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use std::cell::UnsafeCell;
use std::sync::Mutex;
pub struct LazyOnce<T> {
cell: Mutex<UnsafeCell<Option<T>>>,
}
impl<T> LazyOnce<T> {
pub fn new() -> Self {
Self {
cell: Mutex::new(UnsafeCell::new(None)),
}
}
#[must_use]
pub fn get(&self, f: impl FnOnce() -> T) -> &T {
let cell = self.cell.lock().unwrap();
unsafe {
(*cell.get()).get_or_insert_with(f)
}
}
#[must_use]
pub fn try_get(&self) -> Option<&T> {
match self.cell.try_lock() {
Ok(cell) => {
unsafe {
(*cell.get()).as_ref()
}
},
Err(_) => None,
}
}
#[must_use]
pub fn get_mut(&mut self, f: impl FnOnce() -> T) -> &mut T {
let cell = self.cell.lock().unwrap();
unsafe {
(*cell.get()).get_or_insert_with(f)
}
}
pub fn expect(&self, s: &str) -> &T {
let cell = self.cell.lock().unwrap();
unsafe {
(*cell.get()).as_ref().expect(s)
}
}
#[must_use]
pub fn into_inner(self) -> Option<T> {
let cell = self.cell.lock().unwrap();
unsafe {
(*cell.get()).take()
}
}
}
#[test]
fn test_get() {
let l = LazyOnce::new();
assert_eq!(None, l.try_get());
assert_eq!(1, *l.get(|| 1u8));
assert_eq!(1, *l.get(|| 2u8));
assert_eq!(1, *l.try_get().unwrap());
assert_eq!(1, *l.get(|| 3u8));
assert_eq!(1, *l.expect("set"));
}
#[test]
fn test_recursive() {
let l = LazyOnce::new();
let _ = *l.get(|| {
assert!(l.try_get().is_none());
1u8
});
}
#[test]
fn test_mut() {
let mut l = LazyOnce::new();
assert_eq!(1, *l.get_mut(|| 1u8));
assert_eq!(1, *l.get(|| 2u8));
assert_eq!(1, *l.get_mut(|| 3u8));
*l.get_mut(|| 3u8) = 4;
assert_eq!(4, *l.get_mut(|| 3u8));
assert_eq!(4, *l.try_get().unwrap());
assert_eq!(4, *l.expect("set"));
assert_eq!(4, l.into_inner().unwrap());
}
#[test]
fn test_send_and_sync() {
let l = LazyOnce::new();
::std::thread::spawn(move || {
let _ = l.get(|| "hi");
});
fn is_sync<T: Sync>(_: T) {}
is_sync(&LazyOnce::<u8>::new());
}