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 /// Consumes the `Rc`, returning the wrapped pointer.
345 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
346 /// [`Rc::from_raw`][from_raw].
348 /// [from_raw]: struct.Rc.html#method.from_raw
355 /// let x = Rc::new(10);
356 /// let x_ptr = Rc::into_raw(x);
357 /// assert_eq!(unsafe { *x_ptr }, 10);
359 #[stable(feature = "rc_raw", since = "1.17.0")]
360 pub fn into_raw(this: Self) -> *const T {
361 let ptr = unsafe { &mut (*this.ptr.as_mut_ptr()).value as *const _ };
366 /// Constructs an `Rc` from a raw pointer.
368 /// The raw pointer must have been previously returned by a call to a
369 /// [`Rc::into_raw`][into_raw].
371 /// This function is unsafe because improper use may lead to memory problems. For example, a
372 /// double-free may occur if the function is called twice on the same raw pointer.
374 /// [into_raw]: struct.Rc.html#method.into_raw
381 /// let x = Rc::new(10);
382 /// let x_ptr = Rc::into_raw(x);
385 /// // Convert back to an `Rc` to prevent leak.
386 /// let x = Rc::from_raw(x_ptr);
387 /// assert_eq!(*x, 10);
389 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
392 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
394 #[stable(feature = "rc_raw", since = "1.17.0")]
395 pub unsafe fn from_raw(ptr: *const T) -> Self {
396 // To find the corresponding pointer to the `RcBox` we need to subtract the offset of the
397 // `value` field from the pointer.
398 Rc { ptr: Shared::new((ptr as *const u8).offset(-offset_of!(RcBox<T>, value)) as *const _) }
403 /// Constructs a new `Rc<str>` from a string slice.
405 #[unstable(feature = "rustc_private",
406 reason = "for internal use in rustc",
408 pub fn __from_str(value: &str) -> Rc<str> {
410 // Allocate enough space for `RcBox<str>`.
411 let aligned_len = 2 + (value.len() + size_of::<usize>() - 1) / size_of::<usize>();
412 let vec = RawVec::<usize>::with_capacity(aligned_len);
415 // Initialize fields of `RcBox<str>`.
416 *ptr.offset(0) = 1; // strong: Cell::new(1)
417 *ptr.offset(1) = 1; // weak: Cell::new(1)
418 ptr::copy_nonoverlapping(value.as_ptr(), ptr.offset(2) as *mut u8, value.len());
419 // Combine the allocation address and the string length into a fat pointer to `RcBox`.
420 let rcbox_ptr: *mut RcBox<str> = mem::transmute([ptr as usize, value.len()]);
421 assert!(aligned_len * size_of::<usize>() == size_of_val(&*rcbox_ptr));
422 Rc { ptr: Shared::new(rcbox_ptr) }
428 /// Constructs a new `Rc<[T]>` from a `Box<[T]>`.
430 #[unstable(feature = "rustc_private",
431 reason = "for internal use in rustc",
433 pub fn __from_array(value: Box<[T]>) -> Rc<[T]> {
435 let ptr: *mut RcBox<[T]> =
436 mem::transmute([mem::align_of::<RcBox<[T; 1]>>(), value.len()]);
437 // FIXME(custom-DST): creating this invalid &[T] is dubiously defined,
438 // we should have a better way of getting the size/align
439 // of a DST from its unsized part.
440 let ptr = allocate(size_of_val(&*ptr), align_of_val(&*ptr));
441 let ptr: *mut RcBox<[T]> = mem::transmute([ptr as usize, value.len()]);
443 // Initialize the new RcBox.
444 ptr::write(&mut (*ptr).strong, Cell::new(1));
445 ptr::write(&mut (*ptr).weak, Cell::new(1));
446 ptr::copy_nonoverlapping(
448 &mut (*ptr).value as *mut [T] as *mut T,
451 // Free the original allocation without freeing its (moved) contents.
452 box_free(Box::into_raw(value));
454 Rc { ptr: Shared::new(ptr as *const _) }
459 impl<T: ?Sized> Rc<T> {
460 /// Creates a new [`Weak`][weak] pointer to this value.
462 /// [weak]: struct.Weak.html
469 /// let five = Rc::new(5);
471 /// let weak_five = Rc::downgrade(&five);
473 #[stable(feature = "rc_weak", since = "1.4.0")]
474 pub fn downgrade(this: &Self) -> Weak<T> {
476 Weak { ptr: this.ptr }
479 /// Gets the number of [`Weak`][weak] pointers to this value.
481 /// [weak]: struct.Weak.html
488 /// let five = Rc::new(5);
489 /// let _weak_five = Rc::downgrade(&five);
491 /// assert_eq!(1, Rc::weak_count(&five));
494 #[stable(feature = "rc_counts", since = "1.15.0")]
495 pub fn weak_count(this: &Self) -> usize {
499 /// Gets the number of strong (`Rc`) pointers to this value.
506 /// let five = Rc::new(5);
507 /// let _also_five = five.clone();
509 /// assert_eq!(2, Rc::strong_count(&five));
512 #[stable(feature = "rc_counts", since = "1.15.0")]
513 pub fn strong_count(this: &Self) -> usize {
517 /// Returns true if there are no other `Rc` or [`Weak`][weak] pointers to
518 /// this inner value.
520 /// [weak]: struct.Weak.html
522 fn is_unique(this: &Self) -> bool {
523 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
526 /// Returns a mutable reference to the inner value, if there are
527 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
529 /// Returns [`None`] otherwise, because it is not safe to
530 /// mutate a shared value.
532 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
533 /// the inner value when it's shared.
535 /// [weak]: struct.Weak.html
536 /// [`None`]: ../../std/option/enum.Option.html#variant.None
537 /// [make_mut]: struct.Rc.html#method.make_mut
538 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
545 /// let mut x = Rc::new(3);
546 /// *Rc::get_mut(&mut x).unwrap() = 4;
547 /// assert_eq!(*x, 4);
549 /// let _y = x.clone();
550 /// assert!(Rc::get_mut(&mut x).is_none());
553 #[stable(feature = "rc_unique", since = "1.4.0")]
554 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
555 if Rc::is_unique(this) {
556 let inner = unsafe { &mut *this.ptr.as_mut_ptr() };
557 Some(&mut inner.value)
564 #[stable(feature = "ptr_eq", since = "1.17.0")]
565 /// Returns true if the two `Rc`s point to the same value (not
566 /// just values that compare as equal).
573 /// let five = Rc::new(5);
574 /// let same_five = five.clone();
575 /// let other_five = Rc::new(5);
577 /// assert!(Rc::ptr_eq(&five, &same_five));
578 /// assert!(!Rc::ptr_eq(&five, &other_five));
580 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
581 let this_ptr: *const RcBox<T> = *this.ptr;
582 let other_ptr: *const RcBox<T> = *other.ptr;
583 this_ptr == other_ptr
587 impl<T: Clone> Rc<T> {
588 /// Makes a mutable reference into the given `Rc`.
590 /// If there are other `Rc` or [`Weak`][weak] pointers to the same value,
591 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
592 /// ensure unique ownership. This is also referred to as clone-on-write.
594 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
596 /// [weak]: struct.Weak.html
597 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
598 /// [get_mut]: struct.Rc.html#method.get_mut
605 /// let mut data = Rc::new(5);
607 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
608 /// let mut other_data = data.clone(); // Won't clone inner data
609 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
610 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
611 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
613 /// // Now `data` and `other_data` point to different values.
614 /// assert_eq!(*data, 8);
615 /// assert_eq!(*other_data, 12);
618 #[stable(feature = "rc_unique", since = "1.4.0")]
619 pub fn make_mut(this: &mut Self) -> &mut T {
620 if Rc::strong_count(this) != 1 {
621 // Gotta clone the data, there are other Rcs
622 *this = Rc::new((**this).clone())
623 } else if Rc::weak_count(this) != 0 {
624 // Can just steal the data, all that's left is Weaks
626 let mut swap = Rc::new(ptr::read(&(**this.ptr).value));
627 mem::swap(this, &mut swap);
629 // Remove implicit strong-weak ref (no need to craft a fake
630 // Weak here -- we know other Weaks can clean up for us)
635 // This unsafety is ok because we're guaranteed that the pointer
636 // returned is the *only* pointer that will ever be returned to T. Our
637 // reference count is guaranteed to be 1 at this point, and we required
638 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
639 // reference to the inner value.
640 let inner = unsafe { &mut *this.ptr.as_mut_ptr() };
645 #[stable(feature = "rust1", since = "1.0.0")]
646 impl<T: ?Sized> Deref for Rc<T> {
650 fn deref(&self) -> &T {
655 #[stable(feature = "rust1", since = "1.0.0")]
656 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
659 /// This will decrement the strong reference count. If the strong reference
660 /// count reaches zero then the only other references (if any) are
661 /// [`Weak`][weak], so we `drop` the inner value.
663 /// [weak]: struct.Weak.html
672 /// impl Drop for Foo {
673 /// fn drop(&mut self) {
674 /// println!("dropped!");
678 /// let foo = Rc::new(Foo);
679 /// let foo2 = foo.clone();
681 /// drop(foo); // Doesn't print anything
682 /// drop(foo2); // Prints "dropped!"
686 let ptr = self.ptr.as_mut_ptr();
689 if self.strong() == 0 {
690 // destroy the contained object
691 ptr::drop_in_place(&mut (*ptr).value);
693 // remove the implicit "strong weak" pointer now that we've
694 // destroyed the contents.
697 if self.weak() == 0 {
698 deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
705 #[stable(feature = "rust1", since = "1.0.0")]
706 impl<T: ?Sized> Clone for Rc<T> {
707 /// Makes a clone of the `Rc` pointer.
709 /// This creates another pointer to the same inner value, increasing the
710 /// strong reference count.
717 /// let five = Rc::new(5);
722 fn clone(&self) -> Rc<T> {
728 #[stable(feature = "rust1", since = "1.0.0")]
729 impl<T: Default> Default for Rc<T> {
730 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
737 /// let x: Rc<i32> = Default::default();
738 /// assert_eq!(*x, 0);
741 fn default() -> Rc<T> {
742 Rc::new(Default::default())
746 #[stable(feature = "rust1", since = "1.0.0")]
747 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
748 /// Equality for two `Rc`s.
750 /// Two `Rc`s are equal if their inner values are equal.
757 /// let five = Rc::new(5);
759 /// assert!(five == Rc::new(5));
762 fn eq(&self, other: &Rc<T>) -> bool {
766 /// Inequality for two `Rc`s.
768 /// Two `Rc`s are unequal if their inner values are unequal.
775 /// let five = Rc::new(5);
777 /// assert!(five != Rc::new(6));
780 fn ne(&self, other: &Rc<T>) -> bool {
785 #[stable(feature = "rust1", since = "1.0.0")]
786 impl<T: ?Sized + Eq> Eq for Rc<T> {}
788 #[stable(feature = "rust1", since = "1.0.0")]
789 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
790 /// Partial comparison for two `Rc`s.
792 /// The two are compared by calling `partial_cmp()` on their inner values.
798 /// use std::cmp::Ordering;
800 /// let five = Rc::new(5);
802 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
805 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
806 (**self).partial_cmp(&**other)
809 /// Less-than comparison for two `Rc`s.
811 /// The two are compared by calling `<` on their inner values.
818 /// let five = Rc::new(5);
820 /// assert!(five < Rc::new(6));
823 fn lt(&self, other: &Rc<T>) -> bool {
827 /// 'Less than or equal to' comparison for two `Rc`s.
829 /// The two are compared by calling `<=` on their inner values.
836 /// let five = Rc::new(5);
838 /// assert!(five <= Rc::new(5));
841 fn le(&self, other: &Rc<T>) -> bool {
845 /// Greater-than comparison for two `Rc`s.
847 /// The two are compared by calling `>` on their inner values.
854 /// let five = Rc::new(5);
856 /// assert!(five > Rc::new(4));
859 fn gt(&self, other: &Rc<T>) -> bool {
863 /// 'Greater than or equal to' comparison for two `Rc`s.
865 /// The two are compared by calling `>=` on their inner values.
872 /// let five = Rc::new(5);
874 /// assert!(five >= Rc::new(5));
877 fn ge(&self, other: &Rc<T>) -> bool {
882 #[stable(feature = "rust1", since = "1.0.0")]
883 impl<T: ?Sized + Ord> Ord for Rc<T> {
884 /// Comparison for two `Rc`s.
886 /// The two are compared by calling `cmp()` on their inner values.
892 /// use std::cmp::Ordering;
894 /// let five = Rc::new(5);
896 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
899 fn cmp(&self, other: &Rc<T>) -> Ordering {
900 (**self).cmp(&**other)
904 #[stable(feature = "rust1", since = "1.0.0")]
905 impl<T: ?Sized + Hash> Hash for Rc<T> {
906 fn hash<H: Hasher>(&self, state: &mut H) {
907 (**self).hash(state);
911 #[stable(feature = "rust1", since = "1.0.0")]
912 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
913 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
914 fmt::Display::fmt(&**self, f)
918 #[stable(feature = "rust1", since = "1.0.0")]
919 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
920 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
921 fmt::Debug::fmt(&**self, f)
925 #[stable(feature = "rust1", since = "1.0.0")]
926 impl<T: ?Sized> fmt::Pointer for Rc<T> {
927 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
928 fmt::Pointer::fmt(&*self.ptr, f)
932 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
933 impl<T> From<T> for Rc<T> {
934 fn from(t: T) -> Self {
939 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
940 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
941 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
943 /// Since a `Weak` reference does not count towards ownership, it will not
944 /// prevent the inner value from being dropped, and `Weak` itself makes no
945 /// guarantees about the value still being present and may return [`None`]
946 /// when [`upgrade`]d.
948 /// A `Weak` pointer is useful for keeping a temporary reference to the value
949 /// within [`Rc`] without extending its lifetime. It is also used to prevent
950 /// circular references between [`Rc`] pointers, since mutual owning references
951 /// would never allow either [`Arc`] to be dropped. For example, a tree could
952 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
953 /// pointers from children back to their parents.
955 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
957 /// [`Rc`]: struct.Rc.html
958 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
959 /// [`upgrade`]: struct.Weak.html#method.upgrade
960 /// [`Option`]: ../../std/option/enum.Option.html
961 /// [`None`]: ../../std/option/enum.Option.html#variant.None
962 #[stable(feature = "rc_weak", since = "1.4.0")]
963 pub struct Weak<T: ?Sized> {
964 ptr: Shared<RcBox<T>>,
967 #[stable(feature = "rc_weak", since = "1.4.0")]
968 impl<T: ?Sized> !marker::Send for Weak<T> {}
969 #[stable(feature = "rc_weak", since = "1.4.0")]
970 impl<T: ?Sized> !marker::Sync for Weak<T> {}
972 #[unstable(feature = "coerce_unsized", issue = "27732")]
973 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
976 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
977 /// it. Calling [`upgrade`] on the return value always gives [`None`].
979 /// [`upgrade`]: struct.Weak.html#method.upgrade
980 /// [`None`]: ../../std/option/enum.Option.html
985 /// use std::rc::Weak;
987 /// let empty: Weak<i64> = Weak::new();
988 /// assert!(empty.upgrade().is_none());
990 #[stable(feature = "downgraded_weak", since = "1.10.0")]
991 pub fn new() -> Weak<T> {
994 ptr: Shared::new(Box::into_raw(box RcBox {
995 strong: Cell::new(0),
997 value: uninitialized(),
1004 impl<T: ?Sized> Weak<T> {
1005 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1006 /// the lifetime of the value if successful.
1008 /// Returns [`None`] if the value has since been dropped.
1010 /// [`Rc`]: struct.Rc.html
1011 /// [`None`]: ../../std/option/enum.Option.html
1016 /// use std::rc::Rc;
1018 /// let five = Rc::new(5);
1020 /// let weak_five = Rc::downgrade(&five);
1022 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1023 /// assert!(strong_five.is_some());
1025 /// // Destroy all strong pointers.
1026 /// drop(strong_five);
1029 /// assert!(weak_five.upgrade().is_none());
1031 #[stable(feature = "rc_weak", since = "1.4.0")]
1032 pub fn upgrade(&self) -> Option<Rc<T>> {
1033 if self.strong() == 0 {
1037 Some(Rc { ptr: self.ptr })
1042 #[stable(feature = "rc_weak", since = "1.4.0")]
1043 impl<T: ?Sized> Drop for Weak<T> {
1044 /// Drops the `Weak` pointer.
1049 /// use std::rc::Rc;
1053 /// impl Drop for Foo {
1054 /// fn drop(&mut self) {
1055 /// println!("dropped!");
1059 /// let foo = Rc::new(Foo);
1060 /// let weak_foo = Rc::downgrade(&foo);
1061 /// let other_weak_foo = weak_foo.clone();
1063 /// drop(weak_foo); // Doesn't print anything
1064 /// drop(foo); // Prints "dropped!"
1066 /// assert!(other_weak_foo.upgrade().is_none());
1068 fn drop(&mut self) {
1070 let ptr = *self.ptr;
1073 // the weak count starts at 1, and will only go to zero if all
1074 // the strong pointers have disappeared.
1075 if self.weak() == 0 {
1076 deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
1082 #[stable(feature = "rc_weak", since = "1.4.0")]
1083 impl<T: ?Sized> Clone for Weak<T> {
1084 /// Makes a clone of the `Weak` pointer that points to the same value.
1089 /// use std::rc::Rc;
1091 /// let weak_five = Rc::downgrade(&Rc::new(5));
1093 /// weak_five.clone();
1096 fn clone(&self) -> Weak<T> {
1098 Weak { ptr: self.ptr }
1102 #[stable(feature = "rc_weak", since = "1.4.0")]
1103 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1104 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1109 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1110 impl<T> Default for Weak<T> {
1111 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1112 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1114 /// [`upgrade`]: struct.Weak.html#method.upgrade
1115 /// [`None`]: ../../std/option/enum.Option.html
1120 /// use std::rc::Weak;
1122 /// let empty: Weak<i64> = Default::default();
1123 /// assert!(empty.upgrade().is_none());
1125 fn default() -> Weak<T> {
1130 // NOTE: We checked_add here to deal with mem::forget safety. In particular
1131 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1132 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1133 // We abort because this is such a degenerate scenario that we don't care about
1134 // what happens -- no real program should ever experience this.
1136 // This should have negligible overhead since you don't actually need to
1137 // clone these much in Rust thanks to ownership and move-semantics.
1140 trait RcBoxPtr<T: ?Sized> {
1141 fn inner(&self) -> &RcBox<T>;
1144 fn strong(&self) -> usize {
1145 self.inner().strong.get()
1149 fn inc_strong(&self) {
1150 self.inner().strong.set(self.strong().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1154 fn dec_strong(&self) {
1155 self.inner().strong.set(self.strong() - 1);
1159 fn weak(&self) -> usize {
1160 self.inner().weak.get()
1164 fn inc_weak(&self) {
1165 self.inner().weak.set(self.weak().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1169 fn dec_weak(&self) {
1170 self.inner().weak.set(self.weak() - 1);
1174 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1176 fn inner(&self) -> &RcBox<T> {
1178 // Safe to assume this here, as if it weren't true, we'd be breaking
1179 // the contract anyway.
1180 // This allows the null check to be elided in the destructor if we
1181 // manipulated the reference count in the same function.
1182 assume(!(*(&self.ptr as *const _ as *const *const ())).is_null());
1188 impl<T: ?Sized> RcBoxPtr<T> for Weak<T> {
1190 fn inner(&self) -> &RcBox<T> {
1192 // Safe to assume this here, as if it weren't true, we'd be breaking
1193 // the contract anyway.
1194 // This allows the null check to be elided in the destructor if we
1195 // manipulated the reference count in the same function.
1196 assume(!(*(&self.ptr as *const _ as *const *const ())).is_null());
1204 use super::{Rc, Weak};
1205 use std::boxed::Box;
1206 use std::cell::RefCell;
1207 use std::option::Option;
1208 use std::option::Option::{None, Some};
1209 use std::result::Result::{Err, Ok};
1211 use std::clone::Clone;
1212 use std::convert::From;
1216 let x = Rc::new(RefCell::new(5));
1218 *x.borrow_mut() = 20;
1219 assert_eq!(*y.borrow(), 20);
1229 fn test_simple_clone() {
1237 fn test_destructor() {
1238 let x: Rc<Box<_>> = Rc::new(box 5);
1245 let y = Rc::downgrade(&x);
1246 assert!(y.upgrade().is_some());
1252 let y = Rc::downgrade(&x);
1254 assert!(y.upgrade().is_none());
1258 fn weak_self_cyclic() {
1260 x: RefCell<Option<Weak<Cycle>>>,
1263 let a = Rc::new(Cycle { x: RefCell::new(None) });
1264 let b = Rc::downgrade(&a.clone());
1265 *a.x.borrow_mut() = Some(b);
1267 // hopefully we don't double-free (or leak)...
1273 assert!(Rc::is_unique(&x));
1275 assert!(!Rc::is_unique(&x));
1277 assert!(Rc::is_unique(&x));
1278 let w = Rc::downgrade(&x);
1279 assert!(!Rc::is_unique(&x));
1281 assert!(Rc::is_unique(&x));
1285 fn test_strong_count() {
1287 assert!(Rc::strong_count(&a) == 1);
1288 let w = Rc::downgrade(&a);
1289 assert!(Rc::strong_count(&a) == 1);
1290 let b = w.upgrade().expect("upgrade of live rc failed");
1291 assert!(Rc::strong_count(&b) == 2);
1292 assert!(Rc::strong_count(&a) == 2);
1295 assert!(Rc::strong_count(&b) == 1);
1297 assert!(Rc::strong_count(&b) == 2);
1298 assert!(Rc::strong_count(&c) == 2);
1302 fn test_weak_count() {
1304 assert!(Rc::strong_count(&a) == 1);
1305 assert!(Rc::weak_count(&a) == 0);
1306 let w = Rc::downgrade(&a);
1307 assert!(Rc::strong_count(&a) == 1);
1308 assert!(Rc::weak_count(&a) == 1);
1310 assert!(Rc::strong_count(&a) == 1);
1311 assert!(Rc::weak_count(&a) == 0);
1313 assert!(Rc::strong_count(&a) == 2);
1314 assert!(Rc::weak_count(&a) == 0);
1321 assert_eq!(Rc::try_unwrap(x), Ok(3));
1324 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
1326 let _w = Rc::downgrade(&x);
1327 assert_eq!(Rc::try_unwrap(x), Ok(5));
1331 fn into_from_raw() {
1332 let x = Rc::new(box "hello");
1335 let x_ptr = Rc::into_raw(x);
1338 assert_eq!(**x_ptr, "hello");
1340 let x = Rc::from_raw(x_ptr);
1341 assert_eq!(**x, "hello");
1343 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
1349 let mut x = Rc::new(3);
1350 *Rc::get_mut(&mut x).unwrap() = 4;
1353 assert!(Rc::get_mut(&mut x).is_none());
1355 assert!(Rc::get_mut(&mut x).is_some());
1356 let _w = Rc::downgrade(&x);
1357 assert!(Rc::get_mut(&mut x).is_none());
1361 fn test_cowrc_clone_make_unique() {
1362 let mut cow0 = Rc::new(75);
1363 let mut cow1 = cow0.clone();
1364 let mut cow2 = cow1.clone();
1366 assert!(75 == *Rc::make_mut(&mut cow0));
1367 assert!(75 == *Rc::make_mut(&mut cow1));
1368 assert!(75 == *Rc::make_mut(&mut cow2));
1370 *Rc::make_mut(&mut cow0) += 1;
1371 *Rc::make_mut(&mut cow1) += 2;
1372 *Rc::make_mut(&mut cow2) += 3;
1374 assert!(76 == *cow0);
1375 assert!(77 == *cow1);
1376 assert!(78 == *cow2);
1378 // none should point to the same backing memory
1379 assert!(*cow0 != *cow1);
1380 assert!(*cow0 != *cow2);
1381 assert!(*cow1 != *cow2);
1385 fn test_cowrc_clone_unique2() {
1386 let mut cow0 = Rc::new(75);
1387 let cow1 = cow0.clone();
1388 let cow2 = cow1.clone();
1390 assert!(75 == *cow0);
1391 assert!(75 == *cow1);
1392 assert!(75 == *cow2);
1394 *Rc::make_mut(&mut cow0) += 1;
1396 assert!(76 == *cow0);
1397 assert!(75 == *cow1);
1398 assert!(75 == *cow2);
1400 // cow1 and cow2 should share the same contents
1401 // cow0 should have a unique reference
1402 assert!(*cow0 != *cow1);
1403 assert!(*cow0 != *cow2);
1404 assert!(*cow1 == *cow2);
1408 fn test_cowrc_clone_weak() {
1409 let mut cow0 = Rc::new(75);
1410 let cow1_weak = Rc::downgrade(&cow0);
1412 assert!(75 == *cow0);
1413 assert!(75 == *cow1_weak.upgrade().unwrap());
1415 *Rc::make_mut(&mut cow0) += 1;
1417 assert!(76 == *cow0);
1418 assert!(cow1_weak.upgrade().is_none());
1423 let foo = Rc::new(75);
1424 assert_eq!(format!("{:?}", foo), "75");
1429 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
1430 assert_eq!(foo, foo.clone());
1434 fn test_from_owned() {
1436 let foo_rc = Rc::from(foo);
1437 assert!(123 == *foo_rc);
1441 fn test_new_weak() {
1442 let foo: Weak<usize> = Weak::new();
1443 assert!(foo.upgrade().is_none());
1448 let five = Rc::new(5);
1449 let same_five = five.clone();
1450 let other_five = Rc::new(5);
1452 assert!(Rc::ptr_eq(&five, &same_five));
1453 assert!(!Rc::ptr_eq(&five, &other_five));
1457 #[stable(feature = "rust1", since = "1.0.0")]
1458 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
1459 fn borrow(&self) -> &T {
1464 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1465 impl<T: ?Sized> AsRef<T> for Rc<T> {
1466 fn as_ref(&self) -> &T {