# Struct hashbrown::HashMap

source · [−]

    pub struct HashMap<K, V, S = DefaultHashBuilder, A: Allocator + Clone = Global> { /* private fields */ }

Expand description

A hash map implemented with quadratic probing and SIMD lookup.

The default hashing algorithm is currently `AHash`, though this is subject to change at any point in the future. This hash function is very fast for all types of keys, but this algorithm will typically not protect against attacks such as HashDoS.

The hashing algorithm can be replaced on a per-`HashMap` basis using the `default`, `with_hasher`, and `with_capacity_and_hasher` methods. Many alternative algorithms are available on crates.io, such as the `fnv` crate.

It is required that the keys implement the `Eq` and `Hash` traits, although this can frequently be achieved by using `#[derive(PartialEq, Eq, Hash)]`. If you implement these yourself, 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 be equal.

It is a logic error for a key to be modified in such a way that the key’s hash, as determined by the `Hash` trait, or its equality, as determined by the `Eq` trait, changes while it is in the map. This is normally only possible through `Cell`, `RefCell`, global state, I/O, or unsafe code.

It is also a logic error for the `Hash` implementation of a key to panic. This is generally only possible if the trait is implemented manually. If a panic does occur then the contents of the `HashMap` may become corrupted and some items may be dropped from the table.

## Examples

    use hashbrown::HashMap;

    // Type inference lets us omit an explicit type signature (which
    // would be `HashMap<String, String>` in this example).
    let mut book_reviews = HashMap::new();

    // Review some books.
    book_reviews.insert(
        "Adventures of Huckleberry Finn".to_string(),
        "My favorite book.".to_string(),
    );
    book_reviews.insert(
        "Grimms' Fairy Tales".to_string(),
        "Masterpiece.".to_string(),
    );
    book_reviews.insert(
        "Pride and Prejudice".to_string(),
        "Very enjoyable.".to_string(),
    );
    book_reviews.insert(
        "The Adventures of Sherlock Holmes".to_string(),
        "Eye lyked it alot.".to_string(),
    );

    // Check for a specific one.
    // When collections store owned values (String), they can still be
    // queried using references (&str).
    if !book_reviews.contains_key("Les Misérables") {
        println!("We've got {} reviews, but Les Misérables ain't one.",
                 book_reviews.len());
    }

    // oops, this review has a lot of spelling mistakes, let's delete it.
    book_reviews.remove("The Adventures of Sherlock Holmes");

    // Look up the values associated with some keys.
    let to_find = ["Pride and Prejudice", "Alice's Adventure in Wonderland"];
    for &book in &to_find {
        match book_reviews.get(book) {
            Some(review) => println!("{}: {}", book, review),
            None => println!("{} is unreviewed.", book)
        }
    }

    // Look up the value for a key (will panic if the key is not found).
    println!("Review for Jane: {}", book_reviews["Pride and Prejudice"]);

    // Iterate over everything.
    for (book, review) in &book_reviews {
        println!("{}: \"{}\"", book, review);
    }

`HashMap` also implements an `Entry API`, which allows for more complex methods of getting, setting, updating and removing keys and their values:

    use hashbrown::HashMap;

    // type inference lets us omit an explicit type signature (which
    // would be `HashMap<&str, u8>` in this example).
    let mut player_stats = HashMap::new();

    fn random_stat_buff() -> u8 {
        // could actually return some random value here - let's just return
        // some fixed value for now
        42
    }

    // insert a key only if it doesn't already exist
    player_stats.entry("health").or_insert(100);

    // insert a key using a function that provides a new value only if it
    // doesn't already exist
    player_stats.entry("defence").or_insert_with(random_stat_buff);

    // update a key, guarding against the key possibly not being set
    let stat = player_stats.entry("attack").or_insert(100);
    *stat += random_stat_buff();

The easiest way to use `HashMap` with a custom key type is to derive `Eq` and `Hash`. We must also derive `PartialEq`.

    use hashbrown::HashMap;

    #[derive(Hash, Eq, PartialEq, Debug)]
    struct Viking {
        name: String,
        country: String,
    }

    impl Viking {
        /// Creates a new Viking.
        fn new(name: &str, country: &str) -> Viking {
            Viking { name: name.to_string(), country: country.to_string() }
        }
    }

    // Use a HashMap to store the vikings' health points.
    let mut vikings = HashMap::new();

    vikings.insert(Viking::new("Einar", "Norway"), 25);
    vikings.insert(Viking::new("Olaf", "Denmark"), 24);
    vikings.insert(Viking::new("Harald", "Iceland"), 12);

    // Use derived implementation to print the status of the vikings.
    for (viking, health) in &vikings {
        println!("{:?} has {} hp", viking, health);
    }

A `HashMap` with fixed list of elements can be initialized from an array:

    use hashbrown::HashMap;

    let timber_resources: HashMap<&str, i32> = [("Norway", 100), ("Denmark", 50), ("Iceland", 10)]
        .iter().cloned().collect();
    // use the values stored in map

## Implementations

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### impl<K, V> HashMap<K, V, DefaultHashBuilder>

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#### pub fn new() -> Self

Creates an empty `HashMap`.

The hash map is initially created with a capacity of 0, so it will not allocate until it is first inserted into.

##### Examples

    use hashbrown::HashMap;
    let mut map: HashMap<&str, i32> = HashMap::new();
    assert_eq!(map.len(), 0);
    assert_eq!(map.capacity(), 0);

source

#### pub fn with_capacity(capacity: usize) -> Self

Creates an empty `HashMap` with the specified capacity.

The hash map will be able to hold at least `capacity` elements without reallocating. If `capacity` is 0, the hash map will not allocate.

##### Examples

    use hashbrown::HashMap;
    let mut map: HashMap<&str, i32> = HashMap::with_capacity(10);
    assert_eq!(map.len(), 0);
    assert!(map.capacity() >= 10);

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### impl<K, V, A: Allocator + Clone> HashMap<K, V, DefaultHashBuilder, A>

source

#### pub fn new_in(alloc: A) -> Self

Creates an empty `HashMap` using the given allocator.

The hash map is initially created with a capacity of 0, so it will not allocate until it is first inserted into.

source

#### pub fn with_capacity_in(capacity: usize, alloc: A) -> Self

Creates an empty `HashMap` with the specified capacity using the given allocator.

The hash map will be able to hold at least `capacity` elements without reallocating. If `capacity` is 0, the hash map will not allocate.

source

### impl<K, V, S> HashMap<K, V, S>

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#### pub const fn with_hasher(hash_builder: S) -> Self

Creates an empty `HashMap` which will use the given hash builder to hash keys.

The hash map is initially created with a capacity of 0, so it will not allocate until it is first inserted into.

Warning: `hash_builder` is normally randomly generated, and is designed to allow HashMaps to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

The `hash_builder` passed should implement the `BuildHasher` trait for the HashMap to be useful, see its documentation for details.

##### Examples

    use hashbrown::HashMap;
    use hashbrown::hash_map::DefaultHashBuilder;

    let s = DefaultHashBuilder::default();
    let mut map = HashMap::with_hasher(s);
    assert_eq!(map.len(), 0);
    assert_eq!(map.capacity(), 0);

    map.insert(1, 2);

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#### pub fn with_capacity_and_hasher(capacity: usize, hash_builder: S) -> Self

Creates an empty `HashMap` with the specified capacity, using `hash_builder` to hash the keys.

The hash map will be able to hold at least `capacity` elements without reallocating. If `capacity` is 0, the hash map will not allocate.

Warning: `hash_builder` is normally randomly generated, and is designed to allow HashMaps to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

The `hash_builder` passed should implement the `BuildHasher` trait for the HashMap to be useful, see its documentation for details.

##### Examples

    use hashbrown::HashMap;
    use hashbrown::hash_map::DefaultHashBuilder;

    let s = DefaultHashBuilder::default();
    let mut map = HashMap::with_capacity_and_hasher(10, s);
    assert_eq!(map.len(), 0);
    assert!(map.capacity() >= 10);

    map.insert(1, 2);

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### impl<K, V, S, A: Allocator + Clone> HashMap<K, V, S, A>

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#### pub fn allocator(&self) -> &A

Returns a reference to the underlying allocator.

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#### pub fn with_hasher_in(hash_builder: S, alloc: A) -> Self

Creates an empty `HashMap` which will use the given hash builder to hash keys. It will be allocated with the given allocator.

The created map has the default initial capacity.

Warning: `hash_builder` is normally randomly generated, and is designed to allow HashMaps to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

##### Examples

    use hashbrown::HashMap;
    use hashbrown::hash_map::DefaultHashBuilder;

    let s = DefaultHashBuilder::default();
    let mut map = HashMap::with_hasher(s);
    map.insert(1, 2);

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#### pub fn with_capacity_and_hasher_in(
capacity: usize,
hash_builder: S,
alloc: A
) -> Self

Creates an empty `HashMap` with the specified capacity, using `hash_builder` to hash the keys. It will be allocated with the given allocator.

The hash map will be able to hold at least `capacity` elements without reallocating. If `capacity` is 0, the hash map will not allocate.

Warning: `hash_builder` is normally randomly generated, and is designed to allow HashMaps to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

##### Examples

    use hashbrown::HashMap;
    use hashbrown::hash_map::DefaultHashBuilder;

    let s = DefaultHashBuilder::default();
    let mut map = HashMap::with_capacity_and_hasher(10, s);
    map.insert(1, 2);

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#### pub fn hasher(&self) -> &S

Returns a reference to the map’s `BuildHasher`.

##### Examples

    use hashbrown::HashMap;
    use hashbrown::hash_map::DefaultHashBuilder;

    let hasher = DefaultHashBuilder::default();
    let map: HashMap<i32, i32> = HashMap::with_hasher(hasher);
    let hasher: &DefaultHashBuilder = map.hasher();

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#### pub fn capacity(&self) -> usize

Returns the number of elements the map can hold without reallocating.

This number is a lower bound; the `HashMap<K, V>` might be able to hold more, but is guaranteed to be able to hold at least this many.

##### Examples

    use hashbrown::HashMap;
    let map: HashMap<i32, i32> = HashMap::with_capacity(100);
    assert_eq!(map.len(), 0);
    assert!(map.capacity() >= 100);

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#### pub fn keys(&self) -> Keys<'_, K, V>ⓘNotable traits for Keys<'a, K, V>`impl<'a, K, V> Iterator for Keys<'a, K, V> type Item = &'a K;`

An iterator visiting all keys in arbitrary order. The iterator element type is `&'a K`.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);
    assert_eq!(map.len(), 3);
    let mut vec: Vec<&str> = Vec::new();

    for key in map.keys() {
        println!("{}", key);
        vec.push(*key);
    }

    // The `Keys` iterator produces keys in arbitrary order, so the
    // keys must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, ["a", "b", "c"]);

    assert_eq!(map.len(), 3);

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#### pub fn values(&self) -> Values<'_, K, V>ⓘNotable traits for Values<'a, K, V>`impl<'a, K, V> Iterator for Values<'a, K, V> type Item = &'a V;`

An iterator visiting all values in arbitrary order. The iterator element type is `&'a V`.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);
    assert_eq!(map.len(), 3);
    let mut vec: Vec<i32> = Vec::new();

    for val in map.values() {
        println!("{}", val);
        vec.push(*val);
    }

    // The `Values` iterator produces values in arbitrary order, so the
    // values must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, [1, 2, 3]);

    assert_eq!(map.len(), 3);

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#### pub fn values_mut(&mut self) -> ValuesMut<'_, K, V>ⓘNotable traits for ValuesMut<'a, K, V>`impl<'a, K, V> Iterator for ValuesMut<'a, K, V> type Item = &'a mut V;`

An iterator visiting all values mutably in arbitrary order. The iterator element type is `&'a mut V`.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();

    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);

    for val in map.values_mut() {
        *val = *val + 10;
    }

    assert_eq!(map.len(), 3);
    let mut vec: Vec<i32> = Vec::new();

    for val in map.values() {
        println!("{}", val);
        vec.push(*val);
    }

    // The `Values` iterator produces values in arbitrary order, so the
    // values must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, [11, 12, 13]);

    assert_eq!(map.len(), 3);

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#### pub fn iter(&self) -> Iter<'_, K, V>ⓘNotable traits for Iter<'a, K, V>`impl<'a, K, V> Iterator for Iter<'a, K, V> type Item = (&'a K, &'a V);`

An iterator visiting all key-value pairs in arbitrary order. The iterator element type is `(&'a K, &'a V)`.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);
    assert_eq!(map.len(), 3);
    let mut vec: Vec<(&str, i32)> = Vec::new();

    for (key, val) in map.iter() {
        println!("key: {} val: {}", key, val);
        vec.push((*key, *val));
    }

    // The `Iter` iterator produces items in arbitrary order, so the
    // items must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, [("a", 1), ("b", 2), ("c", 3)]);

    assert_eq!(map.len(), 3);

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#### pub fn iter_mut(&mut self) -> IterMut<'_, K, V>ⓘNotable traits for IterMut<'a, K, V>`impl<'a, K, V> Iterator for IterMut<'a, K, V> type Item = (&'a K, &'a mut V);`

An iterator visiting all key-value pairs in arbitrary order, with mutable references to the values. The iterator element type is `(&'a K, &'a mut V)`.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);

    // Update all values
    for (_, val) in map.iter_mut() {
        *val *= 2;
    }

    assert_eq!(map.len(), 3);
    let mut vec: Vec<(&str, i32)> = Vec::new();

    for (key, val) in &map {
        println!("key: {} val: {}", key, val);
        vec.push((*key, *val));
    }

    // The `Iter` iterator produces items in arbitrary order, so the
    // items must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, [("a", 2), ("b", 4), ("c", 6)]);

    assert_eq!(map.len(), 3);

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#### pub fn len(&self) -> usize

Returns the number of elements in the map.

##### Examples

    use hashbrown::HashMap;

    let mut a = HashMap::new();
    assert_eq!(a.len(), 0);
    a.insert(1, "a");
    assert_eq!(a.len(), 1);

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#### pub fn is_empty(&self) -> bool

Returns `true` if the map contains no elements.

##### Examples

    use hashbrown::HashMap;

    let mut a = HashMap::new();
    assert!(a.is_empty());
    a.insert(1, "a");
    assert!(!a.is_empty());

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#### pub fn drain(&mut self) -> Drain<'_, K, V, A>ⓘNotable traits for Drain<'a, K, V, A>`impl<'a, K, V, A: Allocator + Clone> Iterator for Drain<'a, K, V, A> type Item = (K, V);`

Clears the map, returning all key-value pairs as an iterator. Keeps the allocated memory for reuse.

If the returned iterator is dropped before being fully consumed, it drops the remaining key-value pairs. The returned iterator keeps a mutable borrow on the vector to optimize its implementation.

##### Examples

    use hashbrown::HashMap;

    let mut a = HashMap::new();
    a.insert(1, "a");
    a.insert(2, "b");
    let capacity_before_drain = a.capacity();

    for (k, v) in a.drain().take(1) {
        assert!(k == 1 || k == 2);
        assert!(v == "a" || v == "b");
    }

    // As we can see, the map is empty and contains no element.
    assert!(a.is_empty() && a.len() == 0);
    // But map capacity is equal to old one.
    assert_eq!(a.capacity(), capacity_before_drain);

    let mut a = HashMap::new();
    a.insert(1, "a");
    a.insert(2, "b");

    {   // Iterator is dropped without being consumed.
        let d = a.drain();
    }

    // But the map is empty even if we do not use Drain iterator.
    assert!(a.is_empty());

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#### pub fn retain<F>(&mut self, f: F) where
F: FnMut(&K, &mut V) -> bool,

Retains only the elements specified by the predicate. Keeps the allocated memory for reuse.

In other words, remove all pairs `(k, v)` such that `f(&k, &mut v)` returns `false`. The elements are visited in unsorted (and unspecified) order.

##### Examples

    use hashbrown::HashMap;

    let mut map: HashMap<i32, i32> = (0..8).map(|x|(x, x*10)).collect();
    assert_eq!(map.len(), 8);
    let capacity_before_retain = map.capacity();

    map.retain(|&k, _| k % 2 == 0);

    // We can see, that the number of elements inside map is changed.
    assert_eq!(map.len(), 4);
    // But map capacity is equal to old one.
    assert_eq!(map.capacity(), capacity_before_retain);

    let mut vec: Vec<(i32, i32)> = map.iter().map(|(&k, &v)| (k, v)).collect();
    vec.sort_unstable();
    assert_eq!(vec, [(0, 0), (2, 20), (4, 40), (6, 60)]);

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#### pub fn drain_filter<F>(&mut self, f: F) -> DrainFilter<'_, K, V, F, A>ⓘNotable traits for DrainFilter<'_, K, V, F, A>`impl<K, V, F, A> Iterator for DrainFilter<'_, K, V, F, A> where
F: FnMut(&K, &mut V) -> bool,
A: Allocator + Clone, type Item = (K, V);` where
F: FnMut(&K, &mut V) -> bool,

Drains elements which are true under the given predicate, and returns an iterator over the removed items.

In other words, move all pairs `(k, v)` such that `f(&k, &mut v)` returns `true` out into another iterator.

Note that `drain_filter` lets you mutate every value in the filter closure, regardless of whether you choose to keep or remove it.

When the returned DrainedFilter is dropped, any remaining elements that satisfy the predicate are dropped from the table.

It is unspecified how many more elements will be subjected to the closure if a panic occurs in the closure, or a panic occurs while dropping an element, or if the `DrainFilter` value is leaked.

Keeps the allocated memory for reuse.

##### Examples

    use hashbrown::HashMap;

    let mut map: HashMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
    let capacity_before_drain_filter = map.capacity();
    let drained: HashMap<i32, i32> = map.drain_filter(|k, _v| k % 2 == 0).collect();

    let mut evens = drained.keys().cloned().collect::<Vec<_>>();
    let mut odds = map.keys().cloned().collect::<Vec<_>>();
    evens.sort();
    odds.sort();

    assert_eq!(evens, vec![0, 2, 4, 6]);
    assert_eq!(odds, vec![1, 3, 5, 7]);
    // Map capacity is equal to old one.
    assert_eq!(map.capacity(), capacity_before_drain_filter);

    let mut map: HashMap<i32, i32> = (0..8).map(|x| (x, x)).collect();

    {   // Iterator is dropped without being consumed.
        let d = map.drain_filter(|k, _v| k % 2 != 0);
    }

    // But the map lens have been reduced by half
    // even if we do not use DrainFilter iterator.
    assert_eq!(map.len(), 4);

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#### pub fn clear(&mut self)

Clears the map, removing all key-value pairs. Keeps the allocated memory for reuse.

##### Examples

    use hashbrown::HashMap;

    let mut a = HashMap::new();
    a.insert(1, "a");
    let capacity_before_clear = a.capacity();

    a.clear();

    // Map is empty.
    assert!(a.is_empty());
    // But map capacity is equal to old one.
    assert_eq!(a.capacity(), capacity_before_clear);

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#### pub fn into_keys(self) -> IntoKeys<K, V, A>ⓘNotable traits for IntoKeys<K, V, A>`impl<K, V, A: Allocator + Clone> Iterator for IntoKeys<K, V, A> type Item = K;`

Creates a consuming iterator visiting all the keys in arbitrary order. The map cannot be used after calling this. The iterator element type is `K`.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);

    let mut vec: Vec<&str> = map.into_keys().collect();

    // The `IntoKeys` iterator produces keys in arbitrary order, so the
    // keys must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, ["a", "b", "c"]);

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#### pub fn into_values(self) -> IntoValues<K, V, A>ⓘNotable traits for IntoValues<K, V, A>`impl<K, V, A: Allocator + Clone> Iterator for IntoValues<K, V, A> type Item = V;`

Creates a consuming iterator visiting all the values in arbitrary order. The map cannot be used after calling this. The iterator element type is `V`.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);

    let mut vec: Vec<i32> = map.into_values().collect();

    // The `IntoValues` iterator produces values in arbitrary order, so
    // the values must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, [1, 2, 3]);

source

### impl<K, V, S, A> HashMap<K, V, S, A> where
K: Eq + Hash,
S: BuildHasher,
A: Allocator + Clone,

source

#### pub fn reserve(&mut self, additional: usize)

Reserves capacity for at least `additional` more elements to be inserted in the `HashMap`. The collection may reserve more space to avoid frequent reallocations.

##### Panics

Panics if the new allocation size overflows `usize`.

##### Examples

    use hashbrown::HashMap;
    let mut map: HashMap<&str, i32> = HashMap::new();
    // Map is empty and doesn't allocate memory
    assert_eq!(map.capacity(), 0);

    map.reserve(10);

    // And now map can hold at least 10 elements
    assert!(map.capacity() >= 10);

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#### pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>

Tries to reserve capacity for at least `additional` more elements to be inserted in the given `HashMap<K,V>`. The collection may reserve more space to avoid frequent reallocations.

##### Errors

If the capacity overflows, or the allocator reports a failure, then an error is returned.

##### Examples

    use hashbrown::HashMap;

    let mut map: HashMap<&str, isize> = HashMap::new();
    // Map is empty and doesn't allocate memory
    assert_eq!(map.capacity(), 0);

    map.try_reserve(10).expect("why is the test harness OOMing on 10 bytes?");

    // And now map can hold at least 10 elements
    assert!(map.capacity() >= 10);

If the capacity overflows, or the allocator reports a failure, then an error is returned:

    use hashbrown::HashMap;
    use hashbrown::TryReserveError;
    let mut map: HashMap<i32, i32> = HashMap::new();

    match map.try_reserve(usize::MAX) {
        Err(error) => match error {
            TryReserveError::CapacityOverflow => {}
            _ => panic!("TryReserveError::AllocError ?"),
        },
        _ => panic!(),
    }

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#### pub fn shrink_to_fit(&mut self)

Shrinks the capacity of the map as much as possible. It will drop down as much as possible while maintaining the internal rules and possibly leaving some space in accordance with the resize policy.

##### Examples

    use hashbrown::HashMap;

    let mut map: HashMap<i32, i32> = HashMap::with_capacity(100);
    map.insert(1, 2);
    map.insert(3, 4);
    assert!(map.capacity() >= 100);
    map.shrink_to_fit();
    assert!(map.capacity() >= 2);

source

#### pub fn shrink_to(&mut self, min_capacity: usize)

Shrinks the capacity of the map with a lower limit. It will drop down no lower than the supplied limit while maintaining the internal rules and possibly leaving some space in accordance with the resize policy.

This function does nothing if the current capacity is smaller than the supplied minimum capacity.

##### Examples

    use hashbrown::HashMap;

    let mut map: HashMap<i32, i32> = HashMap::with_capacity(100);
    map.insert(1, 2);
    map.insert(3, 4);
    assert!(map.capacity() >= 100);
    map.shrink_to(10);
    assert!(map.capacity() >= 10);
    map.shrink_to(0);
    assert!(map.capacity() >= 2);
    map.shrink_to(10);
    assert!(map.capacity() >= 2);

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#### pub fn entry(&mut self, key: K) -> Entry<'_, K, V, S, A>

Gets the given key’s corresponding entry in the map for in-place manipulation.

##### Examples

    use hashbrown::HashMap;

    let mut letters = HashMap::new();

    for ch in "a short treatise on fungi".chars() {
        let counter = letters.entry(ch).or_insert(0);
        *counter += 1;
    }

    assert_eq!(letters[&'s'], 2);
    assert_eq!(letters[&'t'], 3);
    assert_eq!(letters[&'u'], 1);
    assert_eq!(letters.get(&'y'), None);

source

#### pub fn entry_ref<'a, 'b, Q: ?Sized>(
&'a mut self,
key: &'b Q
) -> EntryRef<'a, 'b, K, Q, V, S, A> where
K: Borrow<Q>,
Q: Hash + Eq,

Gets the given key’s corresponding entry by reference in the map for in-place manipulation.

##### Examples

    use hashbrown::HashMap;

    let mut words: HashMap<String, usize> = HashMap::new();
    let source = ["poneyland", "horseyland", "poneyland", "poneyland"];
    for (i, &s) in source.iter().enumerate() {
        let counter = words.entry_ref(s).or_insert(0);
        *counter += 1;
    }

    assert_eq!(words["poneyland"], 3);
    assert_eq!(words["horseyland"], 1);

source

#### pub fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V> where
K: Borrow<Q>,
Q: Hash + Eq,

Returns a reference to the value corresponding to the key.

The key may be any borrowed form of the map’s key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert(1, "a");
    assert_eq!(map.get(&1), Some(&"a"));
    assert_eq!(map.get(&2), None);

source

#### pub fn get_key_value<Q: ?Sized>(&self, k: &Q) -> Option<(&K, &V)> where
K: Borrow<Q>,
Q: Hash + Eq,

Returns the key-value pair corresponding to the supplied key.

The supplied key may be any borrowed form of the map’s key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert(1, "a");
    assert_eq!(map.get_key_value(&1), Some((&1, &"a")));
    assert_eq!(map.get_key_value(&2), None);

source

#### pub fn get_key_value_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<(&K, &mut V)> where
K: Borrow<Q>,
Q: Hash + Eq,

Returns the key-value pair corresponding to the supplied key, with a mutable reference to value.

The supplied key may be any borrowed form of the map’s key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert(1, "a");
    let (k, v) = map.get_key_value_mut(&1).unwrap();
    assert_eq!(k, &1);
    assert_eq!(v, &mut "a");
    *v = "b";
    assert_eq!(map.get_key_value_mut(&1), Some((&1, &mut "b")));
    assert_eq!(map.get_key_value_mut(&2), None);

source

#### pub fn contains_key<Q: ?Sized>(&self, k: &Q) -> bool where
K: Borrow<Q>,
Q: Hash + Eq,

Returns `true` if the map contains a value for the specified key.

The key may be any borrowed form of the map’s key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert(1, "a");
    assert_eq!(map.contains_key(&1), true);
    assert_eq!(map.contains_key(&2), false);

source

#### pub fn get_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut V> where
K: Borrow<Q>,
Q: Hash + Eq,

Returns a mutable reference to the value corresponding to the key.

The key may be any borrowed form of the map’s key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.insert(1, "a");
    if let Some(x) = map.get_mut(&1) {
        *x = "b";
    }
    assert_eq!(map[&1], "b");

    assert_eq!(map.get_mut(&2), None);

source

#### pub fn get_many_mut<Q: ?Sized, const N: usize>(
&mut self,
ks: [&Q; N]
) -> Option<[&mut V; N]> where
K: Borrow<Q>,
Q: Hash + Eq,

Attempts to get mutable references to `N` values in the map at once.

Returns an array of length `N` with the results of each query. For soundness, at most one mutable reference will be returned to any value. `None` will be returned if any of the keys are duplicates or missing.

##### Examples

    use hashbrown::HashMap;

    let mut libraries = HashMap::new();
    libraries.insert("Bodleian Library".to_string(), 1602);
    libraries.insert("Athenæum".to_string(), 1807);
    libraries.insert("Herzogin-Anna-Amalia-Bibliothek".to_string(), 1691);
    libraries.insert("Library of Congress".to_string(), 1800);

    let got = libraries.get_many_mut([
        "Athenæum",
        "Library of Congress",
    ]);
    assert_eq!(
        got,
        Some([
            &mut 1807,
            &mut 1800,
        ]),
    );

    // Missing keys result in None
    let got = libraries.get_many_mut([
        "Athenæum",
        "New York Public Library",
    ]);
    assert_eq!(got, None);

    // Duplicate keys result in None
    let got = libraries.get_many_mut([
        "Athenæum",
        "Athenæum",
    ]);
    assert_eq!(got, None);

source

#### pub unsafe fn get_many_unchecked_mut<Q: ?Sized, const N: usize>(
&mut self,
ks: [&Q; N]
) -> Option<[&mut V; N]> where
K: Borrow<Q>,
Q: Hash + Eq,

Attempts to get mutable references to `N` values in the map at once, without validating that the values are unique.

Returns an array of length `N` with the results of each query. `None` will be returned if any of the keys are missing.

For a safe alternative see `get_many_mut`.

##### Safety

Calling this method with overlapping keys is undefined behavior even if the resulting references are not used.

##### Examples

    use hashbrown::HashMap;

    let mut libraries = HashMap::new();
    libraries.insert("Bodleian Library".to_string(), 1602);
    libraries.insert("Athenæum".to_string(), 1807);
    libraries.insert("Herzogin-Anna-Amalia-Bibliothek".to_string(), 1691);
    libraries.insert("Library of Congress".to_string(), 1800);

    let got = libraries.get_many_mut([
        "Athenæum",
        "Library of Congress",
    ]);
    assert_eq!(
        got,
        Some([
            &mut 1807,
            &mut 1800,
        ]),
    );

    // Missing keys result in None
    let got = libraries.get_many_mut([
        "Athenæum",
        "New York Public Library",
    ]);
    assert_eq!(got, None);

source

#### pub fn get_many_key_value_mut<Q: ?Sized, const N: usize>(
&mut self,
ks: [&Q; N]
) -> Option<[(&K, &mut V); N]> where
K: Borrow<Q>,
Q: Hash + Eq,

Attempts to get mutable references to `N` values in the map at once, with immutable references to the corresponding keys.

Returns an array of length `N` with the results of each query. For soundness, at most one mutable reference will be returned to any value. `None` will be returned if any of the keys are duplicates or missing.

##### Examples

    use hashbrown::HashMap;

    let mut libraries = HashMap::new();
    libraries.insert("Bodleian Library".to_string(), 1602);
    libraries.insert("Athenæum".to_string(), 1807);
    libraries.insert("Herzogin-Anna-Amalia-Bibliothek".to_string(), 1691);
    libraries.insert("Library of Congress".to_string(), 1800);

    let got = libraries.get_many_key_value_mut([
        "Bodleian Library",
        "Herzogin-Anna-Amalia-Bibliothek",
    ]);
    assert_eq!(
        got,
        Some([
            (&"Bodleian Library".to_string(), &mut 1602),
            (&"Herzogin-Anna-Amalia-Bibliothek".to_string(), &mut 1691),
        ]),
    );
    // Missing keys result in None
    let got = libraries.get_many_key_value_mut([
        "Bodleian Library",
        "Gewandhaus",
    ]);
    assert_eq!(got, None);

    // Duplicate keys result in None
    let got = libraries.get_many_key_value_mut([
        "Bodleian Library",
        "Herzogin-Anna-Amalia-Bibliothek",
        "Herzogin-Anna-Amalia-Bibliothek",
    ]);
    assert_eq!(got, None);

source

#### pub unsafe fn get_many_key_value_unchecked_mut<Q: ?Sized, const N: usize>(
&mut self,
ks: [&Q; N]
) -> Option<[(&K, &mut V); N]> where
K: Borrow<Q>,
Q: Hash + Eq,

Attempts to get mutable references to `N` values in the map at once, with immutable references to the corresponding keys, without validating that the values are unique.

Returns an array of length `N` with the results of each query. `None` will be returned if any of the keys are missing.

For a safe alternative see `get_many_key_value_mut`.

##### Safety

Calling this method with overlapping keys is undefined behavior even if the resulting references are not used.

##### Examples

    use hashbrown::HashMap;

    let mut libraries = HashMap::new();
    libraries.insert("Bodleian Library".to_string(), 1602);
    libraries.insert("Athenæum".to_string(), 1807);
    libraries.insert("Herzogin-Anna-Amalia-Bibliothek".to_string(), 1691);
    libraries.insert("Library of Congress".to_string(), 1800);

    let got = libraries.get_many_key_value_mut([
        "Bodleian Library",
        "Herzogin-Anna-Amalia-Bibliothek",
    ]);
    assert_eq!(
        got,
        Some([
            (&"Bodleian Library".to_string(), &mut 1602),
            (&"Herzogin-Anna-Amalia-Bibliothek".to_string(), &mut 1691),
        ]),
    );
    // Missing keys result in None
    let got = libraries.get_many_key_value_mut([
        "Bodleian Library",
        "Gewandhaus",
    ]);
    assert_eq!(got, None);

source

#### pub fn insert(&mut self, k: K, v: V) -> Option<V>

Inserts a key-value pair into the map.

If the map did not have this key present, `None` is returned.

If the map did have this key present, the value is updated, and the old value is returned. The key is not updated, though; this matters for types that can be `==` without being identical. See the `std::collections` module-level documentation for more.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    assert_eq!(map.insert(37, "a"), None);
    assert_eq!(map.is_empty(), false);

    map.insert(37, "b");
    assert_eq!(map.insert(37, "c"), Some("b"));
    assert_eq!(map[&37], "c");

source

#### pub fn insert_unique_unchecked(&mut self, k: K, v: V) -> (&K, &mut V)

Insert a key-value pair into the map without checking if the key already exists in the map.

Returns a reference to the key and value just inserted.

This operation is safe if a key does not exist in the map.

However, if a key exists in the map already, the behavior is unspecified: this operation may panic, loop forever, or any following operation with the map may panic, loop forever or return arbitrary result.

That said, this operation (and following operations) are guaranteed to not violate memory safety.

This operation is faster than regular insert, because it does not perform lookup before insertion.

This operation is useful during initial population of the map. For example, when constructing a map from another map, we know that keys are unique.

##### Examples

    use hashbrown::HashMap;

    let mut map1 = HashMap::new();
    assert_eq!(map1.insert(1, "a"), None);
    assert_eq!(map1.insert(2, "b"), None);
    assert_eq!(map1.insert(3, "c"), None);
    assert_eq!(map1.len(), 3);

    let mut map2 = HashMap::new();

    for (key, value) in map1.into_iter() {
        map2.insert_unique_unchecked(key, value);
    }

    let (key, value) = map2.insert_unique_unchecked(4, "d");
    assert_eq!(key, &4);
    assert_eq!(value, &mut "d");
    *value = "e";

    assert_eq!(map2[&1], "a");
    assert_eq!(map2[&2], "b");
    assert_eq!(map2[&3], "c");
    assert_eq!(map2[&4], "e");
    assert_eq!(map2.len(), 4);

source

#### pub fn try_insert(
&mut self,
key: K,
value: V
) -> Result<&mut V, OccupiedError<'_, K, V, S, A>>

Tries to insert a key-value pair into the map, and returns a mutable reference to the value in the entry.

##### Errors

If the map already had this key present, nothing is updated, and an error containing the occupied entry and the value is returned.

##### Examples

Basic usage:

    use hashbrown::HashMap;
    use hashbrown::hash_map::OccupiedError;

    let mut map = HashMap::new();
    assert_eq!(map.try_insert(37, "a").unwrap(), &"a");

    match map.try_insert(37, "b") {
        Err(OccupiedError { entry, value }) => {
            assert_eq!(entry.key(), &37);
            assert_eq!(entry.get(), &"a");
            assert_eq!(value, "b");
        }
        _ => panic!()
    }

source

#### pub fn remove<Q: ?Sized>(&mut self, k: &Q) -> Option<V> where
K: Borrow<Q>,
Q: Hash + Eq,

Removes a key from the map, returning the value at the key if the key was previously in the map. Keeps the allocated memory for reuse.

The key may be any borrowed form of the map’s key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    // The map is empty
    assert!(map.is_empty() && map.capacity() == 0);

    map.insert(1, "a");
    let capacity_before_remove = map.capacity();

    assert_eq!(map.remove(&1), Some("a"));
    assert_eq!(map.remove(&1), None);

    // Now map holds none elements but capacity is equal to the old one
    assert!(map.len() == 0 && map.capacity() == capacity_before_remove);

source

#### pub fn remove_entry<Q: ?Sized>(&mut self, k: &Q) -> Option<(K, V)> where
K: Borrow<Q>,
Q: Hash + Eq,

Removes a key from the map, returning the stored key and value if the key was previously in the map. Keeps the allocated memory for reuse.

The key may be any borrowed form of the map’s key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

##### Examples

    use hashbrown::HashMap;

    let mut map = HashMap::new();
    // The map is empty
    assert!(map.is_empty() && map.capacity() == 0);

    map.insert(1, "a");
    let capacity_before_remove = map.capacity();

    assert_eq!(map.remove_entry(&1), Some((1, "a")));
    assert_eq!(map.remove(&1), None);

    // Now map hold none elements but capacity is equal to the old one
    assert!(map.len() == 0 && map.capacity() == capacity_before_remove);

source

### impl<K, V, S, A: Allocator + Clone> HashMap<K, V, S, A>

source

#### pub fn raw_entry_mut(&mut self) -> RawEntryBuilderMut<'_, K, V, S, A>

Creates a raw entry builder for the HashMap.

Raw entries provide the lowest level of control for searching and manipulating a map. They must be manually initialized with a hash and then manually searched. After this, insertions into a vacant entry still require an owned key to be provided.

Raw entries are useful for such exotic situations as:

- Hash memoization
- Deferring the creation of an owned key until it is known to be required
- Using a search key that doesn’t work with the Borrow trait
- Using custom comparison logic without newtype wrappers

Because raw entries provide much more low-level control, it’s much easier to put the HashMap into an inconsistent state which, while memory-safe, will cause the map to produce seemingly random results. Higher-level and more foolproof APIs like `entry` should be preferred when possible.

In particular, the hash used to initialized the raw entry must still be consistent with the hash of the key that is ultimately stored in the entry. This is because implementations of HashMap may need to recompute hashes when resizing, at which point only the keys are available.

Raw entries give mutable access to the keys. This must not be used to modify how the key would compare or hash, as the map will not re-evaluate where the key should go, meaning the keys may become “lost” if their location does not reflect their state. For instance, if you change a key so that the map now contains keys which compare equal, search may start acting erratically, with two keys randomly masking each other. Implementations are free to assume this doesn’t happen (within the limits of memory-safety).

##### Examples

    use core::hash::{BuildHasher, Hash};
    use hashbrown::hash_map::{HashMap, RawEntryMut};

    let mut map = HashMap::new();
    map.extend([("a", 100), ("b", 200), ("c", 300)]);

    fn compute_hash<K: Hash + ?Sized, S: BuildHasher>(hash_builder: &S, key: &K) -> u64 {
        use core::hash::Hasher;
        let mut state = hash_builder.build_hasher();
        key.hash(&mut state);
        state.finish()
    }

    // Existing key (insert and update)
    match map.raw_entry_mut().from_key(&"a") {
        RawEntryMut::Vacant(_) => unreachable!(),
        RawEntryMut::Occupied(mut view) => {
            assert_eq!(view.get(), &100);
            let v = view.get_mut();
            let new_v = (*v) * 10;
            *v = new_v;
            assert_eq!(view.insert(1111), 1000);
        }
    }

    assert_eq!(map[&"a"], 1111);
    assert_eq!(map.len(), 3);

    // Existing key (take)
    let hash = compute_hash(map.hasher(), &"c");
    match map.raw_entry_mut().from_key_hashed_nocheck(hash, &"c") {
        RawEntryMut::Vacant(_) => unreachable!(),
        RawEntryMut::Occupied(view) => {
            assert_eq!(view.remove_entry(), ("c", 300));
        }
    }
    assert_eq!(map.raw_entry().from_key(&"c"), None);
    assert_eq!(map.len(), 2);

    // Nonexistent key (insert and update)
    let key = "d";
    let hash = compute_hash(map.hasher(), &key);
    match map.raw_entry_mut().from_hash(hash, |q| *q == key) {
        RawEntryMut::Occupied(_) => unreachable!(),
        RawEntryMut::Vacant(view) => {
            let (k, value) = view.insert("d", 4000);
            assert_eq!((*k, *value), ("d", 4000));
            *value = 40000;
        }
    }
    assert_eq!(map[&"d"], 40000);
    assert_eq!(map.len(), 3);

    match map.raw_entry_mut().from_hash(hash, |q| *q == key) {
        RawEntryMut::Vacant(_) => unreachable!(),
        RawEntryMut::Occupied(view) => {
            assert_eq!(view.remove_entry(), ("d", 40000));
        }
    }
    assert_eq!(map.get(&"d"), None);
    assert_eq!(map.len(), 2);

source

#### pub fn raw_entry(&self) -> RawEntryBuilder<'_, K, V, S, A>

Creates a raw immutable entry builder for the HashMap.

Raw entries provide the lowest level of control for searching and manipulating a map. They must be manually initialized with a hash and then manually searched.

This is useful for

- Hash memoization
- Using a search key that doesn’t work with the Borrow trait
- Using custom comparison logic without newtype wrappers

Unless you are in such a situation, higher-level and more foolproof APIs like `get` should be preferred.

Immutable raw entries have very limited use; you might instead want `raw_entry_mut`.

##### Examples

    use core::hash::{BuildHasher, Hash};
    use hashbrown::HashMap;

    let mut map = HashMap::new();
    map.extend([("a", 100), ("b", 200), ("c", 300)]);

    fn compute_hash<K: Hash + ?Sized, S: BuildHasher>(hash_builder: &S, key: &K) -> u64 {
        use core::hash::Hasher;
        let mut state = hash_builder.build_hasher();
        key.hash(&mut state);
        state.finish()
    }

    for k in ["a", "b", "c", "d", "e", "f"] {
        let hash = compute_hash(map.hasher(), k);
        let v = map.get(&k).cloned();
        let kv = v.as_ref().map(|v| (&k, v));

        println!("Key: {} and value: {:?}", k, v);

        assert_eq!(map.raw_entry().from_key(&k), kv);
        assert_eq!(map.raw_entry().from_hash(hash, |q| *q == k), kv);
        assert_eq!(map.raw_entry().from_key_hashed_nocheck(hash, &k), kv);
    }

## Trait Implementations

source

### impl<K: Clone, V: Clone, S: Clone, A: Allocator + Clone> Clone for HashMap<K, V, S, A>

source

#### fn clone(&self) -> Self

Returns a copy of the value. Read more

source

#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more

source

### impl<K, V, S, A> Debug for HashMap<K, V, S, A> where
K: Debug,
V: Debug,
A: Allocator + Clone,

source

#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more

source

### impl<K, V, S, A> Default for HashMap<K, V, S, A> where
S: Default,
A: Default + Allocator + Clone,

source

#### fn default() -> Self

Creates an empty `HashMap<K, V, S, A>`, with the `Default` value for the hasher and allocator.

##### Examples

    use hashbrown::HashMap;
    use std::collections::hash_map::RandomState;

    // You can specify all types of HashMap, including hasher and allocator.
    // Created map is empty and don't allocate memory
    let map: HashMap<u32, String> = Default::default();
    assert_eq!(map.capacity(), 0);
    let map: HashMap<u32, String, RandomState> = HashMap::default();
    assert_eq!(map.capacity(), 0);

source

### impl<'a, K, V, S, A> Extend<&'a (K, V)> for HashMap<K, V, S, A> where
K: Eq + Hash + Copy,
V: Copy,
S: BuildHasher,
A: Allocator + Clone,

Inserts all new key-values from the iterator and replaces values with existing keys with new values returned from the iterator.

source

#### fn extend<T: IntoIterator<Item = &'a (K, V)>>(&mut self, iter: T)

Inserts all new key-values from the iterator to existing `HashMap<K, V, S, A>`. Replace values with existing keys with new values returned from the iterator. The keys and values must implement `Copy` trait.

##### Examples

    use hashbrown::hash_map::HashMap;

    let mut map = HashMap::new();
    map.insert(1, 100);

    let arr = [(1, 1), (2, 2)];
    let some_iter = arr.iter();
    map.extend(some_iter);
    // Replace values with existing keys with new values returned from the iterator.
    // So that the map.get(&1) doesn't return Some(&100).
    assert_eq!(map.get(&1), Some(&1));

    let some_vec: Vec<_> = vec![(3, 3), (4, 4)];
    map.extend(&some_vec);

    let some_arr = [(5, 5), (6, 6)];
    map.extend(&some_arr);

    let mut vec: Vec<_> = map.into_iter().collect();
    // The `IntoIter` iterator produces items in arbitrary order, so the
    // items must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, [(1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6)]);

source

#### fn extend_one(&mut self, item: A)

🔬 This is a nightly-only experimental API. (`extend_one`)

Extends a collection with exactly one element.

source

#### fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (`extend_one`)

Reserves capacity in a collection for the given number of additional elements. Read more

source

### impl<'a, K, V, S, A> Extend<(&'a K, &'a V)> for HashMap<K, V, S, A> where
K: Eq + Hash + Copy,
V: Copy,
S: BuildHasher,
A: Allocator + Clone,

Inserts all new key-values from the iterator and replaces values with existing keys with new values returned from the iterator.

source

#### fn extend<T: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: T)

Inserts all new key-values from the iterator to existing `HashMap<K, V, S, A>`. Replace values with existing keys with new values returned from the iterator. The keys and values must implement `Copy` trait.

##### Examples

    use hashbrown::hash_map::HashMap;

    let mut map = HashMap::new();
    map.insert(1, 100);

    let arr = [(1, 1), (2, 2)];
    let some_iter = arr.iter().map(|&(k, v)| (k, v));
    map.extend(some_iter);
    // Replace values with existing keys with new values returned from the iterator.
    // So that the map.get(&1) doesn't return Some(&100).
    assert_eq!(map.get(&1), Some(&1));

    let some_vec: Vec<_> = vec![(3, 3), (4, 4)];
    map.extend(some_vec.iter().map(|&(k, v)| (k, v)));

    let some_arr = [(5, 5), (6, 6)];
    map.extend(some_arr.iter().map(|&(k, v)| (k, v)));

    // You can also extend from another HashMap
    let mut new_map = HashMap::new();
    new_map.extend(&map);
    assert_eq!(new_map, map);

    let mut vec: Vec<_> = new_map.into_iter().collect();
    // The `IntoIter` iterator produces items in arbitrary order, so the
    // items must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, [(1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6)]);

source

#### fn extend_one(&mut self, item: A)

🔬 This is a nightly-only experimental API. (`extend_one`)

Extends a collection with exactly one element.

source

#### fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (`extend_one`)

Reserves capacity in a collection for the given number of additional elements. Read more

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### impl<K, V, S, A> Extend<(K, V)> for HashMap<K, V, S, A> where
K: Eq + Hash,
S: BuildHasher,
A: Allocator + Clone,

Inserts all new key-values from the iterator and replaces values with existing keys with new values returned from the iterator.

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#### fn extend<T: IntoIterator<Item = (K, V)>>(&mut self, iter: T)

Inserts all new key-values from the iterator to existing `HashMap<K, V, S, A>`. Replace values with existing keys with new values returned from the iterator.

##### Examples

    use hashbrown::hash_map::HashMap;

    let mut map = HashMap::new();
    map.insert(1, 100);

    let some_iter = [(1, 1), (2, 2)].into_iter();
    map.extend(some_iter);
    // Replace values with existing keys with new values returned from the iterator.
    // So that the map.get(&1) doesn't return Some(&100).
    assert_eq!(map.get(&1), Some(&1));

    let some_vec: Vec<_> = vec![(3, 3), (4, 4)];
    map.extend(some_vec);

    let some_arr = [(5, 5), (6, 6)];
    map.extend(some_arr);
    let old_map_len = map.len();

    // You can also extend from another HashMap
    let mut new_map = HashMap::new();
    new_map.extend(map);
    assert_eq!(new_map.len(), old_map_len);

    let mut vec: Vec<_> = new_map.into_iter().collect();
    // The `IntoIter` iterator produces items in arbitrary order, so the
    // items must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, [(1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6)]);

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#### fn extend_one(&mut self, item: A)

🔬 This is a nightly-only experimental API. (`extend_one`)

Extends a collection with exactly one element.

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#### fn extend_reserve(&mut self, additional: usize)

🔬 This is a nightly-only experimental API. (`extend_one`)

Reserves capacity in a collection for the given number of additional elements. Read more

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### impl<K, V, A, const N: usize> From<[(K, V); N]> for HashMap<K, V, DefaultHashBuilder, A> where
K: Eq + Hash,
A: Default + Allocator + Clone,

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#### fn from(arr: [(K, V); N]) -> Self

##### Examples

    use hashbrown::HashMap;

    let map1 = HashMap::from([(1, 2), (3, 4)]);
    let map2: HashMap<_, _> = [(1, 2), (3, 4)].into();
    assert_eq!(map1, map2);

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### impl<T, S, A> From<HashMap<T, (), S, A>> for HashSet<T, S, A> where
A: Allocator + Clone,

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#### fn from(map: HashMap<T, (), S, A>) -> Self

Converts to this type from the input type.

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### impl<K, V, S, A> FromIterator<(K, V)> for HashMap<K, V, S, A> where
K: Eq + Hash,
S: BuildHasher + Default,
A: Default + Allocator + Clone,

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#### fn from_iter<T: IntoIterator<Item = (K, V)>>(iter: T) -> Self

Creates a value from an iterator. Read more

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### impl<K, Q: ?Sized, V, S, A> Index<&Q> for HashMap<K, V, S, A> where
K: Eq + Hash + Borrow<Q>,
Q: Eq + Hash,
S: BuildHasher,
A: Allocator + Clone,

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#### fn index(&self, key: &Q) -> &V

Returns a reference to the value corresponding to the supplied key.

##### Panics

Panics if the key is not present in the `HashMap`.

##### Examples

    use hashbrown::HashMap;

    let map: HashMap<_, _> = [("a", "One"), ("b", "Two")].into();

    assert_eq!(map[&"a"], "One");
    assert_eq!(map[&"b"], "Two");

#### type Output = V

The returned type after indexing.

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### impl<'a, K, V, S, A: Allocator + Clone> IntoIterator for &'a HashMap<K, V, S, A>

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#### fn into_iter(self) -> Iter<'a, K, V>ⓘNotable traits for Iter<'a, K, V>`impl<'a, K, V> Iterator for Iter<'a, K, V> type Item = (&'a K, &'a V);`

Creates an iterator over the entries of a `HashMap` in arbitrary order. The iterator element type is `(&'a K, &'a V)`.

Return the same `Iter` struct as by the `iter` method on `HashMap`.

##### Examples

    use hashbrown::HashMap;
    let map_one: HashMap<_, _> = [(1, "a"), (2, "b"), (3, "c")].into();
    let mut map_two = HashMap::new();

    for (key, value) in &map_one {
        println!("Key: {}, Value: {}", key, value);
        map_two.insert_unique_unchecked(*key, *value);
    }

    assert_eq!(map_one, map_two);

#### type Item = (&'a K, &'a V)

The type of the elements being iterated over.

#### type IntoIter = Iter<'a, K, V>

Which kind of iterator are we turning this into?

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### impl<'a, K, V, S, A: Allocator + Clone> IntoIterator for &'a mut HashMap<K, V, S, A>

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#### fn into_iter(self) -> IterMut<'a, K, V>ⓘNotable traits for IterMut<'a, K, V>`impl<'a, K, V> Iterator for IterMut<'a, K, V> type Item = (&'a K, &'a mut V);`

Creates an iterator over the entries of a `HashMap` in arbitrary order with mutable references to the values. The iterator element type is `(&'a K, &'a mut V)`.

Return the same `IterMut` struct as by the `iter_mut` method on `HashMap`.

##### Examples

    use hashbrown::HashMap;
    let mut map: HashMap<_, _> = [("a", 1), ("b", 2), ("c", 3)].into();

    for (key, value) in &mut map {
        println!("Key: {}, Value: {}", key, value);
        *value *= 2;
    }

    let mut vec = map.iter().collect::<Vec<_>>();
    // The `Iter` iterator produces items in arbitrary order, so the
    // items must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, [(&"a", &2), (&"b", &4), (&"c", &6)]);

#### type Item = (&'a K, &'a mut V)

The type of the elements being iterated over.

#### type IntoIter = IterMut<'a, K, V>

Which kind of iterator are we turning this into?

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### impl<K, V, S, A: Allocator + Clone> IntoIterator for HashMap<K, V, S, A>

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#### fn into_iter(self) -> IntoIter<K, V, A>ⓘNotable traits for IntoIter<K, V, A>`impl<K, V, A: Allocator + Clone> Iterator for IntoIter<K, V, A> type Item = (K, V);`

Creates a consuming iterator, that is, one that moves each key-value pair out of the map in arbitrary order. The map cannot be used after calling this.

##### Examples

    use hashbrown::HashMap;

    let map: HashMap<_, _> = [("a", 1), ("b", 2), ("c", 3)].into();

    // Not possible with .iter()
    let mut vec: Vec<(&str, i32)> = map.into_iter().collect();
    // The `IntoIter` iterator produces items in arbitrary order, so
    // the items must be sorted to test them against a sorted array.
    vec.sort_unstable();
    assert_eq!(vec, [("a", 1), ("b", 2), ("c", 3)]);

#### type Item = (K, V)

The type of the elements being iterated over.

#### type IntoIter = IntoIter<K, V, A>

Which kind of iterator are we turning this into?

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### impl<K, V, S, A> PartialEq<HashMap<K, V, S, A>> for HashMap<K, V, S, A> where
K: Eq + Hash,
V: PartialEq,
S: BuildHasher,
A: Allocator + Clone,

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#### fn eq(&self, other: &Self) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more

1.0.0 · source

#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`.

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### impl<K, V, S, A> Eq for HashMap<K, V, S, A> where
K: Eq + Hash,
V: Eq,
S: BuildHasher,
A: Allocator + Clone,

## Auto Trait Implementations

### impl<K, V, S, A> RefUnwindSafe for HashMap<K, V, S, A> where
A: RefUnwindSafe,
K: RefUnwindSafe,
S: RefUnwindSafe,
V: RefUnwindSafe,

### impl<K, V, S, A> Send for HashMap<K, V, S, A> where
A: Send,
K: Send,
S: Send,
V: Send,

### impl<K, V, S, A> Sync for HashMap<K, V, S, A> where
A: Sync,
K: Sync,
S: Sync,
V: Sync,

### impl<K, V, S, A> Unpin for HashMap<K, V, S, A> where
A: Unpin,
K: Unpin,
S: Unpin,
V: Unpin,

### impl<K, V, S, A> UnwindSafe for HashMap<K, V, S, A> where
A: UnwindSafe,
K: UnwindSafe,
S: UnwindSafe,
V: UnwindSafe,

## Blanket Implementations

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### impl<T> Any for T where
T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more

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### impl<T> Borrow<T> for T where
T: ?Sized,

const: unstable · source

#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more

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### impl<T> BorrowMut<T> for T where
T: ?Sized,

const: unstable · source

#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more

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### impl<T> From<T> for T

const: unstable · source

#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T where
U: From<T>,

const: unstable · source

#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> ToOwned for T where
T: Clone,

#### type Owned = T

The resulting type after obtaining ownership.

source

#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more

source

#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more

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### impl<T, U> TryFrom<U> for T where
U: Into<T>,

#### type Error = Infallible

The type returned in the event of a conversion error.

const: unstable · source

#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.

source

### impl<T, U> TryInto<U> for T where
U: TryFrom<T>,

#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.

const: unstable · source

#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.