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;
236 use core::cell::Cell;
237 use core::cmp::Ordering;
239 use core::hash::{Hash, Hasher};
240 use core::intrinsics::abort;
242 use core::marker::{self, Unpin, Unsize, PhantomData};
243 use core::mem::{self, align_of, align_of_val, forget, size_of_val};
244 use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn};
246 use core::ptr::{self, NonNull};
247 use core::slice::{self, from_raw_parts_mut};
248 use core::convert::From;
251 use crate::alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
252 use crate::string::String;
255 struct RcBox<T: ?Sized> {
261 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
264 /// See the [module-level documentation](./index.html) for more details.
266 /// The inherent methods of `Rc` are all associated functions, which means
267 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
268 /// `value.get_mut()`. This avoids conflicts with methods of the inner
271 /// [get_mut]: #method.get_mut
272 #[cfg_attr(not(test), lang = "rc")]
273 #[stable(feature = "rust1", since = "1.0.0")]
274 pub struct Rc<T: ?Sized> {
275 ptr: NonNull<RcBox<T>>,
276 phantom: PhantomData<T>,
279 #[stable(feature = "rust1", since = "1.0.0")]
280 impl<T: ?Sized> !marker::Send for Rc<T> {}
281 #[stable(feature = "rust1", since = "1.0.0")]
282 impl<T: ?Sized> !marker::Sync for Rc<T> {}
284 #[unstable(feature = "coerce_unsized", issue = "27732")]
285 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
287 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
288 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
290 impl<T: ?Sized> Rc<T> {
291 fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
294 phantom: PhantomData,
298 unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
299 Self::from_inner(NonNull::new_unchecked(ptr))
304 /// Constructs a new `Rc<T>`.
311 /// let five = Rc::new(5);
313 #[stable(feature = "rust1", since = "1.0.0")]
314 pub fn new(value: T) -> Rc<T> {
315 // There is an implicit weak pointer owned by all the strong
316 // pointers, which ensures that the weak destructor never frees
317 // the allocation while the strong destructor is running, even
318 // if the weak pointer is stored inside the strong one.
319 Self::from_inner(Box::into_raw_non_null(box RcBox {
320 strong: Cell::new(1),
326 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
327 /// `value` will be pinned in memory and unable to be moved.
328 #[stable(feature = "pin", since = "1.33.0")]
329 pub fn pin(value: T) -> Pin<Rc<T>> {
330 unsafe { Pin::new_unchecked(Rc::new(value)) }
333 /// Returns the contained value, if the `Rc` has exactly one strong reference.
335 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
338 /// This will succeed even if there are outstanding weak references.
340 /// [result]: ../../std/result/enum.Result.html
347 /// let x = Rc::new(3);
348 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
350 /// let x = Rc::new(4);
351 /// let _y = Rc::clone(&x);
352 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
355 #[stable(feature = "rc_unique", since = "1.4.0")]
356 pub fn try_unwrap(this: Self) -> Result<T, Self> {
357 if Rc::strong_count(&this) == 1 {
359 let val = ptr::read(&*this); // copy the contained object
361 // Indicate to Weaks that they can't be promoted by decrementing
362 // the strong count, and then remove the implicit "strong weak"
363 // pointer while also handling drop logic by just crafting a
366 let _weak = Weak { ptr: this.ptr };
376 impl<T: ?Sized> Rc<T> {
377 /// Consumes the `Rc`, returning the wrapped pointer.
379 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
380 /// [`Rc::from_raw`][from_raw].
382 /// [from_raw]: struct.Rc.html#method.from_raw
389 /// let x = Rc::new("hello".to_owned());
390 /// let x_ptr = Rc::into_raw(x);
391 /// assert_eq!(unsafe { &*x_ptr }, "hello");
393 #[stable(feature = "rc_raw", since = "1.17.0")]
394 pub fn into_raw(this: Self) -> *const T {
395 let ptr: *const T = &*this;
400 /// Constructs an `Rc` from a raw pointer.
402 /// The raw pointer must have been previously returned by a call to a
403 /// [`Rc::into_raw`][into_raw].
405 /// This function is unsafe because improper use may lead to memory problems. For example, a
406 /// double-free may occur if the function is called twice on the same raw pointer.
408 /// [into_raw]: struct.Rc.html#method.into_raw
415 /// let x = Rc::new("hello".to_owned());
416 /// let x_ptr = Rc::into_raw(x);
419 /// // Convert back to an `Rc` to prevent leak.
420 /// let x = Rc::from_raw(x_ptr);
421 /// assert_eq!(&*x, "hello");
423 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
426 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
428 #[stable(feature = "rc_raw", since = "1.17.0")]
429 pub unsafe fn from_raw(ptr: *const T) -> Self {
430 let offset = data_offset(ptr);
432 // Reverse the offset to find the original RcBox.
433 let fake_ptr = ptr as *mut RcBox<T>;
434 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
436 Self::from_ptr(rc_ptr)
439 /// Consumes the `Rc`, returning the wrapped pointer as `NonNull<T>`.
444 /// #![feature(rc_into_raw_non_null)]
448 /// let x = Rc::new("hello".to_owned());
449 /// let ptr = Rc::into_raw_non_null(x);
450 /// let deref = unsafe { ptr.as_ref() };
451 /// assert_eq!(deref, "hello");
453 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
455 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
456 // safe because Rc guarantees its pointer is non-null
457 unsafe { NonNull::new_unchecked(Rc::into_raw(this) as *mut _) }
460 /// Creates a new [`Weak`][weak] pointer to this value.
462 /// [weak]: struct.Weak.html
469 /// let five = Rc::new(5);
471 /// let weak_five = Rc::downgrade(&five);
473 #[stable(feature = "rc_weak", since = "1.4.0")]
474 pub fn downgrade(this: &Self) -> Weak<T> {
476 // Make sure we do not create a dangling Weak
477 debug_assert!(!is_dangling(this.ptr));
478 Weak { ptr: this.ptr }
481 /// Gets the number of [`Weak`][weak] pointers to this value.
483 /// [weak]: struct.Weak.html
490 /// let five = Rc::new(5);
491 /// let _weak_five = Rc::downgrade(&five);
493 /// assert_eq!(1, Rc::weak_count(&five));
496 #[stable(feature = "rc_counts", since = "1.15.0")]
497 pub fn weak_count(this: &Self) -> usize {
501 /// Gets the number of strong (`Rc`) pointers to this value.
508 /// let five = Rc::new(5);
509 /// let _also_five = Rc::clone(&five);
511 /// assert_eq!(2, Rc::strong_count(&five));
514 #[stable(feature = "rc_counts", since = "1.15.0")]
515 pub fn strong_count(this: &Self) -> usize {
519 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
520 /// this inner value.
522 /// [weak]: struct.Weak.html
524 fn is_unique(this: &Self) -> bool {
525 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
528 /// Returns a mutable reference to the inner value, if there are
529 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
531 /// Returns [`None`] otherwise, because it is not safe to
532 /// mutate a shared value.
534 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
535 /// the inner value when it's shared.
537 /// [weak]: struct.Weak.html
538 /// [`None`]: ../../std/option/enum.Option.html#variant.None
539 /// [make_mut]: struct.Rc.html#method.make_mut
540 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
547 /// let mut x = Rc::new(3);
548 /// *Rc::get_mut(&mut x).unwrap() = 4;
549 /// assert_eq!(*x, 4);
551 /// let _y = Rc::clone(&x);
552 /// assert!(Rc::get_mut(&mut x).is_none());
555 #[stable(feature = "rc_unique", since = "1.4.0")]
556 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
557 if Rc::is_unique(this) {
559 Some(&mut this.ptr.as_mut().value)
567 #[stable(feature = "ptr_eq", since = "1.17.0")]
568 /// Returns `true` if the two `Rc`s point to the same value (not
569 /// just values that compare as equal).
576 /// let five = Rc::new(5);
577 /// let same_five = Rc::clone(&five);
578 /// let other_five = Rc::new(5);
580 /// assert!(Rc::ptr_eq(&five, &same_five));
581 /// assert!(!Rc::ptr_eq(&five, &other_five));
583 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
584 this.ptr.as_ptr() == other.ptr.as_ptr()
588 impl<T: Clone> Rc<T> {
589 /// Makes a mutable reference into the given `Rc`.
591 /// If there are other `Rc` pointers to the same value, then `make_mut` will
592 /// [`clone`] the inner value to ensure unique ownership. This is also
593 /// referred to as clone-on-write.
595 /// If there are no other `Rc` pointers to this value, then [`Weak`]
596 /// pointers to this value will be dissassociated.
598 /// See also [`get_mut`], which will fail rather than cloning.
600 /// [`Weak`]: struct.Weak.html
601 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
602 /// [`get_mut`]: struct.Rc.html#method.get_mut
609 /// let mut data = Rc::new(5);
611 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
612 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
613 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
614 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
615 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
617 /// // Now `data` and `other_data` point to different values.
618 /// assert_eq!(*data, 8);
619 /// assert_eq!(*other_data, 12);
622 /// [`Weak`] pointers will be dissassociated:
627 /// let mut data = Rc::new(75);
628 /// let weak = Rc::downgrade(&data);
630 /// assert!(75 == *data);
631 /// assert!(75 == *weak.upgrade().unwrap());
633 /// *Rc::make_mut(&mut data) += 1;
635 /// assert!(76 == *data);
636 /// assert!(weak.upgrade().is_none());
639 #[stable(feature = "rc_unique", since = "1.4.0")]
640 pub fn make_mut(this: &mut Self) -> &mut T {
641 if Rc::strong_count(this) != 1 {
642 // Gotta clone the data, there are other Rcs
643 *this = Rc::new((**this).clone())
644 } else if Rc::weak_count(this) != 0 {
645 // Can just steal the data, all that's left is Weaks
647 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
648 mem::swap(this, &mut swap);
650 // Remove implicit strong-weak ref (no need to craft a fake
651 // Weak here -- we know other Weaks can clean up for us)
656 // This unsafety is ok because we're guaranteed that the pointer
657 // returned is the *only* pointer that will ever be returned to T. Our
658 // reference count is guaranteed to be 1 at this point, and we required
659 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
660 // reference to the inner value.
662 &mut this.ptr.as_mut().value
669 #[stable(feature = "rc_downcast", since = "1.29.0")]
670 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
675 /// use std::any::Any;
678 /// fn print_if_string(value: Rc<dyn Any>) {
679 /// if let Ok(string) = value.downcast::<String>() {
680 /// println!("String ({}): {}", string.len(), string);
685 /// let my_string = "Hello World".to_string();
686 /// print_if_string(Rc::new(my_string));
687 /// print_if_string(Rc::new(0i8));
690 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
691 if (*self).is::<T>() {
692 let ptr = self.ptr.cast::<RcBox<T>>();
694 Ok(Rc::from_inner(ptr))
701 impl<T: ?Sized> Rc<T> {
702 /// Allocates an `RcBox<T>` with sufficient space for
703 /// an unsized value where the value has the layout provided.
705 /// The function `mem_to_rcbox` is called with the data pointer
706 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
707 unsafe fn allocate_for_unsized(
708 value_layout: Layout,
709 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>
711 // Calculate layout using the given value layout.
712 // Previously, layout was calculated on the expression
713 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
714 // reference (see #54908).
715 let layout = Layout::new::<RcBox<()>>()
716 .extend(value_layout).unwrap().0
717 .pad_to_align().unwrap();
719 // Allocate for the layout.
720 let mem = Global.alloc(layout)
721 .unwrap_or_else(|_| handle_alloc_error(layout));
723 // Initialize the RcBox
724 let inner = mem_to_rcbox(mem.as_ptr());
725 debug_assert_eq!(Layout::for_value(&*inner), layout);
727 ptr::write(&mut (*inner).strong, Cell::new(1));
728 ptr::write(&mut (*inner).weak, Cell::new(1));
733 /// Allocates an `RcBox<T>` with sufficient space for an unsized value
734 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
735 // Allocate for the `RcBox<T>` using the given value.
736 Self::allocate_for_unsized(
737 Layout::for_value(&*ptr),
738 |mem| set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>,
742 fn from_box(v: Box<T>) -> Rc<T> {
744 let box_unique = Box::into_unique(v);
745 let bptr = box_unique.as_ptr();
747 let value_size = size_of_val(&*bptr);
748 let ptr = Self::allocate_for_ptr(bptr);
750 // Copy value as bytes
751 ptr::copy_nonoverlapping(
752 bptr as *const T as *const u8,
753 &mut (*ptr).value as *mut _ as *mut u8,
756 // Free the allocation without dropping its contents
757 box_free(box_unique);
765 /// Allocates an `RcBox<[T]>` with the given length.
766 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
767 Self::allocate_for_unsized(
768 Layout::array::<T>(len).unwrap(),
769 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>,
774 /// Sets the data pointer of a `?Sized` raw pointer.
776 /// For a slice/trait object, this sets the `data` field and leaves the rest
777 /// unchanged. For a sized raw pointer, this simply sets the pointer.
778 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
779 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
784 /// Copy elements from slice into newly allocated Rc<[T]>
786 /// Unsafe because the caller must either take ownership or bind `T: Copy`
787 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
788 let ptr = Self::allocate_for_slice(v.len());
790 ptr::copy_nonoverlapping(
792 &mut (*ptr).value as *mut [T] as *mut T,
798 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
800 /// Behavior is undefined should the size be wrong.
801 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
802 // Panic guard while cloning T elements.
803 // In the event of a panic, elements that have been written
804 // into the new RcBox will be dropped, then the memory freed.
812 impl<T> Drop for Guard<T> {
815 let slice = from_raw_parts_mut(self.elems, self.n_elems);
816 ptr::drop_in_place(slice);
818 Global.dealloc(self.mem, self.layout);
823 let ptr = Self::allocate_for_slice(len);
825 let mem = ptr as *mut _ as *mut u8;
826 let layout = Layout::for_value(&*ptr);
828 // Pointer to first element
829 let elems = &mut (*ptr).value as *mut [T] as *mut T;
831 let mut guard = Guard {
832 mem: NonNull::new_unchecked(mem),
838 for (i, item) in iter.enumerate() {
839 ptr::write(elems.add(i), item);
843 // All clear. Forget the guard so it doesn't free the new RcBox.
850 /// Specialization trait used for `From<&[T]>`.
851 trait RcFromSlice<T> {
852 fn from_slice(slice: &[T]) -> Self;
855 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
857 default fn from_slice(v: &[T]) -> Self {
859 Self::from_iter_exact(v.iter().cloned(), v.len())
864 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
866 fn from_slice(v: &[T]) -> Self {
867 unsafe { Rc::copy_from_slice(v) }
871 #[stable(feature = "rust1", since = "1.0.0")]
872 impl<T: ?Sized> Deref for Rc<T> {
876 fn deref(&self) -> &T {
881 #[unstable(feature = "receiver_trait", issue = "0")]
882 impl<T: ?Sized> Receiver for Rc<T> {}
884 #[stable(feature = "rust1", since = "1.0.0")]
885 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
888 /// This will decrement the strong reference count. If the strong reference
889 /// count reaches zero then the only other references (if any) are
890 /// [`Weak`], so we `drop` the inner value.
899 /// impl Drop for Foo {
900 /// fn drop(&mut self) {
901 /// println!("dropped!");
905 /// let foo = Rc::new(Foo);
906 /// let foo2 = Rc::clone(&foo);
908 /// drop(foo); // Doesn't print anything
909 /// drop(foo2); // Prints "dropped!"
912 /// [`Weak`]: ../../std/rc/struct.Weak.html
916 if self.strong() == 0 {
917 // destroy the contained object
918 ptr::drop_in_place(self.ptr.as_mut());
920 // remove the implicit "strong weak" pointer now that we've
921 // destroyed the contents.
924 if self.weak() == 0 {
925 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
932 #[stable(feature = "rust1", since = "1.0.0")]
933 impl<T: ?Sized> Clone for Rc<T> {
934 /// Makes a clone of the `Rc` pointer.
936 /// This creates another pointer to the same inner value, increasing the
937 /// strong reference count.
944 /// let five = Rc::new(5);
946 /// let _ = Rc::clone(&five);
949 fn clone(&self) -> Rc<T> {
951 Self::from_inner(self.ptr)
955 #[stable(feature = "rust1", since = "1.0.0")]
956 impl<T: Default> Default for Rc<T> {
957 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
964 /// let x: Rc<i32> = Default::default();
965 /// assert_eq!(*x, 0);
968 fn default() -> Rc<T> {
969 Rc::new(Default::default())
973 #[stable(feature = "rust1", since = "1.0.0")]
974 trait RcEqIdent<T: ?Sized + PartialEq> {
975 fn eq(&self, other: &Rc<T>) -> bool;
976 fn ne(&self, other: &Rc<T>) -> bool;
979 #[stable(feature = "rust1", since = "1.0.0")]
980 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
982 default fn eq(&self, other: &Rc<T>) -> bool {
987 default fn ne(&self, other: &Rc<T>) -> bool {
992 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
993 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
994 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
995 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
996 /// the same value, than two `&T`s.
997 #[stable(feature = "rust1", since = "1.0.0")]
998 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
1000 fn eq(&self, other: &Rc<T>) -> bool {
1001 Rc::ptr_eq(self, other) || **self == **other
1005 fn ne(&self, other: &Rc<T>) -> bool {
1006 !Rc::ptr_eq(self, other) && **self != **other
1010 #[stable(feature = "rust1", since = "1.0.0")]
1011 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1012 /// Equality for two `Rc`s.
1014 /// Two `Rc`s are equal if their inner values are equal.
1016 /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
1022 /// use std::rc::Rc;
1024 /// let five = Rc::new(5);
1026 /// assert!(five == Rc::new(5));
1029 fn eq(&self, other: &Rc<T>) -> bool {
1030 RcEqIdent::eq(self, other)
1033 /// Inequality for two `Rc`s.
1035 /// Two `Rc`s are unequal if their inner values are unequal.
1037 /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
1043 /// use std::rc::Rc;
1045 /// let five = Rc::new(5);
1047 /// assert!(five != Rc::new(6));
1050 fn ne(&self, other: &Rc<T>) -> bool {
1051 RcEqIdent::ne(self, other)
1055 #[stable(feature = "rust1", since = "1.0.0")]
1056 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1058 #[stable(feature = "rust1", since = "1.0.0")]
1059 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1060 /// Partial comparison for two `Rc`s.
1062 /// The two are compared by calling `partial_cmp()` on their inner values.
1067 /// use std::rc::Rc;
1068 /// use std::cmp::Ordering;
1070 /// let five = Rc::new(5);
1072 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1075 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1076 (**self).partial_cmp(&**other)
1079 /// Less-than comparison for two `Rc`s.
1081 /// The two are compared by calling `<` on their inner values.
1086 /// use std::rc::Rc;
1088 /// let five = Rc::new(5);
1090 /// assert!(five < Rc::new(6));
1093 fn lt(&self, other: &Rc<T>) -> bool {
1097 /// 'Less than or equal to' comparison for two `Rc`s.
1099 /// The two are compared by calling `<=` on their inner values.
1104 /// use std::rc::Rc;
1106 /// let five = Rc::new(5);
1108 /// assert!(five <= Rc::new(5));
1111 fn le(&self, other: &Rc<T>) -> bool {
1115 /// Greater-than comparison for two `Rc`s.
1117 /// The two are compared by calling `>` on their inner values.
1122 /// use std::rc::Rc;
1124 /// let five = Rc::new(5);
1126 /// assert!(five > Rc::new(4));
1129 fn gt(&self, other: &Rc<T>) -> bool {
1133 /// 'Greater than or equal to' comparison for two `Rc`s.
1135 /// The two are compared by calling `>=` on their inner values.
1140 /// use std::rc::Rc;
1142 /// let five = Rc::new(5);
1144 /// assert!(five >= Rc::new(5));
1147 fn ge(&self, other: &Rc<T>) -> bool {
1152 #[stable(feature = "rust1", since = "1.0.0")]
1153 impl<T: ?Sized + Ord> Ord for Rc<T> {
1154 /// Comparison for two `Rc`s.
1156 /// The two are compared by calling `cmp()` on their inner values.
1161 /// use std::rc::Rc;
1162 /// use std::cmp::Ordering;
1164 /// let five = Rc::new(5);
1166 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1169 fn cmp(&self, other: &Rc<T>) -> Ordering {
1170 (**self).cmp(&**other)
1174 #[stable(feature = "rust1", since = "1.0.0")]
1175 impl<T: ?Sized + Hash> Hash for Rc<T> {
1176 fn hash<H: Hasher>(&self, state: &mut H) {
1177 (**self).hash(state);
1181 #[stable(feature = "rust1", since = "1.0.0")]
1182 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1183 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1184 fmt::Display::fmt(&**self, f)
1188 #[stable(feature = "rust1", since = "1.0.0")]
1189 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1190 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1191 fmt::Debug::fmt(&**self, f)
1195 #[stable(feature = "rust1", since = "1.0.0")]
1196 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1197 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1198 fmt::Pointer::fmt(&(&**self as *const T), f)
1202 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1203 impl<T> From<T> for Rc<T> {
1204 fn from(t: T) -> Self {
1209 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1210 impl<T: Clone> From<&[T]> for Rc<[T]> {
1212 fn from(v: &[T]) -> Rc<[T]> {
1213 <Self as RcFromSlice<T>>::from_slice(v)
1217 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1218 impl From<&str> for Rc<str> {
1220 fn from(v: &str) -> Rc<str> {
1221 let rc = Rc::<[u8]>::from(v.as_bytes());
1222 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1226 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1227 impl From<String> for Rc<str> {
1229 fn from(v: String) -> Rc<str> {
1234 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1235 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1237 fn from(v: Box<T>) -> Rc<T> {
1242 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1243 impl<T> From<Vec<T>> for Rc<[T]> {
1245 fn from(mut v: Vec<T>) -> Rc<[T]> {
1247 let rc = Rc::copy_from_slice(&v);
1249 // Allow the Vec to free its memory, but not destroy its contents
1257 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1258 impl<T> iter::FromIterator<T> for Rc<[T]> {
1259 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1261 /// # Performance characteristics
1263 /// ## The general case
1265 /// In the general case, collecting into `Rc<[T]>` is done by first
1266 /// collecting into a `Vec<T>`. That is, when writing the following:
1269 /// # use std::rc::Rc;
1270 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1271 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1274 /// this behaves as if we wrote:
1277 /// # use std::rc::Rc;
1278 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1279 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1280 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1281 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1284 /// This will allocate as many times as needed for constructing the `Vec<T>`
1285 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1287 /// ## Iterators of known length
1289 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1290 /// a single allocation will be made for the `Rc<[T]>`. For example:
1293 /// # use std::rc::Rc;
1294 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1295 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1297 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1298 RcFromIter::from_iter(iter.into_iter())
1302 /// Specialization trait used for collecting into `Rc<[T]>`.
1303 trait RcFromIter<T, I> {
1304 fn from_iter(iter: I) -> Self;
1307 impl<T, I: Iterator<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1308 default fn from_iter(iter: I) -> Self {
1309 iter.collect::<Vec<T>>().into()
1313 impl<T, I: iter::TrustedLen<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1314 default fn from_iter(iter: I) -> Self {
1315 // This is the case for a `TrustedLen` iterator.
1316 let (low, high) = iter.size_hint();
1317 if let Some(high) = high {
1320 "TrustedLen iterator's size hint is not exact: {:?}",
1325 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1326 Rc::from_iter_exact(iter, low)
1329 // Fall back to normal implementation.
1330 iter.collect::<Vec<T>>().into()
1335 impl<'a, T: 'a + Clone> RcFromIter<&'a T, slice::Iter<'a, T>> for Rc<[T]> {
1336 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
1337 // Delegate to `impl<T: Clone> From<&[T]> for Rc<[T]>`.
1339 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
1340 // which is even more performant.
1342 // In the fall-back case we have `T: Clone`. This is still better
1343 // than the `TrustedLen` implementation as slices have a known length
1344 // and so we get to avoid calling `size_hint` and avoid the branching.
1345 iter.as_slice().into()
1349 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1350 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
1351 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1353 /// Since a `Weak` reference does not count towards ownership, it will not
1354 /// prevent the inner value from being dropped, and `Weak` itself makes no
1355 /// guarantees about the value still being present and may return [`None`]
1356 /// when [`upgrade`]d.
1358 /// A `Weak` pointer is useful for keeping a temporary reference to the value
1359 /// within [`Rc`] without extending its lifetime. It is also used to prevent
1360 /// circular references between [`Rc`] pointers, since mutual owning references
1361 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1362 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1363 /// pointers from children back to their parents.
1365 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1367 /// [`Rc`]: struct.Rc.html
1368 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1369 /// [`upgrade`]: struct.Weak.html#method.upgrade
1370 /// [`Option`]: ../../std/option/enum.Option.html
1371 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1372 #[stable(feature = "rc_weak", since = "1.4.0")]
1373 pub struct Weak<T: ?Sized> {
1374 // This is a `NonNull` to allow optimizing the size of this type in enums,
1375 // but it is not necessarily a valid pointer.
1376 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1377 // to allocate space on the heap. That's not a value a real pointer
1378 // will ever have because RcBox has alignment at least 2.
1379 ptr: NonNull<RcBox<T>>,
1382 #[stable(feature = "rc_weak", since = "1.4.0")]
1383 impl<T: ?Sized> !marker::Send for Weak<T> {}
1384 #[stable(feature = "rc_weak", since = "1.4.0")]
1385 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1387 #[unstable(feature = "coerce_unsized", issue = "27732")]
1388 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1390 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
1391 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1394 /// Constructs a new `Weak<T>`, without allocating any memory.
1395 /// Calling [`upgrade`] on the return value always gives [`None`].
1397 /// [`upgrade`]: #method.upgrade
1398 /// [`None`]: ../../std/option/enum.Option.html
1403 /// use std::rc::Weak;
1405 /// let empty: Weak<i64> = Weak::new();
1406 /// assert!(empty.upgrade().is_none());
1408 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1409 pub fn new() -> Weak<T> {
1411 ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"),
1415 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1417 /// It is up to the caller to ensure that the object is still alive when accessing it through
1420 /// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
1425 /// #![feature(weak_into_raw)]
1427 /// use std::rc::Rc;
1430 /// let strong = Rc::new("hello".to_owned());
1431 /// let weak = Rc::downgrade(&strong);
1432 /// // Both point to the same object
1433 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1434 /// // The strong here keeps it alive, so we can still access the object.
1435 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1438 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1439 /// // undefined behaviour.
1440 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1443 /// [`null`]: ../../std/ptr/fn.null.html
1444 #[unstable(feature = "weak_into_raw", issue = "60728")]
1445 pub fn as_raw(&self) -> *const T {
1446 match self.inner() {
1447 None => ptr::null(),
1449 let offset = data_offset_sized::<T>();
1450 let ptr = inner as *const RcBox<T>;
1451 // Note: while the pointer we create may already point to dropped value, the
1452 // allocation still lives (it must hold the weak point as long as we are alive).
1453 // Therefore, the offset is OK to do, it won't get out of the allocation.
1454 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1460 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1462 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1463 /// can be turned back into the `Weak<T>` with [`from_raw`].
1465 /// The same restrictions of accessing the target of the pointer as with
1466 /// [`as_raw`] apply.
1471 /// #![feature(weak_into_raw)]
1473 /// use std::rc::{Rc, Weak};
1475 /// let strong = Rc::new("hello".to_owned());
1476 /// let weak = Rc::downgrade(&strong);
1477 /// let raw = weak.into_raw();
1479 /// assert_eq!(1, Rc::weak_count(&strong));
1480 /// assert_eq!("hello", unsafe { &*raw });
1482 /// drop(unsafe { Weak::from_raw(raw) });
1483 /// assert_eq!(0, Rc::weak_count(&strong));
1486 /// [`from_raw`]: struct.Weak.html#method.from_raw
1487 /// [`as_raw`]: struct.Weak.html#method.as_raw
1488 #[unstable(feature = "weak_into_raw", issue = "60728")]
1489 pub fn into_raw(self) -> *const T {
1490 let result = self.as_raw();
1495 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1497 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1498 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1500 /// It takes ownership of one weak count. In case a [`null`] is passed, a dangling [`Weak`] is
1505 /// The pointer must represent one valid weak count. In other words, it must point to `T` which
1506 /// is or *was* managed by an [`Rc`] and the weak count of that [`Rc`] must not have reached
1507 /// 0. It is allowed for the strong count to be 0.
1512 /// #![feature(weak_into_raw)]
1514 /// use std::rc::{Rc, Weak};
1516 /// let strong = Rc::new("hello".to_owned());
1518 /// let raw_1 = Rc::downgrade(&strong).into_raw();
1519 /// let raw_2 = Rc::downgrade(&strong).into_raw();
1521 /// assert_eq!(2, Rc::weak_count(&strong));
1523 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1524 /// assert_eq!(1, Rc::weak_count(&strong));
1528 /// // Decrement the last weak count.
1529 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1532 /// [`null`]: ../../std/ptr/fn.null.html
1533 /// [`into_raw`]: struct.Weak.html#method.into_raw
1534 /// [`upgrade`]: struct.Weak.html#method.upgrade
1535 /// [`Rc`]: struct.Rc.html
1536 /// [`Weak`]: struct.Weak.html
1537 #[unstable(feature = "weak_into_raw", issue = "60728")]
1538 pub unsafe fn from_raw(ptr: *const T) -> Self {
1542 // See Rc::from_raw for details
1543 let offset = data_offset(ptr);
1544 let fake_ptr = ptr as *mut RcBox<T>;
1545 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1547 ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
1553 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1554 let address = ptr.as_ptr() as *mut () as usize;
1555 address == usize::MAX
1558 impl<T: ?Sized> Weak<T> {
1559 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1560 /// the lifetime of the value if successful.
1562 /// Returns [`None`] if the value has since been dropped.
1564 /// [`Rc`]: struct.Rc.html
1565 /// [`None`]: ../../std/option/enum.Option.html
1570 /// use std::rc::Rc;
1572 /// let five = Rc::new(5);
1574 /// let weak_five = Rc::downgrade(&five);
1576 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1577 /// assert!(strong_five.is_some());
1579 /// // Destroy all strong pointers.
1580 /// drop(strong_five);
1583 /// assert!(weak_five.upgrade().is_none());
1585 #[stable(feature = "rc_weak", since = "1.4.0")]
1586 pub fn upgrade(&self) -> Option<Rc<T>> {
1587 let inner = self.inner()?;
1588 if inner.strong() == 0 {
1592 Some(Rc::from_inner(self.ptr))
1596 /// Gets the number of strong (`Rc`) pointers pointing to this value.
1598 /// If `self` was created using [`Weak::new`], this will return 0.
1600 /// [`Weak::new`]: #method.new
1601 #[unstable(feature = "weak_counts", issue = "57977")]
1602 pub fn strong_count(&self) -> usize {
1603 if let Some(inner) = self.inner() {
1610 /// Gets the number of `Weak` pointers pointing to this value.
1612 /// If `self` was created using [`Weak::new`], this will return `None`. If
1613 /// not, the returned value is at least 1, since `self` still points to the
1616 /// [`Weak::new`]: #method.new
1617 #[unstable(feature = "weak_counts", issue = "57977")]
1618 pub fn weak_count(&self) -> Option<usize> {
1619 self.inner().map(|inner| {
1620 if inner.strong() > 0 {
1621 inner.weak() - 1 // subtract the implicit weak ptr
1628 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1629 /// (i.e., when this `Weak` was created by `Weak::new`).
1631 fn inner(&self) -> Option<&RcBox<T>> {
1632 if is_dangling(self.ptr) {
1635 Some(unsafe { self.ptr.as_ref() })
1639 /// Returns `true` if the two `Weak`s point to the same value (not just values
1640 /// that compare as equal).
1644 /// Since this compares pointers it means that `Weak::new()` will equal each
1645 /// other, even though they don't point to any value.
1650 /// #![feature(weak_ptr_eq)]
1651 /// use std::rc::Rc;
1653 /// let first_rc = Rc::new(5);
1654 /// let first = Rc::downgrade(&first_rc);
1655 /// let second = Rc::downgrade(&first_rc);
1657 /// assert!(first.ptr_eq(&second));
1659 /// let third_rc = Rc::new(5);
1660 /// let third = Rc::downgrade(&third_rc);
1662 /// assert!(!first.ptr_eq(&third));
1665 /// Comparing `Weak::new`.
1668 /// #![feature(weak_ptr_eq)]
1669 /// use std::rc::{Rc, Weak};
1671 /// let first = Weak::new();
1672 /// let second = Weak::new();
1673 /// assert!(first.ptr_eq(&second));
1675 /// let third_rc = Rc::new(());
1676 /// let third = Rc::downgrade(&third_rc);
1677 /// assert!(!first.ptr_eq(&third));
1680 #[unstable(feature = "weak_ptr_eq", issue = "55981")]
1681 pub fn ptr_eq(&self, other: &Self) -> bool {
1682 self.ptr.as_ptr() == other.ptr.as_ptr()
1686 #[stable(feature = "rc_weak", since = "1.4.0")]
1687 impl<T: ?Sized> Drop for Weak<T> {
1688 /// Drops the `Weak` pointer.
1693 /// use std::rc::{Rc, Weak};
1697 /// impl Drop for Foo {
1698 /// fn drop(&mut self) {
1699 /// println!("dropped!");
1703 /// let foo = Rc::new(Foo);
1704 /// let weak_foo = Rc::downgrade(&foo);
1705 /// let other_weak_foo = Weak::clone(&weak_foo);
1707 /// drop(weak_foo); // Doesn't print anything
1708 /// drop(foo); // Prints "dropped!"
1710 /// assert!(other_weak_foo.upgrade().is_none());
1712 fn drop(&mut self) {
1713 if let Some(inner) = self.inner() {
1715 // the weak count starts at 1, and will only go to zero if all
1716 // the strong pointers have disappeared.
1717 if inner.weak() == 0 {
1719 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1726 #[stable(feature = "rc_weak", since = "1.4.0")]
1727 impl<T: ?Sized> Clone for Weak<T> {
1728 /// Makes a clone of the `Weak` pointer that points to the same value.
1733 /// use std::rc::{Rc, Weak};
1735 /// let weak_five = Rc::downgrade(&Rc::new(5));
1737 /// let _ = Weak::clone(&weak_five);
1740 fn clone(&self) -> Weak<T> {
1741 if let Some(inner) = self.inner() {
1744 Weak { ptr: self.ptr }
1748 #[stable(feature = "rc_weak", since = "1.4.0")]
1749 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1750 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1755 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1756 impl<T> Default for Weak<T> {
1757 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1758 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1760 /// [`None`]: ../../std/option/enum.Option.html
1761 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
1766 /// use std::rc::Weak;
1768 /// let empty: Weak<i64> = Default::default();
1769 /// assert!(empty.upgrade().is_none());
1771 fn default() -> Weak<T> {
1776 // NOTE: We checked_add here to deal with mem::forget safely. In particular
1777 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1778 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1779 // We abort because this is such a degenerate scenario that we don't care about
1780 // what happens -- no real program should ever experience this.
1782 // This should have negligible overhead since you don't actually need to
1783 // clone these much in Rust thanks to ownership and move-semantics.
1786 trait RcBoxPtr<T: ?Sized> {
1787 fn inner(&self) -> &RcBox<T>;
1790 fn strong(&self) -> usize {
1791 self.inner().strong.get()
1795 fn inc_strong(&self) {
1796 let strong = self.strong();
1798 // We want to abort on overflow instead of dropping the value.
1799 // The reference count will never be zero when this is called;
1800 // nevertheless, we insert an abort here to hint LLVM at
1801 // an otherwise missed optimization.
1802 if strong == 0 || strong == usize::max_value() {
1805 self.inner().strong.set(strong + 1);
1809 fn dec_strong(&self) {
1810 self.inner().strong.set(self.strong() - 1);
1814 fn weak(&self) -> usize {
1815 self.inner().weak.get()
1819 fn inc_weak(&self) {
1820 let weak = self.weak();
1822 // We want to abort on overflow instead of dropping the value.
1823 // The reference count will never be zero when this is called;
1824 // nevertheless, we insert an abort here to hint LLVM at
1825 // an otherwise missed optimization.
1826 if weak == 0 || weak == usize::max_value() {
1829 self.inner().weak.set(weak + 1);
1833 fn dec_weak(&self) {
1834 self.inner().weak.set(self.weak() - 1);
1838 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1840 fn inner(&self) -> &RcBox<T> {
1847 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
1849 fn inner(&self) -> &RcBox<T> {
1856 use super::{Rc, Weak};
1857 use std::boxed::Box;
1858 use std::cell::RefCell;
1859 use std::option::Option::{self, None, Some};
1860 use std::result::Result::{Err, Ok};
1862 use std::clone::Clone;
1863 use std::convert::From;
1867 let x = Rc::new(RefCell::new(5));
1869 *x.borrow_mut() = 20;
1870 assert_eq!(*y.borrow(), 20);
1880 fn test_simple_clone() {
1888 fn test_destructor() {
1889 let x: Rc<Box<_>> = Rc::new(box 5);
1896 let y = Rc::downgrade(&x);
1897 assert!(y.upgrade().is_some());
1903 let y = Rc::downgrade(&x);
1905 assert!(y.upgrade().is_none());
1909 fn weak_self_cyclic() {
1911 x: RefCell<Option<Weak<Cycle>>>,
1914 let a = Rc::new(Cycle { x: RefCell::new(None) });
1915 let b = Rc::downgrade(&a.clone());
1916 *a.x.borrow_mut() = Some(b);
1918 // hopefully we don't double-free (or leak)...
1924 assert!(Rc::is_unique(&x));
1926 assert!(!Rc::is_unique(&x));
1928 assert!(Rc::is_unique(&x));
1929 let w = Rc::downgrade(&x);
1930 assert!(!Rc::is_unique(&x));
1932 assert!(Rc::is_unique(&x));
1936 fn test_strong_count() {
1938 assert!(Rc::strong_count(&a) == 1);
1939 let w = Rc::downgrade(&a);
1940 assert!(Rc::strong_count(&a) == 1);
1941 let b = w.upgrade().expect("upgrade of live rc failed");
1942 assert!(Rc::strong_count(&b) == 2);
1943 assert!(Rc::strong_count(&a) == 2);
1946 assert!(Rc::strong_count(&b) == 1);
1948 assert!(Rc::strong_count(&b) == 2);
1949 assert!(Rc::strong_count(&c) == 2);
1953 fn test_weak_count() {
1955 assert!(Rc::strong_count(&a) == 1);
1956 assert!(Rc::weak_count(&a) == 0);
1957 let w = Rc::downgrade(&a);
1958 assert!(Rc::strong_count(&a) == 1);
1959 assert!(Rc::weak_count(&a) == 1);
1961 assert!(Rc::strong_count(&a) == 1);
1962 assert!(Rc::weak_count(&a) == 0);
1964 assert!(Rc::strong_count(&a) == 2);
1965 assert!(Rc::weak_count(&a) == 0);
1971 assert_eq!(Weak::weak_count(&Weak::<u64>::new()), None);
1972 assert_eq!(Weak::strong_count(&Weak::<u64>::new()), 0);
1975 let w = Rc::downgrade(&a);
1976 assert_eq!(Weak::strong_count(&w), 1);
1977 assert_eq!(Weak::weak_count(&w), Some(1));
1979 assert_eq!(Weak::strong_count(&w), 1);
1980 assert_eq!(Weak::weak_count(&w), Some(2));
1981 assert_eq!(Weak::strong_count(&w2), 1);
1982 assert_eq!(Weak::weak_count(&w2), Some(2));
1984 assert_eq!(Weak::strong_count(&w2), 1);
1985 assert_eq!(Weak::weak_count(&w2), Some(1));
1987 assert_eq!(Weak::strong_count(&w2), 2);
1988 assert_eq!(Weak::weak_count(&w2), Some(1));
1991 assert_eq!(Weak::strong_count(&w2), 0);
1992 assert_eq!(Weak::weak_count(&w2), Some(1));
1999 assert_eq!(Rc::try_unwrap(x), Ok(3));
2002 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
2004 let _w = Rc::downgrade(&x);
2005 assert_eq!(Rc::try_unwrap(x), Ok(5));
2009 fn into_from_raw() {
2010 let x = Rc::new(box "hello");
2013 let x_ptr = Rc::into_raw(x);
2016 assert_eq!(**x_ptr, "hello");
2018 let x = Rc::from_raw(x_ptr);
2019 assert_eq!(**x, "hello");
2021 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
2026 fn test_into_from_raw_unsized() {
2027 use std::fmt::Display;
2028 use std::string::ToString;
2030 let rc: Rc<str> = Rc::from("foo");
2032 let ptr = Rc::into_raw(rc.clone());
2033 let rc2 = unsafe { Rc::from_raw(ptr) };
2035 assert_eq!(unsafe { &*ptr }, "foo");
2036 assert_eq!(rc, rc2);
2038 let rc: Rc<dyn Display> = Rc::new(123);
2040 let ptr = Rc::into_raw(rc.clone());
2041 let rc2 = unsafe { Rc::from_raw(ptr) };
2043 assert_eq!(unsafe { &*ptr }.to_string(), "123");
2044 assert_eq!(rc2.to_string(), "123");
2049 let mut x = Rc::new(3);
2050 *Rc::get_mut(&mut x).unwrap() = 4;
2053 assert!(Rc::get_mut(&mut x).is_none());
2055 assert!(Rc::get_mut(&mut x).is_some());
2056 let _w = Rc::downgrade(&x);
2057 assert!(Rc::get_mut(&mut x).is_none());
2061 fn test_cowrc_clone_make_unique() {
2062 let mut cow0 = Rc::new(75);
2063 let mut cow1 = cow0.clone();
2064 let mut cow2 = cow1.clone();
2066 assert!(75 == *Rc::make_mut(&mut cow0));
2067 assert!(75 == *Rc::make_mut(&mut cow1));
2068 assert!(75 == *Rc::make_mut(&mut cow2));
2070 *Rc::make_mut(&mut cow0) += 1;
2071 *Rc::make_mut(&mut cow1) += 2;
2072 *Rc::make_mut(&mut cow2) += 3;
2074 assert!(76 == *cow0);
2075 assert!(77 == *cow1);
2076 assert!(78 == *cow2);
2078 // none should point to the same backing memory
2079 assert!(*cow0 != *cow1);
2080 assert!(*cow0 != *cow2);
2081 assert!(*cow1 != *cow2);
2085 fn test_cowrc_clone_unique2() {
2086 let mut cow0 = Rc::new(75);
2087 let cow1 = cow0.clone();
2088 let cow2 = cow1.clone();
2090 assert!(75 == *cow0);
2091 assert!(75 == *cow1);
2092 assert!(75 == *cow2);
2094 *Rc::make_mut(&mut cow0) += 1;
2096 assert!(76 == *cow0);
2097 assert!(75 == *cow1);
2098 assert!(75 == *cow2);
2100 // cow1 and cow2 should share the same contents
2101 // cow0 should have a unique reference
2102 assert!(*cow0 != *cow1);
2103 assert!(*cow0 != *cow2);
2104 assert!(*cow1 == *cow2);
2108 fn test_cowrc_clone_weak() {
2109 let mut cow0 = Rc::new(75);
2110 let cow1_weak = Rc::downgrade(&cow0);
2112 assert!(75 == *cow0);
2113 assert!(75 == *cow1_weak.upgrade().unwrap());
2115 *Rc::make_mut(&mut cow0) += 1;
2117 assert!(76 == *cow0);
2118 assert!(cow1_weak.upgrade().is_none());
2123 let foo = Rc::new(75);
2124 assert_eq!(format!("{:?}", foo), "75");
2129 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
2130 assert_eq!(foo, foo.clone());
2134 fn test_from_owned() {
2136 let foo_rc = Rc::from(foo);
2137 assert!(123 == *foo_rc);
2141 fn test_new_weak() {
2142 let foo: Weak<usize> = Weak::new();
2143 assert!(foo.upgrade().is_none());
2148 let five = Rc::new(5);
2149 let same_five = five.clone();
2150 let other_five = Rc::new(5);
2152 assert!(Rc::ptr_eq(&five, &same_five));
2153 assert!(!Rc::ptr_eq(&five, &other_five));
2157 fn test_from_str() {
2158 let r: Rc<str> = Rc::from("foo");
2160 assert_eq!(&r[..], "foo");
2164 fn test_copy_from_slice() {
2165 let s: &[u32] = &[1, 2, 3];
2166 let r: Rc<[u32]> = Rc::from(s);
2168 assert_eq!(&r[..], [1, 2, 3]);
2172 fn test_clone_from_slice() {
2173 #[derive(Clone, Debug, Eq, PartialEq)]
2176 let s: &[X] = &[X(1), X(2), X(3)];
2177 let r: Rc<[X]> = Rc::from(s);
2179 assert_eq!(&r[..], s);
2184 fn test_clone_from_slice_panic() {
2185 use std::string::{String, ToString};
2187 struct Fail(u32, String);
2189 impl Clone for Fail {
2190 fn clone(&self) -> Fail {
2194 Fail(self.0, self.1.clone())
2199 Fail(0, "foo".to_string()),
2200 Fail(1, "bar".to_string()),
2201 Fail(2, "baz".to_string()),
2204 // Should panic, but not cause memory corruption
2205 let _r: Rc<[Fail]> = Rc::from(s);
2209 fn test_from_box() {
2210 let b: Box<u32> = box 123;
2211 let r: Rc<u32> = Rc::from(b);
2213 assert_eq!(*r, 123);
2217 fn test_from_box_str() {
2218 use std::string::String;
2220 let s = String::from("foo").into_boxed_str();
2221 let r: Rc<str> = Rc::from(s);
2223 assert_eq!(&r[..], "foo");
2227 fn test_from_box_slice() {
2228 let s = vec![1, 2, 3].into_boxed_slice();
2229 let r: Rc<[u32]> = Rc::from(s);
2231 assert_eq!(&r[..], [1, 2, 3]);
2235 fn test_from_box_trait() {
2236 use std::fmt::Display;
2237 use std::string::ToString;
2239 let b: Box<dyn Display> = box 123;
2240 let r: Rc<dyn Display> = Rc::from(b);
2242 assert_eq!(r.to_string(), "123");
2246 fn test_from_box_trait_zero_sized() {
2247 use std::fmt::Debug;
2249 let b: Box<dyn Debug> = box ();
2250 let r: Rc<dyn Debug> = Rc::from(b);
2252 assert_eq!(format!("{:?}", r), "()");
2256 fn test_from_vec() {
2257 let v = vec![1, 2, 3];
2258 let r: Rc<[u32]> = Rc::from(v);
2260 assert_eq!(&r[..], [1, 2, 3]);
2264 fn test_downcast() {
2267 let r1: Rc<dyn Any> = Rc::new(i32::max_value());
2268 let r2: Rc<dyn Any> = Rc::new("abc");
2270 assert!(r1.clone().downcast::<u32>().is_err());
2272 let r1i32 = r1.downcast::<i32>();
2273 assert!(r1i32.is_ok());
2274 assert_eq!(r1i32.unwrap(), Rc::new(i32::max_value()));
2276 assert!(r2.clone().downcast::<i32>().is_err());
2278 let r2str = r2.downcast::<&'static str>();
2279 assert!(r2str.is_ok());
2280 assert_eq!(r2str.unwrap(), Rc::new("abc"));
2284 #[stable(feature = "rust1", since = "1.0.0")]
2285 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2286 fn borrow(&self) -> &T {
2291 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2292 impl<T: ?Sized> AsRef<T> for Rc<T> {
2293 fn as_ref(&self) -> &T {
2298 #[stable(feature = "pin", since = "1.33.0")]
2299 impl<T: ?Sized> Unpin for Rc<T> { }
2301 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2302 // Align the unsized value to the end of the `RcBox`.
2303 // Because it is ?Sized, it will always be the last field in memory.
2304 data_offset_align(align_of_val(&*ptr))
2307 /// Computes the offset of the data field within `RcBox`.
2309 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2310 fn data_offset_sized<T>() -> isize {
2311 data_offset_align(align_of::<T>())
2315 fn data_offset_align(align: usize) -> isize {
2316 let layout = Layout::new::<RcBox<()>>();
2317 (layout.size() + layout.padding_needed_for(align)) as isize