quest

Borrow checker Advanced

28  Interior mutability

Sometimes a value must change behind a shared reference -- a counter, a cache, a shared log. Cell and RefCell move the borrow rules to runtime; Rc<RefCell<..>> is the classic shared-mutable combo.

Fix Tracker so record can count through &self: store the hits in a Cell.

use std::cell::Cell;

// BROKEN: record takes &self but tries to mutate. Change `hits` to a
// Cell<u32> so it can update through a shared reference.
struct Tracker { hits: u32 }

impl Tracker {
    fn record(&self) {
        self.hits += 1;
    }
    fn hits(&self) -> u32 {
        self.hits
    }
}

use std::cell::Cell;

struct Tracker { hits: Cell<u32> }

impl Tracker {
    fn record(&self) {
        self.hits.set(self.hits.get() + 1);
    }
    fn hits(&self) -> u32 {
        self.hits.get()
    }
}