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# 全局变量
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在一些场景，我们可能需要全局变量来简化状态共享的代码，包括全局ID，全局数据存储等等，下面我们来一一给出对应的实现方法。
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## 全局唯一ID
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```rust
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use std::sync::atomic::{Ordering, AtomicUsize};
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struct Factory{
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factory_id: usize,
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}
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static GLOBAL_ID_COUNTER: AtomicUsize = AtomicUsize::new(0);
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// This gives large room for ids to overflow
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// This code assumes that your app never would need many factories
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const MAX_ID: usize = usize::MAX / 2;
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fn generate_id()->usize{
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// Check overflow twice to avoid growing of GLOBAL_ID_COUNTER after overflow.
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let current_val = GLOBAL_ID_COUNTER.load(Ordering::Relaxed);
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if current_val > MAX_ID{
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panic!("Factory ids overflowed");
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}
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let next_id = GLOBAL_ID_COUNTER.fetch_add(1, Ordering::Relaxed);
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if next_id > MAX_ID{
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panic!("Factory ids overflowed");
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}
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next_id
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}
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impl Factory{
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fn new()->Self{
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Self{
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factory_id: generate_id()
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}
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}
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}
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```
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## 从函数中返回全局变量
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https://www.reddit.com/r/learnrust/comments/rqn74g/cant_a_function_return_a_reference_to_some_global/
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## lazy_static
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```rust
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use std::{sync::{Mutex, MutexGuard}, thread};
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use std::thread::sleep;
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use std::time::Duration;
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use lazy_static::lazy_static;
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lazy_static! {
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static ref MUTEX1: Mutex<i64> = Mutex::new(0);
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static ref MUTEX2: Mutex<i64> = Mutex::new(0);
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}
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fn main() {
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// Spawn thread and store handles
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let mut children = vec![];
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for i_thread in 0..2 {
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children.push(thread::spawn(move || {
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for _ in 0..1 {
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// Thread 1
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if i_thread % 2 == 0 {
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// Lock mutex1
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// No need to specify type but yes create a dummy variable to prevent rust
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// compiler from being lazy
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let _guard: MutexGuard<i64> = MUTEX1.lock().unwrap();
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// Just log
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println!("Thread {} locked mutex1 and will try to lock the mutex2, after a nap !", i_thread);
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// Here I sleep to let Thread 2 lock mutex2
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sleep(Duration::from_millis(10));
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// Lock mutex 2
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let _guard = MUTEX2.lock().unwrap();
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// Thread 2
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} else {
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// Lock mutex 1
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let _guard = MUTEX2.lock().unwrap();
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println!("Thread {} locked mutex2 and will try to lock the mutex1", i_thread);
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// Here I freeze !
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let _guard = MUTEX1.lock().unwrap();
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}
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}
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}));
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}
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// Wait
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for child in children {
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let _ = child.join();
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}
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println!("This is not printed");
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}
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```
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## box::leak |
