24 Containers library [containers]

24.2 Requirements [container.requirements]

24.2.7 Associative containers [associative.reqmts]

24.2.7.1 General [associative.reqmts.general]

1
#
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).
2
#
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.
3
#
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.
4
#
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.
5
#
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>.
6
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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]
7
#
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 86),
  • (7.4)
    b denotes a value of 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.
8
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A type X meets the 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 map and multimap, the requirements placed on value_type in [container.reqmts] apply instead to key_type and mapped_type.
[Note 3: 
For example, in some cases key_type and mapped_type 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
9
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Result: Key.
🔗
typename X::mapped_type
10
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Result: T.
11
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Remarks: For map and multimap only.
🔗
typename X::value_type
12
#
Result: Key for set and multiset only; pair<const Key, T> for map and multimap only.
13
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Preconditions: X​::​value_type is Cpp17Erasable from X.
🔗
typename X::key_compare
14
#
Result: Compare.
15
#
Preconditions: key_compare is Cpp17CopyConstructible.
🔗
typename X::value_compare
16
#
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
17
#
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)
18
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Effects: Constructs an empty container.
Uses a copy of c as a comparison object.
19
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Complexity: Constant.
🔗
X u = X(); X u;
20
#
Preconditions: key_compare meets the Cpp17DefaultConstructible requirements.
21
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Effects: Constructs an empty container.
Uses Compare() as a comparison object.
22
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Complexity: Constant.
🔗
X(i, j, c)
23
#
Preconditions: value_type is Cpp17EmplaceConstructible into X from *i.
24
#
Effects: Constructs an empty container and inserts elements from the range [i, j) into it; uses c as a comparison object.
25
#
Complexity: in general, where N has the value distance(i, j); linear if [i, j) is sorted with respect to value_comp().
🔗
X(i, j)
26
#
Preconditions: key_compare meets the Cpp17DefaultConstructible requirements.
value_type is Cpp17EmplaceConstructible into X from *i.
27
#
Effects: Constructs an empty container and inserts elements from the range [i, j) into it; uses Compare() as a comparison object.
28
#
Complexity: 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)
29
#
Preconditions: value_type is Cpp17EmplaceConstructible into X from *ranges​::​begin(rg).
30
#
Effects: Constructs an empty container and inserts each element from rg into it.
Uses c as the comparison object.
31
#
Complexity: in general, where N has the value ranges​::​distance(rg); linear if rg is sorted with respect to value_comp().
🔗
X(from_range, rg)
32
#
Preconditions: key_compare meets the Cpp17DefaultConstructible requirements.
value_type is Cpp17EmplaceConstructible into X from *ranges​::​begin(rg).
33
#
Effects: Constructs an empty container and inserts each element from rg into it.
Uses Compare() as the comparison object.
34
#
Complexity: Same as X(from_range, rg, c).
🔗
X(il, c)
35
#
Effects: Equivalent to X(il.begin(), il.end(), c).
🔗
X(il)
36
#
Effects: Equivalent to X(il.begin(), il.end()).
🔗
a = il
37
#
Result: X&
38
#
Preconditions: value_type is Cpp17CopyInsertable into X and Cpp17CopyAssignable.
39
#
Effects: Assigns the range [il.begin(), il.end()) into a.
All existing elements of a are either assigned to or destroyed.
40
#
Complexity: 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()
41
#
Result: X​::​key_compare
42
#
Returns: The comparison object out of which b was constructed.
43
#
Complexity: Constant.
🔗
b.value_comp()
44
#
Result: X​::​value_compare
45
#
Returns: An object of value_compare constructed out of the comparison object.
46
#
Complexity: Constant.
🔗
a_uniq.emplace(args)
47
#
Result: pair<iterator, bool>
48
#
Preconditions: value_type is Cpp17EmplaceConstructible into X from args.
49
#
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.
50
#
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.
51
#
Complexity: Logarithmic.
🔗
a_eq.emplace(args)
52
#
Result: iterator
53
#
Preconditions: value_type is Cpp17EmplaceConstructible into X from args.
54
#
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.
55
#
Returns: An iterator pointing to the newly inserted element.
56
#
Complexity: Logarithmic.
🔗
a.emplace_hint(p, args)
57
#
Result: iterator
58
#
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.
59
#
Returns: An iterator pointing to the element with the key equivalent to the newly inserted element.
60
#
Complexity: Logarithmic in general, but amortized constant if the element is inserted right before p.
🔗
a_uniq.insert(t)
61
#
Result: pair<iterator, bool>
62
#
Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.
63
#
Effects: Inserts t if and only if there is no element in the container with key equivalent to the key of t.
64
#
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.
65
#
Complexity: Logarithmic.
🔗
a_eq.insert(t)
66
#
Result: iterator
67
#
Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.
68
#
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.
69
#
Complexity: Logarithmic.
🔗
a.insert(p, t)
70
#
Result: iterator
71
#
Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X.
72
#
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.
73
#
Returns: An iterator pointing to the element with key equivalent to the key of t.
74
#
Complexity: Logarithmic in general, but amortized constant if t is inserted right before p.
🔗
a.insert(i, j)
75
#
Result: void
76
#
Preconditions: value_type is Cpp17EmplaceConstructible into X from *i.
Neither i nor j are iterators into a.
77
#
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.
78
#
Complexity: , where N has the value distance(i, j).
🔗
a.insert_range(rg)
79
#
Result: void
80
#
Preconditions: value_type is Cpp17EmplaceConstructible into X from *ranges​::​begin(rg).
rg and a do not overlap.
81
#
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.
82
#
Complexity: , where N has the value ranges​::​distance(rg).
🔗
a.insert(il)
83
#
Effects: Equivalent to a.insert(il.begin(), il.end()).
🔗
a_uniq.insert(nh)
84
#
Result: insert_return_type
85
#
Preconditions: nh is empty or a_uniq.get_allocator() == nh.get_allocator() is true.
86
#
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().
87
#
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().
88
#
Complexity: Logarithmic.
🔗
a_eq.insert(nh)
89
#
Result: iterator
90
#
Preconditions: nh is empty or a_eq.get_allocator() == nh.get_allocator() is true.
91
#
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.
92
#
Postconditions: nh is empty.
93
#
Complexity: Logarithmic.
🔗
a.insert(p, nh)
94
#
Result: iterator
95
#
Preconditions: nh is empty or a.get_allocator() == nh.get_allocator() is true.
96
#
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.
97
#
Postconditions: nh is empty if insertion succeeds, unchanged if insertion fails.
98
#
Returns: An iterator pointing to the element with key equivalent to nh.key().
99
#
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:
🔗
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:
🔗
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: , 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:
🔗
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:
🔗
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: , 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:
🔗
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:
🔗
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.

24.2.7.2 Exception safety guarantees [associative.reqmts.except]

1
#
For associative containers, no clear() function throws an exception.
erase(k) does not throw an exception unless that exception is thrown by the container's Compare object (if any).
2
#
For associative containers, if an exception is thrown by any operation from within an insert or emplace function inserting a single element, the insertion has no effect.
3
#
For associative containers, no swap function throws an exception unless that exception is thrown by the swap of the container's Compare object (if any).