Unordered associative containers provide an ability for fast retrieval of data based on keys. The worst-case complexity for most operations is linear, but the average case is much faster. The library provides four unordered associative containers: unordered_set, unordered_map, unordered_multiset, and unordered_multimap.

Unordered associative containers conform to the requirements for Containers, except that the expressions a == b and a != b have different semantics than for the other container types.

Each unordered associative container is parameterized by Key, by a function object type Hash that meets the Hash requirements and acts as a hash function for argument values of type Key, and by a binary predicate Pred that induces an equivalence relation on values of type Key. Additionally, unordered_map and unordered_multimap associate an arbitrary mapped type T with the Key.

The container's object of type Hash — denoted by
hash — is called the *hash function* of the
container. The container's object of type Pred —
denoted by pred — is called the
*key equality predicate* of the container.

Two values k1 and k2 of type Key are considered equivalent if the container's key equality predicate returns true when passed those values. If k1 and k2 are equivalent, the container's hash function shall return the same value for both. [ Note: Thus, when an unordered associative container is instantiated with a non-default Pred parameter it usually needs a non-default Hash parameter as well. — end note ] For any two keys k1 and k2 in the same container, calling pred(k1, k2) shall always return the same value. For any key k in a container, calling hash(k) shall always return the same value.

An unordered associative container supports unique keys if it
may contain at most one element for each key. Otherwise, it supports
equivalent keys. unordered_set and unordered_map
support unique keys. unordered_multiset and unordered_multimap
support equivalent keys. In containers that support equivalent keys,
elements with equivalent keys are adjacent to each other
in the iteration order of the container. Thus, although the absolute order
of elements in an unordered container is not specified, its elements are
grouped into *equivalent-key groups* such that all elements of each
group have equivalent keys. Mutating operations on unordered containers shall
preserve the relative order of elements within each equivalent-key group
unless otherwise specified.

For unordered_set and unordered_multiset the value type is the same as the key type. For unordered_map and unordered_multimap it is pair<const Key, T>.

For unordered containers where the value type is the same as the key type, both iterator and const_iterator are constant iterators. It is unspecified whether or not iterator and const_iterator are the same type. [ Note: iterator and const_iterator have identical semantics in this case, and iterator is convertible to const_iterator. Users can avoid violating the one-definition rule by always using const_iterator in their function parameter lists. — end note ]

The elements of an unordered associative container are organized into buckets. Keys with the same hash code appear in the same bucket. The number of buckets is automatically increased as elements are added to an unordered associative container, so that the average number of elements per bucket is kept below a bound. Rehashing invalidates iterators, changes ordering between elements, and changes which buckets elements appear in, but does not invalidate pointers or references to elements. For unordered_multiset and unordered_multimap, rehashing preserves the relative ordering of equivalent elements.

The unordered associative containers meet all the requirements of Allocator-aware containers, except that for unordered_map and unordered_multimap, the requirements placed on value_type in Table 83 apply instead to key_type and mapped_type. [ Note: For example, key_type and mapped_type are sometimes required to be CopyAssignable even though the associated value_type, pair<const key_type, mapped_type>, is not CopyAssignable. — end note ]

In Table 91: X denotes an unordered associative container class, a denotes a value of type X, a2 denotes a value of a type with nodes compatible with type X (Table 89), b denotes a possibly const value of type X, a_uniq denotes a value of type X when X supports unique keys, a_eq denotes a value of type X when X supports equivalent keys, i and j denote input iterators that refer to value_type, [i, j) denotes a valid range, p and q2 denote valid constant iterators to a, q and q1 denote valid dereferenceable constant iterators to a, r denotes a valid dereferenceable iterator to a, [q1, q2) denotes a valid range in a, il denotes a value of type initializer_list<value_type>, t denotes a value of type X::value_type, k denotes a value of type key_type, hf denotes a possibly const value of type hasher, eq denotes a possibly const value of type key_equal, n denotes a value of type size_type, z denotes a value of type float, and nh denotes a non-const rvalue of type X::node_type.

Table 91 — Unordered associative container requirements (in addition to container)

Expression | Return type | Assertion/note | Complexity |

pre-/post-condition | |||

X::key_type | Key | compile time | |

X::mapped_type (unordered_map and unordered_multimap only) | T | compile time | |

X::value_type (unordered_set and unordered_multiset only) | Key | Requires: value_type is Erasable from X | compile time |

X::value_type (unordered_map and unordered_multimap only) | pair<const Key, T> | Requires: value_type is Erasable from X | compile time |

X::hasher | Hash | Hash shall be a unary function object type such that the expression hf(k) has type size_t. | compile time |

X::key_equal | Pred | Requires: Pred is CopyConstructible. Pred shall be a binary predicate that takes two arguments of type Key. Pred is an equivalence relation. | compile time |

X::local_iterator | An iterator type whose category, value type, difference type, and pointer and reference types are the same as X::iterator's. | A local_iterator object may be used to iterate through a single bucket, but may not be used to iterate across buckets. | compile time |

X::const_local_iterator | An iterator type whose category, value type, difference type, and pointer and reference types are the same as X::const_iterator's. | A const_local_iterator object may be used to iterate through a single bucket, but may not be used to iterate across buckets. | compile time |

X::node_type | a specialization of a node_handle class template, such that the public nested types are the same types as the corresponding types in X. | see [container.node] | compile time |

X(n, hf, eq) X a(n, hf, eq); | X | Effects: Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate. | O(n) |

X(n, hf) X a(n, hf); | X | Requires: key_equal is DefaultConstructible. Effects: Constructs an empty container with at least n buckets, using hf as the hash function and key_equal() as the key equality predicate. | O(n) |

X(n) X a(n); | X | Requires: hasher and key_equal are DefaultConstructible. Effects: Constructs an empty container with at least n buckets, using hasher() as the hash function and key_equal() as the key equality predicate. | O(n) |

X() X a; | X | Requires: hasher and key_equal are DefaultConstructible. Effects: Constructs an empty container with an unspecified number of buckets, using hasher() as the hash function and key_equal() as the key equality predicate. | constant |

X(i, j, n, hf, eq) X a(i, j, n, hf, eq); | X | Requires: value_type is EmplaceConstructible into X from *i. Effects: Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate, and inserts elements from [i, j) into it. | Average case O(N) (N is distance(i, j)), worst case O(N2) |

X(i, j, n, hf) X a(i, j, n, hf); | X | Requires: key_equal is DefaultConstructible.
value_type is EmplaceConstructible into X from *i. Effects: Constructs an empty container with at least n buckets, using hf as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it. | Average case O(N) (N is distance(i, j)), worst case O(N2) |

X(i, j, n) X a(i, j, n); | X | Requires: hasher and key_equal are DefaultConstructible.
value_type is EmplaceConstructible into X from *i. Effects: Constructs an empty container with at least n buckets, using hasher() as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it. | Average case O(N) (N is distance(i, j)), worst case O(N2) |

X(i, j) X a(i, j); | X | Requires: hasher and key_equal are DefaultConstructible.
value_type is EmplaceConstructible into X from *i. Effects: Constructs an empty container with an unspecified number of buckets, using hasher() as the hash function and key_equal() as the key equality predicate, and inserts elements from [i, j) into it. | Average case O(N) (N is distance(i, j)), worst case O(N2) |

X(il) | X | Same as X(il.begin(), il.end()). | Same as X(il.begin(), il.end()). |

X(il, n) | X | Same as X(il.begin(), il.end(), n). | Same as X(il.begin(), il.end(), n). |

X(il, n, hf) | X | Same as X(il.begin(), il.end(), n, hf). | Same as X(il.begin(), il.end(), n, hf). |

X(il, n, hf, eq) | X | Same as X(il.begin(), il.end(), n, hf, eq). | Same as X(il.begin(), il.end(), n, hf, eq). |

X(b) X a(b); | X | Copy constructor. In addition to the requirements of Table 83, copies the hash function, predicate, and maximum load factor. | Average case linear in b.size(), worst case quadratic. |

a = b | X& | Copy assignment operator. In addition to the requirements of Table 83, copies the hash function, predicate, and maximum load factor. | Average case linear in b.size(), worst case quadratic. |

a = il | X& | Requires: value_type is
CopyInsertable into X
and CopyAssignable. Effects: Assigns the range [il.begin(), il.end()) into a. All existing elements of a are either assigned to or destroyed. | Same as a = X(il). |

b.hash_function() | hasher | Returns b's hash function. | constant |

b.key_eq() | key_equal | Returns b's key equality predicate. | constant |

a_uniq. emplace(args) | pair<iterator, bool> |
Requires: value_type shall be EmplaceConstructible into X from args. Effects: Inserts a value_type object t constructed with std::forward<Args>(args)... if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t. | Average case O(1), worst case O(a_uniq.size()). |

a_eq.emplace(args) | iterator |
Requires: value_type shall be EmplaceConstructible into X from args. Effects: Inserts a value_type object t constructed with std::forward<Args>(args)... and returns the iterator pointing to the newly inserted element. | Average case O(1), worst case O(a_eq.size()). |

a.emplace_hint(p, args) | iterator |
Requires: value_type shall be EmplaceConstructible into X from args. Effects: Equivalent to a.emplace( std::forward<Args>(args)...). Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. The const_iterator p is a hint pointing to where the search should start. Implementations are permitted to ignore the hint. | Average case O(1), worst case O(a.size()). |

a_uniq.insert(t) | pair<iterator, bool> | Requires: If t is a non-const rvalue expression, value_type shall be
MoveInsertable into X; otherwise, value_type shall be
CopyInsertable into X. Effects: Inserts t if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key of t. | Average case O(1), worst case O(a_uniq.size()). |

a_eq.insert(t) | iterator | Requires: If t is a non-const rvalue expression, value_type shall be
MoveInsertable into X; otherwise, value_type shall be
CopyInsertable into X. Effects: Inserts t, and returns an iterator pointing to the newly inserted element. | Average case O(1), worst case O(a_eq.size()). |

a.insert(p, t) | iterator | Requires: If t is a non-const rvalue expression, value_type shall be
MoveInsertable into X; otherwise, value_type shall be
CopyInsertable into X. Effects: Equivalent to a.insert(t). Return value is an iterator pointing to the element with the key equivalent to that of t. The iterator p is a hint pointing to where the search should start. Implementations are permitted to ignore the hint. | Average case O(1), worst case O(a.size()). |

a.insert(i, j) | void | Requires: value_type shall be EmplaceConstructible into X from *i. Requires: i and j are not iterators in a. Equivalent to a.insert(t) for each element in [i,j). | Average case O(N), where N is distance(i, j). Worst case O(N(a.size()+1)). |

a.insert(il) | void | Same as a.insert(il.begin(), il.end()). | Same as a.insert( il.begin(), il.end()). |

a_uniq. insert(nh) | insert_return_type |
Requires: nh is empty or
a_uniq.get_allocator() == nh.get_allocator(). Effects: If nh is empty, has no effect. Otherwise, inserts the element owned by nh if and only if there is no element in the container with a key equivalent to nh.key(). Postconditions: If nh is empty, inserted is false, position is end(), and node is empty. Otherwise if the insertion took place, inserted is true, position points to the inserted element, and node is empty; if the insertion failed, inserted is false, node has the previous value of nh, and position points to an element with a key equivalent to nh.key(). | Average case O(1), worst case O(a_uniq.size()). |

a_eq. insert(nh) | iterator |
Requires: nh is empty or
a_eq.get_allocator() == nh.get_allocator(). Effects: If nh is empty, has no effect and returns a_eq.end(). Otherwise, inserts the element owned by nh and returns an iterator pointing to the newly inserted element. Postconditions: nh is empty. | Average case O(1), worst case O(a_eq.size()). |

a.insert(q, nh) | iterator |
Requires: nh is empty or
a.get_allocator() == nh.get_allocator(). Effects: If nh is empty, has no effect and returns a.end(). Otherwise, inserts the element owned by nh if and only if there is no element with key equivalent to nh.key() in containers with unique keys; always inserts the element owned by nh in containers with equivalent keys. Always returns the iterator pointing to the element with key equivalent to nh.key(). The iterator q is a hint pointing to where the search should start. Implementations are permitted to ignore the hint. Postconditions: nh is empty if insertion succeeds, unchanged if insertion fails. | Average case O(1), worst case O(a.size()). |

a.extract(k) | node_type | Removes an element in the container with key equivalent to k. Returns a node_type owning the element if found, otherwise an empty node_type. | Average case O(1), worst case O(a.size()). |

a.extract(q) | node_type | Removes the element pointed to by q. Returns a node_type owning that element. | Average case O(1), worst case O(a.size()). |

a.merge(a2) | void |
Requires: a.get_allocator() == a2.get_allocator(). Attempts to extract each element in a2 and insert it into a using the hash function and key equality predicate of a. In containers with unique keys, if there is an element in a with key equivalent to the key of an element from a2, then that element is not extracted from a2. Postconditions: Pointers and references to the transferred elements of a2 refer to those same elements but as members of a. Iterators referring to the transferred elements and all iterators referring to a will be invalidated, but iterators to elements remaining in a2 will remain valid. Throws: Nothing unless the hash function or key equality predicate throws. | Average case O(N), where N is a2.size(). Worst case O(N*a.size()+N). |

a.erase(k) | size_type | Erases all elements with key equivalent to k. Returns the number of elements erased. | Average case O(a.count(k)). Worst case O(a.size()). |

a.erase(q) | iterator | Erases the element pointed to by q. Returns the iterator immediately following q prior to the erasure. | Average case O(1), worst case O(a.size()). |

a.erase(r) | iterator | Erases the element pointed to by r. Returns the iterator immediately following r prior to the erasure. | Average case O(1), worst case O(a.size()). |

a.erase(q1, q2) | iterator | Erases all elements in the range [q1, q2). Returns the iterator immediately following the erased elements prior to the erasure. | Average case linear in distance(q1, q2), worst case O(a.size()). |

a.clear() | void | Erases all elements in the container. Postconditions: a.empty() returns true | Linear in a.size(). |

b.find(k) | iterator; const_iterator for const b. | Returns an iterator pointing to an element with key equivalent to k, or b.end() if no such element exists. | Average case O(1), worst case O(b.size()). |

b.count(k) | size_type | Returns the number of elements with key equivalent to k. | Average case O(b.count(k)), worst case O(b.size()). |

b.equal_range(k) | pair<iterator, iterator>; pair<const_iterator, const_iterator> for const b. | Returns a range containing all elements with keys equivalent to k. Returns make_pair(b.end(), b.end()) if no such elements exist. | Average case O(b.count(k)). Worst case O(b.size()). |

b.bucket_count() | size_type | Returns the number of buckets that b contains. | Constant |

b.max_bucket_count() | size_type | Returns an upper bound on the number of buckets that b might ever contain. | Constant |

b.bucket(k) | size_type |
Requires: b.bucket_count() > 0. Returns the index of the bucket in which elements with keys equivalent to k would be found, if any such element existed. Postconditions: the return value shall be in the range [0, b.bucket_count()). | Constant |

b.bucket_size(n) | size_type | Requires: n shall be in the range [0, b.bucket_count()). Returns the number of elements in the n th bucket. | O(b.bucket_size(n)) |

b.begin(n) | local_iterator; const_local_iterator for const b. | Requires: n shall be in the range [0, b.bucket_count()). b.begin(n) returns an iterator referring to the first element in the bucket. If the bucket is empty, then b.begin(n) == b.end(n). | Constant |

b.end(n) | local_iterator; const_local_iterator for const b. | Requires: n shall be in the range [0, b.bucket_count()). b.end(n) returns an iterator which is the past-the-end value for the bucket. | Constant |

b.cbegin(n) | const_local_iterator | Requires: n shall be in the range [0, b.bucket_count()). Note: [b.cbegin(n), b.cend(n)) is a valid range containing all of the elements in the n th bucket. | Constant |

b.cend(n) | const_local_iterator | Requires: n shall be in the range [0, b.bucket_count()). | Constant |

b.load_factor() | float | Returns the average number of elements per bucket. | Constant |

b.max_load_factor() | float | Returns a positive number that the container attempts to keep the load factor less than or equal to. The container automatically increases the number of buckets as necessary to keep the load factor below this number. | Constant |

a.max_load_factor(z) | void | Requires: z shall be positive. May change the container's maximum load factor, using z as a hint. | Constant |

a.rehash(n) | void | Postconditions: a.bucket_count() >= a.size() / a.max_load_factor() and a.bucket_count() >= n. | Average case linear in a.size(), worst case quadratic. |

a.reserve(n) | void | Same as a.rehash(ceil(n / a.max_load_factor())). | Average case linear in a.size(), worst case quadratic. |

Two unordered containers a and b compare equal if a.size() == b.size() and, for every equivalent-key group [Ea1, Ea2) obtained from a.equal_range(Ea1), there exists an equivalent-key group [Eb1, Eb2) obtained from b.equal_range(Ea1), such that is_permutation(Ea1, Ea2, Eb1, Eb2) returns true. For unordered_set and unordered_map, the complexity of operator== (i.e., the number of calls to the == operator of the value_type, to the predicate returned by key_eq(), and to the hasher returned by hash_function()) is proportional to N in the average case and to N2 in the worst case, where N is a.size(). For unordered_multiset and unordered_multimap, the complexity of operator== is proportional to ∑E2i in the average case and to N2 in the worst case, where N is a.size(), and Ei is the size of the ith equivalent-key group in a. However, if the respective elements of each corresponding pair of equivalent-key groups Eai and Ebi are arranged in the same order (as is commonly the case, e.g., if a and b are unmodified copies of the same container), then the average-case complexity for unordered_multiset and unordered_multimap becomes proportional to N (but worst-case complexity remains O(N2), e.g., for a pathologically bad hash function). The behavior of a program that uses operator== or operator!= on unordered containers is undefined unless the Hash and Pred function objects respectively have the same behavior for both containers and the equality comparison function for Key is a refinement257 of the partition into equivalent-key groups produced by Pred.

The iterator types iterator and const_iterator of an unordered associative container are of at least the forward iterator category. For unordered associative containers where the key type and value type are the same, both iterator and const_iterator are constant iterators.

The insert and emplace members shall not affect the validity of references to container elements, but may invalidate all iterators to the container. The erase members shall invalidate only iterators and references to the erased elements, and preserve the relative order of the elements that are not erased.

The insert and emplace members shall not affect the validity of iterators if (N+n) <= z * B, where N is the number of elements in the container prior to the insert operation, n is the number of elements inserted, B is the container's bucket count, and z is the container's maximum load factor.

The extract members invalidate only iterators to the removed element, and preserve the relative order of the elements that are not erased; pointers and references to the removed element remain valid. However, accessing the element through such pointers and references while the element is owned by a node_type is undefined behavior. References and pointers to an element obtained while it is owned by a node_type are invalidated if the element is successfully inserted.

A deduction guide for an unordered associative container shall not participate in overload resolution if any of the following are true:

It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.

It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

Equality comparison is a refinement of partitioning if no two objects that compare equal fall into different partitions.

For unordered associative containers, no clear() function throws an exception. erase(k) does not throw an exception unless that exception is thrown by the container's Hash or Pred object (if any).

For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an insert or emplace function inserting a single element, the insertion has no effect.

For unordered associative containers, no swap function throws an exception unless that exception is thrown by the swap of the container's Hash or Pred object (if any).