Struct std::lazy::OnceCell
pub struct OnceCell<T> { /* fields omitted */ }
A cell which can be written to only once.
Unlike RefCell, a OnceCell only provides shared &T references to its value. Unlike Cell, a OnceCell doesn’t require copying or replacing the value to access it.
Examples
#![feature(once_cell)]
use std::lazy::OnceCell;
let cell = OnceCell::new();
assert!(cell.get().is_none());
let value: &String = cell.get_or_init(|| {
"Hello, World!".to_string()
});
assert_eq!(value, "Hello, World!");
assert!(cell.get().is_some());Implementations
impl<T> OnceCell<T>
pub const fn new() -> OnceCell<T>
Creates a new empty cell.
pub fn get(&self) -> Option<&T>
Gets the reference to the underlying value.
Returns None if the cell is empty.
pub fn get_mut(&mut self) -> Option<&mut T>
Gets the mutable reference to the underlying value.
Returns None if the cell is empty.
pub fn set(&self, value: T) -> Result<(), T>
Sets the contents of the cell to value.
Errors
This method returns Ok(()) if the cell was empty and Err(value) if it was full.
Examples
#![feature(once_cell)] use std::lazy::OnceCell; let cell = OnceCell::new(); assert!(cell.get().is_none()); assert_eq!(cell.set(92), Ok(())); assert_eq!(cell.set(62), Err(62)); assert!(cell.get().is_some());
Gets the contents of the cell, initializing it with f if the cell was empty.
Panics
If f panics, the panic is propagated to the caller, and the cell remains uninitialized.
It is an error to reentrantly initialize the cell from f. Doing so results in a panic.
Examples
#![feature(once_cell)] use std::lazy::OnceCell; let cell = OnceCell::new(); let value = cell.get_or_init(|| 92); assert_eq!(value, &92); let value = cell.get_or_init(|| unreachable!()); assert_eq!(value, &92);
Gets the contents of the cell, initializing it with f if the cell was empty. If the cell was empty and f failed, an error is returned.
Panics
If f panics, the panic is propagated to the caller, and the cell remains uninitialized.
It is an error to reentrantly initialize the cell from f. Doing so results in a panic.
Examples
#![feature(once_cell)]
use std::lazy::OnceCell;
let cell = OnceCell::new();
assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
assert!(cell.get().is_none());
let value = cell.get_or_try_init(|| -> Result<i32, ()> {
Ok(92)
});
assert_eq!(value, Ok(&92));
assert_eq!(cell.get(), Some(&92))pub fn into_inner(self) -> Option<T>
Consumes the cell, returning the wrapped value.
Returns None if the cell was empty.
Examples
#![feature(once_cell)]
use std::lazy::OnceCell;
let cell: OnceCell<String> = OnceCell::new();
assert_eq!(cell.into_inner(), None);
let cell = OnceCell::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.into_inner(), Some("hello".to_string()));pub fn take(&mut self) -> Option<T>
Takes the value out of this OnceCell, moving it back to an uninitialized state.
Has no effect and returns None if the OnceCell hasn’t been initialized.
Safety is guaranteed by requiring a mutable reference.
Examples
#![feature(once_cell)]
use std::lazy::OnceCell;
let mut cell: OnceCell<String> = OnceCell::new();
assert_eq!(cell.take(), None);
let mut cell = OnceCell::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.take(), Some("hello".to_string()));
assert_eq!(cell.get(), None);Trait Implementations
pub fn clone(&self) -> OnceCell<T>
Returns a copy of the value. Read more
fn clone_from(&mut self, source: &Self)
Performs copy-assignment from source. Read more
pub fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>
Formats the value using the given formatter. Read more
impl<T> Default for OnceCell<T>
impl<T> From<T> for OnceCell<T>
pub fn from(value: T) -> OnceCell<T>
Performs the conversion.
Auto Trait Implementations
impl<T> !RefUnwindSafe for OnceCell<T>
impl<T> Send for OnceCell<T> where
T: Send,
impl<T> !Sync for OnceCell<T>
impl<T> Unpin for OnceCell<T> where
T: Unpin,
impl<T> UnwindSafe for OnceCell<T> where
T: UnwindSafe,
Blanket Implementations
impl<T> From<!> for T
pub fn from(t: !) -> T
Performs the conversion.
impl<T> From<T> for T
pub fn from(t: T) -> T
Performs the conversion.
pub fn into(self) -> U
Performs the conversion.
type Owned = T
The resulting type after obtaining ownership.
pub fn to_owned(&self) -> T
Creates owned data from borrowed data, usually by cloning. Read more
pub fn clone_into(&self, target: &mut T)
toowned_clone_into #41263)recently added
Uses borrowed data to replace owned data, usually by cloning. Read more
type Error = Infallible
The type returned in the event of a conversion error.
pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>
Performs the conversion.
type Error = <U as TryFrom<T>>::Error
The type returned in the event of a conversion error.
pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>
Performs the conversion.
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/lazy/struct.OnceCell.html