Volatile data in Rust

main.rs

/// Code example of how to have mutable data in an object passed by ref
/// The use case of this snippet is to cache some computed data

use core::cell::RefCell;

struct Cat {
    age: RefCell<Option<i32>>
}


fn display_age(cat: &Cat) {
    if cat.age.borrow().is_none() {
        let mut age_mut = cat.age.borrow_mut();
        *age_mut = Some(7);
    }
    
    print!("The cat is {} yo\n", cat.age.borrow().unwrap());
}


fn main() {
    let c = Cat { age: RefCell::new(None) };
    let felix = Cat { age: RefCell::new(Some(88)) };
    
    display_age(&c);
    display_age(&felix);
}