请问怎么能在铁锈里做一个安全的静电单例？

trait Foo {}struct CBox;impl CBox { /// Stores value in a bound C api, forget(value) fn set<T: Foo>(value: T) { // ... } /// Periodically call this and maybe get a callback invoked fn poll(_: Box<Fn<(EventType, Foo), ()> + Send>) { // ... }}impl Drop for CBox { fn drop(&mut self) { // Safely load all saved Foo's here and discard them, preventing memory leaks }}

#[cfg(test)]mod test { struct IsFoo; impl Foo for IsFoo {} impl Drop for IsFoo { fn drop(&mut self) { Static::touch(); } } #[test] fn test_drops_actually_work() { guard = Static::lock(); // Prevent any other use of Static concurrently Static::reset(); // Set to zero { let c = CBox; c.set(IsFoo); c.set(IsFoo); c.poll(/*...*/); } assert!(Static::get() == 2); // Assert that all expected drops were invoked guard.release(); }}

/// Global instancestatic mut INSTANCE_LOCK: bool = false;static mut INSTANCE: *mut StaticUtils = 0 as *mut StaticUtils;static mut WRITE_LOCK: *mut Semaphore = 0 as *mut Semaphore;static mut LOCK: *mut Semaphore = 0 as *mut Semaphore; /// Generate instances if they don't existunsafe fn init() { if !INSTANCE_LOCK { INSTANCE_LOCK = true; INSTANCE = transmute(box StaticUtils::new()); WRITE_LOCK = transmute(box Semaphore::new(1)); LOCK = transmute(box Semaphore::new(1)); }}

另见懒静这使得事情更符合人体工程学。它所做的与静态的事情本质上是一样的。Once对于每个变量，但将其包装为实现Deref这样您就可以像普通引用一样访问它。用法如下(从文件中):#[macro_use]extern crate lazy_static;use std::collections::HashMap;lazy_static! {     static ref HASHMAP: HashMap<u32, &'static str> = {         let mut m = HashMap::new();         m.insert(0, "foo");         m.insert(1, "bar");         m.insert(2, "baz");         m    };     static ref COUNT: usize = HASHMAP.len();     static ref NUMBER: u32 = times_two(21);}fn times_two(n: u32) -> u32 { n * 2 }fn main() {     println!("The map has {} entries.", *COUNT);     println!("The entry for `0` is \"{}\".", HASHMAP.get(&0).unwrap());     println!("A expensive calculation on a static results in: {}.", *NUMBER);}请注意，autoderef意味着您甚至不必使用*每当您对静态变量调用方法时。变量将在第一次初始化它的Deref德。但是，惰性_静态变量是不可变的(因为它们在引用后面)。如果您想要一个可变的静态，则需要使用Mutex:lazy_static! {     static ref VALUE: Mutex<u64>;}impl Drop for IsFoo {     fn drop(&mut self) {         let mut value = VALUE.lock().unwrap();         *value += 1;     }}#[test]fn test_drops_actually_work() {     // Have to drop the mutex guard to unlock, so we put it in its own scope     {         *VALUE.lock().unwrap() = 0;     }     {         let c = CBox;         c.set(IsFoo);         c.set(IsFoo);         c.poll(/*...*/);     }     assert!(*VALUE.lock().unwrap() == 2); // Assert that all expected drops were invoked}

请问怎么能在铁锈里做一个安全的静电单例？

2回答