Struct RefCell

pub struct RefCell<T: ?Sized> { /* private fields */ }

A mutable memory location with dynamically checked borrow rules

See the module-level documentation for more.

Implementations

impl<T> RefCell<T>

pub const fn new(value: T) -> RefCell<T>

Creates a new RefCell containing value.

Examples

use std::cell::RefCell;

let c = RefCell::new(5);

pub fn into_inner(self) -> T

Consumes the RefCell, returning the wrapped value.

Examples

use std::cell::RefCell;

let c = RefCell::new(5);

let five = c.into_inner();

pub fn replace(&self, t: T) -> T

Replaces the wrapped value with a new one, returning the old value, without deinitializing either one.

This function corresponds to std::mem::replace.

Panics

Panics if the value is currently borrowed.

Examples

use std::cell::RefCell;
let cell = RefCell::new(5);
let old_value = cell.replace(6);
assert_eq!(old_value, 5);
assert_eq!(cell, RefCell::new(6));

pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T

Replaces the wrapped value with a new one computed from f, returning the old value, without deinitializing either one.

Panics

Panics if the value is currently borrowed.

Examples

use std::cell::RefCell;
let cell = RefCell::new(5);
let old_value = cell.replace_with(|&mut old| old + 1);
assert_eq!(old_value, 5);
assert_eq!(cell, RefCell::new(6));

pub fn swap(&self, other: &Self)

Swaps the wrapped value of self with the wrapped value of other, without deinitializing either one.

This function corresponds to std::mem::swap.

Panics

Panics if the value in either RefCell is currently borrowed, or if self and other point to the same RefCell.

Examples

use std::cell::RefCell;
let c = RefCell::new(5);
let d = RefCell::new(6);
c.swap(&d);
assert_eq!(c, RefCell::new(6));
assert_eq!(d, RefCell::new(5));

impl<T: ?Sized> RefCell<T>

pub fn borrow(&self) -> Ref<'_, T>

Immutably borrows the wrapped value.

The borrow lasts until the returned Ref exits scope. Multiple immutable borrows can be taken out at the same time.

Panics

Panics if the value is currently mutably borrowed. For a non-panicking variant, use try_borrow.

Examples

use std::cell::RefCell;

let c = RefCell::new(5);

let borrowed_five = c.borrow();
let borrowed_five2 = c.borrow();

An example of panic:

use std::cell::RefCell;

let c = RefCell::new(5);

let m = c.borrow_mut();
let b = c.borrow(); // this causes a panic

pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError>

Immutably borrows the wrapped value, returning an error if the value is currently mutably borrowed.

The borrow lasts until the returned Ref exits scope. Multiple immutable borrows can be taken out at the same time.

This is the non-panicking variant of borrow.

Examples

use std::cell::RefCell;

let c = RefCell::new(5);

{
    let m = c.borrow_mut();
    assert!(c.try_borrow().is_err());
}

{
    let m = c.borrow();
    assert!(c.try_borrow().is_ok());
}

pub fn borrow_mut(&self) -> RefMut<'_, T>

Mutably borrows the wrapped value.

The borrow lasts until the returned RefMut or all RefMuts derived from it exit scope. The value cannot be borrowed while this borrow is active.

Panics

Panics if the value is currently borrowed. For a non-panicking variant, use try_borrow_mut.

Examples

use std::cell::RefCell;

let c = RefCell::new("hello".to_owned());

*c.borrow_mut() = "bonjour".to_owned();

assert_eq!(&*c.borrow(), "bonjour");

An example of panic:

use std::cell::RefCell;

let c = RefCell::new(5);
let m = c.borrow();

let b = c.borrow_mut(); // this causes a panic

pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError>

Mutably borrows the wrapped value, returning an error if the value is currently borrowed.

The borrow lasts until the returned RefMut or all RefMuts derived from it exit scope. The value cannot be borrowed while this borrow is active.

This is the non-panicking variant of borrow_mut.

Examples

use std::cell::RefCell;

let c = RefCell::new(5);

{
    let m = c.borrow();
    assert!(c.try_borrow_mut().is_err());
}

assert!(c.try_borrow_mut().is_ok());

pub fn as_ptr(&self) -> *mut T

Returns a raw pointer to the underlying data in this cell.

Examples

use std::cell::RefCell;

let c = RefCell::new(5);

let ptr = c.as_ptr();

pub fn get_mut(&mut self) -> &mut T

Returns a mutable reference to the underlying data.

Since this method borrows RefCell mutably, it is statically guaranteed that no borrows to the underlying data exist. The dynamic checks inherent in borrow_mut and most other methods of RefCell are therefore unnecessary.

This method can only be called if RefCell can be mutably borrowed, which in general is only the case directly after the RefCell has been created. In these situations, skipping the aforementioned dynamic borrowing checks may yield better ergonomics and runtime-performance.

In most situations where RefCell is used, it can’t be borrowed mutably. Use borrow_mut to get mutable access to the underlying data then.

Examples

use std::cell::RefCell;

let mut c = RefCell::new(5);
*c.get_mut() += 1;

assert_eq!(c, RefCell::new(6));

pub fn undo_leak(&mut self) -> &mut T

🔬This is a nightly-only experimental API. (cell_leak #69099)

Undo the effect of leaked guards on the borrow state of the RefCell.

This call is similar to get_mut but more specialized. It borrows RefCell mutably to ensure no borrows exist and then resets the state tracking shared borrows. This is relevant if some Ref or RefMut borrows have been leaked.

Examples

#![feature(cell_leak)]
use std::cell::RefCell;

let mut c = RefCell::new(0);
std::mem::forget(c.borrow_mut());

assert!(c.try_borrow().is_err());
c.undo_leak();
assert!(c.try_borrow().is_ok());

pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError>

Immutably borrows the wrapped value, returning an error if the value is currently mutably borrowed.

Safety

Unlike RefCell::borrow, this method is unsafe because it does not return a Ref, thus leaving the borrow flag untouched. Mutably borrowing the RefCell while the reference returned by this method is alive is undefined behaviour.

Examples

use std::cell::RefCell;

let c = RefCell::new(5);

{
    let m = c.borrow_mut();
    assert!(unsafe { c.try_borrow_unguarded() }.is_err());
}

{
    let m = c.borrow();
    assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
}

impl<T: Default> RefCell<T>

pub fn take(&self) -> T

Takes the wrapped value, leaving Default::default() in its place.

Panics

Panics if the value is currently borrowed.

Examples

use std::cell::RefCell;

let c = RefCell::new(5);
let five = c.take();

assert_eq!(five, 5);
assert_eq!(c.into_inner(), 0);

Trait Implementations

impl<T: Clone> Clone for RefCell<T>

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

Panics

Panics if the value is currently mutably borrowed.

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

Panics

Panics if source is currently mutably borrowed.

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

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

Formats the value using the given formatter.

impl<T: Default> Default for RefCell<T>

fn default() -> RefCell<T>

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

impl<T> From<T> for RefCell<T>

fn from(t: T) -> RefCell<T>

Creates a new RefCell<T> containing the given value.

impl<T: ?Sized + Ord> Ord for RefCell<T>

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

Panics

Panics if the value in either RefCell is currently mutably borrowed.

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

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

Panics

Panics if the value in either RefCell is currently mutably borrowed.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

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

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

Panics

Panics if the value in either RefCell is currently mutably borrowed.

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

Panics

Panics if the value in either RefCell is currently mutably borrowed.

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

Panics

Panics if the value in either RefCell is currently mutably borrowed.

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

Panics

Panics if the value in either RefCell is currently mutably borrowed.

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

Panics

Panics if the value in either RefCell is currently mutably borrowed.

impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T>

impl<T: ?Sized + Eq> Eq for RefCell<T>

impl<T: ?Sized> PinCoerceUnsized for RefCell<T>

impl<T> Send for RefCell<T>

where T: Send + ?Sized,

impl<T: ?Sized> !Sync for RefCell<T>