# Struct linked_hash_map::LinkedHashMap

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    pub struct LinkedHashMap<K, V, S = RandomState> { /* private fields */ }

Expand description

A linked hash map.

## Implementations

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### impl<K: Hash + Eq, V> LinkedHashMap<K, V>

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

Creates a linked hash map.

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

Creates an empty linked hash map with the given initial capacity.

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### impl<K: Hash + Eq, V, S: BuildHasher> LinkedHashMap<K, V, S>

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

Creates an empty linked hash map with the given initial hash builder.

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

Creates an empty linked hash map with the given initial capacity and hash builder.

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

Reserves capacity for at least `additional` more elements to be inserted into the map. The map may reserve more space to avoid frequent allocations.

##### Panics

Panics if the new allocation size overflows `usize.`

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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.

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

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

##### Examples

    use linked_hash_map::LinkedHashMap;

    let mut letters = LinkedHashMap::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);

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#### pub fn entries(&mut self) -> Entries<'_, K, V, S>ⓘNotable traits for Entries<'a, K, V, S>`impl<'a, K, V, S: BuildHasher> Iterator for Entries<'a, K, V, S> type Item = OccupiedEntry<'a, K, V, S>;`

Returns an iterator visiting all entries in insertion order. Iterator element type is `OccupiedEntry<K, V, S>`. Allows for removal as well as replacing the entry.

##### Examples

    use linked_hash_map::LinkedHashMap;

    let mut map = LinkedHashMap::new();
    map.insert("a", 10);
    map.insert("c", 30);
    map.insert("b", 20);

    {
        let mut iter = map.entries();
        let mut entry = iter.next().unwrap();
        assert_eq!(&"a", entry.key());
        *entry.get_mut() = 17;
    }

    assert_eq!(&17, map.get(&"a").unwrap());

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

Inserts a key-value pair into the map. If the key already existed, the old value is returned.

##### Examples

    use linked_hash_map::LinkedHashMap;
    let mut map = LinkedHashMap::new();

    map.insert(1, "a");
    map.insert(2, "b");
    assert_eq!(map[&1], "a");
    assert_eq!(map[&2], "b");

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

Checks if the map contains the given key.

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

Returns the value corresponding to the key in the map.

##### Examples

    use linked_hash_map::LinkedHashMap;
    let mut map = LinkedHashMap::new();

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

    assert_eq!(map.get(&1), Some(&"a"));
    assert_eq!(map.get(&2), Some(&"c"));

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

Returns the mutable reference corresponding to the key in the map.

##### Examples

    use linked_hash_map::LinkedHashMap;
    let mut map = LinkedHashMap::new();

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

    *map.get_mut(&1).unwrap() = "c";
    assert_eq!(map.get(&1), Some(&"c"));

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

Returns the value corresponding to the key in the map.

If value is found, it is moved to the end of the list. This operation can be used in implemenation of LRU cache.

##### Examples

    use linked_hash_map::LinkedHashMap;
    let mut map = LinkedHashMap::new();

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

    assert_eq!(map.get_refresh(&2), Some(&mut "b"));

    assert_eq!((&2, &"b"), map.iter().rev().next().unwrap());

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

Removes and returns the value corresponding to the key from the map.

##### Examples

    use linked_hash_map::LinkedHashMap;
    let mut map = LinkedHashMap::new();

    map.insert(2, "a");

    assert_eq!(map.remove(&1), None);
    assert_eq!(map.remove(&2), Some("a"));
    assert_eq!(map.remove(&2), None);
    assert_eq!(map.len(), 0);

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

Returns the maximum number of key-value pairs the map can hold without reallocating.

##### Examples

    use linked_hash_map::LinkedHashMap;
    let mut map: LinkedHashMap<i32, &str> = LinkedHashMap::new();
    let capacity = map.capacity();

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#### pub fn pop_front(&mut self) -> Option<(K, V)>

Removes the first entry.

Can be used in implementation of LRU cache.

##### Examples

    use linked_hash_map::LinkedHashMap;
    let mut map = LinkedHashMap::new();
    map.insert(1, 10);
    map.insert(2, 20);
    map.pop_front();
    assert_eq!(map.get(&1), None);
    assert_eq!(map.get(&2), Some(&20));

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#### pub fn front(&self) -> Option<(&K, &V)>

Gets the first entry.

##### Examples

    use linked_hash_map::LinkedHashMap;
    let mut map = LinkedHashMap::new();
    map.insert(1, 10);
    map.insert(2, 20);
    assert_eq!(map.front(), Some((&1, &10)));

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#### pub fn pop_back(&mut self) -> Option<(K, V)>

Removes the last entry.

##### Examples

    use linked_hash_map::LinkedHashMap;
    let mut map = LinkedHashMap::new();
    map.insert(1, 10);
    map.insert(2, 20);
    map.pop_back();
    assert_eq!(map.get(&1), Some(&10));
    assert_eq!(map.get(&2), None);

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#### pub fn back(&self) -> Option<(&K, &V)>

Gets the last entry.

##### Examples

    use linked_hash_map::LinkedHashMap;
    let mut map = LinkedHashMap::new();
    map.insert(1, 10);
    map.insert(2, 20);
    assert_eq!(map.back(), Some((&2, &20)));

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

Returns the number of key-value pairs in the map.

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

Returns whether the map is currently empty.

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

Returns a reference to the map’s hasher.

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

Clears the map of all key-value pairs.

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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);`

Returns a double-ended iterator visiting all key-value pairs in order of insertion. Iterator element type is `(&'a K, &'a V)`

##### Examples

    use linked_hash_map::LinkedHashMap;

    let mut map = LinkedHashMap::new();
    map.insert("a", 10);
    map.insert("c", 30);
    map.insert("b", 20);

    let mut iter = map.iter();
    assert_eq!((&"a", &10), iter.next().unwrap());
    assert_eq!((&"c", &30), iter.next().unwrap());
    assert_eq!((&"b", &20), iter.next().unwrap());
    assert_eq!(None, iter.next());

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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);`

Returns a double-ended iterator visiting all key-value pairs in order of insertion. Iterator element type is `(&'a K, &'a mut V)`

##### Examples

    use linked_hash_map::LinkedHashMap;

    let mut map = LinkedHashMap::new();
    map.insert("a", 10);
    map.insert("c", 30);
    map.insert("b", 20);

    {
        let mut iter = map.iter_mut();
        let mut entry = iter.next().unwrap();
        assert_eq!(&"a", entry.0);
        *entry.1 = 17;
    }

    assert_eq!(&17, map.get(&"a").unwrap());

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#### pub fn drain(&mut self) -> Drain<'_, K, V>ⓘNotable traits for Drain<'a, K, V>`impl<'a, K, V> Iterator for Drain<'a, K, V> 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.

Current performance implications (why to use this over into_iter()):

- Clears the inner HashMap instead of dropping it
- Puts all drained nodes in the free-list instead of deallocating them
- Avoids deallocating the sentinel node

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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;`

Returns a double-ended iterator visiting all key in order of insertion.

##### Examples

    use linked_hash_map::LinkedHashMap;

    let mut map = LinkedHashMap::new();
    map.insert('a', 10);
    map.insert('c', 30);
    map.insert('b', 20);

    let mut keys = map.keys();
    assert_eq!(&'a', keys.next().unwrap());
    assert_eq!(&'c', keys.next().unwrap());
    assert_eq!(&'b', keys.next().unwrap());
    assert_eq!(None, keys.next());

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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;`

Returns a double-ended iterator visiting all values in order of insertion.

##### Examples

    use linked_hash_map::LinkedHashMap;

    let mut map = LinkedHashMap::new();
    map.insert('a', 10);
    map.insert('c', 30);
    map.insert('b', 20);

    let mut values = map.values();
    assert_eq!(&10, values.next().unwrap());
    assert_eq!(&30, values.next().unwrap());
    assert_eq!(&20, values.next().unwrap());
    assert_eq!(None, values.next());

## Trait Implementations

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### impl<K: Hash + Eq + Clone, V: Clone, S: BuildHasher + Clone> Clone for LinkedHashMap<K, V, S>

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

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

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### impl<A: Debug + Hash + Eq, B: Debug, S: BuildHasher> Debug for LinkedHashMap<A, B, S>

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

Returns a string that lists the key-value pairs in insertion order.

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### impl<K: Hash + Eq, V, S: BuildHasher + Default> Default for LinkedHashMap<K, V, S>

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#### fn default() -> Self

Returns the “default value” for a type. Read more

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### impl<K, V, S> Drop for LinkedHashMap<K, V, S>

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#### fn drop(&mut self)

Executes the destructor for this type. Read more

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

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

Extends a collection with the contents of an iterator. Read more

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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: Hash + Eq, V, S: BuildHasher> Extend<(K, V)> for LinkedHashMap<K, V, S>

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

Extends a collection with the contents of an iterator. Read more

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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: Hash + Eq, V, S: BuildHasher + Default> FromIterator<(K, V)> for LinkedHashMap<K, V, S>

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

Creates a value from an iterator. Read more

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### impl<K: Hash + Eq, V: Hash, S: BuildHasher> Hash for LinkedHashMap<K, V, S>

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#### fn hash<H: Hasher>(&self, h: &mut H)

Feeds this value into the given `Hasher`. Read more

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

Feeds a slice of this type into the given `Hasher`. Read more

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### impl<'a, K, V, S, Q: ?Sized> Index<&'a Q> for LinkedHashMap<K, V, S> where
K: Hash + Eq + Borrow<Q>,
S: BuildHasher,
Q: Eq + Hash,

#### type Output = V

The returned type after indexing.

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

Performs the indexing (`container[index]`) operation. Read more

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### impl<'a, K, V, S, Q: ?Sized> IndexMut<&'a Q> for LinkedHashMap<K, V, S> where
K: Hash + Eq + Borrow<Q>,
S: BuildHasher,
Q: Eq + Hash,

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#### fn index_mut(&mut self, index: &'a Q) -> &mut V

Performs the mutable indexing (`container[index]`) operation. Read more

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### impl<'a, K: Hash + Eq, V, S: BuildHasher> IntoIterator for &'a LinkedHashMap<K, V, S>

#### 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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#### 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 from a value. Read more

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### impl<'a, K: Hash + Eq, V, S: BuildHasher> IntoIterator for &'a mut LinkedHashMap<K, V, S>

#### 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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#### 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 from a value. Read more

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### impl<K: Hash + Eq, V, S: BuildHasher> IntoIterator for LinkedHashMap<K, V, S>

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

The type of the elements being iterated over.

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

Which kind of iterator are we turning this into?

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

Creates an iterator from a value. Read more

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### impl<K: Hash + Eq + Ord, V: Ord, S: BuildHasher> Ord for LinkedHashMap<K, V, S>

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#### fn cmp(&self, other: &Self) -> Ordering

This method returns an `Ordering` between `self` and `other`. Read more

1.21.0 · source

#### fn max(self, other: Self) -> Self

Compares and returns the maximum of two values. Read more

1.21.0 · source

#### fn min(self, other: Self) -> Self

Compares and returns the minimum of two values. Read more

1.50.0 · source

#### fn clamp(self, min: Self, max: Self) -> Self where
Self: PartialOrd<Self>,

Restrict a value to a certain interval. Read more

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### impl<K: Hash + Eq, V: PartialEq, S: BuildHasher> PartialEq<LinkedHashMap<K, V, S>> for LinkedHashMap<K, V, S>

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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: Hash + Eq + PartialOrd, V: PartialOrd, S: BuildHasher> PartialOrd<LinkedHashMap<K, V, S>> for LinkedHashMap<K, V, S>

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#### fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between `self` and `other` values if one exists. Read more

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

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more

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

This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more

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

This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more

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

This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more

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### impl<K: Hash + Eq, V: Eq, S: BuildHasher> Eq for LinkedHashMap<K, V, S>

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### impl<K: Send, V: Send, S: Send> Send for LinkedHashMap<K, V, S>

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### impl<K: Sync, V: Sync, S: Sync> Sync for LinkedHashMap<K, V, S>

## Auto Trait Implementations

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

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

### impl<K, V, S> UnwindSafe for LinkedHashMap<K, V, S> where
K: RefUnwindSafe,
S: UnwindSafe,
V: RefUnwindSafe,

## 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.

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

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

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#### 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.

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### 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.