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 SetCursorStyle

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 AVActiveFormatDescription

impl Hash for AVAppToDevMessageType

impl Hash for AVAudioServiceType

impl Hash for AVCRCId

impl Hash for AVChannel

impl Hash for AVChannelOrder

impl Hash for AVChromaLocation

impl Hash for AVClassCategory

impl Hash for AVCodecID

impl Hash for AVColorPrimaries

impl Hash for AVColorRange

impl Hash for AVColorSpace

impl Hash for AVColorTransferCharacteristic

impl Hash for AVDevToAppMessageType

impl Hash for AVDiscard

impl Hash for AVDownmixType

impl Hash for AVDurationEstimationMethod

impl Hash for AVEscapeMode

impl Hash for AVFieldOrder

impl Hash for AVFilterLink__bindgen_ty_1

impl Hash for AVFrameSideDataType

impl Hash for AVHMACType

impl Hash for AVHWDeviceType

impl Hash for AVHWFrameTransferDirection

impl Hash for AVIODataMarkerType

impl Hash for AVIODirEntryType

impl Hash for AVMatrixEncoding

impl Hash for AVMediaType

impl Hash for AVPacketSideDataType

impl Hash for AVPictureStructure

impl Hash for AVPictureType

impl Hash for AVPixelFormat

impl Hash for AVRounding

impl Hash for AVSampleFormat

impl Hash for AVSideDataParamChangeFlags

impl Hash for AVStereo3DType

impl Hash for AVStereo3DView

impl Hash for AVStreamParseType

impl Hash for AVSubtitleType

impl Hash for AVThreadMessageFlags

impl Hash for AVTimebaseSource

impl Hash for AVTimecodeFlag

impl Hash for DCTTransformType

impl Hash for RDFTransformType

impl Hash for SwrDitherType

impl Hash for SwrEngine

impl Hash for SwrFilterType

impl Hash for _bindgen_ty_2

impl Hash for _bindgen_ty_3

impl Hash for _bindgen_ty_4

impl Hash for _bindgen_ty_5

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,