CODEWITHSUNDEEP
rust
Fred Hors
Fred Hors

Reputation: 4116

Arc<Struct> with fields as HashMap, cannot borrow as mutable, why?

I'm using this struct:

pub struct Store {
  pub player: HashMap<i64, Player>,
  pub team: HashMap<i64, Team>,
}

impl Store {
  pub fn new() -> Arc<Self> {
    Arc::new(Self {
      player: HashMap::new(),
      team: HashMap::new(),
    })
  }
}

sharing it in my resolvers with:

async fn player_by_id(&self, store: Arc<Store>, id: i64) -> Result<()> {
  let team = get_team();

  store.team.insert(id, team.into()); // here I get the error
}

I get the error:

error[E0596]: cannot borrow data in an `Arc` as mutable
   |
84 |         store.team.insert(id, team.into());
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable
   |
   = help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `Arc<Store>`

Why?

Upvotes: 0

Views: 761

Answers (1)

Chayim Friedman
Chayim Friedman

Reputation: 70850

As explained in Rc's documentation,

Shared references in Rust disallow mutation by default, and Rc is no exception: you cannot generally obtain a mutable reference to something inside an Rc.

Arc is the same as Rc in this regard. You need to use a Mutex or RwLock (or atomics) to change its contents.

Upvotes: 3

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