Cell<T>

Learn interior mutability for Copy types with Cell<T>.

Cell<T>

Cell<T> is the lightest form of interior mutability. It gives no references; it takes the value as a whole (get, Copy types only) or puts one in its place (set, replace). Since there is no borrow counting, it never panics:

use std::cell::Cell;

struct Sayac {
    n: Cell<u32>,
}

impl Sayac {
    fn artir(&self) {          // &self is enough!
        self.n.set(self.n.get() + 1);
    }
}

Cell or RefCell?

For small Copy values (counters, flags), Cell is cheaper and safer. If you need a reference to the inner value (pushing to a vector, editing a String), RefCell is needed:

let bayrak = Cell::new(false);
bayrak.set(true);
let eski = bayrak.replace(false);   // returns true
println!("{} {}", eski, bayrak.get());   // true false

Exercise Counter with Cell

Write the Sayac type above with Cell<u32>. For let s = Sayac { n: Cell::new(0) }; (without mut), call artir three times and print the value.

Show example solution

use std::cell::Cell;

struct Sayac {
    n: Cell<u32>,
}

impl Sayac {
    fn artir(&self) {
        self.n.set(self.n.get() + 1);
    }
}

fn main() {
    let s = Sayac { n: Cell::new(0) };
    s.artir();
    s.artir();
    s.artir();
    println!("{}", s.n.get());
}

Test yourself

Can <code>get()</code> be called on a <code>Cell<String></code>?
- Yes, it returns a reference
- It panics
- No; get exists only for Copy types. Use take() or replace()
- Yes, it returns a copy
Show answer

No; get exists only for Copy types. Use take() or replace() — Since Cell can't give references, a non-Copy value can only be taken out by putting another value in its place.

Put the value

fn artir(&self) {
    self.n.____(self.n.get() + 1);
}

Show answer

fn artir(&self) {
    self.n.set(self.n.get() + 1);
}

Method and job

get()

Gives a copy
set(x)

Puts it in place
replace(x)

Returns the old one
take()

Leaves the default

Next lesson: External mutation

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