Update/re-assign struct fields without taking &mut self?

Question

I have the following scenario:

struct Foo {
    complex_data: RwLock<ComplexData>,
    instant: Instant,
}

impl Foo {
    update(&self) {
        let mut complex_data = self.complex_data.write();
        complex_data.mut_update();
        // I can't re-assign Instant since I'm only taking `&self` not `&mut self` 
    }
}

I want to be able to re-assign the value of instant inside the update method without changing update to take &mut self. What's the best way to do this? (Is it to use a Box?)

You don't. That's literally the point of the `mut` mechanism. You're not supposed to. — Silvio Mayolo, Mar 28 '21 at 23:44
Does this answer your question? [Situations where Cell or RefCell is the best choice](https://stackoverflow.com/questions/30831037/situations-where-cell-or-refcell-is-the-best-choice) — Stargateur, Mar 29 '21 at 00:03
@SilvioMayolo Have you heard of concurrent data structures? Sometimes... you want to call a method on a struct in different threads! — Foobar, Mar 29 '21 at 00:15
@Stargateur I read it over, but I haven't been able to implement a non-`Box` solution from the materials present. — Foobar, Mar 29 '21 at 01:06
@Roymunson I didn't use Cell much so I can't tell, anyway the point is that if you don't need share this feature between thread you should use Cell not Mutex. The point of Cell is having interior mutability. Your question is not very clear so It's hard to answer you perfectly. Your use case of update a time is often a good use case for interior mutability so I proposed to use Cell. — Stargateur, Mar 29 '21 at 01:12

Foobar · Answer 1 · 2021-03-29T00:30:37.220

0

I ended up using a Mutex<Box<Instant>>, but I'm not sure if there's a better way. Forcing something to be on the heap seems unnecessary/slow?

struct Foo {
    complex_data: RwLock<ComplexData>,
    instant: Mutex<Instant>,
}

impl Foo {
    update(&self) {
        let mut complex_data = self.complex_data.write();
        complex_data.mut_update();
        // I can't re-assign Instant since I'm only taking `&self` not `&mut self` 
        let mut instant = self.instant.lock();
        *instant = Box::new(Instant::now());
    }
}

edited Mar 29 '21 at 00:30

answered Mar 28 '21 at 23:42

Foobar

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BTW heap is not slow but Mutex is. – Stargateur Mar 29 '21 at 00:09
Yes, I suppose the overhead from the `Mutex` is much more than the overhead from the `Box` – Foobar Mar 29 '21 at 00:31
It doesn't seem that a `Box` is necessary, but without more context in the form of a [mre], it's impossible to say for sure. – trent Mar 29 '21 at 14:05

Update/re-assign struct fields without taking &mut self?

1 Answers1