Rc Reference Counted Smart Pointer

Used for multiple ownership e.g. if multiple edges are pointing to a value in a graph.

Cons list with reference counting

Here a is referenced by b and c

enum List {
    Cons(i32, Rc<List>),

use crate::List::{Cons, Nil};
use std::rc::Rc;

fn main() {
    let a = Rc::new(Cons(5, Rc::new(Cons(10, Rc::new(Nil)))));
    println!("count after creating a = {}", Rc::strong_count(&a));
    let b = Cons(3, Rc::clone(&a));
    let c = Cons(4, Rc::clone(&a));

    println!("{:?}\n{:?}\n{:?}", a, b, c);

Rc::clone(&a) doesn't create a deep copy, it only increments the reference count. It's the same as calling a.clone(), but most implementations create a deep copy, so by convention we distinguish it's coming from Rc.

Rc::strong_count shows the amount of references.