<p align="center"><img src="design/logo.png" alt="once_cell"></p>
[](https://github.com/matklad/once_cell/actions)
[](https://crates.io/crates/once_cell)
[](https://docs.rs/once_cell/)
# Overview
`once_cell` provides two new cell-like types, `unsync::OnceCell` and `sync::OnceCell`. `OnceCell`
might store arbitrary non-`Copy` types, can be assigned to at most once and provide direct access
to the stored contents. In a nutshell, API looks *roughly* like this:
```rust
impl OnceCell<T> {
fn new() -> OnceCell<T> { ... }
fn set(&self, value: T) -> Result<(), T> { ... }
fn get(&self) -> Option<&T> { ... }
}
```
Note that, like with `RefCell` and `Mutex`, the `set` method requires only a shared reference.
Because of the single assignment restriction `get` can return an `&T` instead of `Ref<T>`
or `MutexGuard<T>`.
`once_cell` also has a `Lazy<T>` type, build on top of `OnceCell` which provides the same API as
the `lazy_static!` macro, but without using any macros:
```rust
use std::{sync::Mutex, collections::HashMap};
use once_cell::sync::Lazy;
static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
let mut m = HashMap::new();
m.insert(13, "Spica".to_string());
m.insert(74, "Hoyten".to_string());
Mutex::new(m)
});
fn main() {
println!("{:?}", GLOBAL_DATA.lock().unwrap());
}
```
More patterns and use-cases are in the [docs](https://docs.rs/once_cell/)!
# Related crates
* [double-checked-cell](https://github.com/niklasf/double-checked-cell)
* [lazy-init](https://crates.io/crates/lazy-init)
* [lazycell](https://crates.io/crates/lazycell)
* [mitochondria](https://crates.io/crates/mitochondria)
* [lazy_static](https://crates.io/crates/lazy_static)
* [async_once_cell](https://crates.io/crates/async_once_cell)
* [generic_once_cell](https://crates.io/crates/generic_once_cell) (bring your own mutex)
Parts of `once_cell` API are included into `std` [as of Rust 1.70.0](https://github.com/rust-lang/rust/pull/105587).