What is the right smart pointer to have multiple strong references and allow mutability?

Question

I want to have a structure on the heap with two references; one for me and another from a closure. Note that the code is for the single-threaded case:

use std::rc::Rc;

#[derive(Debug)]
struct Foo {
    val: u32,
}
impl Foo {
    fn set_val(&mut self, val: u32) {
        self.val = val;
    }
}
impl Drop for Foo {
    fn drop(&mut self) {
        println!("we drop {:?}", self);
    }
}

fn need_callback(mut cb: Box<FnMut(u32)>) {
    cb(17);
}

fn create() -> Rc<Foo> {
    let rc = Rc::new(Foo { val: 5 });
    let weak_rc = Rc::downgrade(&rc);
    need_callback(Box::new(move |x| {
        if let Some(mut rc) = weak_rc.upgrade() {
            if let Some(foo) = Rc::get_mut(&mut rc) {
                foo.set_val(x);
            }
        }
    }));
    rc
}

fn main() {
    create();
}

In the real code, need_callback saves the callback to some place, but before that may call cb as need_callback does.

The code shows that std::rc::Rc is not suitable for this task because foo.set_val(x) is never called; I have two strong references and Rc::get_mut gives None in this case.

What smart pointer with reference counting should I use instead of std::rc::Rc to make it possible to call foo.set_val? Maybe it is possible to fix my code and still use std::rc::Rc?

After some thinking, I need something like std::rc::Rc, but weak references should prevent dropping. I can have two weak references and upgrade them to strong when I need mutability.

Because it is a singled-threaded program, I will have only strong reference at a time, so everything will work as expected.

Answer 1

Rc (and its multithreaded counterpart Arc) only concern themselves with ownership. Instead of a single owner, there is now joint ownership, tracked at runtime.

Mutability is a different concept, although closely related to ownership: if you own a value, then you have the ability to mutate it. This is why Rc::get_mut only works when there is a single strong reference - it's the same as saying there is a single owner.

If you need the ability to divide mutability in a way that doesn't match the structure of the program, you can use tools like Cell or RefCell for single-threaded programs:

use std::cell::RefCell;

fn create() -> Rc<RefCell<Foo>> {
    let rc = Rc::new(RefCell::new(Foo { val: 5 }));
    let weak_rc = Rc::downgrade(&rc);
    need_callback(move |x| {
        if let Some(rc) = weak_rc.upgrade() {
            rc.borrow_mut().set_val(x);
        }
    });
    rc
}

Or Mutex, RwLock, or an atomic type in multithreaded contexts:

use std::sync::Mutex;

fn create() -> Rc<Mutex<Foo>> {
    let rc = Rc::new(Mutex::new(Foo { val: 5 }));
    let weak_rc = Rc::downgrade(&rc);
    need_callback(move |x| {
        if let Some(rc) = weak_rc.upgrade() {
            if let Ok(mut foo) = rc.try_lock() {
                foo.set_val(x);
            }
        }
    });
    rc
}

These tools all defer the check that there is only a single mutable reference to runtime, instead of compile time.