Trait core::cmp::PartialOrd

1.0.0 · source ·
pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
    // Required method
    fn partial_cmp(&self, other: &Rhs) -> Option<Ordering>;

    // Provided methods
    fn lt(&self, other: &Rhs) -> bool { ... }
    fn le(&self, other: &Rhs) -> bool { ... }
    fn gt(&self, other: &Rhs) -> bool { ... }
    fn ge(&self, other: &Rhs) -> bool { ... }
}
Expand description

Trait for types that form a partial order.

The lt, le, gt, and ge methods of this trait can be called using the <, <=, >, and >= operators, respectively.

The methods of this trait must be consistent with each other and with those of PartialEq. The following conditions must hold:

  1. a == b if and only if partial_cmp(a, b) == Some(Equal).
  2. a < b if and only if partial_cmp(a, b) == Some(Less)
  3. a > b if and only if partial_cmp(a, b) == Some(Greater)
  4. a <= b if and only if a < b || a == b
  5. a >= b if and only if a > b || a == b
  6. a != b if and only if !(a == b).

Conditions 2–5 above are ensured by the default implementation. Condition 6 is already ensured by PartialEq.

If Ord is also implemented for Self and Rhs, it must also be consistent with partial_cmp (see the documentation of that trait for the exact requirements). It’s easy to accidentally make them disagree by deriving some of the traits and manually implementing others.

The comparison must satisfy, for all a, b and c:

  • transitivity: a < b and b < c implies a < c. The same must hold for both == and >.
  • duality: a < b if and only if b > a.

Note that these requirements mean that the trait itself must be implemented symmetrically and transitively: if T: PartialOrd<U> and U: PartialOrd<V> then U: PartialOrd<T> and T: PartialOrd<V>.

Violating these requirements is a logic error. The behavior resulting from a logic error is not specified, but users of the trait must ensure that such logic errors do not result in undefined behavior. This means that unsafe code must not rely on the correctness of these methods.

Corollaries

The following corollaries follow from the above requirements:

  • irreflexivity of < and >: !(a < a), !(a > a)
  • transitivity of >: if a > b and b > c then a > c
  • duality of partial_cmp: partial_cmp(a, b) == partial_cmp(b, a).map(Ordering::reverse)

Strict and non-strict partial orders

The < and > operators behave according to a strict partial order. However, <= and >= do not behave according to a non-strict partial order. That is because mathematically, a non-strict partial order would require reflexivity, i.e. a <= a would need to be true for every a. This isn’t always the case for types that implement PartialOrd, for example:

let a = f64::sqrt(-1.0);
assert_eq!(a <= a, false);
Run

Derivable

This trait can be used with #[derive].

When derived on structs, it will produce a lexicographic ordering based on the top-to-bottom declaration order of the struct’s members.

When derived on enums, variants are ordered by their discriminants. By default, the discriminant is smallest for variants at the top, and largest for variants at the bottom. Here’s an example:

#[derive(PartialEq, PartialOrd)]
enum E {
    Top,
    Bottom,
}

assert!(E::Top < E::Bottom);
Run

However, manually setting the discriminants can override this default behavior:

#[derive(PartialEq, PartialOrd)]
enum E {
    Top = 2,
    Bottom = 1,
}

assert!(E::Bottom < E::Top);
Run

How can I implement PartialOrd?

PartialOrd only requires implementation of the partial_cmp method, with the others generated from default implementations.

However it remains possible to implement the others separately for types which do not have a total order. For example, for floating point numbers, NaN < 0 == false and NaN >= 0 == false (cf. IEEE 754-2008 section 5.11).

PartialOrd requires your type to be PartialEq.

If your type is Ord, you can implement partial_cmp by using cmp:

use std::cmp::Ordering;

#[derive(Eq)]
struct Person {
    id: u32,
    name: String,
    height: u32,
}

impl PartialOrd for Person {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

impl Ord for Person {
    fn cmp(&self, other: &Self) -> Ordering {
        self.height.cmp(&other.height)
    }
}

impl PartialEq for Person {
    fn eq(&self, other: &Self) -> bool {
        self.height == other.height
    }
}
Run

You may also find it useful to use partial_cmp on your type’s fields. Here is an example of Person types who have a floating-point height field that is the only field to be used for sorting:

use std::cmp::Ordering;

struct Person {
    id: u32,
    name: String,
    height: f64,
}

impl PartialOrd for Person {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        self.height.partial_cmp(&other.height)
    }
}

impl PartialEq for Person {
    fn eq(&self, other: &Self) -> bool {
        self.height == other.height
    }
}
Run

Examples

let x: u32 = 0;
let y: u32 = 1;

assert_eq!(x < y, true);
assert_eq!(x.lt(&y), true);
Run

Required Methods§

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fn partial_cmp(&self, other: &Rhs) -> Option<Ordering>

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

Examples
use std::cmp::Ordering;

let result = 1.0.partial_cmp(&2.0);
assert_eq!(result, Some(Ordering::Less));

let result = 1.0.partial_cmp(&1.0);
assert_eq!(result, Some(Ordering::Equal));

let result = 2.0.partial_cmp(&1.0);
assert_eq!(result, Some(Ordering::Greater));
Run

When comparison is impossible:

let result = f64::NAN.partial_cmp(&1.0);
assert_eq!(result, None);
Run

Provided Methods§

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fn lt(&self, other: &Rhs) -> bool

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

Examples
assert_eq!(1.0 < 1.0, false);
assert_eq!(1.0 < 2.0, true);
assert_eq!(2.0 < 1.0, false);
Run
source

fn le(&self, other: &Rhs) -> bool

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

Examples
assert_eq!(1.0 <= 1.0, true);
assert_eq!(1.0 <= 2.0, true);
assert_eq!(2.0 <= 1.0, false);
Run
source

fn gt(&self, other: &Rhs) -> bool

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

Examples
assert_eq!(1.0 > 1.0, false);
assert_eq!(1.0 > 2.0, false);
assert_eq!(2.0 > 1.0, true);
Run
source

fn ge(&self, other: &Rhs) -> bool

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

Examples
assert_eq!(1.0 >= 1.0, true);
assert_eq!(1.0 >= 2.0, false);
assert_eq!(2.0 >= 1.0, true);
Run

Implementors§

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impl PartialOrd for AsciiChar

1.34.0 · source§

impl PartialOrd for Infallible

1.7.0 · source§

impl PartialOrd for IpAddr

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impl PartialOrd for SocketAddr

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impl PartialOrd for Which

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impl PartialOrd for Ordering

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impl PartialOrd for bool

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impl PartialOrd for char

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impl PartialOrd for f32

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impl PartialOrd for f64

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impl PartialOrd for i8

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impl PartialOrd for i16

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impl PartialOrd for i32

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impl PartialOrd for i64

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impl PartialOrd for i128

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impl PartialOrd for isize

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impl PartialOrd for !

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impl PartialOrd for str

Implements comparison operations on strings.

Strings are compared lexicographically by their byte values. This compares Unicode code points based on their positions in the code charts. This is not necessarily the same as “alphabetical” order, which varies by language and locale. Comparing strings according to culturally-accepted standards requires locale-specific data that is outside the scope of the str type.

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impl PartialOrd for u8

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impl PartialOrd for u16

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impl PartialOrd for u32

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impl PartialOrd for u64

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impl PartialOrd for u128

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impl PartialOrd for ()

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impl PartialOrd for usize

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impl PartialOrd for TypeId

1.27.0 · source§

impl PartialOrd for CpuidResult

Available on x86 or x86-64 only.
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impl PartialOrd for CStr

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impl PartialOrd for Error

1.33.0 · source§

impl PartialOrd for PhantomPinned

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impl PartialOrd for Ipv4Addr

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impl PartialOrd for Ipv6Addr

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impl PartialOrd for SocketAddrV4

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impl PartialOrd for SocketAddrV6

1.34.0 · source§

impl PartialOrd for NonZeroI8

1.34.0 · source§

impl PartialOrd for NonZeroI16

1.34.0 · source§

impl PartialOrd for NonZeroI32

1.34.0 · source§

impl PartialOrd for NonZeroI64

1.34.0 · source§

impl PartialOrd for NonZeroI128

1.34.0 · source§

impl PartialOrd for NonZeroIsize

1.28.0 · source§

impl PartialOrd for NonZeroU8

1.28.0 · source§

impl PartialOrd for NonZeroU16

1.28.0 · source§

impl PartialOrd for NonZeroU32

1.28.0 · source§

impl PartialOrd for NonZeroU64

1.28.0 · source§

impl PartialOrd for NonZeroU128

1.28.0 · source§

impl PartialOrd for NonZeroUsize

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impl PartialOrd for Alignment

1.3.0 · source§

impl PartialOrd for Duration

1.16.0 · source§

impl PartialOrd<IpAddr> for Ipv4Addr

1.16.0 · source§

impl PartialOrd<IpAddr> for Ipv6Addr

1.16.0 · source§

impl PartialOrd<Ipv4Addr> for IpAddr

1.16.0 · source§

impl PartialOrd<Ipv6Addr> for IpAddr

1.10.0 · source§

impl<'a> PartialOrd for Location<'a>

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impl<A, B: ?Sized> PartialOrd<&B> for &Awhere A: PartialOrd<B> + ?Sized,

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impl<A, B: ?Sized> PartialOrd<&mut B> for &mut Awhere A: PartialOrd<B> + ?Sized,

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impl<Dyn: ?Sized> PartialOrd for DynMetadata<Dyn>

1.4.0 · source§

impl<F: FnPtr> PartialOrd for F

1.41.0 · source§

impl<P: Deref, Q: Deref> PartialOrd<Pin<Q>> for Pin<P>where P::Target: PartialOrd<Q::Target>,

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impl<T> PartialOrd for (T₁, T₂, …, Tₙ)where T: ?Sized + PartialOrd,

This trait is implemented for tuples up to twelve items long.

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impl<T, const LANES: usize> PartialOrd for Mask<T, LANES>where T: MaskElement + PartialOrd, LaneCount<LANES>: SupportedLaneCount,

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impl<T, const N: usize> PartialOrd for Simd<T, N>where LaneCount<N>: SupportedLaneCount, T: SimdElement + PartialOrd,

1.10.0 · source§

impl<T: PartialOrd + Copy> PartialOrd for Cell<T>

1.20.0 · source§

impl<T: PartialOrd + ?Sized> PartialOrd for ManuallyDrop<T>

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impl<T: PartialOrd> PartialOrd for Option<T>

1.36.0 · source§

impl<T: PartialOrd> PartialOrd for Poll<T>

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impl<T: PartialOrd> PartialOrd for [T]

Implements comparison of vectors lexicographically.

1.74.0 · source§

impl<T: PartialOrd> PartialOrd for Saturating<T>

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impl<T: PartialOrd> PartialOrd for Wrapping<T>

1.19.0 · source§

impl<T: PartialOrd> PartialOrd for Reverse<T>

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impl<T: PartialOrd, E: PartialOrd> PartialOrd for Result<T, E>

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impl<T: PartialOrd, const N: usize> PartialOrd for [T; N]

1.10.0 · source§

impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T>

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impl<T: ?Sized> PartialOrd for *const T

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impl<T: ?Sized> PartialOrd for *mut T

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impl<T: ?Sized> PartialOrd for PhantomData<T>

1.25.0 · source§

impl<T: ?Sized> PartialOrd for NonNull<T>

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impl<Y: PartialOrd, R: PartialOrd> PartialOrd for CoroutineState<Y, R>