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, 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::{Heap, Alloc, Layout, 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> {
307 // there is an implicit weak pointer owned by all the strong
308 // pointers, which ensures that the weak destructor never frees
309 // the allocation while the strong destructor is running, even
310 // if the weak pointer is stored inside the strong one.
311 ptr: Shared::from(Box::into_unique(box RcBox {
312 strong: Cell::new(1),
319 /// Returns the contained value, if the `Rc` has exactly one strong reference.
321 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
324 /// This will succeed even if there are outstanding weak references.
326 /// [result]: ../../std/result/enum.Result.html
333 /// let x = Rc::new(3);
334 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
336 /// let x = Rc::new(4);
337 /// let _y = Rc::clone(&x);
338 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
341 #[stable(feature = "rc_unique", since = "1.4.0")]
342 pub fn try_unwrap(this: Self) -> Result<T, Self> {
343 if Rc::strong_count(&this) == 1 {
345 let val = ptr::read(&*this); // copy the contained object
347 // Indicate to Weaks that they can't be promoted by decrememting
348 // the strong count, and then remove the implicit "strong weak"
349 // pointer while also handling drop logic by just crafting a
352 let _weak = Weak { ptr: this.ptr };
361 /// Consumes the `Rc`, returning the wrapped pointer.
363 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
364 /// [`Rc::from_raw`][from_raw].
366 /// [from_raw]: struct.Rc.html#method.from_raw
373 /// let x = Rc::new(10);
374 /// let x_ptr = Rc::into_raw(x);
375 /// assert_eq!(unsafe { *x_ptr }, 10);
377 #[stable(feature = "rc_raw", since = "1.17.0")]
378 pub fn into_raw(this: Self) -> *const T {
379 let ptr: *const T = &*this;
384 /// Constructs an `Rc` from a raw pointer.
386 /// The raw pointer must have been previously returned by a call to a
387 /// [`Rc::into_raw`][into_raw].
389 /// This function is unsafe because improper use may lead to memory problems. For example, a
390 /// double-free may occur if the function is called twice on the same raw pointer.
392 /// [into_raw]: struct.Rc.html#method.into_raw
399 /// let x = Rc::new(10);
400 /// let x_ptr = Rc::into_raw(x);
403 /// // Convert back to an `Rc` to prevent leak.
404 /// let x = Rc::from_raw(x_ptr);
405 /// assert_eq!(*x, 10);
407 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
410 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
412 #[stable(feature = "rc_raw", since = "1.17.0")]
413 pub unsafe fn from_raw(ptr: *const T) -> Self {
414 // To find the corresponding pointer to the `RcBox` we need to subtract the offset of the
415 // `value` field from the pointer.
417 let ptr = (ptr as *const u8).offset(-offset_of!(RcBox<T>, value));
419 ptr: Shared::new_unchecked(ptr as *mut u8 as *mut _)
425 /// Constructs a new `Rc<str>` from a string slice.
427 #[unstable(feature = "rustc_private",
428 reason = "for internal use in rustc",
430 pub fn __from_str(value: &str) -> Rc<str> {
432 // Allocate enough space for `RcBox<str>`.
433 let aligned_len = 2 + (value.len() + size_of::<usize>() - 1) / size_of::<usize>();
434 let vec = RawVec::<usize>::with_capacity(aligned_len);
437 // Initialize fields of `RcBox<str>`.
438 *ptr.offset(0) = 1; // strong: Cell::new(1)
439 *ptr.offset(1) = 1; // weak: Cell::new(1)
440 ptr::copy_nonoverlapping(value.as_ptr(), ptr.offset(2) as *mut u8, value.len());
441 // Combine the allocation address and the string length into a fat pointer to `RcBox`.
442 let rcbox_ptr: *mut RcBox<str> = mem::transmute([ptr as usize, value.len()]);
443 assert!(aligned_len * size_of::<usize>() == size_of_val(&*rcbox_ptr));
444 Rc { ptr: Shared::new_unchecked(rcbox_ptr) }
450 /// Constructs a new `Rc<[T]>` from a `Box<[T]>`.
452 #[unstable(feature = "rustc_private",
453 reason = "for internal use in rustc",
455 pub fn __from_array(value: Box<[T]>) -> Rc<[T]> {
457 let ptr: *mut RcBox<[T]> =
458 mem::transmute([mem::align_of::<RcBox<[T; 1]>>(), value.len()]);
459 // FIXME(custom-DST): creating this invalid &[T] is dubiously defined,
460 // we should have a better way of getting the size/align
461 // of a DST from its unsized part.
462 let ptr = Heap.alloc(Layout::for_value(&*ptr))
463 .unwrap_or_else(|e| Heap.oom(e));
464 let ptr: *mut RcBox<[T]> = mem::transmute([ptr as usize, value.len()]);
466 // Initialize the new RcBox.
467 ptr::write(&mut (*ptr).strong, Cell::new(1));
468 ptr::write(&mut (*ptr).weak, Cell::new(1));
469 ptr::copy_nonoverlapping(
471 &mut (*ptr).value as *mut [T] as *mut T,
474 // Free the original allocation without freeing its (moved) contents.
475 box_free(Box::into_raw(value));
477 Rc { ptr: Shared::new_unchecked(ptr as *mut _) }
482 impl<T: ?Sized> Rc<T> {
483 /// Creates a new [`Weak`][weak] pointer to this value.
485 /// [weak]: struct.Weak.html
492 /// let five = Rc::new(5);
494 /// let weak_five = Rc::downgrade(&five);
496 #[stable(feature = "rc_weak", since = "1.4.0")]
497 pub fn downgrade(this: &Self) -> Weak<T> {
499 Weak { ptr: this.ptr }
502 /// Gets the number of [`Weak`][weak] pointers to this value.
504 /// [weak]: struct.Weak.html
511 /// let five = Rc::new(5);
512 /// let _weak_five = Rc::downgrade(&five);
514 /// assert_eq!(1, Rc::weak_count(&five));
517 #[stable(feature = "rc_counts", since = "1.15.0")]
518 pub fn weak_count(this: &Self) -> usize {
522 /// Gets the number of strong (`Rc`) pointers to this value.
529 /// let five = Rc::new(5);
530 /// let _also_five = Rc::clone(&five);
532 /// assert_eq!(2, Rc::strong_count(&five));
535 #[stable(feature = "rc_counts", since = "1.15.0")]
536 pub fn strong_count(this: &Self) -> usize {
540 /// Returns true if there are no other `Rc` or [`Weak`][weak] pointers to
541 /// this inner value.
543 /// [weak]: struct.Weak.html
545 fn is_unique(this: &Self) -> bool {
546 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
549 /// Returns a mutable reference to the inner value, if there are
550 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
552 /// Returns [`None`] otherwise, because it is not safe to
553 /// mutate a shared value.
555 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
556 /// the inner value when it's shared.
558 /// [weak]: struct.Weak.html
559 /// [`None`]: ../../std/option/enum.Option.html#variant.None
560 /// [make_mut]: struct.Rc.html#method.make_mut
561 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
568 /// let mut x = Rc::new(3);
569 /// *Rc::get_mut(&mut x).unwrap() = 4;
570 /// assert_eq!(*x, 4);
572 /// let _y = Rc::clone(&x);
573 /// assert!(Rc::get_mut(&mut x).is_none());
576 #[stable(feature = "rc_unique", since = "1.4.0")]
577 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
578 if Rc::is_unique(this) {
580 Some(&mut this.ptr.as_mut().value)
588 #[stable(feature = "ptr_eq", since = "1.17.0")]
589 /// Returns true if the two `Rc`s point to the same value (not
590 /// just values that compare as equal).
597 /// let five = Rc::new(5);
598 /// let same_five = Rc::clone(&five);
599 /// let other_five = Rc::new(5);
601 /// assert!(Rc::ptr_eq(&five, &same_five));
602 /// assert!(!Rc::ptr_eq(&five, &other_five));
604 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
605 this.ptr.as_ptr() == other.ptr.as_ptr()
609 impl<T: Clone> Rc<T> {
610 /// Makes a mutable reference into the given `Rc`.
612 /// If there are other `Rc` or [`Weak`][weak] pointers to the same value,
613 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
614 /// ensure unique ownership. This is also referred to as clone-on-write.
616 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
618 /// [weak]: struct.Weak.html
619 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
620 /// [get_mut]: struct.Rc.html#method.get_mut
627 /// let mut data = Rc::new(5);
629 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
630 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
631 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
632 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
633 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
635 /// // Now `data` and `other_data` point to different values.
636 /// assert_eq!(*data, 8);
637 /// assert_eq!(*other_data, 12);
640 #[stable(feature = "rc_unique", since = "1.4.0")]
641 pub fn make_mut(this: &mut Self) -> &mut T {
642 if Rc::strong_count(this) != 1 {
643 // Gotta clone the data, there are other Rcs
644 *this = Rc::new((**this).clone())
645 } else if Rc::weak_count(this) != 0 {
646 // Can just steal the data, all that's left is Weaks
648 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
649 mem::swap(this, &mut swap);
651 // Remove implicit strong-weak ref (no need to craft a fake
652 // Weak here -- we know other Weaks can clean up for us)
657 // This unsafety is ok because we're guaranteed that the pointer
658 // returned is the *only* pointer that will ever be returned to T. Our
659 // reference count is guaranteed to be 1 at this point, and we required
660 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
661 // reference to the inner value.
663 &mut this.ptr.as_mut().value
668 #[stable(feature = "rust1", since = "1.0.0")]
669 impl<T: ?Sized> Deref for Rc<T> {
673 fn deref(&self) -> &T {
678 #[stable(feature = "rust1", since = "1.0.0")]
679 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
682 /// This will decrement the strong reference count. If the strong reference
683 /// count reaches zero then the only other references (if any) are
684 /// [`Weak`][weak], so we `drop` the inner value.
686 /// [weak]: struct.Weak.html
695 /// impl Drop for Foo {
696 /// fn drop(&mut self) {
697 /// println!("dropped!");
701 /// let foo = Rc::new(Foo);
702 /// let foo2 = Rc::clone(&foo);
704 /// drop(foo); // Doesn't print anything
705 /// drop(foo2); // Prints "dropped!"
709 let ptr = self.ptr.as_ptr();
712 if self.strong() == 0 {
713 // destroy the contained object
714 ptr::drop_in_place(self.ptr.as_mut());
716 // remove the implicit "strong weak" pointer now that we've
717 // destroyed the contents.
720 if self.weak() == 0 {
721 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr));
728 #[stable(feature = "rust1", since = "1.0.0")]
729 impl<T: ?Sized> Clone for Rc<T> {
730 /// Makes a clone of the `Rc` pointer.
732 /// This creates another pointer to the same inner value, increasing the
733 /// strong reference count.
740 /// let five = Rc::new(5);
742 /// Rc::clone(&five);
745 fn clone(&self) -> Rc<T> {
751 #[stable(feature = "rust1", since = "1.0.0")]
752 impl<T: Default> Default for Rc<T> {
753 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
760 /// let x: Rc<i32> = Default::default();
761 /// assert_eq!(*x, 0);
764 fn default() -> Rc<T> {
765 Rc::new(Default::default())
769 #[stable(feature = "rust1", since = "1.0.0")]
770 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
771 /// Equality for two `Rc`s.
773 /// Two `Rc`s are equal if their inner values are equal.
780 /// let five = Rc::new(5);
782 /// assert!(five == Rc::new(5));
785 fn eq(&self, other: &Rc<T>) -> bool {
789 /// Inequality for two `Rc`s.
791 /// Two `Rc`s are unequal if their inner values are unequal.
798 /// let five = Rc::new(5);
800 /// assert!(five != Rc::new(6));
803 fn ne(&self, other: &Rc<T>) -> bool {
808 #[stable(feature = "rust1", since = "1.0.0")]
809 impl<T: ?Sized + Eq> Eq for Rc<T> {}
811 #[stable(feature = "rust1", since = "1.0.0")]
812 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
813 /// Partial comparison for two `Rc`s.
815 /// The two are compared by calling `partial_cmp()` on their inner values.
821 /// use std::cmp::Ordering;
823 /// let five = Rc::new(5);
825 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
828 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
829 (**self).partial_cmp(&**other)
832 /// Less-than comparison for two `Rc`s.
834 /// The two are compared by calling `<` on their inner values.
841 /// let five = Rc::new(5);
843 /// assert!(five < Rc::new(6));
846 fn lt(&self, other: &Rc<T>) -> bool {
850 /// 'Less than or equal to' comparison for two `Rc`s.
852 /// The two are compared by calling `<=` on their inner values.
859 /// let five = Rc::new(5);
861 /// assert!(five <= Rc::new(5));
864 fn le(&self, other: &Rc<T>) -> bool {
868 /// Greater-than comparison for two `Rc`s.
870 /// The two are compared by calling `>` on their inner values.
877 /// let five = Rc::new(5);
879 /// assert!(five > Rc::new(4));
882 fn gt(&self, other: &Rc<T>) -> bool {
886 /// 'Greater than or equal to' comparison for two `Rc`s.
888 /// The two are compared by calling `>=` on their inner values.
895 /// let five = Rc::new(5);
897 /// assert!(five >= Rc::new(5));
900 fn ge(&self, other: &Rc<T>) -> bool {
905 #[stable(feature = "rust1", since = "1.0.0")]
906 impl<T: ?Sized + Ord> Ord for Rc<T> {
907 /// Comparison for two `Rc`s.
909 /// The two are compared by calling `cmp()` on their inner values.
915 /// use std::cmp::Ordering;
917 /// let five = Rc::new(5);
919 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
922 fn cmp(&self, other: &Rc<T>) -> Ordering {
923 (**self).cmp(&**other)
927 #[stable(feature = "rust1", since = "1.0.0")]
928 impl<T: ?Sized + Hash> Hash for Rc<T> {
929 fn hash<H: Hasher>(&self, state: &mut H) {
930 (**self).hash(state);
934 #[stable(feature = "rust1", since = "1.0.0")]
935 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
936 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
937 fmt::Display::fmt(&**self, f)
941 #[stable(feature = "rust1", since = "1.0.0")]
942 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
943 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
944 fmt::Debug::fmt(&**self, f)
948 #[stable(feature = "rust1", since = "1.0.0")]
949 impl<T: ?Sized> fmt::Pointer for Rc<T> {
950 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
951 fmt::Pointer::fmt(&self.ptr, f)
955 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
956 impl<T> From<T> for Rc<T> {
957 fn from(t: T) -> Self {
962 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
963 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
964 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
966 /// Since a `Weak` reference does not count towards ownership, it will not
967 /// prevent the inner value from being dropped, and `Weak` itself makes no
968 /// guarantees about the value still being present and may return [`None`]
969 /// when [`upgrade`]d.
971 /// A `Weak` pointer is useful for keeping a temporary reference to the value
972 /// within [`Rc`] without extending its lifetime. It is also used to prevent
973 /// circular references between [`Rc`] pointers, since mutual owning references
974 /// would never allow either [`Rc`] to be dropped. For example, a tree could
975 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
976 /// pointers from children back to their parents.
978 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
980 /// [`Rc`]: struct.Rc.html
981 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
982 /// [`upgrade`]: struct.Weak.html#method.upgrade
983 /// [`Option`]: ../../std/option/enum.Option.html
984 /// [`None`]: ../../std/option/enum.Option.html#variant.None
985 #[stable(feature = "rc_weak", since = "1.4.0")]
986 pub struct Weak<T: ?Sized> {
987 ptr: Shared<RcBox<T>>,
990 #[stable(feature = "rc_weak", since = "1.4.0")]
991 impl<T: ?Sized> !marker::Send for Weak<T> {}
992 #[stable(feature = "rc_weak", since = "1.4.0")]
993 impl<T: ?Sized> !marker::Sync for Weak<T> {}
995 #[unstable(feature = "coerce_unsized", issue = "27732")]
996 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
999 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1000 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1002 /// [`upgrade`]: struct.Weak.html#method.upgrade
1003 /// [`None`]: ../../std/option/enum.Option.html
1008 /// use std::rc::Weak;
1010 /// let empty: Weak<i64> = Weak::new();
1011 /// assert!(empty.upgrade().is_none());
1013 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1014 pub fn new() -> Weak<T> {
1017 ptr: Shared::from(Box::into_unique(box RcBox {
1018 strong: Cell::new(0),
1020 value: uninitialized(),
1027 impl<T: ?Sized> Weak<T> {
1028 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1029 /// the lifetime of the value if successful.
1031 /// Returns [`None`] if the value has since been dropped.
1033 /// [`Rc`]: struct.Rc.html
1034 /// [`None`]: ../../std/option/enum.Option.html
1039 /// use std::rc::Rc;
1041 /// let five = Rc::new(5);
1043 /// let weak_five = Rc::downgrade(&five);
1045 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1046 /// assert!(strong_five.is_some());
1048 /// // Destroy all strong pointers.
1049 /// drop(strong_five);
1052 /// assert!(weak_five.upgrade().is_none());
1054 #[stable(feature = "rc_weak", since = "1.4.0")]
1055 pub fn upgrade(&self) -> Option<Rc<T>> {
1056 if self.strong() == 0 {
1060 Some(Rc { ptr: self.ptr })
1065 #[stable(feature = "rc_weak", since = "1.4.0")]
1066 impl<T: ?Sized> Drop for Weak<T> {
1067 /// Drops the `Weak` pointer.
1072 /// use std::rc::{Rc, Weak};
1076 /// impl Drop for Foo {
1077 /// fn drop(&mut self) {
1078 /// println!("dropped!");
1082 /// let foo = Rc::new(Foo);
1083 /// let weak_foo = Rc::downgrade(&foo);
1084 /// let other_weak_foo = Weak::clone(&weak_foo);
1086 /// drop(weak_foo); // Doesn't print anything
1087 /// drop(foo); // Prints "dropped!"
1089 /// assert!(other_weak_foo.upgrade().is_none());
1091 fn drop(&mut self) {
1093 let ptr = self.ptr.as_ptr();
1096 // the weak count starts at 1, and will only go to zero if all
1097 // the strong pointers have disappeared.
1098 if self.weak() == 0 {
1099 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr));
1105 #[stable(feature = "rc_weak", since = "1.4.0")]
1106 impl<T: ?Sized> Clone for Weak<T> {
1107 /// Makes a clone of the `Weak` pointer that points to the same value.
1112 /// use std::rc::{Rc, Weak};
1114 /// let weak_five = Rc::downgrade(&Rc::new(5));
1116 /// Weak::clone(&weak_five);
1119 fn clone(&self) -> Weak<T> {
1121 Weak { ptr: self.ptr }
1125 #[stable(feature = "rc_weak", since = "1.4.0")]
1126 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1127 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1132 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1133 impl<T> Default for Weak<T> {
1134 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1135 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1137 /// [`upgrade`]: struct.Weak.html#method.upgrade
1138 /// [`None`]: ../../std/option/enum.Option.html
1143 /// use std::rc::Weak;
1145 /// let empty: Weak<i64> = Default::default();
1146 /// assert!(empty.upgrade().is_none());
1148 fn default() -> Weak<T> {
1153 // NOTE: We checked_add here to deal with mem::forget safety. In particular
1154 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1155 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1156 // We abort because this is such a degenerate scenario that we don't care about
1157 // what happens -- no real program should ever experience this.
1159 // This should have negligible overhead since you don't actually need to
1160 // clone these much in Rust thanks to ownership and move-semantics.
1163 trait RcBoxPtr<T: ?Sized> {
1164 fn inner(&self) -> &RcBox<T>;
1167 fn strong(&self) -> usize {
1168 self.inner().strong.get()
1172 fn inc_strong(&self) {
1173 self.inner().strong.set(self.strong().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1177 fn dec_strong(&self) {
1178 self.inner().strong.set(self.strong() - 1);
1182 fn weak(&self) -> usize {
1183 self.inner().weak.get()
1187 fn inc_weak(&self) {
1188 self.inner().weak.set(self.weak().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1192 fn dec_weak(&self) {
1193 self.inner().weak.set(self.weak() - 1);
1197 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1199 fn inner(&self) -> &RcBox<T> {
1206 impl<T: ?Sized> RcBoxPtr<T> for Weak<T> {
1208 fn inner(&self) -> &RcBox<T> {
1217 use super::{Rc, Weak};
1218 use std::boxed::Box;
1219 use std::cell::RefCell;
1220 use std::option::Option;
1221 use std::option::Option::{None, Some};
1222 use std::result::Result::{Err, Ok};
1224 use std::clone::Clone;
1225 use std::convert::From;
1229 let x = Rc::new(RefCell::new(5));
1231 *x.borrow_mut() = 20;
1232 assert_eq!(*y.borrow(), 20);
1242 fn test_simple_clone() {
1250 fn test_destructor() {
1251 let x: Rc<Box<_>> = Rc::new(box 5);
1258 let y = Rc::downgrade(&x);
1259 assert!(y.upgrade().is_some());
1265 let y = Rc::downgrade(&x);
1267 assert!(y.upgrade().is_none());
1271 fn weak_self_cyclic() {
1273 x: RefCell<Option<Weak<Cycle>>>,
1276 let a = Rc::new(Cycle { x: RefCell::new(None) });
1277 let b = Rc::downgrade(&a.clone());
1278 *a.x.borrow_mut() = Some(b);
1280 // hopefully we don't double-free (or leak)...
1286 assert!(Rc::is_unique(&x));
1288 assert!(!Rc::is_unique(&x));
1290 assert!(Rc::is_unique(&x));
1291 let w = Rc::downgrade(&x);
1292 assert!(!Rc::is_unique(&x));
1294 assert!(Rc::is_unique(&x));
1298 fn test_strong_count() {
1300 assert!(Rc::strong_count(&a) == 1);
1301 let w = Rc::downgrade(&a);
1302 assert!(Rc::strong_count(&a) == 1);
1303 let b = w.upgrade().expect("upgrade of live rc failed");
1304 assert!(Rc::strong_count(&b) == 2);
1305 assert!(Rc::strong_count(&a) == 2);
1308 assert!(Rc::strong_count(&b) == 1);
1310 assert!(Rc::strong_count(&b) == 2);
1311 assert!(Rc::strong_count(&c) == 2);
1315 fn test_weak_count() {
1317 assert!(Rc::strong_count(&a) == 1);
1318 assert!(Rc::weak_count(&a) == 0);
1319 let w = Rc::downgrade(&a);
1320 assert!(Rc::strong_count(&a) == 1);
1321 assert!(Rc::weak_count(&a) == 1);
1323 assert!(Rc::strong_count(&a) == 1);
1324 assert!(Rc::weak_count(&a) == 0);
1326 assert!(Rc::strong_count(&a) == 2);
1327 assert!(Rc::weak_count(&a) == 0);
1334 assert_eq!(Rc::try_unwrap(x), Ok(3));
1337 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
1339 let _w = Rc::downgrade(&x);
1340 assert_eq!(Rc::try_unwrap(x), Ok(5));
1344 fn into_from_raw() {
1345 let x = Rc::new(box "hello");
1348 let x_ptr = Rc::into_raw(x);
1351 assert_eq!(**x_ptr, "hello");
1353 let x = Rc::from_raw(x_ptr);
1354 assert_eq!(**x, "hello");
1356 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
1362 let mut x = Rc::new(3);
1363 *Rc::get_mut(&mut x).unwrap() = 4;
1366 assert!(Rc::get_mut(&mut x).is_none());
1368 assert!(Rc::get_mut(&mut x).is_some());
1369 let _w = Rc::downgrade(&x);
1370 assert!(Rc::get_mut(&mut x).is_none());
1374 fn test_cowrc_clone_make_unique() {
1375 let mut cow0 = Rc::new(75);
1376 let mut cow1 = cow0.clone();
1377 let mut cow2 = cow1.clone();
1379 assert!(75 == *Rc::make_mut(&mut cow0));
1380 assert!(75 == *Rc::make_mut(&mut cow1));
1381 assert!(75 == *Rc::make_mut(&mut cow2));
1383 *Rc::make_mut(&mut cow0) += 1;
1384 *Rc::make_mut(&mut cow1) += 2;
1385 *Rc::make_mut(&mut cow2) += 3;
1387 assert!(76 == *cow0);
1388 assert!(77 == *cow1);
1389 assert!(78 == *cow2);
1391 // none should point to the same backing memory
1392 assert!(*cow0 != *cow1);
1393 assert!(*cow0 != *cow2);
1394 assert!(*cow1 != *cow2);
1398 fn test_cowrc_clone_unique2() {
1399 let mut cow0 = Rc::new(75);
1400 let cow1 = cow0.clone();
1401 let cow2 = cow1.clone();
1403 assert!(75 == *cow0);
1404 assert!(75 == *cow1);
1405 assert!(75 == *cow2);
1407 *Rc::make_mut(&mut cow0) += 1;
1409 assert!(76 == *cow0);
1410 assert!(75 == *cow1);
1411 assert!(75 == *cow2);
1413 // cow1 and cow2 should share the same contents
1414 // cow0 should have a unique reference
1415 assert!(*cow0 != *cow1);
1416 assert!(*cow0 != *cow2);
1417 assert!(*cow1 == *cow2);
1421 fn test_cowrc_clone_weak() {
1422 let mut cow0 = Rc::new(75);
1423 let cow1_weak = Rc::downgrade(&cow0);
1425 assert!(75 == *cow0);
1426 assert!(75 == *cow1_weak.upgrade().unwrap());
1428 *Rc::make_mut(&mut cow0) += 1;
1430 assert!(76 == *cow0);
1431 assert!(cow1_weak.upgrade().is_none());
1436 let foo = Rc::new(75);
1437 assert_eq!(format!("{:?}", foo), "75");
1442 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
1443 assert_eq!(foo, foo.clone());
1447 fn test_from_owned() {
1449 let foo_rc = Rc::from(foo);
1450 assert!(123 == *foo_rc);
1454 fn test_new_weak() {
1455 let foo: Weak<usize> = Weak::new();
1456 assert!(foo.upgrade().is_none());
1461 let five = Rc::new(5);
1462 let same_five = five.clone();
1463 let other_five = Rc::new(5);
1465 assert!(Rc::ptr_eq(&five, &same_five));
1466 assert!(!Rc::ptr_eq(&five, &other_five));
1470 #[stable(feature = "rust1", since = "1.0.0")]
1471 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
1472 fn borrow(&self) -> &T {
1477 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1478 impl<T: ?Sized> AsRef<T> for Rc<T> {
1479 fn as_ref(&self) -> &T {