src/libcore/option.rs

   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.
   4 //
   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.
  10
  11 //! Optional values
  12 //!
  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:
  17 //!
  18 //! * Initial values
  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
  22 //!   returned on error
  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
  28 //!
  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.
  31 //!
  32 //! ```
  33 //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
  34 //!     if denominator == 0.0 {
  35 //!         None
  36 //!     } else {
  37 //!         Some(numerator / denominator)
  38 //!     }
  39 //! }
  40 //!
  41 //! // The return value of the function is an option
  42 //! let result = divide(2.0, 3.0);
  43 //!
  44 //! // Pattern match to retrieve the value
  45 //! match result {
  46 //!     // The division was valid
  47 //!     Some(x) => println!("Result: {}", x),
  48 //!     // The division was invalid
  49 //!     None    => println!("Cannot divide by 0")
  50 //! }
  51 //! ```
  52 //!
  53 //
  54 // FIXME: Show how `Option` is used in practice, with lots of methods
  55 //
  56 //! # Options and pointers ("nullable" pointers)
  57 //!
  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>>`.
  61 //!
  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
  66 //! not (`None`).
  67 //!
  68 //! ```
  69 //! let optional: Option<Box<i32>> = None;
  70 //! check_optional(&optional);
  71 //!
  72 //! let optional: Option<Box<i32>> = Some(Box::new(9000));
  73 //! check_optional(&optional);
  74 //!
  75 //! fn check_optional(optional: &Option<Box<i32>>) {
  76 //!     match *optional {
  77 //!         Some(ref p) => println!("have value {}", p),
  78 //!         None => println!("have no value")
  79 //!     }
  80 //! }
  81 //! ```
  82 //!
  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.
  87 //!
  88 //! # Examples
  89 //!
  90 //! Basic pattern matching on `Option`:
  91 //!
  92 //! ```
  93 //! let msg = Some("howdy");
  94 //!
  95 //! // Take a reference to the contained string
  96 //! match msg {
  97 //!     Some(ref m) => println!("{}", *m),
  98 //!     None => ()
  99 //! }
 100 //!
 101 //! // Remove the contained string, destroying the Option
 102 //! let unwrapped_msg = match msg {
 103 //!     Some(m) => m,
 104 //!     None => "default message"
 105 //! };
 106 //! ```
 107 //!
 108 //! Initialize a result to `None` before a loop:
 109 //!
 110 //! ```
 111 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
 112 //!
 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"),
 121 //! ];
 122 //!
 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.iter() {
 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);
 133 //!         }
 134 //!         Kingdom::Animal(..) | Kingdom::Plant(..) => ()
 135 //!     }
 136 //! }
 137 //!
 138 //! match name_of_biggest_animal {
 139 //!     Some(name) => println!("the biggest animal is {}", name),
 140 //!     None => println!("there are no animals :(")
 141 //! }
 142 //! ```
 143
 144 #![stable(feature = "rust1", since = "1.0.0")]
 145
 146 use self::Option::*;
 147
 148 use clone::Clone;
 149 use cmp::{Eq, Ord};
 150 use default::Default;
 151 use iter::ExactSizeIterator;
 152 use iter::{Iterator, DoubleEndedIterator, FromIterator, IntoIterator};
 153 use mem;
 154 use ops::FnOnce;
 155 use result::Result::{Ok, Err};
 156 use result::Result;
 157 use slice;
 158
 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`.
 163
 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")]
 167 pub enum Option<T> {
 168     /// No value
 169     #[stable(feature = "rust1", since = "1.0.0")]
 170     None,
 171     /// Some value `T`
 172     #[stable(feature = "rust1", since = "1.0.0")]
 173     Some(T)
 174 }
 175
 176 /////////////////////////////////////////////////////////////////////////////
 177 // Type implementation
 178 /////////////////////////////////////////////////////////////////////////////
 179
 180 impl<T> Option<T> {
 181     /////////////////////////////////////////////////////////////////////////
 182     // Querying the contained values
 183     /////////////////////////////////////////////////////////////////////////
 184
 185     /// Returns `true` if the option is a `Some` value
 186     ///
 187     /// # Examples
 188     ///
 189     /// ```
 190     /// let x: Option<u32> = Some(2);
 191     /// assert_eq!(x.is_some(), true);
 192     ///
 193     /// let x: Option<u32> = None;
 194     /// assert_eq!(x.is_some(), false);
 195     /// ```
 196     #[inline]
 197     #[stable(feature = "rust1", since = "1.0.0")]
 198     pub fn is_some(&self) -> bool {
 199         match *self {
 200             Some(_) => true,
 201             None => false
 202         }
 203     }
 204
 205     /// Returns `true` if the option is a `None` value
 206     ///
 207     /// # Examples
 208     ///
 209     /// ```
 210     /// let x: Option<u32> = Some(2);
 211     /// assert_eq!(x.is_none(), false);
 212     ///
 213     /// let x: Option<u32> = None;
 214     /// assert_eq!(x.is_none(), true);
 215     /// ```
 216     #[inline]
 217     #[stable(feature = "rust1", since = "1.0.0")]
 218     pub fn is_none(&self) -> bool {
 219         !self.is_some()
 220     }
 221
 222     /////////////////////////////////////////////////////////////////////////
 223     // Adapter for working with references
 224     /////////////////////////////////////////////////////////////////////////
 225
 226     /// Converts from `Option<T>` to `Option<&T>`
 227     ///
 228     /// # Examples
 229     ///
 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.
 234     ///
 235     /// ```
 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);
 241     /// ```
 242     #[inline]
 243     #[stable(feature = "rust1", since = "1.0.0")]
 244     pub fn as_ref<'r>(&'r self) -> Option<&'r T> {
 245         match *self {
 246             Some(ref x) => Some(x),
 247             None => None
 248         }
 249     }
 250
 251     /// Converts from `Option<T>` to `Option<&mut T>`
 252     ///
 253     /// # Examples
 254     ///
 255     /// ```
 256     /// let mut x = Some(2);
 257     /// match x.as_mut() {
 258     ///     Some(v) => *v = 42,
 259     ///     None => {},
 260     /// }
 261     /// assert_eq!(x, Some(42));
 262     /// ```
 263     #[inline]
 264     #[stable(feature = "rust1", since = "1.0.0")]
 265     pub fn as_mut<'r>(&'r mut self) -> Option<&'r mut T> {
 266         match *self {
 267             Some(ref mut x) => Some(x),
 268             None => None
 269         }
 270     }
 271
 272     /// Converts from `Option<T>` to `&mut [T]` (without copying)
 273     ///
 274     /// # Examples
 275     ///
 276     /// ```
 277     /// # #![feature(core)]
 278     /// let mut x = Some("Diamonds");
 279     /// {
 280     ///     let v = x.as_mut_slice();
 281     ///     assert!(v == ["Diamonds"]);
 282     ///     v[0] = "Dirt";
 283     ///     assert!(v == ["Dirt"]);
 284     /// }
 285     /// assert_eq!(x, Some("Dirt"));
 286     /// ```
 287     #[inline]
 288     #[unstable(feature = "core",
 289                reason = "waiting for mut conventions")]
 290     pub fn as_mut_slice<'r>(&'r mut self) -> &'r mut [T] {
 291         match *self {
 292             Some(ref mut x) => {
 293                 let result: &mut [T] = slice::mut_ref_slice(x);
 294                 result
 295             }
 296             None => {
 297                 let result: &mut [T] = &mut [];
 298                 result
 299             }
 300         }
 301     }
 302
 303     /////////////////////////////////////////////////////////////////////////
 304     // Getting to contained values
 305     /////////////////////////////////////////////////////////////////////////
 306
 307     /// Unwraps an option, yielding the content of a `Some`
 308     ///
 309     /// # Panics
 310     ///
 311     /// Panics if the value is a `None` with a custom panic message provided by
 312     /// `msg`.
 313     ///
 314     /// # Examples
 315     ///
 316     /// ```
 317     /// let x = Some("value");
 318     /// assert_eq!(x.expect("the world is ending"), "value");
 319     /// ```
 320     ///
 321     /// ```{.should_panic}
 322     /// let x: Option<&str> = None;
 323     /// x.expect("the world is ending"); // panics with `world is ending`
 324     /// ```
 325     #[inline]
 326     #[stable(feature = "rust1", since = "1.0.0")]
 327     pub fn expect(self, msg: &str) -> T {
 328         match self {
 329             Some(val) => val,
 330             None => panic!("{}", msg),
 331         }
 332     }
 333
 334     /// Moves the value `v` out of the `Option<T>` if it is `Some(v)`.
 335     ///
 336     /// # Panics
 337     ///
 338     /// Panics if the self value equals `None`.
 339     ///
 340     /// # Safety note
 341     ///
 342     /// In general, because this function may panic, its use is discouraged.
 343     /// Instead, prefer to use pattern matching and handle the `None`
 344     /// case explicitly.
 345     ///
 346     /// # Examples
 347     ///
 348     /// ```
 349     /// let x = Some("air");
 350     /// assert_eq!(x.unwrap(), "air");
 351     /// ```
 352     ///
 353     /// ```{.should_panic}
 354     /// let x: Option<&str> = None;
 355     /// assert_eq!(x.unwrap(), "air"); // fails
 356     /// ```
 357     #[inline]
 358     #[stable(feature = "rust1", since = "1.0.0")]
 359     pub fn unwrap(self) -> T {
 360         match self {
 361             Some(val) => val,
 362             None => panic!("called `Option::unwrap()` on a `None` value"),
 363         }
 364     }
 365
 366     /// Returns the contained value or a default.
 367     ///
 368     /// # Examples
 369     ///
 370     /// ```
 371     /// assert_eq!(Some("car").unwrap_or("bike"), "car");
 372     /// assert_eq!(None.unwrap_or("bike"), "bike");
 373     /// ```
 374     #[inline]
 375     #[stable(feature = "rust1", since = "1.0.0")]
 376     pub fn unwrap_or(self, def: T) -> T {
 377         match self {
 378             Some(x) => x,
 379             None => def
 380         }
 381     }
 382
 383     /// Returns the contained value or computes it from a closure.
 384     ///
 385     /// # Examples
 386     ///
 387     /// ```
 388     /// let k = 10;
 389     /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
 390     /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
 391     /// ```
 392     #[inline]
 393     #[stable(feature = "rust1", since = "1.0.0")]
 394     pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
 395         match self {
 396             Some(x) => x,
 397             None => f()
 398         }
 399     }
 400
 401     /////////////////////////////////////////////////////////////////////////
 402     // Transforming contained values
 403     /////////////////////////////////////////////////////////////////////////
 404
 405     /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value
 406     ///
 407     /// # Examples
 408     ///
 409     /// Convert an `Option<String>` into an `Option<usize>`, consuming the original:
 410     ///
 411     /// ```
 412     /// let num_as_str: Option<String> = Some("10".to_string());
 413     /// // `Option::map` takes self *by value*, consuming `num_as_str`
 414     /// let num_as_int: Option<usize> = num_as_str.map(|n| n.len());
 415     /// ```
 416     #[inline]
 417     #[stable(feature = "rust1", since = "1.0.0")]
 418     pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
 419         match self {
 420             Some(x) => Some(f(x)),
 421             None => None
 422         }
 423     }
 424
 425     /// Applies a function to the contained value or returns a default.
 426     ///
 427     /// # Examples
 428     ///
 429     /// ```
 430     /// let x = Some("foo");
 431     /// assert_eq!(x.map_or(42, |v| v.len()), 3);
 432     ///
 433     /// let x: Option<&str> = None;
 434     /// assert_eq!(x.map_or(42, |v| v.len()), 42);
 435     /// ```
 436     #[inline]
 437     #[stable(feature = "rust1", since = "1.0.0")]
 438     pub fn map_or<U, F: FnOnce(T) -> U>(self, def: U, f: F) -> U {
 439         match self {
 440             Some(t) => f(t),
 441             None => def
 442         }
 443     }
 444
 445     /// Applies a function to the contained value or computes a default.
 446     ///
 447     /// # Examples
 448     ///
 449     /// ```
 450     /// let k = 21;
 451     ///
 452     /// let x = Some("foo");
 453     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
 454     ///
 455     /// let x: Option<&str> = None;
 456     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
 457     /// ```
 458     #[inline]
 459     #[stable(feature = "rust1", since = "1.0.0")]
 460     pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, def: D, f: F) -> U {
 461         match self {
 462             Some(t) => f(t),
 463             None => def()
 464         }
 465     }
 466
 467     /// Transforms the `Option<T>` into a `Result<T, E>`, mapping `Some(v)` to
 468     /// `Ok(v)` and `None` to `Err(err)`.
 469     ///
 470     /// # Examples
 471     ///
 472     /// ```
 473     /// # #![feature(core)]
 474     /// let x = Some("foo");
 475     /// assert_eq!(x.ok_or(0), Ok("foo"));
 476     ///
 477     /// let x: Option<&str> = None;
 478     /// assert_eq!(x.ok_or(0), Err(0));
 479     /// ```
 480     #[inline]
 481     #[stable(feature = "rust1", since = "1.0.0")]
 482     pub fn ok_or<E>(self, err: E) -> Result<T, E> {
 483         match self {
 484             Some(v) => Ok(v),
 485             None => Err(err),
 486         }
 487     }
 488
 489     /// Transforms the `Option<T>` into a `Result<T, E>`, mapping `Some(v)` to
 490     /// `Ok(v)` and `None` to `Err(err())`.
 491     ///
 492     /// # Examples
 493     ///
 494     /// ```
 495     /// # #![feature(core)]
 496     /// let x = Some("foo");
 497     /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
 498     ///
 499     /// let x: Option<&str> = None;
 500     /// assert_eq!(x.ok_or_else(|| 0), Err(0));
 501     /// ```
 502     #[inline]
 503     #[stable(feature = "rust1", since = "1.0.0")]
 504     pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
 505         match self {
 506             Some(v) => Ok(v),
 507             None => Err(err()),
 508         }
 509     }
 510
 511     /////////////////////////////////////////////////////////////////////////
 512     // Iterator constructors
 513     /////////////////////////////////////////////////////////////////////////
 514
 515     /// Returns an iterator over the possibly contained value.
 516     ///
 517     /// # Examples
 518     ///
 519     /// ```
 520     /// let x = Some(4);
 521     /// assert_eq!(x.iter().next(), Some(&4));
 522     ///
 523     /// let x: Option<u32> = None;
 524     /// assert_eq!(x.iter().next(), None);
 525     /// ```
 526     #[inline]
 527     #[stable(feature = "rust1", since = "1.0.0")]
 528     pub fn iter(&self) -> Iter<T> {
 529         Iter { inner: Item { opt: self.as_ref() } }
 530     }
 531
 532     /// Returns a mutable iterator over the possibly contained value.
 533     ///
 534     /// # Examples
 535     ///
 536     /// ```
 537     /// # #![feature(core)]
 538     /// let mut x = Some(4);
 539     /// match x.iter_mut().next() {
 540     ///     Some(&mut ref mut v) => *v = 42,
 541     ///     None => {},
 542     /// }
 543     /// assert_eq!(x, Some(42));
 544     ///
 545     /// let mut x: Option<u32> = None;
 546     /// assert_eq!(x.iter_mut().next(), None);
 547     /// ```
 548     #[inline]
 549     #[stable(feature = "rust1", since = "1.0.0")]
 550     pub fn iter_mut(&mut self) -> IterMut<T> {
 551         IterMut { inner: Item { opt: self.as_mut() } }
 552     }
 553
 554     /////////////////////////////////////////////////////////////////////////
 555     // Boolean operations on the values, eager and lazy
 556     /////////////////////////////////////////////////////////////////////////
 557
 558     /// Returns `None` if the option is `None`, otherwise returns `optb`.
 559     ///
 560     /// # Examples
 561     ///
 562     /// ```
 563     /// let x = Some(2);
 564     /// let y: Option<&str> = None;
 565     /// assert_eq!(x.and(y), None);
 566     ///
 567     /// let x: Option<u32> = None;
 568     /// let y = Some("foo");
 569     /// assert_eq!(x.and(y), None);
 570     ///
 571     /// let x = Some(2);
 572     /// let y = Some("foo");
 573     /// assert_eq!(x.and(y), Some("foo"));
 574     ///
 575     /// let x: Option<u32> = None;
 576     /// let y: Option<&str> = None;
 577     /// assert_eq!(x.and(y), None);
 578     /// ```
 579     #[inline]
 580     #[stable(feature = "rust1", since = "1.0.0")]
 581     pub fn and<U>(self, optb: Option<U>) -> Option<U> {
 582         match self {
 583             Some(_) => optb,
 584             None => None,
 585         }
 586     }
 587
 588     /// Returns `None` if the option is `None`, otherwise calls `f` with the
 589     /// wrapped value and returns the result.
 590     ///
 591     /// Some languages call this operation flatmap.
 592     ///
 593     /// # Examples
 594     ///
 595     /// ```
 596     /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
 597     /// fn nope(_: u32) -> Option<u32> { None }
 598     ///
 599     /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
 600     /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
 601     /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
 602     /// assert_eq!(None.and_then(sq).and_then(sq), None);
 603     /// ```
 604     #[inline]
 605     #[stable(feature = "rust1", since = "1.0.0")]
 606     pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
 607         match self {
 608             Some(x) => f(x),
 609             None => None,
 610         }
 611     }
 612
 613     /// Returns the option if it contains a value, otherwise returns `optb`.
 614     ///
 615     /// # Examples
 616     ///
 617     /// ```
 618     /// let x = Some(2);
 619     /// let y = None;
 620     /// assert_eq!(x.or(y), Some(2));
 621     ///
 622     /// let x = None;
 623     /// let y = Some(100);
 624     /// assert_eq!(x.or(y), Some(100));
 625     ///
 626     /// let x = Some(2);
 627     /// let y = Some(100);
 628     /// assert_eq!(x.or(y), Some(2));
 629     ///
 630     /// let x: Option<u32> = None;
 631     /// let y = None;
 632     /// assert_eq!(x.or(y), None);
 633     /// ```
 634     #[inline]
 635     #[stable(feature = "rust1", since = "1.0.0")]
 636     pub fn or(self, optb: Option<T>) -> Option<T> {
 637         match self {
 638             Some(_) => self,
 639             None => optb
 640         }
 641     }
 642
 643     /// Returns the option if it contains a value, otherwise calls `f` and
 644     /// returns the result.
 645     ///
 646     /// # Examples
 647     ///
 648     /// ```
 649     /// fn nobody() -> Option<&'static str> { None }
 650     /// fn vikings() -> Option<&'static str> { Some("vikings") }
 651     ///
 652     /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
 653     /// assert_eq!(None.or_else(vikings), Some("vikings"));
 654     /// assert_eq!(None.or_else(nobody), None);
 655     /// ```
 656     #[inline]
 657     #[stable(feature = "rust1", since = "1.0.0")]
 658     pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
 659         match self {
 660             Some(_) => self,
 661             None => f()
 662         }
 663     }
 664
 665     /////////////////////////////////////////////////////////////////////////
 666     // Misc
 667     /////////////////////////////////////////////////////////////////////////
 668
 669     /// Takes the value out of the option, leaving a `None` in its place.
 670     ///
 671     /// # Examples
 672     ///
 673     /// ```
 674     /// let mut x = Some(2);
 675     /// x.take();
 676     /// assert_eq!(x, None);
 677     ///
 678     /// let mut x: Option<u32> = None;
 679     /// x.take();
 680     /// assert_eq!(x, None);
 681     /// ```
 682     #[inline]
 683     #[stable(feature = "rust1", since = "1.0.0")]
 684     pub fn take(&mut self) -> Option<T> {
 685         mem::replace(self, None)
 686     }
 687
 688     /// Converts from `Option<T>` to `&[T]` (without copying)
 689     #[inline]
 690     #[unstable(feature = "as_slice", since = "unsure of the utility here")]
 691     pub fn as_slice<'a>(&'a self) -> &'a [T] {
 692         match *self {
 693             Some(ref x) => slice::ref_slice(x),
 694             None => {
 695                 let result: &[_] = &[];
 696                 result
 697             }
 698         }
 699     }
 700 }
 701
 702 impl<'a, T: Clone> Option<&'a T> {
 703     /// Maps an Option<&T> to an Option<T> by cloning the contents of the Option.
 704     #[stable(feature = "rust1", since = "1.0.0")]
 705     pub fn cloned(self) -> Option<T> {
 706         self.map(|t| t.clone())
 707     }
 708 }
 709
 710 impl<T: Default> Option<T> {
 711     /// Returns the contained value or a default
 712     ///
 713     /// Consumes the `self` argument then, if `Some`, returns the contained
 714     /// value, otherwise if `None`, returns the default value for that
 715     /// type.
 716     ///
 717     /// # Examples
 718     ///
 719     /// Convert a string to an integer, turning poorly-formed strings
 720     /// into 0 (the default value for integers). `parse` converts
 721     /// a string to any other type that implements `FromStr`, returning
 722     /// `None` on error.
 723     ///
 724     /// ```
 725     /// let good_year_from_input = "1909";
 726     /// let bad_year_from_input = "190blarg";
 727     /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
 728     /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
 729     ///
 730     /// assert_eq!(1909, good_year);
 731     /// assert_eq!(0, bad_year);
 732     /// ```
 733     #[inline]
 734     #[stable(feature = "rust1", since = "1.0.0")]
 735     pub fn unwrap_or_default(self) -> T {
 736         match self {
 737             Some(x) => x,
 738             None => Default::default()
 739         }
 740     }
 741 }
 742
 743 /////////////////////////////////////////////////////////////////////////////
 744 // Trait implementations
 745 /////////////////////////////////////////////////////////////////////////////
 746
 747 #[stable(feature = "rust1", since = "1.0.0")]
 748 impl<T> Default for Option<T> {
 749     #[inline]
 750     #[stable(feature = "rust1", since = "1.0.0")]
 751     fn default() -> Option<T> { None }
 752 }
 753
 754 #[stable(feature = "rust1", since = "1.0.0")]
 755 impl<T> IntoIterator for Option<T> {
 756     type Item = T;
 757     type IntoIter = IntoIter<T>;
 758
 759     /// Returns a consuming iterator over the possibly contained value.
 760     ///
 761     /// # Examples
 762     ///
 763     /// ```
 764     /// let x = Some("string");
 765     /// let v: Vec<&str> = x.into_iter().collect();
 766     /// assert_eq!(v, ["string"]);
 767     ///
 768     /// let x = None;
 769     /// let v: Vec<&str> = x.into_iter().collect();
 770     /// assert!(v.is_empty());
 771     /// ```
 772     #[inline]
 773     fn into_iter(self) -> IntoIter<T> {
 774         IntoIter { inner: Item { opt: self } }
 775     }
 776 }
 777
 778 /////////////////////////////////////////////////////////////////////////////
 779 // The Option Iterators
 780 /////////////////////////////////////////////////////////////////////////////
 781
 782 #[derive(Clone)]
 783 struct Item<A> {
 784     opt: Option<A>
 785 }
 786
 787 impl<A> Iterator for Item<A> {
 788     type Item = A;
 789
 790     #[inline]
 791     fn next(&mut self) -> Option<A> {
 792         self.opt.take()
 793     }
 794
 795     #[inline]
 796     fn size_hint(&self) -> (usize, Option<usize>) {
 797         match self.opt {
 798             Some(_) => (1, Some(1)),
 799             None => (0, Some(0)),
 800         }
 801     }
 802 }
 803
 804 impl<A> DoubleEndedIterator for Item<A> {
 805     #[inline]
 806     fn next_back(&mut self) -> Option<A> {
 807         self.opt.take()
 808     }
 809 }
 810
 811 impl<A> ExactSizeIterator for Item<A> {}
 812
 813 /// An iterator over a reference of the contained item in an Option.
 814 #[stable(feature = "rust1", since = "1.0.0")]
 815 pub struct Iter<'a, A: 'a> { inner: Item<&'a A> }
 816
 817 #[stable(feature = "rust1", since = "1.0.0")]
 818 impl<'a, A> Iterator for Iter<'a, A> {
 819     type Item = &'a A;
 820
 821     #[inline]
 822     fn next(&mut self) -> Option<&'a A> { self.inner.next() }
 823     #[inline]
 824     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 825 }
 826
 827 #[stable(feature = "rust1", since = "1.0.0")]
 828 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
 829     #[inline]
 830     fn next_back(&mut self) -> Option<&'a A> { self.inner.next_back() }
 831 }
 832
 833 #[stable(feature = "rust1", since = "1.0.0")]
 834 impl<'a, A> ExactSizeIterator for Iter<'a, A> {}
 835
 836 #[stable(feature = "rust1", since = "1.0.0")]
 837 impl<'a, A> Clone for Iter<'a, A> {
 838     fn clone(&self) -> Iter<'a, A> {
 839         Iter { inner: self.inner.clone() }
 840     }
 841 }
 842
 843 /// An iterator over a mutable reference of the contained item in an Option.
 844 #[stable(feature = "rust1", since = "1.0.0")]
 845 pub struct IterMut<'a, A: 'a> { inner: Item<&'a mut A> }
 846
 847 #[stable(feature = "rust1", since = "1.0.0")]
 848 impl<'a, A> Iterator for IterMut<'a, A> {
 849     type Item = &'a mut A;
 850
 851     #[inline]
 852     fn next(&mut self) -> Option<&'a mut A> { self.inner.next() }
 853     #[inline]
 854     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 855 }
 856
 857 #[stable(feature = "rust1", since = "1.0.0")]
 858 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
 859     #[inline]
 860     fn next_back(&mut self) -> Option<&'a mut A> { self.inner.next_back() }
 861 }
 862
 863 #[stable(feature = "rust1", since = "1.0.0")]
 864 impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}
 865
 866 /// An iterator over the item contained inside an Option.
 867 #[stable(feature = "rust1", since = "1.0.0")]
 868 pub struct IntoIter<A> { inner: Item<A> }
 869
 870 #[stable(feature = "rust1", since = "1.0.0")]
 871 impl<A> Iterator for IntoIter<A> {
 872     type Item = A;
 873
 874     #[inline]
 875     fn next(&mut self) -> Option<A> { self.inner.next() }
 876     #[inline]
 877     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 878 }
 879
 880 #[stable(feature = "rust1", since = "1.0.0")]
 881 impl<A> DoubleEndedIterator for IntoIter<A> {
 882     #[inline]
 883     fn next_back(&mut self) -> Option<A> { self.inner.next_back() }
 884 }
 885
 886 #[stable(feature = "rust1", since = "1.0.0")]
 887 impl<A> ExactSizeIterator for IntoIter<A> {}
 888
 889 /////////////////////////////////////////////////////////////////////////////
 890 // FromIterator
 891 /////////////////////////////////////////////////////////////////////////////
 892
 893 #[stable(feature = "rust1", since = "1.0.0")]
 894 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
 895     /// Takes each element in the `Iterator`: if it is `None`, no further
 896     /// elements are taken, and the `None` is returned. Should no `None` occur, a
 897     /// container with the values of each `Option` is returned.
 898     ///
 899     /// Here is an example which increments every integer in a vector,
 900     /// checking for overflow:
 901     ///
 902     /// ```
 903     /// use std::u16;
 904     ///
 905     /// let v = vec!(1, 2);
 906     /// let res: Option<Vec<u16>> = v.iter().map(|&x: &u16|
 907     ///     if x == u16::MAX { None }
 908     ///     else { Some(x + 1) }
 909     /// ).collect();
 910     /// assert!(res == Some(vec!(2, 3)));
 911     /// ```
 912     #[inline]
 913     #[stable(feature = "rust1", since = "1.0.0")]
 914     fn from_iter<I: IntoIterator<Item=Option<A>>>(iter: I) -> Option<V> {
 915         // FIXME(#11084): This could be replaced with Iterator::scan when this
 916         // performance bug is closed.
 917
 918         struct Adapter<Iter> {
 919             iter: Iter,
 920             found_none: bool,
 921         }
 922
 923         impl<T, Iter: Iterator<Item=Option<T>>> Iterator for Adapter<Iter> {
 924             type Item = T;
 925
 926             #[inline]
 927             fn next(&mut self) -> Option<T> {
 928                 match self.iter.next() {
 929                     Some(Some(value)) => Some(value),
 930                     Some(None) => {
 931                         self.found_none = true;
 932                         None
 933                     }
 934                     None => None,
 935                 }
 936             }
 937         }
 938
 939         let mut adapter = Adapter { iter: iter.into_iter(), found_none: false };
 940         let v: V = FromIterator::from_iter(adapter.by_ref());
 941
 942         if adapter.found_none {
 943             None
 944         } else {
 945             Some(v)
 946         }
 947     }
 948 }