Struct otter_nodejs_tests::inventory::core::cell::OnceCell

source · [−]
pub struct OnceCell<T> { /* private fields */ }
🔬 This is a nightly-only experimental API. (once_cell)
Expand description

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::cell::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

🔬 This is a nightly-only experimental API. (once_cell)

Creates a new empty cell.

🔬 This is a nightly-only experimental API. (once_cell)

Gets the reference to the underlying value.

Returns None if the cell is empty.

🔬 This is a nightly-only experimental API. (once_cell)

Gets the mutable reference to the underlying value.

Returns None if the cell is empty.

🔬 This is a nightly-only experimental API. (once_cell)

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::cell::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());
🔬 This is a nightly-only experimental API. (once_cell)

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::cell::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);
🔬 This is a nightly-only experimental API. (once_cell)

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::cell::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))
🔬 This is a nightly-only experimental API. (once_cell)

Consumes the cell, returning the wrapped value.

Returns None if the cell was empty.

Examples
#![feature(once_cell)]

use std::cell::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()));
🔬 This is a nightly-only experimental API. (once_cell)

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::cell::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

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

Returns the “default value” for a type. Read more

Creates a new OnceCell<T> which already contains the given value.

This method tests for self and other values to be equal, and is used by ==. Read more

This method tests for !=.

Auto Trait Implementations

Blanket Implementations

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Convert &Trait (where Trait: Downcast) to &Any. This is needed since Rust cannot generate &Any’s vtable from &Trait’s. Read more

Convert &mut Trait (where Trait: Downcast) to &Any. This is needed since Rust cannot generate &mut Any’s vtable from &mut Trait’s. Read more

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Returns the argument unchanged.

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That is, this conversion is whatever the implementation of From<T> for U chooses to do.

Should always be Self

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Performs the conversion.

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