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
> {
29 data
: Cell
<Option
<T
>>,
32 impl<T
: Copy
> Ivar
<T
> {
33 pub fn new() -> Ivar
<T
> {
34 Ivar { data: Cell::new(None) }
37 pub fn get(&self) -> Option
<T
> {
41 pub fn fulfill(&self, value
: T
) {
42 assert
!(self.data
.get().is_none(), "Value already set!");
43 self.data
.set(Some(value
));
46 pub fn is_fulfilled(&self) -> bool
{
47 self.data
.get().is_some()
50 pub fn unwrap(&self) -> T
{
55 impl<T
: Copy
+ fmt
::Debug
> fmt
::Debug
for Ivar
<T
> {
56 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
58 Some(val
) => write
!(f
, "Ivar({:?})", val
),
59 None
=> f
.write_str("Ivar(<unfulfilled>)"),
64 impl<T
: Copy
> Clone
for Ivar
<T
> {
65 fn clone(&self) -> Ivar
<T
> {
67 Some(val
) => Ivar { data: Cell::new(Some(val)) }
,