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:
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:
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!
The API of once_cell is being proposed for inclusion in
std.
