[unord.req.general]

Unordered associative containers conform to the requirements for Containers ([container.requirements]), 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 Cpp17Hash requirements ([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 are considered equivalent if the container's key equality predicate pred(k1, k2) is valid and 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 1:

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 2:

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.

In this subclause,

(10.1)
X denotes an unordered associative container class,
(10.2)
a denotes a value of type X,
(10.3)
a2 denotes a value of a type with nodes compatible with type X (Table 75),
(10.4)
b denotes a value of type X or const X,
(10.5)
a_uniq denotes a value of type X when X supports unique keys,
(10.6)
a_eq denotes a value of type X when X supports equivalent keys,
(10.7)
a_tran denotes a value of type X or const X when the qualified-ids X::key_equal::is_transparent and X::hasher::is_transparent are both valid and denote types ([temp.deduct]),
(10.8)
i and j denote input iterators that refer to value_type,
(10.9)
[i, j) denotes a valid range,
(10.10)
rg denotes a value of a type R that models container-compatible-range<value_type>,
(10.11)
p and q2 denote valid constant iterators to a,
(10.12)
q and q1 denote valid dereferenceable constant iterators to a,
(10.13)
r denotes a valid dereferenceable iterator to a,
(10.14)
[q1, q2) denotes a valid range in a,
(10.15)
il denotes a value of type initializer_list<value_type>,
(10.16)
t denotes a value of type X::value_type,
(10.17)
k denotes a value of type key_type,
(10.18)
hf denotes a value of type hasher or const hasher,
(10.19)
eq denotes a value of type key_equal or const key_equal,
(10.20)
ke is a value such that
- (10.20.1)
  eq(r1, ke) == eq(ke, r1),
- (10.20.2)
  hf(r1) == hf(ke) if eq(r1, ke) is true, and
- (10.20.3)
  if any two of eq(r1, ke), eq(r2, ke), and eq(r1, r2) are true, then all three are true,
where r1 and r2 are keys of elements in a_tran,
(10.21)
kx is a value such that
- (10.21.1)
  eq(r1, kx) == eq(kx, r1),
- (10.21.2)
  hf(r1) == hf(kx) if eq(r1, kx) is true,
- (10.21.3)
  if any two of eq(r1, kx), eq(r2, kx), and eq(r1, r2) are true, then all three are true, and
- (10.21.4)
  kx is not convertible to either iterator or const_iterator,
where r1 and r2 are keys of elements in a_tran,
(10.22)
n denotes a value of type size_type,
(10.23)
z denotes a value of type float, and
(10.24)
nh denotes an rvalue of type X::node_type.

A type X meets the unordered associative container requirements if X meets all the requirements of an allocator-aware container ([container.alloc.reqmts]) and the following types, statements, and expressions are well-formed and have the specified semantics, except that for unordered_map and unordered_multimap, the requirements placed on value_type in [container.reqmts] apply instead to key_type and mapped_type.

[Note 3:

For example, key_type and mapped_type sometimes need to be Cpp17CopyAssignable even though the associated value_type, pair<const key_type, mapped_type>, is not Cpp17CopyAssignable.

— end note]

🔗

typename X::key_type

Result: Key.

🔗

typename X::mapped_type

Remarks: For unordered_map and unordered_multimap only.

🔗

typename X::value_type

Result: Key for unordered_set and unordered_multiset only; pair<const Key, T> for unordered_map and unordered_multimap only.

Preconditions: value_type is Cpp17Erasable from X.

🔗

typename X::hasher

Preconditions: Hash is a unary function object type such that the expression hf(k) has type size_t.

🔗

typename X::key_equal

Preconditions: Pred meets the Cpp17CopyConstructible requirements.

Pred is a binary predicate that takes two arguments of type Key.

Pred is an equivalence relation.

🔗

typename X::local_iterator

Result: An iterator type whose category, value type, difference type, and pointer and reference types are the same as X::iterator's.

[Note 4:

A local_iterator object can be used to iterate through a single bucket, but cannot be used to iterate across buckets.

— end note]

🔗

typename X::const_local_iterator

Result: An iterator type whose category, value type, difference type, and pointer and reference types are the same as X::const_iterator's.

[Note 5:

A const_local_iterator object can be used to iterate through a single bucket, but cannot be used to iterate across buckets.

— end note]

🔗

typename X::node_type

Result: A specialization of a node-handle class template ([container.node]), such that the public nested types are the same types as the corresponding types in X.

🔗

X(n, hf, eq)

Effects: Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate.

Complexity:

O (n)

🔗

X(n, hf)

Effects: Constructs an empty container with at least n buckets, using hf as the hash function and key_equal() as the key equality predicate.

Complexity:

O (n)

🔗

X(n)

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

Effects: Constructs an empty container with at least n buckets, using hasher() as the hash function and key_equal() as the key equality predicate.

Complexity:

O (n)

🔗

X a = X();
X a;

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

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.

Complexity: Constant.

🔗

X(i, j, n, hf, eq)

Preconditions: value_type is Cpp17EmplaceConstructible 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.

Complexity: Average case

O (N) (N is distance (i, j)), worst case O ({N 2) .}^{2) .}

🔗

X(i, j, n, hf)

Preconditions: key_equal meets the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible 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.

Complexity: Average case

O (N) (N is distance (i, j)), worst case O ({N 2) .}^{2) .}

🔗

X(i, j, n)

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible 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.

Complexity: Average case

O (N) (N is distance (i, j)), worst case O ({N 2) .}^{2) .}

🔗

X(i, j)

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible 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.

Complexity: Average case

O (N) (N is distance (i, j)), worst case O ({N 2) .}^{2) .}

🔗

X(from_range, rg, n, hf, eq)

Preconditions: value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

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 rg into it.

Complexity: Average case

O (N) (N is ranges ​::​ distance (rg)), worst case O ({N 2) .}^{2) .}

🔗

X(from_range, rg, n, hf)

Preconditions: key_equal meets the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

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 rg into it.

Complexity: Average case

O (N) (N is ranges ​::​ distance (rg)), worst case O ({N 2) .}^{2) .}

🔗

X(from_range, rg, n)

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

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 rg into it.

Complexity: Average case

O (N) (N is ranges ​::​ distance (rg)), worst case O ({N 2) .}^{2) .}

🔗

X(from_range, rg)

Preconditions: hasher and key_equal meet the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

Complexity: Average case

O (N) (N is ranges ​::​ distance (rg)), worst case O ({N 2) .}^{2) .}

🔗

X(il)

Effects: Equivalent to X(il.begin(), il.end().

🔗

X(il, n)

Effects: Equivalent to X(il.begin(), il.end(), n).

🔗

X(il, n, hf)

Effects: Equivalent to X(il.begin(), il.end(), n, hf).

🔗

X(il, n, hf, eq)

Effects: Equivalent to X(il.begin(), il.end(), n, hf, eq).

🔗

X(b)

Effects: In addition to the container requirements ([container.reqmts]), copies the hash function, predicate, and maximum load factor.

Complexity: Average case linear in b.size(), worst case quadratic.

🔗

a = b

Effects: In addition to the container requirements, copies the hash function, predicate, and maximum load factor.

Complexity: Average case linear in b.size(), worst case quadratic.

🔗

a = il

Preconditions: value_type is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

Complexity: Average case linear in il.size(), worst case quadratic.

🔗

b.hash_function()

Complexity: Constant.

🔗

b.key_eq()

Complexity: Constant.

🔗

a_uniq.emplace(args)

Preconditions: value_type is Cpp17EmplaceConstructible 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.

Returns: 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.

Complexity: Average case

O (1), worst case O (a_uniq.size()) .

🔗

a_eq.emplace(args)

Preconditions: value_type is Cpp17EmplaceConstructible into X from args.

Effects: Inserts a value_type object t constructed with std::forward<Args>(args)..

Complexity: Average case

O (1), worst case O (a_eq.size()) .

🔗

a.emplace_hint(p, args)

Effects: Equivalent to a.emplace(std::forward<Args>(args).), except that the const_iterator p is a hint pointing to where the search should start.

Implementations are permitted to ignore the hint.

Returns: The iterator returned by emplace.

🔗

a_uniq.insert(t)

Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.

Effects: Inserts t if and only if there is no element in the container with key equivalent to the key of t.

Returns: 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.

Complexity: Average case

O (1), worst case O (a_uniq.size()) .

🔗

a_eq.insert(t)

Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.

Complexity: Average case

O (1), worst case O (a_eq.size()) .

🔗

a.insert(p, t)

101

Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.

103

Returns: An iterator pointing to the element with the key equivalent to that of t.

105

Complexity: Average case

O (1), worst case O (a.size()) .

🔗

a.insert(i, j)

106

Preconditions: value_type is Cpp17EmplaceConstructible into X from *i.

Neither i nor j are iterators into a.

108

Complexity: Average case

O (N), where N is distance (i, j), worst case O (N (a.size() + 1)) .

🔗

a.insert_range(rg)

110

Preconditions: value_type is Cpp17EmplaceConstructible into X from *ranges::begin(rg).

rg and a do not overlap.

112

Complexity: Average case

O (N), where N is ranges ​::​ distance (rg), worst case O (N (a.size() + 1)) .

🔗

a.insert(il)

114

Effects: Equivalent to a.insert(il.begin(), il.end().

🔗

a_uniq.insert(nh)

115

Preconditions: nh is empty or a_uniq.get_allocator() == nh.get_allocator() is true.

117

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().

118

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().

119

Complexity: Average case

O (1), worst case O (a_uniq.size()) .

🔗

a_eq.insert(nh)

120

Preconditions: nh is empty or a_eq.get_allocator() == nh.get_allocator() is true.

122

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.

123

Complexity: Average case

O (1), worst case O (a_eq.size()) .

🔗

a.insert(q, nh)

125

Preconditions: nh is empty or a.get_allocator() == nh.get_allocator() is true.

127

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.

The iterator q is a hint pointing to where the search should start.

Implementations are permitted to ignore the hint.

128

Complexity: Average case

O (1), worst case O (a.size()) .

🔗

a.extract(k)

131

Returns: A node_type owning the element if found, otherwise an empty node_type.

134

Complexity: Average case

O (1), worst case O (a.size()) .

🔗

a_tran.extract(kx)

135

Returns: A node_type owning the element if found, otherwise an empty node_type.

138

Complexity: Average case

O (1), worst case O (a_tran.size()) .

🔗

a.extract(q)

139

Complexity: Average case

O (1), worst case O (a.size()) .

🔗

a.merge(a2)

143

Effects: 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.

146

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.

147

Complexity: Average case