[associative.reqmts]

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.

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 75),
(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 equivalent 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.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]

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typename X::key_type

Result: Key.

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typename X::mapped_type

Remarks: For map and multimap only.

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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.

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typename X::key_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.

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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.

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X(c)

Complexity: Constant.

🔗

X u = X();
X u;

Complexity: Constant.

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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 () .

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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

Preconditions: value_type is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

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()

Complexity: Constant.

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b.value_comp()

Complexity: Constant.

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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: Logarithmic.

🔗

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)..

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

Complexity: Logarithmic.

🔗

a.emplace_hint(p, args)

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.

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

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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 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)

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.

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a.insert(p, 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 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.

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

🔗

a.insert(i, j)

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)

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().

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a_uniq.insert(nh)

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)

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.

Complexity: Logarithmic.

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a.insert(p, nh)

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.

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

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a.extract(k)

100

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

103

Complexity:

log (a.size())

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a_tran.extract(kx)

104

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())

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a.extract(q)

108

Complexity: Amortized constant.

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a.merge(a2)

112

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.

If abegin and a2begin have the same type, 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

23.2.7.2 Exception safety guarantees [associative.reqmts.except]