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;
241 use core::marker::{self, Unpin, Unsize, PhantomData};
242 use core::mem::{self, align_of, align_of_val, forget, size_of_val};
243 use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn};
245 use core::ptr::{self, NonNull};
246 use core::slice::from_raw_parts_mut;
247 use core::convert::From;
250 use crate::alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
251 use crate::string::String;
254 struct RcBox<T: ?Sized> {
260 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
263 /// See the [module-level documentation](./index.html) for more details.
265 /// The inherent methods of `Rc` are all associated functions, which means
266 /// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
267 /// `value.get_mut()`. This avoids conflicts with methods of the inner
270 /// [get_mut]: #method.get_mut
271 #[cfg_attr(not(test), lang = "rc")]
272 #[stable(feature = "rust1", since = "1.0.0")]
273 pub struct Rc<T: ?Sized> {
274 ptr: NonNull<RcBox<T>>,
275 phantom: PhantomData<T>,
278 #[stable(feature = "rust1", since = "1.0.0")]
279 impl<T: ?Sized> !marker::Send for Rc<T> {}
280 #[stable(feature = "rust1", since = "1.0.0")]
281 impl<T: ?Sized> !marker::Sync for Rc<T> {}
283 #[unstable(feature = "coerce_unsized", issue = "27732")]
284 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
286 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
287 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
290 /// Constructs a new `Rc<T>`.
297 /// let five = Rc::new(5);
299 #[stable(feature = "rust1", since = "1.0.0")]
300 pub fn new(value: T) -> Rc<T> {
302 // there is an implicit weak pointer owned by all the strong
303 // pointers, which ensures that the weak destructor never frees
304 // the allocation while the strong destructor is running, even
305 // if the weak pointer is stored inside the strong one.
306 ptr: Box::into_raw_non_null(box RcBox {
307 strong: Cell::new(1),
311 phantom: PhantomData,
315 /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
316 /// `value` will be pinned in memory and unable to be moved.
317 #[stable(feature = "pin", since = "1.33.0")]
318 pub fn pin(value: T) -> Pin<Rc<T>> {
319 unsafe { Pin::new_unchecked(Rc::new(value)) }
322 /// Returns the contained value, if the `Rc` has exactly one strong reference.
324 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
327 /// This will succeed even if there are outstanding weak references.
329 /// [result]: ../../std/result/enum.Result.html
336 /// let x = Rc::new(3);
337 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
339 /// let x = Rc::new(4);
340 /// let _y = Rc::clone(&x);
341 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
344 #[stable(feature = "rc_unique", since = "1.4.0")]
345 pub fn try_unwrap(this: Self) -> Result<T, Self> {
346 if Rc::strong_count(&this) == 1 {
348 let val = ptr::read(&*this); // copy the contained object
350 // Indicate to Weaks that they can't be promoted by decrementing
351 // the strong count, and then remove the implicit "strong weak"
352 // pointer while also handling drop logic by just crafting a
355 let _weak = Weak { ptr: this.ptr };
365 impl<T: ?Sized> Rc<T> {
366 /// Consumes the `Rc`, returning the wrapped pointer.
368 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
369 /// [`Rc::from_raw`][from_raw].
371 /// [from_raw]: struct.Rc.html#method.from_raw
378 /// let x = Rc::new(10);
379 /// let x_ptr = Rc::into_raw(x);
380 /// assert_eq!(unsafe { *x_ptr }, 10);
382 #[stable(feature = "rc_raw", since = "1.17.0")]
383 pub fn into_raw(this: Self) -> *const T {
384 let ptr: *const T = &*this;
389 /// Constructs an `Rc` from a raw pointer.
391 /// The raw pointer must have been previously returned by a call to a
392 /// [`Rc::into_raw`][into_raw].
394 /// This function is unsafe because improper use may lead to memory problems. For example, a
395 /// double-free may occur if the function is called twice on the same raw pointer.
397 /// [into_raw]: struct.Rc.html#method.into_raw
404 /// let x = Rc::new(10);
405 /// let x_ptr = Rc::into_raw(x);
408 /// // Convert back to an `Rc` to prevent leak.
409 /// let x = Rc::from_raw(x_ptr);
410 /// assert_eq!(*x, 10);
412 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
415 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
417 #[stable(feature = "rc_raw", since = "1.17.0")]
418 pub unsafe fn from_raw(ptr: *const T) -> Self {
419 let offset = data_offset(ptr);
421 // Reverse the offset to find the original RcBox.
422 let fake_ptr = ptr as *mut RcBox<T>;
423 let rc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
426 ptr: NonNull::new_unchecked(rc_ptr),
427 phantom: PhantomData,
431 /// Consumes the `Rc`, returning the wrapped pointer as `NonNull<T>`.
436 /// #![feature(rc_into_raw_non_null)]
440 /// let x = Rc::new(10);
441 /// let ptr = Rc::into_raw_non_null(x);
442 /// let deref = unsafe { *ptr.as_ref() };
443 /// assert_eq!(deref, 10);
445 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
447 pub fn into_raw_non_null(this: Self) -> NonNull<T> {
448 // safe because Rc guarantees its pointer is non-null
449 unsafe { NonNull::new_unchecked(Rc::into_raw(this) as *mut _) }
452 /// Creates a new [`Weak`][weak] pointer to this value.
454 /// [weak]: struct.Weak.html
461 /// let five = Rc::new(5);
463 /// let weak_five = Rc::downgrade(&five);
465 #[stable(feature = "rc_weak", since = "1.4.0")]
466 pub fn downgrade(this: &Self) -> Weak<T> {
468 // Make sure we do not create a dangling Weak
469 debug_assert!(!is_dangling(this.ptr));
470 Weak { ptr: this.ptr }
473 /// Gets the number of [`Weak`][weak] pointers to this value.
475 /// [weak]: struct.Weak.html
482 /// let five = Rc::new(5);
483 /// let _weak_five = Rc::downgrade(&five);
485 /// assert_eq!(1, Rc::weak_count(&five));
488 #[stable(feature = "rc_counts", since = "1.15.0")]
489 pub fn weak_count(this: &Self) -> usize {
493 /// Gets the number of strong (`Rc`) pointers to this value.
500 /// let five = Rc::new(5);
501 /// let _also_five = Rc::clone(&five);
503 /// assert_eq!(2, Rc::strong_count(&five));
506 #[stable(feature = "rc_counts", since = "1.15.0")]
507 pub fn strong_count(this: &Self) -> usize {
511 /// Returns `true` if there are no other `Rc` or [`Weak`][weak] pointers to
512 /// this inner value.
514 /// [weak]: struct.Weak.html
516 fn is_unique(this: &Self) -> bool {
517 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
520 /// Returns a mutable reference to the inner value, if there are
521 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
523 /// Returns [`None`] otherwise, because it is not safe to
524 /// mutate a shared value.
526 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
527 /// the inner value when it's shared.
529 /// [weak]: struct.Weak.html
530 /// [`None`]: ../../std/option/enum.Option.html#variant.None
531 /// [make_mut]: struct.Rc.html#method.make_mut
532 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
539 /// let mut x = Rc::new(3);
540 /// *Rc::get_mut(&mut x).unwrap() = 4;
541 /// assert_eq!(*x, 4);
543 /// let _y = Rc::clone(&x);
544 /// assert!(Rc::get_mut(&mut x).is_none());
547 #[stable(feature = "rc_unique", since = "1.4.0")]
548 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
549 if Rc::is_unique(this) {
551 Some(&mut this.ptr.as_mut().value)
559 #[stable(feature = "ptr_eq", since = "1.17.0")]
560 /// Returns `true` if the two `Rc`s point to the same value (not
561 /// just values that compare as equal).
568 /// let five = Rc::new(5);
569 /// let same_five = Rc::clone(&five);
570 /// let other_five = Rc::new(5);
572 /// assert!(Rc::ptr_eq(&five, &same_five));
573 /// assert!(!Rc::ptr_eq(&five, &other_five));
575 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
576 this.ptr.as_ptr() == other.ptr.as_ptr()
580 impl<T: Clone> Rc<T> {
581 /// Makes a mutable reference into the given `Rc`.
583 /// If there are other `Rc` pointers to the same value, then `make_mut` will
584 /// [`clone`] the inner value to ensure unique ownership. This is also
585 /// referred to as clone-on-write.
587 /// If there are no other `Rc` pointers to this value, then [`Weak`]
588 /// pointers to this value will be dissassociated.
590 /// See also [`get_mut`], which will fail rather than cloning.
592 /// [`Weak`]: struct.Weak.html
593 /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
594 /// [`get_mut`]: struct.Rc.html#method.get_mut
601 /// let mut data = Rc::new(5);
603 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
604 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
605 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
606 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
607 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
609 /// // Now `data` and `other_data` point to different values.
610 /// assert_eq!(*data, 8);
611 /// assert_eq!(*other_data, 12);
614 /// [`Weak`] pointers will be dissassociated:
619 /// let mut data = Rc::new(75);
620 /// let weak = Rc::downgrade(&data);
622 /// assert!(75 == *data);
623 /// assert!(75 == *weak.upgrade().unwrap());
625 /// *Rc::make_mut(&mut data) += 1;
627 /// assert!(76 == *data);
628 /// assert!(weak.upgrade().is_none());
631 #[stable(feature = "rc_unique", since = "1.4.0")]
632 pub fn make_mut(this: &mut Self) -> &mut T {
633 if Rc::strong_count(this) != 1 {
634 // Gotta clone the data, there are other Rcs
635 *this = Rc::new((**this).clone())
636 } else if Rc::weak_count(this) != 0 {
637 // Can just steal the data, all that's left is Weaks
639 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
640 mem::swap(this, &mut swap);
642 // Remove implicit strong-weak ref (no need to craft a fake
643 // Weak here -- we know other Weaks can clean up for us)
648 // This unsafety is ok because we're guaranteed that the pointer
649 // returned is the *only* pointer that will ever be returned to T. Our
650 // reference count is guaranteed to be 1 at this point, and we required
651 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
652 // reference to the inner value.
654 &mut this.ptr.as_mut().value
661 #[stable(feature = "rc_downcast", since = "1.29.0")]
662 /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
667 /// use std::any::Any;
670 /// fn print_if_string(value: Rc<dyn Any>) {
671 /// if let Ok(string) = value.downcast::<String>() {
672 /// println!("String ({}): {}", string.len(), string);
677 /// let my_string = "Hello World".to_string();
678 /// print_if_string(Rc::new(my_string));
679 /// print_if_string(Rc::new(0i8));
682 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
683 if (*self).is::<T>() {
684 let ptr = self.ptr.cast::<RcBox<T>>();
686 Ok(Rc { ptr, phantom: PhantomData })
693 impl<T: ?Sized> Rc<T> {
694 // Allocates an `RcBox<T>` with sufficient space for an unsized value
695 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
696 // Calculate layout using the given value.
697 // Previously, layout was calculated on the expression
698 // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
699 // reference (see #54908).
700 let layout = Layout::new::<RcBox<()>>()
701 .extend(Layout::for_value(&*ptr)).unwrap().0
702 .pad_to_align().unwrap();
704 let mem = Global.alloc(layout)
705 .unwrap_or_else(|_| handle_alloc_error(layout));
707 // Initialize the RcBox
708 let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut RcBox<T>;
709 debug_assert_eq!(Layout::for_value(&*inner), layout);
711 ptr::write(&mut (*inner).strong, Cell::new(1));
712 ptr::write(&mut (*inner).weak, Cell::new(1));
717 fn from_box(v: Box<T>) -> Rc<T> {
719 let box_unique = Box::into_unique(v);
720 let bptr = box_unique.as_ptr();
722 let value_size = size_of_val(&*bptr);
723 let ptr = Self::allocate_for_ptr(bptr);
725 // Copy value as bytes
726 ptr::copy_nonoverlapping(
727 bptr as *const T as *const u8,
728 &mut (*ptr).value as *mut _ as *mut u8,
731 // Free the allocation without dropping its contents
732 box_free(box_unique);
734 Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
739 // Sets the data pointer of a `?Sized` raw pointer.
741 // For a slice/trait object, this sets the `data` field and leaves the rest
742 // unchanged. For a sized raw pointer, this simply sets the pointer.
743 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
744 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
749 // Copy elements from slice into newly allocated Rc<[T]>
751 // Unsafe because the caller must either take ownership or bind `T: Copy`
752 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
753 let v_ptr = v as *const [T];
754 let ptr = Self::allocate_for_ptr(v_ptr);
756 ptr::copy_nonoverlapping(
758 &mut (*ptr).value as *mut [T] as *mut T,
761 Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
765 trait RcFromSlice<T> {
766 fn from_slice(slice: &[T]) -> Self;
769 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
771 default fn from_slice(v: &[T]) -> Self {
772 // Panic guard while cloning T elements.
773 // In the event of a panic, elements that have been written
774 // into the new RcBox will be dropped, then the memory freed.
782 impl<T> Drop for Guard<T> {
785 let slice = from_raw_parts_mut(self.elems, self.n_elems);
786 ptr::drop_in_place(slice);
788 Global.dealloc(self.mem, self.layout.clone());
794 let v_ptr = v as *const [T];
795 let ptr = Self::allocate_for_ptr(v_ptr);
797 let mem = ptr as *mut _ as *mut u8;
798 let layout = Layout::for_value(&*ptr);
800 // Pointer to first element
801 let elems = &mut (*ptr).value as *mut [T] as *mut T;
803 let mut guard = Guard{
804 mem: NonNull::new_unchecked(mem),
810 for (i, item) in v.iter().enumerate() {
811 ptr::write(elems.add(i), item.clone());
815 // All clear. Forget the guard so it doesn't free the new RcBox.
818 Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
823 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
825 fn from_slice(v: &[T]) -> Self {
826 unsafe { Rc::copy_from_slice(v) }
830 #[stable(feature = "rust1", since = "1.0.0")]
831 impl<T: ?Sized> Deref for Rc<T> {
835 fn deref(&self) -> &T {
840 #[unstable(feature = "receiver_trait", issue = "0")]
841 impl<T: ?Sized> Receiver for Rc<T> {}
843 #[stable(feature = "rust1", since = "1.0.0")]
844 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
847 /// This will decrement the strong reference count. If the strong reference
848 /// count reaches zero then the only other references (if any) are
849 /// [`Weak`], so we `drop` the inner value.
858 /// impl Drop for Foo {
859 /// fn drop(&mut self) {
860 /// println!("dropped!");
864 /// let foo = Rc::new(Foo);
865 /// let foo2 = Rc::clone(&foo);
867 /// drop(foo); // Doesn't print anything
868 /// drop(foo2); // Prints "dropped!"
871 /// [`Weak`]: ../../std/rc/struct.Weak.html
875 if self.strong() == 0 {
876 // destroy the contained object
877 ptr::drop_in_place(self.ptr.as_mut());
879 // remove the implicit "strong weak" pointer now that we've
880 // destroyed the contents.
883 if self.weak() == 0 {
884 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
891 #[stable(feature = "rust1", since = "1.0.0")]
892 impl<T: ?Sized> Clone for Rc<T> {
893 /// Makes a clone of the `Rc` pointer.
895 /// This creates another pointer to the same inner value, increasing the
896 /// strong reference count.
903 /// let five = Rc::new(5);
905 /// let _ = Rc::clone(&five);
908 fn clone(&self) -> Rc<T> {
910 Rc { ptr: self.ptr, phantom: PhantomData }
914 #[stable(feature = "rust1", since = "1.0.0")]
915 impl<T: Default> Default for Rc<T> {
916 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
923 /// let x: Rc<i32> = Default::default();
924 /// assert_eq!(*x, 0);
927 fn default() -> Rc<T> {
928 Rc::new(Default::default())
932 #[stable(feature = "rust1", since = "1.0.0")]
933 trait RcEqIdent<T: ?Sized + PartialEq> {
934 fn eq(&self, other: &Rc<T>) -> bool;
935 fn ne(&self, other: &Rc<T>) -> bool;
938 #[stable(feature = "rust1", since = "1.0.0")]
939 impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
941 default fn eq(&self, other: &Rc<T>) -> bool {
946 default fn ne(&self, other: &Rc<T>) -> bool {
951 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
952 /// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
953 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
954 /// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
955 /// the same value, than two `&T`s.
956 #[stable(feature = "rust1", since = "1.0.0")]
957 impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
959 fn eq(&self, other: &Rc<T>) -> bool {
960 Rc::ptr_eq(self, other) || **self == **other
964 fn ne(&self, other: &Rc<T>) -> bool {
965 !Rc::ptr_eq(self, other) && **self != **other
969 #[stable(feature = "rust1", since = "1.0.0")]
970 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
971 /// Equality for two `Rc`s.
973 /// Two `Rc`s are equal if their inner values are equal.
975 /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
983 /// let five = Rc::new(5);
985 /// assert!(five == Rc::new(5));
988 fn eq(&self, other: &Rc<T>) -> bool {
989 RcEqIdent::eq(self, other)
992 /// Inequality for two `Rc`s.
994 /// Two `Rc`s are unequal if their inner values are unequal.
996 /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
1002 /// use std::rc::Rc;
1004 /// let five = Rc::new(5);
1006 /// assert!(five != Rc::new(6));
1009 fn ne(&self, other: &Rc<T>) -> bool {
1010 RcEqIdent::ne(self, other)
1014 #[stable(feature = "rust1", since = "1.0.0")]
1015 impl<T: ?Sized + Eq> Eq for Rc<T> {}
1017 #[stable(feature = "rust1", since = "1.0.0")]
1018 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
1019 /// Partial comparison for two `Rc`s.
1021 /// The two are compared by calling `partial_cmp()` on their inner values.
1026 /// use std::rc::Rc;
1027 /// use std::cmp::Ordering;
1029 /// let five = Rc::new(5);
1031 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
1034 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
1035 (**self).partial_cmp(&**other)
1038 /// Less-than comparison for two `Rc`s.
1040 /// The two are compared by calling `<` on their inner values.
1045 /// use std::rc::Rc;
1047 /// let five = Rc::new(5);
1049 /// assert!(five < Rc::new(6));
1052 fn lt(&self, other: &Rc<T>) -> bool {
1056 /// 'Less than or equal to' comparison for two `Rc`s.
1058 /// The two are compared by calling `<=` on their inner values.
1063 /// use std::rc::Rc;
1065 /// let five = Rc::new(5);
1067 /// assert!(five <= Rc::new(5));
1070 fn le(&self, other: &Rc<T>) -> bool {
1074 /// Greater-than comparison for two `Rc`s.
1076 /// The two are compared by calling `>` on their inner values.
1081 /// use std::rc::Rc;
1083 /// let five = Rc::new(5);
1085 /// assert!(five > Rc::new(4));
1088 fn gt(&self, other: &Rc<T>) -> bool {
1092 /// 'Greater than or equal to' comparison for two `Rc`s.
1094 /// The two are compared by calling `>=` on their inner values.
1099 /// use std::rc::Rc;
1101 /// let five = Rc::new(5);
1103 /// assert!(five >= Rc::new(5));
1106 fn ge(&self, other: &Rc<T>) -> bool {
1111 #[stable(feature = "rust1", since = "1.0.0")]
1112 impl<T: ?Sized + Ord> Ord for Rc<T> {
1113 /// Comparison for two `Rc`s.
1115 /// The two are compared by calling `cmp()` on their inner values.
1120 /// use std::rc::Rc;
1121 /// use std::cmp::Ordering;
1123 /// let five = Rc::new(5);
1125 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1128 fn cmp(&self, other: &Rc<T>) -> Ordering {
1129 (**self).cmp(&**other)
1133 #[stable(feature = "rust1", since = "1.0.0")]
1134 impl<T: ?Sized + Hash> Hash for Rc<T> {
1135 fn hash<H: Hasher>(&self, state: &mut H) {
1136 (**self).hash(state);
1140 #[stable(feature = "rust1", since = "1.0.0")]
1141 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1142 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1143 fmt::Display::fmt(&**self, f)
1147 #[stable(feature = "rust1", since = "1.0.0")]
1148 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1149 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1150 fmt::Debug::fmt(&**self, f)
1154 #[stable(feature = "rust1", since = "1.0.0")]
1155 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1156 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1157 fmt::Pointer::fmt(&(&**self as *const T), f)
1161 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1162 impl<T> From<T> for Rc<T> {
1163 fn from(t: T) -> Self {
1168 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1169 impl<T: Clone> From<&[T]> for Rc<[T]> {
1171 fn from(v: &[T]) -> Rc<[T]> {
1172 <Self as RcFromSlice<T>>::from_slice(v)
1176 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1177 impl From<&str> for Rc<str> {
1179 fn from(v: &str) -> Rc<str> {
1180 let rc = Rc::<[u8]>::from(v.as_bytes());
1181 unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
1185 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1186 impl From<String> for Rc<str> {
1188 fn from(v: String) -> Rc<str> {
1193 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1194 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1196 fn from(v: Box<T>) -> Rc<T> {
1201 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1202 impl<T> From<Vec<T>> for Rc<[T]> {
1204 fn from(mut v: Vec<T>) -> Rc<[T]> {
1206 let rc = Rc::copy_from_slice(&v);
1208 // Allow the Vec to free its memory, but not destroy its contents
1216 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1217 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
1218 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1220 /// Since a `Weak` reference does not count towards ownership, it will not
1221 /// prevent the inner value from being dropped, and `Weak` itself makes no
1222 /// guarantees about the value still being present and may return [`None`]
1223 /// when [`upgrade`]d.
1225 /// A `Weak` pointer is useful for keeping a temporary reference to the value
1226 /// within [`Rc`] without extending its lifetime. It is also used to prevent
1227 /// circular references between [`Rc`] pointers, since mutual owning references
1228 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1229 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1230 /// pointers from children back to their parents.
1232 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1234 /// [`Rc`]: struct.Rc.html
1235 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1236 /// [`upgrade`]: struct.Weak.html#method.upgrade
1237 /// [`Option`]: ../../std/option/enum.Option.html
1238 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1239 #[stable(feature = "rc_weak", since = "1.4.0")]
1240 pub struct Weak<T: ?Sized> {
1241 // This is a `NonNull` to allow optimizing the size of this type in enums,
1242 // but it is not necessarily a valid pointer.
1243 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
1244 // to allocate space on the heap. That's not a value a real pointer
1245 // will ever have because RcBox has alignment at least 2.
1246 ptr: NonNull<RcBox<T>>,
1249 #[stable(feature = "rc_weak", since = "1.4.0")]
1250 impl<T: ?Sized> !marker::Send for Weak<T> {}
1251 #[stable(feature = "rc_weak", since = "1.4.0")]
1252 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1254 #[unstable(feature = "coerce_unsized", issue = "27732")]
1255 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1257 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
1258 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
1261 /// Constructs a new `Weak<T>`, without allocating any memory.
1262 /// Calling [`upgrade`] on the return value always gives [`None`].
1264 /// [`upgrade`]: #method.upgrade
1265 /// [`None`]: ../../std/option/enum.Option.html
1270 /// use std::rc::Weak;
1272 /// let empty: Weak<i64> = Weak::new();
1273 /// assert!(empty.upgrade().is_none());
1275 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1276 pub fn new() -> Weak<T> {
1278 ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"),
1282 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1284 /// It is up to the caller to ensure that the object is still alive when accessing it through
1287 /// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
1292 /// #![feature(weak_into_raw)]
1294 /// use std::rc::{Rc, Weak};
1297 /// let strong = Rc::new(42);
1298 /// let weak = Rc::downgrade(&strong);
1299 /// // Both point to the same object
1300 /// assert!(ptr::eq(&*strong, Weak::as_raw(&weak)));
1301 /// // The strong here keeps it alive, so we can still access the object.
1302 /// assert_eq!(42, unsafe { *Weak::as_raw(&weak) });
1305 /// // But not any more. We can do Weak::as_raw(&weak), but accessing the pointer would lead to
1306 /// // undefined behaviour.
1307 /// // assert_eq!(42, unsafe { *Weak::as_raw(&weak) });
1310 /// [`null`]: ../../std/ptr/fn.null.html
1311 #[unstable(feature = "weak_into_raw", issue = "60728")]
1312 pub fn as_raw(this: &Self) -> *const T {
1313 match this.inner() {
1314 None => ptr::null(),
1316 let offset = data_offset_sized::<T>();
1317 let ptr = inner as *const RcBox<T>;
1318 // Note: while the pointer we create may already point to dropped value, the
1319 // allocation still lives (it must hold the weak point as long as we are alive).
1320 // Therefore, the offset is OK to do, it won't get out of the allocation.
1321 let ptr = unsafe { (ptr as *const u8).offset(offset) };
1327 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1329 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1330 /// can be turned back into the `Weak<T>` with [`from_raw`].
1332 /// The same restrictions of accessing the target of the pointer as with
1333 /// [`as_raw`] apply.
1338 /// #![feature(weak_into_raw)]
1340 /// use std::rc::{Rc, Weak};
1342 /// let strong = Rc::new(42);
1343 /// let weak = Rc::downgrade(&strong);
1344 /// let raw = Weak::into_raw(weak);
1346 /// assert_eq!(1, Rc::weak_count(&strong));
1347 /// assert_eq!(42, unsafe { *raw });
1349 /// drop(unsafe { Weak::from_raw(raw) });
1350 /// assert_eq!(0, Rc::weak_count(&strong));
1353 /// [`from_raw`]: struct.Weak.html#method.from_raw
1354 /// [`as_raw`]: struct.Weak.html#method.as_raw
1355 #[unstable(feature = "weak_into_raw", issue = "60728")]
1356 pub fn into_raw(this: Self) -> *const T {
1357 let result = Self::as_raw(&this);
1362 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
1364 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1365 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1367 /// It takes ownership of one weak count. In case a [`null`] is passed, a dangling [`Weak`] is
1372 /// The pointer must represent one valid weak count. In other words, it must point to `T` which
1373 /// is or *was* managed by an [`Rc`] and the weak count of that [`Rc`] must not have reached
1374 /// 0. It is allowed for the strong count to be 0.
1379 /// #![feature(weak_into_raw)]
1381 /// use std::rc::{Rc, Weak};
1383 /// let strong = Rc::new(42);
1385 /// let raw_1 = Weak::into_raw(Rc::downgrade(&strong));
1386 /// let raw_2 = Weak::into_raw(Rc::downgrade(&strong));
1388 /// assert_eq!(2, Rc::weak_count(&strong));
1390 /// assert_eq!(42, *Weak::upgrade(&unsafe { Weak::from_raw(raw_1) }).unwrap());
1391 /// assert_eq!(1, Rc::weak_count(&strong));
1395 /// // Decrement the last weak count.
1396 /// assert!(Weak::upgrade(&unsafe { Weak::from_raw(raw_2) }).is_none());
1399 /// [`null`]: ../../std/ptr/fn.null.html
1400 /// [`into_raw`]: struct.Weak.html#method.into_raw
1401 /// [`upgrade`]: struct.Weak.html#method.upgrade
1402 /// [`Rc`]: struct.Rc.html
1403 /// [`Weak`]: struct.Weak.html
1404 #[unstable(feature = "weak_into_raw", issue = "60728")]
1405 pub unsafe fn from_raw(ptr: *const T) -> Self {
1409 // See Rc::from_raw for details
1410 let offset = data_offset(ptr);
1411 let fake_ptr = ptr as *mut RcBox<T>;
1412 let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
1414 ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
1420 pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
1421 let address = ptr.as_ptr() as *mut () as usize;
1422 address == usize::MAX
1425 impl<T: ?Sized> Weak<T> {
1426 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1427 /// the lifetime of the value if successful.
1429 /// Returns [`None`] if the value has since been dropped.
1431 /// [`Rc`]: struct.Rc.html
1432 /// [`None`]: ../../std/option/enum.Option.html
1437 /// use std::rc::Rc;
1439 /// let five = Rc::new(5);
1441 /// let weak_five = Rc::downgrade(&five);
1443 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1444 /// assert!(strong_five.is_some());
1446 /// // Destroy all strong pointers.
1447 /// drop(strong_five);
1450 /// assert!(weak_five.upgrade().is_none());
1452 #[stable(feature = "rc_weak", since = "1.4.0")]
1453 pub fn upgrade(&self) -> Option<Rc<T>> {
1454 let inner = self.inner()?;
1455 if inner.strong() == 0 {
1459 Some(Rc { ptr: self.ptr, phantom: PhantomData })
1463 /// Gets the number of strong (`Rc`) pointers pointing to this value.
1465 /// If `self` was created using [`Weak::new`], this will return 0.
1467 /// [`Weak::new`]: #method.new
1468 #[unstable(feature = "weak_counts", issue = "57977")]
1469 pub fn strong_count(&self) -> usize {
1470 if let Some(inner) = self.inner() {
1477 /// Gets the number of `Weak` pointers pointing to this value.
1479 /// If `self` was created using [`Weak::new`], this will return `None`. If
1480 /// not, the returned value is at least 1, since `self` still points to the
1483 /// [`Weak::new`]: #method.new
1484 #[unstable(feature = "weak_counts", issue = "57977")]
1485 pub fn weak_count(&self) -> Option<usize> {
1486 self.inner().map(|inner| {
1487 if inner.strong() > 0 {
1488 inner.weak() - 1 // subtract the implicit weak ptr
1495 /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
1496 /// (i.e., when this `Weak` was created by `Weak::new`).
1498 fn inner(&self) -> Option<&RcBox<T>> {
1499 if is_dangling(self.ptr) {
1502 Some(unsafe { self.ptr.as_ref() })
1506 /// Returns `true` if the two `Weak`s point to the same value (not just values
1507 /// that compare as equal).
1511 /// Since this compares pointers it means that `Weak::new()` will equal each
1512 /// other, even though they don't point to any value.
1517 /// #![feature(weak_ptr_eq)]
1518 /// use std::rc::{Rc, Weak};
1520 /// let first_rc = Rc::new(5);
1521 /// let first = Rc::downgrade(&first_rc);
1522 /// let second = Rc::downgrade(&first_rc);
1524 /// assert!(Weak::ptr_eq(&first, &second));
1526 /// let third_rc = Rc::new(5);
1527 /// let third = Rc::downgrade(&third_rc);
1529 /// assert!(!Weak::ptr_eq(&first, &third));
1532 /// Comparing `Weak::new`.
1535 /// #![feature(weak_ptr_eq)]
1536 /// use std::rc::{Rc, Weak};
1538 /// let first = Weak::new();
1539 /// let second = Weak::new();
1540 /// assert!(Weak::ptr_eq(&first, &second));
1542 /// let third_rc = Rc::new(());
1543 /// let third = Rc::downgrade(&third_rc);
1544 /// assert!(!Weak::ptr_eq(&first, &third));
1547 #[unstable(feature = "weak_ptr_eq", issue = "55981")]
1548 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1549 this.ptr.as_ptr() == other.ptr.as_ptr()
1553 #[stable(feature = "rc_weak", since = "1.4.0")]
1554 impl<T: ?Sized> Drop for Weak<T> {
1555 /// Drops the `Weak` pointer.
1560 /// use std::rc::{Rc, Weak};
1564 /// impl Drop for Foo {
1565 /// fn drop(&mut self) {
1566 /// println!("dropped!");
1570 /// let foo = Rc::new(Foo);
1571 /// let weak_foo = Rc::downgrade(&foo);
1572 /// let other_weak_foo = Weak::clone(&weak_foo);
1574 /// drop(weak_foo); // Doesn't print anything
1575 /// drop(foo); // Prints "dropped!"
1577 /// assert!(other_weak_foo.upgrade().is_none());
1579 fn drop(&mut self) {
1580 if let Some(inner) = self.inner() {
1582 // the weak count starts at 1, and will only go to zero if all
1583 // the strong pointers have disappeared.
1584 if inner.weak() == 0 {
1586 Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
1593 #[stable(feature = "rc_weak", since = "1.4.0")]
1594 impl<T: ?Sized> Clone for Weak<T> {
1595 /// Makes a clone of the `Weak` pointer that points to the same value.
1600 /// use std::rc::{Rc, Weak};
1602 /// let weak_five = Rc::downgrade(&Rc::new(5));
1604 /// let _ = Weak::clone(&weak_five);
1607 fn clone(&self) -> Weak<T> {
1608 if let Some(inner) = self.inner() {
1611 Weak { ptr: self.ptr }
1615 #[stable(feature = "rc_weak", since = "1.4.0")]
1616 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1617 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1622 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1623 impl<T> Default for Weak<T> {
1624 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1625 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1627 /// [`None`]: ../../std/option/enum.Option.html
1628 /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
1633 /// use std::rc::Weak;
1635 /// let empty: Weak<i64> = Default::default();
1636 /// assert!(empty.upgrade().is_none());
1638 fn default() -> Weak<T> {
1643 // NOTE: We checked_add here to deal with mem::forget safely. In particular
1644 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1645 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1646 // We abort because this is such a degenerate scenario that we don't care about
1647 // what happens -- no real program should ever experience this.
1649 // This should have negligible overhead since you don't actually need to
1650 // clone these much in Rust thanks to ownership and move-semantics.
1653 trait RcBoxPtr<T: ?Sized> {
1654 fn inner(&self) -> &RcBox<T>;
1657 fn strong(&self) -> usize {
1658 self.inner().strong.get()
1662 fn inc_strong(&self) {
1663 // We want to abort on overflow instead of dropping the value.
1664 // The reference count will never be zero when this is called;
1665 // nevertheless, we insert an abort here to hint LLVM at
1666 // an otherwise missed optimization.
1667 if self.strong() == 0 || self.strong() == usize::max_value() {
1670 self.inner().strong.set(self.strong() + 1);
1674 fn dec_strong(&self) {
1675 self.inner().strong.set(self.strong() - 1);
1679 fn weak(&self) -> usize {
1680 self.inner().weak.get()
1684 fn inc_weak(&self) {
1685 // We want to abort on overflow instead of dropping the value.
1686 // The reference count will never be zero when this is called;
1687 // nevertheless, we insert an abort here to hint LLVM at
1688 // an otherwise missed optimization.
1689 if self.weak() == 0 || self.weak() == usize::max_value() {
1692 self.inner().weak.set(self.weak() + 1);
1696 fn dec_weak(&self) {
1697 self.inner().weak.set(self.weak() - 1);
1701 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1703 fn inner(&self) -> &RcBox<T> {
1710 impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
1712 fn inner(&self) -> &RcBox<T> {
1719 use super::{Rc, Weak};
1720 use std::boxed::Box;
1721 use std::cell::RefCell;
1722 use std::option::Option::{self, None, Some};
1723 use std::result::Result::{Err, Ok};
1725 use std::clone::Clone;
1726 use std::convert::From;
1730 let x = Rc::new(RefCell::new(5));
1732 *x.borrow_mut() = 20;
1733 assert_eq!(*y.borrow(), 20);
1743 fn test_simple_clone() {
1751 fn test_destructor() {
1752 let x: Rc<Box<_>> = Rc::new(box 5);
1759 let y = Rc::downgrade(&x);
1760 assert!(y.upgrade().is_some());
1766 let y = Rc::downgrade(&x);
1768 assert!(y.upgrade().is_none());
1772 fn weak_self_cyclic() {
1774 x: RefCell<Option<Weak<Cycle>>>,
1777 let a = Rc::new(Cycle { x: RefCell::new(None) });
1778 let b = Rc::downgrade(&a.clone());
1779 *a.x.borrow_mut() = Some(b);
1781 // hopefully we don't double-free (or leak)...
1787 assert!(Rc::is_unique(&x));
1789 assert!(!Rc::is_unique(&x));
1791 assert!(Rc::is_unique(&x));
1792 let w = Rc::downgrade(&x);
1793 assert!(!Rc::is_unique(&x));
1795 assert!(Rc::is_unique(&x));
1799 fn test_strong_count() {
1801 assert!(Rc::strong_count(&a) == 1);
1802 let w = Rc::downgrade(&a);
1803 assert!(Rc::strong_count(&a) == 1);
1804 let b = w.upgrade().expect("upgrade of live rc failed");
1805 assert!(Rc::strong_count(&b) == 2);
1806 assert!(Rc::strong_count(&a) == 2);
1809 assert!(Rc::strong_count(&b) == 1);
1811 assert!(Rc::strong_count(&b) == 2);
1812 assert!(Rc::strong_count(&c) == 2);
1816 fn test_weak_count() {
1818 assert!(Rc::strong_count(&a) == 1);
1819 assert!(Rc::weak_count(&a) == 0);
1820 let w = Rc::downgrade(&a);
1821 assert!(Rc::strong_count(&a) == 1);
1822 assert!(Rc::weak_count(&a) == 1);
1824 assert!(Rc::strong_count(&a) == 1);
1825 assert!(Rc::weak_count(&a) == 0);
1827 assert!(Rc::strong_count(&a) == 2);
1828 assert!(Rc::weak_count(&a) == 0);
1834 assert_eq!(Weak::weak_count(&Weak::<u64>::new()), None);
1835 assert_eq!(Weak::strong_count(&Weak::<u64>::new()), 0);
1838 let w = Rc::downgrade(&a);
1839 assert_eq!(Weak::strong_count(&w), 1);
1840 assert_eq!(Weak::weak_count(&w), Some(1));
1842 assert_eq!(Weak::strong_count(&w), 1);
1843 assert_eq!(Weak::weak_count(&w), Some(2));
1844 assert_eq!(Weak::strong_count(&w2), 1);
1845 assert_eq!(Weak::weak_count(&w2), Some(2));
1847 assert_eq!(Weak::strong_count(&w2), 1);
1848 assert_eq!(Weak::weak_count(&w2), Some(1));
1850 assert_eq!(Weak::strong_count(&w2), 2);
1851 assert_eq!(Weak::weak_count(&w2), Some(1));
1854 assert_eq!(Weak::strong_count(&w2), 0);
1855 assert_eq!(Weak::weak_count(&w2), Some(1));
1862 assert_eq!(Rc::try_unwrap(x), Ok(3));
1865 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
1867 let _w = Rc::downgrade(&x);
1868 assert_eq!(Rc::try_unwrap(x), Ok(5));
1872 fn into_from_raw() {
1873 let x = Rc::new(box "hello");
1876 let x_ptr = Rc::into_raw(x);
1879 assert_eq!(**x_ptr, "hello");
1881 let x = Rc::from_raw(x_ptr);
1882 assert_eq!(**x, "hello");
1884 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
1889 fn test_into_from_raw_unsized() {
1890 use std::fmt::Display;
1891 use std::string::ToString;
1893 let rc: Rc<str> = Rc::from("foo");
1895 let ptr = Rc::into_raw(rc.clone());
1896 let rc2 = unsafe { Rc::from_raw(ptr) };
1898 assert_eq!(unsafe { &*ptr }, "foo");
1899 assert_eq!(rc, rc2);
1901 let rc: Rc<dyn Display> = Rc::new(123);
1903 let ptr = Rc::into_raw(rc.clone());
1904 let rc2 = unsafe { Rc::from_raw(ptr) };
1906 assert_eq!(unsafe { &*ptr }.to_string(), "123");
1907 assert_eq!(rc2.to_string(), "123");
1912 let mut x = Rc::new(3);
1913 *Rc::get_mut(&mut x).unwrap() = 4;
1916 assert!(Rc::get_mut(&mut x).is_none());
1918 assert!(Rc::get_mut(&mut x).is_some());
1919 let _w = Rc::downgrade(&x);
1920 assert!(Rc::get_mut(&mut x).is_none());
1924 fn test_cowrc_clone_make_unique() {
1925 let mut cow0 = Rc::new(75);
1926 let mut cow1 = cow0.clone();
1927 let mut cow2 = cow1.clone();
1929 assert!(75 == *Rc::make_mut(&mut cow0));
1930 assert!(75 == *Rc::make_mut(&mut cow1));
1931 assert!(75 == *Rc::make_mut(&mut cow2));
1933 *Rc::make_mut(&mut cow0) += 1;
1934 *Rc::make_mut(&mut cow1) += 2;
1935 *Rc::make_mut(&mut cow2) += 3;
1937 assert!(76 == *cow0);
1938 assert!(77 == *cow1);
1939 assert!(78 == *cow2);
1941 // none should point to the same backing memory
1942 assert!(*cow0 != *cow1);
1943 assert!(*cow0 != *cow2);
1944 assert!(*cow1 != *cow2);
1948 fn test_cowrc_clone_unique2() {
1949 let mut cow0 = Rc::new(75);
1950 let cow1 = cow0.clone();
1951 let cow2 = cow1.clone();
1953 assert!(75 == *cow0);
1954 assert!(75 == *cow1);
1955 assert!(75 == *cow2);
1957 *Rc::make_mut(&mut cow0) += 1;
1959 assert!(76 == *cow0);
1960 assert!(75 == *cow1);
1961 assert!(75 == *cow2);
1963 // cow1 and cow2 should share the same contents
1964 // cow0 should have a unique reference
1965 assert!(*cow0 != *cow1);
1966 assert!(*cow0 != *cow2);
1967 assert!(*cow1 == *cow2);
1971 fn test_cowrc_clone_weak() {
1972 let mut cow0 = Rc::new(75);
1973 let cow1_weak = Rc::downgrade(&cow0);
1975 assert!(75 == *cow0);
1976 assert!(75 == *cow1_weak.upgrade().unwrap());
1978 *Rc::make_mut(&mut cow0) += 1;
1980 assert!(76 == *cow0);
1981 assert!(cow1_weak.upgrade().is_none());
1986 let foo = Rc::new(75);
1987 assert_eq!(format!("{:?}", foo), "75");
1992 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
1993 assert_eq!(foo, foo.clone());
1997 fn test_from_owned() {
1999 let foo_rc = Rc::from(foo);
2000 assert!(123 == *foo_rc);
2004 fn test_new_weak() {
2005 let foo: Weak<usize> = Weak::new();
2006 assert!(foo.upgrade().is_none());
2011 let five = Rc::new(5);
2012 let same_five = five.clone();
2013 let other_five = Rc::new(5);
2015 assert!(Rc::ptr_eq(&five, &same_five));
2016 assert!(!Rc::ptr_eq(&five, &other_five));
2020 fn test_from_str() {
2021 let r: Rc<str> = Rc::from("foo");
2023 assert_eq!(&r[..], "foo");
2027 fn test_copy_from_slice() {
2028 let s: &[u32] = &[1, 2, 3];
2029 let r: Rc<[u32]> = Rc::from(s);
2031 assert_eq!(&r[..], [1, 2, 3]);
2035 fn test_clone_from_slice() {
2036 #[derive(Clone, Debug, Eq, PartialEq)]
2039 let s: &[X] = &[X(1), X(2), X(3)];
2040 let r: Rc<[X]> = Rc::from(s);
2042 assert_eq!(&r[..], s);
2047 fn test_clone_from_slice_panic() {
2048 use std::string::{String, ToString};
2050 struct Fail(u32, String);
2052 impl Clone for Fail {
2053 fn clone(&self) -> Fail {
2057 Fail(self.0, self.1.clone())
2062 Fail(0, "foo".to_string()),
2063 Fail(1, "bar".to_string()),
2064 Fail(2, "baz".to_string()),
2067 // Should panic, but not cause memory corruption
2068 let _r: Rc<[Fail]> = Rc::from(s);
2072 fn test_from_box() {
2073 let b: Box<u32> = box 123;
2074 let r: Rc<u32> = Rc::from(b);
2076 assert_eq!(*r, 123);
2080 fn test_from_box_str() {
2081 use std::string::String;
2083 let s = String::from("foo").into_boxed_str();
2084 let r: Rc<str> = Rc::from(s);
2086 assert_eq!(&r[..], "foo");
2090 fn test_from_box_slice() {
2091 let s = vec![1, 2, 3].into_boxed_slice();
2092 let r: Rc<[u32]> = Rc::from(s);
2094 assert_eq!(&r[..], [1, 2, 3]);
2098 fn test_from_box_trait() {
2099 use std::fmt::Display;
2100 use std::string::ToString;
2102 let b: Box<dyn Display> = box 123;
2103 let r: Rc<dyn Display> = Rc::from(b);
2105 assert_eq!(r.to_string(), "123");
2109 fn test_from_box_trait_zero_sized() {
2110 use std::fmt::Debug;
2112 let b: Box<dyn Debug> = box ();
2113 let r: Rc<dyn Debug> = Rc::from(b);
2115 assert_eq!(format!("{:?}", r), "()");
2119 fn test_from_vec() {
2120 let v = vec![1, 2, 3];
2121 let r: Rc<[u32]> = Rc::from(v);
2123 assert_eq!(&r[..], [1, 2, 3]);
2127 fn test_downcast() {
2130 let r1: Rc<dyn Any> = Rc::new(i32::max_value());
2131 let r2: Rc<dyn Any> = Rc::new("abc");
2133 assert!(r1.clone().downcast::<u32>().is_err());
2135 let r1i32 = r1.downcast::<i32>();
2136 assert!(r1i32.is_ok());
2137 assert_eq!(r1i32.unwrap(), Rc::new(i32::max_value()));
2139 assert!(r2.clone().downcast::<i32>().is_err());
2141 let r2str = r2.downcast::<&'static str>();
2142 assert!(r2str.is_ok());
2143 assert_eq!(r2str.unwrap(), Rc::new("abc"));
2147 #[stable(feature = "rust1", since = "1.0.0")]
2148 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
2149 fn borrow(&self) -> &T {
2154 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2155 impl<T: ?Sized> AsRef<T> for Rc<T> {
2156 fn as_ref(&self) -> &T {
2161 #[stable(feature = "pin", since = "1.33.0")]
2162 impl<T: ?Sized> Unpin for Rc<T> { }
2164 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
2165 // Align the unsized value to the end of the RcBox.
2166 // Because it is ?Sized, it will always be the last field in memory.
2167 let align = align_of_val(&*ptr);
2168 let layout = Layout::new::<RcBox<()>>();
2169 (layout.size() + layout.padding_needed_for(align)) as isize
2172 /// Computes the offset of the data field within ArcInner.
2174 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2175 fn data_offset_sized<T>() -> isize {
2176 let align = align_of::<T>();
2177 let layout = Layout::new::<RcBox<()>>();
2178 (layout.size() + layout.padding_needed_for(align)) as isize