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this post was submitted on 15 Jun 2023
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Programming
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Only for things that you specifically want shared between threads – namely this (synchronized refcount) is an
std::sync::Arc. What you want to share really depends on the app; in database-backed web services it's quite common to have pretty much zero state shared across threads. Multithreaded environment doesn't imply sharing!The refcount absolutely is shared state across threads.
If Thread#1 thinks the refcount is 5, but Thread#2 thinks the refcount is 0, you've got problems.
Again, only for things that you specifically want shared between threads.
There's no "the" refcount in Rust, anyhow. If you just instantiate some container or your custom data struct, like
let mut x = Vec::new();– it's very local to where you are, it's on the stack, it's not reference counted at runtime at all, you cannot pass it between threads (if it's notSendit cannot EVER cross a thread boundary in safe Rust). The standard library provides two ref-counter containers.Rcis just a basic refcount that is not thread-safe and thus also is notSendand won't ever be allowed to cross the thread boundary in safe Rust.Arcimplements atomic-based thread-safe ref-counting and thus isSend, implementing what you're talking about, but as an opt-in per-object container, not as some behind-the-scenes global feature.