Struct swc_common::sync::OnceCell [−][src]
pub struct OnceCell<T>(_);
Expand description
A thread-safe cell which can be written to only once.
OnceCell
provides &
references to the contents without RAII guards.
Reading a non-None
value out of OnceCell
establishes a
happens-before relationship with a corresponding write. For example, if
thread A initializes the cell with get_or_init(f)
, and thread B
subsequently reads the result of this call, B also observes all the side
effects of f
.
Example
use once_cell::sync::OnceCell;
static CELL: OnceCell<String> = OnceCell::new();
assert!(CELL.get().is_none());
std::thread::spawn(|| {
let value: &String = CELL.get_or_init(|| {
"Hello, World!".to_string()
});
assert_eq!(value, "Hello, World!");
}).join().unwrap();
let value: Option<&String> = CELL.get();
assert!(value.is_some());
assert_eq!(value.unwrap().as_str(), "Hello, World!");
Implementations
Gets the reference to the underlying value.
Returns None
if the cell is empty, or being initialized. This
method never blocks.
Gets the mutable reference to the underlying value.
Returns None
if the cell is empty.
Get the reference to the underlying value, without checking if the cell is initialized.
Safety
Caller must ensure that the cell is in initialized state, and that the contents are acquired by (synchronized to) this thread.
Sets the contents of this cell to value
.
Returns Ok(())
if the cell was empty and Err(value)
if it was
full.
Example
use once_cell::sync::OnceCell;
static CELL: OnceCell<i32> = OnceCell::new();
fn main() {
assert!(CELL.get().is_none());
std::thread::spawn(|| {
assert_eq!(CELL.set(92), Ok(()));
}).join().unwrap();
assert_eq!(CELL.set(62), Err(62));
assert_eq!(CELL.get(), Some(&92));
}
Gets the contents of the cell, initializing it with f
if the cell
was empty.
Many threads may call get_or_init
concurrently with different
initializing functions, but it is guaranteed that only one function
will be executed.
Panics
If f
panics, the panic is propagated to the caller, and the cell
remains uninitialized.
It is an error to reentrantly initialize the cell from f
. The
exact outcome is unspecified. Current implementation deadlocks, but
this may be changed to a panic in the future.
Example
use once_cell::sync::OnceCell;
let cell = OnceCell::new();
let value = cell.get_or_init(|| 92);
assert_eq!(value, &92);
let value = cell.get_or_init(|| unreachable!());
assert_eq!(value, &92);
Gets the contents of the cell, initializing it with f
if
the cell was empty. If the cell was empty and f
failed, an
error is returned.
Panics
If f
panics, the panic is propagated to the caller, and
the cell remains uninitialized.
It is an error to reentrantly initialize the cell from f
.
The exact outcome is unspecified. Current implementation
deadlocks, but this may be changed to a panic in the future.
Example
use once_cell::sync::OnceCell;
let cell = OnceCell::new();
assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
assert!(cell.get().is_none());
let value = cell.get_or_try_init(|| -> Result<i32, ()> {
Ok(92)
});
assert_eq!(value, Ok(&92));
assert_eq!(cell.get(), Some(&92))
Takes the value out of this OnceCell
, moving it back to an uninitialized state.
Has no effect and returns None
if the OnceCell
hasn’t been initialized.
Examples
use once_cell::sync::OnceCell;
let mut cell: OnceCell<String> = OnceCell::new();
assert_eq!(cell.take(), None);
let mut cell = OnceCell::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.take(), Some("hello".to_string()));
assert_eq!(cell.get(), None);
Consumes the OnceCell
, returning the wrapped value. Returns
None
if the cell was empty.
Examples
use once_cell::sync::OnceCell;
let cell: OnceCell<String> = OnceCell::new();
assert_eq!(cell.into_inner(), None);
let cell = OnceCell::new();
cell.set("hello".to_string()).unwrap();
assert_eq!(cell.into_inner(), Some("hello".to_string()));
Trait Implementations
Auto Trait Implementations
impl<T> RefUnwindSafe for OnceCell<T> where
T: UnwindSafe + RefUnwindSafe,
impl<T> UnwindSafe for OnceCell<T> where
T: UnwindSafe,
Blanket Implementations
Mutably borrows from an owned value. Read more
type ROwned = T
type ROwned = T
The owned type, stored in RCow::Owned
pub fn r_borrow(
this: &'a <T as BorrowOwned<'a>>::ROwned
) -> <T as BorrowOwned<'a>>::RBorrowed
type ForSelf = WithMetadata_<T, T>
type ForSelf = WithMetadata_<T, T>
This is always WithMetadata_<Self, Self>
Gets a reference to a field, determined by offset
. Read more
Gets a muatble reference to a field, determined by offset
. Read more
Gets a const pointer to a field,
the field is determined by offset
. Read more
pub fn f_get_mut_ptr<F, A>(&mut self, offset: FieldOffset<S, F, A>) -> *mut F
pub fn f_get_mut_ptr<F, A>(&mut self, offset: FieldOffset<S, F, A>) -> *mut F
Gets a mutable pointer to a field, determined by offset
. Read more
pub fn f_replace<F>(
&mut self,
offset: FieldOffset<S, F, Aligned>,
value: F
) -> F
pub fn f_replace<F>(
&mut self,
offset: FieldOffset<S, F, Aligned>,
value: F
) -> F
Replaces a field (determined by offset
) with value
,
returning the previous value of the field. Read more
Swaps a field (determined by offset
) with the same field in right
. Read more
pub fn f_get_copy<F>(&self, offset: FieldOffset<S, F, Aligned>) -> F where
F: Copy,
pub fn f_get_copy<F>(&self, offset: FieldOffset<S, F, Aligned>) -> F where
F: Copy,
Gets a copy of a field (determined by offset
).
The field is determined by offset
. Read more
pub fn f_replace<F>(
&mut self,
offset: FieldOffset<S, F, Unaligned>,
value: F
) -> F
pub fn f_replace<F>(
&mut self,
offset: FieldOffset<S, F, Unaligned>,
value: F
) -> F
Replaces a field (determined by offset
) with value
,
returning the previous value of the field. Read more
Swaps a field (determined by offset
) with the same field in right
. Read more
pub fn f_get_copy<F>(&self, offset: FieldOffset<S, F, Unaligned>) -> F where
F: Copy,
pub fn f_get_copy<F>(&self, offset: FieldOffset<S, F, Unaligned>) -> F where
F: Copy,
Gets a copy of a field (determined by offset
).
The field is determined by offset
. Read more
impl<T> SelfOps for T where
T: ?Sized,
impl<T> SelfOps for T where
T: ?Sized,
Compares the address of self
with the address of other
. Read more
Emulates the pipeline operator, allowing method syntax in more places. Read more
The same as piped
except that the function takes &Self
Useful for functions that take &Self
instead of Self
. Read more
The same as piped
, except that the function takes &mut Self
.
Useful for functions that take &mut Self
instead of Self
. Read more
Mutates self using a closure taking self by mutable reference, passing it along the method chain. Read more
Observes the value of self, passing it along unmodified. Useful in long method chains. Read more
Performs a conversion with Into
.
using the turbofish .into_::<_>()
syntax. Read more
Performs a reference to reference conversion with AsRef
,
using the turbofish .as_ref_::<_>()
syntax. Read more
Performs a mutable reference to mutable reference conversion with AsMut
,
using the turbofish .as_mut_::<_>()
syntax. Read more
unsafe fn transmute_element<T>(self) -> Self::TransmutedPtr where
Self: CanTransmuteElement<T>,
unsafe fn transmute_element<T>(self) -> Self::TransmutedPtr where
Self: CanTransmuteElement<T>,
Transmutes the element type of this pointer.. Read more
impl<T> TypeIdentity for T where
T: ?Sized,
impl<T> TypeIdentity for T where
T: ?Sized,
type Type = T
type Type = T
This is always Self
.
Converts a value back to the original type.
fn as_type(&self) -> &Self::Type
fn as_type(&self) -> &Self::Type
Converts a reference back to the original type.
fn as_type_mut(&mut self) -> &mut Self::Type
fn as_type_mut(&mut self) -> &mut Self::Type
Converts a mutable reference back to the original type.
fn into_type_box(self: Box<Self, Global>) -> Box<Self::Type, Global>
fn into_type_box(self: Box<Self, Global>) -> Box<Self::Type, Global>
Converts a box back to the original type.
fn into_type_arc(this: Arc<Self>) -> Arc<Self::Type>
fn into_type_arc(this: Arc<Self>) -> Arc<Self::Type>
Converts an Arc back to the original type. Read more
fn into_type_rc(this: Rc<Self>) -> Rc<Self::Type>
fn into_type_rc(this: Rc<Self>) -> Rc<Self::Type>
Converts an Rc back to the original type. Read more
Converts a value back to the original type.
fn from_type_ref(this: &Self::Type) -> &Self
fn from_type_ref(this: &Self::Type) -> &Self
Converts a reference back to the original type.
fn from_type_mut(this: &mut Self::Type) -> &mut Self
fn from_type_mut(this: &mut Self::Type) -> &mut Self
Converts a mutable reference back to the original type.
fn from_type_box(this: Box<Self::Type, Global>) -> Box<Self, Global>
fn from_type_box(this: Box<Self::Type, Global>) -> Box<Self, Global>
Converts a box back to the original type.
fn from_type_arc(this: Arc<Self::Type>) -> Arc<Self>
fn from_type_arc(this: Arc<Self::Type>) -> Arc<Self>
Converts an Arc back to the original type.
fn from_type_rc(this: Rc<Self::Type>) -> Rc<Self>
fn from_type_rc(this: Rc<Self::Type>) -> Rc<Self>
Converts an Rc back to the original type.
Attaches the provided Subscriber
to this type, returning a
WithDispatch
wrapper. Read more
Attaches the current default Subscriber
to this type, returning a
WithDispatch
wrapper. Read more