pub trait Hash {
// Required method
fn hash<H: Hasher>(&self, state: &mut H);
// Provided method
fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)
where 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§
Provided Methods§
1.3.0 · sourcefn hash_slice<H: Hasher>(data: &[Self], state: &mut H)where
Self: Sized,
fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)where
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());Object Safety§
Implementors§
impl Hash for AsciiChar
impl Hash for core::cmp::Ordering
impl Hash for Infallible
impl Hash for IpAddr
impl Hash for Ipv6MulticastScope
impl Hash for SocketAddr
impl Hash for core::sync::atomic::Ordering
impl Hash for bool
impl Hash for char
impl Hash for i8
impl Hash for i16
impl Hash for i32
impl Hash for i64
impl Hash for i128
impl Hash for isize
impl Hash for !
impl Hash for str
impl Hash for u8
impl Hash for u16
impl Hash for u32
impl Hash for u64
impl Hash for u128
impl Hash for ()
impl Hash for usize
impl Hash for Layout
impl Hash for TypeId
impl Hash for CStr
impl Hash for Error
impl Hash for PhantomPinned
impl Hash for Ipv4Addr
impl Hash for Ipv6Addr
impl Hash for SocketAddrV4
impl Hash for SocketAddrV6
impl Hash for NonZeroI8
impl Hash for NonZeroI16
impl Hash for NonZeroI32
impl Hash for NonZeroI64
impl Hash for NonZeroI128
impl Hash for NonZeroIsize
impl Hash for NonZeroU8
impl Hash for NonZeroU16
impl Hash for NonZeroU32
impl Hash for NonZeroU64
impl Hash for NonZeroU128
impl Hash for NonZeroUsize
impl Hash for RangeFull
impl Hash for Alignment
impl Hash for Duration
impl<'a> Hash for Location<'a>
impl<B: Hash, C: Hash> Hash for ControlFlow<B, C>
impl<Dyn: ?Sized> Hash for DynMetadata<Dyn>
impl<F: FnPtr> Hash for F
impl<Idx: Hash> Hash for Range<Idx>
impl<Idx: Hash> Hash for RangeFrom<Idx>
impl<Idx: Hash> Hash for RangeInclusive<Idx>
impl<Idx: Hash> Hash for RangeTo<Idx>
impl<Idx: Hash> Hash for RangeToInclusive<Idx>
impl<Ptr: Deref<Target: Hash>> Hash for Pin<Ptr>
impl<T> Hash for (T₁, T₂, …, Tₙ)
This trait is implemented for tuples up to twelve items long.
impl<T> Hash for Discriminant<T>
impl<T, const N: usize> Hash for Simd<T, N>
impl<T: Hash + ?Sized> Hash for ManuallyDrop<T>
impl<T: Hash> Hash for Bound<T>
impl<T: Hash> Hash for Option<T>
impl<T: Hash> Hash for Poll<T>
impl<T: Hash> Hash for [T]
impl<T: Hash> Hash for Reverse<T>
impl<T: Hash> Hash for Saturating<T>
impl<T: Hash> Hash for Wrapping<T>
impl<T: Hash, E: Hash> Hash for Result<T, E>
impl<T: Hash, const N: usize> Hash for [T; N]
The hash of an array is the same as that of the corresponding slice,
as required by the Borrow implementation.
use std::hash::BuildHasher;
let b = std::hash::RandomState::new();
let a: [u8; 3] = [0xa8, 0x3c, 0x09];
let s: &[u8] = &[0xa8, 0x3c, 0x09];
assert_eq!(b.hash_one(a), b.hash_one(s));