rust 30 行概括 once_cell 的 Lazy

use std::cell::UnsafeCell;

pub struct Global<T> {
  v: UnsafeCell<Option<T>>,
  f: Option<fn()-> T>
}
unsafe impl<T> Sync for Global<T> {}
impl<T> Global<T> {
  pub const fn new(f:fn()->T)-> Global<T> { Global {
    v: UnsafeCell::new(None),
    f: Some(f)
  }}
  fn init_or_get(&self)->&T {
    if let Some(v) = unsafe{self.v.get().as_ref().unwrap_unchecked()} {return v;};
    let val = self.f.unwrap()();
    unsafe {
      *self.v.get() = Some(val);
    }
    self.init_or_get()
  }
}

impl<T> std::ops::Deref for Global<T> {
  type Target = T;
  fn deref(&self) -> &T {
    self.init_or_get()
  }
}

使用

static Test:Global<u8> = Global::new(||2);

fn main() {
  println!("{}",Test.pow(2));
}

预期输出:4