57 lines
2.0 KiB
Markdown
57 lines
2.0 KiB
Markdown
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<p align="center"><img src="design/logo.png" alt="once_cell"></p>
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[](https://github.com/matklad/once_cell/actions)
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[](https://crates.io/crates/once_cell)
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[](https://docs.rs/once_cell/)
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# Overview
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`once_cell` provides two new cell-like types, `unsync::OnceCell` and `sync::OnceCell`. `OnceCell`
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might store arbitrary non-`Copy` types, can be assigned to at most once and provide direct access
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to the stored contents. In a nutshell, API looks *roughly* like this:
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```rust
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impl OnceCell<T> {
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fn new() -> OnceCell<T> { ... }
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fn set(&self, value: T) -> Result<(), T> { ... }
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fn get(&self) -> Option<&T> { ... }
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}
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```
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Note that, like with `RefCell` and `Mutex`, the `set` method requires only a shared reference.
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Because of the single assignment restriction `get` can return an `&T` instead of `Ref<T>`
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or `MutexGuard<T>`.
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`once_cell` also has a `Lazy<T>` type, build on top of `OnceCell` which provides the same API as
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the `lazy_static!` macro, but without using any macros:
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```rust
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use std::{sync::Mutex, collections::HashMap};
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use once_cell::sync::Lazy;
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static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
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let mut m = HashMap::new();
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m.insert(13, "Spica".to_string());
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m.insert(74, "Hoyten".to_string());
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Mutex::new(m)
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});
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fn main() {
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println!("{:?}", GLOBAL_DATA.lock().unwrap());
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}
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```
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More patterns and use-cases are in the [docs](https://docs.rs/once_cell/)!
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# Related crates
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* [double-checked-cell](https://github.com/niklasf/double-checked-cell)
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* [lazy-init](https://crates.io/crates/lazy-init)
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* [lazycell](https://crates.io/crates/lazycell)
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* [mitochondria](https://crates.io/crates/mitochondria)
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* [lazy_static](https://crates.io/crates/lazy_static)
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The API of `once_cell` is being proposed for inclusion in
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[`std`](https://github.com/rust-lang/rfcs/pull/2788).
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