How to access value in RefCell properly

Question

I'm trying to wrap my head around Rc and RefCell in Rust. What I'm trying to achieve is to to have multiple mutable references to the same objects.

I came up with this dummy code:

use std::rc::Rc;
use std::cell::RefCell;

struct Person {
    name: String,
    mother: Option<Rc<RefCell<Person>>>,
    father: Option<Rc<RefCell<Person>>>,
    partner: Option<Rc<RefCell<Person>>>
}

pub fn main () {

    let mut susan = Person {
        name: "Susan".to_string(),
        mother: None,
        father: None,
        partner: None
    };

    let mut boxed_susan = Rc::new(RefCell::new(susan));

    let mut john = Person {
        name: "John".to_string(),
        mother: None,
        father: None,
        partner: Some(boxed_susan.clone())
    };

    let mut boxed_john = Rc::new(RefCell::new(john));

    let mut fred = Person {
        name: "Fred".to_string(),
        mother: Some(boxed_susan.clone()),
        father: Some(boxed_john.clone()),
        partner: None
    };

    fred.mother.unwrap().borrow_mut().name = "Susana".to_string();

    println!("{}", boxed_susan.borrow().name);

    // boxed_john.borrow().partner.unwrap().borrow_mut().name = "Susanna".to_string();
    // println!("{}", boxed_susan.borrow().name);

}

The most interesting part is this:

    fred.mother.unwrap().borrow_mut().name = "Susana".to_string();
    println!("{}", boxed_susan.borrow().name)

I change the name of Freds mother and then print out the name of Susan which should happen to be exactly the same reference. And surprise, surprise it prints out "Susana" so I am assuming that my little experiment of having shared mutable references was successful.

However, now I wanted to mutate it again this time accessing it as the partner of John which should also happen to be exactly the same instance.

Unfortunately when I comment in the following two lines:

// boxed_john.borrow().partner.unwrap().borrow_mut().name = "Susanna".to_string();
// println!("{}", boxed_susan.borrow().name);

I'm running into my old friend cannot move out of dereference of&-pointer. What am I doing wrong here?

Zelmazelten

Answer 1

This will fix it:

boxed_john.borrow().partner.as_ref().unwrap().borrow_mut().name = "Susanna".to_string();

The problem is the unwrap() on the Option<Rc<RefCell>>, which consumes the Option (ie moves out of it), but you only have a borrowed pointer. The as_ref converts the Option(T) to Option(&T) and unwrap converts it to &T, avoiding any move.

Also note: your variables have a lot more mutability than they really need. But I'm sure you're already seeing the compiler warnings for that.

Mosra

This syntax makes me sad. – Gharry

@MichaelFulton The thing which provokes this sadness is contrived design. As Raph Levien says "your variables have a lot more mutability than they really need". – Slain