23 Containers library [containers]

23.2 Requirements [container.requirements]

23.2.8 Unordered associative containers [unord.req]

23.2.8.1 General [unord.req.general]

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 ([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 70),
(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

Result: T.

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

Result: Hash.

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

🔗

typename X::key_equal

Result: Pred.

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)

Preconditions: key_equal meets the Cpp17DefaultConstructible requirements.

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})

🔗

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})

🔗

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})

🔗

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})

🔗

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})

🔗

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})

🔗

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})

🔗

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})

🔗

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

Result: X&

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

Result: X&

Preconditions: value_type is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

Effects: Assigns the range [il.begin(), il.end()) into a.

All existing elements of a are either assigned to or destroyed.

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

🔗

b.hash_function()

Result: hasher

Returns: b's hash function.

Complexity: Constant.

🔗

b.key_eq()

Result: key_equal

Returns: b's key equality predicate.

Complexity: Constant.

🔗

a_uniq.emplace(args)

Result: pair<iterator, bool>

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)

Result: iterator

Preconditions: value_type is Cpp17EmplaceConstructible into X from args.

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

Returns: An iterator pointing to the newly inserted element.

Complexity: Average case

O (1)

, worst case

O (a_eq.size())

🔗

a.emplace_hint(p, args)

Result: iterator

Preconditions: value_type is Cpp17EmplaceConstructible into X from args.

Effects: Equivalent to a.emplace(std::forward<Args>(args)...).

Returns: 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.

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a_uniq.insert(t)

Result: pair<iterator, bool>

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)

Result: iterator

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

100

Effects: Inserts t.

101

Returns: An iterator pointing to the newly inserted element.

102

Complexity: Average case

O (1)

, worst case

O (a_eq.size())

🔗

a.insert(p, t)

103

Result: iterator

104

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

105

Effects: Equivalent to a.insert(t).

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

Implementations are permitted to ignore the hint.

106

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

107

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a.insert(i, j)

108

Result: void

109

Preconditions: value_type is Cpp17EmplaceConstructible into X from *i.

Neither i nor j are iterators into a.

110

Effects: Equivalent to a.insert(t) for each element in [i,j).

111

Complexity: Average case

O (N)

, where N is distance(i, j), worst case

O (N (a.size() + 1))

🔗

a.insert_range(rg)

112

Result: void

113

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

rg and a do not overlap.

114

Effects: Equivalent to a.insert(t) for each element t in rg.

115

Complexity: Average case

O (N)

, where N is ranges::distance(rg), worst case

O (N (a.size() + 1))

🔗

a.insert(il)

116

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

🔗

a_uniq.insert(nh)

117

Result: insert_return_type

118

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

119

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().

120

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().

121

Complexity: Average case

O (1)

, worst case

O (a_uniq.size())

🔗

a_eq.insert(nh)

122

Result: iterator

123

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

124

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.

125

Postconditions: nh is empty.

126

Complexity: Average case

O (1)

, worst case

O (a_eq.size())

🔗

a.insert(q, nh)

127

Result: iterator

128

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

129

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.

130

Postconditions: nh is empty if insertion succeeds, unchanged if insertion fails.

131

Returns: An iterator pointing to the element with key equivalent to nh.key().

132

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a.extract(k)

133

Result: node_type

134

Effects: Removes an element in the container with key equivalent to k.

135

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

136

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a_tran.extract(kx)

137

Result: node_type

138

Effects: Removes an element in the container with key equivalent to kx.

139

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

140

Complexity: Average case

O (1)

, worst case

O (a_tran.size())

🔗

a.extract(q)

141

Result: node_type

142

Effects: Removes the element pointed to by q.

143

Returns: A node_type owning that element.

144

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a.merge(a2)

145

Result: void

146

Preconditions: a.get_allocator() == a2.get_allocator().

147

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.

148

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.

149

Complexity: Average case

O (N)

, where N is a2.size(), worst case

O (N *a.size() + N)

🔗

a.erase(k)

150

Result: size_type

151

Effects: Erases all elements with key equivalent to k.

152

Returns: The number of elements erased.

153

Complexity: Average case

O (a.count(k))

, worst case

O (a.size())

🔗

a_tran.erase(kx)

154

Result: size_type

155

Effects: Erases all elements with key equivalent to kx.

156

Returns: The number of elements erased.

157

Complexity: Average case

O (a_tran.count(kx))

, worst case

O (a_tran.size())

🔗

a.erase(q)

158

Result: iterator

159

Effects: Erases the element pointed to by q.

160

Returns: The iterator immediately following q prior to the erasure.

161

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a.erase(r)

162

Result: iterator

163

Effects: Erases the element pointed to by r.

164

Returns: The iterator immediately following r prior to the erasure.

165

Complexity: Average case

O (1)

, worst case

O (a.size())

🔗

a.erase(q1, q2)

166

Result: iterator

167

Effects: Erases all elements in the range [q1, q2).

168

Returns: The iterator immediately following the erased elements prior to the erasure.

169

Complexity: Average case linear in distance(q1, q2), worst case

O (a.size())

🔗

a.clear()

170

Result: void

171

Effects: Erases all elements in the container.

172

Postconditions: a.empty() is true.

173

Complexity: Linear in a.size().

🔗

b.find(k)

174

Result: iterator; const_iterator for constant b.

175

Returns: An iterator pointing to an element with key equivalent to k, or b.end() if no such element exists.

176

Complexity: Average case

O (1)

, worst case

O (b.size())

🔗

a_tran.find(ke)

177

Result: iterator; const_iterator for constant a_tran.

178

Returns: An iterator pointing to an element with key equivalent to ke, or a_tran.end() if no such element exists.

179

Complexity: Average case

O (1)

, worst case

O (a_tran.size())

🔗

b.count(k)

180

Result: size_type

181

Returns: The number of elements with key equivalent to k.

182

Complexity: Average case

O (b.count(k))

, worst case

O (b.size())

🔗

a_tran.count(ke)

183

Result: size_type

184

Returns: The number of elements with key equivalent to ke.

185

Complexity: Average case

O (a_tran.count(ke))

, worst case

O (a_tran.size())

🔗

b.contains(k)

186

Effects: Equivalent to b.find(k) != b.end().

🔗

a_tran.contains(ke)

187

Effects: Equivalent to a_tran.find(ke) != a_tran.end().

🔗

b.equal_range(k)

188

Result: pair<iterator, iterator>; pair<const_iterator, const_iterator> for constant b.

189

Returns: A range containing all elements with keys equivalent to k.

Returns make_pair(b.end(), b.end()) if no such elements exist.

190

Complexity: Average case

O (b.count(k))

, worst case

O (b.size())

🔗

a_tran.equal_range(ke)

191

Result: pair<iterator, iterator>; pair<const_iterator, const_iterator> for constant a_tran.

192

Returns: A range containing all elements with keys equivalent to ke.

Returns make_pair(a_tran.end(), a_tran.end()) if no such elements exist.

193

Complexity: Average case

O (a_tran.count(ke))

, worst case

O (a_tran.size())

🔗

b.bucket_count()

194

Result: size_type

195

Returns: The number of buckets that b contains.

196

Complexity: Constant.

🔗

b.max_bucket_count()

197

Result: size_type

198

Returns: An upper bound on the number of buckets that b can ever contain.

199

Complexity: Constant.

🔗

b.bucket(k)

200

Result: size_type

201

Preconditions: b.bucket_count() > 0.

202

Returns: The index of the bucket in which elements with keys equivalent to k would be found, if any such element existed.

The return value is in the range [0, b.bucket_count()).

203

Complexity: Constant.

🔗

a_tran.bucket(ke)

204

Result: size_type

205

Preconditions: a_tran.bucket_count() > 0.

206

Postconditions: The return value is in the range [0, a_tran.bucket_count()).

207

Returns: The index of the bucket in which elements with keys equivalent to ke would be found, if any such element existed.

208

Complexity: Constant.

🔗

b.bucket_size(n)

209

Result: size_type

210

Preconditions: n shall be in the range [0, b.bucket_count()).

211

Returns: The number of elements in the

n^{th}

bucket.

212

Complexity:

O (b.bucket_size(n))

🔗

b.begin(n)

213

Result: local_iterator; const_local_iterator for constant b.

214

Preconditions: n is in the range [0, b.bucket_count()).

215

Returns: An iterator referring to the first element in the bucket.

If the bucket is empty, then b.begin(n) == b.end(n).

216

Complexity: Constant.

🔗

b.end(n)

217

Result: local_iterator; const_local_iterator for constant b.

218

Preconditions: n is in the range [0, b.bucket_count()).

219

Returns: An iterator which is the past-the-end value for the bucket.

220

Complexity: Constant.

🔗

b.cbegin(n)

221

Result: const_local_iterator

222

Preconditions: n shall be in the range [0, b.bucket_count()).

223

Returns: An iterator referring to the first element in the bucket.

If the bucket is empty, then b.cbegin(n) == b.cend(n).

224

Complexity: Constant.

🔗

b.cend(n)

225

Result: const_local_iterator

226

Preconditions: n is in the range [0, b.bucket_count()).

227

Returns: An iterator which is the past-the-end value for the bucket.

228

Complexity: Constant.

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b.load_factor()

229

Result: float

230

Returns: The average number of elements per bucket.

231

Complexity: Constant.

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b.max_load_factor()

232

Result: float

233

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.

234

Complexity: Constant.

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a.max_load_factor(z)

235

Result: void

236

Preconditions: z is positive.

May change the container's maximum load factor, using z as a hint.

237

Complexity: Constant.

🔗

a.rehash(n)

238

Result: void

239

Postconditions: a.bucket_count() >= a.size() / a.max_load_factor() and a.bucket_count() >= n.

240

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

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a.reserve(n)

241

Effects: Equivalent to a.rehash(ceil(n / a.max_load_factor())).

242

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

N^{2}

in the worst case, where N is a.size().

For unordered_multiset and unordered_multimap, the complexity of operator== is proportional to

\sum E_{i}^{2}

in the average case and to

N^{2}

in the worst case, where N is a.size(), and

E_{i}

is the size of the

i^{th}

equivalent-key group in a.

However, if the respective elements of each corresponding pair of equivalent-key groups

E a_{i}

and

E b_{i}

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 (N^{2})

, 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 Pred function object has the same behavior for both containers and the equality comparison function for Key is a refinement199 of the partition into equivalent-key groups produced by Pred.

243

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.

244

The insert, insert_range, 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.

245

The insert, insert_range, 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.

246

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.

247

The member function templates find, count, equal_range, contains, extract, erase, and bucket shall not participate in overload resolution unless the qualified-ids Pred::is_transparent and Hash::is_transparent are both valid and denote types ([temp.deduct]).

Additionally, the member function templates extract and erase shall not participate in overload resolution if is_convertible_v<K&&, iterator> || is_convertible_v<K&&, const_iterator> is true, where K is the type substituted as the first template argument.

248

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

(248.1)
It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.
(248.2)
It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.
(248.3)
It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.
(248.4)
It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

199)199)

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