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 //! [`Option`]s 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(...)`][`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 //! if let Some(ref m) = msg {
  97 //!     println!("{}", *m);
  98 //! }
  99 //!
 100 //! // Remove the contained string, destroying the Option
 101 //! let unwrapped_msg = msg.unwrap_or("default message");
 102 //! ```
 103 //!
 104 //! Initialize a result to [`None`] before a loop:
 105 //!
 106 //! ```
 107 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
 108 //!
 109 //! // A list of data to search through.
 110 //! let all_the_big_things = [
 111 //!     Kingdom::Plant(250, "redwood"),
 112 //!     Kingdom::Plant(230, "noble fir"),
 113 //!     Kingdom::Plant(229, "sugar pine"),
 114 //!     Kingdom::Animal(25, "blue whale"),
 115 //!     Kingdom::Animal(19, "fin whale"),
 116 //!     Kingdom::Animal(15, "north pacific right whale"),
 117 //! ];
 118 //!
 119 //! // We're going to search for the name of the biggest animal,
 120 //! // but to start with we've just got `None`.
 121 //! let mut name_of_biggest_animal = None;
 122 //! let mut size_of_biggest_animal = 0;
 123 //! for big_thing in &all_the_big_things {
 124 //!     match *big_thing {
 125 //!         Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
 126 //!             // Now we've found the name of some big animal
 127 //!             size_of_biggest_animal = size;
 128 //!             name_of_biggest_animal = Some(name);
 129 //!         }
 130 //!         Kingdom::Animal(..) | Kingdom::Plant(..) => ()
 131 //!     }
 132 //! }
 133 //!
 134 //! match name_of_biggest_animal {
 135 //!     Some(name) => println!("the biggest animal is {}", name),
 136 //!     None => println!("there are no animals :("),
 137 //! }
 138 //! ```
 139 //!
 140 //! [`Option`]: enum.Option.html
 141 //! [`Some`]: enum.Option.html#variant.Some
 142 //! [`None`]: enum.Option.html#variant.None
 143 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
 144 //! [`i32`]: ../../std/primitive.i32.html
 145
 146 #![stable(feature = "rust1", since = "1.0.0")]
 147
 148 use iter::{FromIterator, FusedIterator, TrustedLen};
 149 use mem;
 150
 151 // Note that this is not a lang item per se, but it has a hidden dependency on
 152 // `Iterator`, which is one. The compiler assumes that the `next` method of
 153 // `Iterator` is an enumeration with one type parameter and two variants,
 154 // which basically means it must be `Option`.
 155
 156 /// The `Option` type. See [the module level documentation](index.html) for more.
 157 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 158 #[stable(feature = "rust1", since = "1.0.0")]
 159 pub enum Option<T> {
 160     /// No value
 161     #[stable(feature = "rust1", since = "1.0.0")]
 162     None,
 163     /// Some value `T`
 164     #[stable(feature = "rust1", since = "1.0.0")]
 165     Some(#[stable(feature = "rust1", since = "1.0.0")] T),
 166 }
 167
 168 /////////////////////////////////////////////////////////////////////////////
 169 // Type implementation
 170 /////////////////////////////////////////////////////////////////////////////
 171
 172 impl<T> Option<T> {
 173     /////////////////////////////////////////////////////////////////////////
 174     // Querying the contained values
 175     /////////////////////////////////////////////////////////////////////////
 176
 177     /// Returns `true` if the option is a `Some` value.
 178     ///
 179     /// # Examples
 180     ///
 181     /// ```
 182     /// let x: Option<u32> = Some(2);
 183     /// assert_eq!(x.is_some(), true);
 184     ///
 185     /// let x: Option<u32> = None;
 186     /// assert_eq!(x.is_some(), false);
 187     /// ```
 188     #[inline]
 189     #[stable(feature = "rust1", since = "1.0.0")]
 190     pub fn is_some(&self) -> bool {
 191         match *self {
 192             Some(_) => true,
 193             None => false,
 194         }
 195     }
 196
 197     /// Returns `true` if the option is a `None` value.
 198     ///
 199     /// # Examples
 200     ///
 201     /// ```
 202     /// let x: Option<u32> = Some(2);
 203     /// assert_eq!(x.is_none(), false);
 204     ///
 205     /// let x: Option<u32> = None;
 206     /// assert_eq!(x.is_none(), true);
 207     /// ```
 208     #[inline]
 209     #[stable(feature = "rust1", since = "1.0.0")]
 210     pub fn is_none(&self) -> bool {
 211         !self.is_some()
 212     }
 213
 214     /////////////////////////////////////////////////////////////////////////
 215     // Adapter for working with references
 216     /////////////////////////////////////////////////////////////////////////
 217
 218     /// Converts from `Option<T>` to `Option<&T>`.
 219     ///
 220     /// # Examples
 221     ///
 222     /// Convert an `Option<String>` into an `Option<usize>`, preserving the original.
 223     /// The [`map`] method takes the `self` argument by value, consuming the original,
 224     /// so this technique uses `as_ref` to first take an `Option` to a reference
 225     /// to the value inside the original.
 226     ///
 227     /// [`map`]: enum.Option.html#method.map
 228     ///
 229     /// ```
 230     /// let num_as_str: Option<String> = Some("10".to_string());
 231     /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
 232     /// // then consume *that* with `map`, leaving `num_as_str` on the stack.
 233     /// let num_as_int: Option<usize> = num_as_str.as_ref().map(|n| n.len());
 234     /// println!("still can print num_as_str: {:?}", num_as_str);
 235     /// ```
 236     #[inline]
 237     #[stable(feature = "rust1", since = "1.0.0")]
 238     pub fn as_ref(&self) -> Option<&T> {
 239         match *self {
 240             Some(ref x) => Some(x),
 241             None => None,
 242         }
 243     }
 244
 245     /// Converts from `Option<T>` to `Option<&mut T>`.
 246     ///
 247     /// # Examples
 248     ///
 249     /// ```
 250     /// let mut x = Some(2);
 251     /// match x.as_mut() {
 252     ///     Some(v) => *v = 42,
 253     ///     None => {},
 254     /// }
 255     /// assert_eq!(x, Some(42));
 256     /// ```
 257     #[inline]
 258     #[stable(feature = "rust1", since = "1.0.0")]
 259     pub fn as_mut(&mut self) -> Option<&mut T> {
 260         match *self {
 261             Some(ref mut x) => Some(x),
 262             None => None,
 263         }
 264     }
 265
 266     /////////////////////////////////////////////////////////////////////////
 267     // Getting to contained values
 268     /////////////////////////////////////////////////////////////////////////
 269
 270     /// Unwraps an option, yielding the content of a `Some`.
 271     ///
 272     /// # Panics
 273     ///
 274     /// Panics if the value is a `None` with a custom panic message provided by
 275     /// `msg`.
 276     ///
 277     /// # Examples
 278     ///
 279     /// ```
 280     /// let x = Some("value");
 281     /// assert_eq!(x.expect("the world is ending"), "value");
 282     /// ```
 283     ///
 284     /// ```{.should_panic}
 285     /// let x: Option<&str> = None;
 286     /// x.expect("the world is ending"); // panics with `the world is ending`
 287     /// ```
 288     #[inline]
 289     #[stable(feature = "rust1", since = "1.0.0")]
 290     pub fn expect(self, msg: &str) -> T {
 291         match self {
 292             Some(val) => val,
 293             None => expect_failed(msg),
 294         }
 295     }
 296
 297     /// Moves the value `v` out of the `Option<T>` if it is `Some(v)`.
 298     ///
 299     /// In general, because this function may panic, its use is discouraged.
 300     /// Instead, prefer to use pattern matching and handle the `None`
 301     /// case explicitly.
 302     ///
 303     /// # Panics
 304     ///
 305     /// Panics if the self value equals `None`.
 306     ///
 307     /// # Examples
 308     ///
 309     /// ```
 310     /// let x = Some("air");
 311     /// assert_eq!(x.unwrap(), "air");
 312     /// ```
 313     ///
 314     /// ```{.should_panic}
 315     /// let x: Option<&str> = None;
 316     /// assert_eq!(x.unwrap(), "air"); // fails
 317     /// ```
 318     #[inline]
 319     #[stable(feature = "rust1", since = "1.0.0")]
 320     pub fn unwrap(self) -> T {
 321         match self {
 322             Some(val) => val,
 323             None => panic!("called `Option::unwrap()` on a `None` value"),
 324         }
 325     }
 326
 327     /// Returns the contained value or a default.
 328     ///
 329     /// # Examples
 330     ///
 331     /// ```
 332     /// assert_eq!(Some("car").unwrap_or("bike"), "car");
 333     /// assert_eq!(None.unwrap_or("bike"), "bike");
 334     /// ```
 335     #[inline]
 336     #[stable(feature = "rust1", since = "1.0.0")]
 337     pub fn unwrap_or(self, def: T) -> T {
 338         match self {
 339             Some(x) => x,
 340             None => def,
 341         }
 342     }
 343
 344     /// Returns the contained value or computes it from a closure.
 345     ///
 346     /// # Examples
 347     ///
 348     /// ```
 349     /// let k = 10;
 350     /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
 351     /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
 352     /// ```
 353     #[inline]
 354     #[stable(feature = "rust1", since = "1.0.0")]
 355     pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
 356         match self {
 357             Some(x) => x,
 358             None => f(),
 359         }
 360     }
 361
 362     /////////////////////////////////////////////////////////////////////////
 363     // Transforming contained values
 364     /////////////////////////////////////////////////////////////////////////
 365
 366     /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
 367     ///
 368     /// # Examples
 369     ///
 370     /// Convert an `Option<String>` into an `Option<usize>`, consuming the original:
 371     ///
 372     /// ```
 373     /// let maybe_some_string = Some(String::from("Hello, World!"));
 374     /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
 375     /// let maybe_some_len = maybe_some_string.map(|s| s.len());
 376     ///
 377     /// assert_eq!(maybe_some_len, Some(13));
 378     /// ```
 379     #[inline]
 380     #[stable(feature = "rust1", since = "1.0.0")]
 381     pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
 382         match self {
 383             Some(x) => Some(f(x)),
 384             None => None,
 385         }
 386     }
 387
 388     /// Applies a function to the contained value (if any),
 389     /// or returns a `default` (if not).
 390     ///
 391     /// # Examples
 392     ///
 393     /// ```
 394     /// let x = Some("foo");
 395     /// assert_eq!(x.map_or(42, |v| v.len()), 3);
 396     ///
 397     /// let x: Option<&str> = None;
 398     /// assert_eq!(x.map_or(42, |v| v.len()), 42);
 399     /// ```
 400     #[inline]
 401     #[stable(feature = "rust1", since = "1.0.0")]
 402     pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
 403         match self {
 404             Some(t) => f(t),
 405             None => default,
 406         }
 407     }
 408
 409     /// Applies a function to the contained value (if any),
 410     /// or computes a `default` (if not).
 411     ///
 412     /// # Examples
 413     ///
 414     /// ```
 415     /// let k = 21;
 416     ///
 417     /// let x = Some("foo");
 418     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
 419     ///
 420     /// let x: Option<&str> = None;
 421     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
 422     /// ```
 423     #[inline]
 424     #[stable(feature = "rust1", since = "1.0.0")]
 425     pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
 426         match self {
 427             Some(t) => f(t),
 428             None => default(),
 429         }
 430     }
 431
 432     /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping `Some(v)` to
 433     /// [`Ok(v)`] and `None` to [`Err(err)`][Err].
 434     ///
 435     /// [`Result<T, E>`]: ../../std/result/enum.Result.html
 436     /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
 437     /// [Err]: ../../std/result/enum.Result.html#variant.Err
 438     ///
 439     /// # Examples
 440     ///
 441     /// ```
 442     /// let x = Some("foo");
 443     /// assert_eq!(x.ok_or(0), Ok("foo"));
 444     ///
 445     /// let x: Option<&str> = None;
 446     /// assert_eq!(x.ok_or(0), Err(0));
 447     /// ```
 448     #[inline]
 449     #[stable(feature = "rust1", since = "1.0.0")]
 450     pub fn ok_or<E>(self, err: E) -> Result<T, E> {
 451         match self {
 452             Some(v) => Ok(v),
 453             None => Err(err),
 454         }
 455     }
 456
 457     /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping `Some(v)` to
 458     /// [`Ok(v)`] and `None` to [`Err(err())`][Err].
 459     ///
 460     /// [`Result<T, E>`]: ../../std/result/enum.Result.html
 461     /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
 462     /// [Err]: ../../std/result/enum.Result.html#variant.Err
 463     ///
 464     /// # Examples
 465     ///
 466     /// ```
 467     /// let x = Some("foo");
 468     /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
 469     ///
 470     /// let x: Option<&str> = None;
 471     /// assert_eq!(x.ok_or_else(|| 0), Err(0));
 472     /// ```
 473     #[inline]
 474     #[stable(feature = "rust1", since = "1.0.0")]
 475     pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
 476         match self {
 477             Some(v) => Ok(v),
 478             None => Err(err()),
 479         }
 480     }
 481
 482     /////////////////////////////////////////////////////////////////////////
 483     // Iterator constructors
 484     /////////////////////////////////////////////////////////////////////////
 485
 486     /// Returns an iterator over the possibly contained value.
 487     ///
 488     /// # Examples
 489     ///
 490     /// ```
 491     /// let x = Some(4);
 492     /// assert_eq!(x.iter().next(), Some(&4));
 493     ///
 494     /// let x: Option<u32> = None;
 495     /// assert_eq!(x.iter().next(), None);
 496     /// ```
 497     #[inline]
 498     #[stable(feature = "rust1", since = "1.0.0")]
 499     pub fn iter(&self) -> Iter<T> {
 500         Iter { inner: Item { opt: self.as_ref() } }
 501     }
 502
 503     /// Returns a mutable iterator over the possibly contained value.
 504     ///
 505     /// # Examples
 506     ///
 507     /// ```
 508     /// let mut x = Some(4);
 509     /// match x.iter_mut().next() {
 510     ///     Some(v) => *v = 42,
 511     ///     None => {},
 512     /// }
 513     /// assert_eq!(x, Some(42));
 514     ///
 515     /// let mut x: Option<u32> = None;
 516     /// assert_eq!(x.iter_mut().next(), None);
 517     /// ```
 518     #[inline]
 519     #[stable(feature = "rust1", since = "1.0.0")]
 520     pub fn iter_mut(&mut self) -> IterMut<T> {
 521         IterMut { inner: Item { opt: self.as_mut() } }
 522     }
 523
 524     /////////////////////////////////////////////////////////////////////////
 525     // Boolean operations on the values, eager and lazy
 526     /////////////////////////////////////////////////////////////////////////
 527
 528     /// Returns `None` if the option is `None`, otherwise returns `optb`.
 529     ///
 530     /// # Examples
 531     ///
 532     /// ```
 533     /// let x = Some(2);
 534     /// let y: Option<&str> = None;
 535     /// assert_eq!(x.and(y), None);
 536     ///
 537     /// let x: Option<u32> = None;
 538     /// let y = Some("foo");
 539     /// assert_eq!(x.and(y), None);
 540     ///
 541     /// let x = Some(2);
 542     /// let y = Some("foo");
 543     /// assert_eq!(x.and(y), Some("foo"));
 544     ///
 545     /// let x: Option<u32> = None;
 546     /// let y: Option<&str> = None;
 547     /// assert_eq!(x.and(y), None);
 548     /// ```
 549     #[inline]
 550     #[stable(feature = "rust1", since = "1.0.0")]
 551     pub fn and<U>(self, optb: Option<U>) -> Option<U> {
 552         match self {
 553             Some(_) => optb,
 554             None => None,
 555         }
 556     }
 557
 558     /// Returns `None` if the option is `None`, otherwise calls `f` with the
 559     /// wrapped value and returns the result.
 560     ///
 561     /// Some languages call this operation flatmap.
 562     ///
 563     /// # Examples
 564     ///
 565     /// ```
 566     /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
 567     /// fn nope(_: u32) -> Option<u32> { None }
 568     ///
 569     /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
 570     /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
 571     /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
 572     /// assert_eq!(None.and_then(sq).and_then(sq), None);
 573     /// ```
 574     #[inline]
 575     #[stable(feature = "rust1", since = "1.0.0")]
 576     pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
 577         match self {
 578             Some(x) => f(x),
 579             None => None,
 580         }
 581     }
 582
 583     /// Returns the option if it contains a value, otherwise returns `optb`.
 584     ///
 585     /// # Examples
 586     ///
 587     /// ```
 588     /// let x = Some(2);
 589     /// let y = None;
 590     /// assert_eq!(x.or(y), Some(2));
 591     ///
 592     /// let x = None;
 593     /// let y = Some(100);
 594     /// assert_eq!(x.or(y), Some(100));
 595     ///
 596     /// let x = Some(2);
 597     /// let y = Some(100);
 598     /// assert_eq!(x.or(y), Some(2));
 599     ///
 600     /// let x: Option<u32> = None;
 601     /// let y = None;
 602     /// assert_eq!(x.or(y), None);
 603     /// ```
 604     #[inline]
 605     #[stable(feature = "rust1", since = "1.0.0")]
 606     pub fn or(self, optb: Option<T>) -> Option<T> {
 607         match self {
 608             Some(_) => self,
 609             None => optb,
 610         }
 611     }
 612
 613     /// Returns the option if it contains a value, otherwise calls `f` and
 614     /// returns the result.
 615     ///
 616     /// # Examples
 617     ///
 618     /// ```
 619     /// fn nobody() -> Option<&'static str> { None }
 620     /// fn vikings() -> Option<&'static str> { Some("vikings") }
 621     ///
 622     /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
 623     /// assert_eq!(None.or_else(vikings), Some("vikings"));
 624     /// assert_eq!(None.or_else(nobody), None);
 625     /// ```
 626     #[inline]
 627     #[stable(feature = "rust1", since = "1.0.0")]
 628     pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
 629         match self {
 630             Some(_) => self,
 631             None => f(),
 632         }
 633     }
 634
 635     /////////////////////////////////////////////////////////////////////////
 636     // Misc
 637     /////////////////////////////////////////////////////////////////////////
 638
 639     /// Takes the value out of the option, leaving a `None` in its place.
 640     ///
 641     /// # Examples
 642     ///
 643     /// ```
 644     /// let mut x = Some(2);
 645     /// x.take();
 646     /// assert_eq!(x, None);
 647     ///
 648     /// let mut x: Option<u32> = None;
 649     /// x.take();
 650     /// assert_eq!(x, None);
 651     /// ```
 652     #[inline]
 653     #[stable(feature = "rust1", since = "1.0.0")]
 654     pub fn take(&mut self) -> Option<T> {
 655         mem::replace(self, None)
 656     }
 657 }
 658
 659 impl<'a, T: Clone> Option<&'a T> {
 660     /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
 661     /// option.
 662     #[stable(feature = "rust1", since = "1.0.0")]
 663     pub fn cloned(self) -> Option<T> {
 664         self.map(|t| t.clone())
 665     }
 666 }
 667
 668 impl<T: Default> Option<T> {
 669     /// Returns the contained value or a default
 670     ///
 671     /// Consumes the `self` argument then, if `Some`, returns the contained
 672     /// value, otherwise if `None`, returns the default value for that
 673     /// type.
 674     ///
 675     /// # Examples
 676     ///
 677     /// Convert a string to an integer, turning poorly-formed strings
 678     /// into 0 (the default value for integers). `parse` converts
 679     /// a string to any other type that implements `FromStr`, returning
 680     /// `None` on error.
 681     ///
 682     /// ```
 683     /// let good_year_from_input = "1909";
 684     /// let bad_year_from_input = "190blarg";
 685     /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
 686     /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
 687     ///
 688     /// assert_eq!(1909, good_year);
 689     /// assert_eq!(0, bad_year);
 690     /// ```
 691     #[inline]
 692     #[stable(feature = "rust1", since = "1.0.0")]
 693     pub fn unwrap_or_default(self) -> T {
 694         match self {
 695             Some(x) => x,
 696             None => Default::default(),
 697         }
 698     }
 699 }
 700
 701 // This is a separate function to reduce the code size of .expect() itself.
 702 #[inline(never)]
 703 #[cold]
 704 fn expect_failed(msg: &str) -> ! {
 705     panic!("{}", msg)
 706 }
 707
 708
 709 /////////////////////////////////////////////////////////////////////////////
 710 // Trait implementations
 711 /////////////////////////////////////////////////////////////////////////////
 712
 713 #[stable(feature = "rust1", since = "1.0.0")]
 714 impl<T> Default for Option<T> {
 715     /// Returns None.
 716     #[inline]
 717     fn default() -> Option<T> { None }
 718 }
 719
 720 #[stable(feature = "rust1", since = "1.0.0")]
 721 impl<T> IntoIterator for Option<T> {
 722     type Item = T;
 723     type IntoIter = IntoIter<T>;
 724
 725     /// Returns a consuming iterator over the possibly contained value.
 726     ///
 727     /// # Examples
 728     ///
 729     /// ```
 730     /// let x = Some("string");
 731     /// let v: Vec<&str> = x.into_iter().collect();
 732     /// assert_eq!(v, ["string"]);
 733     ///
 734     /// let x = None;
 735     /// let v: Vec<&str> = x.into_iter().collect();
 736     /// assert!(v.is_empty());
 737     /// ```
 738     #[inline]
 739     fn into_iter(self) -> IntoIter<T> {
 740         IntoIter { inner: Item { opt: self } }
 741     }
 742 }
 743
 744 #[stable(since = "1.4.0", feature = "option_iter")]
 745 impl<'a, T> IntoIterator for &'a Option<T> {
 746     type Item = &'a T;
 747     type IntoIter = Iter<'a, T>;
 748
 749     fn into_iter(self) -> Iter<'a, T> {
 750         self.iter()
 751     }
 752 }
 753
 754 #[stable(since = "1.4.0", feature = "option_iter")]
 755 impl<'a, T> IntoIterator for &'a mut Option<T> {
 756     type Item = &'a mut T;
 757     type IntoIter = IterMut<'a, T>;
 758
 759     fn into_iter(mut self) -> IterMut<'a, T> {
 760         self.iter_mut()
 761     }
 762 }
 763
 764 #[stable(since = "1.12.0", feature = "option_from")]
 765 impl<T> From<T> for Option<T> {
 766     fn from(val: T) -> Option<T> {
 767         Some(val)
 768     }
 769 }
 770
 771 /////////////////////////////////////////////////////////////////////////////
 772 // The Option Iterators
 773 /////////////////////////////////////////////////////////////////////////////
 774
 775 #[derive(Clone, Debug)]
 776 struct Item<A> {
 777     opt: Option<A>
 778 }
 779
 780 impl<A> Iterator for Item<A> {
 781     type Item = A;
 782
 783     #[inline]
 784     fn next(&mut self) -> Option<A> {
 785         self.opt.take()
 786     }
 787
 788     #[inline]
 789     fn size_hint(&self) -> (usize, Option<usize>) {
 790         match self.opt {
 791             Some(_) => (1, Some(1)),
 792             None => (0, Some(0)),
 793         }
 794     }
 795 }
 796
 797 impl<A> DoubleEndedIterator for Item<A> {
 798     #[inline]
 799     fn next_back(&mut self) -> Option<A> {
 800         self.opt.take()
 801     }
 802 }
 803
 804 impl<A> ExactSizeIterator for Item<A> {}
 805 impl<A> FusedIterator for Item<A> {}
 806 unsafe impl<A> TrustedLen for Item<A> {}
 807
 808 /// An iterator over a reference of the contained item in an [`Option`].
 809 ///
 810 /// [`Option`]: enum.Option.html
 811 #[stable(feature = "rust1", since = "1.0.0")]
 812 #[derive(Debug)]
 813 pub struct Iter<'a, A: 'a> { inner: Item<&'a A> }
 814
 815 #[stable(feature = "rust1", since = "1.0.0")]
 816 impl<'a, A> Iterator for Iter<'a, A> {
 817     type Item = &'a A;
 818
 819     #[inline]
 820     fn next(&mut self) -> Option<&'a A> { self.inner.next() }
 821     #[inline]
 822     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 823 }
 824
 825 #[stable(feature = "rust1", since = "1.0.0")]
 826 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
 827     #[inline]
 828     fn next_back(&mut self) -> Option<&'a A> { self.inner.next_back() }
 829 }
 830
 831 #[stable(feature = "rust1", since = "1.0.0")]
 832 impl<'a, A> ExactSizeIterator for Iter<'a, A> {}
 833
 834 #[unstable(feature = "fused", issue = "35602")]
 835 impl<'a, A> FusedIterator for Iter<'a, A> {}
 836
 837 #[unstable(feature = "trusted_len", issue = "37572")]
 838 unsafe impl<'a, A> TrustedLen for Iter<'a, A> {}
 839
 840 #[stable(feature = "rust1", since = "1.0.0")]
 841 impl<'a, A> Clone for Iter<'a, A> {
 842     fn clone(&self) -> Iter<'a, A> {
 843         Iter { inner: self.inner.clone() }
 844     }
 845 }
 846
 847 /// An iterator over a mutable reference of the contained item in an [`Option`].
 848 ///
 849 /// [`Option`]: enum.Option.html
 850 #[stable(feature = "rust1", since = "1.0.0")]
 851 #[derive(Debug)]
 852 pub struct IterMut<'a, A: 'a> { inner: Item<&'a mut A> }
 853
 854 #[stable(feature = "rust1", since = "1.0.0")]
 855 impl<'a, A> Iterator for IterMut<'a, A> {
 856     type Item = &'a mut A;
 857
 858     #[inline]
 859     fn next(&mut self) -> Option<&'a mut A> { self.inner.next() }
 860     #[inline]
 861     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 862 }
 863
 864 #[stable(feature = "rust1", since = "1.0.0")]
 865 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
 866     #[inline]
 867     fn next_back(&mut self) -> Option<&'a mut A> { self.inner.next_back() }
 868 }
 869
 870 #[stable(feature = "rust1", since = "1.0.0")]
 871 impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}
 872
 873 #[unstable(feature = "fused", issue = "35602")]
 874 impl<'a, A> FusedIterator for IterMut<'a, A> {}
 875 #[unstable(feature = "trusted_len", issue = "37572")]
 876 unsafe impl<'a, A> TrustedLen for IterMut<'a, A> {}
 877
 878 /// An iterator over the item contained inside an [`Option`].
 879 ///
 880 /// [`Option`]: enum.Option.html
 881 #[derive(Clone, Debug)]
 882 #[stable(feature = "rust1", since = "1.0.0")]
 883 pub struct IntoIter<A> { inner: Item<A> }
 884
 885 #[stable(feature = "rust1", since = "1.0.0")]
 886 impl<A> Iterator for IntoIter<A> {
 887     type Item = A;
 888
 889     #[inline]
 890     fn next(&mut self) -> Option<A> { self.inner.next() }
 891     #[inline]
 892     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 893 }
 894
 895 #[stable(feature = "rust1", since = "1.0.0")]
 896 impl<A> DoubleEndedIterator for IntoIter<A> {
 897     #[inline]
 898     fn next_back(&mut self) -> Option<A> { self.inner.next_back() }
 899 }
 900
 901 #[stable(feature = "rust1", since = "1.0.0")]
 902 impl<A> ExactSizeIterator for IntoIter<A> {}
 903
 904 #[unstable(feature = "fused", issue = "35602")]
 905 impl<A> FusedIterator for IntoIter<A> {}
 906
 907 #[unstable(feature = "trusted_len", issue = "37572")]
 908 unsafe impl<A> TrustedLen for IntoIter<A> {}
 909
 910 /////////////////////////////////////////////////////////////////////////////
 911 // FromIterator
 912 /////////////////////////////////////////////////////////////////////////////
 913
 914 #[stable(feature = "rust1", since = "1.0.0")]
 915 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
 916     /// Takes each element in the `Iterator`: if it is `None`, no further
 917     /// elements are taken, and the `None` is returned. Should no `None` occur, a
 918     /// container with the values of each `Option` is returned.
 919     ///
 920     /// Here is an example which increments every integer in a vector,
 921     /// checking for overflow:
 922     ///
 923     /// ```
 924     /// use std::u16;
 925     ///
 926     /// let v = vec![1, 2];
 927     /// let res: Option<Vec<u16>> = v.iter().map(|&x: &u16|
 928     ///     if x == u16::MAX { None }
 929     ///     else { Some(x + 1) }
 930     /// ).collect();
 931     /// assert!(res == Some(vec![2, 3]));
 932     /// ```
 933     #[inline]
 934     fn from_iter<I: IntoIterator<Item=Option<A>>>(iter: I) -> Option<V> {
 935         // FIXME(#11084): This could be replaced with Iterator::scan when this
 936         // performance bug is closed.
 937
 938         struct Adapter<Iter> {
 939             iter: Iter,
 940             found_none: bool,
 941         }
 942
 943         impl<T, Iter: Iterator<Item=Option<T>>> Iterator for Adapter<Iter> {
 944             type Item = T;
 945
 946             #[inline]
 947             fn next(&mut self) -> Option<T> {
 948                 match self.iter.next() {
 949                     Some(Some(value)) => Some(value),
 950                     Some(None) => {
 951                         self.found_none = true;
 952                         None
 953                     }
 954                     None => None,
 955                 }
 956             }
 957         }
 958
 959         let mut adapter = Adapter { iter: iter.into_iter(), found_none: false };
 960         let v: V = FromIterator::from_iter(adapter.by_ref());
 961
 962         if adapter.found_none {
 963             None
 964         } else {
 965             Some(v)
 966         }
 967     }
 968 }