Pass reference of read-only vector to thread

---

• Arc allows us to have reference counting.
• Here the clone only copies the reference and not the whole data structure.

examples/threads/pass-reference-to-vector/src/main.rs

use std::sync::Arc;

macro_rules! prt {
    ($text: expr, $var: expr) => {
        println!("{:11} {:?} {:p} {:?}", $text, $var, &$var, $var.as_ptr());
    };
}

fn main() {
    let animals = Arc::new(vec![
        String::from("crab"),
        String::from("ant"),
        String::from("cat"),
        String::from("dog"),
        String::from("bat"),
    ]);

    prt!("Before:", animals);
    let mut handles = vec![];
    for _ in 1..=3 {
        let animals = animals.clone();
        handles.push(std::thread::spawn(move || {
            list_animals(&animals);
        }));
    }
    prt!("Started:", animals);

    for handle in handles {
        handle.join().unwrap();
    }
    prt!("After:", animals);
}

fn list_animals(animals: &Vec<String>) {
    prt!(format!("{:?}", std::thread::current().id()), animals);
    //for animal in animals {
    //    println!("{:?} {}", std::thread::current().id(), animal);
    //}
}

Before:     ["crab", "ant", "cat", "dog", "bat"] 0x7ffc60e9fbf8 0x6225d6993480
Started:    ["crab", "ant", "cat", "dog", "bat"] 0x7ffc60e9fbf8 0x6225d6993480
ThreadId(2) ["crab", "ant", "cat", "dog", "bat"] 0x75e378bff678 0x6225d6993480
ThreadId(3) ["crab", "ant", "cat", "dog", "bat"] 0x75e3787ff678 0x6225d6993480
ThreadId(4) ["crab", "ant", "cat", "dog", "bat"] 0x75e373dff678 0x6225d6993480
After:      ["crab", "ant", "cat", "dog", "bat"] 0x7ffc60e9fbf8 0x6225d6993480