Rust Programming: Hash Maps and Ownership

For types that implement the Copy trait, like i32, the values are copied into the hash map. This means that the hash map will have its own copy of the value, and the original value will still be available.

For owned values like String, the values will be moved and the hash map will be the owner of those values. This means that the hash map will take ownership of the value, and the original value will no longer be available.

Example:

Rust

use std::collections::HashMap;

fn main() {
    let mut map = HashMap::new();

    // Insert a copy of an i32 value.
    map.insert(1, 2);

    // Insert an owned String value.
    let field_name = String::from("Favorite color");
    map.insert(2, field_name);

    // We can still use the i32 value.
    println!("{:?}", map.get(&1)); // Outputs: Some(2)

    // We can't use the String value anymore.
    // println!("{:?}", map.get(&2)); // Error: cannot borrow `field_name` as mutable because it is also borrowed as immutable

    // We can get a reference to the String value, but we can't modify it.
    let field_value = map.get(&2).unwrap();
    println!("{}", field_value); // Outputs: "Favorite color"
}

Why does HashMap take ownership of owned values?

There are a few reasons why HashMap takes ownership of owned values. First, it makes the code more efficient. If HashMap didn't take ownership of owned values, it would need to copy them every time the hash map was resized or rehashed. Second, it helps to prevent memory leaks. If HashMap didn't take ownership of owned values, it would be possible for the programmer to forget to drop the values, which would lead to a memory leak.

How to deal with ownership when using HashMap

There are a few things you can do to deal with ownership when using HashMap:

• Use types that implement the Copy trait for the keys and values in your hash map. This will allow the hash map to copy the values, and you will still be able to use the original values.
• Use references to owned values for the keys and values in your hash map. This will allow you to use the owned values outside of the hash map, but you must make sure that the owned values are valid for at least as long as the hash map is valid.
• Use a smart pointer, such as Arc or Rc, to share ownership of owned values between the hash map and other parts of your code.