interior_mutability_example.rs

use std::cell::Cell;

#[derive(Debug)]
struct Ab {
    a: i32,
    b: i32,
    cache: Cell<Option<i32>>
}

impl Ab {
    fn new(a: i32, b: i32) -> Self {
        Self { a, b, cache: Cell::new(None)}
    }

    fn a_plus_b(&self) -> i32 {
        match self.cache.get() {
            None => {
                let sum = self.a + self.b;
                println!("Calculating sum");
                self.cache.set(Some(sum));
                sum
            },
            Some(val) => {
                println!("Using cached value");
                val
            }
        }
    }
}


fn main() {
    let ab = Ab::new(4, 7);
    println!("ab = {:?}", ab);
    println!("ab.a_plus_b() = {:?}", ab.a_plus_b());
    println!("ab.a_plus_b() = {:?}", ab.a_plus_b());
    println!("ab = {:?}", ab);
}

// output:
//   ab = ab { a: 4, b: 7, cache: cell { value: none } }
//   Calculating sum
//   ab.a_plus_b() = 11
//   using cached value
//   ab.a_plus_b() = 11
//   ab = ab { a: 4, b: 7, cache: cell { value: some(11) } }