[](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 { fn new() -> OnceCell { ... } 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` or `MutexGuard`. `once_cell` also has a `Lazy` 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>> = 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) The API of `once_cell` is being proposed for inclusion in [`std`](https://github.com/rust-lang/rfcs/pull/2788).