Rust Lifetime Issue with Threads [duplicate] - rust

Question

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Closed 4 years ago.
Here is the code:
use std::collections::HashMap;
use std::thread;
type MAP = HashMap<String, String>;
fn handle_n_times(count: i32, map: &mut MAP) {
for i in 0..count {
thread::spawn(move || {
map.insert(format!("key-{}", i), format!("value-{}", i));
});
}
}
fn main() {
let mut map: MAP = MAP::new();
handle_n_times(5, &mut map);
}
I cannot compile:
error[E0621]: explicit lifetime required in the type of `map`
--> src/main.rs:8:9
|
6 | fn handle_n_times(count: i32, map: &mut MAP) {
| -------- help: add explicit lifetime `'static` to the type of `map`: `&'static mut std::collections::HashMap<std::string::String, std::string::String>`
7 | for i in 0..count {
8 | thread::spawn(move || {
| ^^^^^^^^^^^^^ lifetime `'static` required
The hint it provides (add a &'static) is not helpful.
Question 1: How to make this code work?
Question 2: After 1 solved, I want to use std::sync::Arc to make map thread-safe, what's the Rust way to do it?

Answer 1

Question 1:
It is not thread safe to share variables into new threads. In Rust you can not compile this code. You can make the code work via using Arc<Mutex<Map>>
Question 2:
You need to change method signature as follows:
handle_n_times(count: i32, arc_map: Arc<Mutex<Map>>)
In case you are moving ownership in your thread::spawn, you need to clone Arc value just before sending it to other thread with
let clone_arc = arc_map.clone();
When you want to insert an item to the map you need to get reference from the Arc<Mutex> via lock and unwrap:
let map = clone_arc.lock().unwrap();
After that you can call map.insert with no explicit lifetime requirements.
Here is the full working code as an example