# Enum ethcore_util::standard::option::Option 1.0.0 [−] [src]

    pub enum Option<T> {
        None,
        Some(T),
    }

1.0.0

The `Option` type. See the module level documentation for more.

## Variants

`None`

No value

`Some(T)`

Some value `T`

## Methods

### `impl<T> Option<T>`

#### `fn is_some(&self) -> bool`

Returns `true` if the option is a `Some` value.

# Examples

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

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

#### `fn is_none(&self) -> bool`

Returns `true` if the option is a `None` value.

# Examples

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

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

#### `fn as_ref(&self) -> Option<&T>`

Converts from `Option<T>` to `Option<&T>`.

# Examples

Convert an `Option<String>` into an `Option<usize>`, preserving the original. The `map` method takes the `self` argument by value, consuming the original, so this technique uses `as_ref` to first take an `Option` to a reference to the value inside the original.

    let num_as_str: Option<String> = Some("10".to_string());
    // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
    // then consume *that* with `map`, leaving `num_as_str` on the stack.
    let num_as_int: Option<usize> = num_as_str.as_ref().map(|n| n.len());
    println!("still can print num_as_str: {:?}", num_as_str);

#### `fn as_mut(&mut self) -> Option<&mut T>`

Converts from `Option<T>` to `Option<&mut T>`.

# Examples

    let mut x = Some(2);
    match x.as_mut() {
        Some(v) => *v = 42,
        None => {},
    }
    assert_eq!(x, Some(42));

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

Unwraps an option, yielding the content of a `Some`.

# Panics

Panics if the value is a `None` with a custom panic message provided by `msg`.

# Examples

    let x = Some("value");
    assert_eq!(x.expect("the world is ending"), "value");

    let x: Option<&str> = None;
    x.expect("the world is ending"); // panics with `the world is ending`

#### `fn unwrap(self) -> T`

Moves the value `v` out of the `Option<T>` if it is `Some(v)`.

In general, because this function may panic, its use is discouraged. Instead, prefer to use pattern matching and handle the `None` case explicitly.

# Panics

Panics if the self value equals `None`.

# Examples

    let x = Some("air");
    assert_eq!(x.unwrap(), "air");

    let x: Option<&str> = None;
    assert_eq!(x.unwrap(), "air"); // fails

#### `fn unwrap_or(self, def: T) -> T`

Returns the contained value or a default.

# Examples

    assert_eq!(Some("car").unwrap_or("bike"), "car");
    assert_eq!(None.unwrap_or("bike"), "bike");

#### `fn unwrap_or_else<F>(self, f: F) -> T where F: FnOnce() -> T`

Returns the contained value or computes it from a closure.

# Examples

    let k = 10;
    assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
    assert_eq!(None.unwrap_or_else(|| 2 * k), 20);

#### `fn map<U, F>(self, f: F) -> Option<U> where F: FnOnce(T) -> U`

Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.

# Examples

Convert an `Option<String>` into an `Option<usize>`, consuming the original:

    let maybe_some_string = Some(String::from("Hello, World!"));
    // `Option::map` takes self *by value*, consuming `maybe_some_string`
    let maybe_some_len = maybe_some_string.map(|s| s.len());

    assert_eq!(maybe_some_len, Some(13));

#### `fn map_or<U, F>(self, default: U, f: F) -> U where F: FnOnce(T) -> U`

Applies a function to the contained value (if any), or returns a `default` (if not).

# Examples

    let x = Some("foo");
    assert_eq!(x.map_or(42, |v| v.len()), 3);

    let x: Option<&str> = None;
    assert_eq!(x.map_or(42, |v| v.len()), 42);

#### `fn map_or_else<U, D, F>(self, default: D, f: F) -> U where D: FnOnce() -> U, F: FnOnce(T) -> U`

Applies a function to the contained value (if any), or computes a `default` (if not).

# Examples

    let k = 21;

    let x = Some("foo");
    assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);

    let x: Option<&str> = None;
    assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);

#### `fn ok_or<E>(self, err: E) -> Result<T, E>`

Transforms the `Option<T>` into a `Result<T, E>`, mapping `Some(v)` to `Ok(v)` and `None` to `Err(err)`.

# Examples

    let x = Some("foo");
    assert_eq!(x.ok_or(0), Ok("foo"));

    let x: Option<&str> = None;
    assert_eq!(x.ok_or(0), Err(0));

#### `fn ok_or_else<E, F>(self, err: F) -> Result<T, E> where F: FnOnce() -> E`

Transforms the `Option<T>` into a `Result<T, E>`, mapping `Some(v)` to `Ok(v)` and `None` to `Err(err())`.

# Examples

    let x = Some("foo");
    assert_eq!(x.ok_or_else(|| 0), Ok("foo"));

    let x: Option<&str> = None;
    assert_eq!(x.ok_or_else(|| 0), Err(0));

#### `fn iter(&self) -> Iter<T>`

Returns an iterator over the possibly contained value.

# Examples

    let x = Some(4);
    assert_eq!(x.iter().next(), Some(&4));

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

#### `fn iter_mut(&mut self) -> IterMut<T>`

Returns a mutable iterator over the possibly contained value.

# Examples

    let mut x = Some(4);
    match x.iter_mut().next() {
        Some(v) => *v = 42,
        None => {},
    }
    assert_eq!(x, Some(42));

    let mut x: Option<u32> = None;
    assert_eq!(x.iter_mut().next(), None);

#### `fn and<U>(self, optb: Option<U>) -> Option<U>`

Returns `None` if the option is `None`, otherwise returns `optb`.

# Examples

    let x = Some(2);
    let y: Option<&str> = None;
    assert_eq!(x.and(y), None);

    let x: Option<u32> = None;
    let y = Some("foo");
    assert_eq!(x.and(y), None);

    let x = Some(2);
    let y = Some("foo");
    assert_eq!(x.and(y), Some("foo"));

    let x: Option<u32> = None;
    let y: Option<&str> = None;
    assert_eq!(x.and(y), None);

#### `fn and_then<U, F>(self, f: F) -> Option<U> where F: FnOnce(T) -> Option<U>`

Returns `None` if the option is `None`, otherwise calls `f` with the wrapped value and returns the result.

Some languages call this operation flatmap.

# Examples

    fn sq(x: u32) -> Option<u32> { Some(x * x) }
    fn nope(_: u32) -> Option<u32> { None }

    assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
    assert_eq!(Some(2).and_then(sq).and_then(nope), None);
    assert_eq!(Some(2).and_then(nope).and_then(sq), None);
    assert_eq!(None.and_then(sq).and_then(sq), None);

#### `fn or(self, optb: Option<T>) -> Option<T>`

Returns the option if it contains a value, otherwise returns `optb`.

# Examples

    let x = Some(2);
    let y = None;
    assert_eq!(x.or(y), Some(2));

    let x = None;
    let y = Some(100);
    assert_eq!(x.or(y), Some(100));

    let x = Some(2);
    let y = Some(100);
    assert_eq!(x.or(y), Some(2));

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

#### `fn or_else<F>(self, f: F) -> Option<T> where F: FnOnce() -> Option<T>`

Returns the option if it contains a value, otherwise calls `f` and returns the result.

# Examples

    fn nobody() -> Option<&'static str> { None }
    fn vikings() -> Option<&'static str> { Some("vikings") }

    assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
    assert_eq!(None.or_else(vikings), Some("vikings"));
    assert_eq!(None.or_else(nobody), None);

#### `fn take(&mut self) -> Option<T>`

Takes the value out of the option, leaving a `None` in its place.

# Examples

    let mut x = Some(2);
    x.take();
    assert_eq!(x, None);

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

### `impl<'a, T> Option<&'a T> where T: Clone`

#### `fn cloned(self) -> Option<T>`

Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the option.

### `impl<T> Option<T> where T: Default`

#### `fn unwrap_or_default(self) -> T`

Returns the contained value or a default

Consumes the `self` argument then, if `Some`, returns the contained value, otherwise if `None`, returns the default value for that type.

# Examples

Convert a string to an integer, turning poorly-formed strings into 0 (the default value for integers). `parse` converts a string to any other type that implements `FromStr`, returning `None` on error.

    let good_year_from_input = "1909";
    let bad_year_from_input = "190blarg";
    let good_year = good_year_from_input.parse().ok().unwrap_or_default();
    let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();

    assert_eq!(1909, good_year);
    assert_eq!(0, bad_year);

## Trait Implementations

### `impl<T> HeapSizeOf for Option<T> where T: HeapSizeOf`

#### `fn heap_size_of_children(&self) -> usize`

Measure the size of any heap-allocated structures that hang off this value, but not the space taken up by the value itself (i.e. what size_of::<T> measures, more or less); that space is handled by the implementation of HeapSizeOf for Box<T> below. Read more

### `impl<T> Eq for Option<T> where T: Eq`

### `impl<T> Hash for Option<T> where T: Hash`

#### `fn hash<__HT>(&self, __arg_0: &mut __HT) where __HT: Hasher`

Feeds this value into the state given, updating the hasher as necessary.

#### `fn hash_slice<H>(data: &[Self], state: &mut H) where H: Hasher`
1.3.0

Feeds a slice of this type into the state provided.

### `impl<T> PartialOrd<Option<T>> for Option<T> where T: PartialOrd<T>`

#### `fn partial_cmp(&self, __arg_0: &Option<T>) -> Option<Ordering>`

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

#### `fn lt(&self, __arg_0: &Option<T>) -> bool`

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more

#### `fn le(&self, __arg_0: &Option<T>) -> bool`

This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more

#### `fn gt(&self, __arg_0: &Option<T>) -> bool`

This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more

#### `fn ge(&self, __arg_0: &Option<T>) -> bool`

This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more

### `impl<T> From<T> for Option<T>`
1.12.0

#### `fn from(val: T) -> Option<T>`

Performs the conversion.

### `impl<A, V> FromIterator<Option<A>> for Option<V> where V: FromIterator<A>`

#### `fn from_iter<I>(iter: I) -> Option<V> where I: IntoIterator<Item=Option<A>>`

Takes each element in the `Iterator`: if it is `None`, no further elements are taken, and the `None` is returned. Should no `None` occur, a container with the values of each `Option` is returned.

Here is an example which increments every integer in a vector, checking for overflow:

    use std::u16;

    let v = vec![1, 2];
    let res: Option<Vec<u16>> = v.iter().map(|&x: &u16|
        if x == u16::MAX { None }
        else { Some(x + 1) }
    ).collect();
    assert!(res == Some(vec![2, 3]));

### `impl<T> Copy for Option<T> where T: Copy`

### `impl<T> Clone for Option<T> where T: Clone`

#### `fn clone(&self) -> Option<T>`

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

### `impl<T> Ord for Option<T> where T: Ord`

#### `fn cmp(&self, __arg_0: &Option<T>) -> Ordering`

This method returns an `Ordering` between `self` and `other`. Read more

### `impl<T> Default for Option<T>`

#### `fn default() -> Option<T>`

Returns None.

### `impl<T> Debug for Option<T> where T: Debug`

#### `fn fmt(&self, __arg_0: &mut Formatter) -> Result<(), Error>`

Formats the value using the given formatter.

### `impl<T> PartialEq<Option<T>> for Option<T> where T: PartialEq<T>`

#### `fn eq(&self, __arg_0: &Option<T>) -> bool`

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more

#### `fn ne(&self, __arg_0: &Option<T>) -> bool`

This method tests for `!=`.

### `impl<T> IntoIterator for Option<T>`

#### `type Item = T`

The type of the elements being iterated over.

#### `type IntoIter = IntoIter<T>`

Which kind of iterator are we turning this into?

#### `fn into_iter(self) -> IntoIter<T>`

Returns a consuming iterator over the possibly contained value.

# Examples

    let x = Some("string");
    let v: Vec<&str> = x.into_iter().collect();
    assert_eq!(v, ["string"]);

    let x = None;
    let v: Vec<&str> = x.into_iter().collect();
    assert!(v.is_empty());

### `impl<'a, T> IntoIterator for &'a Option<T>`
1.4.0

#### `type Item = &'a T`

The type of the elements being iterated over.

#### `type IntoIter = Iter<'a, T>`

Which kind of iterator are we turning this into?

#### `fn into_iter(self) -> Iter<'a, T>`

Creates an iterator from a value. Read more

### `impl<'a, T> IntoIterator for &'a mut Option<T>`
1.4.0

#### `type Item = &'a mut T`

The type of the elements being iterated over.

#### `type IntoIter = IterMut<'a, T>`

Which kind of iterator are we turning this into?

#### `fn into_iter(self) -> IterMut<'a, T>`

Creates an iterator from a value. Read more

### `impl<T> Decodable for Option<T> where T: Decodable`

#### `fn decode<D>(decoder: &D) -> Result<Option<T>, DecoderError> where D: Decoder`

### `impl<T> Encodable for Option<T> where T: Encodable`

#### `fn rlp_append(&self, s: &mut RlpStream)`

### `impl<T> Rand for Option<T> where T: Rand`

#### `fn rand<R>(rng: &mut R) -> Option<T> where R: Rng`

### `impl<T> Encodable for Option<T> where T: Encodable`

#### `fn encode<S>(&self, s: &mut S) -> Result<(), S::Error> where S: Encoder`

### `impl<A> ToJson for Option<A> where A: ToJson`

#### `fn to_json(&self) -> Json`

### `impl<T> Decodable for Option<T> where T: Decodable`

#### `fn decode<D>(d: &mut D) -> Result<Option<T>, D::Error> where D: Decoder`

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