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;
258 struct RcBox<T: ?Sized> {
264 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
267 /// See the [module-level documentation](./index.html) for more details.
269 /// The inherent methods of `Rc` are all associated functions, which means
270 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
271 /// `value.get_mut()`. This avoids conflicts with methods of the inner
274 /// [get_mut]: #method.get_mut
275 #[cfg_attr(not(test), lang = "rc")]
276 #[stable(feature = "rust1", since = "1.0.0")]
277 pub struct Rc<T: ?Sized> {
278 ptr: NonNull<RcBox<T>>,
279 phantom: PhantomData<T>,
282 #[stable(feature = "rust1", since = "1.0.0")]
283 impl<T: ?Sized> !marker::Send for Rc<T> {}
284 #[stable(feature = "rust1", since = "1.0.0")]
285 impl<T: ?Sized> !marker::Sync for Rc<T> {}
287 #[unstable(feature = "coerce_unsized", issue = "27732")]
288 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
290 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
291 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
293 impl<T: ?Sized> Rc<T> {
294 fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
297 phantom: PhantomData,
301 unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
302 Self::from_inner(NonNull::new_unchecked(ptr))
307 /// Constructs a new `Rc<T>`.
314 /// let five = Rc::new(5);
316 #[stable(feature = "rust1", since = "1.0.0")]
317 pub fn new(value: T) -> Rc<T> {
318 // There is an implicit weak pointer owned by all the strong
319 // pointers, which ensures that the weak destructor never frees
320 // the allocation while the strong destructor is running, even
321 // if the weak pointer is stored inside the strong one.
322 Self::from_inner(Box::into_raw_non_null(box RcBox {
323 strong: Cell::new(1),
329 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
330 /// `value` will be pinned in memory and unable to be moved.
331 #[stable(feature = "pin", since = "1.33.0")]
332 pub fn pin(value: T) -> Pin<Rc<T>> {
333 unsafe { Pin::new_unchecked(Rc::new(value)) }
336 /// Returns the contained value, if the `Rc` has exactly one strong reference.
338 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
341 /// This will succeed even if there are outstanding weak references.
343 /// [result]: ../../std/result/enum.Result.html
350 /// let x = Rc::new(3);
351 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
353 /// let x = Rc::new(4);
354 /// let _y = Rc::clone(&x);
355 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
358 #[stable(feature = "rc_unique", since = "1.4.0")]
359 pub fn try_unwrap(this: Self) -> Result<T, Self> {
360 if Rc::strong_count(&this) == 1 {
362 let val = ptr::read(&*this); // copy the contained object
364 // Indicate to Weaks that they can't be promoted by decrementing
365 // the strong count, and then remove the implicit "strong weak"
366 // pointer while also handling drop logic by just crafting a
369 let _weak = Weak { ptr: this.ptr };
379 impl<T: ?Sized> Rc<T> {
380 /// Consumes the `Rc`, returning the wrapped pointer.
382 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
383 /// [`Rc::from_raw`][from_raw].
385 /// [from_raw]: struct.Rc.html#method.from_raw
392 /// let x = Rc::new("hello".to_owned());
393 /// let x_ptr = Rc::into_raw(x);
394 /// assert_eq!(unsafe { &*x_ptr }, "hello");
396 #[stable(feature = "rc_raw", since = "1.17.0")]
397 pub fn into_raw(this: Self) -> *const T {
398 let ptr: *const T = &*this;
403 /// Constructs an `Rc` from a raw pointer.
405 /// The raw pointer must have been previously returned by a call to a
406 /// [`Rc::into_raw`][into_raw].
408 /// This function is unsafe because improper use may lead to memory problems. For example, a
409 /// double-free may occur if the function is called twice on the same raw pointer.
411 /// [into_raw]: struct.Rc.html#method.into_raw
418 /// let x = Rc::new("hello".to_owned());
419 /// let x_ptr = Rc::into_raw(x);
422 /// // Convert back to an `Rc` to prevent leak.
423 /// let x = Rc::from_raw(x_ptr);
424 /// assert_eq!(&*x, "hello");
426 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
429 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
431 #[stable(feature = "rc_raw", since = "1.17.0")]
432 pub unsafe fn from_raw(ptr: *const T) -> Self {
433 let offset = data_offset(ptr);
435 // Reverse the offset to find the original RcBox.
436 let fake_ptr = ptr as *mut RcBox<T>;
437 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
439 Self::from_ptr(rc_ptr)
442 /// Consumes the `Rc`, returning the wrapped pointer as `NonNull<T>`.
447 /// #![feature(rc_into_raw_non_null)]
451 /// let x = Rc::new("hello".to_owned());
452 /// let ptr = Rc::into_raw_non_null(x);
453 /// let deref = unsafe { ptr.as_ref() };
454 /// assert_eq!(deref, "hello");
456 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
458 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
459 // safe because Rc guarantees its pointer is non-null
460 unsafe { NonNull::new_unchecked(Rc::into_raw(this) as *mut _) }
463 /// Creates a new [`Weak`][weak] pointer to this value.
465 /// [weak]: struct.Weak.html
472 /// let five = Rc::new(5);
474 /// let weak_five = Rc::downgrade(&five);
476 #[stable(feature = "rc_weak", since = "1.4.0")]
477 pub fn downgrade(this: &Self) -> Weak<T> {
479 // Make sure we do not create a dangling Weak
480 debug_assert!(!is_dangling(this.ptr));
481 Weak { ptr: this.ptr }
484 /// Gets the number of [`Weak`][weak] pointers to this value.
486 /// [weak]: struct.Weak.html
493 /// let five = Rc::new(5);
494 /// let _weak_five = Rc::downgrade(&five);
496 /// assert_eq!(1, Rc::weak_count(&five));
499 #[stable(feature = "rc_counts", since = "1.15.0")]
500 pub fn weak_count(this: &Self) -> usize {
504 /// Gets the number of strong (`Rc`) pointers to this value.
511 /// let five = Rc::new(5);
512 /// let _also_five = Rc::clone(&five);
514 /// assert_eq!(2, Rc::strong_count(&five));
517 #[stable(feature = "rc_counts", since = "1.15.0")]
518 pub fn strong_count(this: &Self) -> usize {
522 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
523 /// this inner value.
525 /// [weak]: struct.Weak.html
527 fn is_unique(this: &Self) -> bool {
528 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
531 /// Returns a mutable reference to the inner value, if there are
532 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
534 /// Returns [`None`] otherwise, because it is not safe to
535 /// mutate a shared value.
537 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
538 /// the inner value when it's shared.
540 /// [weak]: struct.Weak.html
541 /// [`None`]: ../../std/option/enum.Option.html#variant.None
542 /// [make_mut]: struct.Rc.html#method.make_mut
543 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
550 /// let mut x = Rc::new(3);
551 /// *Rc::get_mut(&mut x).unwrap() = 4;
552 /// assert_eq!(*x, 4);
554 /// let _y = Rc::clone(&x);
555 /// assert!(Rc::get_mut(&mut x).is_none());
558 #[stable(feature = "rc_unique", since = "1.4.0")]
559 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
560 if Rc::is_unique(this) {
562 Some(&mut this.ptr.as_mut().value)
570 #[stable(feature = "ptr_eq", since = "1.17.0")]
571 /// Returns `true` if the two `Rc`s point to the same value (not
572 /// just values that compare as equal).
579 /// let five = Rc::new(5);
580 /// let same_five = Rc::clone(&five);
581 /// let other_five = Rc::new(5);
583 /// assert!(Rc::ptr_eq(&five, &same_five));
584 /// assert!(!Rc::ptr_eq(&five, &other_five));
586 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
587 this.ptr.as_ptr() == other.ptr.as_ptr()
591 impl<T: Clone> Rc<T> {
592 /// Makes a mutable reference into the given `Rc`.
594 /// If there are other `Rc` pointers to the same value, then `make_mut` will
595 /// [`clone`] the inner value to ensure unique ownership. This is also
596 /// referred to as clone-on-write.
598 /// If there are no other `Rc` pointers to this value, then [`Weak`]
599 /// pointers to this value will be dissassociated.
601 /// See also [`get_mut`], which will fail rather than cloning.
603 /// [`Weak`]: struct.Weak.html
604 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
605 /// [`get_mut`]: struct.Rc.html#method.get_mut
612 /// let mut data = Rc::new(5);
614 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
615 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
616 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
617 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
618 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
620 /// // Now `data` and `other_data` point to different values.
621 /// assert_eq!(*data, 8);
622 /// assert_eq!(*other_data, 12);
625 /// [`Weak`] pointers will be dissassociated:
630 /// let mut data = Rc::new(75);
631 /// let weak = Rc::downgrade(&data);
633 /// assert!(75 == *data);
634 /// assert!(75 == *weak.upgrade().unwrap());
636 /// *Rc::make_mut(&mut data) += 1;
638 /// assert!(76 == *data);
639 /// assert!(weak.upgrade().is_none());
642 #[stable(feature = "rc_unique", since = "1.4.0")]
643 pub fn make_mut(this: &mut Self) -> &mut T {
644 if Rc::strong_count(this) != 1 {
645 // Gotta clone the data, there are other Rcs
646 *this = Rc::new((**this).clone())
647 } else if Rc::weak_count(this) != 0 {
648 // Can just steal the data, all that's left is Weaks
650 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
651 mem::swap(this, &mut swap);
653 // Remove implicit strong-weak ref (no need to craft a fake
654 // Weak here -- we know other Weaks can clean up for us)
659 // This unsafety is ok because we're guaranteed that the pointer
660 // returned is the *only* pointer that will ever be returned to T. Our
661 // reference count is guaranteed to be 1 at this point, and we required
662 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
663 // reference to the inner value.
665 &mut this.ptr.as_mut().value
672 #[stable(feature = "rc_downcast", since = "1.29.0")]
673 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
678 /// use std::any::Any;
681 /// fn print_if_string(value: Rc<dyn Any>) {
682 /// if let Ok(string) = value.downcast::<String>() {
683 /// println!("String ({}): {}", string.len(), string);
688 /// let my_string = "Hello World".to_string();
689 /// print_if_string(Rc::new(my_string));
690 /// print_if_string(Rc::new(0i8));
693 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
694 if (*self).is::<T>() {
695 let ptr = self.ptr.cast::<RcBox<T>>();
697 Ok(Rc::from_inner(ptr))
704 impl<T: ?Sized> Rc<T> {
705 /// Allocates an `RcBox<T>` with sufficient space for
706 /// an unsized value where the value has the layout provided.
708 /// The function `mem_to_rcbox` is called with the data pointer
709 /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
710 unsafe fn allocate_for_unsized(
711 value_layout: Layout,
712 mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>
714 // Calculate layout using the given value layout.
715 // Previously, layout was calculated on the expression
716 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
717 // reference (see #54908).
718 let layout = Layout::new::<RcBox<()>>()
719 .extend(value_layout).unwrap().0
720 .pad_to_align().unwrap();
722 // Allocate for the layout.
723 let mem = Global.alloc(layout)
724 .unwrap_or_else(|_| handle_alloc_error(layout));
726 // Initialize the RcBox
727 let inner = mem_to_rcbox(mem.as_ptr());
728 debug_assert_eq!(Layout::for_value(&*inner), layout);
730 ptr::write(&mut (*inner).strong, Cell::new(1));
731 ptr::write(&mut (*inner).weak, Cell::new(1));
736 /// Allocates an `RcBox<T>` with sufficient space for an unsized value
737 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
738 // Allocate for the `RcBox<T>` using the given value.
739 Self::allocate_for_unsized(
740 Layout::for_value(&*ptr),
741 |mem| set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>,
745 fn from_box(v: Box<T>) -> Rc<T> {
747 let box_unique = Box::into_unique(v);
748 let bptr = box_unique.as_ptr();
750 let value_size = size_of_val(&*bptr);
751 let ptr = Self::allocate_for_ptr(bptr);
753 // Copy value as bytes
754 ptr::copy_nonoverlapping(
755 bptr as *const T as *const u8,
756 &mut (*ptr).value as *mut _ as *mut u8,
759 // Free the allocation without dropping its contents
760 box_free(box_unique);
768 /// Allocates an `RcBox<[T]>` with the given length.
769 unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
770 Self::allocate_for_unsized(
771 Layout::array::<T>(len).unwrap(),
772 |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>,
777 /// Sets the data pointer of a `?Sized` raw pointer.
779 /// For a slice/trait object, this sets the `data` field and leaves the rest
780 /// unchanged. For a sized raw pointer, this simply sets the pointer.
781 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
782 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
787 /// Copy elements from slice into newly allocated Rc<[T]>
789 /// Unsafe because the caller must either take ownership or bind `T: Copy`
790 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
791 let ptr = Self::allocate_for_slice(v.len());
793 ptr::copy_nonoverlapping(
795 &mut (*ptr).value as *mut [T] as *mut T,
801 /// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
803 /// Behavior is undefined should the size be wrong.
804 unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
805 // Panic guard while cloning T elements.
806 // In the event of a panic, elements that have been written
807 // into the new RcBox will be dropped, then the memory freed.
815 impl<T> Drop for Guard<T> {
818 let slice = from_raw_parts_mut(self.elems, self.n_elems);
819 ptr::drop_in_place(slice);
821 Global.dealloc(self.mem, self.layout);
826 let ptr = Self::allocate_for_slice(len);
828 let mem = ptr as *mut _ as *mut u8;
829 let layout = Layout::for_value(&*ptr);
831 // Pointer to first element
832 let elems = &mut (*ptr).value as *mut [T] as *mut T;
834 let mut guard = Guard {
835 mem: NonNull::new_unchecked(mem),
841 for (i, item) in iter.enumerate() {
842 ptr::write(elems.add(i), item);
846 // All clear. Forget the guard so it doesn't free the new RcBox.
853 /// Specialization trait used for `From<&[T]>`.
854 trait RcFromSlice<T> {
855 fn from_slice(slice: &[T]) -> Self;
858 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
860 default fn from_slice(v: &[T]) -> Self {
862 Self::from_iter_exact(v.iter().cloned(), v.len())
867 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
869 fn from_slice(v: &[T]) -> Self {
870 unsafe { Rc::copy_from_slice(v) }
874 #[stable(feature = "rust1", since = "1.0.0")]
875 impl<T: ?Sized> Deref for Rc<T> {
879 fn deref(&self) -> &T {
884 #[unstable(feature = "receiver_trait", issue = "0")]
885 impl<T: ?Sized> Receiver for Rc<T> {}
887 #[stable(feature = "rust1", since = "1.0.0")]
888 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
891 /// This will decrement the strong reference count. If the strong reference
892 /// count reaches zero then the only other references (if any) are
893 /// [`Weak`], so we `drop` the inner value.
902 /// impl Drop for Foo {
903 /// fn drop(&mut self) {
904 /// println!("dropped!");
908 /// let foo = Rc::new(Foo);
909 /// let foo2 = Rc::clone(&foo);
911 /// drop(foo); // Doesn't print anything
912 /// drop(foo2); // Prints "dropped!"
915 /// [`Weak`]: ../../std/rc/struct.Weak.html
919 if self.strong() == 0 {
920 // destroy the contained object
921 ptr::drop_in_place(self.ptr.as_mut());
923 // remove the implicit "strong weak" pointer now that we've
924 // destroyed the contents.
927 if self.weak() == 0 {
928 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
935 #[stable(feature = "rust1", since = "1.0.0")]
936 impl<T: ?Sized> Clone for Rc<T> {
937 /// Makes a clone of the `Rc` pointer.
939 /// This creates another pointer to the same inner value, increasing the
940 /// strong reference count.
947 /// let five = Rc::new(5);
949 /// let _ = Rc::clone(&five);
952 fn clone(&self) -> Rc<T> {
954 Self::from_inner(self.ptr)
958 #[stable(feature = "rust1", since = "1.0.0")]
959 impl<T: Default> Default for Rc<T> {
960 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
967 /// let x: Rc<i32> = Default::default();
968 /// assert_eq!(*x, 0);
971 fn default() -> Rc<T> {
972 Rc::new(Default::default())
976 #[stable(feature = "rust1", since = "1.0.0")]
977 trait RcEqIdent<T: ?Sized + PartialEq> {
978 fn eq(&self, other: &Rc<T>) -> bool;
979 fn ne(&self, other: &Rc<T>) -> bool;
982 #[stable(feature = "rust1", since = "1.0.0")]
983 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
985 default fn eq(&self, other: &Rc<T>) -> bool {
990 default fn ne(&self, other: &Rc<T>) -> bool {
995 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
996 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
997 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
998 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
999 /// the same value, than two `&T`s.
1000 #[stable(feature = "rust1", since = "1.0.0")]
1001 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
1003 fn eq(&self, other: &Rc<T>) -> bool {
1004 Rc::ptr_eq(self, other) || **self == **other
1008 fn ne(&self, other: &Rc<T>) -> bool {
1009 !Rc::ptr_eq(self, other) && **self != **other
1013 #[stable(feature = "rust1", since = "1.0.0")]
1014 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
1015 /// Equality for two `Rc`s.
1017 /// Two `Rc`s are equal if their inner values are equal.
1019 /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
1025 /// use std::rc::Rc;
1027 /// let five = Rc::new(5);
1029 /// assert!(five == Rc::new(5));
1032 fn eq(&self, other: &Rc<T>) -> bool {
1033 RcEqIdent::eq(self, other)
1036 /// Inequality for two `Rc`s.
1038 /// Two `Rc`s are unequal if their inner values are unequal.
1040 /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
1046 /// use std::rc::Rc;
1048 /// let five = Rc::new(5);
1050 /// assert!(five != Rc::new(6));
1053 fn ne(&self, other: &Rc<T>) -> bool {
1054 RcEqIdent::ne(self, other)
1058 #[stable(feature = "rust1", since = "1.0.0")]
1059 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1061 #[stable(feature = "rust1", since = "1.0.0")]
1062 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1063 /// Partial comparison for two `Rc`s.
1065 /// The two are compared by calling `partial_cmp()` on their inner values.
1070 /// use std::rc::Rc;
1071 /// use std::cmp::Ordering;
1073 /// let five = Rc::new(5);
1075 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1078 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1079 (**self).partial_cmp(&**other)
1082 /// Less-than comparison for two `Rc`s.
1084 /// The two are compared by calling `<` on their inner values.
1089 /// use std::rc::Rc;
1091 /// let five = Rc::new(5);
1093 /// assert!(five < Rc::new(6));
1096 fn lt(&self, other: &Rc<T>) -> bool {
1100 /// 'Less than or equal to' comparison for two `Rc`s.
1102 /// The two are compared by calling `<=` on their inner values.
1107 /// use std::rc::Rc;
1109 /// let five = Rc::new(5);
1111 /// assert!(five <= Rc::new(5));
1114 fn le(&self, other: &Rc<T>) -> bool {
1118 /// Greater-than comparison for two `Rc`s.
1120 /// The two are compared by calling `>` on their inner values.
1125 /// use std::rc::Rc;
1127 /// let five = Rc::new(5);
1129 /// assert!(five > Rc::new(4));
1132 fn gt(&self, other: &Rc<T>) -> bool {
1136 /// 'Greater than or equal to' comparison for two `Rc`s.
1138 /// The two are compared by calling `>=` on their inner values.
1143 /// use std::rc::Rc;
1145 /// let five = Rc::new(5);
1147 /// assert!(five >= Rc::new(5));
1150 fn ge(&self, other: &Rc<T>) -> bool {
1155 #[stable(feature = "rust1", since = "1.0.0")]
1156 impl<T: ?Sized + Ord> Ord for Rc<T> {
1157 /// Comparison for two `Rc`s.
1159 /// The two are compared by calling `cmp()` on their inner values.
1164 /// use std::rc::Rc;
1165 /// use std::cmp::Ordering;
1167 /// let five = Rc::new(5);
1169 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1172 fn cmp(&self, other: &Rc<T>) -> Ordering {
1173 (**self).cmp(&**other)
1177 #[stable(feature = "rust1", since = "1.0.0")]
1178 impl<T: ?Sized + Hash> Hash for Rc<T> {
1179 fn hash<H: Hasher>(&self, state: &mut H) {
1180 (**self).hash(state);
1184 #[stable(feature = "rust1", since = "1.0.0")]
1185 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1186 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1187 fmt::Display::fmt(&**self, f)
1191 #[stable(feature = "rust1", since = "1.0.0")]
1192 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1193 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1194 fmt::Debug::fmt(&**self, f)
1198 #[stable(feature = "rust1", since = "1.0.0")]
1199 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1200 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1201 fmt::Pointer::fmt(&(&**self as *const T), f)
1205 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1206 impl<T> From<T> for Rc<T> {
1207 fn from(t: T) -> Self {
1212 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1213 impl<T: Clone> From<&[T]> for Rc<[T]> {
1215 fn from(v: &[T]) -> Rc<[T]> {
1216 <Self as RcFromSlice<T>>::from_slice(v)
1220 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1221 impl From<&str> for Rc<str> {
1223 fn from(v: &str) -> Rc<str> {
1224 let rc = Rc::<[u8]>::from(v.as_bytes());
1225 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1229 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1230 impl From<String> for Rc<str> {
1232 fn from(v: String) -> Rc<str> {
1237 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1238 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1240 fn from(v: Box<T>) -> Rc<T> {
1245 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1246 impl<T> From<Vec<T>> for Rc<[T]> {
1248 fn from(mut v: Vec<T>) -> Rc<[T]> {
1250 let rc = Rc::copy_from_slice(&v);
1252 // Allow the Vec to free its memory, but not destroy its contents
1260 #[stable(feature = "shared_from_iter", since = "1.37.0")]
1261 impl<T> iter::FromIterator<T> for Rc<[T]> {
1262 /// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
1264 /// # Performance characteristics
1266 /// ## The general case
1268 /// In the general case, collecting into `Rc<[T]>` is done by first
1269 /// collecting into a `Vec<T>`. That is, when writing the following:
1272 /// # use std::rc::Rc;
1273 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
1274 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1277 /// this behaves as if we wrote:
1280 /// # use std::rc::Rc;
1281 /// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
1282 /// .collect::<Vec<_>>() // The first set of allocations happens here.
1283 /// .into(); // A second allocation for `Rc<[T]>` happens here.
1284 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
1287 /// This will allocate as many times as needed for constructing the `Vec<T>`
1288 /// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
1290 /// ## Iterators of known length
1292 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
1293 /// a single allocation will be made for the `Rc<[T]>`. For example:
1296 /// # use std::rc::Rc;
1297 /// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
1298 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
1300 fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
1301 RcFromIter::from_iter(iter.into_iter())
1305 /// Specialization trait used for collecting into `Rc<[T]>`.
1306 trait RcFromIter<T, I> {
1307 fn from_iter(iter: I) -> Self;
1310 impl<T, I: Iterator<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1311 default fn from_iter(iter: I) -> Self {
1312 iter.collect::<Vec<T>>().into()
1316 impl<T, I: iter::TrustedLen<Item = T>> RcFromIter<T, I> for Rc<[T]> {
1317 default fn from_iter(iter: I) -> Self {
1318 // This is the case for a `TrustedLen` iterator.
1319 let (low, high) = iter.size_hint();
1320 if let Some(high) = high {
1323 "TrustedLen iterator's size hint is not exact: {:?}",
1328 // SAFETY: We need to ensure that the iterator has an exact length and we have.
1329 Rc::from_iter_exact(iter, low)
1332 // Fall back to normal implementation.
1333 iter.collect::<Vec<T>>().into()
1338 impl<'a, T: 'a + Clone> RcFromIter<&'a T, slice::Iter<'a, T>> for Rc<[T]> {
1339 fn from_iter(iter: slice::Iter<'a, T>) -> Self {
1340 // Delegate to `impl<T: Clone> From<&[T]> for Rc<[T]>`.
1342 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
1343 // which is even more performant.
1345 // In the fall-back case we have `T: Clone`. This is still better
1346 // than the `TrustedLen` implementation as slices have a known length
1347 // and so we get to avoid calling `size_hint` and avoid the branching.
1348 iter.as_slice().into()
1352 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1353 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
1354 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1356 /// Since a `Weak` reference does not count towards ownership, it will not
1357 /// prevent the inner value from being dropped, and `Weak` itself makes no
1358 /// guarantees about the value still being present and may return [`None`]
1359 /// when [`upgrade`]d.
1361 /// A `Weak` pointer is useful for keeping a temporary reference to the value
1362 /// within [`Rc`] without extending its lifetime. It is also used to prevent
1363 /// circular references between [`Rc`] pointers, since mutual owning references
1364 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1365 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1366 /// pointers from children back to their parents.
1368 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1370 /// [`Rc`]: struct.Rc.html
1371 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1372 /// [`upgrade`]: struct.Weak.html#method.upgrade
1373 /// [`Option`]: ../../std/option/enum.Option.html
1374 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1375 #[stable(feature = "rc_weak", since = "1.4.0")]
1376 pub struct Weak<T: ?Sized> {
1377 // This is a `NonNull` to allow optimizing the size of this type in enums,
1378 // but it is not necessarily a valid pointer.
1379 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1380 // to allocate space on the heap. That's not a value a real pointer
1381 // will ever have because RcBox has alignment at least 2.
1382 ptr: NonNull<RcBox<T>>,
1385 #[stable(feature = "rc_weak", since = "1.4.0")]
1386 impl<T: ?Sized> !marker::Send for Weak<T> {}
1387 #[stable(feature = "rc_weak", since = "1.4.0")]
1388 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1390 #[unstable(feature = "coerce_unsized", issue = "27732")]
1391 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1393 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
1394 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1397 /// Constructs a new `Weak<T>`, without allocating any memory.
1398 /// Calling [`upgrade`] on the return value always gives [`None`].
1400 /// [`upgrade`]: #method.upgrade
1401 /// [`None`]: ../../std/option/enum.Option.html
1406 /// use std::rc::Weak;
1408 /// let empty: Weak<i64> = Weak::new();
1409 /// assert!(empty.upgrade().is_none());
1411 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1412 pub fn new() -> Weak<T> {
1414 ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"),
1418 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1420 /// It is up to the caller to ensure that the object is still alive when accessing it through
1423 /// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
1428 /// #![feature(weak_into_raw)]
1430 /// use std::rc::Rc;
1433 /// let strong = Rc::new("hello".to_owned());
1434 /// let weak = Rc::downgrade(&strong);
1435 /// // Both point to the same object
1436 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1437 /// // The strong here keeps it alive, so we can still access the object.
1438 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1441 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1442 /// // undefined behaviour.
1443 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1446 /// [`null`]: ../../std/ptr/fn.null.html
1447 #[unstable(feature = "weak_into_raw", issue = "60728")]
1448 pub fn as_raw(&self) -> *const T {
1449 match self.inner() {
1450 None => ptr::null(),
1452 let offset = data_offset_sized::<T>();
1453 let ptr = inner as *const RcBox<T>;
1454 // Note: while the pointer we create may already point to dropped value, the
1455 // allocation still lives (it must hold the weak point as long as we are alive).
1456 // Therefore, the offset is OK to do, it won't get out of the allocation.
1457 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1463 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1465 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1466 /// can be turned back into the `Weak<T>` with [`from_raw`].
1468 /// The same restrictions of accessing the target of the pointer as with
1469 /// [`as_raw`] apply.
1474 /// #![feature(weak_into_raw)]
1476 /// use std::rc::{Rc, Weak};
1478 /// let strong = Rc::new("hello".to_owned());
1479 /// let weak = Rc::downgrade(&strong);
1480 /// let raw = weak.into_raw();
1482 /// assert_eq!(1, Rc::weak_count(&strong));
1483 /// assert_eq!("hello", unsafe { &*raw });
1485 /// drop(unsafe { Weak::from_raw(raw) });
1486 /// assert_eq!(0, Rc::weak_count(&strong));
1489 /// [`from_raw`]: struct.Weak.html#method.from_raw
1490 /// [`as_raw`]: struct.Weak.html#method.as_raw
1491 #[unstable(feature = "weak_into_raw", issue = "60728")]
1492 pub fn into_raw(self) -> *const T {
1493 let result = self.as_raw();
1498 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1500 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1501 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1503 /// It takes ownership of one weak count. In case a [`null`] is passed, a dangling [`Weak`] is
1508 /// The pointer must represent one valid weak count. In other words, it must point to `T` which
1509 /// is or *was* managed by an [`Rc`] and the weak count of that [`Rc`] must not have reached
1510 /// 0. It is allowed for the strong count to be 0.
1515 /// #![feature(weak_into_raw)]
1517 /// use std::rc::{Rc, Weak};
1519 /// let strong = Rc::new("hello".to_owned());
1521 /// let raw_1 = Rc::downgrade(&strong).into_raw();
1522 /// let raw_2 = Rc::downgrade(&strong).into_raw();
1524 /// assert_eq!(2, Rc::weak_count(&strong));
1526 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1527 /// assert_eq!(1, Rc::weak_count(&strong));
1531 /// // Decrement the last weak count.
1532 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1535 /// [`null`]: ../../std/ptr/fn.null.html
1536 /// [`into_raw`]: struct.Weak.html#method.into_raw
1537 /// [`upgrade`]: struct.Weak.html#method.upgrade
1538 /// [`Rc`]: struct.Rc.html
1539 /// [`Weak`]: struct.Weak.html
1540 #[unstable(feature = "weak_into_raw", issue = "60728")]
1541 pub unsafe fn from_raw(ptr: *const T) -> Self {
1545 // See Rc::from_raw for details
1546 let offset = data_offset(ptr);
1547 let fake_ptr = ptr as *mut RcBox<T>;
1548 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1550 ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
1556 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1557 let address = ptr.as_ptr() as *mut () as usize;
1558 address == usize::MAX
1561 impl<T: ?Sized> Weak<T> {
1562 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1563 /// the lifetime of the value if successful.
1565 /// Returns [`None`] if the value has since been dropped.
1567 /// [`Rc`]: struct.Rc.html
1568 /// [`None`]: ../../std/option/enum.Option.html
1573 /// use std::rc::Rc;
1575 /// let five = Rc::new(5);
1577 /// let weak_five = Rc::downgrade(&five);
1579 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1580 /// assert!(strong_five.is_some());
1582 /// // Destroy all strong pointers.
1583 /// drop(strong_five);
1586 /// assert!(weak_five.upgrade().is_none());
1588 #[stable(feature = "rc_weak", since = "1.4.0")]
1589 pub fn upgrade(&self) -> Option<Rc<T>> {
1590 let inner = self.inner()?;
1591 if inner.strong() == 0 {
1595 Some(Rc::from_inner(self.ptr))
1599 /// Gets the number of strong (`Rc`) pointers pointing to this value.
1601 /// If `self` was created using [`Weak::new`], this will return 0.
1603 /// [`Weak::new`]: #method.new
1604 #[unstable(feature = "weak_counts", issue = "57977")]
1605 pub fn strong_count(&self) -> usize {
1606 if let Some(inner) = self.inner() {
1613 /// Gets the number of `Weak` pointers pointing to this value.
1615 /// If `self` was created using [`Weak::new`], this will return `None`. If
1616 /// not, the returned value is at least 1, since `self` still points to the
1619 /// [`Weak::new`]: #method.new
1620 #[unstable(feature = "weak_counts", issue = "57977")]
1621 pub fn weak_count(&self) -> Option<usize> {
1622 self.inner().map(|inner| {
1623 if inner.strong() > 0 {
1624 inner.weak() - 1 // subtract the implicit weak ptr
1631 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1632 /// (i.e., when this `Weak` was created by `Weak::new`).
1634 fn inner(&self) -> Option<&RcBox<T>> {
1635 if is_dangling(self.ptr) {
1638 Some(unsafe { self.ptr.as_ref() })
1642 /// Returns `true` if the two `Weak`s point to the same value (not just values
1643 /// that compare as equal).
1647 /// Since this compares pointers it means that `Weak::new()` will equal each
1648 /// other, even though they don't point to any value.
1653 /// #![feature(weak_ptr_eq)]
1654 /// use std::rc::Rc;
1656 /// let first_rc = Rc::new(5);
1657 /// let first = Rc::downgrade(&first_rc);
1658 /// let second = Rc::downgrade(&first_rc);
1660 /// assert!(first.ptr_eq(&second));
1662 /// let third_rc = Rc::new(5);
1663 /// let third = Rc::downgrade(&third_rc);
1665 /// assert!(!first.ptr_eq(&third));
1668 /// Comparing `Weak::new`.
1671 /// #![feature(weak_ptr_eq)]
1672 /// use std::rc::{Rc, Weak};
1674 /// let first = Weak::new();
1675 /// let second = Weak::new();
1676 /// assert!(first.ptr_eq(&second));
1678 /// let third_rc = Rc::new(());
1679 /// let third = Rc::downgrade(&third_rc);
1680 /// assert!(!first.ptr_eq(&third));
1683 #[unstable(feature = "weak_ptr_eq", issue = "55981")]
1684 pub fn ptr_eq(&self, other: &Self) -> bool {
1685 self.ptr.as_ptr() == other.ptr.as_ptr()
1689 #[stable(feature = "rc_weak", since = "1.4.0")]
1690 impl<T: ?Sized> Drop for Weak<T> {
1691 /// Drops the `Weak` pointer.
1696 /// use std::rc::{Rc, Weak};
1700 /// impl Drop for Foo {
1701 /// fn drop(&mut self) {
1702 /// println!("dropped!");
1706 /// let foo = Rc::new(Foo);
1707 /// let weak_foo = Rc::downgrade(&foo);
1708 /// let other_weak_foo = Weak::clone(&weak_foo);
1710 /// drop(weak_foo); // Doesn't print anything
1711 /// drop(foo); // Prints "dropped!"
1713 /// assert!(other_weak_foo.upgrade().is_none());
1715 fn drop(&mut self) {
1716 if let Some(inner) = self.inner() {
1718 // the weak count starts at 1, and will only go to zero if all
1719 // the strong pointers have disappeared.
1720 if inner.weak() == 0 {
1722 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1729 #[stable(feature = "rc_weak", since = "1.4.0")]
1730 impl<T: ?Sized> Clone for Weak<T> {
1731 /// Makes a clone of the `Weak` pointer that points to the same value.
1736 /// use std::rc::{Rc, Weak};
1738 /// let weak_five = Rc::downgrade(&Rc::new(5));
1740 /// let _ = Weak::clone(&weak_five);
1743 fn clone(&self) -> Weak<T> {
1744 if let Some(inner) = self.inner() {
1747 Weak { ptr: self.ptr }
1751 #[stable(feature = "rc_weak", since = "1.4.0")]
1752 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1753 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1758 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1759 impl<T> Default for Weak<T> {
1760 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1761 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1763 /// [`None`]: ../../std/option/enum.Option.html
1764 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
1769 /// use std::rc::Weak;
1771 /// let empty: Weak<i64> = Default::default();
1772 /// assert!(empty.upgrade().is_none());
1774 fn default() -> Weak<T> {
1779 // NOTE: We checked_add here to deal with mem::forget safely. In particular
1780 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1781 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1782 // We abort because this is such a degenerate scenario that we don't care about
1783 // what happens -- no real program should ever experience this.
1785 // This should have negligible overhead since you don't actually need to
1786 // clone these much in Rust thanks to ownership and move-semantics.
1789 trait RcBoxPtr<T: ?Sized> {
1790 fn inner(&self) -> &RcBox<T>;
1793 fn strong(&self) -> usize {
1794 self.inner().strong.get()
1798 fn inc_strong(&self) {
1799 let strong = self.strong();
1801 // We want to abort on overflow instead of dropping the value.
1802 // The reference count will never be zero when this is called;
1803 // nevertheless, we insert an abort here to hint LLVM at
1804 // an otherwise missed optimization.
1805 if strong == 0 || strong == usize::max_value() {
1808 self.inner().strong.set(strong + 1);
1812 fn dec_strong(&self) {
1813 self.inner().strong.set(self.strong() - 1);
1817 fn weak(&self) -> usize {
1818 self.inner().weak.get()
1822 fn inc_weak(&self) {
1823 let weak = self.weak();
1825 // We want to abort on overflow instead of dropping the value.
1826 // The reference count will never be zero when this is called;
1827 // nevertheless, we insert an abort here to hint LLVM at
1828 // an otherwise missed optimization.
1829 if weak == 0 || weak == usize::max_value() {
1832 self.inner().weak.set(weak + 1);
1836 fn dec_weak(&self) {
1837 self.inner().weak.set(self.weak() - 1);
1841 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1843 fn inner(&self) -> &RcBox<T> {
1850 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
1852 fn inner(&self) -> &RcBox<T> {
1857 #[stable(feature = "rust1", since = "1.0.0")]
1858 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
1859 fn borrow(&self) -> &T {
1864 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1865 impl<T: ?Sized> AsRef<T> for Rc<T> {
1866 fn as_ref(&self) -> &T {
1871 #[stable(feature = "pin", since = "1.33.0")]
1872 impl<T: ?Sized> Unpin for Rc<T> { }
1874 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
1875 // Align the unsized value to the end of the `RcBox`.
1876 // Because it is ?Sized, it will always be the last field in memory.
1877 data_offset_align(align_of_val(&*ptr))
1880 /// Computes the offset of the data field within `RcBox`.
1882 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
1883 fn data_offset_sized<T>() -> isize {
1884 data_offset_align(align_of::<T>())
1888 fn data_offset_align(align: usize) -> isize {
1889 let layout = Layout::new::<RcBox<()>>();
1890 (layout.size() + layout.padding_needed_for(align)) as isize