24 Containers library [containers]

24.2 Requirements [container.requirements]

24.2.1 Preamble [container.requirements.pre]

Containers are objects that store other objects.

They control allocation and deallocation of these objects through constructors, destructors, insert and erase operations.

All of the complexity requirements in this Clause are stated solely in terms of the number of operations on the contained objects.

[Example 1:

The copy constructor of type vector<vector<int>> has linear complexity, even though the complexity of copying each contained vector<int> is itself linear.

— end example]

Allocator-aware containers ([container.alloc.reqmts]) other than basic_string construct elements using the function allocator_traits<allocator_type>::rebind_traits<U>::construct and destroy elements using the function allocator_traits<allocator_type>::rebind_traits<U>::destroy ([allocator.traits.members]), where U is either allocator_type::value_type or an internal type used by the container.

These functions are called only for the container's element type, not for internal types used by the container.

[Note 1:

This means, for example, that a node-based container would need to construct nodes containing aligned buffers and call construct to place the element into the buffer.

— end note]

24.2.2 General containers [container.requirements.general]

24.2.2.1 Introduction [container.intro.reqmts]

In subclause [container.requirements.general],

(1.1)
X denotes a container class containing objects of type T,
(1.2)
a denotes a value of type X,
(1.3)
b and c denote values of type (possibly const) X,
(1.4)
i and j denote values of type (possibly const) X::iterator,
(1.5)
u denotes an identifier,
(1.6)
v denotes an lvalue of type (possibly const) X or an rvalue of type const X,
(1.7)
s and t denote non-const lvalues of type X, and
(1.8)
rv denotes a non-const rvalue of type X.

The following exposition-only concept is used in the definition of containers: template<class R, class T> concept container-compatible-range = // exposition only ranges::input_range<R> && convertible_to<ranges::range_reference_t<R>, T>;

24.2.2.2 Container requirements [container.reqmts]

A type X meets the container requirements if the following types, statements, and expressions are well-formed and have the specified semantics.

🔗

typename X::value_type

Result: T

Preconditions: T is Cpp17Erasable from X (see [container.alloc.reqmts], below).

🔗

typename X::reference

Result: T&

🔗

typename X::const_reference

Result: const T&

🔗

typename X::iterator

Result: A type that meets the forward iterator requirements ([forward.iterators]) with value type T.

The type X::iterator is convertible to X::const_iterator.

🔗

typename X::const_iterator

Result: A type that meets the requirements of a constant iterator and those of a forward iterator with value type T.

🔗

typename X::difference_type

Result: A signed integer type, identical to the difference type of X::iterator and X::const_iterator.

🔗

typename X::size_type

Result: An unsigned integer type that can represent any non-negative value of X::difference_type.

🔗

X u;
X u = X();

Postconditions: u.empty()

Complexity: Constant.

🔗

X u(v);
X u = v;

Preconditions: T is Cpp17CopyInsertable into X (see below).

Postconditions: u == v.

Complexity: Linear.

🔗

X u(rv);
X u = rv;

Postconditions: u is equal to the value that rv had before this construction.

Complexity: Linear for array and constant for all other standard containers.

🔗

t = v;

Result: X&.

Postconditions: t == v.

Complexity: Linear.

🔗

t = rv

Result: X&.

Effects: All existing elements of t are either move assigned to or destroyed.

Postconditions: If t and rv do not refer to the same object, t is equal to the value that rv had before this assignment.

Complexity: Linear.

🔗

a.~X()

Result: void.

Effects: Destroys every element of a; any memory obtained is deallocated.

Complexity: Linear.

🔗

b.begin()

Result: iterator; const_iterator for constant b.

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

Complexity: Constant.

🔗

b.end()

Result: iterator; const_iterator for constant b.

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

Complexity: Constant.

🔗

b.cbegin()

Result: const_iterator.

Returns: const_cast<X const&>(b).begin()

Complexity: Constant.

🔗

b.cend()

Result: const_iterator.

Returns: const_cast<X const&>(b).end()

Complexity: Constant.

🔗

i <=> j

Result: strong_ordering.

Constraints: X::iterator meets the random access iterator requirements.

Complexity: Constant.

🔗

c == b

Preconditions: T meets the Cpp17EqualityComparable requirements.

Result: bool.

Returns: equal(c.begin(), c.end(), b.begin(), b.end())

[Note 1:

The algorithm equal is defined in [alg.equal].

— end note]

Complexity: Constant if c.size() != b.size(), linear otherwise.

Remarks: == is an equivalence relation.

🔗

c != b

Effects: Equivalent to !(c == b).

🔗

t.swap(s)

Result: void.

Effects: Exchanges the contents of t and s.

Complexity: Linear for array and constant for all other standard containers.

🔗

swap(t, s)

Effects: Equivalent to t.swap(s).

🔗

c.size()

Result: size_type.

Returns: distance(c.begin(), c.end()), i.e., the number of elements in the container.

Complexity: Constant.

Remarks: The number of elements is defined by the rules of constructors, inserts, and erases.

🔗

c.max_size()

Result: size_type.

Returns: distance(begin(), end()) for the largest possible container.

Complexity: Constant.

🔗

c.empty()

Result: bool.

Returns: c.begin() == c.end()

Complexity: Constant.

Remarks: If the container is empty, then c.empty() is true.

In the expressions i == j i != j i < j i <= j i >= j i > j i <=> j i - j where i and j denote objects of a container's iterator type, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.

Unless otherwise specified, all containers defined in this Clause obtain memory using an allocator (see [allocator.requirements]).

[Note 2:

In particular, containers and iterators do not store references to allocated elements other than through the allocator's pointer type, i.e., as objects of type P or pointer_traits<P>::template rebind<unspecified>, where P is allocator_traits<allocator_type>::pointer.

— end note]

Copy constructors for these container types obtain an allocator by calling allocator_traits<allocator_type>::select_on_container_copy_construction on the allocator belonging to the container being copied.

Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved.

Such move construction of the allocator shall not exit via an exception.

All other constructors for these container types take a const allocator_type& argument.

[Note 3:

If an invocation of a constructor uses the default value of an optional allocator argument, then the allocator type must support value-initialization.

— end note]

A copy of this allocator is used for any memory allocation and element construction performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced.

The allocator may be replaced only via assignment or swap().

Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only if

(64.1)
allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value,
(64.2)
allocator_traits<allocator_type>::propagate_on_container_move_assignment::value, or
(64.3)
allocator_traits<allocator_type>::propagate_on_container_swap::value

is true within the implementation of the corresponding container operation.

In all container types defined in this Clause, the member get_allocator() returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement.

The expression a.swap(b), for containers a and b of a standard container type other than array, shall exchange the values of a and b without invoking any move, copy, or swap operations on the individual container elements.

Any Compare, Pred, or Hash types belonging to a and b shall meet the Cpp17Swappable requirements and shall be exchanged by calling swap as described in [swappable.requirements].

If allocator_traits<allocator_type>::propagate_on_container_swap::value is true, then allocator_type shall meet the Cpp17Swappable requirements and the allocators of a and b shall also be exchanged by calling swap as described in [swappable.requirements].

Otherwise, the allocators shall not be swapped, and the behavior is undefined unless a.get_allocator() == b.get_allocator().

Every iterator referring to an element in one container before the swap shall refer to the same element in the other container after the swap.

It is unspecified whether an iterator with value a.end() before the swap will have value b.end() after the swap.

Unless otherwise specified (see [associative.reqmts.except], [unord.req.except], [deque.modifiers], and [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:

(66.1)
If an exception is thrown by an insert() or emplace() function while inserting a single element, that function has no effects.
(66.2)
If an exception is thrown by a push_back(), push_front(), emplace_back(), or emplace_front() function, that function has no effects.
(66.3)
No erase(), clear(), pop_back() or pop_front() function throws an exception.
(66.4)
No copy constructor or assignment operator of a returned iterator throws an exception.
(66.5)
No swap() function throws an exception.
(66.6)
No swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped.

[Note 4:
The end() iterator does not refer to any element, so it can be invalidated.
— end note]

Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container.

A contiguous container is a container whose member types iterator and const_iterator meet the Cpp17RandomAccessIterator requirements ([random.access.iterators]) and model contiguous_iterator ([iterator.concept.contiguous]).

The behavior of certain container member functions and deduction guides depends on whether types qualify as input iterators or allocators.

The extent to which an implementation determines that a type cannot be an input iterator is unspecified, except that as a minimum integral types shall not qualify as input iterators.

Likewise, the extent to which an implementation determines that a type cannot be an allocator is unspecified, except that as a minimum a type A shall not qualify as an allocator unless it meets both of the following conditions:

(69.1)
The qualified-id A::value_type is valid and denotes a type ([temp.deduct]).
(69.2)
The expression declval<A&>().allocate(size_t{}) is well-formed when treated as an unevaluated operand.

24.2.2.3 Reversible container requirements [container.rev.reqmts]

A type X meets the reversible container requirements if X meets the container requirements, the iterator type of X belongs to the bidirectional or random access iterator categories ([iterator.requirements]), and the following types and expressions are well-formed and have the specified semantics.

🔗

typename X::reverse_iterator

Result: The type reverse_iterator<X::iterator>, an iterator type whose value type is T.

🔗

typename X::const_reverse_iterator

Result: The type reverse_iterator<X::const_iterator>, a constant iterator type whose value type is T.

🔗

a.rbegin()

Result: reverse_iterator; const_reverse_iterator for constant a.

Returns: reverse_iterator(end())

Complexity: Constant.

🔗

a.rend()

Result: reverse_iterator; const_reverse_iterator for constant a.

Returns: reverse_iterator(begin())

Complexity: Constant.

🔗

a.crbegin()

Result: const_reverse_iterator.

Returns: const_cast<X const&>(a).rbegin()

Complexity: Constant.

🔗

a.crend()

Result: const_reverse_iterator.

Returns: const_cast<X const&>(a).rend()

Complexity: Constant.

24.2.2.4 Optional container requirements [container.opt.reqmts]

The following operations are provided for some types of containers but not others.

Those containers for which the listed operations are provided shall implement the semantics as described unless otherwise stated.

If the iterators passed to lexicographical_compare_three_way meet the constexpr iterator requirements ([iterator.requirements.general]) then the operations described below are implemented by constexpr functions.

🔗

a <=> b

Result: synth-three-way-result<X::value_type>.

Preconditions: Either T models three_way_comparable, or < is defined for values of type (possibly const) T and < is a total ordering relationship.

Returns: lexicographical_compare_three_way(a.begin(), a.end(), b.begin(), b.end(),
synth-three-way)

[Note 1:

The algorithm lexicographical_compare_three_way is defined in [algorithms].

— end note]

Complexity: Linear.

24.2.2.5 Allocator-aware containers [container.alloc.reqmts]

Except for array, all of the containers defined in [containers], [stacktrace.basic], [basic.string], and [re.results] meet the additional requirements of an allocator-aware container, as described below.

Given an allocator type A and given a container type X having a value_type identical to T and an allocator_type identical to allocator_traits<A>::rebind_alloc<T> and given an lvalue m of type A, a pointer p of type T*, an expression v that denotes an lvalue of type T or const T or an rvalue of type const T, and an rvalue rv of type T, the following terms are defined.

If X is not allocator-aware or is a specialization of basic_string, the terms below are defined as if A were allocator<T> — no allocator object needs to be created and user specializations of allocator<T> are not instantiated:

(2.1)
T is Cpp17DefaultInsertable into X means that the following expression is well-formed: allocator_traits<A>::construct(m, p)
(2.2)
An element of X is default-inserted if it is initialized by evaluation of the expression allocator_traits<A>::construct(m, p) where p is the address of the uninitialized storage for the element allocated within X.
(2.3)
T is Cpp17MoveInsertable into X means that the following expression is well-formed: allocator_traits<A>::construct(m, p, rv) and its evaluation causes the following postcondition to hold: The value of *p is equivalent to the value of rv before the evaluation.
[Note 1:
rv remains a valid object.

Its state is unspecified.
— end note]
(2.4)
T is Cpp17CopyInsertable into X means that, in addition to T being Cpp17MoveInsertable into X, the following expression is well-formed: allocator_traits<A>::construct(m, p, v) and its evaluation causes the following postcondition to hold: The value of v is unchanged and is equivalent to *p.
(2.5)
T is Cpp17EmplaceConstructible into X from args, for zero or more arguments args, means that the following expression is well-formed: allocator_traits<A>::construct(m, p, args)
(2.6)
T is Cpp17Erasable from X means that the following expression is well-formed: allocator_traits<A>::destroy(m, p)

[Note 2:

A container calls allocator_traits<A>::construct(m, p, args) to construct an element at p using args, with m == get_allocator().

The default construct in allocator will call ::new((void*)p) T(args), but specialized allocators can choose a different definition.

— end note]

In this subclause,

(3.1)
X denotes an allocator-aware container class with a value_type of T using an allocator of type A,
(3.2)
u denotes a variable,
(3.3)
a and b denote non-const lvalues of type X,
(3.4)
c denotes an lvalue of type const X,
(3.5)
t denotes an lvalue or a const rvalue of type X,
(3.6)
rv denotes a non-const rvalue of type X, and
(3.7)
m is a value of type A.

A type X meets the allocator-aware container requirements if X meets the container requirements and the following types, statements, and expressions are well-formed and have the specified semantics.

🔗

typename X::allocator_type

Result: A

Mandates: allocator_type::value_type is the same as X::value_type.

🔗

c.get_allocator()

Result: A

Complexity: Constant.

🔗

X u;
X u = X();

Preconditions: A meets the Cpp17DefaultConstructible requirements.

Postconditions: u.empty() returns true, u.get_allocator() == A().

Complexity: Constant.

🔗

X u(m);

Postconditions: u.empty() returns true, u.get_allocator() == m.

Complexity: Constant.

🔗

X u(t, m);

Preconditions: T is Cpp17CopyInsertable into X.

Postconditions: u == t, u.get_allocator() == m

Complexity: Linear.

🔗

X u(rv);

Postconditions: u has the same elements as rv had before this construction; the value of u.get_allocator() is the same as the value of rv.get_allocator() before this construction.

Complexity: Constant.

🔗

X u(rv, m);

Preconditions: T is Cpp17MoveInsertable into X.

Postconditions: u has the same elements, or copies of the elements, that rv had before this construction, u.get_allocator() == m.

Complexity: Constant if m == rv.get_allocator(), otherwise linear.

🔗

a = t

Result: X&.

Preconditions: T is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

Postconditions: a == t is true.

Complexity: Linear.

🔗

a = rv

Result: X&.

Preconditions: If allocator_traits<allocator_type>::propagate_on_container_move_assignment::value is false, T is Cpp17MoveInsertable into X and Cpp17MoveAssignable.

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

Postconditions: If a and rv do not refer to the same object, a is equal to the value that rv had before this assignment.

Complexity: Linear.

🔗

a.swap(b)

Result: void

Effects: Exchanges the contents of a and b.

Complexity: Constant.

24.2.3 Container data races [container.requirements.dataraces]

For purposes of avoiding data races ([res.on.data.races]), implementations shall consider the following functions to be const: begin, end, rbegin, rend, front, back, data, find, lower_bound, upper_bound, equal_range, at and, except in associative or unordered associative containers, operator[].

Notwithstanding [res.on.data.races], implementations are required to avoid data races when the contents of the contained object in different elements in the same container, excepting vector<bool>, are modified concurrently.

[Note 1:

For a vector<int> x with a size greater than one, x[1] = 5 and *x.begin() = 10 can be executed concurrently without a data race, but x[0] = 5 and *x.begin() = 10 executed concurrently can result in a data race.

As an exception to the general rule, for a vector<bool> y, y[0] = true can race with y[1] = true.

— end note]

24.2.4 Sequence containers [sequence.reqmts]

A sequence container organizes a finite set of objects, all of the same type, into a strictly linear arrangement.

The library provides four basic kinds of sequence containers: vector, forward_list, list, and deque.

In addition, array is provided as a sequence container which provides limited sequence operations because it has a fixed number of elements.

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

In this subclause,

(2.1)
X denotes a sequence container class,
(2.2)
a denotes a value of type X containing elements of type T,
(2.3)
u denotes the name of a variable being declared,
(2.4)
A denotes X::allocator_type if the qualified-id X::allocator_type is valid and denotes a type ([temp.deduct]) and allocator<T> if it doesn't,
(2.5)
i and j denote iterators that meet the Cpp17InputIterator requirements and refer to elements implicitly convertible to value_type,
(2.6)
[i, j) denotes a valid range,
(2.7)
rg denotes a value of a type R that models container-compatible-range<T>,
(2.8)
il designates an object of type initializer_list<value_type>,
(2.9)
n denotes a value of type X::size_type,
(2.10)
p denotes a valid constant iterator to a,
(2.11)
q denotes a valid dereferenceable constant iterator to a,
(2.12)
[q1, q2) denotes a valid range of constant iterators in a,
(2.13)
t denotes an lvalue or a const rvalue of X::value_type, and
(2.14)
rv denotes a non-const rvalue of X::value_type.
(2.15)
Args denotes a template parameter pack;
(2.16)
args denotes a function parameter pack with the pattern Args&&.

The complexities of the expressions are sequence dependent.

A type X meets the sequence container requirements if X meets the container requirements and the following statements and expressions are well-formed and have the specified semantics.

🔗

X u(n, t);

Preconditions: T is Cpp17CopyInsertable into X.

Effects: Constructs a sequence container with n copies of t.

Postconditions: distance(u.begin(), u.end()) == n is true.

🔗

X u(i, j);

Preconditions: T is Cpp17EmplaceConstructible into X from *i.

For vector, if the iterator does not meet the Cpp17ForwardIterator requirements ([forward.iterators]), T is also Cpp17MoveInsertable into X.

Effects: Constructs a sequence container equal to the range [i, j).

Each iterator in the range [i, j) is dereferenced exactly once.

Postconditions: distance(u.begin(), u.end()) == distance(i, j) is true.

🔗

X(from_range, rg)

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

For vector, if R models neither ranges::sized_range nor ranges::forward_range, T is also Cpp17MoveInsertable into X.

Effects: Constructs a sequence container equal to the range rg.

Each iterator in the range rg is dereferenced exactly once.

Postconditions: distance(begin(), end()) == ranges::distance(rg) is true.

🔗

X(il)

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

🔗

a = il

Result: X&.

Preconditions: T 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.

Returns: *this.

🔗

a.emplace(p, args)

Result: iterator.

Preconditions: T is Cpp17EmplaceConstructible into X from args.

For vector and deque, T is also Cpp17MoveInsertable into X and Cpp17MoveAssignable.

Effects: Inserts an object of type T constructed with std::forward<Args>(args)... before p.

[Note 1:

args can directly or indirectly refer to a value in a.

— end note]

Returns: An iterator that points to the new element constructed from args into a.

🔗

a.insert(p, t)

Result: iterator.

Preconditions: T is Cpp17CopyInsertable into X.

For vector and deque, T is also Cpp17CopyAssignable.

Effects: Inserts a copy of t before p.

Returns: An iterator that points to the copy of t inserted into a.

🔗

a.insert(p, rv)

Result: iterator.

Preconditions: T is Cpp17MoveInsertable into X.

For vector and deque, T is also Cpp17MoveAssignable.

Effects: Inserts a copy of rv before p.

Returns: An iterator that points to the copy of rv inserted into a.

🔗

a.insert(p, n, t)

Result: iterator.

Preconditions: T is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

Effects: Inserts n copies of t before p.

Returns: An iterator that points to the copy of the first element inserted into a, or p if n == 0.

🔗

a.insert(p, i, j)

Result: iterator.

Preconditions: T is Cpp17EmplaceConstructible into X from *i.

For vector and deque, T is also Cpp17MoveInsertable into X, and T meets the Cpp17MoveConstructible, Cpp17MoveAssignable, and Cpp17Swappable ([swappable.requirements]) requirements.

Neither i nor j are iterators into a.

Effects: Inserts copies of elements in [i, j) before p.

Each iterator in the range [i, j) shall be dereferenced exactly once.

Returns: An iterator that points to the copy of the first element inserted into a, or p if i == j.

🔗

a.insert_range(p, rg)

Result: iterator.

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

For vector and deque, T is also Cpp17MoveInsertable into X, and T meets the Cpp17MoveConstructible, Cpp17MoveAssignable, and Cpp17Swappable ([swappable.requirements]) requirements.

rg and a do not overlap.

Effects: Inserts copies of elements in rg before p.

Each iterator in the range rg is dereferenced exactly once.

Returns: An iterator that points to the copy of the first element inserted into a, or p if rg is empty.

🔗

a.insert(p, il)

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

🔗

a.erase(q)

Result: iterator.

Preconditions: For vector and deque, T is Cpp17MoveAssignable.

Effects: Erases the element pointed to by q.

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

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

🔗

a.erase(q1, q2)

Result: iterator.

Preconditions: For vector and deque, T is Cpp17MoveAssignable.

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

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

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

🔗

a.clear()

Result: void

Effects: Destroys all elements in a.

Invalidates all references, pointers, and iterators referring to the elements of a and may invalidate the past-the-end iterator.

Postconditions: a.empty() is true.

Complexity: Linear.

🔗

a.assign(i, j)

Result: void

Preconditions: T is Cpp17EmplaceConstructible into X from *i and assignable from *i.

For vector, if the iterator does not meet the forward iterator requirements ([forward.iterators]), T is also Cpp17MoveInsertable into X.

Neither i nor j are iterators into a.

Effects: Replaces elements in a with a copy of [i, j).

Invalidates all references, pointers and iterators referring to the elements of a.

For vector and deque, also invalidates the past-the-end iterator.

Each iterator in the range [i, j) is dereferenced exactly once.

🔗

a.assign_range(rg)

Result: void

Mandates: assignable_from<T&, ranges::range_reference_t<R>> is modeled.

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

For vector, if R models neither ranges::sized_range nor ranges::forward_range, T is also Cpp17MoveInsertable into X.

rg and a do not overlap.

Effects: Replaces elements in a with a copy of each element in rg.

Invalidates all references, pointers, and iterators referring to the elements of a.

For vector and deque, also invalidates the past-the-end iterator.

Each iterator in the range rg is dereferenced exactly once.

🔗

a.assign(il)

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

🔗

a.assign(n, t)

Result: void

Preconditions: T is Cpp17CopyInsertable into X and Cpp17CopyAssignable.

t is not a reference into a.

Effects: Replaces elements in a with n copies of t.

Invalidates all references, pointers and iterators referring to the elements of a.

For vector and deque, also invalidates the past-the-end iterator.

For every sequence container defined in this Clause and in [strings]:

(67.1)
If the constructor template<class InputIterator> X(InputIterator first, InputIterator last, const allocator_type& alloc = allocator_type()); is called with a type InputIterator that does not qualify as an input iterator, then the constructor shall not participate in overload resolution.
(67.2)
If the member functions of the forms: template<class InputIterator> return-type F(const_iterator p, InputIterator first, InputIterator last); // such as insert template<class InputIterator> return-type F(InputIterator first, InputIterator last); // such as append, assign template<class InputIterator> return-type F(const_iterator i1, const_iterator i2, InputIterator first, InputIterator last); // such as replace are called with a type InputIterator that does not qualify as an input iterator, then these functions shall not participate in overload resolution.
(67.3)
A deduction guide for a sequence container shall not participate in overload resolution if it has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter, or if it has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

The following operations are provided for some types of sequence containers but not others.

Operations other than prepend_range and append_range are implemented so as to take amortized constant time.

🔗

a.front()

Result: reference; const_reference for constant a.

Returns: *a.begin()

Remarks: Required for basic_string, array, deque, forward_list, list, and vector.

🔗

a.back()

Result: reference; const_reference for constant a.

Effects: Equivalent to: auto tmp = a.end(); --tmp; return *tmp;

Remarks: Required for basic_string, array, deque, list, and vector.

🔗

a.emplace_front(args)

Result: reference

Preconditions: T is Cpp17EmplaceConstructible into X from args.

Effects: Prepends an object of type T constructed with std::forward<Args>(args)....

Returns: a.front().

Remarks: Required for deque, forward_list, and list.

🔗

a.emplace_back(args)

Result: reference

Preconditions: T is Cpp17EmplaceConstructible into X from args.

For vector, T is also Cpp17MoveInsertable into X.

Effects: Appends an object of type T constructed with std::forward<Args>(args)....

Returns: a.back().

Remarks: Required for deque, list, and vector.

🔗

a.push_front(t)

Result: void

Preconditions: T is Cpp17CopyInsertable into X.

Effects: Prepends a copy of t.

Remarks: Required for deque, forward_list, and list.

🔗

a.push_front(rv)

Result: void

Preconditions: T is Cpp17MoveInsertable into X.

Effects: Prepends a copy of rv.

Remarks: Required for deque, forward_list, and list.

🔗

a.prepend_range(rg)

Result: void

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

For deque, T is also Cpp17MoveInsertable into X, and T meets the Cpp17MoveConstructible, Cpp17MoveAssignable, and Cpp17Swappable ([swappable.requirements]) requirements.

Effects: Inserts copies of elements in rg before begin().

Each iterator in the range rg is dereferenced exactly once.

[Note 2:

The order of elements in rg is not reversed.

— end note]

Remarks: Required for deque, forward_list, and list.

🔗

a.push_back(t)

Result: void

Preconditions: T is Cpp17CopyInsertable into X.

Effects: Appends a copy of t.

100

Remarks: Required for basic_string, deque, list, and vector.

🔗

a.push_back(rv)

101

Result: void

102

Preconditions: T is Cpp17MoveInsertable into X.

103

Effects: Appends a copy of rv.

104

Remarks: Required for basic_string, deque, list, and vector.

🔗

a.append_range(rg)

105

Result: void

106

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

For vector, T is also Cpp17MoveInsertable into X.

107

Effects: Inserts copies of elements in rg before end().

Each iterator in the range rg is dereferenced exactly once.

108

Remarks: Required for deque, list, and vector.

🔗

a.pop_front()

109

Result: void

110

Preconditions: a.empty() is false.

111

Effects: Destroys the first element.

112

Remarks: Required for deque, forward_list, and list.

🔗

a.pop_back()

113

Result: void

114

Preconditions: a.empty() is false.

115

Effects: Destroys the last element.

116

Remarks: Required for basic_string, deque, list, and vector.

🔗

a[n]

117

Result: reference; const_reference for constant a

118

Effects: Equivalent to: return *(a.begin() + n);

119

Remarks: Required for basic_string, array, deque, and vector.

🔗

a.at(n)

120

Result: reference; const_reference for constant a

121

Returns: *(a.begin() + n)

122

Throws: out_of_range if n >= a.size().

123

Remarks: Required for basic_string, array, deque, and vector.

24.2.5 Node handles [container.node]

24.2.5.1 Overview [container.node.overview]

A node handle is an object that accepts ownership of a single element from an associative container ([associative.reqmts]) or an unordered associative container ([unord.req]).

It may be used to transfer that ownership to another container with compatible nodes.

Containers with compatible nodes have the same node handle type.

Elements may be transferred in either direction between container types in the same row of Table 83 .

Table 83: Container types with compatible nodes [tab:container.node.compat]

🔗	map<K, T, C1, A>	map<K, T, C2, A>
🔗	map<K, T, C1, A>	multimap<K, T, C2, A>
🔗	set<K, C1, A>	set<K, C2, A>
🔗	set<K, C1, A>	multiset<K, C2, A>
🔗	unordered_map<K, T, H1, E1, A>	unordered_map<K, T, H2, E2, A>
🔗	unordered_map<K, T, H1, E1, A>	unordered_multimap<K, T, H2, E2, A>
🔗	unordered_set<K, H1, E1, A>	unordered_set<K, H2, E2, A>
🔗	unordered_set<K, H1, E1, A>	unordered_multiset<K, H2, E2, A>

If a node handle is not empty, then it contains an allocator that is equal to the allocator of the container when the element was extracted.

If a node handle is empty, it contains no allocator.

Class node-handle is for exposition only.

If a user-defined specialization of pair exists for pair<const Key, T> or pair<Key, T>, where Key is the container's key_type and T is the container's mapped_type, the behavior of operations involving node handles is undefined.

template<unspecified> class node-handle { public: // These type declarations are described in [associative.reqmts] and [unord.req]. using value_type = see below; // not present for map containers using key_type = see below; // not present for set containers using mapped_type = see below; // not present for set containers using allocator_type = see below; private: using container_node_type = unspecified; // exposition only using ator_traits = allocator_traits<allocator_type>; // exposition only typename ator_traits::template rebind_traits<container_node_type>::pointer ptr_; // exposition only optional<allocator_type> alloc_; // exposition only public: // [container.node.cons], constructors, copy, and assignment constexpr node-handle() noexcept : ptr_(), alloc_() {} node-handle(node-handle&&) noexcept; node-handle& operator=(node-handle&&); // [container.node.dtor], destructor ~node-handle(); // [container.node.observers], observers value_type& value() const; // not present for map containers key_type& key() const; // not present for set containers mapped_type& mapped() const; // not present for set containers allocator_type get_allocator() const; explicit operator bool() const noexcept; [[nodiscard]] bool empty() const noexcept; // [container.node.modifiers], modifiers void swap(node-handle&) noexcept(ator_traits::propagate_on_container_swap::value || ator_traits::is_always_equal::value); friend void swap(node-handle& x, node-handle& y) noexcept(noexcept(x.swap(y))) { x.swap(y); } };

24.2.5.2 Constructors, copy, and assignment [container.node.cons]

🔗

node-handle(node-handle&& nh) noexcept;

Effects: Constructs a node-handle object initializing ptr_ with nh.ptr_.

Move constructs alloc_ with nh.alloc_.

Assigns nullptr to nh.ptr_ and assigns nullopt to nh.alloc_.

🔗

node-handle& operator=(node-handle&& nh);

Preconditions: Either !alloc_, or ator_traits::propagate_on_container_move_assignment::value is true, or alloc_ == nh.alloc_.

Effects:

(3.1)
If ptr_ != nullptr, destroys the value_type subobject in the container_node_type object pointed to by ptr_ by calling ator_traits::destroy, then deallocates ptr_ by calling ator_traits::template rebind_traits<container_node_type>::deallocate.
(3.2)
Assigns nh.ptr_ to ptr_.
(3.3)
If !alloc_ or ator_traits::propagate_on_container_move_assignment::value is true,
move assigns nh.alloc_ to alloc_.
(3.4)
Assigns nullptr to nh.ptr_ and assigns nullopt to nh.alloc_.

Returns: *this.

Throws: Nothing.

24.2.5.3 Destructor [container.node.dtor]

🔗

~node-handle();

Effects: If ptr_ != nullptr, destroys the value_type subobject in the container_node_type object pointed to by ptr_ by calling ator_traits::destroy, then deallocates ptr_ by calling ator_traits::template rebind_traits<container_node_type>::deallocate.

24.2.5.4 Observers [container.node.observers]

🔗

value_type& value() const;

Preconditions: empty() == false.

Returns: A reference to the value_type subobject in the container_node_type object pointed to by ptr_.

Throws: Nothing.

🔗

key_type& key() const;

Preconditions: empty() == false.

Returns: A non-const reference to the key_type member of the value_type subobject in the container_node_type object pointed to by ptr_.

Throws: Nothing.

Remarks: Modifying the key through the returned reference is permitted.

🔗

mapped_type& mapped() const;

Preconditions: empty() == false.

Returns: A reference to the mapped_type member of the value_type subobject in the container_node_type object pointed to by ptr_.

Throws: Nothing.

🔗

allocator_type get_allocator() const;

Preconditions: empty() == false.

Returns: *alloc_.

Throws: Nothing.

🔗

explicit operator bool() const noexcept;

Returns: ptr_ != nullptr.

🔗

[[nodiscard]] bool empty() const noexcept;

Returns: ptr_ == nullptr.

24.2.5.5 Modifiers [container.node.modifiers]

🔗

void swap(node-handle& nh)
  noexcept(ator_traits::propagate_on_container_swap::value ||
           ator_traits::is_always_equal::value);

Preconditions: !alloc_, or !nh.alloc_, or ator_traits::propagate_on_container_swap::value is true, or alloc_ == nh.alloc_.

Effects: Calls swap(ptr_, nh.ptr_).

If !alloc_, or !nh.alloc_, or ator_traits::propagate_on_container_swap::value is true calls swap(alloc_, nh.alloc_).

24.2.6 Insert return type [container.insert.return]

The associative containers with unique keys and the unordered containers with unique keys have a member function insert that returns a nested type insert_return_type.

That return type is a specialization of the template specified in this subclause.

template<class Iterator, class NodeType> struct insert-return-type { Iterator position; bool inserted; NodeType node; };

The name insert-return-type is exposition only.

insert-return-type has the template parameters, data members, and special members specified above.

It has no base classes or members other than those specified.

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 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 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.

24.2.7.2 Exception safety guarantees [associative.reqmts.except]

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).

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.

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).

24.2.8 Unordered associative containers [unord.req]

24.2.8.1 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 83),
(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.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.alloc.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.

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().

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.

🔗

b.load_factor()

229

Result: float

230

Returns: The average number of elements per bucket.

231

Complexity: Constant.

🔗

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.

🔗

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.

🔗

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 refinement207 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]).

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.

207)207)

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

24.2.8.2 Exception safety guarantees [unord.req.except]

For unordered associative containers, no clear() function throws an exception.

erase(k) does not throw an exception unless that exception is thrown by the container's Hash or Pred object (if any).

For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an insert or emplace function inserting a single element, the insertion has no effect.

For unordered associative containers, no swap function throws an exception unless that exception is thrown by the swap of the container's Hash or Pred object (if any).

For unordered associative containers, if an exception is thrown from within a rehash() function other than by the container's hash function or comparison function, the rehash() function has no effect.