You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
57 lines
2.0 KiB
57 lines
2.0 KiB
4 months ago
|
<p align="center"><img src="design/logo.png" alt="once_cell"></p>
|
||
|
|
|
||
|
|
|
||
|
|
[](https://travis-ci.org/matklad/once_cell)
|
||
|
|
[](https://crates.io/crates/once_cell)
|
||
|
|
[](https://docs.rs/once_cell/)
|
||
|
|
|
||
|
|
# 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:
|
||
|
|
|
||
|
|
```rust
|
||
|
|
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:
|
||
|
|
|
||
|
|
```rust
|
||
|
|
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](https://docs.rs/once_cell/)!
|
||
|
|
|
||
|
|
# Related crates
|
||
|
|
|
||
|
|
* [double-checked-cell](https://github.com/niklasf/double-checked-cell)
|
||
|
|
* [lazy-init](https://crates.io/crates/lazy-init)
|
||
|
|
* [lazycell](https://crates.io/crates/lazycell)
|
||
|
|
* [mitochondria](https://crates.io/crates/mitochondria)
|
||
|
|
* [lazy_static](https://crates.io/crates/lazy_static)
|
||
|
|
|
||
|
|
The API of `once_cell` is being proposed for inclusion in
|
||
|
|
[`std`](https://github.com/rust-lang/rfcs/pull/2788).
|