无答案的答案
一般来说,避免全局状态。相反,尽早在某个地方构建对象(也许在main
),然后将该对象的可变引用传递到需要它的地方。这通常会让你的代码更容易推理,并且不需要太多的向后弯腰。
在决定是否需要全局可变变量之前,请仔细照照镜子。在极少数情况下它是有用的,因此值得了解如何做。
还想做一个吗...?
Tips
在以下解决方案中:
- 如果您删除Mutex https://doc.rust-lang.org/std/sync/struct.Mutex.html那么你有一个全局单例,没有任何可变性.
- 您还可以使用RwLock https://doc.rust-lang.org/std/sync/struct.RwLock.html代替
Mutex
to 允许多个并发读者.
Using lazy-static
The 惰性静态 https://crates.io/crates/lazy_staticcrate 可以消除手动创建单例的一些苦差事。这是一个全局可变向量:
use lazy_static::lazy_static; // 1.4.0
use std::sync::Mutex;
lazy_static! {
static ref ARRAY: Mutex<Vec<u8>> = Mutex::new(vec![]);
}
fn do_a_call() {
ARRAY.lock().unwrap().push(1);
}
fn main() {
do_a_call();
do_a_call();
do_a_call();
println!("called {}", ARRAY.lock().unwrap().len());
}
Using once_cell
The 一次细胞 https://crates.io/crates/once_cellcrate 可以消除手动创建单例的一些苦差事。这是一个全局可变向量:
use once_cell::sync::Lazy; // 1.3.1
use std::sync::Mutex;
static ARRAY: Lazy<Mutex<Vec<u8>>> = Lazy::new(|| Mutex::new(vec![]));
fn do_a_call() {
ARRAY.lock().unwrap().push(1);
}
fn main() {
do_a_call();
do_a_call();
do_a_call();
println!("called {}", ARRAY.lock().unwrap().len());
}
Using std::sync::LazyLock
标准库位于过程 https://github.com/rust-lang/rust/issues/74465添加的once_cell
的功能,目前称为LazyLock https://doc.rust-lang.org/nightly/std/sync/struct.LazyLock.html:
#![feature(once_cell)] // 1.67.0-nightly
use std::sync::{LazyLock, Mutex};
static ARRAY: LazyLock<Mutex<Vec<u8>>> = LazyLock::new(|| Mutex::new(vec![]));
fn do_a_call() {
ARRAY.lock().unwrap().push(1);
}
fn main() {
do_a_call();
do_a_call();
do_a_call();
println!("called {}", ARRAY.lock().unwrap().len());
}
Using std::sync::OnceLock
LazyLock
仍然不稳定,但是OnceLock https://doc.rust-lang.org/stable/std/sync/struct.OnceLock.html从 Rust 1.70.0 开始稳定。您可以使用它在稳定版上获得无依赖实现:
use std::sync::{OnceLock, Mutex};
fn array() -> &'static Mutex<Vec<u8>> {
static ARRAY: OnceLock<Mutex<Vec<u8>>> = OnceLock::new();
ARRAY.get_or_init(|| Mutex::new(vec![]))
}
fn do_a_call() {
array().lock().unwrap().push(1);
}
fn main() {
do_a_call();
do_a_call();
do_a_call();
println!("called {}", array().lock().unwrap().len());
}
一个特例:原子
如果你只需要跟踪一个整数值,你可以直接使用atomic https://doc.rust-lang.org/std/sync/atomic/:
use std::sync::atomic::{AtomicUsize, Ordering};
static CALL_COUNT: AtomicUsize = AtomicUsize::new(0);
fn do_a_call() {
CALL_COUNT.fetch_add(1, Ordering::SeqCst);
}
fn main() {
do_a_call();
do_a_call();
do_a_call();
println!("called {}", CALL_COUNT.load(Ordering::SeqCst));
}
手动、无依赖性实施
有几种现有的静态实现,例如Rust 1.0 的实现stdin https://github.com/rust-lang/rust/blob/2a8cb678e61e91c160d80794b5fdd723d0d4211c/src/libstd/io/stdio.rs#L217-L247。这与适应现代 Rust 的想法相同,例如使用MaybeUninit
以避免分配和不必要的间接。您还应该看看现代的实现io::Lazy https://github.com/rust-lang/rust/blob/1.42.0/src/libstd/io/lazy.rs。我已经对每一行的作用进行了内联评论。
use std::sync::{Mutex, Once};
use std::time::Duration;
use std::{mem::MaybeUninit, thread};
struct SingletonReader {
// Since we will be used in many threads, we need to protect
// concurrent access
inner: Mutex<u8>,
}
fn singleton() -> &'static SingletonReader {
// Create an uninitialized static
static mut SINGLETON: MaybeUninit<SingletonReader> = MaybeUninit::uninit();
static ONCE: Once = Once::new();
unsafe {
ONCE.call_once(|| {
// Make it
let singleton = SingletonReader {
inner: Mutex::new(0),
};
// Store it to the static var, i.e. initialize it
SINGLETON.write(singleton);
});
// Now we give out a shared reference to the data, which is safe to use
// concurrently.
SINGLETON.assume_init_ref()
}
}
fn main() {
// Let's use the singleton in a few threads
let threads: Vec<_> = (0..10)
.map(|i| {
thread::spawn(move || {
thread::sleep(Duration::from_millis(i * 10));
let s = singleton();
let mut data = s.inner.lock().unwrap();
*data = i as u8;
})
})
.collect();
// And let's check the singleton every so often
for _ in 0u8..20 {
thread::sleep(Duration::from_millis(5));
let s = singleton();
let data = s.inner.lock().unwrap();
println!("It is: {}", *data);
}
for thread in threads.into_iter() {
thread.join().unwrap();
}
}
这打印出:
It is: 0
It is: 1
It is: 1
It is: 2
It is: 2
It is: 3
It is: 3
It is: 4
It is: 4
It is: 5
It is: 5
It is: 6
It is: 6
It is: 7
It is: 7
It is: 8
It is: 8
It is: 9
It is: 9
It is: 9
此代码使用 Rust 1.55.0 编译。
所有这些工作都是lazy-static 或once_cell 为您做的。
“全球”的含义
请注意,您仍然可以使用正常的 Rust 范围和模块级隐私来控制对static
or lazy_static
多变的。这意味着您可以在模块中甚至函数内部声明它,并且在该模块/函数之外无法访问它。这有利于控制访问:
use lazy_static::lazy_static; // 1.2.0
fn only_here() {
lazy_static! {
static ref NAME: String = String::from("hello, world!");
}
println!("{}", &*NAME);
}
fn not_here() {
println!("{}", &*NAME);
}
error[E0425]: cannot find value `NAME` in this scope
--> src/lib.rs:12:22
|
12 | println!("{}", &*NAME);
| ^^^^ not found in this scope
然而,该变量仍然是全局的,因为整个程序中都存在它的一个实例。