Lazy Init

OnceCell⮳ is a cell which can be written to only once.

The corresponding Sync version of OnceCell<T> is OnceLock<T>.

use std::cell::OnceCell;

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

    let value: &String = cell.get_or_init(|| "Hello, World!".to_string());
    assert_eq!(value, "Hello, World!");
    assert!(cell.get().is_some());
}

Older library

(lib.rs)⮳

once_cell provides two cell-like types, unsync::OnceCell and sync::OnceCell. A OnceCell might store arbitrary non-Copy types, can be assigned to at most once and provides direct access to the stored contents. The sync flavor is thread-safe.

once_cell also has a Lazy<T> type, build on top of OnceCell:

use std::collections::HashMap;
use std::sync::Mutex;

use once_cell::sync::Lazy;

// must be static, not const
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());
}

See also