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")]
249 use core::cell::Cell;
250 use core::cmp::Ordering;
252 use core::hash::{Hash, Hasher};
253 use core::intrinsics::abort;
255 use core::marker::Unsize;
256 use core::mem::{self, forget, size_of_val, uninitialized};
257 use core::ops::Deref;
258 use core::ops::CoerceUnsized;
259 use core::ptr::{self, Shared};
260 use core::convert::From;
262 use heap::{Heap, Alloc, Layout, box_free};
266 struct RcBox<T: ?Sized> {
272 /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
275 /// See the [module-level documentation](./index.html) for more details.
277 /// The inherent methods of `Rc` are all associated functions, which means
278 /// that you have to call them as e.g. [`Rc::get_mut(&mut value)`][get_mut] instead of
279 /// `value.get_mut()`. This avoids conflicts with methods of the inner
282 /// [get_mut]: #method.get_mut
283 #[stable(feature = "rust1", since = "1.0.0")]
284 pub struct Rc<T: ?Sized> {
285 ptr: Shared<RcBox<T>>,
288 #[stable(feature = "rust1", since = "1.0.0")]
289 impl<T: ?Sized> !marker::Send for Rc<T> {}
290 #[stable(feature = "rust1", since = "1.0.0")]
291 impl<T: ?Sized> !marker::Sync for Rc<T> {}
293 #[unstable(feature = "coerce_unsized", issue = "27732")]
294 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
297 /// Constructs a new `Rc<T>`.
304 /// let five = Rc::new(5);
306 #[stable(feature = "rust1", since = "1.0.0")]
307 pub fn new(value: T) -> Rc<T> {
309 // there is an implicit weak pointer owned by all the strong
310 // pointers, which ensures that the weak destructor never frees
311 // the allocation while the strong destructor is running, even
312 // if the weak pointer is stored inside the strong one.
313 ptr: Shared::from(Box::into_unique(box RcBox {
314 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_unchecked(ptr as *mut u8 as *mut _)
426 impl<T: ?Sized> Rc<T> {
427 /// Creates a new [`Weak`][weak] pointer to this value.
429 /// [weak]: struct.Weak.html
436 /// let five = Rc::new(5);
438 /// let weak_five = Rc::downgrade(&five);
440 #[stable(feature = "rc_weak", since = "1.4.0")]
441 pub fn downgrade(this: &Self) -> Weak<T> {
443 Weak { ptr: this.ptr }
446 /// Gets the number of [`Weak`][weak] pointers to this value.
448 /// [weak]: struct.Weak.html
455 /// let five = Rc::new(5);
456 /// let _weak_five = Rc::downgrade(&five);
458 /// assert_eq!(1, Rc::weak_count(&five));
461 #[stable(feature = "rc_counts", since = "1.15.0")]
462 pub fn weak_count(this: &Self) -> usize {
466 /// Gets the number of strong (`Rc`) pointers to this value.
473 /// let five = Rc::new(5);
474 /// let _also_five = Rc::clone(&five);
476 /// assert_eq!(2, Rc::strong_count(&five));
479 #[stable(feature = "rc_counts", since = "1.15.0")]
480 pub fn strong_count(this: &Self) -> usize {
484 /// Returns true if there are no other `Rc` or [`Weak`][weak] pointers to
485 /// this inner value.
487 /// [weak]: struct.Weak.html
489 fn is_unique(this: &Self) -> bool {
490 Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
493 /// Returns a mutable reference to the inner value, if there are
494 /// no other `Rc` or [`Weak`][weak] pointers to the same value.
496 /// Returns [`None`] otherwise, because it is not safe to
497 /// mutate a shared value.
499 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
500 /// the inner value when it's shared.
502 /// [weak]: struct.Weak.html
503 /// [`None`]: ../../std/option/enum.Option.html#variant.None
504 /// [make_mut]: struct.Rc.html#method.make_mut
505 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
512 /// let mut x = Rc::new(3);
513 /// *Rc::get_mut(&mut x).unwrap() = 4;
514 /// assert_eq!(*x, 4);
516 /// let _y = Rc::clone(&x);
517 /// assert!(Rc::get_mut(&mut x).is_none());
520 #[stable(feature = "rc_unique", since = "1.4.0")]
521 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
522 if Rc::is_unique(this) {
524 Some(&mut this.ptr.as_mut().value)
532 #[stable(feature = "ptr_eq", since = "1.17.0")]
533 /// Returns true if the two `Rc`s point to the same value (not
534 /// just values that compare as equal).
541 /// let five = Rc::new(5);
542 /// let same_five = Rc::clone(&five);
543 /// let other_five = Rc::new(5);
545 /// assert!(Rc::ptr_eq(&five, &same_five));
546 /// assert!(!Rc::ptr_eq(&five, &other_five));
548 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
549 this.ptr.as_ptr() == other.ptr.as_ptr()
553 impl<T: Clone> Rc<T> {
554 /// Makes a mutable reference into the given `Rc`.
556 /// If there are other `Rc` or [`Weak`][weak] pointers to the same value,
557 /// then `make_mut` will invoke [`clone`][clone] on the inner value to
558 /// ensure unique ownership. This is also referred to as clone-on-write.
560 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
562 /// [weak]: struct.Weak.html
563 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
564 /// [get_mut]: struct.Rc.html#method.get_mut
571 /// let mut data = Rc::new(5);
573 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
574 /// let mut other_data = Rc::clone(&data); // Won't clone inner data
575 /// *Rc::make_mut(&mut data) += 1; // Clones inner data
576 /// *Rc::make_mut(&mut data) += 1; // Won't clone anything
577 /// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
579 /// // Now `data` and `other_data` point to different values.
580 /// assert_eq!(*data, 8);
581 /// assert_eq!(*other_data, 12);
584 #[stable(feature = "rc_unique", since = "1.4.0")]
585 pub fn make_mut(this: &mut Self) -> &mut T {
586 if Rc::strong_count(this) != 1 {
587 // Gotta clone the data, there are other Rcs
588 *this = Rc::new((**this).clone())
589 } else if Rc::weak_count(this) != 0 {
590 // Can just steal the data, all that's left is Weaks
592 let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
593 mem::swap(this, &mut swap);
595 // Remove implicit strong-weak ref (no need to craft a fake
596 // Weak here -- we know other Weaks can clean up for us)
601 // This unsafety is ok because we're guaranteed that the pointer
602 // returned is the *only* pointer that will ever be returned to T. Our
603 // reference count is guaranteed to be 1 at this point, and we required
604 // the `Rc<T>` itself to be `mut`, so we're returning the only possible
605 // reference to the inner value.
607 &mut this.ptr.as_mut().value
614 #[unstable(feature = "rc_downcast", issue = "44608")]
615 /// Attempt to downcast the `Rc<Any>` to a concrete type.
620 /// #![feature(rc_downcast)]
621 /// use std::any::Any;
624 /// fn print_if_string(value: Rc<Any>) {
625 /// if let Ok(string) = value.downcast::<String>() {
626 /// println!("String ({}): {}", string.len(), string);
631 /// let my_string = "Hello World".to_string();
632 /// print_if_string(Rc::new(my_string));
633 /// print_if_string(Rc::new(0i8));
636 pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<Any>> {
637 if (*self).is::<T>() {
638 // avoid the pointer arithmetic in from_raw
640 let raw: *const RcBox<Any> = self.ptr.as_ptr();
643 ptr: Shared::new_unchecked(raw as *const RcBox<T> as *mut _),
652 impl<T: ?Sized> Rc<T> {
653 // Allocates an `RcBox<T>` with sufficient space for an unsized value
654 unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
655 // Create a fake RcBox to find allocation size and alignment
656 let fake_ptr = ptr as *mut RcBox<T>;
658 let layout = Layout::for_value(&*fake_ptr);
660 let mem = Heap.alloc(layout)
661 .unwrap_or_else(|e| Heap.oom(e));
663 // Initialize the real RcBox
664 let inner = set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>;
666 ptr::write(&mut (*inner).strong, Cell::new(1));
667 ptr::write(&mut (*inner).weak, Cell::new(1));
672 fn from_box(v: Box<T>) -> Rc<T> {
674 let bptr = Box::into_raw(v);
676 let value_size = size_of_val(&*bptr);
677 let ptr = Self::allocate_for_ptr(bptr);
679 // Copy value as bytes
680 ptr::copy_nonoverlapping(
681 bptr as *const T as *const u8,
682 &mut (*ptr).value as *mut _ as *mut u8,
685 // Free the allocation without dropping its contents
688 Rc { ptr: Shared::new_unchecked(ptr) }
693 // Sets the data pointer of a `?Sized` raw pointer.
695 // For a slice/trait object, this sets the `data` field and leaves the rest
696 // unchanged. For a sized raw pointer, this simply sets the pointer.
697 unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
698 ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
703 // Copy elements from slice into newly allocated Rc<[T]>
705 // Unsafe because the caller must either take ownership or bind `T: Copy`
706 unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
707 let v_ptr = v as *const [T];
708 let ptr = Self::allocate_for_ptr(v_ptr);
710 ptr::copy_nonoverlapping(
712 &mut (*ptr).value as *mut [T] as *mut T,
715 Rc { ptr: Shared::new_unchecked(ptr) }
719 trait RcFromSlice<T> {
720 fn from_slice(slice: &[T]) -> Self;
723 impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
725 default fn from_slice(v: &[T]) -> Self {
726 // Panic guard while cloning T elements.
727 // In the event of a panic, elements that have been written
728 // into the new RcBox will be dropped, then the memory freed.
736 impl<T> Drop for Guard<T> {
738 use core::slice::from_raw_parts_mut;
741 let slice = from_raw_parts_mut(self.elems, self.n_elems);
742 ptr::drop_in_place(slice);
744 Heap.dealloc(self.mem, self.layout.clone());
750 let v_ptr = v as *const [T];
751 let ptr = Self::allocate_for_ptr(v_ptr);
753 let mem = ptr as *mut _ as *mut u8;
754 let layout = Layout::for_value(&*ptr);
756 // Pointer to first element
757 let elems = &mut (*ptr).value as *mut [T] as *mut T;
759 let mut guard = Guard{
766 for (i, item) in v.iter().enumerate() {
767 ptr::write(elems.offset(i as isize), item.clone());
771 // All clear. Forget the guard so it doesn't free the new RcBox.
774 Rc { ptr: Shared::new_unchecked(ptr) }
779 impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
781 fn from_slice(v: &[T]) -> Self {
782 unsafe { Rc::copy_from_slice(v) }
786 #[stable(feature = "rust1", since = "1.0.0")]
787 impl<T: ?Sized> Deref for Rc<T> {
791 fn deref(&self) -> &T {
796 #[stable(feature = "rust1", since = "1.0.0")]
797 unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
800 /// This will decrement the strong reference count. If the strong reference
801 /// count reaches zero then the only other references (if any) are
802 /// [`Weak`][weak], so we `drop` the inner value.
804 /// [weak]: struct.Weak.html
813 /// impl Drop for Foo {
814 /// fn drop(&mut self) {
815 /// println!("dropped!");
819 /// let foo = Rc::new(Foo);
820 /// let foo2 = Rc::clone(&foo);
822 /// drop(foo); // Doesn't print anything
823 /// drop(foo2); // Prints "dropped!"
827 let ptr = self.ptr.as_ptr();
830 if self.strong() == 0 {
831 // destroy the contained object
832 ptr::drop_in_place(self.ptr.as_mut());
834 // remove the implicit "strong weak" pointer now that we've
835 // destroyed the contents.
838 if self.weak() == 0 {
839 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr));
846 #[stable(feature = "rust1", since = "1.0.0")]
847 impl<T: ?Sized> Clone for Rc<T> {
848 /// Makes a clone of the `Rc` pointer.
850 /// This creates another pointer to the same inner value, increasing the
851 /// strong reference count.
858 /// let five = Rc::new(5);
860 /// Rc::clone(&five);
863 fn clone(&self) -> Rc<T> {
869 #[stable(feature = "rust1", since = "1.0.0")]
870 impl<T: Default> Default for Rc<T> {
871 /// Creates a new `Rc<T>`, with the `Default` value for `T`.
878 /// let x: Rc<i32> = Default::default();
879 /// assert_eq!(*x, 0);
882 fn default() -> Rc<T> {
883 Rc::new(Default::default())
887 #[stable(feature = "rust1", since = "1.0.0")]
888 impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
889 /// Equality for two `Rc`s.
891 /// Two `Rc`s are equal if their inner values are equal.
898 /// let five = Rc::new(5);
900 /// assert!(five == Rc::new(5));
903 fn eq(&self, other: &Rc<T>) -> bool {
907 /// Inequality for two `Rc`s.
909 /// Two `Rc`s are unequal if their inner values are unequal.
916 /// let five = Rc::new(5);
918 /// assert!(five != Rc::new(6));
921 fn ne(&self, other: &Rc<T>) -> bool {
926 #[stable(feature = "rust1", since = "1.0.0")]
927 impl<T: ?Sized + Eq> Eq for Rc<T> {}
929 #[stable(feature = "rust1", since = "1.0.0")]
930 impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
931 /// Partial comparison for two `Rc`s.
933 /// The two are compared by calling `partial_cmp()` on their inner values.
939 /// use std::cmp::Ordering;
941 /// let five = Rc::new(5);
943 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
946 fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
947 (**self).partial_cmp(&**other)
950 /// Less-than comparison for two `Rc`s.
952 /// The two are compared by calling `<` on their inner values.
959 /// let five = Rc::new(5);
961 /// assert!(five < Rc::new(6));
964 fn lt(&self, other: &Rc<T>) -> bool {
968 /// 'Less than or equal to' comparison for two `Rc`s.
970 /// The two are compared by calling `<=` on their inner values.
977 /// let five = Rc::new(5);
979 /// assert!(five <= Rc::new(5));
982 fn le(&self, other: &Rc<T>) -> bool {
986 /// Greater-than comparison for two `Rc`s.
988 /// The two are compared by calling `>` on their inner values.
995 /// let five = Rc::new(5);
997 /// assert!(five > Rc::new(4));
1000 fn gt(&self, other: &Rc<T>) -> bool {
1004 /// 'Greater than or equal to' comparison for two `Rc`s.
1006 /// The two are compared by calling `>=` on their inner values.
1011 /// use std::rc::Rc;
1013 /// let five = Rc::new(5);
1015 /// assert!(five >= Rc::new(5));
1018 fn ge(&self, other: &Rc<T>) -> bool {
1023 #[stable(feature = "rust1", since = "1.0.0")]
1024 impl<T: ?Sized + Ord> Ord for Rc<T> {
1025 /// Comparison for two `Rc`s.
1027 /// The two are compared by calling `cmp()` on their inner values.
1032 /// use std::rc::Rc;
1033 /// use std::cmp::Ordering;
1035 /// let five = Rc::new(5);
1037 /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
1040 fn cmp(&self, other: &Rc<T>) -> Ordering {
1041 (**self).cmp(&**other)
1045 #[stable(feature = "rust1", since = "1.0.0")]
1046 impl<T: ?Sized + Hash> Hash for Rc<T> {
1047 fn hash<H: Hasher>(&self, state: &mut H) {
1048 (**self).hash(state);
1052 #[stable(feature = "rust1", since = "1.0.0")]
1053 impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
1054 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1055 fmt::Display::fmt(&**self, f)
1059 #[stable(feature = "rust1", since = "1.0.0")]
1060 impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
1061 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1062 fmt::Debug::fmt(&**self, f)
1066 #[stable(feature = "rust1", since = "1.0.0")]
1067 impl<T: ?Sized> fmt::Pointer for Rc<T> {
1068 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1069 fmt::Pointer::fmt(&self.ptr, f)
1073 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1074 impl<T> From<T> for Rc<T> {
1075 fn from(t: T) -> Self {
1080 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1081 impl<'a, T: Clone> From<&'a [T]> for Rc<[T]> {
1083 fn from(v: &[T]) -> Rc<[T]> {
1084 <Self as RcFromSlice<T>>::from_slice(v)
1088 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1089 impl<'a> From<&'a str> for Rc<str> {
1091 fn from(v: &str) -> Rc<str> {
1092 unsafe { mem::transmute(<Rc<[u8]>>::from(v.as_bytes())) }
1096 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1097 impl From<String> for Rc<str> {
1099 fn from(v: String) -> Rc<str> {
1104 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1105 impl<T: ?Sized> From<Box<T>> for Rc<T> {
1107 fn from(v: Box<T>) -> Rc<T> {
1112 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1113 impl<T> From<Vec<T>> for Rc<[T]> {
1115 fn from(mut v: Vec<T>) -> Rc<[T]> {
1117 let rc = Rc::copy_from_slice(&v);
1119 // Allow the Vec to free its memory, but not destroy its contents
1127 /// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
1128 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
1129 /// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
1131 /// Since a `Weak` reference does not count towards ownership, it will not
1132 /// prevent the inner value from being dropped, and `Weak` itself makes no
1133 /// guarantees about the value still being present and may return [`None`]
1134 /// when [`upgrade`]d.
1136 /// A `Weak` pointer is useful for keeping a temporary reference to the value
1137 /// within [`Rc`] without extending its lifetime. It is also used to prevent
1138 /// circular references between [`Rc`] pointers, since mutual owning references
1139 /// would never allow either [`Rc`] to be dropped. For example, a tree could
1140 /// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
1141 /// pointers from children back to their parents.
1143 /// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
1145 /// [`Rc`]: struct.Rc.html
1146 /// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
1147 /// [`upgrade`]: struct.Weak.html#method.upgrade
1148 /// [`Option`]: ../../std/option/enum.Option.html
1149 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1150 #[stable(feature = "rc_weak", since = "1.4.0")]
1151 pub struct Weak<T: ?Sized> {
1152 ptr: Shared<RcBox<T>>,
1155 #[stable(feature = "rc_weak", since = "1.4.0")]
1156 impl<T: ?Sized> !marker::Send for Weak<T> {}
1157 #[stable(feature = "rc_weak", since = "1.4.0")]
1158 impl<T: ?Sized> !marker::Sync for Weak<T> {}
1160 #[unstable(feature = "coerce_unsized", issue = "27732")]
1161 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
1164 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1165 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1167 /// [`upgrade`]: struct.Weak.html#method.upgrade
1168 /// [`None`]: ../../std/option/enum.Option.html
1173 /// use std::rc::Weak;
1175 /// let empty: Weak<i64> = Weak::new();
1176 /// assert!(empty.upgrade().is_none());
1178 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1179 pub fn new() -> Weak<T> {
1182 ptr: Shared::from(Box::into_unique(box RcBox {
1183 strong: Cell::new(0),
1185 value: uninitialized(),
1192 impl<T: ?Sized> Weak<T> {
1193 /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
1194 /// the lifetime of the value if successful.
1196 /// Returns [`None`] if the value has since been dropped.
1198 /// [`Rc`]: struct.Rc.html
1199 /// [`None`]: ../../std/option/enum.Option.html
1204 /// use std::rc::Rc;
1206 /// let five = Rc::new(5);
1208 /// let weak_five = Rc::downgrade(&five);
1210 /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
1211 /// assert!(strong_five.is_some());
1213 /// // Destroy all strong pointers.
1214 /// drop(strong_five);
1217 /// assert!(weak_five.upgrade().is_none());
1219 #[stable(feature = "rc_weak", since = "1.4.0")]
1220 pub fn upgrade(&self) -> Option<Rc<T>> {
1221 if self.strong() == 0 {
1225 Some(Rc { ptr: self.ptr })
1230 #[stable(feature = "rc_weak", since = "1.4.0")]
1231 impl<T: ?Sized> Drop for Weak<T> {
1232 /// Drops the `Weak` pointer.
1237 /// use std::rc::{Rc, Weak};
1241 /// impl Drop for Foo {
1242 /// fn drop(&mut self) {
1243 /// println!("dropped!");
1247 /// let foo = Rc::new(Foo);
1248 /// let weak_foo = Rc::downgrade(&foo);
1249 /// let other_weak_foo = Weak::clone(&weak_foo);
1251 /// drop(weak_foo); // Doesn't print anything
1252 /// drop(foo); // Prints "dropped!"
1254 /// assert!(other_weak_foo.upgrade().is_none());
1256 fn drop(&mut self) {
1258 let ptr = self.ptr.as_ptr();
1261 // the weak count starts at 1, and will only go to zero if all
1262 // the strong pointers have disappeared.
1263 if self.weak() == 0 {
1264 Heap.dealloc(ptr as *mut u8, Layout::for_value(&*ptr));
1270 #[stable(feature = "rc_weak", since = "1.4.0")]
1271 impl<T: ?Sized> Clone for Weak<T> {
1272 /// Makes a clone of the `Weak` pointer that points to the same value.
1277 /// use std::rc::{Rc, Weak};
1279 /// let weak_five = Rc::downgrade(&Rc::new(5));
1281 /// Weak::clone(&weak_five);
1284 fn clone(&self) -> Weak<T> {
1286 Weak { ptr: self.ptr }
1290 #[stable(feature = "rc_weak", since = "1.4.0")]
1291 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
1292 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1297 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1298 impl<T> Default for Weak<T> {
1299 /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
1300 /// it. Calling [`upgrade`] on the return value always gives [`None`].
1302 /// [`upgrade`]: struct.Weak.html#method.upgrade
1303 /// [`None`]: ../../std/option/enum.Option.html
1308 /// use std::rc::Weak;
1310 /// let empty: Weak<i64> = Default::default();
1311 /// assert!(empty.upgrade().is_none());
1313 fn default() -> Weak<T> {
1318 // NOTE: We checked_add here to deal with mem::forget safety. In particular
1319 // if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
1320 // you can free the allocation while outstanding Rcs (or Weaks) exist.
1321 // We abort because this is such a degenerate scenario that we don't care about
1322 // what happens -- no real program should ever experience this.
1324 // This should have negligible overhead since you don't actually need to
1325 // clone these much in Rust thanks to ownership and move-semantics.
1328 trait RcBoxPtr<T: ?Sized> {
1329 fn inner(&self) -> &RcBox<T>;
1332 fn strong(&self) -> usize {
1333 self.inner().strong.get()
1337 fn inc_strong(&self) {
1338 self.inner().strong.set(self.strong().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1342 fn dec_strong(&self) {
1343 self.inner().strong.set(self.strong() - 1);
1347 fn weak(&self) -> usize {
1348 self.inner().weak.get()
1352 fn inc_weak(&self) {
1353 self.inner().weak.set(self.weak().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
1357 fn dec_weak(&self) {
1358 self.inner().weak.set(self.weak() - 1);
1362 impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
1364 fn inner(&self) -> &RcBox<T> {
1371 impl<T: ?Sized> RcBoxPtr<T> for Weak<T> {
1373 fn inner(&self) -> &RcBox<T> {
1382 use super::{Rc, Weak};
1383 use std::boxed::Box;
1384 use std::cell::RefCell;
1385 use std::option::Option;
1386 use std::option::Option::{None, Some};
1387 use std::result::Result::{Err, Ok};
1389 use std::clone::Clone;
1390 use std::convert::From;
1394 let x = Rc::new(RefCell::new(5));
1396 *x.borrow_mut() = 20;
1397 assert_eq!(*y.borrow(), 20);
1407 fn test_simple_clone() {
1415 fn test_destructor() {
1416 let x: Rc<Box<_>> = Rc::new(box 5);
1423 let y = Rc::downgrade(&x);
1424 assert!(y.upgrade().is_some());
1430 let y = Rc::downgrade(&x);
1432 assert!(y.upgrade().is_none());
1436 fn weak_self_cyclic() {
1438 x: RefCell<Option<Weak<Cycle>>>,
1441 let a = Rc::new(Cycle { x: RefCell::new(None) });
1442 let b = Rc::downgrade(&a.clone());
1443 *a.x.borrow_mut() = Some(b);
1445 // hopefully we don't double-free (or leak)...
1451 assert!(Rc::is_unique(&x));
1453 assert!(!Rc::is_unique(&x));
1455 assert!(Rc::is_unique(&x));
1456 let w = Rc::downgrade(&x);
1457 assert!(!Rc::is_unique(&x));
1459 assert!(Rc::is_unique(&x));
1463 fn test_strong_count() {
1465 assert!(Rc::strong_count(&a) == 1);
1466 let w = Rc::downgrade(&a);
1467 assert!(Rc::strong_count(&a) == 1);
1468 let b = w.upgrade().expect("upgrade of live rc failed");
1469 assert!(Rc::strong_count(&b) == 2);
1470 assert!(Rc::strong_count(&a) == 2);
1473 assert!(Rc::strong_count(&b) == 1);
1475 assert!(Rc::strong_count(&b) == 2);
1476 assert!(Rc::strong_count(&c) == 2);
1480 fn test_weak_count() {
1482 assert!(Rc::strong_count(&a) == 1);
1483 assert!(Rc::weak_count(&a) == 0);
1484 let w = Rc::downgrade(&a);
1485 assert!(Rc::strong_count(&a) == 1);
1486 assert!(Rc::weak_count(&a) == 1);
1488 assert!(Rc::strong_count(&a) == 1);
1489 assert!(Rc::weak_count(&a) == 0);
1491 assert!(Rc::strong_count(&a) == 2);
1492 assert!(Rc::weak_count(&a) == 0);
1499 assert_eq!(Rc::try_unwrap(x), Ok(3));
1502 assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
1504 let _w = Rc::downgrade(&x);
1505 assert_eq!(Rc::try_unwrap(x), Ok(5));
1509 fn into_from_raw() {
1510 let x = Rc::new(box "hello");
1513 let x_ptr = Rc::into_raw(x);
1516 assert_eq!(**x_ptr, "hello");
1518 let x = Rc::from_raw(x_ptr);
1519 assert_eq!(**x, "hello");
1521 assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
1527 let mut x = Rc::new(3);
1528 *Rc::get_mut(&mut x).unwrap() = 4;
1531 assert!(Rc::get_mut(&mut x).is_none());
1533 assert!(Rc::get_mut(&mut x).is_some());
1534 let _w = Rc::downgrade(&x);
1535 assert!(Rc::get_mut(&mut x).is_none());
1539 fn test_cowrc_clone_make_unique() {
1540 let mut cow0 = Rc::new(75);
1541 let mut cow1 = cow0.clone();
1542 let mut cow2 = cow1.clone();
1544 assert!(75 == *Rc::make_mut(&mut cow0));
1545 assert!(75 == *Rc::make_mut(&mut cow1));
1546 assert!(75 == *Rc::make_mut(&mut cow2));
1548 *Rc::make_mut(&mut cow0) += 1;
1549 *Rc::make_mut(&mut cow1) += 2;
1550 *Rc::make_mut(&mut cow2) += 3;
1552 assert!(76 == *cow0);
1553 assert!(77 == *cow1);
1554 assert!(78 == *cow2);
1556 // none should point to the same backing memory
1557 assert!(*cow0 != *cow1);
1558 assert!(*cow0 != *cow2);
1559 assert!(*cow1 != *cow2);
1563 fn test_cowrc_clone_unique2() {
1564 let mut cow0 = Rc::new(75);
1565 let cow1 = cow0.clone();
1566 let cow2 = cow1.clone();
1568 assert!(75 == *cow0);
1569 assert!(75 == *cow1);
1570 assert!(75 == *cow2);
1572 *Rc::make_mut(&mut cow0) += 1;
1574 assert!(76 == *cow0);
1575 assert!(75 == *cow1);
1576 assert!(75 == *cow2);
1578 // cow1 and cow2 should share the same contents
1579 // cow0 should have a unique reference
1580 assert!(*cow0 != *cow1);
1581 assert!(*cow0 != *cow2);
1582 assert!(*cow1 == *cow2);
1586 fn test_cowrc_clone_weak() {
1587 let mut cow0 = Rc::new(75);
1588 let cow1_weak = Rc::downgrade(&cow0);
1590 assert!(75 == *cow0);
1591 assert!(75 == *cow1_weak.upgrade().unwrap());
1593 *Rc::make_mut(&mut cow0) += 1;
1595 assert!(76 == *cow0);
1596 assert!(cow1_weak.upgrade().is_none());
1601 let foo = Rc::new(75);
1602 assert_eq!(format!("{:?}", foo), "75");
1607 let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
1608 assert_eq!(foo, foo.clone());
1612 fn test_from_owned() {
1614 let foo_rc = Rc::from(foo);
1615 assert!(123 == *foo_rc);
1619 fn test_new_weak() {
1620 let foo: Weak<usize> = Weak::new();
1621 assert!(foo.upgrade().is_none());
1626 let five = Rc::new(5);
1627 let same_five = five.clone();
1628 let other_five = Rc::new(5);
1630 assert!(Rc::ptr_eq(&five, &same_five));
1631 assert!(!Rc::ptr_eq(&five, &other_five));
1635 fn test_from_str() {
1636 let r: Rc<str> = Rc::from("foo");
1638 assert_eq!(&r[..], "foo");
1642 fn test_copy_from_slice() {
1643 let s: &[u32] = &[1, 2, 3];
1644 let r: Rc<[u32]> = Rc::from(s);
1646 assert_eq!(&r[..], [1, 2, 3]);
1650 fn test_clone_from_slice() {
1651 #[derive(Clone, Debug, Eq, PartialEq)]
1654 let s: &[X] = &[X(1), X(2), X(3)];
1655 let r: Rc<[X]> = Rc::from(s);
1657 assert_eq!(&r[..], s);
1662 fn test_clone_from_slice_panic() {
1663 use std::string::{String, ToString};
1665 struct Fail(u32, String);
1667 impl Clone for Fail {
1668 fn clone(&self) -> Fail {
1672 Fail(self.0, self.1.clone())
1677 Fail(0, "foo".to_string()),
1678 Fail(1, "bar".to_string()),
1679 Fail(2, "baz".to_string()),
1682 // Should panic, but not cause memory corruption
1683 let _r: Rc<[Fail]> = Rc::from(s);
1687 fn test_from_box() {
1688 let b: Box<u32> = box 123;
1689 let r: Rc<u32> = Rc::from(b);
1691 assert_eq!(*r, 123);
1695 fn test_from_box_str() {
1696 use std::string::String;
1698 let s = String::from("foo").into_boxed_str();
1699 let r: Rc<str> = Rc::from(s);
1701 assert_eq!(&r[..], "foo");
1705 fn test_from_box_slice() {
1706 let s = vec![1, 2, 3].into_boxed_slice();
1707 let r: Rc<[u32]> = Rc::from(s);
1709 assert_eq!(&r[..], [1, 2, 3]);
1713 fn test_from_box_trait() {
1714 use std::fmt::Display;
1715 use std::string::ToString;
1717 let b: Box<Display> = box 123;
1718 let r: Rc<Display> = Rc::from(b);
1720 assert_eq!(r.to_string(), "123");
1724 fn test_from_box_trait_zero_sized() {
1725 use std::fmt::Debug;
1727 let b: Box<Debug> = box ();
1728 let r: Rc<Debug> = Rc::from(b);
1730 assert_eq!(format!("{:?}", r), "()");
1734 fn test_from_vec() {
1735 let v = vec![1, 2, 3];
1736 let r: Rc<[u32]> = Rc::from(v);
1738 assert_eq!(&r[..], [1, 2, 3]);
1742 fn test_downcast() {
1745 let r1: Rc<Any> = Rc::new(i32::max_value());
1746 let r2: Rc<Any> = Rc::new("abc");
1748 assert!(r1.clone().downcast::<u32>().is_err());
1750 let r1i32 = r1.downcast::<i32>();
1751 assert!(r1i32.is_ok());
1752 assert_eq!(r1i32.unwrap(), Rc::new(i32::max_value()));
1754 assert!(r2.clone().downcast::<i32>().is_err());
1756 let r2str = r2.downcast::<&'static str>();
1757 assert!(r2str.is_ok());
1758 assert_eq!(r2str.unwrap(), Rc::new("abc"));
1762 #[stable(feature = "rust1", since = "1.0.0")]
1763 impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
1764 fn borrow(&self) -> &T {
1769 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1770 impl<T: ?Sized> AsRef<T> for Rc<T> {
1771 fn as_ref(&self) -> &T {