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 GLOBALDATA: Lazy>> = Lazy::new(|| { let mut m = HashMap::new(); m.insert(13, "Spica".tostring()); 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!

Overview

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