Struct OnceOption 

pub struct OnceOption<T> { /* private fields */ }

OnceOption represents an optional value. Differently from Option, an empty OnceOption cannot be re-set to contain a value.

Check the crate level documentation for more details.

Implementations

impl<T> OnceOption<T>

pub const NONE: Self

A constant representing a OnceOption that doesn’t contain any value. Since a OnceOption cannot be set to contain a value after it has been emptied, this constant is provided as a helper, but is of dubious utility.

Examples

let x: OnceOption<u32> = OnceOption::NONE;
assert_eq!(x.is_some(), false);

pub const fn is_some(&self) -> bool

Returns true if the once-option contains a value.

Examples

let x: OnceOption<u32> = OnceOption(1912);
assert_eq!(x.is_some(), true);

let x: OnceOption<u32> = OnceOption::NONE;
assert_eq!(x.is_some(), false);

pub const fn is_none(&self) -> bool

Returns true if the option is empty.

Examples

let x: Option<u32> = Some(2);
assert_eq!(x.is_none(), false);

let x: Option<u32> = None;
assert_eq!(x.is_none(), true);

pub fn expect(self, msg: &str) -> T

Returns the contained value, consuming the self value.

Panics

Panics if the value is empty with a custom panic message provided by msg.

Examples

let x = OnceOption("value");
assert_eq!(x.expect("fruits are healthy"), "value");

let x = OnceOption::<&str>::NONE;
x.expect("fruits are healthy"); // panics with `fruits are healthy`

pub fn expect_none(self, msg: &str)

Panics if the OnceOption is not empty and does contain a value.

Panics

Panics if the value is not empty, with a custom panic message provided by msg.

Examples

let x = OnceOption("value");
assert_eq!(x.expect("fruits are healthy"), "value");

let x = OnceOption::<&str>("something something");
x.expect_none("fruits are healthy"); // panics with `fruits are healthy`

pub fn take(&mut self) -> T

Takes the value out of the OnceOption, leaving it empty. If the OnceOption is already empty, this function will panic.

Note that this operation cannot be reversed: once a OnceOption becomes empty, it cannot get a value again.

Examples

let mut x = OnceOption(2);
let y = x.take();
assert!(x.is_none());
assert_eq!(y, 2);

let mut x = OnceOption(2);
let y = x.take();
let w = x.take(); // this panics!

pub fn unwrap(self) -> T

Returns the contained value, consuming the self value. If the OnceOption is empty, this function will panic.

Examples

let mut x = OnceOption(2);
assert_eq!(*x, 2);
x.replace(5);
assert_eq!(*x, 5);

let mut x: OnceOption<u32> = OnceOption::NONE;
x.unwrap(); // this panics!

pub fn replace(&mut self, value: T) -> T

Replaces the actual value in the option by the value given in parameter, returning the old value if present, or panic’ing otherwise.

Examples

let mut x = OnceOption(2);
let old = x.replace(5);
assert_eq!(x.take(), 5);
assert_eq!(old, 2);

let mut x: OnceOption<i32> = OnceOption::NONE;
let _ = x.replace(3); // this panics

Trait Implementations

impl<T: Binary> Binary for OnceOption<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: Clone> Clone for OnceOption<T>

fn clone(&self) -> OnceOption<T>

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: Debug> Debug for OnceOption<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T> Default for OnceOption<T>

fn default() -> OnceOption<T>

Returns an empty OnceOption.

Examples

let opt: OnceOption<u32> = OnceOption::default();
assert!(opt.is_none());

impl<T> Deref for OnceOption<T>

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

impl<T> DerefMut for OnceOption<T>

fn deref_mut(&mut self) -> &mut T

Mutably dereferences the value.

impl<T: Display> Display for OnceOption<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T> From<Option<T>> for OnceOption<T>

fn from(val: Option<T>) -> OnceOption<T>

Moves an option val into a new OnceOption.

Examples

let o: OnceOption<u8> = OnceOption::from(Some(67));

assert_eq!(67, *o);

impl<T> From<T> for OnceOption<T>

fn from(val: T) -> OnceOption<T>

Moves val into a new OnceOption.

Examples

let o: OnceOption<u8> = OnceOption::from(67);

assert_eq!(67, *o);

Alternatively:

let o: OnceOption<u8> = 67.into();

assert_eq!(67, *o);

impl<T: Hash> Hash for OnceOption<T>

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<T: LowerExp> LowerExp for OnceOption<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: LowerHex> LowerHex for OnceOption<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: Octal> Octal for OnceOption<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: Ord> Ord for OnceOption<T>

fn cmp(&self, other: &OnceOption<T>) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl<T: PartialEq> PartialEq for OnceOption<T>

fn eq(&self, other: &OnceOption<T>) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T: PartialOrd> PartialOrd for OnceOption<T>

fn partial_cmp(&self, other: &OnceOption<T>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl<T: UpperExp> UpperExp for OnceOption<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: UpperHex> UpperHex for OnceOption<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: Copy> Copy for OnceOption<T>

impl<T: Eq> Eq for OnceOption<T>

impl<T> StructuralPartialEq for OnceOption<T>