Trait std::cmp::PartialOrd

pub trait PartialOrd<Rhs = Self>: PartialEq<Rhs> where    Rhs: ?Sized, {
    fn partial_cmp(&self, other: &Rhs) -> Option<Ordering>;

    fn lt(&self, other: &Rhs) -> bool { ... }
    fn le(&self, other: &Rhs) -> bool { ... }
    fn gt(&self, other: &Rhs) -> bool { ... }
    fn ge(&self, other: &Rhs) -> bool { ... }
}

Trait for values that can be compared for a sort-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 in the following sense:

• a == b if and only if partial_cmp(a, b) == Some(Equal).
• a < b if and only if partial_cmp(a, b) == Some(Less) (ensured by the default implementation).
• a > b if and only if partial_cmp(a, b) == Some(Greater) (ensured by the default implementation).
• a <= b if and only if a < b || a == b (ensured by the default implementation).
• a >= b if and only if a > b || a == b (ensured by the default implementation).
• a != b if and only if !(a == b) (already part of 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>.

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)

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 top-to-bottom discriminant order.

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
    }
}

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
    }
}

Examples

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

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

Required methods

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));

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

Provided methods

let result = 1.0 < 2.0;
assert_eq!(result, true);

let result = 2.0 < 1.0;
assert_eq!(result, false);

let result = 1.0 <= 2.0;
assert_eq!(result, true);

let result = 2.0 <= 2.0;
assert_eq!(result, true);

let result = 1.0 > 2.0;
assert_eq!(result, false);

let result = 2.0 > 2.0;
assert_eq!(result, false);

let result = 2.0 >= 1.0;
assert_eq!(result, true);

let result = 2.0 >= 2.0;
assert_eq!(result, true);

Implementations on Foreign Types

Implementors