double-checked-cell

A thread-safe lazily initialized cell using double-checked locking.

Introduction

Provides a memory location that can be safely shared between threads and initialized at most once. Once the cell is initialized it becomes immutable.

If you do not need to change the value after initialization DoubleCheckedCell<T> is more efficient than a Mutex<Option<T>>.

extern crate double_checked_cell;

use double_checked_cell::DoubleCheckedCell;

fn main() {
    let cell = DoubleCheckedCell::new();

    // The cell starts uninitialized.
    assert_eq!(cell.get(), None);

    // Perform potentially expensive initialization.
    let value = cell.get_or_init(|| 21 + 21);
    assert_eq!(*value, 42);
    assert_eq!(cell.get(), Some(&42));

    // The cell is already initialized.
    let value = cell.get_or_init(|| {
        panic!("initilization does not run again")
    });
    assert_eq!(*value, 42);
    assert_eq!(cell.get(), Some(&42));
}

Related crates

These crates are similar but distinct by design:

lazy-init – Based on a LazyTransform<T, U> which can lazily consume T to produce an U. Therefore can not support fallible initialization.
lazycell – AtomicLazyCell does not support lazy initialization (unlike its non-thread-safe counterpart LazyCell using LazyCell::borrow_with()).

Documentation

Read the documentation

Changelog

1.1.0
- Fix unsoundness: DoubleCheckedCell<T> where T: !Send cannot be Sync.
1.0.1
- Ignore unused_unsafe warning due to UnsafeCell::into_inner() no longer beeing unsafe.
1.0.0
- Initial release.

License