Fast, initializable, and thread safe static variables
Borrows the excellent ZST based tagging implementation to guarantee cells are initialized exactly once before an access is attempted.
This is implemented via the [TaggedCell] and a Tag type, which must be unique for each instance of the [TaggedCell] for safe operation. The [TaggedCell] must then be set up via [init()][TaggedCell::init], which initializes the underlying data using a user provided function or closure, and then returns a special zero-sized [Init] tag used to access the Cell's data.
use tagged_cell::TaggedCell;
struct FooTag;
static FOO: TaggedCell<usize, FooTag> = unsafe {TaggedCell::new()};
// Initialize the cell's data and retrieve a tag
let tag = FOO.init(|| 27);
// The tag is required to obtain a shared reference to the data
assert_eq!(*FOO.get(tag), 27);
To ensure unique tag types are used for each cell, and to 'wrap' the unsafe call, 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);
When unique tag types are used, attempting to access a [TaggedCell] before it is initialized will cause a compilation error.
use tagged_cell::tagged_cell;
tagged_cell!{
static BAZ: TaggedCell<usize>, _> = TaggedCell::new();
}
tagged_cell!{
static QUX: TaggedCell<usize>, _> = TaggedCell::new();
}
// read before init is not possible
BAZ.get(Init{BAZ::TagType});
let qux_tag = QUX.init(|| 35);
// using the wrong tag throws an error
BAZ.get(qux_tag);
To allow for usage across threads, only the first invocation of [init()][TaggedCell::init] will initialize the Cell's data. All future [init()][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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