Interior Mutability Concepts

Cell

Mutating a Copy Value

Cell<T> lets code update a small Copy value through a shared owner.

Program

Play the program to choose the starting count, store it in a Cell, and update it without making the binding mutable.

cell_counter.rs 
use std::cell::Cell;

fn main() {
    let start = ;
    let counter = Cell::new(start);
    counter.set(counter.get() + 2);
    let current = counter.get();
    println!("{current}");
}

use std::cell::Cell;

fn main() {
    let start = ;
    let counter = Cell::new(start);
    counter.set(counter.get() + 2);
    let current = counter.get();
    println!("{current}");
}

use std::cell::Cell;

fn main() {
    let start = ;
    let counter = Cell::new(start);
    counter.set(counter.get() + 2);
    let current = counter.get();
    println!("{current}");
}
Cell<T> `Cell<T>` supports interior mutation for values that can be copied in and out.
shared owner The `counter` binding is not `mut`; the cell owns the mutation operation.
get and set `get` copies the current value, and `set` replaces it.