Merge "[LSC] Add LOCAL_LICENSE_KINDS to external/rust/crates/once_cell"
tree: 6e7343b6e19bf6a035fc087285e5cbbc291b9bdb
  1. .cargo_vcs_info.json
  2. .github/
  3. .gitignore
  4. Android.bp
  5. CHANGELOG.md
  6. Cargo.toml
  7. Cargo.toml.orig
  8. LICENSE-APACHE
  9. LICENSE-MIT
  10. METADATA
  11. MODULE_LICENSE_APACHE2
  12. OWNERS
  13. README.md
  14. TEST_MAPPING
  15. bors.toml
  16. examples/
  17. patches/
  18. src/
  19. tests/
README.md

Build Status Crates.io API reference

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:

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!

Related crates

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