1 //! Functionality for ordering and comparison.
3 //! This module contains various tools for ordering and comparing values. In
6 //! * [`Eq`] and [`PartialEq`] are traits that allow you to define total and
7 //! partial equality between values, respectively. Implementing them overloads
8 //! the `==` and `!=` operators.
9 //! * [`Ord`] and [`PartialOrd`] are traits that allow you to define total and
10 //! partial orderings between values, respectively. Implementing them overloads
11 //! the `<`, `<=`, `>`, and `>=` operators.
12 //! * [`Ordering`] is an enum returned by the main functions of [`Ord`] and
13 //! [`PartialOrd`], and describes an ordering.
14 //! * [`Reverse`] is a struct that allows you to easily reverse an ordering.
15 //! * [`max`] and [`min`] are functions that build off of [`Ord`] and allow you
16 //! to find the maximum or minimum of two values.
18 //! For more details, see the respective documentation of each item in the list.
23 #![stable(feature = "rust1", since = "1.0.0")]
25 use self::Ordering::*;
27 /// Trait for equality comparisons which are [partial equivalence
28 /// relations](https://en.wikipedia.org/wiki/Partial_equivalence_relation).
30 /// This trait allows for partial equality, for types that do not have a full
31 /// equivalence relation. For example, in floating point numbers `NaN != NaN`,
32 /// so floating point types implement `PartialEq` but not [`trait@Eq`].
34 /// Formally, the equality must be (for all `a`, `b`, `c` of type `A`, `B`,
37 /// - **Symmetric**: if `A: PartialEq<B>` and `B: PartialEq<A>`, then **`a == b`
38 /// implies `b == a`**; and
40 /// - **Transitive**: if `A: PartialEq<B>` and `B: PartialEq<C>` and `A:
41 /// PartialEq<C>`, then **`a == b` and `b == c` implies `a == c`**.
43 /// Note that the `B: PartialEq<A>` (symmetric) and `A: PartialEq<C>`
44 /// (transitive) impls are not forced to exist, but these requirements apply
45 /// whenever they do exist.
49 /// This trait can be used with `#[derive]`. When `derive`d on structs, two
50 /// instances are equal if all fields are equal, and not equal if any fields
51 /// are not equal. When `derive`d on enums, each variant is equal to itself
52 /// and not equal to the other variants.
54 /// ## How can I implement `PartialEq`?
56 /// `PartialEq` only requires the [`eq`] method to be implemented; [`ne`] is defined
57 /// in terms of it by default. Any manual implementation of [`ne`] *must* respect
58 /// the rule that [`eq`] is a strict inverse of [`ne`]; that is, `!(a == b)` if and
61 /// Implementations of `PartialEq`, [`PartialOrd`], and [`Ord`] *must* agree with
62 /// each other. It's easy to accidentally make them disagree by deriving some
63 /// of the traits and manually implementing others.
65 /// An example implementation for a domain in which two books are considered
66 /// the same book if their ISBN matches, even if the formats differ:
77 /// format: BookFormat,
80 /// impl PartialEq for Book {
81 /// fn eq(&self, other: &Self) -> bool {
82 /// self.isbn == other.isbn
86 /// let b1 = Book { isbn: 3, format: BookFormat::Paperback };
87 /// let b2 = Book { isbn: 3, format: BookFormat::Ebook };
88 /// let b3 = Book { isbn: 10, format: BookFormat::Paperback };
90 /// assert!(b1 == b2);
91 /// assert!(b1 != b3);
94 /// ## How can I compare two different types?
96 /// The type you can compare with is controlled by `PartialEq`'s type parameter.
97 /// For example, let's tweak our previous code a bit:
100 /// // The derive implements <BookFormat> == <BookFormat> comparisons
101 /// #[derive(PartialEq)]
102 /// enum BookFormat {
110 /// format: BookFormat,
113 /// // Implement <Book> == <BookFormat> comparisons
114 /// impl PartialEq<BookFormat> for Book {
115 /// fn eq(&self, other: &BookFormat) -> bool {
116 /// self.format == *other
120 /// // Implement <BookFormat> == <Book> comparisons
121 /// impl PartialEq<Book> for BookFormat {
122 /// fn eq(&self, other: &Book) -> bool {
123 /// *self == other.format
127 /// let b1 = Book { isbn: 3, format: BookFormat::Paperback };
129 /// assert!(b1 == BookFormat::Paperback);
130 /// assert!(BookFormat::Ebook != b1);
133 /// By changing `impl PartialEq for Book` to `impl PartialEq<BookFormat> for Book`,
134 /// we allow `BookFormat`s to be compared with `Book`s.
136 /// A comparison like the one above, which ignores some fields of the struct,
137 /// can be dangerous. It can easily lead to an unintended violation of the
138 /// requirements for a partial equivalence relation. For example, if we kept
139 /// the above implementation of `PartialEq<Book>` for `BookFormat` and added an
140 /// implementation of `PartialEq<Book>` for `Book` (either via a `#[derive]` or
141 /// via the manual implementation from the first example) then the result would
142 /// violate transitivity:
145 /// #[derive(PartialEq)]
146 /// enum BookFormat {
152 /// #[derive(PartialEq)]
155 /// format: BookFormat,
158 /// impl PartialEq<BookFormat> for Book {
159 /// fn eq(&self, other: &BookFormat) -> bool {
160 /// self.format == *other
164 /// impl PartialEq<Book> for BookFormat {
165 /// fn eq(&self, other: &Book) -> bool {
166 /// *self == other.format
171 /// let b1 = Book { isbn: 1, format: BookFormat::Paperback };
172 /// let b2 = Book { isbn: 2, format: BookFormat::Paperback };
174 /// assert!(b1 == BookFormat::Paperback);
175 /// assert!(BookFormat::Paperback == b2);
177 /// // The following should hold by transitivity but doesn't.
178 /// assert!(b1 == b2); // <-- PANICS
188 /// assert_eq!(x == y, false);
189 /// assert_eq!(x.eq(&y), false);
192 /// [`eq`]: PartialEq::eq
193 /// [`ne`]: PartialEq::ne
195 #[stable(feature = "rust1", since = "1.0.0")]
198 #[rustc_on_unimplemented(
199 message = "can't compare `{Self}` with `{Rhs}`",
200 label = "no implementation for `{Self} == {Rhs}`"
202 pub trait PartialEq<Rhs: ?Sized = Self> {
203 /// This method tests for `self` and `other` values to be equal, and is used
206 #[stable(feature = "rust1", since = "1.0.0")]
207 fn eq(&self, other: &Rhs) -> bool;
209 /// This method tests for `!=`.
212 #[stable(feature = "rust1", since = "1.0.0")]
213 fn ne(&self, other: &Rhs) -> bool {
218 /// Derive macro generating an impl of the trait `PartialEq`.
219 #[rustc_builtin_macro]
220 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
221 #[allow_internal_unstable(core_intrinsics, structural_match)]
222 pub macro PartialEq($item:item) {
223 /* compiler built-in */
226 /// Trait for equality comparisons which are [equivalence relations](
227 /// https://en.wikipedia.org/wiki/Equivalence_relation).
229 /// This means, that in addition to `a == b` and `a != b` being strict inverses, the equality must
230 /// be (for all `a`, `b` and `c`):
232 /// - reflexive: `a == a`;
233 /// - symmetric: `a == b` implies `b == a`; and
234 /// - transitive: `a == b` and `b == c` implies `a == c`.
236 /// This property cannot be checked by the compiler, and therefore `Eq` implies
237 /// [`PartialEq`], and has no extra methods.
241 /// This trait can be used with `#[derive]`. When `derive`d, because `Eq` has
242 /// no extra methods, it is only informing the compiler that this is an
243 /// equivalence relation rather than a partial equivalence relation. Note that
244 /// the `derive` strategy requires all fields are `Eq`, which isn't
247 /// ## How can I implement `Eq`?
249 /// If you cannot use the `derive` strategy, specify that your type implements
250 /// `Eq`, which has no methods:
253 /// enum BookFormat { Paperback, Hardback, Ebook }
256 /// format: BookFormat,
258 /// impl PartialEq for Book {
259 /// fn eq(&self, other: &Self) -> bool {
260 /// self.isbn == other.isbn
263 /// impl Eq for Book {}
267 #[stable(feature = "rust1", since = "1.0.0")]
268 pub trait Eq: PartialEq<Self> {
269 // this method is used solely by #[deriving] to assert
270 // that every component of a type implements #[deriving]
271 // itself, the current deriving infrastructure means doing this
272 // assertion without using a method on this trait is nearly
275 // This should never be implemented by hand.
278 #[stable(feature = "rust1", since = "1.0.0")]
279 fn assert_receiver_is_total_eq(&self) {}
282 /// Derive macro generating an impl of the trait `Eq`.
283 #[rustc_builtin_macro]
284 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
285 #[allow_internal_unstable(core_intrinsics, derive_eq, structural_match)]
286 pub macro Eq($item:item) {
287 /* compiler built-in */
290 // FIXME: this struct is used solely by #[derive] to
291 // assert that every component of a type implements Eq.
293 // This struct should never appear in user code.
295 #[allow(missing_debug_implementations)]
296 #[unstable(feature = "derive_eq", reason = "deriving hack, should not be public", issue = "none")]
297 pub struct AssertParamIsEq<T: Eq + ?Sized> {
298 _field: crate::marker::PhantomData<T>,
301 /// An `Ordering` is the result of a comparison between two values.
306 /// use std::cmp::Ordering;
308 /// let result = 1.cmp(&2);
309 /// assert_eq!(Ordering::Less, result);
311 /// let result = 1.cmp(&1);
312 /// assert_eq!(Ordering::Equal, result);
314 /// let result = 2.cmp(&1);
315 /// assert_eq!(Ordering::Greater, result);
317 #[derive(Clone, Copy, PartialEq, Debug, Hash)]
318 #[stable(feature = "rust1", since = "1.0.0")]
320 /// An ordering where a compared value is less than another.
321 #[stable(feature = "rust1", since = "1.0.0")]
323 /// An ordering where a compared value is equal to another.
324 #[stable(feature = "rust1", since = "1.0.0")]
326 /// An ordering where a compared value is greater than another.
327 #[stable(feature = "rust1", since = "1.0.0")]
332 /// Returns `true` if the ordering is the `Equal` variant.
337 /// #![feature(ordering_helpers)]
338 /// use std::cmp::Ordering;
340 /// assert_eq!(Ordering::Less.is_eq(), false);
341 /// assert_eq!(Ordering::Equal.is_eq(), true);
342 /// assert_eq!(Ordering::Greater.is_eq(), false);
346 #[unstable(feature = "ordering_helpers", issue = "79885")]
347 pub const fn is_eq(self) -> bool {
348 matches!(self, Equal)
351 /// Returns `true` if the ordering is not the `Equal` variant.
356 /// #![feature(ordering_helpers)]
357 /// use std::cmp::Ordering;
359 /// assert_eq!(Ordering::Less.is_ne(), true);
360 /// assert_eq!(Ordering::Equal.is_ne(), false);
361 /// assert_eq!(Ordering::Greater.is_ne(), true);
365 #[unstable(feature = "ordering_helpers", issue = "79885")]
366 pub const fn is_ne(self) -> bool {
367 !matches!(self, Equal)
370 /// Returns `true` if the ordering is the `Less` variant.
375 /// #![feature(ordering_helpers)]
376 /// use std::cmp::Ordering;
378 /// assert_eq!(Ordering::Less.is_lt(), true);
379 /// assert_eq!(Ordering::Equal.is_lt(), false);
380 /// assert_eq!(Ordering::Greater.is_lt(), false);
384 #[unstable(feature = "ordering_helpers", issue = "79885")]
385 pub const fn is_lt(self) -> bool {
389 /// Returns `true` if the ordering is the `Greater` variant.
394 /// #![feature(ordering_helpers)]
395 /// use std::cmp::Ordering;
397 /// assert_eq!(Ordering::Less.is_gt(), false);
398 /// assert_eq!(Ordering::Equal.is_gt(), false);
399 /// assert_eq!(Ordering::Greater.is_gt(), true);
403 #[unstable(feature = "ordering_helpers", issue = "79885")]
404 pub const fn is_gt(self) -> bool {
405 matches!(self, Greater)
408 /// Returns `true` if the ordering is either the `Less` or `Equal` variant.
413 /// #![feature(ordering_helpers)]
414 /// use std::cmp::Ordering;
416 /// assert_eq!(Ordering::Less.is_le(), true);
417 /// assert_eq!(Ordering::Equal.is_le(), true);
418 /// assert_eq!(Ordering::Greater.is_le(), false);
422 #[unstable(feature = "ordering_helpers", issue = "79885")]
423 pub const fn is_le(self) -> bool {
424 !matches!(self, Greater)
427 /// Returns `true` if the ordering is either the `Greater` or `Equal` variant.
432 /// #![feature(ordering_helpers)]
433 /// use std::cmp::Ordering;
435 /// assert_eq!(Ordering::Less.is_ge(), false);
436 /// assert_eq!(Ordering::Equal.is_ge(), true);
437 /// assert_eq!(Ordering::Greater.is_ge(), true);
441 #[unstable(feature = "ordering_helpers", issue = "79885")]
442 pub const fn is_ge(self) -> bool {
443 !matches!(self, Less)
446 /// Reverses the `Ordering`.
448 /// * `Less` becomes `Greater`.
449 /// * `Greater` becomes `Less`.
450 /// * `Equal` becomes `Equal`.
457 /// use std::cmp::Ordering;
459 /// assert_eq!(Ordering::Less.reverse(), Ordering::Greater);
460 /// assert_eq!(Ordering::Equal.reverse(), Ordering::Equal);
461 /// assert_eq!(Ordering::Greater.reverse(), Ordering::Less);
464 /// This method can be used to reverse a comparison:
467 /// let data: &mut [_] = &mut [2, 10, 5, 8];
469 /// // sort the array from largest to smallest.
470 /// data.sort_by(|a, b| a.cmp(b).reverse());
472 /// let b: &mut [_] = &mut [10, 8, 5, 2];
473 /// assert!(data == b);
477 #[rustc_const_stable(feature = "const_ordering", since = "1.48.0")]
478 #[stable(feature = "rust1", since = "1.0.0")]
479 pub const fn reverse(self) -> Ordering {
487 /// Chains two orderings.
489 /// Returns `self` when it's not `Equal`. Otherwise returns `other`.
494 /// use std::cmp::Ordering;
496 /// let result = Ordering::Equal.then(Ordering::Less);
497 /// assert_eq!(result, Ordering::Less);
499 /// let result = Ordering::Less.then(Ordering::Equal);
500 /// assert_eq!(result, Ordering::Less);
502 /// let result = Ordering::Less.then(Ordering::Greater);
503 /// assert_eq!(result, Ordering::Less);
505 /// let result = Ordering::Equal.then(Ordering::Equal);
506 /// assert_eq!(result, Ordering::Equal);
508 /// let x: (i64, i64, i64) = (1, 2, 7);
509 /// let y: (i64, i64, i64) = (1, 5, 3);
510 /// let result = x.0.cmp(&y.0).then(x.1.cmp(&y.1)).then(x.2.cmp(&y.2));
512 /// assert_eq!(result, Ordering::Less);
516 #[rustc_const_stable(feature = "const_ordering", since = "1.48.0")]
517 #[stable(feature = "ordering_chaining", since = "1.17.0")]
518 pub const fn then(self, other: Ordering) -> Ordering {
525 /// Chains the ordering with the given function.
527 /// Returns `self` when it's not `Equal`. Otherwise calls `f` and returns
533 /// use std::cmp::Ordering;
535 /// let result = Ordering::Equal.then_with(|| Ordering::Less);
536 /// assert_eq!(result, Ordering::Less);
538 /// let result = Ordering::Less.then_with(|| Ordering::Equal);
539 /// assert_eq!(result, Ordering::Less);
541 /// let result = Ordering::Less.then_with(|| Ordering::Greater);
542 /// assert_eq!(result, Ordering::Less);
544 /// let result = Ordering::Equal.then_with(|| Ordering::Equal);
545 /// assert_eq!(result, Ordering::Equal);
547 /// let x: (i64, i64, i64) = (1, 2, 7);
548 /// let y: (i64, i64, i64) = (1, 5, 3);
549 /// let result = x.0.cmp(&y.0).then_with(|| x.1.cmp(&y.1)).then_with(|| x.2.cmp(&y.2));
551 /// assert_eq!(result, Ordering::Less);
555 #[stable(feature = "ordering_chaining", since = "1.17.0")]
556 pub fn then_with<F: FnOnce() -> Ordering>(self, f: F) -> Ordering {
564 /// A helper struct for reverse ordering.
566 /// This struct is a helper to be used with functions like [`Vec::sort_by_key`] and
567 /// can be used to reverse order a part of a key.
569 /// [`Vec::sort_by_key`]: ../../std/vec/struct.Vec.html#method.sort_by_key
574 /// use std::cmp::Reverse;
576 /// let mut v = vec![1, 2, 3, 4, 5, 6];
577 /// v.sort_by_key(|&num| (num > 3, Reverse(num)));
578 /// assert_eq!(v, vec![3, 2, 1, 6, 5, 4]);
580 #[derive(PartialEq, Eq, Debug, Copy, Clone, Default, Hash)]
581 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]
583 pub struct Reverse<T>(#[stable(feature = "reverse_cmp_key", since = "1.19.0")] pub T);
585 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]
586 impl<T: PartialOrd> PartialOrd for Reverse<T> {
588 fn partial_cmp(&self, other: &Reverse<T>) -> Option<Ordering> {
589 other.0.partial_cmp(&self.0)
593 fn lt(&self, other: &Self) -> bool {
597 fn le(&self, other: &Self) -> bool {
601 fn gt(&self, other: &Self) -> bool {
605 fn ge(&self, other: &Self) -> bool {
610 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]
611 impl<T: Ord> Ord for Reverse<T> {
613 fn cmp(&self, other: &Reverse<T>) -> Ordering {
618 /// Trait for types that form a [total order](https://en.wikipedia.org/wiki/Total_order).
620 /// An order is a total order if it is (for all `a`, `b` and `c`):
622 /// - total and asymmetric: exactly one of `a < b`, `a == b` or `a > b` is true; and
623 /// - transitive, `a < b` and `b < c` implies `a < c`. The same must hold for both `==` and `>`.
627 /// This trait can be used with `#[derive]`. When `derive`d on structs, it will produce a
628 /// [lexicographic](https://en.wikipedia.org/wiki/Lexicographic_order) ordering based on the top-to-bottom declaration order of the struct's members.
629 /// When `derive`d on enums, variants are ordered by their top-to-bottom discriminant order.
631 /// ## Lexicographical comparison
633 /// Lexicographical comparison is an operation with the following properties:
634 /// - Two sequences are compared element by element.
635 /// - The first mismatching element defines which sequence is lexicographically less or greater than the other.
636 /// - If one sequence is a prefix of another, the shorter sequence is lexicographically less than the other.
637 /// - If two sequence have equivalent elements and are of the same length, then the sequences are lexicographically equal.
638 /// - An empty sequence is lexicographically less than any non-empty sequence.
639 /// - Two empty sequences are lexicographically equal.
641 /// ## How can I implement `Ord`?
643 /// `Ord` requires that the type also be [`PartialOrd`] and [`Eq`] (which requires [`PartialEq`]).
645 /// Then you must define an implementation for [`cmp`]. You may find it useful to use
646 /// [`cmp`] on your type's fields.
648 /// Implementations of [`PartialEq`], [`PartialOrd`], and `Ord` *must*
649 /// agree with each other. That is, `a.cmp(b) == Ordering::Equal` if
650 /// and only if `a == b` and `Some(a.cmp(b)) == a.partial_cmp(b)` for
651 /// all `a` and `b`. It's easy to accidentally make them disagree by
652 /// deriving some of the traits and manually implementing others.
654 /// Here's an example where you want to sort people by height only, disregarding `id`
658 /// use std::cmp::Ordering;
667 /// impl Ord for Person {
668 /// fn cmp(&self, other: &Self) -> Ordering {
669 /// self.height.cmp(&other.height)
673 /// impl PartialOrd for Person {
674 /// fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
675 /// Some(self.cmp(other))
679 /// impl PartialEq for Person {
680 /// fn eq(&self, other: &Self) -> bool {
681 /// self.height == other.height
686 /// [`cmp`]: Ord::cmp
691 #[stable(feature = "rust1", since = "1.0.0")]
692 pub trait Ord: Eq + PartialOrd<Self> {
693 /// This method returns an [`Ordering`] between `self` and `other`.
695 /// By convention, `self.cmp(&other)` returns the ordering matching the expression
696 /// `self <operator> other` if true.
701 /// use std::cmp::Ordering;
703 /// assert_eq!(5.cmp(&10), Ordering::Less);
704 /// assert_eq!(10.cmp(&5), Ordering::Greater);
705 /// assert_eq!(5.cmp(&5), Ordering::Equal);
708 #[stable(feature = "rust1", since = "1.0.0")]
709 fn cmp(&self, other: &Self) -> Ordering;
711 /// Compares and returns the maximum of two values.
713 /// Returns the second argument if the comparison determines them to be equal.
718 /// assert_eq!(2, 1.max(2));
719 /// assert_eq!(2, 2.max(2));
721 #[stable(feature = "ord_max_min", since = "1.21.0")]
724 fn max(self, other: Self) -> Self
728 max_by(self, other, Ord::cmp)
731 /// Compares and returns the minimum of two values.
733 /// Returns the first argument if the comparison determines them to be equal.
738 /// assert_eq!(1, 1.min(2));
739 /// assert_eq!(2, 2.min(2));
741 #[stable(feature = "ord_max_min", since = "1.21.0")]
744 fn min(self, other: Self) -> Self
748 min_by(self, other, Ord::cmp)
751 /// Restrict a value to a certain interval.
753 /// Returns `max` if `self` is greater than `max`, and `min` if `self` is
754 /// less than `min`. Otherwise this returns `self`.
758 /// Panics if `min > max`.
763 /// assert!((-3).clamp(-2, 1) == -2);
764 /// assert!(0.clamp(-2, 1) == 0);
765 /// assert!(2.clamp(-2, 1) == 1);
768 #[stable(feature = "clamp", since = "1.50.0")]
769 fn clamp(self, min: Self, max: Self) -> Self
776 } else if self > max {
784 /// Derive macro generating an impl of the trait `Ord`.
785 #[rustc_builtin_macro]
786 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
787 #[allow_internal_unstable(core_intrinsics)]
788 pub macro Ord($item:item) {
789 /* compiler built-in */
792 #[stable(feature = "rust1", since = "1.0.0")]
793 impl Eq for Ordering {}
795 #[stable(feature = "rust1", since = "1.0.0")]
796 impl Ord for Ordering {
798 fn cmp(&self, other: &Ordering) -> Ordering {
799 (*self as i32).cmp(&(*other as i32))
803 #[stable(feature = "rust1", since = "1.0.0")]
804 impl PartialOrd for Ordering {
806 fn partial_cmp(&self, other: &Ordering) -> Option<Ordering> {
807 (*self as i32).partial_cmp(&(*other as i32))
811 /// Trait for values that can be compared for a sort-order.
813 /// The comparison must satisfy, for all `a`, `b` and `c`:
815 /// - asymmetry: if `a < b` then `!(a > b)`, as well as `a > b` implying `!(a < b)`; and
816 /// - transitivity: `a < b` and `b < c` implies `a < c`. The same must hold for both `==` and `>`.
818 /// Note that these requirements mean that the trait itself must be implemented symmetrically and
819 /// transitively: if `T: PartialOrd<U>` and `U: PartialOrd<V>` then `U: PartialOrd<T>` and `T:
824 /// This trait can be used with `#[derive]`. When `derive`d on structs, it will produce a
825 /// lexicographic ordering based on the top-to-bottom declaration order of the struct's members.
826 /// When `derive`d on enums, variants are ordered by their top-to-bottom discriminant order.
828 /// ## How can I implement `PartialOrd`?
830 /// `PartialOrd` only requires implementation of the [`partial_cmp`] method, with the others
831 /// generated from default implementations.
833 /// However it remains possible to implement the others separately for types which do not have a
834 /// total order. For example, for floating point numbers, `NaN < 0 == false` and `NaN >= 0 ==
835 /// false` (cf. IEEE 754-2008 section 5.11).
837 /// `PartialOrd` requires your type to be [`PartialEq`].
839 /// Implementations of [`PartialEq`], `PartialOrd`, and [`Ord`] *must* agree with each other. It's
840 /// easy to accidentally make them disagree by deriving some of the traits and manually
841 /// implementing others.
843 /// If your type is [`Ord`], you can implement [`partial_cmp`] by using [`cmp`]:
846 /// use std::cmp::Ordering;
855 /// impl PartialOrd for Person {
856 /// fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
857 /// Some(self.cmp(other))
861 /// impl Ord for Person {
862 /// fn cmp(&self, other: &Self) -> Ordering {
863 /// self.height.cmp(&other.height)
867 /// impl PartialEq for Person {
868 /// fn eq(&self, other: &Self) -> bool {
869 /// self.height == other.height
874 /// You may also find it useful to use [`partial_cmp`] on your type's fields. Here
875 /// is an example of `Person` types who have a floating-point `height` field that
876 /// is the only field to be used for sorting:
879 /// use std::cmp::Ordering;
887 /// impl PartialOrd for Person {
888 /// fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
889 /// self.height.partial_cmp(&other.height)
893 /// impl PartialEq for Person {
894 /// fn eq(&self, other: &Self) -> bool {
895 /// self.height == other.height
906 /// assert_eq!(x < y, true);
907 /// assert_eq!(x.lt(&y), true);
910 /// [`partial_cmp`]: PartialOrd::partial_cmp
911 /// [`cmp`]: Ord::cmp
912 #[lang = "partial_ord"]
913 #[stable(feature = "rust1", since = "1.0.0")]
918 #[rustc_on_unimplemented(
919 message = "can't compare `{Self}` with `{Rhs}`",
920 label = "no implementation for `{Self} < {Rhs}` and `{Self} > {Rhs}`"
922 pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
923 /// This method returns an ordering between `self` and `other` values if one exists.
928 /// use std::cmp::Ordering;
930 /// let result = 1.0.partial_cmp(&2.0);
931 /// assert_eq!(result, Some(Ordering::Less));
933 /// let result = 1.0.partial_cmp(&1.0);
934 /// assert_eq!(result, Some(Ordering::Equal));
936 /// let result = 2.0.partial_cmp(&1.0);
937 /// assert_eq!(result, Some(Ordering::Greater));
940 /// When comparison is impossible:
943 /// let result = f64::NAN.partial_cmp(&1.0);
944 /// assert_eq!(result, None);
947 #[stable(feature = "rust1", since = "1.0.0")]
948 fn partial_cmp(&self, other: &Rhs) -> Option<Ordering>;
950 /// This method tests less than (for `self` and `other`) and is used by the `<` operator.
955 /// let result = 1.0 < 2.0;
956 /// assert_eq!(result, true);
958 /// let result = 2.0 < 1.0;
959 /// assert_eq!(result, false);
963 #[stable(feature = "rust1", since = "1.0.0")]
964 fn lt(&self, other: &Rhs) -> bool {
965 matches!(self.partial_cmp(other), Some(Less))
968 /// This method tests less than or equal to (for `self` and `other`) and is used by the `<=`
974 /// let result = 1.0 <= 2.0;
975 /// assert_eq!(result, true);
977 /// let result = 2.0 <= 2.0;
978 /// assert_eq!(result, true);
982 #[stable(feature = "rust1", since = "1.0.0")]
983 fn le(&self, other: &Rhs) -> bool {
984 // Pattern `Some(Less | Eq)` optimizes worse than negating `None | Some(Greater)`.
985 !matches!(self.partial_cmp(other), None | Some(Greater))
988 /// This method tests greater than (for `self` and `other`) and is used by the `>` operator.
993 /// let result = 1.0 > 2.0;
994 /// assert_eq!(result, false);
996 /// let result = 2.0 > 2.0;
997 /// assert_eq!(result, false);
1001 #[stable(feature = "rust1", since = "1.0.0")]
1002 fn gt(&self, other: &Rhs) -> bool {
1003 matches!(self.partial_cmp(other), Some(Greater))
1006 /// This method tests greater than or equal to (for `self` and `other`) and is used by the `>=`
1012 /// let result = 2.0 >= 1.0;
1013 /// assert_eq!(result, true);
1015 /// let result = 2.0 >= 2.0;
1016 /// assert_eq!(result, true);
1020 #[stable(feature = "rust1", since = "1.0.0")]
1021 fn ge(&self, other: &Rhs) -> bool {
1022 matches!(self.partial_cmp(other), Some(Greater | Equal))
1026 /// Derive macro generating an impl of the trait `PartialOrd`.
1027 #[rustc_builtin_macro]
1028 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
1029 #[allow_internal_unstable(core_intrinsics)]
1030 pub macro PartialOrd($item:item) {
1031 /* compiler built-in */
1034 /// Compares and returns the minimum of two values.
1036 /// Returns the first argument if the comparison determines them to be equal.
1038 /// Internally uses an alias to [`Ord::min`].
1045 /// assert_eq!(1, cmp::min(1, 2));
1046 /// assert_eq!(2, cmp::min(2, 2));
1050 #[stable(feature = "rust1", since = "1.0.0")]
1051 pub fn min<T: Ord>(v1: T, v2: T) -> T {
1055 /// Returns the minimum of two values with respect to the specified comparison function.
1057 /// Returns the first argument if the comparison determines them to be equal.
1062 /// #![feature(cmp_min_max_by)]
1066 /// assert_eq!(cmp::min_by(-2, 1, |x: &i32, y: &i32| x.abs().cmp(&y.abs())), 1);
1067 /// assert_eq!(cmp::min_by(-2, 2, |x: &i32, y: &i32| x.abs().cmp(&y.abs())), -2);
1071 #[unstable(feature = "cmp_min_max_by", issue = "64460")]
1072 pub fn min_by<T, F: FnOnce(&T, &T) -> Ordering>(v1: T, v2: T, compare: F) -> T {
1073 match compare(&v1, &v2) {
1074 Ordering::Less | Ordering::Equal => v1,
1075 Ordering::Greater => v2,
1079 /// Returns the element that gives the minimum value from the specified function.
1081 /// Returns the first argument if the comparison determines them to be equal.
1086 /// #![feature(cmp_min_max_by)]
1090 /// assert_eq!(cmp::min_by_key(-2, 1, |x: &i32| x.abs()), 1);
1091 /// assert_eq!(cmp::min_by_key(-2, 2, |x: &i32| x.abs()), -2);
1095 #[unstable(feature = "cmp_min_max_by", issue = "64460")]
1096 pub fn min_by_key<T, F: FnMut(&T) -> K, K: Ord>(v1: T, v2: T, mut f: F) -> T {
1097 min_by(v1, v2, |v1, v2| f(v1).cmp(&f(v2)))
1100 /// Compares and returns the maximum of two values.
1102 /// Returns the second argument if the comparison determines them to be equal.
1104 /// Internally uses an alias to [`Ord::max`].
1111 /// assert_eq!(2, cmp::max(1, 2));
1112 /// assert_eq!(2, cmp::max(2, 2));
1116 #[stable(feature = "rust1", since = "1.0.0")]
1117 pub fn max<T: Ord>(v1: T, v2: T) -> T {
1121 /// Returns the maximum of two values with respect to the specified comparison function.
1123 /// Returns the second argument if the comparison determines them to be equal.
1128 /// #![feature(cmp_min_max_by)]
1132 /// assert_eq!(cmp::max_by(-2, 1, |x: &i32, y: &i32| x.abs().cmp(&y.abs())), -2);
1133 /// assert_eq!(cmp::max_by(-2, 2, |x: &i32, y: &i32| x.abs().cmp(&y.abs())), 2);
1137 #[unstable(feature = "cmp_min_max_by", issue = "64460")]
1138 pub fn max_by<T, F: FnOnce(&T, &T) -> Ordering>(v1: T, v2: T, compare: F) -> T {
1139 match compare(&v1, &v2) {
1140 Ordering::Less | Ordering::Equal => v2,
1141 Ordering::Greater => v1,
1145 /// Returns the element that gives the maximum value from the specified function.
1147 /// Returns the second argument if the comparison determines them to be equal.
1152 /// #![feature(cmp_min_max_by)]
1156 /// assert_eq!(cmp::max_by_key(-2, 1, |x: &i32| x.abs()), -2);
1157 /// assert_eq!(cmp::max_by_key(-2, 2, |x: &i32| x.abs()), 2);
1161 #[unstable(feature = "cmp_min_max_by", issue = "64460")]
1162 pub fn max_by_key<T, F: FnMut(&T) -> K, K: Ord>(v1: T, v2: T, mut f: F) -> T {
1163 max_by(v1, v2, |v1, v2| f(v1).cmp(&f(v2)))
1166 // Implementation of PartialEq, Eq, PartialOrd and Ord for primitive types
1168 use crate::cmp::Ordering::{self, Equal, Greater, Less};
1169 use crate::hint::unreachable_unchecked;
1171 macro_rules! partial_eq_impl {
1173 #[stable(feature = "rust1", since = "1.0.0")]
1174 impl PartialEq for $t {
1176 fn eq(&self, other: &$t) -> bool { (*self) == (*other) }
1178 fn ne(&self, other: &$t) -> bool { (*self) != (*other) }
1183 #[stable(feature = "rust1", since = "1.0.0")]
1184 impl PartialEq for () {
1186 fn eq(&self, _other: &()) -> bool {
1190 fn ne(&self, _other: &()) -> bool {
1196 bool char usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64
1199 macro_rules! eq_impl {
1201 #[stable(feature = "rust1", since = "1.0.0")]
1206 eq_impl! { () bool char usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
1208 macro_rules! partial_ord_impl {
1210 #[stable(feature = "rust1", since = "1.0.0")]
1211 impl PartialOrd for $t {
1213 fn partial_cmp(&self, other: &$t) -> Option<Ordering> {
1214 match (self <= other, self >= other) {
1215 (false, false) => None,
1216 (false, true) => Some(Greater),
1217 (true, false) => Some(Less),
1218 (true, true) => Some(Equal),
1222 fn lt(&self, other: &$t) -> bool { (*self) < (*other) }
1224 fn le(&self, other: &$t) -> bool { (*self) <= (*other) }
1226 fn ge(&self, other: &$t) -> bool { (*self) >= (*other) }
1228 fn gt(&self, other: &$t) -> bool { (*self) > (*other) }
1233 #[stable(feature = "rust1", since = "1.0.0")]
1234 impl PartialOrd for () {
1236 fn partial_cmp(&self, _: &()) -> Option<Ordering> {
1241 #[stable(feature = "rust1", since = "1.0.0")]
1242 impl PartialOrd for bool {
1244 fn partial_cmp(&self, other: &bool) -> Option<Ordering> {
1245 Some(self.cmp(other))
1249 partial_ord_impl! { f32 f64 }
1251 macro_rules! ord_impl {
1253 #[stable(feature = "rust1", since = "1.0.0")]
1254 impl PartialOrd for $t {
1256 fn partial_cmp(&self, other: &$t) -> Option<Ordering> {
1257 Some(self.cmp(other))
1260 fn lt(&self, other: &$t) -> bool { (*self) < (*other) }
1262 fn le(&self, other: &$t) -> bool { (*self) <= (*other) }
1264 fn ge(&self, other: &$t) -> bool { (*self) >= (*other) }
1266 fn gt(&self, other: &$t) -> bool { (*self) > (*other) }
1269 #[stable(feature = "rust1", since = "1.0.0")]
1272 fn cmp(&self, other: &$t) -> Ordering {
1273 // The order here is important to generate more optimal assembly.
1274 // See <https://github.com/rust-lang/rust/issues/63758> for more info.
1275 if *self < *other { Less }
1276 else if *self == *other { Equal }
1283 #[stable(feature = "rust1", since = "1.0.0")]
1286 fn cmp(&self, _other: &()) -> Ordering {
1291 #[stable(feature = "rust1", since = "1.0.0")]
1294 fn cmp(&self, other: &bool) -> Ordering {
1295 // Casting to i8's and converting the difference to an Ordering generates
1296 // more optimal assembly.
1297 // See <https://github.com/rust-lang/rust/issues/66780> for more info.
1298 match (*self as i8) - (*other as i8) {
1302 // SAFETY: bool as i8 returns 0 or 1, so the difference can't be anything else
1303 _ => unsafe { unreachable_unchecked() },
1308 ord_impl! { char usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
1310 #[unstable(feature = "never_type", issue = "35121")]
1311 impl PartialEq for ! {
1312 fn eq(&self, _: &!) -> bool {
1317 #[unstable(feature = "never_type", issue = "35121")]
1320 #[unstable(feature = "never_type", issue = "35121")]
1321 impl PartialOrd for ! {
1322 fn partial_cmp(&self, _: &!) -> Option<Ordering> {
1327 #[unstable(feature = "never_type", issue = "35121")]
1329 fn cmp(&self, _: &!) -> Ordering {
1336 #[stable(feature = "rust1", since = "1.0.0")]
1337 impl<A: ?Sized, B: ?Sized> PartialEq<&B> for &A
1342 fn eq(&self, other: &&B) -> bool {
1343 PartialEq::eq(*self, *other)
1346 fn ne(&self, other: &&B) -> bool {
1347 PartialEq::ne(*self, *other)
1350 #[stable(feature = "rust1", since = "1.0.0")]
1351 impl<A: ?Sized, B: ?Sized> PartialOrd<&B> for &A
1356 fn partial_cmp(&self, other: &&B) -> Option<Ordering> {
1357 PartialOrd::partial_cmp(*self, *other)
1360 fn lt(&self, other: &&B) -> bool {
1361 PartialOrd::lt(*self, *other)
1364 fn le(&self, other: &&B) -> bool {
1365 PartialOrd::le(*self, *other)
1368 fn gt(&self, other: &&B) -> bool {
1369 PartialOrd::gt(*self, *other)
1372 fn ge(&self, other: &&B) -> bool {
1373 PartialOrd::ge(*self, *other)
1376 #[stable(feature = "rust1", since = "1.0.0")]
1377 impl<A: ?Sized> Ord for &A
1382 fn cmp(&self, other: &Self) -> Ordering {
1383 Ord::cmp(*self, *other)
1386 #[stable(feature = "rust1", since = "1.0.0")]
1387 impl<A: ?Sized> Eq for &A where A: Eq {}
1391 #[stable(feature = "rust1", since = "1.0.0")]
1392 impl<A: ?Sized, B: ?Sized> PartialEq<&mut B> for &mut A
1397 fn eq(&self, other: &&mut B) -> bool {
1398 PartialEq::eq(*self, *other)
1401 fn ne(&self, other: &&mut B) -> bool {
1402 PartialEq::ne(*self, *other)
1405 #[stable(feature = "rust1", since = "1.0.0")]
1406 impl<A: ?Sized, B: ?Sized> PartialOrd<&mut B> for &mut A
1411 fn partial_cmp(&self, other: &&mut B) -> Option<Ordering> {
1412 PartialOrd::partial_cmp(*self, *other)
1415 fn lt(&self, other: &&mut B) -> bool {
1416 PartialOrd::lt(*self, *other)
1419 fn le(&self, other: &&mut B) -> bool {
1420 PartialOrd::le(*self, *other)
1423 fn gt(&self, other: &&mut B) -> bool {
1424 PartialOrd::gt(*self, *other)
1427 fn ge(&self, other: &&mut B) -> bool {
1428 PartialOrd::ge(*self, *other)
1431 #[stable(feature = "rust1", since = "1.0.0")]
1432 impl<A: ?Sized> Ord for &mut A
1437 fn cmp(&self, other: &Self) -> Ordering {
1438 Ord::cmp(*self, *other)
1441 #[stable(feature = "rust1", since = "1.0.0")]
1442 impl<A: ?Sized> Eq for &mut A where A: Eq {}
1444 #[stable(feature = "rust1", since = "1.0.0")]
1445 impl<A: ?Sized, B: ?Sized> PartialEq<&mut B> for &A
1450 fn eq(&self, other: &&mut B) -> bool {
1451 PartialEq::eq(*self, *other)
1454 fn ne(&self, other: &&mut B) -> bool {
1455 PartialEq::ne(*self, *other)
1459 #[stable(feature = "rust1", since = "1.0.0")]
1460 impl<A: ?Sized, B: ?Sized> PartialEq<&B> for &mut A
1465 fn eq(&self, other: &&B) -> bool {
1466 PartialEq::eq(*self, *other)
1469 fn ne(&self, other: &&B) -> bool {
1470 PartialEq::ne(*self, *other)