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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.
4 //
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.
10
11 use std::fmt;
12 use std::cell::Cell;
13
14 /// A write-once variable. When constructed, it is empty, and
15 /// can only be set once.
16 ///
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
22 /// concurrency type.
23 ///
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.
27 #[derive(PartialEq)]
28 pub struct Ivar<T: Copy> {
29 data: Cell<Option<T>>,
30 }
31
32 impl<T: Copy> Ivar<T> {
33 pub fn new() -> Ivar<T> {
34 Ivar { data: Cell::new(None) }
35 }
36
37 pub fn get(&self) -> Option<T> {
38 self.data.get()
39 }
40
41 pub fn fulfill(&self, value: T) {
42 assert!(self.data.get().is_none(), "Value already set!");
43 self.data.set(Some(value));
44 }
45
46 pub fn is_fulfilled(&self) -> bool {
47 self.data.get().is_some()
48 }
49
50 pub fn unwrap(&self) -> T {
51 self.get().unwrap()
52 }
53 }
54
55 impl<T: Copy + fmt::Debug> fmt::Debug for Ivar<T> {
56 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
57 match self.get() {
58 Some(val) => write!(f, "Ivar({:?})", val),
59 None => f.write_str("Ivar(<unfulfilled>)"),
60 }
61 }
62 }
63
64 impl<T: Copy> Clone for Ivar<T> {
65 fn clone(&self) -> Ivar<T> {
66 match self.get() {
67 Some(val) => Ivar { data: Cell::new(Some(val)) },
68 None => Ivar::new(),
69 }
70 }
71 }