Struct alloc::rc::Rc

pub struct Rc<T: ?Sized> { /* fields omitted */ }

A single-threaded reference-counting pointer. ‘Rc’ stands for ‘Reference Counted’.

See the module-level documentation for more details.

The inherent methods of Rc are all associated functions, which means that you have to call them as e.g., Rc::get_mut(&mut value) instead of value.get_mut(). This avoids conflicts with methods of the inner type T.

Implementations

impl<T> Rc<T>

pub fn new(value: T) -> Rc<T>

Constructs a new Rc<T>.

Examples

use std::rc::Rc;

let five = Rc::new(5);

pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Rc<T>

🔬 This is a nightly-only experimental API. (arc_new_cyclic #75861)

Constructs a new Rc<T> using a weak reference to itself. Attempting to upgrade the weak reference before this function returns will result in a None value. However, the weak reference may be cloned freely and stored for use at a later time.

Examples

#![feature(arc_new_cyclic)]
#![allow(dead_code)]
use std::rc::{Rc, Weak};

struct Gadget {
    self_weak: Weak<Self>,
    // ... more fields
}
impl Gadget {
    pub fn new() -> Rc<Self> {
        Rc::new_cyclic(|self_weak| {
            Gadget { self_weak: self_weak.clone(), /* ... */ }
        })
    }
}

pub fn new_uninit() -> Rc<MaybeUninit<T>>

🔬 This is a nightly-only experimental API. (new_uninit #63291)

Constructs a new Rc with uninitialized contents.

Examples

#![feature(new_uninit)]
#![feature(get_mut_unchecked)]

use std::rc::Rc;

let mut five = Rc::<u32>::new_uninit();

let five = unsafe {
    // Deferred initialization:
    Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5)

pub fn new_zeroed() -> Rc<MaybeUninit<T>>

🔬 This is a nightly-only experimental API. (new_uninit #63291)

Constructs a new Rc with uninitialized contents, with the memory being filled with 0 bytes.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(new_uninit)]

use std::rc::Rc;

let zero = Rc::<u32>::new_zeroed();
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0)

pub fn try_new(value: T) -> Result<Rc<T>, AllocError>

🔬 This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new Rc<T>, returning an error if the allocation fails

Examples

#![feature(allocator_api)]
use std::rc::Rc;

let five = Rc::try_new(5);

pub fn try_new_uninit() -> Result<Rc<MaybeUninit<T>>, AllocError>

🔬 This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new Rc with uninitialized contents, returning an error if the allocation fails

Examples

#![feature(allocator_api, new_uninit)]
#![feature(get_mut_unchecked)]

use std::rc::Rc;

let mut five = Rc::<u32>::try_new_uninit()?;

let five = unsafe {
    // Deferred initialization:
    Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5);

pub fn try_new_zeroed() -> Result<Rc<MaybeUninit<T>>, AllocError>

🔬 This is a nightly-only experimental API. (allocator_api #32838)

Constructs a new Rc with uninitialized contents, with the memory being filled with 0 bytes, returning an error if the allocation fails

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(allocator_api, new_uninit)]

use std::rc::Rc;

let zero = Rc::<u32>::try_new_zeroed()?;
let zero = unsafe { zero.assume_init() };

assert_eq!(*zero, 0);

pub fn pin(value: T) -> Pin<Rc<T>>

Constructs a new Pin<Rc<T>>. If T does not implement Unpin, then value will be pinned in memory and unable to be moved.

pub fn try_unwrap(this: Self) -> Result<T, Self>

Returns the inner value, if the Rc has exactly one strong reference.

Otherwise, an Err is returned with the same Rc that was passed in.

This will succeed even if there are outstanding weak references.

Examples

use std::rc::Rc;

let x = Rc::new(3);
assert_eq!(Rc::try_unwrap(x), Ok(3));

let x = Rc::new(4);
let _y = Rc::clone(&x);
assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);

impl<T> Rc<[T]>

pub fn new_uninit_slice(len: usize) -> Rc<[MaybeUninit<T>]>

🔬 This is a nightly-only experimental API. (new_uninit #63291)

Constructs a new reference-counted slice with uninitialized contents.

Examples

#![feature(new_uninit)]
#![feature(get_mut_unchecked)]

use std::rc::Rc;

let mut values = Rc::<[u32]>::new_uninit_slice(3);

let values = unsafe {
    // Deferred initialization:
    Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
    Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
    Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);

    values.assume_init()
};

assert_eq!(*values, [1, 2, 3])

pub fn new_zeroed_slice(len: usize) -> Rc<[MaybeUninit<T>]>

🔬 This is a nightly-only experimental API. (new_uninit #63291)

Constructs a new reference-counted slice with uninitialized contents, with the memory being filled with 0 bytes.

See MaybeUninit::zeroed for examples of correct and incorrect usage of this method.

Examples

#![feature(new_uninit)]

use std::rc::Rc;

let values = Rc::<[u32]>::new_zeroed_slice(3);
let values = unsafe { values.assume_init() };

assert_eq!(*values, [0, 0, 0])

impl<T> Rc<MaybeUninit<T>>

pub unsafe fn assume_init(self) -> Rc<T>

🔬 This is a nightly-only experimental API. (new_uninit #63291)

Converts to Rc<T>.

Safety

As with MaybeUninit::assume_init, it is up to the caller to guarantee that the inner value really is in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior.

Examples

#![feature(new_uninit)]
#![feature(get_mut_unchecked)]

use std::rc::Rc;

let mut five = Rc::<u32>::new_uninit();

let five = unsafe {
    // Deferred initialization:
    Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);

    five.assume_init()
};

assert_eq!(*five, 5)

impl<T> Rc<[MaybeUninit<T>]>

pub unsafe fn assume_init(self) -> Rc<[T]>

🔬 This is a nightly-only experimental API. (new_uninit #63291)

Converts to Rc<[T]>.

Safety

As with MaybeUninit::assume_init, it is up to the caller to guarantee that the inner value really is in an initialized state. Calling this when the content is not yet fully initialized causes immediate undefined behavior.

Examples

#![feature(new_uninit)]
#![feature(get_mut_unchecked)]

use std::rc::Rc;

let mut values = Rc::<[u32]>::new_uninit_slice(3);

let values = unsafe {
    // Deferred initialization:
    Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
    Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
    Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);

    values.assume_init()
};

assert_eq!(*values, [1, 2, 3])

impl<T: ?Sized> Rc<T>

pub fn into_raw(this: Self) -> *const T

Consumes the Rc, returning the wrapped pointer.

To avoid a memory leak the pointer must be converted back to an Rc using Rc::from_raw.

Examples

use std::rc::Rc;

let x = Rc::new("hello".to_owned());
let x_ptr = Rc::into_raw(x);
assert_eq!(unsafe { &*x_ptr }, "hello");

pub fn as_ptr(this: &Self) -> *const T

Provides a raw pointer to the data.

The counts are not affected in any way and the Rc is not consumed. The pointer is valid for as long there are strong counts in the Rc.

Examples

use std::rc::Rc;

let x = Rc::new("hello".to_owned());
let y = Rc::clone(&x);
let x_ptr = Rc::as_ptr(&x);
assert_eq!(x_ptr, Rc::as_ptr(&y));
assert_eq!(unsafe { &*x_ptr }, "hello");

pub unsafe fn from_raw(ptr: *const T) -> Self

Constructs an Rc<T> from a raw pointer.

The raw pointer must have been previously returned by a call to Rc<U>::into_raw where U must have the same size and alignment as T. This is trivially true if U is T. Note that if U is not T but has the same size and alignment, this is basically like transmuting references of different types. See mem::transmute for more information on what restrictions apply in this case.

The user of from_raw has to make sure a specific value of T is only dropped once.

This function is unsafe because improper use may lead to memory unsafety, even if the returned Rc<T> is never accessed.

Examples

use std::rc::Rc;

let x = Rc::new("hello".to_owned());
let x_ptr = Rc::into_raw(x);

unsafe {
    // Convert back to an `Rc` to prevent leak.
    let x = Rc::from_raw(x_ptr);
    assert_eq!(&*x, "hello");

    // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
}

// The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!

pub fn downgrade(this: &Self) -> Weak<T>

Creates a new Weak pointer to this allocation.

Examples

use std::rc::Rc;

let five = Rc::new(5);

let weak_five = Rc::downgrade(&five);

pub fn weak_count(this: &Self) -> usize

Gets the number of Weak pointers to this allocation.

Examples

use std::rc::Rc;

let five = Rc::new(5);
let _weak_five = Rc::downgrade(&five);

assert_eq!(1, Rc::weak_count(&five));

pub fn strong_count(this: &Self) -> usize

Gets the number of strong (Rc) pointers to this allocation.

Examples

use std::rc::Rc;

let five = Rc::new(5);
let _also_five = Rc::clone(&five);

assert_eq!(2, Rc::strong_count(&five));

pub unsafe fn increment_strong_count(ptr: *const T)

Increments the strong reference count on the Rc<T> associated with the provided pointer by one.

Safety

The pointer must have been obtained through Rc::into_raw, and the associated Rc instance must be valid (i.e. the strong count must be at least 1) for the duration of this method.

Examples

use std::rc::Rc;

let five = Rc::new(5);

unsafe {
    let ptr = Rc::into_raw(five);
    Rc::increment_strong_count(ptr);

    let five = Rc::from_raw(ptr);
    assert_eq!(2, Rc::strong_count(&five));
}

pub unsafe fn decrement_strong_count(ptr: *const T)

Decrements the strong reference count on the Rc<T> associated with the provided pointer by one.

Safety

The pointer must have been obtained through Rc::into_raw, and the associated Rc instance must be valid (i.e. the strong count must be at least 1) when invoking this method. This method can be used to release the final Rc and backing storage, but should not be called after the final Rc has been released.

Examples

use std::rc::Rc;

let five = Rc::new(5);

unsafe {
    let ptr = Rc::into_raw(five);
    Rc::increment_strong_count(ptr);

    let five = Rc::from_raw(ptr);
    assert_eq!(2, Rc::strong_count(&five));
    Rc::decrement_strong_count(ptr);
    assert_eq!(1, Rc::strong_count(&five));
}

pub fn get_mut(this: &mut Self) -> Option<&mut T>

Returns a mutable reference into the given Rc, if there are no other Rc or Weak pointers to the same allocation.

Returns None otherwise, because it is not safe to mutate a shared value.

See also make_mut, which will clone the inner value when there are other Rc pointers.

Examples

use std::rc::Rc;

let mut x = Rc::new(3);
*Rc::get_mut(&mut x).unwrap() = 4;
assert_eq!(*x, 4);

let _y = Rc::clone(&x);
assert!(Rc::get_mut(&mut x).is_none());

pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T

🔬 This is a nightly-only experimental API. (get_mut_unchecked #63292)

Returns a mutable reference into the given Rc, without any check.

See also get_mut, which is safe and does appropriate checks.

Safety

Any other Rc or Weak pointers to the same allocation must not be dereferenced for the duration of the returned borrow. This is trivially the case if no such pointers exist, for example immediately after Rc::new.

Examples

#![feature(get_mut_unchecked)]

use std::rc::Rc;

let mut x = Rc::new(String::new());
unsafe {
    Rc::get_mut_unchecked(&mut x).push_str("foo")
}
assert_eq!(*x, "foo");

pub fn ptr_eq(this: &Self, other: &Self) -> bool

Returns true if the two Rcs point to the same allocation (in a vein similar to ptr::eq).

Examples

use std::rc::Rc;

let five = Rc::new(5);
let same_five = Rc::clone(&five);
let other_five = Rc::new(5);

assert!(Rc::ptr_eq(&five, &same_five));
assert!(!Rc::ptr_eq(&five, &other_five));

impl<T: Clone> Rc<T>

pub fn make_mut(this: &mut Self) -> &mut T

Makes a mutable reference into the given Rc.

If there are other Rc pointers to the same allocation, then make_mut will clone the inner value to a new allocation to ensure unique ownership. This is also referred to as clone-on-write.

However, if there are no other Rc pointers to this allocation, but some Weak pointers, then the Weak pointers will be disassociated and the inner value will not be cloned.

See also get_mut, which will fail rather than cloning the inner value or diassociating Weak pointers.

Examples

use std::rc::Rc;

let mut data = Rc::new(5);

*Rc::make_mut(&mut data) += 1;         // Won't clone anything
let mut other_data = Rc::clone(&data); // Won't clone inner data
*Rc::make_mut(&mut data) += 1;         // Clones inner data
*Rc::make_mut(&mut data) += 1;         // Won't clone anything
*Rc::make_mut(&mut other_data) *= 2;   // Won't clone anything

// Now `data` and `other_data` point to different allocations.
assert_eq!(*data, 8);
assert_eq!(*other_data, 12);

Weak pointers will be disassociated:

use std::rc::Rc;

let mut data = Rc::new(75);
let weak = Rc::downgrade(&data);

assert!(75 == *data);
assert!(75 == *weak.upgrade().unwrap());

*Rc::make_mut(&mut data) += 1;

assert!(76 == *data);
assert!(weak.upgrade().is_none());

impl Rc<dyn Any>

pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>>

Attempt to downcast the Rc<dyn Any> to a concrete type.

Examples

use std::any::Any;
use std::rc::Rc;

fn print_if_string(value: Rc<dyn Any>) {
    if let Ok(string) = value.downcast::<String>() {
        println!("String ({}): {}", string.len(), string);
    }
}

let my_string = "Hello World".to_string();
print_if_string(Rc::new(my_string));
print_if_string(Rc::new(0i8));

Trait Implementations

impl<T: ?Sized> AsRef<T> for Rc<T>

fn as_ref(&self) -> &T

Performs the conversion.

impl<T: ?Sized> Borrow<T> for Rc<T>

fn borrow(&self) -> &T

Immutably borrows from an owned value.

impl<T: ?Sized> Clone for Rc<T>

fn clone(&self) -> Rc<T>

Makes a clone of the Rc pointer.

This creates another pointer to the same allocation, increasing the strong reference count.

Examples

use std::rc::Rc;

let five = Rc::new(5);

let _ = Rc::clone(&five);

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: ?Sized + Debug> Debug for Rc<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: Default> Default for Rc<T>

fn default() -> Rc<T>

Creates a new Rc<T>, with the Default value for T.

Examples

use std::rc::Rc;

let x: Rc<i32> = Default::default();
assert_eq!(*x, 0);

impl<T: ?Sized> Deref for Rc<T>

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

impl<T: ?Sized + Display> Display for Rc<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: ?Sized> Drop for Rc<T>

fn drop(&mut self)

Drops the Rc.

This will decrement the strong reference count. If the strong reference count reaches zero then the only other references (if any) are Weak, so we drop the inner value.

Examples

use std::rc::Rc;

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("dropped!");
    }
}

let foo  = Rc::new(Foo);
let foo2 = Rc::clone(&foo);

drop(foo);    // Doesn't print anything
drop(foo2);   // Prints "dropped!"

impl<T: Clone> From<&'_ [T]> for Rc<[T]>

fn from(v: &[T]) -> Rc<[T]>

Allocate a reference-counted slice and fill it by cloning v’s items.

Example

let original: &[i32] = &[1, 2, 3];
let shared: Rc<[i32]> = Rc::from(original);
assert_eq!(&[1, 2, 3], &shared[..]);

impl From<&'_ str> for Rc<str>

fn from(v: &str) -> Rc<str>

Allocate a reference-counted string slice and copy v into it.

Example

let shared: Rc<str> = Rc::from("statue");
assert_eq!("statue", &shared[..]);

impl<T: ?Sized> From<Box<T, Global>> for Rc<T>

fn from(v: Box<T>) -> Rc<T>

Move a boxed object to a new, reference counted, allocation.

Example

let original: Box<i32> = Box::new(1);
let shared: Rc<i32> = Rc::from(original);
assert_eq!(1, *shared);

impl<'a, B: ?Sized> From<Cow<'a, B>> for Rc<B> where
    B: ToOwned + ?Sized,
    Rc<B>: From<&'a B> + From<B::Owned>, 

fn from(cow: Cow<'a, B>) -> Rc<B>

Create a reference-counted pointer from a clone-on-write pointer by copying its content.

Example

let cow: Cow<str> = Cow::Borrowed("eggplant");
let shared: Rc<str> = Rc::from(cow);
assert_eq!("eggplant", &shared[..]);

impl From<String> for Rc<str>

fn from(v: String) -> Rc<str>

Allocate a reference-counted string slice and copy v into it.

Example

let original: String = "statue".to_owned();
let shared: Rc<str> = Rc::from(original);
assert_eq!("statue", &shared[..]);

impl<T> From<T> for Rc<T>

fn from(t: T) -> Self

Converts a generic type T into an Rc<T>

The conversion allocates on the heap and moves t from the stack into it.

Example

let x = 5;
let rc = Rc::new(5);

assert_eq!(Rc::from(x), rc);

impl<T> From<Vec<T, Global>> for Rc<[T]>

fn from(v: Vec<T>) -> Rc<[T]>

Allocate a reference-counted slice and move v’s items into it.

Example

let original: Box<Vec<i32>> = Box::new(vec![1, 2, 3]);
let shared: Rc<Vec<i32>> = Rc::from(original);
assert_eq!(vec![1, 2, 3], *shared);

impl<T> FromIterator<T> for Rc<[T]>

fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self

Takes each element in the Iterator and collects it into an Rc<[T]>.

Performance characteristics

The general case

In the general case, collecting into Rc<[T]> is done by first collecting into a Vec<T>. That is, when writing the following:

let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();

this behaves as if we wrote:

let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
    .collect::<Vec<_>>() // The first set of allocations happens here.
    .into(); // A second allocation for `Rc<[T]>` happens here.

This will allocate as many times as needed for constructing the Vec<T> and then it will allocate once for turning the Vec<T> into the Rc<[T]>.

Iterators of known length

When your Iterator implements TrustedLen and is of an exact size, a single allocation will be made for the Rc<[T]>. For example:

let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.

impl<T: ?Sized + Hash> Hash for Rc<T>

fn hash<H: Hasher>(&self, state: &mut H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given Hasher.

impl<T: ?Sized + Ord> Ord for Rc<T>

fn cmp(&self, other: &Rc<T>) -> Ordering

Comparison for two Rcs.

The two are compared by calling cmp() on their inner values.

Examples

use std::rc::Rc;
use std::cmp::Ordering;

let five = Rc::new(5);

assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

Restrict a value to a certain interval.

impl<T: ?Sized + PartialEq> PartialEq<Rc<T>> for Rc<T>

fn eq(&self, other: &Rc<T>) -> bool

Equality for two Rcs.

Two Rcs are equal if their inner values are equal, even if they are stored in different allocation.

If T also implements Eq (implying reflexivity of equality), two Rcs that point to the same allocation are always equal.

Examples

use std::rc::Rc;

let five = Rc::new(5);

assert!(five == Rc::new(5));

fn ne(&self, other: &Rc<T>) -> bool

Inequality for two Rcs.

Two Rcs are unequal if their inner values are unequal.

If T also implements Eq (implying reflexivity of equality), two Rcs that point to the same allocation are never unequal.

Examples

use std::rc::Rc;

let five = Rc::new(5);

assert!(five != Rc::new(6));

impl<T: ?Sized + PartialOrd> PartialOrd<Rc<T>> for Rc<T>

fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering>

Partial comparison for two Rcs.

The two are compared by calling partial_cmp() on their inner values.

Examples

use std::rc::Rc;
use std::cmp::Ordering;

let five = Rc::new(5);

assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));

fn lt(&self, other: &Rc<T>) -> bool

Less-than comparison for two Rcs.

The two are compared by calling < on their inner values.

Examples

use std::rc::Rc;

let five = Rc::new(5);

assert!(five < Rc::new(6));

fn le(&self, other: &Rc<T>) -> bool

‘Less than or equal to’ comparison for two Rcs.

The two are compared by calling <= on their inner values.

Examples

use std::rc::Rc;

let five = Rc::new(5);

assert!(five <= Rc::new(5));

fn gt(&self, other: &Rc<T>) -> bool

Greater-than comparison for two Rcs.

The two are compared by calling > on their inner values.

Examples

use std::rc::Rc;

let five = Rc::new(5);

assert!(five > Rc::new(4));

fn ge(&self, other: &Rc<T>) -> bool

‘Greater than or equal to’ comparison for two Rcs.

The two are compared by calling >= on their inner values.

Examples

use std::rc::Rc;

let five = Rc::new(5);

assert!(five >= Rc::new(5));

impl<T: ?Sized> Pointer for Rc<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]>

type Error = Rc<[T]>

The type returned in the event of a conversion error.

fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error>

Performs the conversion.

impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T>

impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T>

impl<T: ?Sized + Eq> Eq for Rc<T>

impl<T: ?Sized> !Send for Rc<T>

impl<T: ?Sized> !Sync for Rc<T>

impl<T: ?Sized> Unpin for Rc<T>

impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Rc<T>