Trait Hash

1.0.0 · Source

pub trait Hash {
    // Required method
    fn hash<H>(&self, state: &mut H)
       where H: Hasher;

    // Provided method
    fn hash_slice<H>(data: &[Self], state: &mut H)
       where H: Hasher,
             Self: Sized { ... }
}

Expand description

A hashable type.

Types implementing Hash are able to be hashed with an instance of Hasher.

§Implementing `Hash`

You can derive Hash with #[derive(Hash)] if all fields implement Hash. The resulting hash will be the combination of the values from calling hash on each field.

#[derive(Hash)]
struct Rustacean {
    name: String,
    country: String,
}

If you need more control over how a value is hashed, you can of course implement the Hash trait yourself:

use std::hash::{Hash, Hasher};

struct Person {
    id: u32,
    name: String,
    phone: u64,
}

impl Hash for Person {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.id.hash(state);
        self.phone.hash(state);
    }
}

§`Hash` and `Eq`

When implementing both Hash and Eq, it is important that the following property holds:

k1 == k2 -> hash(k1) == hash(k2)

In other words, if two keys are equal, their hashes must also be equal. HashMap and HashSet both rely on this behavior.

Thankfully, you won’t need to worry about upholding this property when deriving both Eq and Hash with #[derive(PartialEq, Eq, Hash)].

Violating this property 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.

§Prefix collisions

Implementations of hash should ensure that the data they pass to the Hasher are prefix-free. That is, values which are not equal should cause two different sequences of values to be written, and neither of the two sequences should be a prefix of the other.

For example, the standard implementation of Hash for &str passes an extra 0xFF byte to the Hasher so that the values ("ab", "c") and ("a", "bc") hash differently.

§Portability

Due to differences in endianness and type sizes, data fed by Hash to a Hasher should not be considered portable across platforms. Additionally the data passed by most standard library types should not be considered stable between compiler versions.

This means tests shouldn’t probe hard-coded hash values or data fed to a Hasher and instead should check consistency with Eq.

Serialization formats intended to be portable between platforms or compiler versions should either avoid encoding hashes or only rely on Hash and Hasher implementations that provide additional guarantees.

Required Methods§

1.0.0 · Source

fn hash<H>(&self, state: &mut H)
where H: Hasher,

Feeds this value into the given Hasher.

§Examples

use std::hash::{DefaultHasher, Hash, Hasher};

let mut hasher = DefaultHasher::new();
7920.hash(&mut hasher);
println!("Hash is {:x}!", hasher.finish());

Provided Methods§

1.3.0 · Source

fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

This method is meant as a convenience, but its implementation is also explicitly left unspecified. It isn’t guaranteed to be equivalent to repeated calls of hash and implementations of Hash should keep that in mind and call hash themselves if the slice isn’t treated as a whole unit in the PartialEq implementation.

For example, a VecDeque implementation might naïvely call as_slices and then hash_slice on each slice, but this is wrong since the two slices can change with a call to make_contiguous without affecting the PartialEq result. Since these slices aren’t treated as singular units, and instead part of a larger deque, this method cannot be used.

§Examples

use std::hash::{DefaultHasher, Hash, Hasher};

let mut hasher = DefaultHasher::new();
let numbers = [6, 28, 496, 8128];
Hash::hash_slice(&numbers, &mut hasher);
println!("Hash is {:x}!", hasher.finish());

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.

Trait HashCopy item path

§Implementing Hash

§Hash and Eq

§Prefix collisions

§Portability

Required Methods§

fn hash<H>(&self, state: &mut H)where H: Hasher,

§Examples

Provided Methods§

fn hash_slice<H>(data: &[Self], state: &mut H)where H: Hasher, Self: Sized,

§Examples

Dyn Compatibility§

Implementors§

impl Hash for CursorType

impl Hash for geng::Key

impl Hash for geng::MouseButton

impl Hash for AnsiColor

impl Hash for Color

impl Hash for ValueHint

impl Hash for ContextKind

impl Hash for geng::prelude::cli::prelude::clap::error::ErrorKind

impl Hash for geng::prelude::futures::io::ErrorKind

impl Hash for PollNext

impl Hash for AsciiChar

impl Hash for core::cmp::Ordering

impl Hash for Infallible

impl Hash for core::net::ip_addr::IpAddr

impl Hash for Ipv6MulticastScope

impl Hash for SocketAddr

impl Hash for IntErrorKind

impl Hash for core::sync::atomic::Ordering

impl Hash for ChmapPosition

impl Hash for ChmapType

impl Hash for ElemIface

impl Hash for ElemType

impl Hash for SelemChannelId

impl Hash for alsa::pcm::Access

impl Hash for AudioTstampType

impl Hash for alsa::pcm::Format

impl Hash for alsa::pcm::State

impl Hash for TstampType

impl Hash for alsa::seq::EventType

impl Hash for CheckedCastError

impl Hash for PodCastError

impl Hash for byteorder::BigEndian

impl Hash for byteorder::LittleEndian

impl Hash for HostId

impl Hash for CursorIcon

impl Hash for WriteStyle

impl Hash for RoundingMode

impl Hash for BlockType

impl Hash for EnvironmentMap

impl Hash for LevelMode

impl Hash for LineOrder

impl Hash for SampleType

impl Hash for BlockDescription

impl Hash for DistanceMode

impl Hash for gif::reader::decoder::Version

impl Hash for gilrs::ev::Axis

impl Hash for AxisOrBtn

impl Hash for Button

impl Hash for gimli::common::Format

impl Hash for SectionId

impl Hash for RunTimeEndian

impl Hash for glutin::error::ErrorKind

impl Hash for ExtensionType

impl Hash for GeneralCategory

impl Hash for BufferFormat

impl Hash for image::color::ColorType

impl Hash for ExtendedColorType

impl Hash for ImageFormatHint

impl Hash for LimitErrorKind

impl Hash for ParameterErrorKind

impl Hash for UnsupportedErrorKind

impl Hash for image::flat::Error

impl Hash for NormalForm

impl Hash for image::image::ImageFormat

impl Hash for IpAddrRange

impl Hash for IpNet

impl Hash for IpSubnets

Trait Hash

§Implementing `Hash`

§`Hash` and `Eq`

fn hash<H>(&self, state: &mut H)
where H: Hasher,

fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,