24 Containers library [containers]

24.2 Requirements [container.requirements]

24.2.7 Associative containers [associative.reqmts]

24.2.7.1 General [associative.reqmts.general]

Associative containers provide fast retrieval of data based on keys.

The library provides four basic kinds of associative containers: set, multiset, map and multimap.

The library also provides container adaptors that make it easy to construct abstract data types, such as flat_maps, flat_multimaps, flat_sets, or flat_multisets, out of the basic sequence container kinds (or out of other program-defined sequence containers).

Each associative container is parameterized on Key and an ordering relation Compare that induces a strict weak ordering ([alg.sorting]) on elements of Key.

In addition, map and multimap associate an arbitrary mapped type T with the Key.

The object of type Compare is called the comparison object of a container.

The phrase “equivalence of keys” means the equivalence relation imposed by the comparison object.

That is, two keys k1 and k2 are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false.

[Note 1:

This is not necessarily the same as the result of k1 == k2.

— end note]

For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value.

An associative container supports unique keys if it may contain at most one element for each key.

Otherwise, it supports equivalent keys.

The set and map classes support unique keys; the multiset and multimap classes support equivalent keys.

For multiset and multimap, insert, emplace, and erase preserve the relative ordering of equivalent elements.

For set and multiset the value type is the same as the key type.

For map and multimap it is equal to pair<const Key, T>.

iterator of an associative container is of the bidirectional iterator category.

For associative 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]

In this subclause,

(7.1)
X denotes an associative container class,
(7.2)
a denotes a value of type X,
(7.3)
a2 denotes a value of a type with nodes compatible with type X (Table 83),
(7.4)
b denotes a value or type X or const X,
(7.5)
u denotes the name of a variable being declared,
(7.6)
a_uniq denotes a value of type X when X supports unique keys,
(7.7)
a_eq denotes a value of type X when X supports multiple keys,
(7.8)
a_tran denotes a value of type X or const X when the qualified-id X::key_compare::is_transparent is valid and denotes a type ([temp.deduct]),
(7.9)
i and j meet the Cpp17InputIterator requirements and refer to elements implicitly convertible to value_type,
(7.10)
[i, j) denotes a valid range,
(7.11)
rg denotes a value of a type R that models container-compatible-range<value_type>,
(7.12)
p denotes a valid constant iterator to a,
(7.13)
q denotes a valid dereferenceable constant iterator to a,
(7.14)
r denotes a valid dereferenceable iterator to a,
(7.15)
[q1, q2) denotes a valid range of constant iterators in a,
(7.16)
il designates an object of type initializer_list<value_type>,
(7.17)
t denotes a value of type X::value_type,
(7.18)
k denotes a value of type X::key_type, and
(7.19)
c denotes a value of type X::key_compare or const X::key_compare;
(7.20)
kl is a value such that a is partitioned ([alg.sorting]) with respect to c(x, kl), with x the key value of e and e in a;
(7.21)
ku is a value such that a is partitioned with respect to !c(ku, x), with x the key value of e and e in a;
(7.22)
ke is a value such that a is partitioned with respect to c(x, ke) and !c(ke, x), with c(x, ke) implying !c(ke, x) and with x the key value of e and e in a;
(7.23)
kx is a value such that
- (7.23.1)
  a is partitioned with respect to c(x, kx) and !c(kx, x), with c(x, kx) implying !c(kx, x) and with x the key value of e and e in a, and
- (7.23.2)
  kx is not convertible to either iterator or const_iterator; and
(7.24)
A denotes the storage allocator used by X, if any, or allocator<X::value_type> otherwise,
(7.25)
m denotes an allocator of a type convertible to A, and nh denotes a non-const rvalue of type X::node_type.

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

[Note 3:

For example, in some cases key_type and mapped_type are required 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 map and multimap only.

🔗

typename X::value_type

Result: Key for set and multiset only; pair<const Key, T> for map and multimap only.

Preconditions: X::value_type is Cpp17Erasable from X.

🔗

typename X::key_compare

Result: Compare.

Preconditions: key_compare is Cpp17CopyConstructible.

🔗

typename X::value_compare

Result: A binary predicate type.

It is the same as key_compare for set and multiset; is an ordering relation on pairs induced by the first component (i.e., Key) for map and multimap.

🔗

typename X::node_type

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

🔗

X(c)

Effects: Constructs an empty container.

Uses a copy of c as a comparison object.

Complexity: Constant.

🔗

X u = X();
X u;

Preconditions: key_compare meets the Cpp17DefaultConstructible requirements.

Effects: Constructs an empty container.

Uses Compare() as a comparison object.

Complexity: Constant.

🔗

X(i, j, c)

Preconditions: value_type is Cpp17EmplaceConstructible into X from *i.

Effects: Constructs an empty container and inserts elements from the range [i, j) into it; uses c as a comparison object.

Complexity:

N log N

in general, where N has the value distance(i, j); linear if [i, j) is sorted with respect to value_comp().

🔗

X(i, j)

Preconditions: key_compare meets the Cpp17DefaultConstructible requirements.

value_type is Cpp17EmplaceConstructible into X from *i.

Effects: Constructs an empty container and inserts elements from the range [i, j) into it; uses Compare() as a comparison object.

Complexity:

N log N

in general, where N has the value distance(i, j); linear if [i, j) is sorted with respect to value_comp().

🔗

X(from_range, rg, c)

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

Effects: Constructs an empty container and inserts each element from rg into it.

Uses c as the comparison object.

Complexity:

N log N

in general, where N has the value ranges::distance(rg); linear if rg is sorted with respect to value_comp().

🔗

X(from_range, rg)

Preconditions: key_compare meets the Cpp17DefaultConstructible requirements.

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

Effects: Constructs an empty container and inserts each element from rg into it.

Uses Compare() as the comparison object.

Complexity: Same as X(from_range, rg, c).

🔗

X(il, c)

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

🔗

X(il)

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

🔗

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:

N log N

in general, where N has the value il.size() + a.size(); linear if [il.begin(), il.end()) is sorted with respect to value_comp().

🔗

b.key_comp()

Result: X::key_compare

Returns: The comparison object out of which b was constructed.

Complexity: Constant.

🔗

b.value_comp()

Result: X::value_compare

Returns: An object of value_compare constructed out of the comparison object.

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: Logarithmic.

🔗

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)....

If a range containing elements equivalent to t exists in a_eq, t is inserted at the end of that range.

Returns: An iterator pointing to the newly inserted element.

Complexity: Logarithmic.

🔗

a.emplace_hint(p, args)

Result: iterator

Effects: Equivalent to a.emplace(std::forward<Args>(args)...), except that the element is inserted as close as possible to the position just prior to p.

Returns: An iterator pointing to the element with the key equivalent to the newly inserted element.

Complexity: Logarithmic in general, but amortized constant if the element is inserted right before p.

🔗

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 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: Logarithmic.

🔗

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.

Effects: Inserts t and returns the iterator pointing to the newly inserted element.

If a range containing elements equivalent to t exists in a_eq, t is inserted at the end of that range.

Complexity: Logarithmic.

🔗

a.insert(p, t)

Result: iterator

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 with key equivalent to the key of t in containers with unique keys; always inserts t in containers with equivalent keys.

t is inserted as close as possible to the position just prior to p.

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

Complexity: Logarithmic in general, but amortized constant if t is inserted right before p.

🔗

a.insert(i, j)

Result: void

Preconditions: value_type is Cpp17EmplaceConstructible into X from *i.

Neither i nor j are iterators into a.

Effects: Inserts each element from the range [i, j) if and only if there is no element with key equivalent to the key of that element in containers with unique keys; always inserts that element in containers with equivalent keys.

Complexity:

N log (a.size() + N)

, where N has the value distance(i, j).

🔗

a.insert_range(rg)

Result: void

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

rg and a do not overlap.

Effects: Inserts each element from rg if and only if there is no element with key equivalent to the key of that element in containers with unique keys; always inserts that element in containers with equivalent keys.

Complexity:

N log (a.size() + N)

, where N has the value ranges::distance(rg).

🔗

a.insert(il)

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

🔗

a_uniq.insert(nh)

Result: insert_return_type

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

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().

Returns: 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().

Complexity: Logarithmic.

🔗

a_eq.insert(nh)

Result: iterator

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

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.

If a range containing elements with keys equivalent to nh.key() exists in a_eq, the element is inserted at the end of that range.

Postconditions: nh is empty.

Complexity: Logarithmic.

🔗

a.insert(p, nh)

Result: iterator

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

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 element is inserted as close as possible to the position just prior to p.

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

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

Complexity: Logarithmic in general, but amortized constant if the element is inserted right before p.

🔗

a.extract(k)

100

Result: node_type

101

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

102

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

103

Complexity:

log (a.size())

🔗

a_tran.extract(kx)

104

Result: node_type

105

Effects: Removes the first element in the container with key r such that !c(r, kx) && !c(kx, r) is true.

106

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

107

Complexity:

log (a_tran.size())

🔗

a.extract(q)

108

Result: node_type

109

Effects: Removes the element pointed to by q.

110

Returns: A node_type owning that element.

111

Complexity: Amortized constant.

🔗

a.merge(a2)

112

Result: void

113

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

114

Effects: Attempts to extract each element in a2 and insert it into a using the comparison object 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.

115

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 will continue to refer to their elements, but they now behave as iterators into a, not into a2.

116

Throws: Nothing unless the comparison object throws.

117

Complexity:

N log (a.size()+ N)

, where N has the value a2.size().

🔗

a.erase(k)

118

Result: size_type

119

Effects: Erases all elements in the container with key equivalent to k.

120

Returns: The number of erased elements.

121

Complexity:

log (a.size()) + a.count(k)

🔗

a_tran.erase(kx)

122

Result: size_type

123

Effects: Erases all elements in the container with key r such that !c(r, kx) && !c(kx, r) is true.

124

Returns: The number of erased elements.

125

Complexity:

log (a_tran.size()) + a_tran.count(kx)

🔗

a.erase(q)

126

Result: iterator

127

Effects: Erases the element pointed to by q.

128

Returns: An iterator pointing to the element immediately following q prior to the element being erased.

If no such element exists, returns a.end().

129

Complexity: Amortized constant.

🔗

a.erase(r)

130

Result: iterator

131

Effects: Erases the element pointed to by r.

132

Returns: An iterator pointing to the element immediately following r prior to the element being erased.

If no such element exists, returns a.end().

133

Complexity: Amortized constant.

🔗

a.erase(q1, q2)

134

Result: iterator

135

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

136

Returns: An iterator pointing to the element pointed to by q2 prior to any elements being erased.

If no such element exists, a.end() is returned.

137

Complexity:

log (a.size()) + N

, where N has the value distance(q1, q2).

🔗

a.clear()

138

Effects: Equivalent to a.erase(a.begin(), a.end()).

139

Postconditions: a.empty() is true.

140

Complexity: Linear in a.size().

🔗

b.find(k)

141

Result: iterator; const_iterator for constant b.

142

Returns: An iterator pointing to an element with the key equivalent to k, or b.end() if such an element is not found.

143

Complexity: Logarithmic.

🔗

a_tran.find(ke)

144

Result: iterator; const_iterator for constant a_tran.

145

Returns: An iterator pointing to an element with key r such that !c(r, ke) && !c(ke, r) is true, or a_tran.end() if such an element is not found.

146

Complexity: Logarithmic.

🔗

b.count(k)

147

Result: size_type

148

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

149

Complexity:

log (b.size()) + b.count(k)

🔗

a_tran.count(ke)

150

Result: size_type

151

Returns: The number of elements with key r such that !c(r, ke) && !c(ke, r).

152

Complexity:

log (a_tran.size()) + a_tran.count(ke)

🔗

b.contains(k)

153

Result: bool

154

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

🔗

a_tran.contains(ke)

155

Result: bool

156

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

🔗

b.lower_bound(k)

157

Result: iterator; const_iterator for constant b.

158

Returns: An iterator pointing to the first element with key not less than k, or b.end() if such an element is not found.

159

Complexity: Logarithmic.

🔗

a_tran.lower_bound(kl)

160

Result: iterator; const_iterator for constant a_tran.

161

Returns: An iterator pointing to the first element with key r such that !c(r, kl), or a_tran.end() if such an element is not found.

162

Complexity: Logarithmic.

🔗

b.upper_bound(k)

163

Result: iterator; const_iterator for constant b.

164

Returns: An iterator pointing to the first element with key greater than k, or b.end() if such an element is not found.

165

Complexity: Logarithmic,

🔗

a_tran.upper_bound(ku)

166

Result: iterator; const_iterator for constant a_tran.

167

Returns: An iterator pointing to the first element with key r such that c(ku, r), or a_tran.end() if such an element is not found.

168

Complexity: Logarithmic.

🔗

b.equal_range(k)

169

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

170

Effects: Equivalent to: return make_pair(b.lower_bound(k), b.upper_bound(k));

171

Complexity: Logarithmic.

🔗

a_tran.equal_range(ke)

172

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

173

Effects: Equivalent to: return make_pair(a_tran.lower_bound(ke), a_tran.upper_bound(ke));

174

Complexity: Logarithmic.

175

The insert, insert_range, and emplace members shall not affect the validity of iterators and references to the container, and the erase members shall invalidate only iterators and references to the erased elements.

176

The extract members invalidate only iterators to the removed element; 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.

177

The fundamental property of iterators of associative containers is that they iterate through the containers in the non-descending order of keys where non-descending is defined by the comparison that was used to construct them.

For any two dereferenceable iterators i and j such that distance from i to j is positive, the following condition holds: value_comp(*j, *i) == false

178

For associative containers with unique keys the stronger condition holds: value_comp(*i, *j) != false

179

When an associative container is constructed by passing a comparison object the container shall not store a pointer or reference to the passed object, even if that object is passed by reference.

When an associative container is copied, through either a copy constructor or an assignment operator, the target container shall then use the comparison object from the container being copied, as if that comparison object had been passed to the target container in its constructor.

180

The member function templates find, count, contains, lower_bound, upper_bound, equal_range, erase, and extract shall not participate in overload resolution unless the qualified-id Compare::is_transparent is valid and denotes a type ([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.

181

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

(181.1)
It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.
(181.2)
It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.
(181.3)
It has a Compare template parameter and a type that qualifies as an allocator is deduced for that parameter.