Generate sequential IDs for each instance of a struct

Question

I\'m writing a system where I have a collection of Objects, and each Object has a unique integral ID. Here\'s how I would do it in C++:

<         


        

Answer 1

nor are global mutable variables safe

Your C++ example seems like it would have thread-safety issues, but I don't know enough C++ to be sure.

However, only unsynchronized global mutable variables are trouble. If you don't care about cross-thread issues, you can use a thread-local:

use std::cell::Cell;

#[derive(Debug)]
struct Monster {
    id: usize,
    health: u8,
}

thread_local!(static MONSTER_ID: Cell = Cell::new(0));

impl Monster {
    fn new(health: u8) -> Monster {
        MONSTER_ID.with(|thread_id| {
            let id = thread_id.get();
            thread_id.set(id + 1);
            Monster { id, health }
        })
    }
}

fn main() {
    let gnome = Monster::new(41);
    let troll = Monster::new(42);

    println!("gnome {:?}", gnome);
    println!("troll {:?}", troll);
}

If you do want something that works better with multiple threads, check out bluss' answer, which shows how to use an atomic variable.