pub struct OnceCell<T>(_);
Expand description

A thread-safe cell which can be written to only once.

OnceCell provides & references to the contents without RAII guards.

Reading a non-None value out of OnceCell establishes a happens-before relationship with a corresponding write. For example, if thread A initializes the cell with get_or_init(f), and thread B subsequently reads the result of this call, B also observes all the side effects of f.

Example

use once_cell::sync::OnceCell;

static CELL: OnceCell<String> = OnceCell::new();
assert!(CELL.get().is_none());

std::thread::spawn(|| {
    let value: &String = CELL.get_or_init(|| {
        "Hello, World!".to_string()
    });
    assert_eq!(value, "Hello, World!");
}).join().unwrap();

let value: Option<&String> = CELL.get();
assert!(value.is_some());
assert_eq!(value.unwrap().as_str(), "Hello, World!");

Implementations

Creates a new empty cell.

Creates a new initialized cell.

Gets the reference to the underlying value.

Returns None if the cell is empty, or being initialized. This method never blocks.

Gets the reference to the underlying value, blocking the current thread until it is set.

use once_cell::sync::OnceCell;

let mut cell = std::sync::Arc::new(OnceCell::new());
let t = std::thread::spawn({
    let cell = std::sync::Arc::clone(&cell);
    move || cell.set(92).unwrap()
});

// Returns immediately, but might return None.
let _value_or_none = cell.get();

// Will return 92, but might block until the other thread does `.set`.
let value: &u32 = cell.wait();
assert_eq!(*value, 92);
t.join().unwrap();

Gets the mutable reference to the underlying value.

Returns None if the cell is empty.

This method is allowed to violate the invariant of writing to a OnceCell at most once because it requires &mut access to self. As with all interior mutability, &mut access permits arbitrary modification:

use once_cell::sync::OnceCell;

let mut cell: OnceCell<u32> = OnceCell::new();
cell.set(92).unwrap();
cell = OnceCell::new();

Get the reference to the underlying value, without checking if the cell is initialized.

Safety

Caller must ensure that the cell is in initialized state, and that the contents are acquired by (synchronized to) this thread.

Sets the contents of this cell to value.

Returns Ok(()) if the cell was empty and Err(value) if it was full.

Example
use once_cell::sync::OnceCell;

static CELL: OnceCell<i32> = OnceCell::new();

fn main() {
    assert!(CELL.get().is_none());

    std::thread::spawn(|| {
        assert_eq!(CELL.set(92), Ok(()));
    }).join().unwrap();

    assert_eq!(CELL.set(62), Err(62));
    assert_eq!(CELL.get(), Some(&92));
}

Like set, but also returns a reference to the final cell value.

Example
use once_cell::unsync::OnceCell;

let cell = OnceCell::new();
assert!(cell.get().is_none());

assert_eq!(cell.try_insert(92), Ok(&92));
assert_eq!(cell.try_insert(62), Err((&92, 62)));

assert!(cell.get().is_some());

Gets the contents of the cell, initializing it with f if the cell was empty.

Many threads may call get_or_init concurrently with different initializing functions, but it is guaranteed that only one function will be executed.

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. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

Example
use once_cell::sync::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. The exact outcome is unspecified. Current implementation deadlocks, but this may be changed to a panic in the future.

Example
use once_cell::sync::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))

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.

Examples
use once_cell::sync::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);

This method is allowed to violate the invariant of writing to a OnceCell at most once because it requires &mut access to self. As with all interior mutability, &mut access permits arbitrary modification:

use once_cell::sync::OnceCell;

let mut cell: OnceCell<u32> = OnceCell::new();
cell.set(92).unwrap();
cell = OnceCell::new();

Consumes the OnceCell, returning the wrapped value. Returns None if the cell was empty.

Examples
use once_cell::sync::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()));

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
Converts to this type from the input type.
This method tests for self and other values to be equal, and is used by ==. Read more
This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more
Converts to this type from the input type.

Returns the argument unchanged.

Calls U::from(self).

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

The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning. Read more
Uses borrowed data to replace owned data, usually by cloning. Read more
The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.