1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
14 /// A write-once variable. When constructed, it is empty, and
15 /// can only be set once.
17 /// Ivars ensure that data that can only be initialised once. A full
18 /// implementation is used for concurrency and blocks on a read of an
19 /// unfulfilled value. This implementation is more minimal and panics
20 /// if you attempt to read the value before it has been set. It is also
21 /// not `Sync`, but may be extended in the future to be usable as a true
24 /// The `T: Copy` bound is not strictly needed, but it is required by
25 /// Cell (so removing it would require using UnsafeCell), and it
26 /// suffices for the current purposes.
28 pub struct Ivar<T: Copy> {
32 impl<T: Copy> Ivar<T> {
33 pub fn new() -> Ivar<T> {
39 pub fn get(&self) -> Option<T> {
43 pub fn fulfill(&self, value: T) {
44 assert!(self.data.get().is_none(),
45 "Value already set!");
46 self.data.set(Some(value));
49 pub fn is_fulfilled(&self) -> bool {
50 self.data.get().is_some()
53 pub fn unwrap(&self) -> T {
58 impl<T: Copy+fmt::Debug> fmt::Debug for Ivar<T> {
59 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
61 Some(val) => write!(f, "Ivar({:?})", val),
62 None => f.write_str("Ivar(<unfulfilled>)")
67 impl<T: Copy> Clone for Ivar<T> {
68 fn clone(&self) -> Ivar<T> {
70 Some(val) => Ivar { data: Cell::new(Some(val)) },