Hacker News Reader

Comment by baby | Hacker News Reader

so const Mutex means you can do this IIUC:

    const THING: Mutex<u64> = Mutex::new(4);

without using the once_cell crate. This is pretty cool! Also I realize that the Rust playground is still using 1.62.0

https://play.rust-lang.org/?version=beta&mode=debug&edition=...

View on HN

Wouldn’t you want to use static instead of const, otherwise you might get a new unique mutex at each usage? I think there’s a Clippy lint for that.

The playground rebuilds overnight (roughly after the nightly is released); releases don’t have as fixed timing so I tend to trigger a rebuild when I notice. This HN post was delivered before the git tag was published even, so I’ve started it.

Playground is updated now.

Yes. If I say FOUR is a const Mutex with a 4 in it, whenever I talk about FOUR I get a completely new unlocked Mutex with a 4 in it, unrelated to any other Mutex regardless of whether it was made the same way.

This might be what you want if you, for some reason, need to make different Mutexes with a 4 in them all the time. But, if you actually meant one Mutex, which intially has a 4 in it, then that's a static variable not a constant.

https://play.rust-lang.org/?version=beta&mode=debug&edition=...

Change FOUR to be static instead of const, and now when we print y each time it's gone up by ten because we're using the same Mutex which is what we presumably intended.

What's the difference between CONST and STATIC fundamentally?

A const is like a `#define` in C or C++, while a static is a global variable.

If those terms mean anything to you: const is an rvalue, static is an lvalue.

Reply on news.ycombinator.com