Struct alloc::rc::Rc

#[lang = "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.

Methods

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_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 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 contained 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])

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

Constructs an Rc from a raw pointer.

The raw pointer must have been previously returned by a call to a Rc::into_raw.

This function is unsafe because improper use may lead to memory problems. For example, a double-free may occur if the function is called twice on the same raw pointer.

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 into_raw_non_null(this: Self) -> NonNull<T>

🔬 This is a nightly-only experimental API. (rc_into_raw_non_null #47336)

Consumes the Rc, returning the wrapped pointer as NonNull<T>.

Examples

#![feature(rc_into_raw_non_null)]

use std::rc::Rc;

let x = Rc::new("hello".to_owned());
let ptr = Rc::into_raw_non_null(x);
let deref = unsafe { ptr.as_ref() };
assert_eq!(deref, "hello");

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

Creates a new Weak pointer to this value.

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 value.

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 value.

Examples

use std::rc::Rc;

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

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

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

Returns a mutable reference to the inner value, if there are no other Rc or Weak pointers to the same value.

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

See also make_mut, which will clone the inner value when it's shared.

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 to the inner value, without any check.

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

Safety

Any other Rc or Weak pointers to the same value 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 value (not just values that compare as equal).

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 value, then make_mut will clone the inner value to ensure unique ownership. This is also referred to as clone-on-write.

If there are no other Rc pointers to this value, then Weak pointers to this value will be dissassociated.

See also get_mut, which will fail rather than cloning.

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 values.
assert_eq!(*data, 8);
assert_eq!(*other_data, 12);

Weak pointers will be dissassociated:

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);
    }
}

fn main() {
    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 + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T>

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

fn from(t: T) -> Self

Performs the conversion.

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

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

Performs the conversion.

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

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

Performs the conversion.

impl From<String> for Rc<str>

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

Performs the conversion.

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

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

Performs the conversion.

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

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

Performs the conversion.

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

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.

If T also implements Eq, two Rcs that point to the same value 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, two Rcs that point to the same value are never unequal.

Examples

use std::rc::Rc;

let five = Rc::new(5);

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

impl<T: ?Sized + Eq> Eq for Rc<T>

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

🔬 This is a nightly-only experimental API. (clamp #44095)

Restrict a value to a certain interval.

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 + 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> Deref for Rc<T>

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

impl<T: ?Sized> Receiver for Rc<T>

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: ?Sized> Unpin for Rc<T>

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

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

Formats the value using the given formatter.

impl<T: ?Sized + Display> Display 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]> where
    [T; N]: LengthAtMost32, 

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> AsRef<T> for Rc<T>

fn as_ref(&self) -> &T

Performs the conversion.

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> Pointer for Rc<T>

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

Formats the value using the given formatter.

impl<T: ?Sized> !Send for Rc<T>

impl<T: ?Sized> !Sync for Rc<T>

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 inner value, 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: 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> Borrow<T> for Rc<T>

fn borrow(&self) -> &T

Immutably borrows from an owned value.