pub struct Rc<T: ?Sized> { /* private fields */ }Expand description
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
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(), /* ... */ }
})
}
}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)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)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);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);Constructs a new Pin<Rc<T>>. If T does not implement Unpin, then
value will be pinned in memory and unable to be moved.
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);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])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])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)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])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");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");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!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));
}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));
}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());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");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());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
Performs copy-assignment from source. Read more
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!"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.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));