# Trait std::cmp::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 + PartialOrd { ... }
    }

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).

It’s easy to accidentally make `cmp` and `partial_cmp` disagree by deriving some of the traits and manually implementing others.

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

When `derive`d 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, Eq, PartialOrd, Ord)]
    enum E {
        Top,
        Bottom,
    }

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

Run

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);

Run

### 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` and `Eq` (which requires `PartialEq`).

Then you must define an implementation for `cmp`. You may find it useful to use `cmp` on your type’s fields.

Here’s an example where you want to sort people by height only, disregarding `id` and `name`:

    use std::cmp::Ordering;

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

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

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

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

Run

## Required Methods§

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#### 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);

Run

## Provided Methods§

1.21.0 · source

#### fn max(self, other: Self) -> Selfwhere 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);

Run
1.21.0 · source

#### fn min(self, other: Self) -> Selfwhere 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);

Run
1.50.0 · source

#### fn clamp(self, min: Self, max: Self) -> Selfwhere Self: Sized + PartialOrd,

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);

Run

## Object Safety§

This trait is not object safe.

## Implementors§

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### impl Ord for AsciiChar

1.34.0 · source§

### impl Ord for Infallible

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### impl Ord for ErrorKind

1.7.0 · source§

### impl Ord for IpAddr

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### impl Ord for SocketAddr

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### impl Ord for Which

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### impl Ord for Ordering

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### impl Ord for bool

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### impl Ord for char

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### impl Ord for i8

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### impl Ord for i16

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### impl Ord for i32

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### impl Ord for i64

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### impl Ord for i128

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### impl Ord for isize

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### impl Ord for !

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### 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.

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### impl Ord for u8

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### impl Ord for u16

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### impl Ord for u32

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### impl Ord for u64

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### impl Ord for u128

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### impl Ord for ()

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### impl Ord for usize

1.27.0 · source§

### impl Ord for CpuidResult

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### impl Ord for TypeId

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### impl Ord for CStr

1.64.0 · source§

### impl Ord for CString

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### impl Ord for OsStr

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### impl Ord for OsString

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### impl Ord for Error

1.33.0 · source§

### impl Ord for PhantomPinned

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### impl Ord for Ipv4Addr

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### impl Ord for Ipv6Addr

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### impl Ord for SocketAddrV4

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### impl Ord for SocketAddrV6

1.34.0 · source§

### impl Ord for NonZeroI8

1.34.0 · source§

### impl Ord for NonZeroI16

1.34.0 · source§

### impl Ord for NonZeroI32

1.34.0 · source§

### impl Ord for NonZeroI64

1.34.0 · source§

### impl Ord for NonZeroI128

1.34.0 · source§

### impl Ord for NonZeroIsize

1.28.0 · source§

### impl Ord for NonZeroU8

1.28.0 · source§

### impl Ord for NonZeroU16

1.28.0 · source§

### impl Ord for NonZeroU32

1.28.0 · source§

### impl Ord for NonZeroU64

1.28.0 · source§

### impl Ord for NonZeroU128

1.28.0 · source§

### impl Ord for NonZeroUsize

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### impl Ord for Components<'_>

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### impl Ord for Path

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### impl Ord for PathBuf

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### impl Ord for PrefixComponent<'_>

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### impl Ord for Alignment

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### impl Ord for String

1.3.0 · source§

### impl Ord for Duration

1.8.0 · source§

### impl Ord for Instant

1.8.0 · source§

### impl Ord for SystemTime

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

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

1.10.0 · source§

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

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

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

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

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

1.4.0 · source§

### impl<F> Ord for Fwhere F: FnPtr,

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

1.41.0 · source§

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

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

1.36.0 · source§

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

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

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

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

Implements comparison of vectors lexicographically.

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### 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,

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

1.20.0 · source§

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

1.74.0 · source§

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

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### 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,

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

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

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

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

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

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

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

Implements ordering of vectors, lexicographically.

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

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

Implements comparison of arrays lexicographically.

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

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