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sp_std::cmp

Trait Ord

1.0.0 · Source
pub trait Ord: Eq + PartialOrd {
    // Required method
    fn cmp(&self, other: &Self) -> Ordering;

    // Provided methods
    fn max(self, other: Self) -> Self
       where Self: Sized { ... }
    fn min(self, other: Self) -> Self
       where Self: Sized { ... }
    fn clamp(self, min: Self, max: Self) -> Self
       where Self: Sized { ... }
}
Expand description

Trait for types that form a total order.

Implementations must be consistent with the PartialOrd implementation, and ensure max, min, and clamp are consistent with cmp:

  • partial_cmp(a, b) == Some(cmp(a, b)).
  • max(a, b) == max_by(a, b, cmp) (ensured by the default implementation).
  • min(a, b) == min_by(a, b, cmp) (ensured by the default implementation).
  • For a.clamp(min, max), see the method docs (ensured by the default implementation).

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

From the above and the requirements of PartialOrd, it follows that for all a, b and c:

  • exactly one of a < b, a == b or a > b is true; and
  • < is transitive: a < b and b < c implies a < c. The same must hold for both == and >.

Mathematically speaking, the < operator defines a strict weak order. In cases where == conforms to mathematical equality, it also defines a strict total order.

§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 primarily by their discriminants. Secondarily, they are ordered by their fields. By default, the discriminant is smallest for variants at the top, and largest for variants at the bottom. Here’s an example:

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

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

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

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

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

§Lexicographical comparison

Lexicographical comparison is an operation with the following properties:

  • Two sequences are compared element by element.
  • The first mismatching element defines which sequence is lexicographically less or greater than the other.
  • If one sequence is a prefix of another, the shorter sequence is lexicographically less than the other.
  • If two sequences have equivalent elements and are of the same length, then the sequences are lexicographically equal.
  • An empty sequence is lexicographically less than any non-empty sequence.
  • Two empty sequences are lexicographically equal.

§How can I implement Ord?

Ord requires that the type also be PartialOrd, PartialEq, and Eq.

Because Ord implies a stronger ordering relationship than PartialOrd, and both Ord and PartialOrd must agree, you must choose how to implement Ord first. You can choose to derive it, or implement it manually. If you derive it, you should derive all four traits. If you implement it manually, you should manually implement all four traits, based on the implementation of Ord.

Here’s an example where you want to define the Character comparison by health and experience only, disregarding the field mana:

use std::cmp::Ordering;

struct Character {
    health: u32,
    experience: u32,
    mana: f32,
}

impl Ord for Character {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        self.experience
            .cmp(&other.experience)
            .then(self.health.cmp(&other.health))
    }
}

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

impl PartialEq for Character {
    fn eq(&self, other: &Self) -> bool {
        self.health == other.health && self.experience == other.experience
    }
}

impl Eq for Character {}

If all you need is to slice::sort a type by a field value, it can be simpler to use slice::sort_by_key.

§Examples of incorrect Ord implementations

use std::cmp::Ordering;

#[derive(Debug)]
struct Character {
    health: f32,
}

impl Ord for Character {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        if self.health < other.health {
            Ordering::Less
        } else if self.health > other.health {
            Ordering::Greater
        } else {
            Ordering::Equal
        }
    }
}

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

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

impl Eq for Character {}

let a = Character { health: 4.5 };
let b = Character { health: f32::NAN };

// Mistake: floating-point values do not form a total order and using the built-in comparison
// operands to implement `Ord` irregardless of that reality does not change it. Use
// `f32::total_cmp` if you need a total order for floating-point values.

// Reflexivity requirement of `Ord` is not given.
assert!(a == a);
assert!(b != b);

// Antisymmetry requirement of `Ord` is not given. Only one of a < c and c < a is allowed to be
// true, not both or neither.
assert_eq!((a < b) as u8 + (b < a) as u8, 0);
use std::cmp::Ordering;

#[derive(Debug)]
struct Character {
    health: u32,
    experience: u32,
}

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

impl Ord for Character {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        if self.health < 50 {
            self.health.cmp(&other.health)
        } else {
            self.experience.cmp(&other.experience)
        }
    }
}

// For performance reasons implementing `PartialEq` this way is not the idiomatic way, but it
// ensures consistent behavior between `PartialEq`, `PartialOrd` and `Ord` in this example.
impl PartialEq for Character {
    fn eq(&self, other: &Self) -> bool {
        self.cmp(other) == Ordering::Equal
    }
}

impl Eq for Character {}

let a = Character {
    health: 3,
    experience: 5,
};
let b = Character {
    health: 10,
    experience: 77,
};
let c = Character {
    health: 143,
    experience: 2,
};

// Mistake: The implementation of `Ord` compares different fields depending on the value of
// `self.health`, the resulting order is not total.

// Transitivity requirement of `Ord` is not given. If a is smaller than b and b is smaller than
// c, by transitive property a must also be smaller than c.
assert!(a < b && b < c && c < a);

// Antisymmetry requirement of `Ord` is not given. Only one of a < c and c < a is allowed to be
// true, not both or neither.
assert_eq!((a < c) as u8 + (c < a) as u8, 2);

The documentation of PartialOrd contains further examples, for example it’s wrong for PartialOrd and PartialEq to disagree.

Required Methods§

1.0.0 · Source

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

By convention, self.cmp(&other) returns the ordering matching the expression self <operator> other if true.

§Examples
use std::cmp::Ordering;

assert_eq!(5.cmp(&10), Ordering::Less);
assert_eq!(10.cmp(&5), Ordering::Greater);
assert_eq!(5.cmp(&5), Ordering::Equal);

Provided Methods§

1.21.0 · Source

fn max(self, other: Self) -> Self
where Self: Sized,

Compares and returns the maximum of two values.

Returns the second argument if the comparison determines them to be equal.

§Examples
assert_eq!(1.max(2), 2);
assert_eq!(2.max(2), 2);
1.21.0 · Source

fn min(self, other: Self) -> Self
where Self: Sized,

Compares and returns the minimum of two values.

Returns the first argument if the comparison determines them to be equal.

§Examples
assert_eq!(1.min(2), 1);
assert_eq!(2.min(2), 2);
1.50.0 · Source

fn clamp(self, min: Self, max: Self) -> Self
where Self: Sized,

Restrict a value to a certain interval.

Returns max if self is greater than max, and min if self is less than min. Otherwise this returns self.

§Panics

Panics if min > max.

§Examples
assert_eq!((-3).clamp(-2, 1), -2);
assert_eq!(0.clamp(-2, 1), 0);
assert_eq!(2.clamp(-2, 1), 1);

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors§

1.34.0 · Source§

impl Ord for Infallible

1.0.0 · Source§

impl Ord for Ordering

Source§

impl Ord for AsciiChar

1.7.0 · Source§

impl Ord for IpAddr

1.0.0 · Source§

impl Ord for SocketAddr

1.0.0 · Source§

impl Ord for ErrorKind

1.0.0 · Source§

impl Ord for bool

1.0.0 · Source§

impl Ord for char

1.0.0 · Source§

impl Ord for i8

1.0.0 · Source§

impl Ord for i16

1.0.0 · Source§

impl Ord for i32

1.0.0 · Source§

impl Ord for i64

1.0.0 · Source§

impl Ord for i128

1.0.0 · Source§

impl Ord for isize

Source§

impl Ord for !

1.0.0 · Source§

impl Ord for str

Implements ordering of strings.

Strings are ordered lexicographically by their byte values. This orders 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. Sorting strings according to culturally-accepted standards requires locale-specific data that is outside the scope of the str type.

1.0.0 · Source§

impl Ord for u8

1.0.0 · Source§

impl Ord for u16

1.0.0 · Source§

impl Ord for u32

1.0.0 · Source§

impl Ord for u64

1.0.0 · Source§

impl Ord for u128

1.0.0 · Source§

impl Ord for ()

1.0.0 · Source§

impl Ord for usize

1.0.0 · Source§

impl Ord for TypeId

1.0.0 · Source§

impl Ord for Error

1.33.0 · Source§

impl Ord for PhantomPinned

Source§

impl Ord for Alignment

1.3.0 · Source§

impl Ord for Duration

1.64.0 · Source§

impl Ord for CString

1.0.0 · Source§

impl Ord for String

1.27.0 · Source§

impl Ord for CpuidResult

1.0.0 · Source§

impl Ord for CStr

1.0.0 · Source§

impl Ord for Ipv4Addr

1.0.0 · Source§

impl Ord for Ipv6Addr

1.0.0 · Source§

impl Ord for SocketAddrV4

1.0.0 · Source§

impl Ord for SocketAddrV6

1.0.0 · Source§

impl Ord for OsStr

1.0.0 · Source§

impl Ord for OsString

1.0.0 · Source§

impl Ord for Components<'_>

1.0.0 · Source§

impl Ord for Path

1.0.0 · Source§

impl Ord for PathBuf

1.0.0 · Source§

impl Ord for PrefixComponent<'_>

1.8.0 · Source§

impl Ord for Instant

1.8.0 · Source§

impl Ord for SystemTime

1.0.0 · Source§

impl<'a> Ord for Component<'a>

1.0.0 · Source§

impl<'a> Ord for Prefix<'a>

1.10.0 · Source§

impl<'a> Ord for Location<'a>

1.0.0 · Source§

impl<A> Ord for &A
where A: Ord + ?Sized,

1.0.0 · Source§

impl<A> Ord for &mut A
where A: Ord + ?Sized,

1.0.0 · Source§

impl<B> Ord for Cow<'_, B>
where B: Ord + ToOwned + ?Sized,

Source§

impl<Dyn> Ord for DynMetadata<Dyn>
where Dyn: ?Sized,

1.4.0 · Source§

impl<F> Ord for F
where F: FnPtr,

1.0.0 · Source§

impl<K, V, A> Ord for BTreeMap<K, V, A>
where K: Ord, V: Ord, A: Allocator + Clone,

1.41.0 · Source§

impl<Ptr> Ord for Pin<Ptr>
where Ptr: Deref, <Ptr as Deref>::Target: Ord,

1.0.0 · Source§

impl<T> Ord for Option<T>
where T: Ord,

1.36.0 · Source§

impl<T> Ord for Poll<T>
where T: Ord,

1.0.0 · Source§

impl<T> Ord for *const T
where T: ?Sized,

1.0.0 · Source§

impl<T> Ord for *mut T
where T: ?Sized,

1.0.0 · Source§

impl<T> Ord for [T]
where T: Ord,

Implements comparison of slices lexicographically.

1.0.0 · Source§

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

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

1.10.0 · Source§

impl<T> Ord for Cell<T>
where T: Ord + Copy,

1.10.0 · Source§

impl<T> Ord for RefCell<T>
where T: Ord + ?Sized,

1.0.0 · Source§

impl<T> Ord for PhantomData<T>
where T: ?Sized,

1.20.0 · Source§

impl<T> Ord for ManuallyDrop<T>
where T: Ord + ?Sized,

1.28.0 · Source§

impl<T> Ord for NonZero<T>

1.74.0 · Source§

impl<T> Ord for Saturating<T>
where T: Ord,

1.0.0 · Source§

impl<T> Ord for Wrapping<T>
where T: Ord,

1.25.0 · Source§

impl<T> Ord for NonNull<T>
where T: ?Sized,

1.19.0 · Source§

impl<T> Ord for Reverse<T>
where T: Ord,

1.0.0 · Source§

impl<T, A> Ord for Box<T, A>
where T: Ord + ?Sized, A: Allocator,

1.0.0 · Source§

impl<T, A> Ord for BTreeSet<T, A>
where T: Ord, A: Allocator + Clone,

1.0.0 · Source§

impl<T, A> Ord for VecDeque<T, A>
where T: Ord, A: Allocator,

1.0.0 · Source§

impl<T, A> Ord for Rc<T, A>
where T: Ord + ?Sized, A: Allocator,

1.0.0 · Source§

impl<T, A> Ord for Arc<T, A>
where T: Ord + ?Sized, A: Allocator,

1.0.0 · Source§

impl<T, A> Ord for Vec<T, A>
where T: Ord, A: Allocator,

Implements ordering of vectors, lexicographically.

1.0.0 · Source§

impl<T, A> Ord for LinkedList<T, A>
where T: Ord, A: Allocator,

1.0.0 · Source§

impl<T, E> Ord for Result<T, E>
where T: Ord, E: Ord,

1.0.0 · Source§

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

Implements comparison of arrays lexicographically.

Source§

impl<T, const N: usize> Ord for Simd<T, N>

Source§

impl<Y, R> Ord for CoroutineState<Y, R>
where Y: Ord, R: Ord,

impl Ord for ProxyType

impl Ord for ProxyType

impl Ord for HashedLaneId

impl Ord for LegacyLaneId

impl Ord for ParaHead

impl Ord for ParaId

impl<Hash: Ord, Number: Ord> Ord for HeaderId<Hash, Number>

impl Ord for BridgeId

impl Ord for ProxyType

impl Ord for ProxyType

impl Ord for ProxyType

impl Ord for InboundState

impl Ord for CountBound

impl Ord for SizeBound

impl Ord for CrateVersion

impl<AssetId: Ord> Ord for NativeOrWithId<AssetId>

impl Ord for ProxyType

impl Ord for Version

impl Ord for Cid

impl Ord for Multihash

impl<N: Ord + Copy + Clone + PartialOrd + Ord + Eq + PartialEq + Encode + Decode> Ord for TimeSlot<N>

impl<Balance: Ord> Ord for StorageDeposit<Balance>

impl Ord for CallFlags

impl Ord for ReturnFlags

impl Ord for Conviction

impl Ord for Conviction

impl<AccountId, Balance, Solution> Ord for SignedSubmission<AccountId, Balance, Solution>
where AccountId: Ord, Balance: Ord + HasCompact, Solution: Ord, RawSolution<Solution>: Ord,

impl<S: Ord> Ord for RawSolution<S>

impl Ord for Public

impl Ord for TimeSlot

impl<AccountId: Ord, ProxyType: Ord, BlockNumber: Ord> Ord for ProxyDefinition<AccountId, ProxyType, BlockNumber>

impl<Balance: Ord> Ord for StorageDeposit<Balance>

impl Ord for CallFlags

impl Ord for ReturnFlags

impl Ord for StorageFlags

impl Ord for ProxyType

impl Ord for ProxyType

impl<ConsumerIdentifier: Ord, MaxConsumers: Ord + Get<u32>> Ord for RemoteLockedFungibleRecord<ConsumerIdentifier, MaxConsumers>

impl Ord for ProxyType

impl Ord for ProxyType

impl Ord for Priority

impl Ord for HeadData

impl Ord for Id

impl Ord for Sibling

impl Ord for ChunkIndex

impl Ord for CoreIndex

impl Ord for GroupIndex

impl Ord for SlotRange

impl Ord for PartsOf57600

impl Ord for ProxyType

impl<Hash: Ord, Number: Ord> Ord for EpochIdentifier<Hash, Number>

impl Ord for SetId

impl Ord for Roles

impl Ord for Direction

impl Ord for PublicKey

impl Ord for Multiaddr

impl Ord for Multihash

impl Ord for PeerId

impl Ord for SeqID

impl Ord for Public

impl Ord for Public

impl Ord for Public

impl Ord for Public

impl Ord for Public

impl Ord for Public

impl Ord for BigUint

impl Ord for FixedI128

impl Ord for FixedI64

impl Ord for FixedU128

impl Ord for FixedU64

impl Ord for PerU16

impl Ord for Perbill

impl Ord for Percent

impl Ord for Permill

impl Ord for Perquintill

impl Ord for Rational128

impl<Block: BlockT> Ord for HashAndNumber<Block>

impl Ord for Public

impl Ord for Public

impl Ord for Public

impl Ord for Payload

impl<TBlockNumber> Ord for Commitment<TBlockNumber>
where TBlockNumber: Ord,

impl Ord for Slot

impl Ord for SlotDuration

impl Ord for CallContext

impl Ord for AccountId32

impl Ord for CryptoTypeId

impl Ord for KeyTypeId

impl Ord for Capabilities

impl Ord for Duration

impl Ord for Timestamp

impl Ord for Bytes

impl Ord for OpaquePeerId

impl<const N: usize, T> Ord for CryptoBytes<N, T>

impl Ord for Keyring

impl Ord for Keyring

impl Ord for Keyring

impl Ord for MultiSigner

impl<'a> Ord for OpaqueDigestItemId<'a>

impl<T: Ord, D: Get<T>> Ord for TypeWithDefault<T, D>

impl<AccountId: Ord, Balance: Ord + HasCompact> Ord for Exposure<AccountId, Balance>

impl<AccountId: Ord, Balance: Ord + HasCompact> Ord for ExposurePage<AccountId, Balance>

impl<AccountId: Ord, Balance: Ord + HasCompact> Ord for IndividualExposure<AccountId, Balance>

impl<Balance: Ord + HasCompact + MaxEncodedLen> Ord for PagedExposureMetadata<Balance>

impl Ord for ChildInfo

impl Ord for StorageData

impl Ord for StorageKey

impl Ord for Timestamp

impl Ord for AssetId

impl Ord for BodyId

impl Ord for BodyPart

impl Ord for Fungibility

impl Ord for Junction

impl Ord for Junctions

impl Ord for NetworkId

impl Ord for AssetFilter

impl Ord for Fungibility

impl Ord for Junction

impl Ord for Junctions

impl Ord for NetworkId

impl Ord for WildAsset

impl Ord for AssetFilter

impl Ord for Fungibility

impl Ord for Junction

impl Ord for Junctions

impl Ord for NetworkId

impl Ord for WildAsset

impl Ord for Ancestor

impl Ord for MultiAsset

impl Ord for MultiAssets

impl Ord for Parent

impl Ord for ParentThen

impl Ord for Ancestor

impl Ord for Asset

impl Ord for AssetId

impl Ord for Assets

impl Ord for Location

impl Ord for Parent

impl Ord for ParentThen

impl Ord for Ancestor

impl Ord for Asset

impl Ord for AssetId

impl Ord for Assets

impl Ord for Location

impl Ord for Parent

impl Ord for ParentThen

impl<Interior: Ord> Ord for AncestorThen<Interior>

impl<Interior: Ord> Ord for AncestorThen<Interior>

impl<Interior: Ord> Ord for AncestorThen<Interior>

impl<V: Ord> Ord for StorageQuery<V>

impl Ord for ProxyType