RefCell and interior mutability in Rust

RefCell moves the borrow check to run time, so a shared value can still be mutated.

use std::cell::RefCell;

struct Log {
    entries: RefCell<Vec<String>>,
}

impl Log {
    fn record(&self, line: &str) {
        self.entries.borrow_mut().push(line.to_string());
    }

    fn count(&self) -> usize {
        self.entries.borrow().len()
    }
}

fn main() {
    let log = Log { entries: RefCell::new(Vec::new()) };
    log.record("started");
    log.record("finished");
    println!("{} entries", log.count());
    println!("{:?}", log.entries.borrow());
}

How it works

1. borrow_mut panics if a borrow is already outstanding.
2. The value itself needs no mut binding.
3. Keep the borrows short to keep the panics away.

Keywords and builtins used here

• Log
• RefCell
• String
• Vec
• count
• fn
• impl
• let
• main
• record
• self
• str
• struct
• use
• usize

The run, in numbers

Lines

23

Characters to type

417

Tokens

147

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 80 seconds.

Type this snippet

Step 1 of 3 in Interior mutability, step 7 of 11 in Lifetimes & interior mutability.

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