1 // Copyright 2012-2014 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.
13 //! Type `Option` represents an optional value: every `Option`
14 //! is either `Some` and contains a value, or `None`, and
15 //! does not. `Option` types are very common in Rust code, as
16 //! they have a number of uses:
19 //! * Return values for functions that are not defined
20 //! over their entire input range (partial functions)
21 //! * Return value for otherwise reporting simple errors, where `None` is
23 //! * Optional struct fields
24 //! * Struct fields that can be loaned or "taken"
25 //! * Optional function arguments
26 //! * Nullable pointers
27 //! * Swapping things out of difficult situations
29 //! Options are commonly paired with pattern matching to query the presence
30 //! of a value and take action, always accounting for the `None` case.
33 //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
34 //! if denominator == 0.0 {
37 //! Some(numerator / denominator)
41 //! // The return value of the function is an option
42 //! let result = divide(2.0, 3.0);
44 //! // Pattern match to retrieve the value
46 //! // The division was valid
47 //! Some(x) => println!("Result: {}", x),
48 //! // The division was invalid
49 //! None => println!("Cannot divide by 0"),
54 // FIXME: Show how `Option` is used in practice, with lots of methods
56 //! # Options and pointers ("nullable" pointers)
58 //! Rust's pointer types must always point to a valid location; there are
59 //! no "null" pointers. Instead, Rust has *optional* pointers, like
60 //! the optional owned box, `Option<Box<T>>`.
62 //! The following example uses `Option` to create an optional box of
63 //! `i32`. Notice that in order to use the inner `i32` value first the
64 //! `check_optional` function needs to use pattern matching to
65 //! determine whether the box has a value (i.e. it is `Some(...)`) or
69 //! let optional: Option<Box<i32>> = None;
70 //! check_optional(&optional);
72 //! let optional: Option<Box<i32>> = Some(Box::new(9000));
73 //! check_optional(&optional);
75 //! fn check_optional(optional: &Option<Box<i32>>) {
77 //! Some(ref p) => println!("have value {}", p),
78 //! None => println!("have no value"),
83 //! This usage of `Option` to create safe nullable pointers is so
84 //! common that Rust does special optimizations to make the
85 //! representation of `Option<Box<T>>` a single pointer. Optional pointers
86 //! in Rust are stored as efficiently as any other pointer type.
90 //! Basic pattern matching on `Option`:
93 //! let msg = Some("howdy");
95 //! // Take a reference to the contained string
97 //! Some(ref m) => println!("{}", *m),
101 //! // Remove the contained string, destroying the Option
102 //! let unwrapped_msg = match msg {
104 //! None => "default message",
108 //! Initialize a result to `None` before a loop:
111 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
113 //! // A list of data to search through.
114 //! let all_the_big_things = [
115 //! Kingdom::Plant(250, "redwood"),
116 //! Kingdom::Plant(230, "noble fir"),
117 //! Kingdom::Plant(229, "sugar pine"),
118 //! Kingdom::Animal(25, "blue whale"),
119 //! Kingdom::Animal(19, "fin whale"),
120 //! Kingdom::Animal(15, "north pacific right whale"),
123 //! // We're going to search for the name of the biggest animal,
124 //! // but to start with we've just got `None`.
125 //! let mut name_of_biggest_animal = None;
126 //! let mut size_of_biggest_animal = 0;
127 //! for big_thing in &all_the_big_things {
128 //! match *big_thing {
129 //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
130 //! // Now we've found the name of some big animal
131 //! size_of_biggest_animal = size;
132 //! name_of_biggest_animal = Some(name);
134 //! Kingdom::Animal(..) | Kingdom::Plant(..) => ()
138 //! match name_of_biggest_animal {
139 //! Some(name) => println!("the biggest animal is {}", name),
140 //! None => println!("there are no animals :("),
144 #![stable(feature = "rust1", since = "1.0.0")]
150 use default::Default;
151 use iter::ExactSizeIterator;
152 use iter::{Iterator, DoubleEndedIterator, FromIterator, IntoIterator};
155 use result::Result::{Ok, Err};
159 // Note that this is not a lang item per se, but it has a hidden dependency on
160 // `Iterator`, which is one. The compiler assumes that the `next` method of
161 // `Iterator` is an enumeration with one type parameter and two variants,
162 // which basically means it must be `Option`.
164 /// The `Option` type. See [the module level documentation](index.html) for more.
165 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
166 #[stable(feature = "rust1", since = "1.0.0")]
169 #[stable(feature = "rust1", since = "1.0.0")]
172 #[stable(feature = "rust1", since = "1.0.0")]
176 /////////////////////////////////////////////////////////////////////////////
177 // Type implementation
178 /////////////////////////////////////////////////////////////////////////////
181 /////////////////////////////////////////////////////////////////////////
182 // Querying the contained values
183 /////////////////////////////////////////////////////////////////////////
185 /// Returns `true` if the option is a `Some` value
190 /// let x: Option<u32> = Some(2);
191 /// assert_eq!(x.is_some(), true);
193 /// let x: Option<u32> = None;
194 /// assert_eq!(x.is_some(), false);
197 #[stable(feature = "rust1", since = "1.0.0")]
198 pub fn is_some(&self) -> bool {
205 /// Returns `true` if the option is a `None` value
210 /// let x: Option<u32> = Some(2);
211 /// assert_eq!(x.is_none(), false);
213 /// let x: Option<u32> = None;
214 /// assert_eq!(x.is_none(), true);
217 #[stable(feature = "rust1", since = "1.0.0")]
218 pub fn is_none(&self) -> bool {
222 /////////////////////////////////////////////////////////////////////////
223 // Adapter for working with references
224 /////////////////////////////////////////////////////////////////////////
226 /// Converts from `Option<T>` to `Option<&T>`
230 /// Convert an `Option<String>` into an `Option<usize>`, preserving the original.
231 /// The `map` method takes the `self` argument by value, consuming the original,
232 /// so this technique uses `as_ref` to first take an `Option` to a reference
233 /// to the value inside the original.
236 /// let num_as_str: Option<String> = Some("10".to_string());
237 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
238 /// // then consume *that* with `map`, leaving `num_as_str` on the stack.
239 /// let num_as_int: Option<usize> = num_as_str.as_ref().map(|n| n.len());
240 /// println!("still can print num_as_str: {:?}", num_as_str);
243 #[stable(feature = "rust1", since = "1.0.0")]
244 pub fn as_ref(&self) -> Option<&T> {
246 Some(ref x) => Some(x),
251 /// Converts from `Option<T>` to `Option<&mut T>`
256 /// let mut x = Some(2);
257 /// match x.as_mut() {
258 /// Some(v) => *v = 42,
261 /// assert_eq!(x, Some(42));
264 #[stable(feature = "rust1", since = "1.0.0")]
265 pub fn as_mut(&mut self) -> Option<&mut T> {
267 Some(ref mut x) => Some(x),
272 /// Converts from `Option<T>` to `&mut [T]` (without copying)
277 /// #![feature(as_slice)]
279 /// let mut x = Some("Diamonds");
281 /// let v = x.as_mut_slice();
282 /// assert!(v == ["Diamonds"]);
284 /// assert!(v == ["Dirt"]);
286 /// assert_eq!(x, Some("Dirt"));
289 #[unstable(feature = "as_slice",
290 reason = "waiting for mut conventions",
292 #[deprecated(since = "1.4.0", reason = "niche API, unclear of usefulness")]
294 pub fn as_mut_slice(&mut self) -> &mut [T] {
297 let result: &mut [T] = slice::mut_ref_slice(x);
301 let result: &mut [T] = &mut [];
307 /////////////////////////////////////////////////////////////////////////
308 // Getting to contained values
309 /////////////////////////////////////////////////////////////////////////
311 /// Unwraps an option, yielding the content of a `Some`
315 /// Panics if the value is a `None` with a custom panic message provided by
321 /// let x = Some("value");
322 /// assert_eq!(x.expect("the world is ending"), "value");
325 /// ```{.should_panic}
326 /// let x: Option<&str> = None;
327 /// x.expect("the world is ending"); // panics with `the world is ending`
330 #[stable(feature = "rust1", since = "1.0.0")]
331 pub fn expect(self, msg: &str) -> T {
334 None => panic!("{}", msg),
338 /// Moves the value `v` out of the `Option<T>` if it is `Some(v)`.
342 /// Panics if the self value equals `None`.
346 /// In general, because this function may panic, its use is discouraged.
347 /// Instead, prefer to use pattern matching and handle the `None`
353 /// let x = Some("air");
354 /// assert_eq!(x.unwrap(), "air");
357 /// ```{.should_panic}
358 /// let x: Option<&str> = None;
359 /// assert_eq!(x.unwrap(), "air"); // fails
362 #[stable(feature = "rust1", since = "1.0.0")]
363 pub fn unwrap(self) -> T {
366 None => panic!("called `Option::unwrap()` on a `None` value"),
370 /// Returns the contained value or a default.
375 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
376 /// assert_eq!(None.unwrap_or("bike"), "bike");
379 #[stable(feature = "rust1", since = "1.0.0")]
380 pub fn unwrap_or(self, def: T) -> T {
387 /// Returns the contained value or computes it from a closure.
393 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
394 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
397 #[stable(feature = "rust1", since = "1.0.0")]
398 pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
405 /////////////////////////////////////////////////////////////////////////
406 // Transforming contained values
407 /////////////////////////////////////////////////////////////////////////
409 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value
413 /// Convert an `Option<String>` into an `Option<usize>`, consuming the original:
416 /// let maybe_some_string = Some(String::from("Hello, World!"));
417 /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
418 /// let maybe_some_len = maybe_some_string.map(|s| s.len());
420 /// assert_eq!(maybe_some_len, Some(13));
423 #[stable(feature = "rust1", since = "1.0.0")]
424 pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
426 Some(x) => Some(f(x)),
431 /// Applies a function to the contained value (if any),
432 /// or returns a `default` (if not).
437 /// let x = Some("foo");
438 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
440 /// let x: Option<&str> = None;
441 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
444 #[stable(feature = "rust1", since = "1.0.0")]
445 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
452 /// Applies a function to the contained value (if any),
453 /// or computes a `default` (if not).
460 /// let x = Some("foo");
461 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
463 /// let x: Option<&str> = None;
464 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
467 #[stable(feature = "rust1", since = "1.0.0")]
468 pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
475 /// Transforms the `Option<T>` into a `Result<T, E>`, mapping `Some(v)` to
476 /// `Ok(v)` and `None` to `Err(err)`.
481 /// let x = Some("foo");
482 /// assert_eq!(x.ok_or(0), Ok("foo"));
484 /// let x: Option<&str> = None;
485 /// assert_eq!(x.ok_or(0), Err(0));
488 #[stable(feature = "rust1", since = "1.0.0")]
489 pub fn ok_or<E>(self, err: E) -> Result<T, E> {
496 /// Transforms the `Option<T>` into a `Result<T, E>`, mapping `Some(v)` to
497 /// `Ok(v)` and `None` to `Err(err())`.
502 /// let x = Some("foo");
503 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
505 /// let x: Option<&str> = None;
506 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
509 #[stable(feature = "rust1", since = "1.0.0")]
510 pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
517 /////////////////////////////////////////////////////////////////////////
518 // Iterator constructors
519 /////////////////////////////////////////////////////////////////////////
521 /// Returns an iterator over the possibly contained value.
527 /// assert_eq!(x.iter().next(), Some(&4));
529 /// let x: Option<u32> = None;
530 /// assert_eq!(x.iter().next(), None);
533 #[stable(feature = "rust1", since = "1.0.0")]
534 pub fn iter(&self) -> Iter<T> {
535 Iter { inner: Item { opt: self.as_ref() } }
538 /// Returns a mutable iterator over the possibly contained value.
543 /// let mut x = Some(4);
544 /// match x.iter_mut().next() {
545 /// Some(v) => *v = 42,
548 /// assert_eq!(x, Some(42));
550 /// let mut x: Option<u32> = None;
551 /// assert_eq!(x.iter_mut().next(), None);
554 #[stable(feature = "rust1", since = "1.0.0")]
555 pub fn iter_mut(&mut self) -> IterMut<T> {
556 IterMut { inner: Item { opt: self.as_mut() } }
559 /////////////////////////////////////////////////////////////////////////
560 // Boolean operations on the values, eager and lazy
561 /////////////////////////////////////////////////////////////////////////
563 /// Returns `None` if the option is `None`, otherwise returns `optb`.
569 /// let y: Option<&str> = None;
570 /// assert_eq!(x.and(y), None);
572 /// let x: Option<u32> = None;
573 /// let y = Some("foo");
574 /// assert_eq!(x.and(y), None);
577 /// let y = Some("foo");
578 /// assert_eq!(x.and(y), Some("foo"));
580 /// let x: Option<u32> = None;
581 /// let y: Option<&str> = None;
582 /// assert_eq!(x.and(y), None);
585 #[stable(feature = "rust1", since = "1.0.0")]
586 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
593 /// Returns `None` if the option is `None`, otherwise calls `f` with the
594 /// wrapped value and returns the result.
596 /// Some languages call this operation flatmap.
601 /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
602 /// fn nope(_: u32) -> Option<u32> { None }
604 /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
605 /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
606 /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
607 /// assert_eq!(None.and_then(sq).and_then(sq), None);
610 #[stable(feature = "rust1", since = "1.0.0")]
611 pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
618 /// Returns the option if it contains a value, otherwise returns `optb`.
625 /// assert_eq!(x.or(y), Some(2));
628 /// let y = Some(100);
629 /// assert_eq!(x.or(y), Some(100));
632 /// let y = Some(100);
633 /// assert_eq!(x.or(y), Some(2));
635 /// let x: Option<u32> = None;
637 /// assert_eq!(x.or(y), None);
640 #[stable(feature = "rust1", since = "1.0.0")]
641 pub fn or(self, optb: Option<T>) -> Option<T> {
648 /// Returns the option if it contains a value, otherwise calls `f` and
649 /// returns the result.
654 /// fn nobody() -> Option<&'static str> { None }
655 /// fn vikings() -> Option<&'static str> { Some("vikings") }
657 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
658 /// assert_eq!(None.or_else(vikings), Some("vikings"));
659 /// assert_eq!(None.or_else(nobody), None);
662 #[stable(feature = "rust1", since = "1.0.0")]
663 pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
670 /////////////////////////////////////////////////////////////////////////
672 /////////////////////////////////////////////////////////////////////////
674 /// Takes the value out of the option, leaving a `None` in its place.
679 /// let mut x = Some(2);
681 /// assert_eq!(x, None);
683 /// let mut x: Option<u32> = None;
685 /// assert_eq!(x, None);
688 #[stable(feature = "rust1", since = "1.0.0")]
689 pub fn take(&mut self) -> Option<T> {
690 mem::replace(self, None)
693 /// Converts from `Option<T>` to `&[T]` (without copying)
695 #[unstable(feature = "as_slice", reason = "unsure of the utility here",
697 #[deprecated(since = "1.4.0", reason = "niche API, unclear of usefulness")]
699 pub fn as_slice(&self) -> &[T] {
701 Some(ref x) => slice::ref_slice(x),
703 let result: &[_] = &[];
710 impl<'a, T: Clone> Option<&'a T> {
711 /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
713 #[stable(feature = "rust1", since = "1.0.0")]
714 pub fn cloned(self) -> Option<T> {
715 self.map(|t| t.clone())
719 impl<T: Default> Option<T> {
720 /// Returns the contained value or a default
722 /// Consumes the `self` argument then, if `Some`, returns the contained
723 /// value, otherwise if `None`, returns the default value for that
728 /// Convert a string to an integer, turning poorly-formed strings
729 /// into 0 (the default value for integers). `parse` converts
730 /// a string to any other type that implements `FromStr`, returning
734 /// let good_year_from_input = "1909";
735 /// let bad_year_from_input = "190blarg";
736 /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
737 /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
739 /// assert_eq!(1909, good_year);
740 /// assert_eq!(0, bad_year);
743 #[stable(feature = "rust1", since = "1.0.0")]
744 pub fn unwrap_or_default(self) -> T {
747 None => Default::default(),
752 /////////////////////////////////////////////////////////////////////////////
753 // Trait implementations
754 /////////////////////////////////////////////////////////////////////////////
756 #[stable(feature = "rust1", since = "1.0.0")]
757 impl<T> Default for Option<T> {
759 #[stable(feature = "rust1", since = "1.0.0")]
760 fn default() -> Option<T> { None }
763 #[stable(feature = "rust1", since = "1.0.0")]
764 impl<T> IntoIterator for Option<T> {
766 type IntoIter = IntoIter<T>;
768 /// Returns a consuming iterator over the possibly contained value.
773 /// let x = Some("string");
774 /// let v: Vec<&str> = x.into_iter().collect();
775 /// assert_eq!(v, ["string"]);
778 /// let v: Vec<&str> = x.into_iter().collect();
779 /// assert!(v.is_empty());
782 fn into_iter(self) -> IntoIter<T> {
783 IntoIter { inner: Item { opt: self } }
787 #[stable(since = "1.4.0", feature = "option_iter")]
788 impl<'a, T> IntoIterator for &'a Option<T> {
790 type IntoIter = Iter<'a, T>;
792 fn into_iter(self) -> Iter<'a, T> {
797 #[stable(since = "1.4.0", feature = "option_iter")]
798 impl<'a, T> IntoIterator for &'a mut Option<T> {
799 type Item = &'a mut T;
800 type IntoIter = IterMut<'a, T>;
802 fn into_iter(mut self) -> IterMut<'a, T> {
807 /////////////////////////////////////////////////////////////////////////////
808 // The Option Iterators
809 /////////////////////////////////////////////////////////////////////////////
816 impl<A> Iterator for Item<A> {
820 fn next(&mut self) -> Option<A> {
825 fn size_hint(&self) -> (usize, Option<usize>) {
827 Some(_) => (1, Some(1)),
828 None => (0, Some(0)),
833 impl<A> DoubleEndedIterator for Item<A> {
835 fn next_back(&mut self) -> Option<A> {
840 impl<A> ExactSizeIterator for Item<A> {}
842 /// An iterator over a reference of the contained item in an Option.
843 #[stable(feature = "rust1", since = "1.0.0")]
844 pub struct Iter<'a, A: 'a> { inner: Item<&'a A> }
846 #[stable(feature = "rust1", since = "1.0.0")]
847 impl<'a, A> Iterator for Iter<'a, A> {
851 fn next(&mut self) -> Option<&'a A> { self.inner.next() }
853 fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
856 #[stable(feature = "rust1", since = "1.0.0")]
857 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
859 fn next_back(&mut self) -> Option<&'a A> { self.inner.next_back() }
862 #[stable(feature = "rust1", since = "1.0.0")]
863 impl<'a, A> ExactSizeIterator for Iter<'a, A> {}
865 #[stable(feature = "rust1", since = "1.0.0")]
866 impl<'a, A> Clone for Iter<'a, A> {
867 fn clone(&self) -> Iter<'a, A> {
868 Iter { inner: self.inner.clone() }
872 /// An iterator over a mutable reference of the contained item in an Option.
873 #[stable(feature = "rust1", since = "1.0.0")]
874 pub struct IterMut<'a, A: 'a> { inner: Item<&'a mut A> }
876 #[stable(feature = "rust1", since = "1.0.0")]
877 impl<'a, A> Iterator for IterMut<'a, A> {
878 type Item = &'a mut A;
881 fn next(&mut self) -> Option<&'a mut A> { self.inner.next() }
883 fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
886 #[stable(feature = "rust1", since = "1.0.0")]
887 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
889 fn next_back(&mut self) -> Option<&'a mut A> { self.inner.next_back() }
892 #[stable(feature = "rust1", since = "1.0.0")]
893 impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}
895 /// An iterator over the item contained inside an Option.
897 #[stable(feature = "rust1", since = "1.0.0")]
898 pub struct IntoIter<A> { inner: Item<A> }
900 #[stable(feature = "rust1", since = "1.0.0")]
901 impl<A> Iterator for IntoIter<A> {
905 fn next(&mut self) -> Option<A> { self.inner.next() }
907 fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
910 #[stable(feature = "rust1", since = "1.0.0")]
911 impl<A> DoubleEndedIterator for IntoIter<A> {
913 fn next_back(&mut self) -> Option<A> { self.inner.next_back() }
916 #[stable(feature = "rust1", since = "1.0.0")]
917 impl<A> ExactSizeIterator for IntoIter<A> {}
919 /////////////////////////////////////////////////////////////////////////////
921 /////////////////////////////////////////////////////////////////////////////
923 #[stable(feature = "rust1", since = "1.0.0")]
924 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
925 /// Takes each element in the `Iterator`: if it is `None`, no further
926 /// elements are taken, and the `None` is returned. Should no `None` occur, a
927 /// container with the values of each `Option` is returned.
929 /// Here is an example which increments every integer in a vector,
930 /// checking for overflow:
935 /// let v = vec!(1, 2);
936 /// let res: Option<Vec<u16>> = v.iter().map(|&x: &u16|
937 /// if x == u16::MAX { None }
938 /// else { Some(x + 1) }
940 /// assert!(res == Some(vec!(2, 3)));
943 #[stable(feature = "rust1", since = "1.0.0")]
944 fn from_iter<I: IntoIterator<Item=Option<A>>>(iter: I) -> Option<V> {
945 // FIXME(#11084): This could be replaced with Iterator::scan when this
946 // performance bug is closed.
948 struct Adapter<Iter> {
953 impl<T, Iter: Iterator<Item=Option<T>>> Iterator for Adapter<Iter> {
957 fn next(&mut self) -> Option<T> {
958 match self.iter.next() {
959 Some(Some(value)) => Some(value),
961 self.found_none = true;
969 let mut adapter = Adapter { iter: iter.into_iter(), found_none: false };
970 let v: V = FromIterator::from_iter(adapter.by_ref());
972 if adapter.found_none {