tagged_cell
Fast, initializable, and thread safe static variables
Overview
Borrows the excellent ZST based tagging implementation (linked below) to guarantee the cell is initialized exactly once before an access is attempted. https://www.hardmo.de/article/2021-03-14-zst-proof-types.md#proof-of-work
This is implemented via TaggedCell and a Tag type, which must be unique for each instance of the TaggedCell for safe operation. The TaggedCell must then be initialized via TaggedCell::init(), which initializes the underlying data using a user provided function or closure, and then returns a special zero-sized Init
used to access the Cell's data.
To ensure unique tag types are used for each cell, the tagged_cell! macro is provided. The macro creates a new tag type based on the variable's name, and applies it in the declaration.
use tagged_cell::tagged_cell;
tagged_cell!{
static BAR: TaggedCell<Vec<usize>, _> = TaggedCell::new();
}
let tag = BAR.init(|| vec![0, 10, 20]);
let vec = BAR.get(tag);
assert_eq!(vec[2], 20);
To allow for usage across threads, only the first invocation of TaggedCell::init will initialize the Cell's data. All future TaggedCell::init calls will just return a new tag. It is undetermined which thread will initialize the Cell's data.
use std::thread;
use tagged_cell::tagged_cell;
tagged_cell!{
static TABLE: TaggedCell<Vec<usize>, _> = TaggedCell::new();
}
thread::spawn(move || {
let tag = TABLE.init(|| vec![0, 10, 20]);
let table = TABLE.get(tag);
assert_eq!(table[2], 20);
});
thread::spawn(move || {
let tag = TABLE.init(|| vec![0, 10, 20]);
let table = TABLE.get(tag);
assert_eq!(table[1], 10);
});
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