unwrap() = 4;
724 /// assert_eq!(*x, 4);
726 /// let _y = Rc::clone(&x);
727 /// assert!(Rc::get_mut(&mut x).is_none());
730 #[stable(feature = "rc_unique", since = "1.4.0")]
731 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
732 if Rc::is_unique(this) {
734 Some(Rc::get_mut_unchecked(this))
741 /// Returns a mutable reference to the inner value,
742 /// without any check.
744 /// See also [`get_mut`], which is safe and does appropriate checks.
746 /// [`get_mut`]: struct.Rc.html#method.get_mut
750 /// There must be no other `Rc` or [`Weak`] pointers to the same value.
751 /// This is the case for example immediately after `Rc::new`.
756 /// #![feature(get_mut_unchecked)]
760 /// let mut x = Rc::new(String::new());
762 /// Rc::get_mut_unchecked(&mut x).push_str("foo")
764 /// assert_eq!(*x, "foo");
767 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
768 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
769 &mut this.ptr.as_mut().value
773 #[stable(feature = "ptr_eq", since = "1.17.0")]
774 /// Returns `true` if the two `Rc`s point to the same value (not
775 /// just values that compare as equal).
782 /// let five = Rc::new(5);
783 /// let same_five = Rc::clone(&five);
784 /// let other_five = Rc::new(5);
786 /// assert!(Rc::ptr_eq(&five, &same_five));
787 /// assert!(!Rc::ptr_eq(&five, &other_five));
789 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
790 this.ptr.as_ptr() == other.ptr.as_ptr()
794 impl<T: Clone> Rc<T> {
795 /// Makes a mutable reference into the given `Rc`.
797 /// If there are other `Rc` pointers to the same value, then `make_mut` will
798 /// [`clone`] the inner value to ensure unique ownership. This is also
799 /// referred to as clone-on-write.
801 /// If there are no other `Rc` pointers to this value, then [`Weak`]
802 /// pointers to this value will be dissassociated.
804 /// See also [`get_mut`], which will fail rather than cloning.
806 /// [`Weak`]: struct.Weak.html
807 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
808 /// [`get_mut`]: struct.Rc.html#method.get_mut
815 /// let mut data = Rc::new(5);
817 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
818 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
819 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
820 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
821 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
823 /// // Now `data` and `other_data` point to different values.
824 /// assert_eq!(*data, 8);
825 /// assert_eq!(*other_data, 12);
828 /// [`Weak`] pointers will be dissassociated:
833 /// let mut data = Rc::new(75);
834 /// let weak = Rc::downgrade(&data);
836 /// assert!(75 == *data);
837 /// assert!(75 == *weak.upgrade().unwrap());
839 /// *Rc::make_mut(&mut data) += 1;
841 /// assert!(76 == *data);
842 /// assert!(weak.upgrade().is_none());
845 #[stable(feature = "rc_unique", since = "1.4.0")]
846 pub fn make_mut(this: &mut Self) -> &mut T {
847 if Rc::strong_count(this) != 1 {
848 // Gotta clone the data, there are other Rcs
849 *this = Rc::new((**this).clone())
850 } else if Rc::weak_count(this) != 0 {
851 // Can just steal the data, all that's left is Weaks
853 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
854 mem::swap(this, &mut swap);
856 // Remove implicit strong-weak ref (no need to craft a fake
857 // Weak here -- we know other Weaks can clean up for us)
862 // This unsafety is ok because we're guaranteed that the pointer
863 // returned is the *only* pointer that will ever be returned to T. Our
864 // reference count is guaranteed to be 1 at this point, and we required
865 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
866 // reference to the inner value.
868 &mut this.ptr.as_mut().value
875 #[stable(feature = "rc_downcast", since = "1.29.0")]
876 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
881 /// use std::any::Any;
884 /// fn print_if_string(value: Rc<dyn Any>) {
885 /// if let Ok(string) = value.downcast::<String>() {
886 /// println!("String ({}): {}", string.len(), string);
891 /// let my_string = "Hello World".to_string();
892 /// print_if_string(Rc::new(my_string));
893 /// print_if_string(Rc::new(0i8));
896 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
897 if (*self).is::<T>() {
898 let ptr = self.ptr.cast::<RcBox<T>>();
900 Ok(Rc::from_inner(ptr))
907 impl<T: ?Sized> Rc<T> {
908 /// Allocates an `RcBox<T>` with sufficient space for
909 /// an unsized value where the value has the layout provided.
911 /// The function `mem_to_rcbox` is called with the data pointer
912 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
913 unsafe fn allocate_for_unsized(
914 value_layout: Layout,
915 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>
917 // Calculate layout using the given value layout.
918 // Previously, layout was calculated on the expression
919 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
920 // reference (see #54908).
921 let layout = Layout::new::<RcBox<()>>()
922 .extend(value_layout).unwrap().0
923 .pad_to_align().unwrap();
925 // Allocate for the layout.
926 let mem = Global.alloc(layout)
927 .unwrap_or_else(|_| handle_alloc_error(layout));
929 // Initialize the RcBox
930 let inner = mem_to_rcbox(mem.as_ptr());
931 debug_assert_eq!(Layout::for_value(&*inner), layout);
933 ptr::write(&mut (*inner).strong, Cell::new(1));
934 ptr::write(&mut (*inner).weak, Cell::new(1));
939 /// Allocates an `RcBox<T>` with sufficient space for an unsized value
940 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
941 // Allocate for the `RcBox<T>` using the given value.
942 Self::allocate_for_unsized(
943 Layout::for_value(&*ptr),
944 |mem| set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>,
948 fn from_box(v: Box<T>) -> Rc<T> {
950 let box_unique = Box::into_unique(v);
951 let bptr = box_unique.as_ptr();
953 let value_size = size_of_val(&*bptr);
954 let ptr = Self::allocate_for_ptr(bptr);
956 // Copy value as bytes
957 ptr::copy_nonoverlapping(
958 bptr as *const T as *const u8,
959 &mut (*ptr).value as *mut _ as *mut u8,
962 // Free the allocation without dropping its contents
963 box_free(box_unique);
971 /// Allocates an `RcBox<[T]>` with the given length.
972 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
973 Self::allocate_for_unsized(
974 Layout::array::<T>(len).unwrap(),
975 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>,
980 /// Sets the data pointer of a `?Sized` raw pointer.
982 /// For a slice/trait object, this sets the `data` field and leaves the rest
983 /// unchanged. For a sized raw pointer, this simply sets the pointer.
984 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
985 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
990 /// Copy elements from slice into newly allocated Rc<[T]>
992 /// Unsafe because the caller must either take ownership or bind `T: Copy`
993 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
994 let ptr = Self::allocate_for_slice(v.len());
996 ptr::copy_nonoverlapping(
998 &mut (*ptr).value as *mut [T] as *mut T,
1004 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
1006 /// Behavior is undefined should the size be wrong.
1007 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
1008 // Panic guard while cloning T elements.
1009 // In the event of a panic, elements that have been written
1010 // into the new RcBox will be dropped, then the memory freed.
1018 impl<T> Drop for Guard<T> {
1019 fn drop(&mut self) {
1021 let slice = from_raw_parts_mut(self.elems, self.n_elems);
1022 ptr::drop_in_place(slice);
1024 Global.dealloc(self.mem, self.layout);
1029 let ptr = Self::allocate_for_slice(len);
1031 let mem = ptr as *mut _ as *mut u8;
1032 let layout = Layout::for_value(&*ptr);
1034 // Pointer to first element
1035 let elems = &mut (*ptr).value as *mut [T] as *mut T;
1037 let mut guard = Guard {
1038 mem: NonNull::new_unchecked(mem),
1044 for (i, item) in iter.enumerate() {
1045 ptr::write(elems.add(i), item);
1049 // All clear. Forget the guard so it doesn't free the new RcBox.
1056 /// Specialization trait used for `From<&[T]>`.
1057 trait RcFromSlice<T> {
1058 fn from_slice(slice: &[T]) -> Self;
1061 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
1063 default fn from_slice(v: &[T]) -> Self {
1065 Self::from_iter_exact(v.iter().cloned(), v.len())
1070 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
1072 fn from_slice(v: &[T]) -> Self {
1073 unsafe { Rc::copy_from_slice(v) }
1077 #[stable(feature = "rust1", since = "1.0.0")]
1078 impl<T: ?Sized> Deref for Rc<T> {
1082 fn deref(&self) -> &T {
1087 #[unstable(feature = "receiver_trait", issue = "0")]
1088 impl<T: ?Sized> Receiver for Rc<T> {}
1090 #[stable(feature = "rust1", since = "1.0.0")]
1091 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
1094 /// This will decrement the strong reference count. If the strong reference
1095 /// count reaches zero then the only other references (if any) are
1096 /// [`Weak`], so we `drop` the inner value.
1101 /// use std::rc::Rc;
1105 /// impl Drop for Foo {
1106 /// fn drop(&mut self) {
1107 /// println!("dropped!");
1111 /// let foo = Rc::new(Foo);
1112 /// let foo2 = Rc::clone(&foo);
1114 /// drop(foo); // Doesn't print anything
1115 /// drop(foo2); // Prints "dropped!"
1118 /// [`Weak`]: ../../std/rc/struct.Weak.html
1119 fn drop(&mut self) {
1122 if self.strong() == 0 {
1123 // destroy the contained object
1124 ptr::drop_in_place(self.ptr.as_mut());
1126 // remove the implicit "strong weak" pointer now that we've
1127 // destroyed the contents.
1130 if self.weak() == 0 {
1131 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1138 #[stable(feature = "rust1", since = "1.0.0")]
1139 impl<T: ?Sized> Clone for Rc<T> {
1140 /// Makes a clone of the `Rc` pointer.
1142 /// This creates another pointer to the same inner value, increasing the
1143 /// strong reference count.
1148 /// use std::rc::Rc;
1150 /// let five = Rc::new(5);
1152 /// let _ = Rc::clone(&five);
1155 fn clone(&self) -> Rc<T> {
1157 Self::from_inner(self.ptr)
1161 #[stable(feature = "rust1", since = "1.0.0")]
1162 impl<T: Default> Default for Rc<T> {
1163 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
1168 /// use std::rc::Rc;
1170 /// let x: Rc<i32> = Default::default();
1171 /// assert_eq!(*x, 0);
1174 fn default() -> Rc<T> {
1175 Rc::new(Default::default())
1179 #[stable(feature = "rust1", since = "1.0.0")]
1180 trait RcEqIdent<T: ?Sized + PartialEq> {
1181 fn eq(&self, other: &Rc<T>) -> bool;
1182 fn ne(&self, other: &Rc<T>) -> bool;
1185 #[stable(feature = "rust1", since = "1.0.0")]
1186 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
1188 default fn eq(&self, other: &Rc<T>) -> bool {
1193 default fn ne(&self, other: &Rc<T>) -> bool {
1198 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1199 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
1200 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1201 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
1202 /// the same value, than two `&T`s.
1203 #[stable(feature = "rust1", since = "1.0.0")]
1204 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
1206 fn eq(&self, other: &Rc<T>) -> bool {
1207 Rc::ptr_eq(self, other) || **self == **other
1211 fn ne(&self, other: &Rc<T>) -> bool {
1212 !Rc::ptr_eq(self, other) && **self != **other
1216 #[stable(feature = "rust1", since = "1.0.0")]
1217 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1218 /// Equality for two `Rc`s.
1220 /// Two `Rc`s are equal if their inner values are equal.
1222 /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
1228 /// use std::rc::Rc;
1230 /// let five = Rc::new(5);
1232 /// assert!(five == Rc::new(5));
1235 fn eq(&self, other: &Rc<T>) -> bool {
1236 RcEqIdent::eq(self, other)
1239 /// Inequality for two `Rc`s.
1241 /// Two `Rc`s are unequal if their inner values are unequal.
1243 /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
1249 /// use std::rc::Rc;
1251 /// let five = Rc::new(5);
1253 /// assert!(five != Rc::new(6));
1256 fn ne(&self, other: &Rc<T>) -> bool {
1257 RcEqIdent::ne(self, other)
1261 #[stable(feature = "rust1", since = "1.0.0")]
1262 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1264 #[stable(feature = "rust1", since = "1.0.0")]
1265 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1266 /// Partial comparison for two `Rc`s.
1268 /// The two are compared by calling `partial_cmp()` on their inner values.
1273 /// use std::rc::Rc;
1274 /// use std::cmp::Ordering;
1276 /// let five = Rc::new(5);
1278 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1281 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1282 (**self).partial_cmp(&**other)
1285 /// Less-than comparison for two `Rc`s.
1287 /// The two are compared by calling `<` on their inner values.
1292 /// use std::rc::Rc;
1294 /// let five = Rc::new(5);
1296 /// assert!(five < Rc::new(6));
1299 fn lt(&self, other: &Rc<T>) -> bool {
1303 /// 'Less than or equal to' comparison for two `Rc`s.
1305 /// The two are compared by calling `<=` on their inner values.
1310 /// use std::rc::Rc;
1312 /// let five = Rc::new(5);
1314 /// assert!(five <= Rc::new(5));
1317 fn le(&self, other: &Rc<T>) -> bool {
1321 /// Greater-than comparison for two `Rc`s.
1323 /// The two are compared by calling `>` on their inner values.
1328 /// use std::rc::Rc;
1330 /// let five = Rc::new(5);
1332 /// assert!(five > Rc::new(4));
1335 fn gt(&self, other: &Rc<T>) -> bool {
1339 /// 'Greater than or equal to' comparison for two `Rc`s.
1341 /// The two are compared by calling `>=` on their inner values.
1346 /// use std::rc::Rc;
1348 /// let five = Rc::new(5);
1350 /// assert!(five >= Rc::new(5));
1353 fn ge(&self, other: &Rc<T>) -> bool {
1358 #[stable(feature = "rust1", since = "1.0.0")]
1359 impl<T: ?Sized + Ord> Ord for Rc<T> {
1360 /// Comparison for two `Rc`s.
1362 /// The two are compared by calling `cmp()` on their inner values.
1367 /// use std::rc::Rc;
1368 /// use std::cmp::Ordering;
1370 /// let five = Rc::new(5);
1372 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1375 fn cmp(&self, other: &Rc<T>) -> Ordering {
1376 (**self).cmp(&**other)
1380 #[stable(feature = "rust1", since = "1.0.0")]
1381 impl<T: ?Sized + Hash> Hash for Rc<T> {
1382 fn hash<H: Hasher>(&self, state: &mut H) {
1383 (**self).hash(state);
1387 #[stable(feature = "rust1", since = "1.0.0")]
1388 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1389 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1390 fmt::Display::fmt(&**self, f)
1394 #[stable(feature = "rust1", since = "1.0.0")]
1395 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1396 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1397 fmt::Debug::fmt(&**self, f)
1401 #[stable(feature = "rust1", since = "1.0.0")]
1402 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1403 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1404 fmt::Pointer::fmt(&(&**self as *const T), f)
1408 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1409 impl<T> From<T> for Rc<T> {
1410 fn from(t: T) -> Self {
1415 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1416 impl<T: Clone> From<&[T]> for Rc<[T]> {
1418 fn from(v: &[T]) -> Rc<[T]> {
1419 <Self as RcFromSlice<T>>::from_slice(v)
1423 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1424 impl From<&str> for Rc<str> {
1426 fn from(v: &str) -> Rc<str> {
1427 let rc = Rc::<[u8]>::from(v.as_bytes());
1428 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1432 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1433 impl From<String> for Rc<str> {
1435 fn from(v: String) -> Rc<str> {
1440 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1441 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1443 fn from(v: Box<T>) -> Rc<T> {
1448 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1449 impl<T> From<Vec<T>> for Rc<[T]> {
1451 fn from(mut v: Vec<T>) -> Rc<[T]> {
1453 let rc = Rc::copy_from_slice(&v);
1455 // Allow the Vec to free its memory, but not destroy its contents
1463 #[unstable(feature = "boxed_slice_try_from", issue = "0")]
1464 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]>
1466 [T; N]: LengthAtMost32,
1468 type Error = Rc<[T]>;
1470 fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
1471 if boxed_slice.len() == N {
1472 Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
1479 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1480 impl<T> iter::FromIterator<T> for Rc<[T]> {
1481 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1483 /// # Performance characteristics
1485 /// ## The general case
1487 /// In the general case, collecting into `Rc<[T]>` is done by first
1488 /// collecting into a `Vec<T>`. That is, when writing the following:
1491 /// # use std::rc::Rc;
1492 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1493 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1496 /// this behaves as if we wrote:
1499 /// # use std::rc::Rc;
1500 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1501 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1502 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1503 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1506 /// This will allocate as many times as needed for constructing the `Vec<T>`
1507 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1509 /// ## Iterators of known length
1511 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1512 /// a single allocation will be made for the `Rc<[T]>`. For example:
1515 /// # use std::rc::Rc;
1516 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1517 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1519 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1520 RcFromIter::from_iter(iter.into_iter())
1524 /// Specialization trait used for collecting into `Rc<[T]>`.
1525 trait RcFromIter<T, I> {
1526 fn from_iter(iter: I) -> Self;
1529 impl<T, I: Iterator<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1530 default fn from_iter(iter: I) -> Self {
1531 iter.collect::<Vec<T>>().into()
1535 impl<T, I: iter::TrustedLen<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1536 default fn from_iter(iter: I) -> Self {
1537 // This is the case for a `TrustedLen` iterator.
1538 let (low, high) = iter.size_hint();
1539 if let Some(high) = high {
1542 "TrustedLen iterator's size hint is not exact: {:?}",
1547 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1548 Rc::from_iter_exact(iter, low)
1551 // Fall back to normal implementation.
1552 iter.collect::<Vec<T>>().into()
1557 impl<'a, T: 'a + Clone> RcFromIter<&'a T, slice::Iter<'a, T>> for Rc<[T]> {
1558 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
1559 // Delegate to `impl<T: Clone> From<&[T]> for Rc<[T]>`.
1561 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
1562 // which is even more performant.
1564 // In the fall-back case we have `T: Clone`. This is still better
1565 // than the `TrustedLen` implementation as slices have a known length
1566 // and so we get to avoid calling `size_hint` and avoid the branching.
1567 iter.as_slice().into()
1571 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1572 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
1573 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1575 /// Since a `Weak` reference does not count towards ownership, it will not
1576 /// prevent the inner value from being dropped, and `Weak` itself makes no
1577 /// guarantees about the value still being present and may return [`None`]
1578 /// when [`upgrade`]d.
1580 /// A `Weak` pointer is useful for keeping a temporary reference to the value
1581 /// within [`Rc`] without extending its lifetime. It is also used to prevent
1582 /// circular references between [`Rc`] pointers, since mutual owning references
1583 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1584 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1585 /// pointers from children back to their parents.
1587 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1589 /// [`Rc`]: struct.Rc.html
1590 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1591 /// [`upgrade`]: struct.Weak.html#method.upgrade
1592 /// [`Option`]: ../../std/option/enum.Option.html
1593 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1594 #[stable(feature = "rc_weak", since = "1.4.0")]
1595 pub struct Weak<T: ?Sized> {
1596 // This is a `NonNull` to allow optimizing the size of this type in enums,
1597 // but it is not necessarily a valid pointer.
1598 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1599 // to allocate space on the heap. That's not a value a real pointer
1600 // will ever have because RcBox has alignment at least 2.
1601 ptr: NonNull<RcBox<T>>,
1604 #[stable(feature = "rc_weak", since = "1.4.0")]
1605 impl<T: ?Sized> !marker::Send for Weak<T> {}
1606 #[stable(feature = "rc_weak", since = "1.4.0")]
1607 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1609 #[unstable(feature = "coerce_unsized", issue = "27732")]
1610 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1612 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
1613 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1616 /// Constructs a new `Weak<T>`, without allocating any memory.
1617 /// Calling [`upgrade`] on the return value always gives [`None`].
1619 /// [`upgrade`]: #method.upgrade
1620 /// [`None`]: ../../std/option/enum.Option.html
1625 /// use std::rc::Weak;
1627 /// let empty: Weak<i64> = Weak::new();
1628 /// assert!(empty.upgrade().is_none());
1630 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1631 pub fn new() -> Weak<T> {
1633 ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"),
1637 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1639 /// It is up to the caller to ensure that the object is still alive when accessing it through
1642 /// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
1647 /// #![feature(weak_into_raw)]
1649 /// use std::rc::Rc;
1652 /// let strong = Rc::new("hello".to_owned());
1653 /// let weak = Rc::downgrade(&strong);
1654 /// // Both point to the same object
1655 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1656 /// // The strong here keeps it alive, so we can still access the object.
1657 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1660 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1661 /// // undefined behaviour.
1662 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1665 /// [`null`]: ../../std/ptr/fn.null.html
1666 #[unstable(feature = "weak_into_raw", issue = "60728")]
1667 pub fn as_raw(&self) -> *const T {
1668 match self.inner() {
1669 None => ptr::null(),
1671 let offset = data_offset_sized::<T>();
1672 let ptr = inner as *const RcBox<T>;
1673 // Note: while the pointer we create may already point to dropped value, the
1674 // allocation still lives (it must hold the weak point as long as we are alive).
1675 // Therefore, the offset is OK to do, it won't get out of the allocation.
1676 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1682 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1684 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1685 /// can be turned back into the `Weak<T>` with [`from_raw`].
1687 /// The same restrictions of accessing the target of the pointer as with
1688 /// [`as_raw`] apply.
1693 /// #![feature(weak_into_raw)]
1695 /// use std::rc::{Rc, Weak};
1697 /// let strong = Rc::new("hello".to_owned());
1698 /// let weak = Rc::downgrade(&strong);
1699 /// let raw = weak.into_raw();
1701 /// assert_eq!(1, Rc::weak_count(&strong));
1702 /// assert_eq!("hello", unsafe { &*raw });
1704 /// drop(unsafe { Weak::from_raw(raw) });
1705 /// assert_eq!(0, Rc::weak_count(&strong));
1708 /// [`from_raw`]: struct.Weak.html#method.from_raw
1709 /// [`as_raw`]: struct.Weak.html#method.as_raw
1710 #[unstable(feature = "weak_into_raw", issue = "60728")]
1711 pub fn into_raw(self) -> *const T {
1712 let result = self.as_raw();
1717 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1719 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1720 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1722 /// It takes ownership of one weak count. In case a [`null`] is passed, a dangling [`Weak`] is
1727 /// The pointer must represent one valid weak count. In other words, it must point to `T` which
1728 /// is or *was* managed by an [`Rc`] and the weak count of that [`Rc`] must not have reached
1729 /// 0. It is allowed for the strong count to be 0.
1734 /// #![feature(weak_into_raw)]
1736 /// use std::rc::{Rc, Weak};
1738 /// let strong = Rc::new("hello".to_owned());
1740 /// let raw_1 = Rc::downgrade(&strong).into_raw();
1741 /// let raw_2 = Rc::downgrade(&strong).into_raw();
1743 /// assert_eq!(2, Rc::weak_count(&strong));
1745 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1746 /// assert_eq!(1, Rc::weak_count(&strong));
1750 /// // Decrement the last weak count.
1751 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1754 /// [`null`]: ../../std/ptr/fn.null.html
1755 /// [`into_raw`]: struct.Weak.html#method.into_raw
1756 /// [`upgrade`]: struct.Weak.html#method.upgrade
1757 /// [`Rc`]: struct.Rc.html
1758 /// [`Weak`]: struct.Weak.html
1759 #[unstable(feature = "weak_into_raw", issue = "60728")]
1760 pub unsafe fn from_raw(ptr: *const T) -> Self {
1764 // See Rc::from_raw for details
1765 let offset = data_offset(ptr);
1766 let fake_ptr = ptr as *mut RcBox<T>;
1767 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1769 ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
1775 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1776 let address = ptr.as_ptr() as *mut () as usize;
1777 address == usize::MAX
1780 impl<T: ?Sized> Weak<T> {
1781 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1782 /// the lifetime of the value if successful.
1784 /// Returns [`None`] if the value has since been dropped.
1786 /// [`Rc`]: struct.Rc.html
1787 /// [`None`]: ../../std/option/enum.Option.html
1792 /// use std::rc::Rc;
1794 /// let five = Rc::new(5);
1796 /// let weak_five = Rc::downgrade(&five);
1798 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1799 /// assert!(strong_five.is_some());
1801 /// // Destroy all strong pointers.
1802 /// drop(strong_five);
1805 /// assert!(weak_five.upgrade().is_none());
1807 #[stable(feature = "rc_weak", since = "1.4.0")]
1808 pub fn upgrade(&self) -> Option<Rc<T>> {
1809 let inner = self.inner()?;
1810 if inner.strong() == 0 {
1814 Some(Rc::from_inner(self.ptr))
1818 /// Gets the number of strong (`Rc`) pointers pointing to this value.
1820 /// If `self` was created using [`Weak::new`], this will return 0.
1822 /// [`Weak::new`]: #method.new
1823 #[unstable(feature = "weak_counts", issue = "57977")]
1824 pub fn strong_count(&self) -> usize {
1825 if let Some(inner) = self.inner() {
1832 /// Gets the number of `Weak` pointers pointing to this value.
1834 /// If `self` was created using [`Weak::new`], this will return `None`. If
1835 /// not, the returned value is at least 1, since `self` still points to the
1838 /// [`Weak::new`]: #method.new
1839 #[unstable(feature = "weak_counts", issue = "57977")]
1840 pub fn weak_count(&self) -> Option<usize> {
1841 self.inner().map(|inner| {
1842 if inner.strong() > 0 {
1843 inner.weak() - 1 // subtract the implicit weak ptr
1850 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1851 /// (i.e., when this `Weak` was created by `Weak::new`).
1853 fn inner(&self) -> Option<&RcBox<T>> {
1854 if is_dangling(self.ptr) {
1857 Some(unsafe { self.ptr.as_ref() })
1861 /// Returns `true` if the two `Weak`s point to the same value (not just values
1862 /// that compare as equal).
1866 /// Since this compares pointers it means that `Weak::new()` will equal each
1867 /// other, even though they don't point to any value.
1872 /// #![feature(weak_ptr_eq)]
1873 /// use std::rc::Rc;
1875 /// let first_rc = Rc::new(5);
1876 /// let first = Rc::downgrade(&first_rc);
1877 /// let second = Rc::downgrade(&first_rc);
1879 /// assert!(first.ptr_eq(&second));
1881 /// let third_rc = Rc::new(5);
1882 /// let third = Rc::downgrade(&third_rc);
1884 /// assert!(!first.ptr_eq(&third));
1887 /// Comparing `Weak::new`.
1890 /// #![feature(weak_ptr_eq)]
1891 /// use std::rc::{Rc, Weak};
1893 /// let first = Weak::new();
1894 /// let second = Weak::new();
1895 /// assert!(first.ptr_eq(&second));
1897 /// let third_rc = Rc::new(());
1898 /// let third = Rc::downgrade(&third_rc);
1899 /// assert!(!first.ptr_eq(&third));
1902 #[unstable(feature = "weak_ptr_eq", issue = "55981")]
1903 pub fn ptr_eq(&self, other: &Self) -> bool {
1904 self.ptr.as_ptr() == other.ptr.as_ptr()
1908 #[stable(feature = "rc_weak", since = "1.4.0")]
1909 impl<T: ?Sized> Drop for Weak<T> {
1910 /// Drops the `Weak` pointer.
1915 /// use std::rc::{Rc, Weak};
1919 /// impl Drop for Foo {
1920 /// fn drop(&mut self) {
1921 /// println!("dropped!");
1925 /// let foo = Rc::new(Foo);
1926 /// let weak_foo = Rc::downgrade(&foo);
1927 /// let other_weak_foo = Weak::clone(&weak_foo);
1929 /// drop(weak_foo); // Doesn't print anything
1930 /// drop(foo); // Prints "dropped!"
1932 /// assert!(other_weak_foo.upgrade().is_none());
1934 fn drop(&mut self) {
1935 if let Some(inner) = self.inner() {
1937 // the weak count starts at 1, and will only go to zero if all
1938 // the strong pointers have disappeared.
1939 if inner.weak() == 0 {
1941 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1948 #[stable(feature = "rc_weak", since = "1.4.0")]
1949 impl<T: ?Sized> Clone for Weak<T> {
1950 /// Makes a clone of the `Weak` pointer that points to the same value.
1955 /// use std::rc::{Rc, Weak};
1957 /// let weak_five = Rc::downgrade(&Rc::new(5));
1959 /// let _ = Weak::clone(&weak_five);
1962 fn clone(&self) -> Weak<T> {
1963 if let Some(inner) = self.inner() {
1966 Weak { ptr: self.ptr }
1970 #[stable(feature = "rc_weak", since = "1.4.0")]
1971 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1972 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1977 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1978 impl<T> Default for Weak<T> {
1979 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1980 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1982 /// [`None`]: ../../std/option/enum.Option.html
1983 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
1988 /// use std::rc::Weak;
1990 /// let empty: Weak<i64> = Default::default();
1991 /// assert!(empty.upgrade().is_none());
1993 fn default() -> Weak<T> {
1998 // NOTE: We checked_add here to deal with mem::forget safely. In particular
1999 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
2000 // you can free the allocation while outstanding Rcs (or Weaks) exist.
2001 // We abort because this is such a degenerate scenario that we don't care about
2002 // what happens -- no real program should ever experience this.
2004 // This should have negligible overhead since you don't actually need to
2005 // clone these much in Rust thanks to ownership and move-semantics.
2008 trait RcBoxPtr<T: ?Sized> {
2009 fn inner(&self) -> &RcBox<T>;
2012 fn strong(&self) -> usize {
2013 self.inner().strong.get()
2017 fn inc_strong(&self) {
2018 let strong = self.strong();
2020 // We want to abort on overflow instead of dropping the value.
2021 // The reference count will never be zero when this is called;
2022 // nevertheless, we insert an abort here to hint LLVM at
2023 // an otherwise missed optimization.
2024 if strong == 0 || strong == usize::max_value() {
2027 self.inner().strong.set(strong + 1);
2031 fn dec_strong(&self) {
2032 self.inner().strong.set(self.strong() - 1);
2036 fn weak(&self) -> usize {
2037 self.inner().weak.get()
2041 fn inc_weak(&self) {
2042 let weak = self.weak();
2044 // We want to abort on overflow instead of dropping the value.
2045 // The reference count will never be zero when this is called;
2046 // nevertheless, we insert an abort here to hint LLVM at
2047 // an otherwise missed optimization.
2048 if weak == 0 || weak == usize::max_value() {
2051 self.inner().weak.set(weak + 1);
2055 fn dec_weak(&self) {
2056 self.inner().weak.set(self.weak() - 1);
2060 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
2062 fn inner(&self) -> &RcBox<T> {
2069 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
2071 fn inner(&self) -> &RcBox<T> {
2076 #[stable(feature = "rust1", since = "1.0.0")]
2077 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2078 fn borrow(&self) -> &T {
2083 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2084 impl<T: ?Sized> AsRef<T> for Rc<T> {
2085 fn as_ref(&self) -> &T {
2090 #[stable(feature = "pin", since = "1.33.0")]
2091 impl<T: ?Sized> Unpin for Rc<T> { }
2093 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2094 // Align the unsized value to the end of the `RcBox`.
2095 // Because it is ?Sized, it will always be the last field in memory.
2096 data_offset_align(align_of_val(&*ptr))
2099 /// Computes the offset of the data field within `RcBox`.
2101 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2102 fn data_offset_sized<T>() -> isize {
2103 data_offset_align(align_of::<T>())
2107 fn data_offset_align(align: usize) -> isize {
2108 let layout = Layout::new::<RcBox<()>>();
2109 (layout.size() + layout.padding_needed_for(align)) as isize