Demonstration of Rc and Weak references

rc.rs

use std::cell::RefCell;
use std::rc::Weak;

#[derive(Debug)]
pub struct Node<T> {
    data: T,
    // must be a RefCell because we want to store non Copy type `Vec`
    // while allowing interior mutability (necessary for this cyclic type)
    // this doesn't necessarily need to be weak, it is just for learning
    // purposes
    edges: RefCell<Vec<Weak<Node<T>>>>,
}

impl<T> Node<T> {
    pub fn new(new_data: T) -> Node<T> {
        Node {
            data: new_data,
            edges: RefCell::new(vec![]),
        }
    }

    pub fn new_with_children(new_data: T, children: Vec<Weak<Node<T>>>) -> Node<T> {
        Node {
            data: new_data,
            edges: RefCell::new(children),
        }
    }

    pub fn add_child(&self, node: Weak<Node<T>>) {
        self.edges.borrow_mut().push(node);
    }

    pub fn find(&self, query: &T) -> bool
    where
        T: Eq,
    {
        if self.data == *query {
            return true;
        }
        for n in self.edges.borrow().iter() {
            if let Some(i) = n.upgrade() {
                if i.find(query) {
                    return true;
                }
            } else {
                println!("no strong references to this value so it was dropped")
            }
        }
        return false;
    }
}

#[cfg(test)]
mod tests {
    use crate::Node;
    use std::rc::Rc;

    #[test]
    fn find_test() {
        let n1 = Node::new(1);
        let n3 = Node::new(3);

        let rc3 = Rc::new(n3);

        // the first element in `children` will not be upgradable because
        // there is no strong references to it. The second element in
        // children has `rc3` as a strong reference so it won't be dropped until
        // the test function terminates
        let graph = Rc::new(Node::new_with_children(
            5,
            vec![Rc::downgrade(&Rc::new(n1)), Rc::downgrade(&rc3)],
        ));
        println!("{:?}", graph);

        let result = graph.find(&1);
        assert_eq!(true, result);
    }
}