Weak References

Weak<T> is a non-owning reference to Rc/Arc data - breaks cycles via upgrade() to Option<Rc/Arc>.

Recipe

use std::rc::{Rc, Weak};
 
struct Node { value: i32, parent: Option<Weak<Node>>, child: Option<Rc<Node>> }

When to reach for this: Parent pointers in trees/graphs without leaking Rc cycles.

Working Example

use std::rc::{Rc, Weak};
 
struct Owner { name: String }
struct Observer { owner: Weak<Owner> }
 
fn main() {
    let owner = Rc::new(Owner { name: "app".into() });
    let obs = Observer { owner: Rc::downgrade(&owner) };
    if let Some(o) = obs.owner.upgrade() {
        println!("{}", o.name);
    }
}

What this demonstrates:

• downgrade from Rc to Weak
• upgrade returns None if owner dropped
• Observer does not keep owner alive

Deep Dive

Cycles: A Rc-> B Rc-> A leak memory. Make one direction Weak. Arc::downgrade same for threads.

Gotchas

• Forgot upgrade check - None means gone. Fix: Handle absent owner.
• Upgrade keeps alive temporarily - Drop Rc from upgrade promptly if not needed.
• Weak without strong - Value dropped. Fix: Ensure some strong exists.
• Mutex weak patterns - Rare - usually Arc.
• upgrade in tight loop - Atomic cost on Arc.

Alternatives

FAQs

Related

• Rc & Arc
• Rc<RefCell<T>>
• Fighting the Borrow Checker
• Ownership Best Practices

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.