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. 'Rc' stands for 'Reference
16 //! The type [`Rc<T>`][`Rc`] provides shared ownership of a value of type `T`,
17 //! allocated in the heap. Invoking [`clone`][clone] on [`Rc`] produces a new
18 //! pointer to the same value in the heap. When the last [`Rc`] pointer to a
19 //! given value is destroyed, the pointed-to value is also destroyed.
21 //! Shared references in Rust disallow mutation by default, and [`Rc`]
22 //! is no exception: you cannot obtain a mutable reference to
23 //! something inside an [`Rc`]. If you need mutability, put a [`Cell`]
24 //! or [`RefCell`] inside the [`Rc`]; see [an example of mutability
25 //! inside an Rc][mutability].
27 //! [`Rc`] uses non-atomic reference counting. This means that overhead is very
28 //! low, but an [`Rc`] cannot be sent between threads, and consequently [`Rc`]
29 //! does not implement [`Send`][send]. As a result, the Rust compiler
30 //! will check *at compile time* that you are not sending [`Rc`]s between
31 //! threads. If you need multi-threaded, atomic reference counting, use
32 //! [`sync::Arc`][arc].
34 //! The [`downgrade`][downgrade] method can be used to create a non-owning
35 //! [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
36 //! to an [`Rc`], but this will return [`None`] if the value has
37 //! already been dropped.
39 //! A cycle between [`Rc`] pointers will never be deallocated. For this reason,
40 //! [`Weak`] is used to break cycles. For example, a tree could have strong
41 //! [`Rc`] pointers from parent nodes to children, and [`Weak`] pointers from
42 //! children back to their parents.
44 //! `Rc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
45 //! so you can call `T`'s methods on a value of type [`Rc<T>`][`Rc`]. To avoid name
46 //! clashes with `T`'s methods, the methods of [`Rc<T>`][`Rc`] itself are [associated
47 //! functions][assoc], called using function-like syntax:
51 //! let my_rc = Rc::new(());
53 //! Rc::downgrade(&my_rc);
56 //! [`Weak<T>`][`Weak`] does not auto-dereference to `T`, because the value may have
57 //! already been destroyed.
59 //! # Cloning references
61 //! Creating a new reference from an existing reference counted pointer is done using the
62 //! `Clone` trait implemented for [`Rc<T>`][`Rc`] and [`Weak<T>`][`Weak`].
66 //! let foo = Rc::new(vec![1.0, 2.0, 3.0]);
67 //! // The two syntaxes below are equivalent.
68 //! let a = foo.clone();
69 //! let b = Rc::clone(&foo);
70 //! // a and b both point to the same memory location as foo.
73 //! The `Rc::clone(&from)` syntax is the most idiomatic because it conveys more explicitly
74 //! the meaning of the code. In the example above, this syntax makes it easier to see that
75 //! this code is creating a new reference rather than copying the whole content of foo.
79 //! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`.
80 //! We want to have our `Gadget`s point to their `Owner`. We can't do this with
81 //! unique ownership, because more than one gadget may belong to the same
82 //! `Owner`. [`Rc`] allows us to share an `Owner` between multiple `Gadget`s,
83 //! and have the `Owner` remain allocated as long as any `Gadget` points at it.
90 //! // ...other fields
96 //! // ...other fields
100 //! // Create a reference-counted `Owner`.
101 //! let gadget_owner: Rc<Owner> = Rc::new(
103 //! name: "Gadget Man".to_string(),
107 //! // Create `Gadget`s belonging to `gadget_owner`. Cloning the `Rc<Owner>`
108 //! // value gives us a new pointer to the same `Owner` value, incrementing
109 //! // the reference count in the process.
110 //! let gadget1 = Gadget {
112 //! owner: Rc::clone(&gadget_owner),
114 //! let gadget2 = Gadget {
116 //! owner: Rc::clone(&gadget_owner),
119 //! // Dispose of our local variable `gadget_owner`.
120 //! drop(gadget_owner);
122 //! // Despite dropping `gadget_owner`, we're still able to print out the name
123 //! // of the `Owner` of the `Gadget`s. This is because we've only dropped a
124 //! // single `Rc<Owner>`, not the `Owner` it points to. As long as there are
125 //! // other `Rc<Owner>` values pointing at the same `Owner`, it will remain
126 //! // allocated. The field projection `gadget1.owner.name` works because
127 //! // `Rc<Owner>` automatically dereferences to `Owner`.
128 //! println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name);
129 //! println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name);
131 //! // At the end of the function, `gadget1` and `gadget2` are destroyed, and
132 //! // with them the last counted references to our `Owner`. Gadget Man now
133 //! // gets destroyed as well.
137 //! If our requirements change, and we also need to be able to traverse from
138 //! `Owner` to `Gadget`, we will run into problems. An [`Rc`] pointer from `Owner`
139 //! to `Gadget` introduces a cycle between the values. This means that their
140 //! reference counts can never reach 0, and the values will remain allocated
141 //! forever: a memory leak. In order to get around this, we can use [`Weak`]
144 //! Rust actually makes it somewhat difficult to produce this loop in the first
145 //! place. In order to end up with two values that point at each other, one of
146 //! them needs to be mutable. This is difficult because [`Rc`] enforces
147 //! memory safety by only giving out shared references to the value it wraps,
148 //! and these don't allow direct mutation. We need to wrap the part of the
149 //! value we wish to mutate in a [`RefCell`], which provides *interior
150 //! mutability*: a method to achieve mutability through a shared reference.
151 //! [`RefCell`] enforces Rust's borrowing rules at runtime.
155 //! use std::rc::Weak;
156 //! use std::cell::RefCell;
160 //! gadgets: RefCell<Vec<Weak<Gadget>>>,
161 //! // ...other fields
166 //! owner: Rc<Owner>,
167 //! // ...other fields
171 //! // Create a reference-counted `Owner`. Note that we've put the `Owner`'s
172 //! // vector of `Gadget`s inside a `RefCell` so that we can mutate it through
173 //! // a shared reference.
174 //! let gadget_owner: Rc<Owner> = Rc::new(
176 //! name: "Gadget Man".to_string(),
177 //! gadgets: RefCell::new(vec![]),
181 //! // Create `Gadget`s belonging to `gadget_owner`, as before.
182 //! let gadget1 = Rc::new(
185 //! owner: Rc::clone(&gadget_owner),
188 //! let gadget2 = Rc::new(
191 //! owner: Rc::clone(&gadget_owner),
195 //! // Add the `Gadget`s to their `Owner`.
197 //! let mut gadgets = gadget_owner.gadgets.borrow_mut();
198 //! gadgets.push(Rc::downgrade(&gadget1));
199 //! gadgets.push(Rc::downgrade(&gadget2));
201 //! // `RefCell` dynamic borrow ends here.
204 //! // Iterate over our `Gadget`s, printing their details out.
205 //! for gadget_weak in gadget_owner.gadgets.borrow().iter() {
207 //! // `gadget_weak` is a `Weak<Gadget>`. Since `Weak` pointers can't
208 //! // guarantee the value is still allocated, we need to call
209 //! // `upgrade`, which returns an `Option<Rc<Gadget>>`.
211 //! // In this case we know the value still exists, so we simply
212 //! // `unwrap` the `Option`. In a more complicated program, you might
213 //! // need graceful error handling for a `None` result.
215 //! let gadget = gadget_weak.upgrade().unwrap();
216 //! println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
219 //! // At the end of the function, `gadget_owner`, `gadget1`, and `gadget2`
220 //! // are destroyed. There are now no strong (`Rc`) pointers to the
221 //! // gadgets, so they are destroyed. This zeroes the reference count on
222 //! // Gadget Man, so he gets destroyed as well.
226 //! [`Rc`]: struct.Rc.html
227 //! [`Weak`]: struct.Weak.html
228 //! [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
229 //! [`Cell`]: ../../std/cell/struct.Cell.html
230 //! [`RefCell`]: ../../std/cell/struct.RefCell.html
231 //! [send]: ../../std/marker/trait.Send.html
232 //! [arc]: ../../std/sync/struct.Arc.html
233 //! [`Deref`]: ../../std/ops/trait.Deref.html
234 //! [downgrade]: struct.Rc.html#method.downgrade
235 //! [upgrade]: struct.Weak.html#method.upgrade
236 //! [`None`]: ../../std/option/enum.Option.html#variant.None
237 //! [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
238 //! [mutability]: ../../std/cell/index.html#introducing-mutability-inside-of-something-immutable
240 #![stable(feature = "rust1", since = "1.0.0")]
248 use core::cell::Cell;
249 use core::cmp::Ordering;
251 use core::hash::{Hash, Hasher};
252 use core::intrinsics::abort;
254 use core::marker::Unsize;
255 use core::mem::{self, align_of_val, forget, size_of, size_of_val, uninitialized};
256 use core::ops::Deref;
257 use core::ops::CoerceUnsized;
258 use core::ptr::{self, Shared};
259 use core::convert::From;
261 use heap::{allocate, deallocate, box_free};
264 struct RcBox<T: ?Sized> {
270 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
273 /// See the [module-level documentation](./index.html) for more details.
275 /// The inherent methods of `Rc` are all associated functions, which means
276 /// that you have to call them as e.g. [`Rc::get_mut(&mut value)`][get_mut] instead of
277 /// `value.get_mut()`. This avoids conflicts with methods of the inner
280 /// [get_mut]: #method.get_mut
281 #[stable(feature = "rust1", since = "1.0.0")]
282 pub struct Rc<T: ?Sized> {
283 ptr: Shared<RcBox<T>>,
286 #[stable(feature = "rust1", since = "1.0.0")]
287 impl<T: ?Sized> !marker::Send for Rc<T> {}
288 #[stable(feature = "rust1", since = "1.0.0")]
289 impl<T: ?Sized> !marker::Sync for Rc<T> {}
291 #[unstable(feature = "coerce_unsized", issue = "27732")]
292 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
295 /// Constructs a new `Rc<T>`.
302 /// let five = Rc::new(5);
304 #[stable(feature = "rust1", since = "1.0.0")]
305 pub fn new(value: T) -> Rc<T> {
308 // there is an implicit weak pointer owned by all the strong
309 // pointers, which ensures that the weak destructor never frees
310 // the allocation while the strong destructor is running, even
311 // if the weak pointer is stored inside the strong one.
312 ptr: Shared::new(Box::into_raw(box RcBox {
313 strong: Cell::new(1),
321 /// Returns the contained value, if the `Rc` has exactly one strong reference.
323 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
326 /// This will succeed even if there are outstanding weak references.
328 /// [result]: ../../std/result/enum.Result.html
335 /// let x = Rc::new(3);
336 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
338 /// let x = Rc::new(4);
339 /// let _y = Rc::clone(&x);
340 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
343 #[stable(feature = "rc_unique", since = "1.4.0")]
344 pub fn try_unwrap(this: Self) -> Result<T, Self> {
345 if Rc::strong_count(&this) == 1 {
347 let val = ptr::read(&*this); // copy the contained object
349 // Indicate to Weaks that they can't be promoted by decrememting
350 // the strong count, and then remove the implicit "strong weak"
351 // pointer while also handling drop logic by just crafting a
354 let _weak = Weak { ptr: this.ptr };
363 /// Consumes the `Rc`, returning the wrapped pointer.
365 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
366 /// [`Rc::from_raw`][from_raw].
368 /// [from_raw]: struct.Rc.html#method.from_raw
375 /// let x = Rc::new(10);
376 /// let x_ptr = Rc::into_raw(x);
377 /// assert_eq!(unsafe { *x_ptr }, 10);
379 #[stable(feature = "rc_raw", since = "1.17.0")]
380 pub fn into_raw(this: Self) -> *const T {
381 let ptr: *const T = &*this;
386 /// Constructs an `Rc` from a raw pointer.
388 /// The raw pointer must have been previously returned by a call to a
389 /// [`Rc::into_raw`][into_raw].
391 /// This function is unsafe because improper use may lead to memory problems. For example, a
392 /// double-free may occur if the function is called twice on the same raw pointer.
394 /// [into_raw]: struct.Rc.html#method.into_raw
401 /// let x = Rc::new(10);
402 /// let x_ptr = Rc::into_raw(x);
405 /// // Convert back to an `Rc` to prevent leak.
406 /// let x = Rc::from_raw(x_ptr);
407 /// assert_eq!(*x, 10);
409 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
412 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
414 #[stable(feature = "rc_raw", since = "1.17.0")]
415 pub unsafe fn from_raw(ptr: *const T) -> Self {
416 // To find the corresponding pointer to the `RcBox` we need to subtract the offset of the
417 // `value` field from the pointer.
419 let ptr = (ptr as *const u8).offset(-offset_of!(RcBox<T>, value));
421 ptr: Shared::new(ptr as *mut u8 as *mut _)
427 /// Constructs a new `Rc<str>` from a string slice.
429 #[unstable(feature = "rustc_private",
430 reason = "for internal use in rustc",
432 pub fn __from_str(value: &str) -> Rc<str> {
434 // Allocate enough space for `RcBox<str>`.
435 let aligned_len = 2 + (value.len() + size_of::<usize>() - 1) / size_of::<usize>();
436 let vec = RawVec::<usize>::with_capacity(aligned_len);
439 // Initialize fields of `RcBox<str>`.
440 *ptr.offset(0) = 1; // strong: Cell::new(1)
441 *ptr.offset(1) = 1; // weak: Cell::new(1)
442 ptr::copy_nonoverlapping(value.as_ptr(), ptr.offset(2) as *mut u8, value.len());
443 // Combine the allocation address and the string length into a fat pointer to `RcBox`.
444 let rcbox_ptr: *mut RcBox<str> = mem::transmute([ptr as usize, value.len()]);
445 assert!(aligned_len * size_of::<usize>() == size_of_val(&*rcbox_ptr));
446 Rc { ptr: Shared::new(rcbox_ptr) }
452 /// Constructs a new `Rc<[T]>` from a `Box<[T]>`.
454 #[unstable(feature = "rustc_private",
455 reason = "for internal use in rustc",
457 pub fn __from_array(value: Box<[T]>) -> Rc<[T]> {
459 let ptr: *mut RcBox<[T]> =
460 mem::transmute([mem::align_of::<RcBox<[T; 1]>>(), value.len()]);
461 // FIXME(custom-DST): creating this invalid &[T] is dubiously defined,
462 // we should have a better way of getting the size/align
463 // of a DST from its unsized part.
464 let ptr = allocate(size_of_val(&*ptr), align_of_val(&*ptr));
465 let ptr: *mut RcBox<[T]> = mem::transmute([ptr as usize, value.len()]);
467 // Initialize the new RcBox.
468 ptr::write(&mut (*ptr).strong, Cell::new(1));
469 ptr::write(&mut (*ptr).weak, Cell::new(1));
470 ptr::copy_nonoverlapping(
472 &mut (*ptr).value as *mut [T] as *mut T,
475 // Free the original allocation without freeing its (moved) contents.
476 box_free(Box::into_raw(value));
478 Rc { ptr: Shared::new(ptr as *mut _) }
483 impl<T: ?Sized> Rc<T> {
484 /// Creates a new [`Weak`][weak] pointer to this value.
486 /// [weak]: struct.Weak.html
493 /// let five = Rc::new(5);
495 /// let weak_five = Rc::downgrade(&five);
497 #[stable(feature = "rc_weak", since = "1.4.0")]
498 pub fn downgrade(this: &Self) -> Weak<T> {
500 Weak { ptr: this.ptr }
503 /// Gets the number of [`Weak`][weak] pointers to this value.
505 /// [weak]: struct.Weak.html
512 /// let five = Rc::new(5);
513 /// let _weak_five = Rc::downgrade(&five);
515 /// assert_eq!(1, Rc::weak_count(&five));
518 #[stable(feature = "rc_counts", since = "1.15.0")]
519 pub fn weak_count(this: &Self) -> usize {
523 /// Gets the number of strong (`Rc`) pointers to this value.
530 /// let five = Rc::new(5);
531 /// let _also_five = Rc::clone(&five);
533 /// assert_eq!(2, Rc::strong_count(&five));
536 #[stable(feature = "rc_counts", since = "1.15.0")]
537 pub fn strong_count(this: &Self) -> usize {
541 /// Returns true if there are no other `Rc` or [`Weak`][weak] pointers to
542 /// this inner value.
544 /// [weak]: struct.Weak.html
546 fn is_unique(this: &Self) -> bool {
547 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
550 /// Returns a mutable reference to the inner value, if there are
551 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
553 /// Returns [`None`] otherwise, because it is not safe to
554 /// mutate a shared value.
556 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
557 /// the inner value when it's shared.
559 /// [weak]: struct.Weak.html
560 /// [`None`]: ../../std/option/enum.Option.html#variant.None
561 /// [make_mut]: struct.Rc.html#method.make_mut
562 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
569 /// let mut x = Rc::new(3);
570 /// *Rc::get_mut(&mut x).unwrap() = 4;
571 /// assert_eq!(*x, 4);
573 /// let _y = Rc::clone(&x);
574 /// assert!(Rc::get_mut(&mut x).is_none());
577 #[stable(feature = "rc_unique", since = "1.4.0")]
578 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
579 if Rc::is_unique(this) {
581 Some(&mut this.ptr.as_mut().value)
589 #[stable(feature = "ptr_eq", since = "1.17.0")]
590 /// Returns true if the two `Rc`s point to the same value (not
591 /// just values that compare as equal).
598 /// let five = Rc::new(5);
599 /// let same_five = Rc::clone(&five);
600 /// let other_five = Rc::new(5);
602 /// assert!(Rc::ptr_eq(&five, &same_five));
603 /// assert!(!Rc::ptr_eq(&five, &other_five));
605 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
606 this.ptr.as_ptr() == other.ptr.as_ptr()
610 impl<T: Clone> Rc<T> {
611 /// Makes a mutable reference into the given `Rc`.
613 /// If there are other `Rc` or [`Weak`][weak] pointers to the same value,
614 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
615 /// ensure unique ownership. This is also referred to as clone-on-write.
617 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
619 /// [weak]: struct.Weak.html
620 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
621 /// [get_mut]: struct.Rc.html#method.get_mut
628 /// let mut data = Rc::new(5);
630 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
631 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
632 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
633 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
634 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
636 /// // Now `data` and `other_data` point to different values.
637 /// assert_eq!(*data, 8);
638 /// assert_eq!(*other_data, 12);
641 #[stable(feature = "rc_unique", since = "1.4.0")]
642 pub fn make_mut(this: &mut Self) -> &mut T {
643 if Rc::strong_count(this) != 1 {
644 // Gotta clone the data, there are other Rcs
645 *this = Rc::new((**this).clone())
646 } else if Rc::weak_count(this) != 0 {
647 // Can just steal the data, all that's left is Weaks
649 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
650 mem::swap(this, &mut swap);
652 // Remove implicit strong-weak ref (no need to craft a fake
653 // Weak here -- we know other Weaks can clean up for us)
658 // This unsafety is ok because we're guaranteed that the pointer
659 // returned is the *only* pointer that will ever be returned to T. Our
660 // reference count is guaranteed to be 1 at this point, and we required
661 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
662 // reference to the inner value.
664 &mut this.ptr.as_mut().value
669 #[stable(feature = "rust1", since = "1.0.0")]
670 impl<T: ?Sized> Deref for Rc<T> {
674 fn deref(&self) -> &T {
679 #[stable(feature = "rust1", since = "1.0.0")]
680 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
683 /// This will decrement the strong reference count. If the strong reference
684 /// count reaches zero then the only other references (if any) are
685 /// [`Weak`][weak], so we `drop` the inner value.
687 /// [weak]: struct.Weak.html
696 /// impl Drop for Foo {
697 /// fn drop(&mut self) {
698 /// println!("dropped!");
702 /// let foo = Rc::new(Foo);
703 /// let foo2 = Rc::clone(&foo);
705 /// drop(foo); // Doesn't print anything
706 /// drop(foo2); // Prints "dropped!"
710 let ptr = self.ptr.as_ptr();
713 if self.strong() == 0 {
714 // destroy the contained object
715 ptr::drop_in_place(self.ptr.as_mut());
717 // remove the implicit "strong weak" pointer now that we've
718 // destroyed the contents.
721 if self.weak() == 0 {
722 deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
729 #[stable(feature = "rust1", since = "1.0.0")]
730 impl<T: ?Sized> Clone for Rc<T> {
731 /// Makes a clone of the `Rc` pointer.
733 /// This creates another pointer to the same inner value, increasing the
734 /// strong reference count.
741 /// let five = Rc::new(5);
743 /// Rc::clone(&five);
746 fn clone(&self) -> Rc<T> {
752 #[stable(feature = "rust1", since = "1.0.0")]
753 impl<T: Default> Default for Rc<T> {
754 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
761 /// let x: Rc<i32> = Default::default();
762 /// assert_eq!(*x, 0);
765 fn default() -> Rc<T> {
766 Rc::new(Default::default())
770 #[stable(feature = "rust1", since = "1.0.0")]
771 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
772 /// Equality for two `Rc`s.
774 /// Two `Rc`s are equal if their inner values are equal.
781 /// let five = Rc::new(5);
783 /// assert!(five == Rc::new(5));
786 fn eq(&self, other: &Rc<T>) -> bool {
790 /// Inequality for two `Rc`s.
792 /// Two `Rc`s are unequal if their inner values are unequal.
799 /// let five = Rc::new(5);
801 /// assert!(five != Rc::new(6));
804 fn ne(&self, other: &Rc<T>) -> bool {
809 #[stable(feature = "rust1", since = "1.0.0")]
810 impl<T: ?Sized + Eq> Eq for Rc<T> {}
812 #[stable(feature = "rust1", since = "1.0.0")]
813 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
814 /// Partial comparison for two `Rc`s.
816 /// The two are compared by calling `partial_cmp()` on their inner values.
822 /// use std::cmp::Ordering;
824 /// let five = Rc::new(5);
826 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
829 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
830 (**self).partial_cmp(&**other)
833 /// Less-than comparison for two `Rc`s.
835 /// The two are compared by calling `<` on their inner values.
842 /// let five = Rc::new(5);
844 /// assert!(five < Rc::new(6));
847 fn lt(&self, other: &Rc<T>) -> bool {
851 /// 'Less than or equal to' comparison for two `Rc`s.
853 /// The two are compared by calling `<=` on their inner values.
860 /// let five = Rc::new(5);
862 /// assert!(five <= Rc::new(5));
865 fn le(&self, other: &Rc<T>) -> bool {
869 /// Greater-than comparison for two `Rc`s.
871 /// The two are compared by calling `>` on their inner values.
878 /// let five = Rc::new(5);
880 /// assert!(five > Rc::new(4));
883 fn gt(&self, other: &Rc<T>) -> bool {
887 /// 'Greater than or equal to' comparison for two `Rc`s.
889 /// The two are compared by calling `>=` on their inner values.
896 /// let five = Rc::new(5);
898 /// assert!(five >= Rc::new(5));
901 fn ge(&self, other: &Rc<T>) -> bool {
906 #[stable(feature = "rust1", since = "1.0.0")]
907 impl<T: ?Sized + Ord> Ord for Rc<T> {
908 /// Comparison for two `Rc`s.
910 /// The two are compared by calling `cmp()` on their inner values.
916 /// use std::cmp::Ordering;
918 /// let five = Rc::new(5);
920 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
923 fn cmp(&self, other: &Rc<T>) -> Ordering {
924 (**self).cmp(&**other)
928 #[stable(feature = "rust1", since = "1.0.0")]
929 impl<T: ?Sized + Hash> Hash for Rc<T> {
930 fn hash<H: Hasher>(&self, state: &mut H) {
931 (**self).hash(state);
935 #[stable(feature = "rust1", since = "1.0.0")]
936 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
937 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
938 fmt::Display::fmt(&**self, f)
942 #[stable(feature = "rust1", since = "1.0.0")]
943 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
944 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
945 fmt::Debug::fmt(&**self, f)
949 #[stable(feature = "rust1", since = "1.0.0")]
950 impl<T: ?Sized> fmt::Pointer for Rc<T> {
951 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
952 fmt::Pointer::fmt(&self.ptr, f)
956 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
957 impl<T> From<T> for Rc<T> {
958 fn from(t: T) -> Self {
963 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
964 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
965 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
967 /// Since a `Weak` reference does not count towards ownership, it will not
968 /// prevent the inner value from being dropped, and `Weak` itself makes no
969 /// guarantees about the value still being present and may return [`None`]
970 /// when [`upgrade`]d.
972 /// A `Weak` pointer is useful for keeping a temporary reference to the value
973 /// within [`Rc`] without extending its lifetime. It is also used to prevent
974 /// circular references between [`Rc`] pointers, since mutual owning references
975 /// would never allow either [`Arc`] to be dropped. For example, a tree could
976 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
977 /// pointers from children back to their parents.
979 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
981 /// [`Rc`]: struct.Rc.html
982 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
983 /// [`upgrade`]: struct.Weak.html#method.upgrade
984 /// [`Option`]: ../../std/option/enum.Option.html
985 /// [`None`]: ../../std/option/enum.Option.html#variant.None
986 #[stable(feature = "rc_weak", since = "1.4.0")]
987 pub struct Weak<T: ?Sized> {
988 ptr: Shared<RcBox<T>>,
991 #[stable(feature = "rc_weak", since = "1.4.0")]
992 impl<T: ?Sized> !marker::Send for Weak<T> {}
993 #[stable(feature = "rc_weak", since = "1.4.0")]
994 impl<T: ?Sized> !marker::Sync for Weak<T> {}
996 #[unstable(feature = "coerce_unsized", issue = "27732")]
997 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1000 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1001 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1003 /// [`upgrade`]: struct.Weak.html#method.upgrade
1004 /// [`None`]: ../../std/option/enum.Option.html
1009 /// use std::rc::Weak;
1011 /// let empty: Weak<i64> = Weak::new();
1012 /// assert!(empty.upgrade().is_none());
1014 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1015 pub fn new() -> Weak<T> {
1018 ptr: Shared::new(Box::into_raw(box RcBox {
1019 strong: Cell::new(0),
1021 value: uninitialized(),
1028 impl<T: ?Sized> Weak<T> {
1029 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1030 /// the lifetime of the value if successful.
1032 /// Returns [`None`] if the value has since been dropped.
1034 /// [`Rc`]: struct.Rc.html
1035 /// [`None`]: ../../std/option/enum.Option.html
1040 /// use std::rc::Rc;
1042 /// let five = Rc::new(5);
1044 /// let weak_five = Rc::downgrade(&five);
1046 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1047 /// assert!(strong_five.is_some());
1049 /// // Destroy all strong pointers.
1050 /// drop(strong_five);
1053 /// assert!(weak_five.upgrade().is_none());
1055 #[stable(feature = "rc_weak", since = "1.4.0")]
1056 pub fn upgrade(&self) -> Option<Rc<T>> {
1057 if self.strong() == 0 {
1061 Some(Rc { ptr: self.ptr })
1066 #[stable(feature = "rc_weak", since = "1.4.0")]
1067 impl<T: ?Sized> Drop for Weak<T> {
1068 /// Drops the `Weak` pointer.
1073 /// use std::rc::{Rc, Weak};
1077 /// impl Drop for Foo {
1078 /// fn drop(&mut self) {
1079 /// println!("dropped!");
1083 /// let foo = Rc::new(Foo);
1084 /// let weak_foo = Rc::downgrade(&foo);
1085 /// let other_weak_foo = Weak::clone(&weak_foo);
1087 /// drop(weak_foo); // Doesn't print anything
1088 /// drop(foo); // Prints "dropped!"
1090 /// assert!(other_weak_foo.upgrade().is_none());
1092 fn drop(&mut self) {
1094 let ptr = self.ptr.as_ptr();
1097 // the weak count starts at 1, and will only go to zero if all
1098 // the strong pointers have disappeared.
1099 if self.weak() == 0 {
1100 deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
1106 #[stable(feature = "rc_weak", since = "1.4.0")]
1107 impl<T: ?Sized> Clone for Weak<T> {
1108 /// Makes a clone of the `Weak` pointer that points to the same value.
1113 /// use std::rc::{Rc, Weak};
1115 /// let weak_five = Rc::downgrade(&Rc::new(5));
1117 /// Weak::clone(&weak_five);
1120 fn clone(&self) -> Weak<T> {
1122 Weak { ptr: self.ptr }
1126 #[stable(feature = "rc_weak", since = "1.4.0")]
1127 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1128 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1133 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1134 impl<T> Default for Weak<T> {
1135 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1136 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1138 /// [`upgrade`]: struct.Weak.html#method.upgrade
1139 /// [`None`]: ../../std/option/enum.Option.html
1144 /// use std::rc::Weak;
1146 /// let empty: Weak<i64> = Default::default();
1147 /// assert!(empty.upgrade().is_none());
1149 fn default() -> Weak<T> {
1154 // NOTE: We checked_add here to deal with mem::forget safety. In particular
1155 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1156 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1157 // We abort because this is such a degenerate scenario that we don't care about
1158 // what happens -- no real program should ever experience this.
1160 // This should have negligible overhead since you don't actually need to
1161 // clone these much in Rust thanks to ownership and move-semantics.
1164 trait RcBoxPtr<T: ?Sized> {
1165 fn inner(&self) -> &RcBox<T>;
1168 fn strong(&self) -> usize {
1169 self.inner().strong.get()
1173 fn inc_strong(&self) {
1174 self.inner().strong.set(self.strong().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1178 fn dec_strong(&self) {
1179 self.inner().strong.set(self.strong() - 1);
1183 fn weak(&self) -> usize {
1184 self.inner().weak.get()
1188 fn inc_weak(&self) {
1189 self.inner().weak.set(self.weak().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1193 fn dec_weak(&self) {
1194 self.inner().weak.set(self.weak() - 1);
1198 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1200 fn inner(&self) -> &RcBox<T> {
1207 impl<T: ?Sized> RcBoxPtr<T> for Weak<T> {
1209 fn inner(&self) -> &RcBox<T> {
1218 use super::{Rc, Weak};
1219 use std::boxed::Box;
1220 use std::cell::RefCell;
1221 use std::option::Option;
1222 use std::option::Option::{None, Some};
1223 use std::result::Result::{Err, Ok};
1225 use std::clone::Clone;
1226 use std::convert::From;
1230 let x = Rc::new(RefCell::new(5));
1232 *x.borrow_mut() = 20;
1233 assert_eq!(*y.borrow(), 20);
1243 fn test_simple_clone() {
1251 fn test_destructor() {
1252 let x: Rc<Box<_>> = Rc::new(box 5);
1259 let y = Rc::downgrade(&x);
1260 assert!(y.upgrade().is_some());
1266 let y = Rc::downgrade(&x);
1268 assert!(y.upgrade().is_none());
1272 fn weak_self_cyclic() {
1274 x: RefCell<Option<Weak<Cycle>>>,
1277 let a = Rc::new(Cycle { x: RefCell::new(None) });
1278 let b = Rc::downgrade(&a.clone());
1279 *a.x.borrow_mut() = Some(b);
1281 // hopefully we don't double-free (or leak)...
1287 assert!(Rc::is_unique(&x));
1289 assert!(!Rc::is_unique(&x));
1291 assert!(Rc::is_unique(&x));
1292 let w = Rc::downgrade(&x);
1293 assert!(!Rc::is_unique(&x));
1295 assert!(Rc::is_unique(&x));
1299 fn test_strong_count() {
1301 assert!(Rc::strong_count(&a) == 1);
1302 let w = Rc::downgrade(&a);
1303 assert!(Rc::strong_count(&a) == 1);
1304 let b = w.upgrade().expect("upgrade of live rc failed");
1305 assert!(Rc::strong_count(&b) == 2);
1306 assert!(Rc::strong_count(&a) == 2);
1309 assert!(Rc::strong_count(&b) == 1);
1311 assert!(Rc::strong_count(&b) == 2);
1312 assert!(Rc::strong_count(&c) == 2);
1316 fn test_weak_count() {
1318 assert!(Rc::strong_count(&a) == 1);
1319 assert!(Rc::weak_count(&a) == 0);
1320 let w = Rc::downgrade(&a);
1321 assert!(Rc::strong_count(&a) == 1);
1322 assert!(Rc::weak_count(&a) == 1);
1324 assert!(Rc::strong_count(&a) == 1);
1325 assert!(Rc::weak_count(&a) == 0);
1327 assert!(Rc::strong_count(&a) == 2);
1328 assert!(Rc::weak_count(&a) == 0);
1335 assert_eq!(Rc::try_unwrap(x), Ok(3));
1338 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
1340 let _w = Rc::downgrade(&x);
1341 assert_eq!(Rc::try_unwrap(x), Ok(5));
1345 fn into_from_raw() {
1346 let x = Rc::new(box "hello");
1349 let x_ptr = Rc::into_raw(x);
1352 assert_eq!(**x_ptr, "hello");
1354 let x = Rc::from_raw(x_ptr);
1355 assert_eq!(**x, "hello");
1357 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
1363 let mut x = Rc::new(3);
1364 *Rc::get_mut(&mut x).unwrap() = 4;
1367 assert!(Rc::get_mut(&mut x).is_none());
1369 assert!(Rc::get_mut(&mut x).is_some());
1370 let _w = Rc::downgrade(&x);
1371 assert!(Rc::get_mut(&mut x).is_none());
1375 fn test_cowrc_clone_make_unique() {
1376 let mut cow0 = Rc::new(75);
1377 let mut cow1 = cow0.clone();
1378 let mut cow2 = cow1.clone();
1380 assert!(75 == *Rc::make_mut(&mut cow0));
1381 assert!(75 == *Rc::make_mut(&mut cow1));
1382 assert!(75 == *Rc::make_mut(&mut cow2));
1384 *Rc::make_mut(&mut cow0) += 1;
1385 *Rc::make_mut(&mut cow1) += 2;
1386 *Rc::make_mut(&mut cow2) += 3;
1388 assert!(76 == *cow0);
1389 assert!(77 == *cow1);
1390 assert!(78 == *cow2);
1392 // none should point to the same backing memory
1393 assert!(*cow0 != *cow1);
1394 assert!(*cow0 != *cow2);
1395 assert!(*cow1 != *cow2);
1399 fn test_cowrc_clone_unique2() {
1400 let mut cow0 = Rc::new(75);
1401 let cow1 = cow0.clone();
1402 let cow2 = cow1.clone();
1404 assert!(75 == *cow0);
1405 assert!(75 == *cow1);
1406 assert!(75 == *cow2);
1408 *Rc::make_mut(&mut cow0) += 1;
1410 assert!(76 == *cow0);
1411 assert!(75 == *cow1);
1412 assert!(75 == *cow2);
1414 // cow1 and cow2 should share the same contents
1415 // cow0 should have a unique reference
1416 assert!(*cow0 != *cow1);
1417 assert!(*cow0 != *cow2);
1418 assert!(*cow1 == *cow2);
1422 fn test_cowrc_clone_weak() {
1423 let mut cow0 = Rc::new(75);
1424 let cow1_weak = Rc::downgrade(&cow0);
1426 assert!(75 == *cow0);
1427 assert!(75 == *cow1_weak.upgrade().unwrap());
1429 *Rc::make_mut(&mut cow0) += 1;
1431 assert!(76 == *cow0);
1432 assert!(cow1_weak.upgrade().is_none());
1437 let foo = Rc::new(75);
1438 assert_eq!(format!("{:?}", foo), "75");
1443 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
1444 assert_eq!(foo, foo.clone());
1448 fn test_from_owned() {
1450 let foo_rc = Rc::from(foo);
1451 assert!(123 == *foo_rc);
1455 fn test_new_weak() {
1456 let foo: Weak<usize> = Weak::new();
1457 assert!(foo.upgrade().is_none());
1462 let five = Rc::new(5);
1463 let same_five = five.clone();
1464 let other_five = Rc::new(5);
1466 assert!(Rc::ptr_eq(&five, &same_five));
1467 assert!(!Rc::ptr_eq(&five, &other_five));
1471 #[stable(feature = "rust1", since = "1.0.0")]
1472 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
1473 fn borrow(&self) -> &T {
1478 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1479 impl<T: ?Sized> AsRef<T> for Rc<T> {
1480 fn as_ref(&self) -> &T {