Trait Hash

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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

core 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 devela::_core::ascii::Char

impl Hash for Ipv6MulticastScope

impl Hash for CheckedCastError

impl Hash for PodCastError

impl Hash for devela::_dep::crossterm::event::Event

impl Hash for devela::_dep::crossterm::event::KeyCode

impl Hash for KeyEventKind

impl Hash for MediaKeyCode

impl Hash for ModifierKeyCode

impl Hash for devela::_dep::crossterm::event::MouseButton

impl Hash for MouseEventKind

impl Hash for Attribute

impl Hash for devela::_dep::crossterm::style::Color

impl Hash for Colored

impl Hash for ClearType

impl Hash for FltkErrorKind

impl Hash for devela::_dep::gilrs::Axis

impl Hash for devela::_dep::gilrs::Button

impl Hash for AxisOrBtn

impl Hash for ColorType

impl Hash for ExtendedColorType

impl Hash for ImageFormat

impl Hash for ImageFormatHint

impl Hash for LimitErrorKind

impl Hash for ParameterErrorKind

impl Hash for UnsupportedErrorKind

impl Hash for devela::_dep::image::flat::Error

impl Hash for NormalForm

impl Hash for FilterType

impl Hash for devela::_dep::image::metadata::Orientation

impl Hash for Era

impl Hash for Weekday

impl Hash for RoundMode

impl Hash for Unit

impl Hash for Meridiem

impl Hash for PiecesOffset

impl Hash for Dst

impl Hash for EqFilterKind

impl Hash for WhenToStart

impl Hash for PlaybackState

impl Hash for TrackPlaybackState

impl Hash for CursorIcon

impl Hash for FilterMode

impl Hash for devela::_dep::miniquad::KeyCode

impl Hash for MipmapFilterMode

impl Hash for devela::_dep::miniquad::MouseButton

impl Hash for RawId

impl Hash for TextureFormat

impl Hash for TouchPhase

impl Hash for devela::_dep::miniquad::log::Level

impl Hash for HostId

impl Hash for IoringMsgringCmds

impl Hash for IoringOp

impl Hash for IoringRegisterOp

impl Hash for IoringRestrictionOp

impl Hash for Direction

impl Hash for Shutdown

impl Hash for SocketAddrAny

impl Hash for Timeout

impl Hash for Action

impl Hash for OptionalActions

impl Hash for QueueSelector

impl Hash for devela::_dep::rustix::thread::ClockId

impl Hash for TimerfdClockId

impl Hash for devela::_dep::sdl2::audio::AudioFormat

impl Hash for AudioStatus

impl Hash for devela::_dep::sdl2::controller::Axis

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,