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_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};
265 struct RcBox<T: ?Sized> {
271 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
274 /// See the [module-level documentation](./index.html) for more details.
276 /// The inherent methods of `Rc` are all associated functions, which means
277 /// that you have to call them as e.g. [`Rc::get_mut(&mut value)`][get_mut] instead of
278 /// `value.get_mut()`. This avoids conflicts with methods of the inner
281 /// [get_mut]: #method.get_mut
282 #[stable(feature = "rust1", since = "1.0.0")]
283 pub struct Rc<T: ?Sized> {
284 ptr: Shared<RcBox<T>>,
287 #[stable(feature = "rust1", since = "1.0.0")]
288 impl<T: ?Sized> !marker::Send for Rc<T> {}
289 #[stable(feature = "rust1", since = "1.0.0")]
290 impl<T: ?Sized> !marker::Sync for Rc<T> {}
292 #[unstable(feature = "coerce_unsized", issue = "27732")]
293 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
296 /// Constructs a new `Rc<T>`.
303 /// let five = Rc::new(5);
305 #[stable(feature = "rust1", since = "1.0.0")]
306 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::from(Box::into_unique(box RcBox {
313 strong: Cell::new(1),
320 /// Returns the contained value, if the `Rc` has exactly one strong reference.
322 /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
325 /// This will succeed even if there are outstanding weak references.
327 /// [result]: ../../std/result/enum.Result.html
334 /// let x = Rc::new(3);
335 /// assert_eq!(Rc::try_unwrap(x), Ok(3));
337 /// let x = Rc::new(4);
338 /// let _y = Rc::clone(&x);
339 /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
342 #[stable(feature = "rc_unique", since = "1.4.0")]
343 pub fn try_unwrap(this: Self) -> Result<T, Self> {
344 if Rc::strong_count(&this) == 1 {
346 let val = ptr::read(&*this); // copy the contained object
348 // Indicate to Weaks that they can't be promoted by decrememting
349 // the strong count, and then remove the implicit "strong weak"
350 // pointer while also handling drop logic by just crafting a
353 let _weak = Weak { ptr: this.ptr };
362 /// Consumes the `Rc`, returning the wrapped pointer.
364 /// To avoid a memory leak the pointer must be converted back to an `Rc` using
365 /// [`Rc::from_raw`][from_raw].
367 /// [from_raw]: struct.Rc.html#method.from_raw
374 /// let x = Rc::new(10);
375 /// let x_ptr = Rc::into_raw(x);
376 /// assert_eq!(unsafe { *x_ptr }, 10);
378 #[stable(feature = "rc_raw", since = "1.17.0")]
379 pub fn into_raw(this: Self) -> *const T {
380 let ptr: *const T = &*this;
385 /// Constructs an `Rc` from a raw pointer.
387 /// The raw pointer must have been previously returned by a call to a
388 /// [`Rc::into_raw`][into_raw].
390 /// This function is unsafe because improper use may lead to memory problems. For example, a
391 /// double-free may occur if the function is called twice on the same raw pointer.
393 /// [into_raw]: struct.Rc.html#method.into_raw
400 /// let x = Rc::new(10);
401 /// let x_ptr = Rc::into_raw(x);
404 /// // Convert back to an `Rc` to prevent leak.
405 /// let x = Rc::from_raw(x_ptr);
406 /// assert_eq!(*x, 10);
408 /// // Further calls to `Rc::from_raw(x_ptr)` would be memory unsafe.
411 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
413 #[stable(feature = "rc_raw", since = "1.17.0")]
414 pub unsafe fn from_raw(ptr: *const T) -> Self {
415 // To find the corresponding pointer to the `RcBox` we need to subtract the offset of the
416 // `value` field from the pointer.
418 let ptr = (ptr as *const u8).offset(-offset_of!(RcBox<T>, value));
420 ptr: Shared::new_unchecked(ptr as *mut u8 as *mut _)
425 impl<T: ?Sized> Rc<T> {
426 /// Creates a new [`Weak`][weak] pointer to this value.
428 /// [weak]: struct.Weak.html
435 /// let five = Rc::new(5);
437 /// let weak_five = Rc::downgrade(&five);
439 #[stable(feature = "rc_weak", since = "1.4.0")]
440 pub fn downgrade(this: &Self) -> Weak<T> {
442 Weak { ptr: this.ptr }
445 /// Gets the number of [`Weak`][weak] pointers to this value.
447 /// [weak]: struct.Weak.html
454 /// let five = Rc::new(5);
455 /// let _weak_five = Rc::downgrade(&five);
457 /// assert_eq!(1, Rc::weak_count(&five));
460 #[stable(feature = "rc_counts", since = "1.15.0")]
461 pub fn weak_count(this: &Self) -> usize {
465 /// Gets the number of strong (`Rc`) pointers to this value.
472 /// let five = Rc::new(5);
473 /// let _also_five = Rc::clone(&five);
475 /// assert_eq!(2, Rc::strong_count(&five));
478 #[stable(feature = "rc_counts", since = "1.15.0")]
479 pub fn strong_count(this: &Self) -> usize {
483 /// Returns true if there are no other `Rc` or [`Weak`][weak] pointers to
484 /// this inner value.
486 /// [weak]: struct.Weak.html
488 fn is_unique(this: &Self) -> bool {
489 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
492 /// Returns a mutable reference to the inner value, if there are
493 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
495 /// Returns [`None`] otherwise, because it is not safe to
496 /// mutate a shared value.
498 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
499 /// the inner value when it's shared.
501 /// [weak]: struct.Weak.html
502 /// [`None`]: ../../std/option/enum.Option.html#variant.None
503 /// [make_mut]: struct.Rc.html#method.make_mut
504 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
511 /// let mut x = Rc::new(3);
512 /// *Rc::get_mut(&mut x).unwrap() = 4;
513 /// assert_eq!(*x, 4);
515 /// let _y = Rc::clone(&x);
516 /// assert!(Rc::get_mut(&mut x).is_none());
519 #[stable(feature = "rc_unique", since = "1.4.0")]
520 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
521 if Rc::is_unique(this) {
523 Some(&mut this.ptr.as_mut().value)
531 #[stable(feature = "ptr_eq", since = "1.17.0")]
532 /// Returns true if the two `Rc`s point to the same value (not
533 /// just values that compare as equal).
540 /// let five = Rc::new(5);
541 /// let same_five = Rc::clone(&five);
542 /// let other_five = Rc::new(5);
544 /// assert!(Rc::ptr_eq(&five, &same_five));
545 /// assert!(!Rc::ptr_eq(&five, &other_five));
547 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
548 this.ptr.as_ptr() == other.ptr.as_ptr()
552 impl<T: Clone> Rc<T> {
553 /// Makes a mutable reference into the given `Rc`.
555 /// If there are other `Rc` or [`Weak`][weak] pointers to the same value,
556 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
557 /// ensure unique ownership. This is also referred to as clone-on-write.
559 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
561 /// [weak]: struct.Weak.html
562 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
563 /// [get_mut]: struct.Rc.html#method.get_mut
570 /// let mut data = Rc::new(5);
572 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
573 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
574 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
575 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
576 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
578 /// // Now `data` and `other_data` point to different values.
579 /// assert_eq!(*data, 8);
580 /// assert_eq!(*other_data, 12);
583 #[stable(feature = "rc_unique", since = "1.4.0")]
584 pub fn make_mut(this: &mut Self) -> &mut T {
585 if Rc::strong_count(this) != 1 {
586 // Gotta clone the data, there are other Rcs
587 *this = Rc::new((**this).clone())
588 } else if Rc::weak_count(this) != 0 {
589 // Can just steal the data, all that's left is Weaks
591 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
592 mem::swap(this, &mut swap);
594 // Remove implicit strong-weak ref (no need to craft a fake
595 // Weak here -- we know other Weaks can clean up for us)
600 // This unsafety is ok because we're guaranteed that the pointer
601 // returned is the *only* pointer that will ever be returned to T. Our
602 // reference count is guaranteed to be 1 at this point, and we required
603 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
604 // reference to the inner value.
606 &mut this.ptr.as_mut().value
611 impl<T: ?Sized> Rc<T> {
612 // Allocates an `RcBox<T>` with sufficient space for an unsized value
613 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
614 // Create a fake RcBox to find allocation size and alignment
615 let fake_ptr = ptr as *mut RcBox<T>;
617 let layout = Layout::for_value(&*fake_ptr);
619 let mem = Heap.alloc(layout)
620 .unwrap_or_else(|e| Heap.oom(e));
622 // Initialize the real RcBox
623 let inner = set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>;
625 ptr::write(&mut (*inner).strong, Cell::new(1));
626 ptr::write(&mut (*inner).weak, Cell::new(1));
631 fn from_box(v: Box<T>) -> Rc<T> {
633 let bptr = Box::into_raw(v);
635 let value_size = size_of_val(&*bptr);
636 let ptr = Self::allocate_for_ptr(bptr);
638 // Copy value as bytes
639 ptr::copy_nonoverlapping(
640 bptr as *const T as *const u8,
641 &mut (*ptr).value as *mut _ as *mut u8,
644 // Free the allocation without dropping its contents
647 Rc { ptr: Shared::new_unchecked(ptr) }
652 // Sets the data pointer of a `?Sized` raw pointer.
654 // For a slice/trait object, this sets the `data` field and leaves the rest
655 // unchanged. For a sized raw pointer, this simply sets the pointer.
656 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
657 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
662 // Copy elements from slice into newly allocated Rc<[T]>
664 // Unsafe because the caller must either take ownership or bind `T: Copy`
665 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
666 let v_ptr = v as *const [T];
667 let ptr = Self::allocate_for_ptr(v_ptr);
669 ptr::copy_nonoverlapping(
671 &mut (*ptr).value as *mut [T] as *mut T,
674 Rc { ptr: Shared::new_unchecked(ptr) }
678 trait RcFromSlice<T> {
679 fn from_slice(slice: &[T]) -> Self;
682 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
684 default fn from_slice(v: &[T]) -> Self {
685 // Panic guard while cloning T elements.
686 // In the event of a panic, elements that have been written
687 // into the new RcBox will be dropped, then the memory freed.
695 impl<T> Drop for Guard<T> {
697 use core::slice::from_raw_parts_mut;
700 let slice = from_raw_parts_mut(self.elems, self.n_elems);
701 ptr::drop_in_place(slice);
703 Heap.dealloc(self.mem, self.layout.clone());
709 let v_ptr = v as *const [T];
710 let ptr = Self::allocate_for_ptr(v_ptr);
712 let mem = ptr as *mut _ as *mut u8;
713 let layout = Layout::for_value(&*ptr);
715 // Pointer to first element
716 let elems = &mut (*ptr).value as *mut [T] as *mut T;
718 let mut guard = Guard{
725 for (i, item) in v.iter().enumerate() {
726 ptr::write(elems.offset(i as isize), item.clone());
730 // All clear. Forget the guard so it doesn't free the new RcBox.
733 Rc { ptr: Shared::new_unchecked(ptr) }
738 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
740 fn from_slice(v: &[T]) -> Self {
741 unsafe { Rc::copy_from_slice(v) }
745 #[stable(feature = "rust1", since = "1.0.0")]
746 impl<T: ?Sized> Deref for Rc<T> {
750 fn deref(&self) -> &T {
755 #[stable(feature = "rust1", since = "1.0.0")]
756 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
759 /// This will decrement the strong reference count. If the strong reference
760 /// count reaches zero then the only other references (if any) are
761 /// [`Weak`][weak], so we `drop` the inner value.
763 /// [weak]: struct.Weak.html
772 /// impl Drop for Foo {
773 /// fn drop(&mut self) {
774 /// println!("dropped!");
778 /// let foo = Rc::new(Foo);
779 /// let foo2 = Rc::clone(&foo);
781 /// drop(foo); // Doesn't print anything
782 /// drop(foo2); // Prints "dropped!"
786 let ptr = self.ptr.as_ptr();
789 if self.strong() == 0 {
790 // destroy the contained object
791 ptr::drop_in_place(self.ptr.as_mut());
793 // remove the implicit "strong weak" pointer now that we've
794 // destroyed the contents.
797 if self.weak() == 0 {
798 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr));
805 #[stable(feature = "rust1", since = "1.0.0")]
806 impl<T: ?Sized> Clone for Rc<T> {
807 /// Makes a clone of the `Rc` pointer.
809 /// This creates another pointer to the same inner value, increasing the
810 /// strong reference count.
817 /// let five = Rc::new(5);
819 /// Rc::clone(&five);
822 fn clone(&self) -> Rc<T> {
828 #[stable(feature = "rust1", since = "1.0.0")]
829 impl<T: Default> Default for Rc<T> {
830 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
837 /// let x: Rc<i32> = Default::default();
838 /// assert_eq!(*x, 0);
841 fn default() -> Rc<T> {
842 Rc::new(Default::default())
846 #[stable(feature = "rust1", since = "1.0.0")]
847 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
848 /// Equality for two `Rc`s.
850 /// Two `Rc`s are equal if their inner values are equal.
857 /// let five = Rc::new(5);
859 /// assert!(five == Rc::new(5));
862 fn eq(&self, other: &Rc<T>) -> bool {
866 /// Inequality for two `Rc`s.
868 /// Two `Rc`s are unequal if their inner values are unequal.
875 /// let five = Rc::new(5);
877 /// assert!(five != Rc::new(6));
880 fn ne(&self, other: &Rc<T>) -> bool {
885 #[stable(feature = "rust1", since = "1.0.0")]
886 impl<T: ?Sized + Eq> Eq for Rc<T> {}
888 #[stable(feature = "rust1", since = "1.0.0")]
889 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
890 /// Partial comparison for two `Rc`s.
892 /// The two are compared by calling `partial_cmp()` on their inner values.
898 /// use std::cmp::Ordering;
900 /// let five = Rc::new(5);
902 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
905 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
906 (**self).partial_cmp(&**other)
909 /// Less-than comparison for two `Rc`s.
911 /// The two are compared by calling `<` on their inner values.
918 /// let five = Rc::new(5);
920 /// assert!(five < Rc::new(6));
923 fn lt(&self, other: &Rc<T>) -> bool {
927 /// 'Less than or equal to' comparison for two `Rc`s.
929 /// The two are compared by calling `<=` on their inner values.
936 /// let five = Rc::new(5);
938 /// assert!(five <= Rc::new(5));
941 fn le(&self, other: &Rc<T>) -> bool {
945 /// Greater-than comparison for two `Rc`s.
947 /// The two are compared by calling `>` on their inner values.
954 /// let five = Rc::new(5);
956 /// assert!(five > Rc::new(4));
959 fn gt(&self, other: &Rc<T>) -> bool {
963 /// 'Greater than or equal to' comparison for two `Rc`s.
965 /// The two are compared by calling `>=` on their inner values.
972 /// let five = Rc::new(5);
974 /// assert!(five >= Rc::new(5));
977 fn ge(&self, other: &Rc<T>) -> bool {
982 #[stable(feature = "rust1", since = "1.0.0")]
983 impl<T: ?Sized + Ord> Ord for Rc<T> {
984 /// Comparison for two `Rc`s.
986 /// The two are compared by calling `cmp()` on their inner values.
992 /// use std::cmp::Ordering;
994 /// let five = Rc::new(5);
996 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
999 fn cmp(&self, other: &Rc<T>) -> Ordering {
1000 (**self).cmp(&**other)
1004 #[stable(feature = "rust1", since = "1.0.0")]
1005 impl<T: ?Sized + Hash> Hash for Rc<T> {
1006 fn hash<H: Hasher>(&self, state: &mut H) {
1007 (**self).hash(state);
1011 #[stable(feature = "rust1", since = "1.0.0")]
1012 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1013 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1014 fmt::Display::fmt(&**self, f)
1018 #[stable(feature = "rust1", since = "1.0.0")]
1019 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1020 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1021 fmt::Debug::fmt(&**self, f)
1025 #[stable(feature = "rust1", since = "1.0.0")]
1026 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1027 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1028 fmt::Pointer::fmt(&self.ptr, f)
1032 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1033 impl<T> From<T> for Rc<T> {
1034 fn from(t: T) -> Self {
1039 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1040 impl<'a, T: Clone> From<&'a [T]> for Rc<[T]> {
1042 fn from(v: &[T]) -> Rc<[T]> {
1043 <Self as RcFromSlice<T>>::from_slice(v)
1047 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1048 impl<'a> From<&'a str> for Rc<str> {
1050 fn from(v: &str) -> Rc<str> {
1051 unsafe { mem::transmute(<Rc<[u8]>>::from(v.as_bytes())) }
1055 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1056 impl From<String> for Rc<str> {
1058 fn from(v: String) -> Rc<str> {
1063 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1064 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1066 fn from(v: Box<T>) -> Rc<T> {
1071 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1072 impl<T> From<Vec<T>> for Rc<[T]> {
1074 fn from(mut v: Vec<T>) -> Rc<[T]> {
1076 let rc = Rc::copy_from_slice(&v);
1078 // Allow the Vec to free its memory, but not destroy its contents
1086 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1087 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
1088 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1090 /// Since a `Weak` reference does not count towards ownership, it will not
1091 /// prevent the inner value from being dropped, and `Weak` itself makes no
1092 /// guarantees about the value still being present and may return [`None`]
1093 /// when [`upgrade`]d.
1095 /// A `Weak` pointer is useful for keeping a temporary reference to the value
1096 /// within [`Rc`] without extending its lifetime. It is also used to prevent
1097 /// circular references between [`Rc`] pointers, since mutual owning references
1098 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1099 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1100 /// pointers from children back to their parents.
1102 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1104 /// [`Rc`]: struct.Rc.html
1105 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1106 /// [`upgrade`]: struct.Weak.html#method.upgrade
1107 /// [`Option`]: ../../std/option/enum.Option.html
1108 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1109 #[stable(feature = "rc_weak", since = "1.4.0")]
1110 pub struct Weak<T: ?Sized> {
1111 ptr: Shared<RcBox<T>>,
1114 #[stable(feature = "rc_weak", since = "1.4.0")]
1115 impl<T: ?Sized> !marker::Send for Weak<T> {}
1116 #[stable(feature = "rc_weak", since = "1.4.0")]
1117 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1119 #[unstable(feature = "coerce_unsized", issue = "27732")]
1120 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1123 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1124 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1126 /// [`upgrade`]: struct.Weak.html#method.upgrade
1127 /// [`None`]: ../../std/option/enum.Option.html
1132 /// use std::rc::Weak;
1134 /// let empty: Weak<i64> = Weak::new();
1135 /// assert!(empty.upgrade().is_none());
1137 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1138 pub fn new() -> Weak<T> {
1141 ptr: Shared::from(Box::into_unique(box RcBox {
1142 strong: Cell::new(0),
1144 value: uninitialized(),
1151 impl<T: ?Sized> Weak<T> {
1152 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1153 /// the lifetime of the value if successful.
1155 /// Returns [`None`] if the value has since been dropped.
1157 /// [`Rc`]: struct.Rc.html
1158 /// [`None`]: ../../std/option/enum.Option.html
1163 /// use std::rc::Rc;
1165 /// let five = Rc::new(5);
1167 /// let weak_five = Rc::downgrade(&five);
1169 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1170 /// assert!(strong_five.is_some());
1172 /// // Destroy all strong pointers.
1173 /// drop(strong_five);
1176 /// assert!(weak_five.upgrade().is_none());
1178 #[stable(feature = "rc_weak", since = "1.4.0")]
1179 pub fn upgrade(&self) -> Option<Rc<T>> {
1180 if self.strong() == 0 {
1184 Some(Rc { ptr: self.ptr })
1189 #[stable(feature = "rc_weak", since = "1.4.0")]
1190 impl<T: ?Sized> Drop for Weak<T> {
1191 /// Drops the `Weak` pointer.
1196 /// use std::rc::{Rc, Weak};
1200 /// impl Drop for Foo {
1201 /// fn drop(&mut self) {
1202 /// println!("dropped!");
1206 /// let foo = Rc::new(Foo);
1207 /// let weak_foo = Rc::downgrade(&foo);
1208 /// let other_weak_foo = Weak::clone(&weak_foo);
1210 /// drop(weak_foo); // Doesn't print anything
1211 /// drop(foo); // Prints "dropped!"
1213 /// assert!(other_weak_foo.upgrade().is_none());
1215 fn drop(&mut self) {
1217 let ptr = self.ptr.as_ptr();
1220 // the weak count starts at 1, and will only go to zero if all
1221 // the strong pointers have disappeared.
1222 if self.weak() == 0 {
1223 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr));
1229 #[stable(feature = "rc_weak", since = "1.4.0")]
1230 impl<T: ?Sized> Clone for Weak<T> {
1231 /// Makes a clone of the `Weak` pointer that points to the same value.
1236 /// use std::rc::{Rc, Weak};
1238 /// let weak_five = Rc::downgrade(&Rc::new(5));
1240 /// Weak::clone(&weak_five);
1243 fn clone(&self) -> Weak<T> {
1245 Weak { ptr: self.ptr }
1249 #[stable(feature = "rc_weak", since = "1.4.0")]
1250 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1251 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1256 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1257 impl<T> Default for Weak<T> {
1258 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1259 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1261 /// [`upgrade`]: struct.Weak.html#method.upgrade
1262 /// [`None`]: ../../std/option/enum.Option.html
1267 /// use std::rc::Weak;
1269 /// let empty: Weak<i64> = Default::default();
1270 /// assert!(empty.upgrade().is_none());
1272 fn default() -> Weak<T> {
1277 // NOTE: We checked_add here to deal with mem::forget safety. In particular
1278 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1279 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1280 // We abort because this is such a degenerate scenario that we don't care about
1281 // what happens -- no real program should ever experience this.
1283 // This should have negligible overhead since you don't actually need to
1284 // clone these much in Rust thanks to ownership and move-semantics.
1287 trait RcBoxPtr<T: ?Sized> {
1288 fn inner(&self) -> &RcBox<T>;
1291 fn strong(&self) -> usize {
1292 self.inner().strong.get()
1296 fn inc_strong(&self) {
1297 self.inner().strong.set(self.strong().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1301 fn dec_strong(&self) {
1302 self.inner().strong.set(self.strong() - 1);
1306 fn weak(&self) -> usize {
1307 self.inner().weak.get()
1311 fn inc_weak(&self) {
1312 self.inner().weak.set(self.weak().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1316 fn dec_weak(&self) {
1317 self.inner().weak.set(self.weak() - 1);
1321 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1323 fn inner(&self) -> &RcBox<T> {
1330 impl<T: ?Sized> RcBoxPtr<T> for Weak<T> {
1332 fn inner(&self) -> &RcBox<T> {
1341 use super::{Rc, Weak};
1342 use std::boxed::Box;
1343 use std::cell::RefCell;
1344 use std::option::Option;
1345 use std::option::Option::{None, Some};
1346 use std::result::Result::{Err, Ok};
1348 use std::clone::Clone;
1349 use std::convert::From;
1353 let x = Rc::new(RefCell::new(5));
1355 *x.borrow_mut() = 20;
1356 assert_eq!(*y.borrow(), 20);
1366 fn test_simple_clone() {
1374 fn test_destructor() {
1375 let x: Rc<Box<_>> = Rc::new(box 5);
1382 let y = Rc::downgrade(&x);
1383 assert!(y.upgrade().is_some());
1389 let y = Rc::downgrade(&x);
1391 assert!(y.upgrade().is_none());
1395 fn weak_self_cyclic() {
1397 x: RefCell<Option<Weak<Cycle>>>,
1400 let a = Rc::new(Cycle { x: RefCell::new(None) });
1401 let b = Rc::downgrade(&a.clone());
1402 *a.x.borrow_mut() = Some(b);
1404 // hopefully we don't double-free (or leak)...
1410 assert!(Rc::is_unique(&x));
1412 assert!(!Rc::is_unique(&x));
1414 assert!(Rc::is_unique(&x));
1415 let w = Rc::downgrade(&x);
1416 assert!(!Rc::is_unique(&x));
1418 assert!(Rc::is_unique(&x));
1422 fn test_strong_count() {
1424 assert!(Rc::strong_count(&a) == 1);
1425 let w = Rc::downgrade(&a);
1426 assert!(Rc::strong_count(&a) == 1);
1427 let b = w.upgrade().expect("upgrade of live rc failed");
1428 assert!(Rc::strong_count(&b) == 2);
1429 assert!(Rc::strong_count(&a) == 2);
1432 assert!(Rc::strong_count(&b) == 1);
1434 assert!(Rc::strong_count(&b) == 2);
1435 assert!(Rc::strong_count(&c) == 2);
1439 fn test_weak_count() {
1441 assert!(Rc::strong_count(&a) == 1);
1442 assert!(Rc::weak_count(&a) == 0);
1443 let w = Rc::downgrade(&a);
1444 assert!(Rc::strong_count(&a) == 1);
1445 assert!(Rc::weak_count(&a) == 1);
1447 assert!(Rc::strong_count(&a) == 1);
1448 assert!(Rc::weak_count(&a) == 0);
1450 assert!(Rc::strong_count(&a) == 2);
1451 assert!(Rc::weak_count(&a) == 0);
1458 assert_eq!(Rc::try_unwrap(x), Ok(3));
1461 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
1463 let _w = Rc::downgrade(&x);
1464 assert_eq!(Rc::try_unwrap(x), Ok(5));
1468 fn into_from_raw() {
1469 let x = Rc::new(box "hello");
1472 let x_ptr = Rc::into_raw(x);
1475 assert_eq!(**x_ptr, "hello");
1477 let x = Rc::from_raw(x_ptr);
1478 assert_eq!(**x, "hello");
1480 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
1486 let mut x = Rc::new(3);
1487 *Rc::get_mut(&mut x).unwrap() = 4;
1490 assert!(Rc::get_mut(&mut x).is_none());
1492 assert!(Rc::get_mut(&mut x).is_some());
1493 let _w = Rc::downgrade(&x);
1494 assert!(Rc::get_mut(&mut x).is_none());
1498 fn test_cowrc_clone_make_unique() {
1499 let mut cow0 = Rc::new(75);
1500 let mut cow1 = cow0.clone();
1501 let mut cow2 = cow1.clone();
1503 assert!(75 == *Rc::make_mut(&mut cow0));
1504 assert!(75 == *Rc::make_mut(&mut cow1));
1505 assert!(75 == *Rc::make_mut(&mut cow2));
1507 *Rc::make_mut(&mut cow0) += 1;
1508 *Rc::make_mut(&mut cow1) += 2;
1509 *Rc::make_mut(&mut cow2) += 3;
1511 assert!(76 == *cow0);
1512 assert!(77 == *cow1);
1513 assert!(78 == *cow2);
1515 // none should point to the same backing memory
1516 assert!(*cow0 != *cow1);
1517 assert!(*cow0 != *cow2);
1518 assert!(*cow1 != *cow2);
1522 fn test_cowrc_clone_unique2() {
1523 let mut cow0 = Rc::new(75);
1524 let cow1 = cow0.clone();
1525 let cow2 = cow1.clone();
1527 assert!(75 == *cow0);
1528 assert!(75 == *cow1);
1529 assert!(75 == *cow2);
1531 *Rc::make_mut(&mut cow0) += 1;
1533 assert!(76 == *cow0);
1534 assert!(75 == *cow1);
1535 assert!(75 == *cow2);
1537 // cow1 and cow2 should share the same contents
1538 // cow0 should have a unique reference
1539 assert!(*cow0 != *cow1);
1540 assert!(*cow0 != *cow2);
1541 assert!(*cow1 == *cow2);
1545 fn test_cowrc_clone_weak() {
1546 let mut cow0 = Rc::new(75);
1547 let cow1_weak = Rc::downgrade(&cow0);
1549 assert!(75 == *cow0);
1550 assert!(75 == *cow1_weak.upgrade().unwrap());
1552 *Rc::make_mut(&mut cow0) += 1;
1554 assert!(76 == *cow0);
1555 assert!(cow1_weak.upgrade().is_none());
1560 let foo = Rc::new(75);
1561 assert_eq!(format!("{:?}", foo), "75");
1566 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
1567 assert_eq!(foo, foo.clone());
1571 fn test_from_owned() {
1573 let foo_rc = Rc::from(foo);
1574 assert!(123 == *foo_rc);
1578 fn test_new_weak() {
1579 let foo: Weak<usize> = Weak::new();
1580 assert!(foo.upgrade().is_none());
1585 let five = Rc::new(5);
1586 let same_five = five.clone();
1587 let other_five = Rc::new(5);
1589 assert!(Rc::ptr_eq(&five, &same_five));
1590 assert!(!Rc::ptr_eq(&five, &other_five));
1594 fn test_from_str() {
1595 let r: Rc<str> = Rc::from("foo");
1597 assert_eq!(&r[..], "foo");
1601 fn test_copy_from_slice() {
1602 let s: &[u32] = &[1, 2, 3];
1603 let r: Rc<[u32]> = Rc::from(s);
1605 assert_eq!(&r[..], [1, 2, 3]);
1609 fn test_clone_from_slice() {
1610 #[derive(Clone, Debug, Eq, PartialEq)]
1613 let s: &[X] = &[X(1), X(2), X(3)];
1614 let r: Rc<[X]> = Rc::from(s);
1616 assert_eq!(&r[..], s);
1621 fn test_clone_from_slice_panic() {
1622 use std::string::{String, ToString};
1624 struct Fail(u32, String);
1626 impl Clone for Fail {
1627 fn clone(&self) -> Fail {
1631 Fail(self.0, self.1.clone())
1636 Fail(0, "foo".to_string()),
1637 Fail(1, "bar".to_string()),
1638 Fail(2, "baz".to_string()),
1641 // Should panic, but not cause memory corruption
1642 let _r: Rc<[Fail]> = Rc::from(s);
1646 fn test_from_box() {
1647 let b: Box<u32> = box 123;
1648 let r: Rc<u32> = Rc::from(b);
1650 assert_eq!(*r, 123);
1654 fn test_from_box_str() {
1655 use std::string::String;
1657 let s = String::from("foo").into_boxed_str();
1658 let r: Rc<str> = Rc::from(s);
1660 assert_eq!(&r[..], "foo");
1664 fn test_from_box_slice() {
1665 let s = vec![1, 2, 3].into_boxed_slice();
1666 let r: Rc<[u32]> = Rc::from(s);
1668 assert_eq!(&r[..], [1, 2, 3]);
1672 fn test_from_box_trait() {
1673 use std::fmt::Display;
1674 use std::string::ToString;
1676 let b: Box<Display> = box 123;
1677 let r: Rc<Display> = Rc::from(b);
1679 assert_eq!(r.to_string(), "123");
1683 fn test_from_box_trait_zero_sized() {
1684 use std::fmt::Debug;
1686 let b: Box<Debug> = box ();
1687 let r: Rc<Debug> = Rc::from(b);
1689 assert_eq!(format!("{:?}", r), "()");
1693 fn test_from_vec() {
1694 let v = vec![1, 2, 3];
1695 let r: Rc<[u32]> = Rc::from(v);
1697 assert_eq!(&r[..], [1, 2, 3]);
1701 #[stable(feature = "rust1", since = "1.0.0")]
1702 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
1703 fn borrow(&self) -> &T {
1708 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1709 impl<T: ?Sized> AsRef<T> for Rc<T> {
1710 fn as_ref(&self) -> &T {