为结构的每个实例生成顺序 ID [英] Generate sequential IDs for each instance of a struct

问题描述

我正在编写一个系统，其中我有一个 Object 的集合，每个 Object 都有一个唯一的完整 ID.这是我在 C++ 中的做法:

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++:

class Object {
public:
  Object(): id_(nextId_++) { }

private:
  int id_;
  static int nextId_;
}

int Object::nextId_ = 1;

这显然不是线程安全的，但如果我想要它，我可以将 nextId_ 设为 std::atomic_int，或者在 周围包裹一个互斥锁nextId_++ 表达式.

This is obviously not thread_safe, but if I wanted it to be, I could make nextId_ an std::atomic_int, or wrap a mutex around the nextId_++ expression.

我将如何在(最好是安全的)Rust 中做到这一点?没有静态结构成员，全局可变变量也不安全.我总是可以将 nextId 传递到 new 函数中，但是这些对象将被分配到很多地方，我不想通过管道传递 nextId 号码随处可见.想法?

How would I do this in (preferably safe) Rust? There's no static struct members, nor are global mutable variables safe. I could always pass nextId into the new function, but these objects are going to be allocated in a number of places, and I would prefer not to pipe the nextId number hither and yon. Thoughts?

推荐答案

全局可变变量也不安全

nor are global mutable variables safe

您的 C++ 示例似乎存在线程安全问题，但我对 C++ 的了解不够确定.

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

然而，只有未同步的全局可变变量是有问题的.如果你不关心跨线程问题，你可以使用线程本地:

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<usize> = 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);
}

如果您确实想要更适合多线程的东西，请查看 bluss 的答案，其中显示了如何使用一个原子变量.

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

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