I was looking for something that would compile to the magical cmpxchg instruction. After digging into the documentation, I couldn't find anything that would accomplish this for Cell.
Maybe this is an antipattern?
After digging into the code I added the following to Cell's implementation to see if it would work.
pub fn cmp_and_set(&self, old: T, new: T) -> T {
unsafe {
::intrinsics::atomic_cxchg(self.value.get(), old, new)
}
}
// ^ there are many many reasons why this is not so great
// this is just an example of what I'm looking for
simple usage
fn main() {
let c0 = Cell::new(10);
let val0 = c0.cmp_and_set(11, 5);
assert_eq!(c0.get(), 5);
let val1 = c0.cmp_and_set(10, 42);
assert_eq!(c0.get(), 42);
}
As far as I can tell, for very basic cases it works, but again there are many many reasons why the particular implementation is less than stellar. The fact that I edited the standard library to get what I was looking for means I'm certainly attempting to implement some sort of antipattern.
This was prompted from re-reading the following from The Rust Programming Language
It is still possible to violate your own invariants using this wrapper, so be careful when using it. If a field is wrapped in
Cell, it's a nice indicator that the chunk of data is mutable and may not stay the same between the time you first read it and when you intend to use it.
TL;DR: No, there's not, because it's unnecessary.
Compare and Set is only valuable in two actors (or more) are modifying the object in parallel.
While Cell allows internal mutability, it is not thread-safe, so you will never be in a situation where two actors will be attempting to modify it in parallel.
Therefore, you can just use get(), compare, and set() if it suits you. And nobody will change the value between your get() and set(), providing you do not call other code yourself.
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