Rust：在 hashmap 参考中插入 uint

Question

I am new to Rust and I have this error at compilation time but I don't get it

error[E0614]: type `Option<u32>` cannot be dereferenced
 --> src/main.rs:9:5
  |
9 |     *mymap.insert("hello world", 0);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Here is my code simplified to reproduce the issue:

use std::collections::HashMap;

fn main() {
    let mymap: HashMap<&str, u32> = HashMap::new();
    f(&mymap)
}

fn f(mymap: &HashMap<&str, u32>) {
    *mymap.insert("hello world", 0);
}

Also the following does not work either

*mymap.insert("hello world", &0);

I don't find the root cause of my problem by googling it, I think I don't have the words. It looks like some borrowing issue.

Answer 1

You're actually not dereferencing mymap , you're actually dereferencing the result of insert() , because dereferencing (ie * ) have a weaker precedence than a method call .

So it complains about dereferencing an Option<u32> because that's what insert() returns. If you actually wanted to dereference mymap you'd have to write (*mymap).insert("hello world", 0); . However, that is not needed in Rust.

If you remove the * then you will get a second problem, which is that you're attempting to mutate mymap , which is an immutable reference. So to be able to insert, ie mutate mymap in f , you need to pass it a mutable reference, ie mymap: &mut HashMap<&str, u32> .

use std::collections::HashMap;

fn main() {
    let mut mymap: HashMap<&str, u32> = HashMap::new();
    f(&mut mymap)
}

fn f(mymap: &mut HashMap<&str, u32>) {
    mymap.insert("hello world", 0);
}

Answer 2

You don't need to dereference references on method calls, Rust will automatically do that for you. Also, you need to pass a mutable reference of mymap so you can actually perform the insert . Fixed example:

use std::collections::HashMap;

fn main() {
    let mut mymap: HashMap<&str, u32> = HashMap::new();
    f(&mut mymap)
}

fn f(mymap: &mut HashMap<&str, u32>) {
    mymap.insert("hello world", 0);
}

playground