1 // Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
13 //! Single-threaded reference-counting pointers.
15 //! The type [`Rc<T>`][`Rc`] provides shared ownership of a value of type `T`,
16 //! allocated in the heap. Invoking [`clone`][clone] on [`Rc`] produces a new
17 //! pointer to the same value in the heap. When the last [`Rc`] pointer to a
18 //! given value is destroyed, the pointed-to value is also destroyed.
20 //! Shared references in Rust disallow mutation by default, and [`Rc`]
21 //! is no exception: you cannot obtain a mutable reference to
22 //! something inside an [`Rc`]. If you need mutability, put a [`Cell`]
23 //! or [`RefCell`] inside the [`Rc`]; see [an example of mutability
24 //! inside an Rc][mutability].
26 //! [`Rc`] uses non-atomic reference counting. This means that overhead is very
27 //! low, but an [`Rc`] cannot be sent between threads, and consequently [`Rc`]
28 //! does not implement [`Send`][send]. As a result, the Rust compiler
29 //! will check *at compile time* that you are not sending [`Rc`]s between
30 //! threads. If you need multi-threaded, atomic reference counting, use
31 //! [`sync::Arc`][arc].
33 //! The [`downgrade`][downgrade] method can be used to create a non-owning
34 //! [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
35 //! to an [`Rc`], but this will return [`None`] if the value has
36 //! already been dropped.
38 //! A cycle between [`Rc`] pointers will never be deallocated. For this reason,
39 //! [`Weak`] is used to break cycles. For example, a tree could have strong
40 //! [`Rc`] pointers from parent nodes to children, and [`Weak`] pointers from
41 //! children back to their parents.
43 //! `Rc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
44 //! so you can call `T`'s methods on a value of type [`Rc<T>`][`Rc`]. To avoid name
45 //! clashes with `T`'s methods, the methods of [`Rc<T>`][`Rc`] itself are [associated
46 //! functions][assoc], called using function-like syntax:
50 //! let my_rc = Rc::new(());
52 //! Rc::downgrade(&my_rc);
55 //! [`Weak<T>`][`Weak`] does not auto-dereference to `T`, because the value may have
56 //! already been destroyed.
60 //! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`.
61 //! We want to have our `Gadget`s point to their `Owner`. We can't do this with
62 //! unique ownership, because more than one gadget may belong to the same
63 //! `Owner`. [`Rc`] allows us to share an `Owner` between multiple `Gadget`s,
64 //! and have the `Owner` remain allocated as long as any `Gadget` points at it.
71 //! // ...other fields
77 //! // ...other fields
81 //! // Create a reference-counted `Owner`.
82 //! let gadget_owner: Rc<Owner> = Rc::new(
84 //! name: "Gadget Man".to_string(),
88 //! // Create `Gadget`s belonging to `gadget_owner`. Cloning the `Rc<Owner>`
89 //! // value gives us a new pointer to the same `Owner` value, incrementing
90 //! // the reference count in the process.
91 //! let gadget1 = Gadget {
93 //! owner: gadget_owner.clone(),
95 //! let gadget2 = Gadget {
97 //! owner: gadget_owner.clone(),
100 //! // Dispose of our local variable `gadget_owner`.
101 //! drop(gadget_owner);
103 //! // Despite dropping `gadget_owner`, we're still able to print out the name
104 //! // of the `Owner` of the `Gadget`s. This is because we've only dropped a
105 //! // single `Rc<Owner>`, not the `Owner` it points to. As long as there are
106 //! // other `Rc<Owner>` values pointing at the same `Owner`, it will remain
107 //! // allocated. The field projection `gadget1.owner.name` works because
108 //! // `Rc<Owner>` automatically dereferences to `Owner`.
109 //! println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name);
110 //! println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name);
112 //! // At the end of the function, `gadget1` and `gadget2` are destroyed, and
113 //! // with them the last counted references to our `Owner`. Gadget Man now
114 //! // gets destroyed as well.
118 //! If our requirements change, and we also need to be able to traverse from
119 //! `Owner` to `Gadget`, we will run into problems. An [`Rc`] pointer from `Owner`
120 //! to `Gadget` introduces a cycle between the values. This means that their
121 //! reference counts can never reach 0, and the values will remain allocated
122 //! forever: a memory leak. In order to get around this, we can use [`Weak`]
125 //! Rust actually makes it somewhat difficult to produce this loop in the first
126 //! place. In order to end up with two values that point at each other, one of
127 //! them needs to be mutable. This is difficult because [`Rc`] enforces
128 //! memory safety by only giving out shared references to the value it wraps,
129 //! and these don't allow direct mutation. We need to wrap the part of the
130 //! value we wish to mutate in a [`RefCell`], which provides *interior
131 //! mutability*: a method to achieve mutability through a shared reference.
132 //! [`RefCell`] enforces Rust's borrowing rules at runtime.
136 //! use std::rc::Weak;
137 //! use std::cell::RefCell;
141 //! gadgets: RefCell<Vec<Weak<Gadget>>>,
142 //! // ...other fields
147 //! owner: Rc<Owner>,
148 //! // ...other fields
152 //! // Create a reference-counted `Owner`. Note that we've put the `Owner`'s
153 //! // vector of `Gadget`s inside a `RefCell` so that we can mutate it through
154 //! // a shared reference.
155 //! let gadget_owner: Rc<Owner> = Rc::new(
157 //! name: "Gadget Man".to_string(),
158 //! gadgets: RefCell::new(vec![]),
162 //! // Create `Gadget`s belonging to `gadget_owner`, as before.
163 //! let gadget1 = Rc::new(
166 //! owner: gadget_owner.clone(),
169 //! let gadget2 = Rc::new(
172 //! owner: gadget_owner.clone(),
176 //! // Add the `Gadget`s to their `Owner`.
178 //! let mut gadgets = gadget_owner.gadgets.borrow_mut();
179 //! gadgets.push(Rc::downgrade(&gadget1));
180 //! gadgets.push(Rc::downgrade(&gadget2));
182 //! // `RefCell` dynamic borrow ends here.
185 //! // Iterate over our `Gadget`s, printing their details out.
186 //! for gadget_weak in gadget_owner.gadgets.borrow().iter() {
188 //! // `gadget_weak` is a `Weak<Gadget>`. Since `Weak` pointers can't
189 //! // guarantee the value is still allocated, we need to call
190 //! // `upgrade`, which returns an `Option<Rc<Gadget>>`.
192 //! // In this case we know the value still exists, so we simply
193 //! // `unwrap` the `Option`. In a more complicated program, you might
194 //! // need graceful error handling for a `None` result.
196 //! let gadget = gadget_weak.upgrade().unwrap();
197 //! println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
200 //! // At the end of the function, `gadget_owner`, `gadget1`, and `gadget2`
201 //! // are destroyed. There are now no strong (`Rc`) pointers to the
202 //! // gadgets, so they are destroyed. This zeroes the reference count on
203 //! // Gadget Man, so he gets destroyed as well.
207 //! [`Rc`]: struct.Rc.html
208 //! [`Weak`]: struct.Weak.html
209 //! [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
210 //! [`Cell`]: ../../std/cell/struct.Cell.html
211 //! [`RefCell`]: ../../std/cell/struct.RefCell.html
212 //! [send]: ../../std/marker/trait.Send.html
213 //! [arc]: ../../std/sync/struct.Arc.html
214 //! [`Deref`]: ../../std/ops/trait.Deref.html
215 //! [downgrade]: struct.Rc.html#method.downgrade
216 //! [upgrade]: struct.Weak.html#method.upgrade
217 //! [`None`]: ../../std/option/enum.Option.html#variant.None
218 //! [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
219 //! [mutability]: ../../std/cell/index.html#introducing-mutability-inside-of-something-immutable
221 #![stable(feature = "rust1", since = "1.0.0")]
229 use core::cell::Cell;
230 use core::cmp::Ordering;
232 use core::hash::{Hash, Hasher};
233 use core::intrinsics::{abort, assume};
235 use core::marker::Unsize;
236 use core::mem::{self, align_of_val, forget, size_of, size_of_val, uninitialized};
237 use core::ops::Deref;
238 use core::ops::CoerceUnsized;
239 use core::ptr::{self, Shared};
240 use core::convert::From;
242 use heap::{allocate, deallocate, box_free};
245 struct RcBox<T: ?Sized> {
251 /// A single-threaded reference-counting pointer.
253 /// See the [module-level documentation](./index.html) for more details.
255 /// The inherent methods of `Rc` are all associated functions, which means
256 /// that you have to call them as e.g. [`Rc::get_mut(&value)`][get_mut] instead of
257 /// `value.get_mut()`. This avoids conflicts with methods of the inner
260 /// [get_mut]: #method.get_mut
261 #[stable(feature = "rust1", since = "1.0.0")]
262 pub struct Rc<T: ?Sized> {
263 ptr: Shared<RcBox<T>>,
266 #[stable(feature = "rust1", since = "1.0.0")]
267 impl<T: ?Sized> !marker::Send for Rc<T> {}
268 #[stable(feature = "rust1", since = "1.0.0")]
269 impl<T: ?Sized> !marker::Sync for Rc<T> {}
271 #[unstable(feature = "coerce_unsized", issue = "27732")]
272 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
275 /// Constructs a new `Rc<T>`.
282 /// let five = Rc::new(5);
284 #[stable(feature = "rust1", since = "1.0.0")]
285 pub fn new(value: T) -> Rc<T> {
288 // there is an implicit weak pointer owned by all the strong
289 // pointers, which ensures that the weak destructor never frees
290 // the allocation while the strong destructor is running, even
291 // if the weak pointer is stored inside the strong one.
292 ptr: Shared::new(Box::into_raw(box RcBox {
293 strong: Cell::new(1),
301 /// Returns the contained value, if the `Rc` has exactly one strong reference.
303 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
306 /// This will succeed even if there are outstanding weak references.
308 /// [result]: ../../std/result/enum.Result.html
315 /// let x = Rc::new(3);
316 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
318 /// let x = Rc::new(4);
319 /// let _y = x.clone();
320 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
323 #[stable(feature = "rc_unique", since = "1.4.0")]
324 pub fn try_unwrap(this: Self) -> Result<T, Self> {
325 if Rc::strong_count(&this) == 1 {
327 let val = ptr::read(&*this); // copy the contained object
329 // Indicate to Weaks that they can't be promoted by decrememting
330 // the strong count, and then remove the implicit "strong weak"
331 // pointer while also handling drop logic by just crafting a
334 let _weak = Weak { ptr: this.ptr };
343 /// Checks whether [`Rc::try_unwrap`][try_unwrap] would return
346 /// [try_unwrap]: struct.Rc.html#method.try_unwrap
347 /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
348 #[unstable(feature = "rc_would_unwrap",
349 reason = "just added for niche usecase",
351 #[rustc_deprecated(since = "1.15.0", reason = "too niche; use `strong_count` instead")]
352 pub fn would_unwrap(this: &Self) -> bool {
353 Rc::strong_count(&this) == 1
356 /// Consumes the `Rc`, returning the wrapped pointer.
358 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
359 /// [`Rc::from_raw`][from_raw].
361 /// [from_raw]: struct.Rc.html#method.from_raw
368 /// let x = Rc::new(10);
369 /// let x_ptr = Rc::into_raw(x);
370 /// assert_eq!(unsafe { *x_ptr }, 10);
372 #[stable(feature = "rc_raw", since = "1.17.0")]
373 pub fn into_raw(this: Self) -> *const T {
374 let ptr = unsafe { &mut (*this.ptr.as_mut_ptr()).value as *const _ };
379 /// Constructs an `Rc` from a raw pointer.
381 /// The raw pointer must have been previously returned by a call to a
382 /// [`Rc::into_raw`][into_raw].
384 /// This function is unsafe because improper use may lead to memory problems. For example, a
385 /// double-free may occur if the function is called twice on the same raw pointer.
387 /// [into_raw]: struct.Rc.html#method.into_raw
394 /// let x = Rc::new(10);
395 /// let x_ptr = Rc::into_raw(x);
398 /// // Convert back to an `Rc` to prevent leak.
399 /// let x = Rc::from_raw(x_ptr);
400 /// assert_eq!(*x, 10);
402 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
405 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
407 #[stable(feature = "rc_raw", since = "1.17.0")]
408 pub unsafe fn from_raw(ptr: *const T) -> Self {
409 // To find the corresponding pointer to the `RcBox` we need to subtract the offset of the
410 // `value` field from the pointer.
411 Rc { ptr: Shared::new((ptr as *const u8).offset(-offset_of!(RcBox<T>, value)) as *const _) }
416 /// Constructs a new `Rc<str>` from a string slice.
418 #[unstable(feature = "rustc_private",
419 reason = "for internal use in rustc",
421 pub fn __from_str(value: &str) -> Rc<str> {
423 // Allocate enough space for `RcBox<str>`.
424 let aligned_len = 2 + (value.len() + size_of::<usize>() - 1) / size_of::<usize>();
425 let vec = RawVec::<usize>::with_capacity(aligned_len);
428 // Initialize fields of `RcBox<str>`.
429 *ptr.offset(0) = 1; // strong: Cell::new(1)
430 *ptr.offset(1) = 1; // weak: Cell::new(1)
431 ptr::copy_nonoverlapping(value.as_ptr(), ptr.offset(2) as *mut u8, value.len());
432 // Combine the allocation address and the string length into a fat pointer to `RcBox`.
433 let rcbox_ptr: *mut RcBox<str> = mem::transmute([ptr as usize, value.len()]);
434 assert!(aligned_len * size_of::<usize>() == size_of_val(&*rcbox_ptr));
435 Rc { ptr: Shared::new(rcbox_ptr) }
441 /// Constructs a new `Rc<[T]>` from a `Box<[T]>`.
443 #[unstable(feature = "rustc_private",
444 reason = "for internal use in rustc",
446 pub fn __from_array(value: Box<[T]>) -> Rc<[T]> {
448 let ptr: *mut RcBox<[T]> =
449 mem::transmute([mem::align_of::<RcBox<[T; 1]>>(), value.len()]);
450 // FIXME(custom-DST): creating this invalid &[T] is dubiously defined,
451 // we should have a better way of getting the size/align
452 // of a DST from its unsized part.
453 let ptr = allocate(size_of_val(&*ptr), align_of_val(&*ptr));
454 let ptr: *mut RcBox<[T]> = mem::transmute([ptr as usize, value.len()]);
456 // Initialize the new RcBox.
457 ptr::write(&mut (*ptr).strong, Cell::new(1));
458 ptr::write(&mut (*ptr).weak, Cell::new(1));
459 ptr::copy_nonoverlapping(
461 &mut (*ptr).value as *mut [T] as *mut T,
464 // Free the original allocation without freeing its (moved) contents.
465 box_free(Box::into_raw(value));
467 Rc { ptr: Shared::new(ptr as *const _) }
472 impl<T: ?Sized> Rc<T> {
473 /// Creates a new [`Weak`][weak] pointer to this value.
475 /// [weak]: struct.Weak.html
482 /// let five = Rc::new(5);
484 /// let weak_five = Rc::downgrade(&five);
486 #[stable(feature = "rc_weak", since = "1.4.0")]
487 pub fn downgrade(this: &Self) -> Weak<T> {
489 Weak { ptr: this.ptr }
492 /// Gets the number of [`Weak`][weak] pointers to this value.
494 /// [weak]: struct.Weak.html
501 /// let five = Rc::new(5);
502 /// let _weak_five = Rc::downgrade(&five);
504 /// assert_eq!(1, Rc::weak_count(&five));
507 #[stable(feature = "rc_counts", since = "1.15.0")]
508 pub fn weak_count(this: &Self) -> usize {
512 /// Gets the number of strong (`Rc`) pointers to this value.
519 /// let five = Rc::new(5);
520 /// let _also_five = five.clone();
522 /// assert_eq!(2, Rc::strong_count(&five));
525 #[stable(feature = "rc_counts", since = "1.15.0")]
526 pub fn strong_count(this: &Self) -> usize {
530 /// Returns true if there are no other `Rc` or [`Weak`][weak] pointers to
531 /// this inner value.
533 /// [weak]: struct.Weak.html
535 #[unstable(feature = "is_unique", reason = "uniqueness has unclear meaning",
537 #[rustc_deprecated(since = "1.15.0",
538 reason = "too niche; use `strong_count` and `weak_count` instead")]
539 pub fn is_unique(this: &Self) -> bool {
540 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
543 /// Returns a mutable reference to the inner value, if there are
544 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
546 /// Returns [`None`] otherwise, because it is not safe to
547 /// mutate a shared value.
549 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
550 /// the inner value when it's shared.
552 /// [weak]: struct.Weak.html
553 /// [`None`]: ../../std/option/enum.Option.html#variant.None
554 /// [make_mut]: struct.Rc.html#method.make_mut
555 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
562 /// let mut x = Rc::new(3);
563 /// *Rc::get_mut(&mut x).unwrap() = 4;
564 /// assert_eq!(*x, 4);
566 /// let _y = x.clone();
567 /// assert!(Rc::get_mut(&mut x).is_none());
570 #[stable(feature = "rc_unique", since = "1.4.0")]
571 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
572 if Rc::is_unique(this) {
573 let inner = unsafe { &mut *this.ptr.as_mut_ptr() };
574 Some(&mut inner.value)
581 #[stable(feature = "ptr_eq", since = "1.17.0")]
582 /// Returns true if the two `Rc`s point to the same value (not
583 /// just values that compare as equal).
590 /// let five = Rc::new(5);
591 /// let same_five = five.clone();
592 /// let other_five = Rc::new(5);
594 /// assert!(Rc::ptr_eq(&five, &same_five));
595 /// assert!(!Rc::ptr_eq(&five, &other_five));
597 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
598 let this_ptr: *const RcBox<T> = *this.ptr;
599 let other_ptr: *const RcBox<T> = *other.ptr;
600 this_ptr == other_ptr
604 impl<T: Clone> Rc<T> {
605 /// Makes a mutable reference into the given `Rc`.
607 /// If there are other `Rc` or [`Weak`][weak] pointers to the same value,
608 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
609 /// ensure unique ownership. This is also referred to as clone-on-write.
611 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
613 /// [weak]: struct.Weak.html
614 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
615 /// [get_mut]: struct.Rc.html#method.get_mut
622 /// let mut data = Rc::new(5);
624 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
625 /// let mut other_data = data.clone(); // Won't clone inner data
626 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
627 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
628 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
630 /// // Now `data` and `other_data` point to different values.
631 /// assert_eq!(*data, 8);
632 /// assert_eq!(*other_data, 12);
635 #[stable(feature = "rc_unique", since = "1.4.0")]
636 pub fn make_mut(this: &mut Self) -> &mut T {
637 if Rc::strong_count(this) != 1 {
638 // Gotta clone the data, there are other Rcs
639 *this = Rc::new((**this).clone())
640 } else if Rc::weak_count(this) != 0 {
641 // Can just steal the data, all that's left is Weaks
643 let mut swap = Rc::new(ptr::read(&(**this.ptr).value));
644 mem::swap(this, &mut swap);
646 // Remove implicit strong-weak ref (no need to craft a fake
647 // Weak here -- we know other Weaks can clean up for us)
652 // This unsafety is ok because we're guaranteed that the pointer
653 // returned is the *only* pointer that will ever be returned to T. Our
654 // reference count is guaranteed to be 1 at this point, and we required
655 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
656 // reference to the inner value.
657 let inner = unsafe { &mut *this.ptr.as_mut_ptr() };
662 #[stable(feature = "rust1", since = "1.0.0")]
663 impl<T: ?Sized> Deref for Rc<T> {
667 fn deref(&self) -> &T {
672 #[stable(feature = "rust1", since = "1.0.0")]
673 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
676 /// This will decrement the strong reference count. If the strong reference
677 /// count reaches zero then the only other references (if any) are
678 /// [`Weak`][weak], so we `drop` the inner value.
680 /// [weak]: struct.Weak.html
689 /// impl Drop for Foo {
690 /// fn drop(&mut self) {
691 /// println!("dropped!");
695 /// let foo = Rc::new(Foo);
696 /// let foo2 = foo.clone();
698 /// drop(foo); // Doesn't print anything
699 /// drop(foo2); // Prints "dropped!"
703 let ptr = self.ptr.as_mut_ptr();
706 if self.strong() == 0 {
707 // destroy the contained object
708 ptr::drop_in_place(&mut (*ptr).value);
710 // remove the implicit "strong weak" pointer now that we've
711 // destroyed the contents.
714 if self.weak() == 0 {
715 deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
722 #[stable(feature = "rust1", since = "1.0.0")]
723 impl<T: ?Sized> Clone for Rc<T> {
724 /// Makes a clone of the `Rc` pointer.
726 /// This creates another pointer to the same inner value, increasing the
727 /// strong reference count.
734 /// let five = Rc::new(5);
739 fn clone(&self) -> Rc<T> {
745 #[stable(feature = "rust1", since = "1.0.0")]
746 impl<T: Default> Default for Rc<T> {
747 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
754 /// let x: Rc<i32> = Default::default();
755 /// assert_eq!(*x, 0);
758 fn default() -> Rc<T> {
759 Rc::new(Default::default())
763 #[stable(feature = "rust1", since = "1.0.0")]
764 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
765 /// Equality for two `Rc`s.
767 /// Two `Rc`s are equal if their inner values are equal.
774 /// let five = Rc::new(5);
776 /// assert!(five == Rc::new(5));
779 fn eq(&self, other: &Rc<T>) -> bool {
783 /// Inequality for two `Rc`s.
785 /// Two `Rc`s are unequal if their inner values are unequal.
792 /// let five = Rc::new(5);
794 /// assert!(five != Rc::new(6));
797 fn ne(&self, other: &Rc<T>) -> bool {
802 #[stable(feature = "rust1", since = "1.0.0")]
803 impl<T: ?Sized + Eq> Eq for Rc<T> {}
805 #[stable(feature = "rust1", since = "1.0.0")]
806 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
807 /// Partial comparison for two `Rc`s.
809 /// The two are compared by calling `partial_cmp()` on their inner values.
815 /// use std::cmp::Ordering;
817 /// let five = Rc::new(5);
819 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
822 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
823 (**self).partial_cmp(&**other)
826 /// Less-than comparison for two `Rc`s.
828 /// The two are compared by calling `<` on their inner values.
835 /// let five = Rc::new(5);
837 /// assert!(five < Rc::new(6));
840 fn lt(&self, other: &Rc<T>) -> bool {
844 /// 'Less than or equal to' comparison for two `Rc`s.
846 /// The two are compared by calling `<=` on their inner values.
853 /// let five = Rc::new(5);
855 /// assert!(five <= Rc::new(5));
858 fn le(&self, other: &Rc<T>) -> bool {
862 /// Greater-than comparison for two `Rc`s.
864 /// The two are compared by calling `>` on their inner values.
871 /// let five = Rc::new(5);
873 /// assert!(five > Rc::new(4));
876 fn gt(&self, other: &Rc<T>) -> bool {
880 /// 'Greater than or equal to' comparison for two `Rc`s.
882 /// The two are compared by calling `>=` on their inner values.
889 /// let five = Rc::new(5);
891 /// assert!(five >= Rc::new(5));
894 fn ge(&self, other: &Rc<T>) -> bool {
899 #[stable(feature = "rust1", since = "1.0.0")]
900 impl<T: ?Sized + Ord> Ord for Rc<T> {
901 /// Comparison for two `Rc`s.
903 /// The two are compared by calling `cmp()` on their inner values.
909 /// use std::cmp::Ordering;
911 /// let five = Rc::new(5);
913 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
916 fn cmp(&self, other: &Rc<T>) -> Ordering {
917 (**self).cmp(&**other)
921 #[stable(feature = "rust1", since = "1.0.0")]
922 impl<T: ?Sized + Hash> Hash for Rc<T> {
923 fn hash<H: Hasher>(&self, state: &mut H) {
924 (**self).hash(state);
928 #[stable(feature = "rust1", since = "1.0.0")]
929 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
930 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
931 fmt::Display::fmt(&**self, f)
935 #[stable(feature = "rust1", since = "1.0.0")]
936 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
937 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
938 fmt::Debug::fmt(&**self, f)
942 #[stable(feature = "rust1", since = "1.0.0")]
943 impl<T: ?Sized> fmt::Pointer for Rc<T> {
944 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
945 fmt::Pointer::fmt(&*self.ptr, f)
949 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
950 impl<T> From<T> for Rc<T> {
951 fn from(t: T) -> Self {
956 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
957 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
958 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
960 /// Since a `Weak` reference does not count towards ownership, it will not
961 /// prevent the inner value from being dropped, and `Weak` itself makes no
962 /// guarantees about the value still being present and may return [`None`]
963 /// when [`upgrade`]d.
965 /// A `Weak` pointer is useful for keeping a temporary reference to the value
966 /// within [`Rc`] without extending its lifetime. It is also used to prevent
967 /// circular references between [`Rc`] pointers, since mutual owning references
968 /// would never allow either [`Arc`] to be dropped. For example, a tree could
969 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
970 /// pointers from children back to their parents.
972 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
974 /// [`Rc`]: struct.Rc.html
975 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
976 /// [`upgrade`]: struct.Weak.html#method.upgrade
977 /// [`Option`]: ../../std/option/enum.Option.html
978 /// [`None`]: ../../std/option/enum.Option.html#variant.None
979 #[stable(feature = "rc_weak", since = "1.4.0")]
980 pub struct Weak<T: ?Sized> {
981 ptr: Shared<RcBox<T>>,
984 #[stable(feature = "rc_weak", since = "1.4.0")]
985 impl<T: ?Sized> !marker::Send for Weak<T> {}
986 #[stable(feature = "rc_weak", since = "1.4.0")]
987 impl<T: ?Sized> !marker::Sync for Weak<T> {}
989 #[unstable(feature = "coerce_unsized", issue = "27732")]
990 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
993 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
994 /// it. Calling [`upgrade`] on the return value always gives [`None`].
996 /// [`upgrade`]: struct.Weak.html#method.upgrade
997 /// [`None`]: ../../std/option/enum.Option.html
1002 /// use std::rc::Weak;
1004 /// let empty: Weak<i64> = Weak::new();
1005 /// assert!(empty.upgrade().is_none());
1007 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1008 pub fn new() -> Weak<T> {
1011 ptr: Shared::new(Box::into_raw(box RcBox {
1012 strong: Cell::new(0),
1014 value: uninitialized(),
1021 impl<T: ?Sized> Weak<T> {
1022 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1023 /// the lifetime of the value if successful.
1025 /// Returns [`None`] if the value has since been dropped.
1027 /// [`Rc`]: struct.Rc.html
1028 /// [`None`]: ../../std/option/enum.Option.html
1033 /// use std::rc::Rc;
1035 /// let five = Rc::new(5);
1037 /// let weak_five = Rc::downgrade(&five);
1039 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1040 /// assert!(strong_five.is_some());
1042 /// // Destroy all strong pointers.
1043 /// drop(strong_five);
1046 /// assert!(weak_five.upgrade().is_none());
1048 #[stable(feature = "rc_weak", since = "1.4.0")]
1049 pub fn upgrade(&self) -> Option<Rc<T>> {
1050 if self.strong() == 0 {
1054 Some(Rc { ptr: self.ptr })
1059 #[stable(feature = "rc_weak", since = "1.4.0")]
1060 impl<T: ?Sized> Drop for Weak<T> {
1061 /// Drops the `Weak` pointer.
1066 /// use std::rc::Rc;
1070 /// impl Drop for Foo {
1071 /// fn drop(&mut self) {
1072 /// println!("dropped!");
1076 /// let foo = Rc::new(Foo);
1077 /// let weak_foo = Rc::downgrade(&foo);
1078 /// let other_weak_foo = weak_foo.clone();
1080 /// drop(weak_foo); // Doesn't print anything
1081 /// drop(foo); // Prints "dropped!"
1083 /// assert!(other_weak_foo.upgrade().is_none());
1085 fn drop(&mut self) {
1087 let ptr = *self.ptr;
1090 // the weak count starts at 1, and will only go to zero if all
1091 // the strong pointers have disappeared.
1092 if self.weak() == 0 {
1093 deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
1099 #[stable(feature = "rc_weak", since = "1.4.0")]
1100 impl<T: ?Sized> Clone for Weak<T> {
1101 /// Makes a clone of the `Weak` pointer that points to the same value.
1106 /// use std::rc::Rc;
1108 /// let weak_five = Rc::downgrade(&Rc::new(5));
1110 /// weak_five.clone();
1113 fn clone(&self) -> Weak<T> {
1115 Weak { ptr: self.ptr }
1119 #[stable(feature = "rc_weak", since = "1.4.0")]
1120 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1121 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1126 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1127 impl<T> Default for Weak<T> {
1128 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1129 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1131 /// [`upgrade`]: struct.Weak.html#method.upgrade
1132 /// [`None`]: ../../std/option/enum.Option.html
1137 /// use std::rc::Weak;
1139 /// let empty: Weak<i64> = Default::default();
1140 /// assert!(empty.upgrade().is_none());
1142 fn default() -> Weak<T> {
1147 // NOTE: We checked_add here to deal with mem::forget safety. In particular
1148 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1149 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1150 // We abort because this is such a degenerate scenario that we don't care about
1151 // what happens -- no real program should ever experience this.
1153 // This should have negligible overhead since you don't actually need to
1154 // clone these much in Rust thanks to ownership and move-semantics.
1157 trait RcBoxPtr<T: ?Sized> {
1158 fn inner(&self) -> &RcBox<T>;
1161 fn strong(&self) -> usize {
1162 self.inner().strong.get()
1166 fn inc_strong(&self) {
1167 self.inner().strong.set(self.strong().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1171 fn dec_strong(&self) {
1172 self.inner().strong.set(self.strong() - 1);
1176 fn weak(&self) -> usize {
1177 self.inner().weak.get()
1181 fn inc_weak(&self) {
1182 self.inner().weak.set(self.weak().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1186 fn dec_weak(&self) {
1187 self.inner().weak.set(self.weak() - 1);
1191 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1193 fn inner(&self) -> &RcBox<T> {
1195 // Safe to assume this here, as if it weren't true, we'd be breaking
1196 // the contract anyway.
1197 // This allows the null check to be elided in the destructor if we
1198 // manipulated the reference count in the same function.
1199 assume(!(*(&self.ptr as *const _ as *const *const ())).is_null());
1205 impl<T: ?Sized> RcBoxPtr<T> for Weak<T> {
1207 fn inner(&self) -> &RcBox<T> {
1209 // Safe to assume this here, as if it weren't true, we'd be breaking
1210 // the contract anyway.
1211 // This allows the null check to be elided in the destructor if we
1212 // manipulated the reference count in the same function.
1213 assume(!(*(&self.ptr as *const _ as *const *const ())).is_null());
1221 use super::{Rc, Weak};
1222 use std::boxed::Box;
1223 use std::cell::RefCell;
1224 use std::option::Option;
1225 use std::option::Option::{None, Some};
1226 use std::result::Result::{Err, Ok};
1228 use std::clone::Clone;
1229 use std::convert::From;
1233 let x = Rc::new(RefCell::new(5));
1235 *x.borrow_mut() = 20;
1236 assert_eq!(*y.borrow(), 20);
1246 fn test_simple_clone() {
1254 fn test_destructor() {
1255 let x: Rc<Box<_>> = Rc::new(box 5);
1262 let y = Rc::downgrade(&x);
1263 assert!(y.upgrade().is_some());
1269 let y = Rc::downgrade(&x);
1271 assert!(y.upgrade().is_none());
1275 fn weak_self_cyclic() {
1277 x: RefCell<Option<Weak<Cycle>>>,
1280 let a = Rc::new(Cycle { x: RefCell::new(None) });
1281 let b = Rc::downgrade(&a.clone());
1282 *a.x.borrow_mut() = Some(b);
1284 // hopefully we don't double-free (or leak)...
1290 assert!(Rc::is_unique(&x));
1292 assert!(!Rc::is_unique(&x));
1294 assert!(Rc::is_unique(&x));
1295 let w = Rc::downgrade(&x);
1296 assert!(!Rc::is_unique(&x));
1298 assert!(Rc::is_unique(&x));
1302 fn test_strong_count() {
1304 assert!(Rc::strong_count(&a) == 1);
1305 let w = Rc::downgrade(&a);
1306 assert!(Rc::strong_count(&a) == 1);
1307 let b = w.upgrade().expect("upgrade of live rc failed");
1308 assert!(Rc::strong_count(&b) == 2);
1309 assert!(Rc::strong_count(&a) == 2);
1312 assert!(Rc::strong_count(&b) == 1);
1314 assert!(Rc::strong_count(&b) == 2);
1315 assert!(Rc::strong_count(&c) == 2);
1319 fn test_weak_count() {
1321 assert!(Rc::strong_count(&a) == 1);
1322 assert!(Rc::weak_count(&a) == 0);
1323 let w = Rc::downgrade(&a);
1324 assert!(Rc::strong_count(&a) == 1);
1325 assert!(Rc::weak_count(&a) == 1);
1327 assert!(Rc::strong_count(&a) == 1);
1328 assert!(Rc::weak_count(&a) == 0);
1330 assert!(Rc::strong_count(&a) == 2);
1331 assert!(Rc::weak_count(&a) == 0);
1338 assert_eq!(Rc::try_unwrap(x), Ok(3));
1341 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
1343 let _w = Rc::downgrade(&x);
1344 assert_eq!(Rc::try_unwrap(x), Ok(5));
1348 fn into_from_raw() {
1349 let x = Rc::new(box "hello");
1352 let x_ptr = Rc::into_raw(x);
1355 assert_eq!(**x_ptr, "hello");
1357 let x = Rc::from_raw(x_ptr);
1358 assert_eq!(**x, "hello");
1360 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
1366 let mut x = Rc::new(3);
1367 *Rc::get_mut(&mut x).unwrap() = 4;
1370 assert!(Rc::get_mut(&mut x).is_none());
1372 assert!(Rc::get_mut(&mut x).is_some());
1373 let _w = Rc::downgrade(&x);
1374 assert!(Rc::get_mut(&mut x).is_none());
1378 fn test_cowrc_clone_make_unique() {
1379 let mut cow0 = Rc::new(75);
1380 let mut cow1 = cow0.clone();
1381 let mut cow2 = cow1.clone();
1383 assert!(75 == *Rc::make_mut(&mut cow0));
1384 assert!(75 == *Rc::make_mut(&mut cow1));
1385 assert!(75 == *Rc::make_mut(&mut cow2));
1387 *Rc::make_mut(&mut cow0) += 1;
1388 *Rc::make_mut(&mut cow1) += 2;
1389 *Rc::make_mut(&mut cow2) += 3;
1391 assert!(76 == *cow0);
1392 assert!(77 == *cow1);
1393 assert!(78 == *cow2);
1395 // none should point to the same backing memory
1396 assert!(*cow0 != *cow1);
1397 assert!(*cow0 != *cow2);
1398 assert!(*cow1 != *cow2);
1402 fn test_cowrc_clone_unique2() {
1403 let mut cow0 = Rc::new(75);
1404 let cow1 = cow0.clone();
1405 let cow2 = cow1.clone();
1407 assert!(75 == *cow0);
1408 assert!(75 == *cow1);
1409 assert!(75 == *cow2);
1411 *Rc::make_mut(&mut cow0) += 1;
1413 assert!(76 == *cow0);
1414 assert!(75 == *cow1);
1415 assert!(75 == *cow2);
1417 // cow1 and cow2 should share the same contents
1418 // cow0 should have a unique reference
1419 assert!(*cow0 != *cow1);
1420 assert!(*cow0 != *cow2);
1421 assert!(*cow1 == *cow2);
1425 fn test_cowrc_clone_weak() {
1426 let mut cow0 = Rc::new(75);
1427 let cow1_weak = Rc::downgrade(&cow0);
1429 assert!(75 == *cow0);
1430 assert!(75 == *cow1_weak.upgrade().unwrap());
1432 *Rc::make_mut(&mut cow0) += 1;
1434 assert!(76 == *cow0);
1435 assert!(cow1_weak.upgrade().is_none());
1440 let foo = Rc::new(75);
1441 assert_eq!(format!("{:?}", foo), "75");
1446 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
1447 assert_eq!(foo, foo.clone());
1451 fn test_from_owned() {
1453 let foo_rc = Rc::from(foo);
1454 assert!(123 == *foo_rc);
1458 fn test_new_weak() {
1459 let foo: Weak<usize> = Weak::new();
1460 assert!(foo.upgrade().is_none());
1465 let five = Rc::new(5);
1466 let same_five = five.clone();
1467 let other_five = Rc::new(5);
1469 assert!(Rc::ptr_eq(&five, &same_five));
1470 assert!(!Rc::ptr_eq(&five, &other_five));
1474 #[stable(feature = "rust1", since = "1.0.0")]
1475 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
1476 fn borrow(&self) -> &T {
1481 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1482 impl<T: ?Sized> AsRef<T> for Rc<T> {
1483 fn as_ref(&self) -> &T {