22 Containers library [containers]

22.1 General [containers.general]

This Clause describes components that C++ programs may use to organize collections of information.

The following subclauses describe container requirements, and components for sequence containers and associative containers, as summarized in Table 72 .

Table 72: Containers library summary [tab:containers.summary]

	Subclause	Header
[container.requirements]	Requirements
[sequences]	Sequence containers	<array>, <deque>, <forward_list>, <list>, <vector>
[associative]	Associative containers	<map>, <set>
[unord]	Unordered associative containers	<unordered_map>, <unordered_set>
[container.adaptors]	Container adaptors	<queue>, <stack>
[views]	Views	<span>

22.2 Container requirements [container.requirements]

22.2.1 General container requirements [container.requirements.general]

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

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

]

For the components affected by this subclause that declare an allocator_type, objects stored in these components shall be constructed using the function allocator_traits<allocator_type>::rebind_traits::construct and destroyed using the function allocator_traits<allocator_type>::rebind_traits::destroy, 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

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

— end note

]

In Tables 73, 74, and 75 X denotes a container class containing objects of type T, a and b denote values of type X, i and j denote values of type (possibly const) X::iterator, u denotes an identifier, r denotes a non-const value of type X, and rv denotes a non-const rvalue of type X.

Table 73: Container requirements [tab:container.req]

Expression	Return type	Operational	Assertion/note	Complexity
		semantics	pre-/post-condition
X::value_type	T		Preconditions: T is Cpp17Erasable from X (see [container.requirements.general], below)	compile time
X::reference	T&			compile time
X::const_reference	const T&			compile time
X::iterator	iterator type whose value type is T		any iterator category that meets the forward iterator requirements. convertible to X::const_iterator.	compile time
X::const_iterator	constant iterator type whose value type is T		any iterator category that meets the forward iterator requirements.	compile time
X::difference_type	signed integer type		is identical to the difference type of X::iterator and X::const_iterator	compile time
X::size_type	unsigned integer type		size_type can represent any non-negative value of difference_type	compile time
X u;			Postconditions: u.empty()	constant
X()			Postconditions: X().empty()	constant
X(a)			Preconditions: T is Cpp17CopyInsertable into X (see below). Postconditions: a == X(a).	linear
X u(a); X u = a;			Preconditions: T is Cpp17CopyInsertable into X (see below). Postconditions: u == a	linear
X u(rv); X u = rv;			Postconditions: u is equal to the value that rv had before this construction	(Note B)
a = rv	X&	All existing elements of a are either move assigned to or destroyed	Postconditions: a is equal to the value that rv had before this assignment	linear
a.~X()	void		Effects: destroys every element of a; any memory obtained is deallocated.	linear
a.begin()	iterator; const_iterator for constant a			constant
a.end()	iterator; const_iterator for constant a			constant
a.cbegin()	const_iterator	const_cast<X const&>(a).begin();		constant
a.cend()	const_iterator	const_cast<X const&>(a).end();		constant
i <=> j	strong_ordering		Constraints: X::iterator meets the random access iterator requirements.	constant
a == b	convertible to bool	== is an equivalence relation. equal(a.begin(), a.end(), b.begin(), b.end())	Preconditions: T meets the Cpp17EqualityComparable requirements	Constant if a.size() != b.size(), linear otherwise
a != b	convertible to bool	Equivalent to !(a == b)		linear
a.swap(b)	void		Effects: exchanges the contents of a and b	(Note A)
swap(a, b)	void	Equivalent to a.swap(b)		(Note A)
r = a	X&		Postconditions: r == a.	linear
a.size()	size_type	distance(a.begin(), a.end())		constant
a.max_size()	size_type	distance(begin(), end()) for the largest possible container		constant
a.empty()	convertible to bool	a.begin() == a.end()		constant

Those entries marked “(Note A)” or “(Note B)” have linear complexity for array and have constant complexity for all other standard containers.

[ Note

The algorithm equal() is defined in [algorithms].

— end note

]

The member function size() returns the number of elements in the container.

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

begin() returns an iterator referring to the first element in the container.

end() returns an iterator which is the past-the-end value for the container.

If the container is empty, then begin() == end().

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

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::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

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 allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value, allocator_traits<allocator_type>::propagate_on_container_move_assignment::value, or 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.

Lvalues of any Compare, Pred, or Hash types belonging to a and b shall be swappable 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 lvalues of type allocator_type shall be swappable 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.

If the iterator type of a container belongs to the bidirectional or random access iterator categories, the container is called reversible and meets the additional requirements in Table 74 .

Table 74: Reversible container requirements [tab:container.rev.req]

Expression	Return type	Assertion/note	Complexity
		pre-/post-condition
X::reverse_iterator	iterator type whose value type is T	reverse_iterator<iterator>	compile time
X::const_reverse_iterator	constant iterator type whose value type is T	reverse_iterator<const_iterator>	compile time
a.rbegin()	reverse_iterator; const_reverse_iterator for constant a	reverse_iterator(end())	constant
a.rend()	reverse_iterator; const_reverse_iterator for constant a	reverse_iterator(begin())	constant
a.crbegin()	const_reverse_iterator	const_cast<X const&>(a).rbegin()	constant
a.crend()	const_reverse_iterator	const_cast<X const&>(a).rend()	constant

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:

(11.1)
if an exception is thrown by an insert() or emplace() function while inserting a single element, that function has no effects.
(11.2)
if an exception is thrown by a push_back(), push_front(), emplace_back(), or emplace_front() function, that function has no effects.
(11.3)
no erase(), clear(), pop_back() or pop_front() function throws an exception.
(11.4)
no copy constructor or assignment operator of a returned iterator throws an exception.
(11.5)
no swap() function throws an exception.
(11.6)
no swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped.

[ Note
:
The end() iterator does not refer to any element, so it may 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]).

Table 75 lists operations that are provided for some types of containers but not others.

Those containers for which the listed operations are provided shall implement the semantics described in Table 75 unless otherwise stated.

If the iterators passed to lexicographical_compare_three_way meet the constexpr iterator requirements ([iterator.requirements.general]) then the operations described in Table 75 are implemented by constexpr functions.

Table 75: Optional container operations [tab:container.opt]

Expression	Return type	Operational	Assertion/note	Complexity
		semantics	pre-/post-condition
a <=> b	synth-three-way-result<value_type>	lexicographical_compare_three_way(a.begin(), a.end(), b.begin(), b.end(), synth-three-way)	Preconditions: Either <=> is defined for values of type (possibly const) T, or < is defined for values of type (possibly const) T and < is a total ordering relationship.	linear

[ Note

The algorithm lexicographical_compare_three_way is defined in [algorithms].

— end note

]

All of the containers defined in this Clause and in [basic.string] except array meet the additional requirements of an allocator-aware container, as described in Table 76 .

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 of type (possibly const) T, and an rvalue rv of type T, the following terms are defined.

If X is not allocator-aware, 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:

(15.1)
T is Cpp17DefaultInsertable into X means that the following expression is well-formed:
```
allocator_traits<A>::construct(m, p)
```
(15.2)
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.
(15.3)
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
:
rv remains a valid object.

Its state is unspecified
— end note
]
(15.4)
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.
(15.5)
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)
```
(15.6)
T is Cpp17Erasable from X means that the following expression is well-formed:
```
allocator_traits<A>::destroy(m, p)
```

[ Note

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 may choose a different definition.

— end note

]

In Table 76, X denotes an allocator-aware container class with a value_type of T using allocator of type A, u denotes a variable, a and b denote non-const lvalues of type X, t denotes an lvalue or a const rvalue of type X, rv denotes a non-const rvalue of type X, and m is a value of type A.

Table 76: Allocator-aware container requirements [tab:container.alloc.req]

Expression	Return type	Assertion/note	Complexity
		pre-/post-condition
allocator_type	A	Mandates: allocator_type::value_type is the same as X::value_type.	compile time
get_- allocator()	A		constant
X() X u;		Preconditions: A meets the Cpp17DefaultConstructible requirements. Postconditions: u.empty() returns true, u.get_allocator() == A()	constant
X(m)		Postconditions: u.empty() returns true,	constant
X u(m);		u.get_allocator() == m
X(t, m) X u(t, m);		Preconditions: T is Cpp17CopyInsertable into X. Postconditions: u == t, u.get_allocator() == m	linear
X(rv) 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.	constant
X(rv, m) 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	constant if m == rv.get_allocator(), otherwise linear
a = t	X&	Preconditions: T is Cpp17CopyInsertable into X and Cpp17CopyAssignable. Postconditions: a == t	linear
a = rv	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: a is equal to the value that rv had before this assignment.	linear
a.swap(b)	void	Effects: exchanges the contents of a and b	constant

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:

(17.1)
The qualified-id A::value_type is valid and denotes a type ([temp.deduct]).
(17.2)
The expression declval<A&>().allocate(size_t{}) is well-formed when treated as an unevaluated operand.

22.2.2 Container data races [container.requirements.dataraces]

For purposes of avoiding 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

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 may result in a data race.

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

— end note

]

22.2.3 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 or queues, out of the basic sequence container kinds (or out of other kinds of sequence containers that the user might define).

[ Note

The sequence containers offer the programmer different complexity trade-offs and should be used accordingly.

vector is the type of sequence container that should be used by default.

array should be used when the container has a fixed size known during translation.

list or forward_list should be used when there are frequent insertions and deletions from the middle of the sequence.

deque is the data structure of choice when most insertions and deletions take place at the beginning or at the end of the sequence.

When choosing a container, remember vector is best; leave a comment to explain if you choose from the rest!

— end note

]

In Tables 77 and 78, X denotes a sequence container class, a denotes a value of type X containing elements of type T, u denotes the name of a variable being declared, 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, i and j denote iterators that meet the Cpp17InputIterator requirements and refer to elements implicitly convertible to value_type, [i, j) denotes a valid range, il designates an object of type initializer_list<value_type>, n denotes a value of type X::size_type, p denotes a valid constant iterator to a, q denotes a valid dereferenceable constant iterator to a, [q1, q2) denotes a valid range of constant iterators in a, t denotes an lvalue or a const rvalue of X::value_type, and rv denotes a non-const rvalue of X::value_type.

Args denotes a template parameter pack; args denotes a function parameter pack with the pattern Args&&.

The complexities of the expressions are sequence dependent.

Table 77: Sequence container requirements (in addition to container) [tab:container.seq.req]

Expression	Return type	Assertion/note
		pre-/post-condition
X(n, t) X u(n, t);		Preconditions: T is Cpp17CopyInsertable into X. Postconditions: distance(begin(), end()) == n Effects: Constructs a sequence container with n copies of t
X(i, j) 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. Postconditions: distance(begin(), end()) == distance(i, j) Effects: Constructs a sequence container equal to the range [i, j). Each iterator in the range [i, j) is dereferenced exactly once.
X(il)		Equivalent to X(il.begin(), il.end())
a = il	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)	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 : args may directly or indirectly refer to a value in a. — end note ]
a.insert(p,t)	iterator	Preconditions: T is Cpp17CopyInsertable into X. For vector and deque, T is also Cpp17CopyAssignable. Effects: Inserts a copy of t before p.
a.insert(p,rv)	iterator	Preconditions: T is Cpp17MoveInsertable into X. For vector and deque, T is also Cpp17MoveAssignable. Effects: Inserts a copy of rv before p.
a.insert(p,n,t)	iterator	Preconditions: T is Cpp17CopyInsertable into X and Cpp17CopyAssignable. Effects: Inserts n copies of t before p.
a.insert(p,i,j)	iterator	Preconditions: T is Cpp17EmplaceConstructible into X from *i. For vector and deque, T is also Cpp17MoveInsertable into X, Cpp17MoveConstructible, Cpp17MoveAssignable, and swappable ([swappable.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.
a.insert(p, il)	iterator	a.insert(p, il.begin(), il.end()).
a.erase(q)	iterator	Preconditions: For vector and deque, T is Cpp17MoveAssignable. Effects: Erases the element pointed to by q.
a.erase(q1,q2)	iterator	Preconditions: For vector and deque, T is Cpp17MoveAssignable. Effects: Erases the elements in the range [q1, q2).
a.clear()	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)	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) shall be dereferenced exactly once.
a.assign(il)	void	a.assign(il.begin(), il.end()).
a.assign(n,t)	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.

The iterator returned from a.insert(p, t) points to the copy of t inserted into a.

The iterator returned from a.insert(p, rv) points to the copy of rv inserted into a.

The iterator returned from a.insert(p, n, t) points to the copy of the first element inserted into a, or p if n == 0.

The iterator returned from a.insert(p, i, j) points to the copy of the first element inserted into a, or p if i == j.

The iterator returned from a.insert(p, il) points to the copy of the first element inserted into a, or p if il is empty.

The iterator returned from a.emplace(p, args) points to the new element constructed from args into a.

The iterator returned from a.erase(q) points to the element immediately following q prior to the element being erased.

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

The iterator returned by a.erase(q1, q2) points to the element pointed to by q2 prior to any elements being erased.

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

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

(13.1)
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.

(13.2)

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.

(13.3)
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.

Table 78 lists operations that are provided for some types of sequence containers but not others.

An implementation shall provide these operations for all container types shown in the “container” column, and shall implement them so as to take amortized constant time.

Table 78: Optional sequence container operations [tab:container.seq.opt]

Expression	Return type	Operational semantics	Container
a.front()	reference; const_reference for constant a	*a.begin()	basic_string, array, deque, forward_list, list, vector
a.back()	reference; const_reference for constant a	{ auto tmp = a.end(); --tmp; return *tmp; }	basic_string, array, deque, list, vector
a.emplace_front(args)	reference	Effects: Prepends an object of type T constructed with std::forward<Args>(args).... Preconditions: T is Cpp17EmplaceConstructible into X from args. Returns: a.front().	deque, forward_list, list
a.emplace_back(args)	reference	Effects: Appends an object of type T constructed with std::forward<Args>(args).... Preconditions: T is Cpp17EmplaceConstructible into X from args. For vector, T is also Cpp17MoveInsertable into X. Returns: a.back().	deque, list, vector
a.push_front(t)	void	Effects: Prepends a copy of t. Preconditions: T is Cpp17CopyInsertable into X.	deque, forward_list, list
a.push_front(rv)	void	Effects: Prepends a copy of rv. Preconditions: T is Cpp17MoveInsertable into X.	deque, forward_list, list
a.push_back(t)	void	Effects: Appends a copy of t. Preconditions: T is Cpp17CopyInsertable into X.	basic_string, deque, list, vector
a.push_back(rv)	void	Effects: Appends a copy of rv. Preconditions: T is Cpp17MoveInsertable into X.	basic_string, deque, list, vector
a.pop_front()	void	Effects: Destroys the first element. Preconditions: a.empty() is false.	deque, forward_list, list
a.pop_back()	void	Effects: Destroys the last element. Preconditions: a.empty() is false.	basic_string, deque, list, vector
a[n]	reference; const_reference for constant a	*(a.begin() + n)	basic_string, array, deque, vector
a.at(n)	reference; const_reference for constant a	*(a.begin() + n)	basic_string, array, deque, vector

The member function at() provides bounds-checked access to container elements.

at() throws out_of_range if n >= a.size().

22.2.4 Node handles [container.node]

22.2.4.1 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 79 .

Table 79: 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 Tables 80 and 81.
  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;
  using ator_traits = allocator_traits<allocator_type>;

  typename ator_traits::template rebind_traits<container_node_type>::pointer ptr_;
  optional<allocator_type> alloc_;

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);
  }
};

22.2.4.2 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.

22.2.4.3 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.

22.2.4.4 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.

22.2.4.5 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_).

22.2.5 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.

22.2.6 Associative containers [associative.reqmts]

Associative containers provide fast retrieval of data based on keys.

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

Each associative container is parameterized on Key and an ordering relation Compare that induces a strict weak ordering 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

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

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 associative containers meet all the requirements of Allocator-aware containers, except that for map and multimap, the requirements placed on value_type in Table 76 apply instead to key_type and mapped_type.

[ Note

For example, in some cases key_type and mapped_type are required to be Cpp17CopyAssignable even though the associated value_type, pair<const key_type, mapped_type>, is not Cpp17CopyAssignable.

— end note

]

In Table 80, X denotes an associative container class, a denotes a value of type X, a2 denotes a value of a type with nodes compatible with type X (Table 79), b denotes a possibly const value of type X, u denotes the name of a variable being declared, a_uniq denotes a value of type X when X supports unique keys, a_eq denotes a value of type X when X supports multiple keys, a_tran denotes a possibly const value of type X when the qualified-id X::key_compare::is_transparent is valid and denotes a type ([temp.deduct]), i and j meet the Cpp17InputIterator requirements and refer to elements implicitly convertible to value_type, [i, j) denotes a valid range, p denotes a valid constant iterator to a, q denotes a valid dereferenceable constant iterator to a, r denotes a valid dereferenceable iterator to a, [q1, q2) denotes a valid range of constant iterators in a, il designates an object of type initializer_list<value_type>, t denotes a value of type X::value_type, k denotes a value of type X::key_type and c denotes a possibly const value of type X::key_compare; kl is a value such that a is partitioned ([alg.sorting]) with respect to c(r, kl), with r the key value of e and e in a; ku is a value such that a is partitioned with respect to !c(ku, r); ke is a value such that a is partitioned with respect to c(r, ke) and !c(ke, r), with c(r, ke) implying !c(ke, r).

A denotes the storage allocator used by X, if any, or allocator<X::value_type> otherwise, m denotes an allocator of a type convertible to A, and nh denotes a non-const rvalue of type X::node_type.

Table 80: Associative container requirements (in addition to container) [tab:container.assoc.req]

Expression	Return type	Assertion/note	Complexity
		pre-/post-condition
X::key_type	Key		compile time
X::mapped_type (map and multimap only)	T		compile time
X::value_type (set and multiset only)	Key	Preconditions: value_type is Cpp17Erasable from X	compile time
X::value_type (map and multimap only)	pair<const Key, T>	Preconditions: value_type is Cpp17Erasable from X	compile time
X::key_compare	Compare	Preconditions: key_compare is Cpp17CopyConstructible.	compile time
X::value_compare	a binary predicate type	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.	compile time
X::node_type	a specialization of a node-handle class template, such that the public nested types are the same types as the corresponding types in X.	see [container.node]	compile time
X(c) X u(c);		Effects: Constructs an empty container. Uses a copy of c as a comparison object.	constant
X() X u;		Preconditions: key_compare meets the Cpp17DefaultConstructible requirements. Effects: Constructs an empty container. Uses Compare() as a comparison object	constant
X(i,j,c) X u(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.	$N log N$ in general, where N has the value distance(i, j); linear if [i, j) is sorted with value_comp()
X(i,j) X u(i,j);		Preconditions: key_compare meets the Cpp17DefaultConstructible requirements. value_type is Cpp17EmplaceConstructible into X from *i. Effects: Same as above, but uses Compare() as a comparison object.	same as above
X(il)		same as X(il.begin(), il.end())	same as X(il.begin(), il.end())
X(il,c)		same as X(il.begin(), il.end(), c)	same as X(il.begin(), il.end(), c)
a = il	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.	$N log N$ in general, where N has the value il.size() + a.size(); linear if [il.begin(), il.end()) is sorted with value_comp()
b.key_comp()	X::key_compare	Returns: the comparison object out of which b was constructed.	constant
b.value_comp()	X::value_compare	Returns: an object of value_compare constructed out of the comparison object	constant
a_uniq.emplace(args)	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. 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.	logarithmic
a_eq.emplace(args)	iterator	Preconditions: value_type is Cpp17EmplaceConstructible into X from args. Effects: Inserts a value_type object t constructed with std::forward<Args>(args)... 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.	logarithmic
a.emplace_hint(p, args)	iterator	equivalent to a.emplace( std::forward<Args>(args)...). Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. The element is inserted as close as possible to the position just prior to p.	logarithmic in general, but amortized constant if the element is inserted right before p
a_uniq.insert(t)	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. 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.	logarithmic
a_eq.insert(t)	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.	logarithmic
a.insert(p, t)	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. Always returns the iterator pointing to the element with key equivalent to the key of t. t is inserted as close as possible to the position just prior to p.	logarithmic in general, but amortized constant if t is inserted right before p.
a.insert(i, j)	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.	$N log (a.size() + N)$ , where N has the value distance(i, j)
a.insert(il)	void	equivalent to a.insert(il.begin(), il.end())
a_uniq.insert(nh)	insert_return_type	Preconditions: nh is empty or a_uniq.get_allocator() == nh.get_allocator(). 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(). Postconditions: If nh is empty, inserted is false, position is end(), and node is empty. Otherwise if the insertion took place, inserted is true, position points to the inserted element, and node is empty; if the insertion failed, inserted is false, node has the previous value of nh, and position points to an element with a key equivalent to nh.key().	logarithmic
a_eq.insert(nh)	iterator	Preconditions: nh is empty or a_eq.get_allocator() == nh.get_allocator(). 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.	logarithmic
a.insert(p, nh)	iterator	Preconditions: nh is empty or a.get_allocator() == nh.get_allocator(). 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. Always returns the iterator pointing to the element with key equivalent to nh.key(). 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.	logarithmic in general, but amortized constant if the element is inserted right before p.
a.extract(k)	node_type	Effects: Removes the first element in the container with key equivalent to k. Returns: A node_type owning the element if found, otherwise an empty node_type.	log(a.size())
a.extract(q)	node_type	Effects: Removes the element pointed to by q. Returns: A node_type owning that element.	amortized constant
a.merge(a2)	void	Preconditions: a.get_allocator() == a2.get_allocator(). 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. 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. Throws: Nothing unless the comparison object throws.	$N log (a.size()+ N)$ , where N has the value a2.size().
a.erase(k)	size_type	Effects: Erases all elements in the container with key equivalent to k. Returns: The number of erased elements.	$log (a.size()) + a.count(k)$
a.erase(q)	iterator	Effects: Erases the element pointed to by q. Returns: An iterator pointing to the element immediately following q prior to the element being erased. If no such element exists, returns a.end().	amortized constant
a.erase(r)	iterator	Effects: Erases the element pointed to by r. Returns: An iterator pointing to the element immediately following r prior to the element being erased. If no such element exists, returns a.end().	amortized constant
a.erase( q1, q2)	iterator	Effects: Erases all the elements in the range [q1, q2). 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.	$log (a.size()) + N$ , where N has the value distance(q1, q2).
a.clear()	void	Effects: Equivalent to a.erase(a.begin(), a.end()). Postconditions: a.empty() is true.	linear in a.size().
b.find(k)	iterator; const_iterator for constant b.	Returns: An iterator pointing to an element with the key equivalent to k, or b.end() if such an element is not found.	logarithmic
a_tran. find(ke)	iterator; const_iterator for constant a_tran.	Returns: An iterator pointing to an element with key r such that !c(r, ke) && !c(ke, r), or a_tran.end() if such an element is not found.	logarithmic
b.count(k)	size_type	Returns: The number of elements with key equivalent to k.	$log (b.size()) + b.count(k)$
a_tran. count(ke)	size_type	Returns: The number of elements with key r such that !c(r, ke) && !c(ke, r)	$log (a_tran.size()) + a_tran.count(ke)$
b. contains(k)	bool	Effects: Equivalent to: return b.find(k) != b.end();	logarithmic
a_tran. contains(ke)	bool	Effects: Equivalent to: return a_tran.find(ke) != a_tran.end();	logarithmic
b.lower_bound(k)	iterator; const_iterator for constant b.	Returns: An iterator pointing to the first element with key not less than k, or b.end() if such an element is not found.	logarithmic
a_tran. lower_bound(kl)	iterator; const_iterator for constant a_tran.	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.	logarithmic
b.upper_bound(k)	iterator; const_iterator for constant b.	Returns: An iterator pointing to the first element with key greater than k, or b.end() if such an element is not found.	logarithmic
a_tran. upper_bound(ku)	iterator; const_iterator for constant a_tran.	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.	logarithmic
b.equal_range(k)	pair<iterator, iterator>; pair<const_iterator, const_iterator> for constant b.	Effects: Equivalent to: return make_pair(b.lower_bound(k), b.upper_bound(k));	logarithmic
a_tran. equal_range(ke)	pair<iterator, iterator>; pair<const_iterator, const_iterator> for constant a_tran.	Effects: Equivalent to: return make_pair( a_tran.lower_bound(ke), a_tran.upper_bound(ke));	logarithmic

The insert 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.

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.

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

For associative containers with unique keys the stronger condition holds:

value_comp(*i, *j) != false

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.

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

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

(15.1)
It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.
(15.2)
It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.
(15.3)
It has a Compare template parameter and a type that qualifies as an allocator is deduced for that parameter.

22.2.6.1 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).

22.2.7 Unordered associative containers [unord.req]

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, 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

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

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.

The unordered associative containers meet all the requirements of Allocator-aware containers, except that for unordered_map and unordered_multimap, the requirements placed on value_type in Table 76 apply instead to key_type and mapped_type.

[ Note

For example, key_type and mapped_type are sometimes required to be Cpp17CopyAssignable even though the associated value_type, pair<const key_type, mapped_type>, is not Cpp17CopyAssignable.

— end note

]

In Table 81:

(11.1)
X denotes an unordered associative container class,
(11.2)
a denotes a value of type X,
(11.3)
a2 denotes a value of a type with nodes compatible with type X (Table 79),
(11.4)
b denotes a possibly const value of type X,
(11.5)
a_uniq denotes a value of type X when X supports unique keys,
(11.6)
a_eq denotes a value of type X when X supports equivalent keys,
(11.7)
a_tran denotes a possibly const value of type X when the qualified-ids X::key_equal::is_transparent and X::hasher::is_transparent are both valid and denote types ([temp.deduct]),
(11.8)
i and j denote input iterators that refer to value_type,
(11.9)
[i, j) denotes a valid range,
(11.10)
p and q2 denote valid constant iterators to a,
(11.11)
q and q1 denote valid dereferenceable constant iterators to a,
(11.12)
r denotes a valid dereferenceable iterator to a,
(11.13)
[q1, q2) denotes a valid range in a,
(11.14)
il denotes a value of type initializer_list<value_type>,
(11.15)
t denotes a value of type X::value_type,
(11.16)
k denotes a value of type key_type,
(11.17)
hf denotes a possibly const value of type hasher,
(11.18)
eq denotes a possibly const value of type key_equal,
(11.19)
ke is a value such that
- (11.19.1)
  eq(r1, ke) == eq(ke, r1)
- (11.19.2)
  hf(r1) == hf(ke) if eq(r1, ke) is true, and
- (11.19.3)
  (eq(r1, ke) && eq(r1, r2)) == eq(r2, ke)
where r1 and r2 are keys of elements in a_tran,
(11.20)
n denotes a value of type size_type,
(11.21)
z denotes a value of type float, and
(11.22)
nh denotes a non-const rvalue of type X::node_type.

Table 81: Unordered associative container requirements (in addition to container) [tab:container.hash.req]

Expression	Return type	Assertion/note	Complexity
		pre-/post-condition
X::key_type	Key		compile time
X::mapped_type (unordered_map and unordered_multimap only)	T		compile time
X::value_type (unordered_set and unordered_multiset only)	Key	Preconditions: value_type is Cpp17Erasable from X	compile time
X::value_type (unordered_map and unordered_multimap only)	pair<const Key, T>	Preconditions: value_type is Cpp17Erasable from X	compile time
X::hasher	Hash	Preconditions: Hash is a unary function object type such that the expression hf(k) has type size_t.	compile time
X::key_equal	Pred	Preconditions: Pred meets the Cpp17CopyConstructible requirements. Pred is a binary predicate that takes two arguments of type Key. Pred is an equivalence relation.	compile time
X::local_iterator	An iterator type whose category, value type, difference type, and pointer and reference types are the same as X::iterator's.	A local_iterator object may be used to iterate through a single bucket, but may not be used to iterate across buckets.	compile time
X::const_local_iterator	An iterator type whose category, value type, difference type, and pointer and reference types are the same as X::const_iterator's.	A const_local_iterator object may be used to iterate through a single bucket, but may not be used to iterate across buckets.	compile time
X::node_type	a specialization of a node-handle class template, such that the public nested types are the same types as the corresponding types in X.	see [container.node]	compile time
X(n, hf, eq) X a(n, hf, eq);	X	Effects: Constructs an empty container with at least n buckets, using hf as the hash function and eq as the key equality predicate.	$O (n)$
X(n, hf) X a(n, hf);	X	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.	$O (n)$
X(n) X a(n);	X	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.	$O (n)$
X() X a;	X	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.	constant
X(i, j, n, hf, eq) X a(i, j, n, hf, eq);	X	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.	Average case $O (N)$ (N is distance(i, j)), worst case $O (N^{2})$
X(i, j, n, hf) X a(i, j, n, hf);	X	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.	Average case $O (N)$ (N is distance(i, j)), worst case $O (N^{2})$
X(i, j, n) X a(i, j, n);	X	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.	Average case $O (N)$ (N is distance(i, j)), worst case $O (N^{2})$
X(i, j) X a(i, j);	X	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.	Average case $O (N)$ (N is distance(i, j)), worst case $O (N^{2})$
X(il)	X	Same as X(il.begin(), il.end()).	Same as X(il.begin(), il.end()).
X(il, n)	X	Same as X(il.begin(), il.end(), n).	Same as X(il.begin(), il.end(), n).
X(il, n, hf)	X	Same as X(il.begin(), il.end(), n, hf).	Same as X(il.begin(), il.end(), n, hf).
X(il, n, hf, eq)	X	Same as X(il.begin(), il.end(), n, hf, eq).	Same as X(il.begin(), il.end(), n, hf, eq).
X(b) X a(b);	X	Copy constructor. In addition to the requirements of Table 73, copies the hash function, predicate, and maximum load factor.	Average case linear in b.size(), worst case quadratic.
a = b	X&	Copy assignment operator. In addition to the requirements of Table 73, copies the hash function, predicate, and maximum load factor.	Average case linear in b.size(), worst case quadratic.
a = il	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.	Same as a = X(il).
b.hash_function()	hasher	Returns: b's hash function.	constant
b.key_eq()	key_equal	Returns: b's key equality predicate.	constant
a_uniq. emplace(args)	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. 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.	Average case $O (1)$ , worst case $O (a_uniq. size())$ .
a_eq.emplace(args)	iterator	Preconditions: value_type is Cpp17EmplaceConstructible into X from args. Effects: Inserts a value_type object t constructed with std::forward<Args>(args)... and returns the iterator pointing to the newly inserted element.	Average case $O (1)$ , worst case $O (a_eq. size())$ .
a.emplace_hint(p, args)	iterator	Preconditions: value_type is Cpp17EmplaceConstructible into X from args. Effects: Equivalent to a.emplace( std::forward<Args>(args)...). Return value is 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.	Average case $O (1)$ , worst case $O (a. size())$ .
a_uniq.insert(t)	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. 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.	Average case $O (1)$ , worst case $O (a_uniq. size())$ .
a_eq.insert(t)	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 an iterator pointing to the newly inserted element.	Average case $O (1)$ , worst case $O (a_eq. size())$ .
a.insert(p, t)	iterator	Preconditions: If t is a non-const rvalue, value_type is Cpp17MoveInsertable into X; otherwise, value_type is Cpp17CopyInsertable into X. Effects: Equivalent to a.insert(t). Return value is an iterator pointing to the element with the key equivalent to that of t. The iterator p is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.	Average case $O (1)$ , worst case $O (a.size())$ .
a.insert(i, j)	void	Preconditions: value_type is Cpp17EmplaceConstructible into X from *i. Neither i nor j are iterators into a. Effects: Equivalent to a.insert(t) for each element in [i,j).	Average case $O (N)$ , where N is distance(i, j), worst case $O (N (a.size() + 1))$ .
a.insert(il)	void	Same as a.insert(il.begin(), il.end()).	Same as a.insert( il.begin(), il.end()).
a_uniq. insert(nh)	insert_return_type	Preconditions: nh is empty or a_uniq.get_allocator() == nh.get_allocator(). 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(). Postconditions: If nh is empty, inserted is false, position is end(), and node is empty. Otherwise if the insertion took place, inserted is true, position points to the inserted element, and node is empty; if the insertion failed, inserted is false, node has the previous value of nh, and position points to an element with a key equivalent to nh.key().	Average case $O (1)$ , worst case $O (a_uniq. size())$ .
a_eq. insert(nh)	iterator	Preconditions: nh is empty or a_eq.get_allocator() == nh.get_allocator(). 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. Postconditions: nh is empty.	Average case $O (1)$ , worst case $O (a_eq. size())$ .
a.insert(q, nh)	iterator	Preconditions: nh is empty or a.get_allocator() == nh.get_allocator(). 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. Always returns the iterator pointing to the element with key equivalent to nh.key(). The iterator q is a hint pointing to where the search should start. Implementations are permitted to ignore the hint. Postconditions: nh is empty if insertion succeeds, unchanged if insertion fails.	Average case $O (1)$ , worst case $O (a.size())$ .
a.extract(k)	node_type	Effects: Removes an element in the container with key equivalent to k. Returns: A node_type owning the element if found, otherwise an empty node_type.	Average case $O (1)$ , worst case $O (a.size())$ .
a.extract(q)	node_type	Effects: Removes the element pointed to by q. Returns: A node_type owning that element.	Average case $O (1)$ , worst case $O (a.size())$ .
a.merge(a2)	void	Preconditions: a.get_allocator() == a2.get_allocator(). 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. 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.	Average case $O (N)$ , where N is a2.size(), worst case $O (N *a.size() + N)$ .
a.erase(k)	size_type	Effects: Erases all elements with key equivalent to k. Returns: The number of elements erased.	Average case $O (a.count(k))$ , worst case $O (a.size())$ .
a.erase(q)	iterator	Effects: Erases the element pointed to by q. Returns: The iterator immediately following q prior to the erasure.	Average case $O (1)$ , worst case $O (a.size())$ .
a.erase(r)	iterator	Effects: Erases the element pointed to by r. Returns: The iterator immediately following r prior to the erasure.	Average case $O (1)$ , worst case $O (a.size())$ .
a.erase(q1, q2)	iterator	Effects: Erases all elements in the range [q1, q2). Returns: The iterator immediately following the erased elements prior to the erasure.	Average case linear in distance(q1, q2), worst case $O (a.size())$ .
a.clear()	void	Effects: Erases all elements in the container. Postconditions: a.empty() is true	Linear in a.size().
b.find(k)	iterator; const_iterator for const b.	Returns: An iterator pointing to an element with key equivalent to k, or b.end() if no such element exists.	Average case $O (1)$ , worst case $O (b.size())$ .
a_tran.find(ke)	iterator; const_iterator for const a_tran.	Returns: An iterator pointing to an element with key equivalent to ke, or a_tran.end() if no such element exists.	Average case $O (1)$ , worst case $O (a_tran. size())$ .
b.count(k)	size_type	Returns: The number of elements with key equivalent to k.	Average case $O (b.count(k))$ , worst case $O (b.size())$ .
a_tran.count(ke)	size_type	Returns: The number of elements with key equivalent to ke.	Average case $O (a_tran. count(ke))$ , worst case $O (a_tran. size())$ .
b.contains(k)	bool	Effects: Equivalent to b.find(k) != b.end()	Average case $O (1)$ , worst case $O (b.size())$ .
a_tran.contains(ke)	bool	Effects: Equivalent to a_tran.find(ke) != a_tran.end()	Average case $O (1)$ , worst case $O (a_tran. size())$ .
b.equal_range(k)	pair<iterator, iterator>; pair<const_iterator, const_iterator> for const b.	Returns: A range containing all elements with keys equivalent to k. Returns make_pair(b.end(), b.end()) if no such elements exist.	Average case $O (b.count(k))$ , worst case $O (b.size())$ .
a_tran.equal_range(ke)	pair<iterator, iterator>; pair<const_iterator, const_iterator> for const a_tran.	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.	Average case $O (a_tran. count(ke))$ , worst case $O (a_tran. size())$ .
b.bucket_count()	size_type	Returns: The number of buckets that b contains.	Constant
b.max_bucket_count()	size_type	Returns: An upper bound on the number of buckets that b might ever contain.	Constant
b.bucket(k)	size_type	Preconditions: b.bucket_count() > 0. Returns: The index of the bucket in which elements with keys equivalent to k would be found, if any such element existed. Postconditions: The return value shall be in the range [0, b.bucket_count()).	Constant
b.bucket_size(n)	size_type	Preconditions: n shall be in the range [0, b.bucket_count()). Returns: The number of elements in the $n^{th}$ bucket.	$O (b.bucket_size(n))$
b.begin(n)	local_iterator; const_local_iterator for const b.	Preconditions: n is in the range [0, b.bucket_count()). Returns: An iterator referring to the first element in the bucket. If the bucket is empty, then b.begin(n) == b.end(n).	Constant
b.end(n)	local_iterator; const_local_iterator for const b.	Preconditions: n is in the range [0, b.bucket_count()). Returns: An iterator which is the past-the-end value for the bucket.	Constant
b.cbegin(n)	const_local_iterator	Preconditions: n shall be in the range [0, b.bucket_count()). Returns: An iterator referring to the first element in the bucket. If the bucket is empty, then b.cbegin(n) == b.cend(n).	Constant
b.cend(n)	const_local_iterator	Preconditions: n is in the range [0, b.bucket_count()). Returns: An iterator which is the past-the-end value for the bucket.	Constant
b.load_factor()	float	Returns: The average number of elements per bucket.	Constant
b.max_load_factor()	float	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.	Constant
a.max_load_factor(z)	void	Preconditions: z is positive. May change the container's maximum load factor, using z as a hint.	Constant
a.rehash(n)	void	Postconditions: a.bucket_count() >= a.size() / a.max_load_factor() and a.bucket_count() >= n.	Average case linear in a.size(), worst case quadratic.
a.reserve(n)	void	Same as a.rehash(ceil(n / a.max_load_factor())).	Average case linear in a.size(), worst case quadratic.

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 refinement224 of the partition into equivalent-key groups produced by Pred.

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.

The insert 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.

The insert 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.

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.

The member function templates find, count, equal_range, and contains 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]).

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

(18.1)
It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.
(18.2)
It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.
(18.3)
It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.
(18.4)
It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

224)

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

⮥

22.2.7.1 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.

22.3 Sequence containers [sequences]

22.3.1 In general [sequences.general]

The headers <array> ([array.syn]), <deque> ([deque.syn]), <forward_list> ([forward.list.syn]), <list> ([list.syn]), and <vector> ([vector.syn]) define class templates that meet the requirements for sequence containers.

The following exposition-only alias template may appear in deduction guides for sequence containers:

template<class InputIterator>
  using iter-value-type = typename iterator_traits<InputIterator>::value_type;  // exposition only

22.3.2 Header <array> synopsis [array.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  // [array], class template array
  template<class T, size_t N> struct array;

  template<class T, size_t N>
    constexpr bool operator==(const array<T, N>& x, const array<T, N>& y);
  template<class T, size_t N>
    constexpr synth-three-way-result<T>
      operator<=>(const array<T, N>& x, const array<T, N>& y);

  // [array.special], specialized algorithms
  template<class T, size_t N>
    constexpr void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));

  // [array.creation], array creation functions
  template<class T, size_t N>
    constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]);
  template<class T, size_t N>
    constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);

  // [array.tuple], tuple interface
  template<class T> struct tuple_size;
  template<size_t I, class T> struct tuple_element;
  template<class T, size_t N>
    struct tuple_size<array<T, N>>;
  template<size_t I, class T, size_t N>
    struct tuple_element<I, array<T, N>>;
  template<size_t I, class T, size_t N>
    constexpr T& get(array<T, N>&) noexcept;
  template<size_t I, class T, size_t N>
    constexpr T&& get(array<T, N>&&) noexcept;
  template<size_t I, class T, size_t N>
    constexpr const T& get(const array<T, N>&) noexcept;
  template<size_t I, class T, size_t N>
    constexpr const T&& get(const array<T, N>&&) noexcept;
}

22.3.3 Header <deque> synopsis [deque.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  // [deque], class template deque
  template<class T, class Allocator = allocator<T>> class deque;

  template<class T, class Allocator>
    bool operator==(const deque<T, Allocator>& x, const deque<T, Allocator>& y);
  template<class T, class Allocator>
    synth-three-way-result<T> operator<=>(const deque<T, Allocator>& x,
                                          const deque<T, Allocator>& y);

  template<class T, class Allocator>
    void swap(deque<T, Allocator>& x, deque<T, Allocator>& y)
      noexcept(noexcept(x.swap(y)));

  template<class T, class Allocator, class U>
    typename deque<T, Allocator>::size_type
      erase(deque<T, Allocator>& c, const U& value);
  template<class T, class Allocator, class Predicate>
    typename deque<T, Allocator>::size_type
      erase_if(deque<T, Allocator>& c, Predicate pred);

  namespace pmr {
    template<class T>
      using deque = std::deque<T, polymorphic_allocator<T>>;
  }
}

22.3.4 Header <forward_list> synopsis [forward.list.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  // [forwardlist], class template forward_list
  template<class T, class Allocator = allocator<T>> class forward_list;

  template<class T, class Allocator>
    bool operator==(const forward_list<T, Allocator>& x, const forward_list<T, Allocator>& y);
  template<class T, class Allocator>
    synth-three-way-result<T> operator<=>(const forward_list<T, Allocator>& x,
                                          const forward_list<T, Allocator>& y);

  template<class T, class Allocator>
    void swap(forward_list<T, Allocator>& x, forward_list<T, Allocator>& y)
      noexcept(noexcept(x.swap(y)));

  template<class T, class Allocator, class U>
    typename forward_list<T, Allocator>::size_type
      erase(forward_list<T, Allocator>& c, const U& value);
  template<class T, class Allocator, class Predicate>
    typename forward_list<T, Allocator>::size_type
      erase_if(forward_list<T, Allocator>& c, Predicate pred);

  namespace pmr {
    template<class T>
      using forward_list = std::forward_list<T, polymorphic_allocator<T>>;
  }
}

22.3.5 Header <list> synopsis [list.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  // [list], class template list
  template<class T, class Allocator = allocator<T>> class list;

  template<class T, class Allocator>
    bool operator==(const list<T, Allocator>& x, const list<T, Allocator>& y);
  template<class T, class Allocator>
    synth-three-way-result<T> operator<=>(const list<T, Allocator>& x,
                                          const list<T, Allocator>& y);

  template<class T, class Allocator>
    void swap(list<T, Allocator>& x, list<T, Allocator>& y)
      noexcept(noexcept(x.swap(y)));

  template<class T, class Allocator, class U>
    typename list<T, Allocator>::size_type
      erase(list<T, Allocator>& c, const U& value);
  template<class T, class Allocator, class Predicate>
    typename list<T, Allocator>::size_type
      erase_if(list<T, Allocator>& c, Predicate pred);

  namespace pmr {
    template<class T>
      using list = std::list<T, polymorphic_allocator<T>>;
  }
}

22.3.6 Header <vector> synopsis [vector.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  // [vector], class template vector
  template<class T, class Allocator = allocator<T>> class vector;

  template<class T, class Allocator>
    constexpr bool operator==(const vector<T, Allocator>& x, const vector<T, Allocator>& y);
  template<class T, class Allocator>
    constexpr synth-three-way-result<T> operator<=>(const vector<T, Allocator>& x,
                                                    const vector<T, Allocator>& y);

  template<class T, class Allocator>
    constexpr void swap(vector<T, Allocator>& x, vector<T, Allocator>& y)
      noexcept(noexcept(x.swap(y)));

  template<class T, class Allocator, class U>
    constexpr typename vector<T, Allocator>::size_type
      erase(vector<T, Allocator>& c, const U& value);
  template<class T, class Allocator, class Predicate>
    constexpr typename vector<T, Allocator>::size_type
      erase_if(vector<T, Allocator>& c, Predicate pred);

  // [vector.bool], class vector<bool>
  template<class Allocator> class vector<bool, Allocator>;

  // hash support
  template<class T> struct hash;
  template<class Allocator> struct hash<vector<bool, Allocator>>;

  namespace pmr {
    template<class T>
      using vector = std::vector<T, polymorphic_allocator<T>>;
  }
}

22.3.7 Class template array [array]

22.3.7.1 Overview [array.overview]

The header <array> defines a class template for storing fixed-size sequences of objects.

An array is a contiguous container .

An instance of array<T, N> stores N elements of type T, so that size() == N is an invariant.

An array is an aggregate that can be list-initialized with up to N elements whose types are convertible to T.

An array meets all of the requirements of a container and of a reversible container ([container.requirements]), except that a default constructed array object is not empty and that swap does not have constant complexity.

An array meets some of the requirements of a sequence container .

Descriptions are provided here only for operations on array that are not described in one of these tables and for operations where there is additional semantic information.

array<T, N> is a structural type ([temp.param]) if T is a structural type.

Two values a1 and a2 of type array<T, N> are template-argument-equivalent ([temp.type]) if and only if each pair of corresponding elements in a1 and a2 are template-argument-equivalent.

The types iterator and const_iterator meet the constexpr iterator requirements ([iterator.requirements.general]).

namespace std {
  template<class T, size_t N>
  struct array {
    // types
    using value_type             = T;
    using pointer                = T*;
    using const_pointer          = const T*;
    using reference              = T&;
    using const_reference        = const T&;
    using size_type              = size_t;
    using difference_type        = ptrdiff_t;
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

    // no explicit construct/copy/destroy for aggregate type

    constexpr void fill(const T& u);
    constexpr void swap(array&) noexcept(is_nothrow_swappable_v<T>);

    // iterators
    constexpr iterator               begin() noexcept;
    constexpr const_iterator         begin() const noexcept;
    constexpr iterator               end() noexcept;
    constexpr const_iterator         end() const noexcept;

    constexpr reverse_iterator       rbegin() noexcept;
    constexpr const_reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator       rend() noexcept;
    constexpr const_reverse_iterator rend() const noexcept;

    constexpr const_iterator         cbegin() const noexcept;
    constexpr const_iterator         cend() const noexcept;
    constexpr const_reverse_iterator crbegin() const noexcept;
    constexpr const_reverse_iterator crend() const noexcept;

    // capacity
    [[nodiscard]] constexpr bool empty() const noexcept;
    constexpr size_type size() const noexcept;
    constexpr size_type max_size() const noexcept;

    // element access
    constexpr reference       operator[](size_type n);
    constexpr const_reference operator[](size_type n) const;
    constexpr reference       at(size_type n);
    constexpr const_reference at(size_type n) const;
    constexpr reference       front();
    constexpr const_reference front() const;
    constexpr reference       back();
    constexpr const_reference back() const;

    constexpr T *       data() noexcept;
    constexpr const T * data() const noexcept;
  };

  template<class T, class... U>
    array(T, U...) -> array<T, 1 + sizeof...(U)>;
}

22.3.7.2 Constructors, copy, and assignment [array.cons]

The conditions for an aggregate shall be met.

Class array relies on the implicitly-declared special member functions ([class.default.ctor], [class.dtor], and [class.copy.ctor]) to conform to the container requirements table in [container.requirements].

In addition to the requirements specified in the container requirements table, the implicit move constructor and move assignment operator for array require that T be Cpp17MoveConstructible or Cpp17MoveAssignable, respectively.

🔗

template<class T, class... U>
  array(T, U...) -> array<T, 1 + sizeof...(U)>;

Mandates: (is_same_v<T, U> && ...) is true.

22.3.7.3 Member functions [array.members]

🔗

constexpr size_type size() const noexcept;

Returns: N.

🔗

constexpr T* data() noexcept;
constexpr const T* data() const noexcept;

Returns: A pointer such that [data(), data() + size()) is a valid range.

For a non-empty array, data() == addressof(front()).

🔗

constexpr void fill(const T& u);

Effects: As if by fill_n(begin(), N, u).

🔗

constexpr void swap(array& y) noexcept(is_nothrow_swappable_v<T>);

Effects: Equivalent to swap_ranges(begin(), end(), y.begin()).

[ Note

Unlike the swap function for other containers, array::swap takes linear time, may exit via an exception, and does not cause iterators to become associated with the other container.

— end note

]

22.3.7.4 Specialized algorithms [array.special]

🔗

template<class T, size_t N>
  constexpr void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));

Constraints: N == 0 or is_swappable_v<T> is true.

Effects: As if by x.swap(y).

Complexity: Linear in N.

22.3.7.5 Zero-sized arrays [array.zero]

array shall provide support for the special case N == 0.

In the case that N == 0, begin() == end() == unique value.

The return value of data() is unspecified.

The effect of calling front() or back() for a zero-sized array is undefined.

Member function swap() shall have a non-throwing exception specification.

22.3.7.6 Array creation functions [array.creation]

🔗

template<class T, size_t N>
  constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]);

Mandates: is_array_v<T> is false and is_constructible_v<T, T&> is true.

Preconditions: T meets the Cpp17CopyConstructible requirements.

Returns: {{ a[0], …, a[N - 1] }}.

🔗

template<class T, size_t N>
  constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);

Mandates: is_array_v<T> is false and is_move_constructible_v<T> is true.

Preconditions: T meets the Cpp17MoveConstructible requirements.

Returns: {{ std::move(a[0]), …, std::move(a[N - 1]) }}.

22.3.7.7 Tuple interface [array.tuple]

🔗

template<class T, size_t N>
  struct tuple_size<array<T, N>> : integral_constant<size_t, N> { };

🔗

template<size_t I, class T, size_t N>
  struct tuple_element<I, array<T, N>> {
    using type = T;
  };

Mandates: I < N is true.

🔗

template<size_t I, class T, size_t N>
  constexpr T& get(array<T, N>& a) noexcept;
template<size_t I, class T, size_t N>
  constexpr T&& get(array<T, N>&& a) noexcept;
template<size_t I, class T, size_t N>
  constexpr const T& get(const array<T, N>& a) noexcept;
template<size_t I, class T, size_t N>
  constexpr const T&& get(const array<T, N>&& a) noexcept;

Mandates: I < N is true.

Returns: A reference to the

I^{th}

element of a, where indexing is zero-based.

22.3.8 Class template deque [deque]

22.3.8.1 Overview [deque.overview]

A deque is a sequence container that supports random access iterators ([random.access.iterators]).

In addition, it supports constant time insert and erase operations at the beginning or the end; insert and erase in the middle take linear time.

That is, a deque is especially optimized for pushing and popping elements at the beginning and end.

Storage management is handled automatically.

A deque meets all of the requirements of a container, of a reversible container (given in tables in [container.requirements]), of a sequence container, including the optional sequence container requirements ([sequence.reqmts]), and of an allocator-aware container (Table 76).

Descriptions are provided here only for operations on deque that are not described in one of these tables or for operations where there is additional semantic information.

namespace std {
  template<class T, class Allocator = allocator<T>>
  class deque {
  public:
    // types
    using value_type             = T;
    using allocator_type         = Allocator;
    using pointer                = typename allocator_traits<Allocator>::pointer;
    using const_pointer          = typename allocator_traits<Allocator>::const_pointer;
    using reference              = value_type&;
    using const_reference        = const value_type&;
    using size_type              = implementation-defined; // see [container.requirements]
    using difference_type        = implementation-defined; // see [container.requirements]
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

    // [deque.cons], construct/copy/destroy
    deque() : deque(Allocator()) { }
    explicit deque(const Allocator&);
    explicit deque(size_type n, const Allocator& = Allocator());
    deque(size_type n, const T& value, const Allocator& = Allocator());
    template<class InputIterator>
      deque(InputIterator first, InputIterator last, const Allocator& = Allocator());
    deque(const deque& x);
    deque(deque&&);
    deque(const deque&, const Allocator&);
    deque(deque&&, const Allocator&);
    deque(initializer_list<T>, const Allocator& = Allocator());

    ~deque();
    deque& operator=(const deque& x);
    deque& operator=(deque&& x)
      noexcept(allocator_traits<Allocator>::is_always_equal::value);
    deque& operator=(initializer_list<T>);
    template<class InputIterator>
      void assign(InputIterator first, InputIterator last);
    void assign(size_type n, const T& t);
    void assign(initializer_list<T>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator               begin() noexcept;
    const_iterator         begin() const noexcept;
    iterator               end() noexcept;
    const_iterator         end() const noexcept;
    reverse_iterator       rbegin() noexcept;
    const_reverse_iterator rbegin() const noexcept;
    reverse_iterator       rend() noexcept;
    const_reverse_iterator rend() const noexcept;

    const_iterator         cbegin() const noexcept;
    const_iterator         cend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;

    // [deque.capacity], capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;
    void      resize(size_type sz);
    void      resize(size_type sz, const T& c);
    void      shrink_to_fit();

    // element access
    reference       operator[](size_type n);
    const_reference operator[](size_type n) const;
    reference       at(size_type n);
    const_reference at(size_type n) const;
    reference       front();
    const_reference front() const;
    reference       back();
    const_reference back() const;

    // [deque.modifiers], modifiers
    template<class... Args> reference emplace_front(Args&&... args);
    template<class... Args> reference emplace_back(Args&&... args);
    template<class... Args> iterator emplace(const_iterator position, Args&&... args);

    void push_front(const T& x);
    void push_front(T&& x);
    void push_back(const T& x);
    void push_back(T&& x);

    iterator insert(const_iterator position, const T& x);
    iterator insert(const_iterator position, T&& x);
    iterator insert(const_iterator position, size_type n, const T& x);
    template<class InputIterator>
      iterator insert(const_iterator position, InputIterator first, InputIterator last);
    iterator insert(const_iterator position, initializer_list<T>);

    void pop_front();
    void pop_back();

    iterator erase(const_iterator position);
    iterator erase(const_iterator first, const_iterator last);
    void     swap(deque&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value);
    void     clear() noexcept;
  };

  template<class InputIterator, class Allocator = allocator<iter-value-type<InputIterator>>>
    deque(InputIterator, InputIterator, Allocator = Allocator())
      -> deque<iter-value-type<InputIterator>, Allocator>;

  // swap
  template<class T, class Allocator>
    void swap(deque<T, Allocator>& x, deque<T, Allocator>& y)
      noexcept(noexcept(x.swap(y)));
}

22.3.8.2 Constructors, copy, and assignment [deque.cons]

🔗

explicit deque(const Allocator&);

Effects: Constructs an empty deque, using the specified allocator.

Complexity: Constant.

🔗

explicit deque(size_type n, const Allocator& = Allocator());

Effects: Constructs a deque with n default-inserted elements using the specified allocator.

Preconditions: T is Cpp17DefaultInsertable into *this.

Complexity: Linear in n.

🔗

deque(size_type n, const T& value, const Allocator& = Allocator());

Effects: Constructs a deque with n copies of value, using the specified allocator.

Preconditions: T is Cpp17CopyInsertable into *this.

Complexity: Linear in n.

🔗

template<class InputIterator>
  deque(InputIterator first, InputIterator last, const Allocator& = Allocator());

Effects: Constructs a deque equal to the range [first, last), using the specified allocator.

Complexity: Linear in distance(first, last).

22.3.8.3 Capacity [deque.capacity]

🔗

void resize(size_type sz);

Preconditions: T is Cpp17MoveInsertable and Cpp17DefaultInsertable into *this.

Effects: If sz < size(), erases the last size() - sz elements from the sequence.

Otherwise, appends sz - size() default-inserted elements to the sequence.

🔗

void resize(size_type sz, const T& c);

Preconditions: T is Cpp17CopyInsertable into *this.

Effects: If sz < size(), erases the last size() - sz elements from the sequence.

Otherwise, appends sz - size() copies of c to the sequence.

🔗

void shrink_to_fit();

Preconditions: T is Cpp17MoveInsertable into *this.

Effects: shrink_to_fit is a non-binding request to reduce memory use but does not change the size of the sequence.

[ Note

The request is non-binding to allow latitude for implementation-specific optimizations.

— end note

]

If the size is equal to the old capacity, or if an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T, then there are no effects.

Complexity: If the size is not equal to the old capacity, linear in the size of the sequence; otherwise constant.

Remarks: If the size is not equal to the old capacity, then invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator.

22.3.8.4 Modifiers [deque.modifiers]

🔗

iterator insert(const_iterator position, const T& x);
iterator insert(const_iterator position, T&& x);
iterator insert(const_iterator position, size_type n, const T& x);
template<class InputIterator>
  iterator insert(const_iterator position,
                  InputIterator first, InputIterator last);
iterator insert(const_iterator position, initializer_list<T>);

template<class... Args> reference emplace_front(Args&&... args);
template<class... Args> reference emplace_back(Args&&... args);
template<class... Args> iterator emplace(const_iterator position, Args&&... args);
void push_front(const T& x);
void push_front(T&& x);
void push_back(const T& x);
void push_back(T&& x);

Effects: An insertion in the middle of the deque invalidates all the iterators and references to elements of the deque.

An insertion at either end of the deque invalidates all the iterators to the deque, but has no effect on the validity of references to elements of the deque.

Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of T there are no effects.

If an exception is thrown while inserting a single element at either end, there are no effects.

Otherwise, if an exception is thrown by the move constructor of a non-Cpp17CopyInsertable T, the effects are unspecified.

Complexity: The complexity is linear in the number of elements inserted plus the lesser of the distances to the beginning and end of the deque.

Inserting a single element at either the beginning or end of a deque always takes constant time and causes a single call to a constructor of T.

🔗

iterator erase(const_iterator position);
iterator erase(const_iterator first, const_iterator last);
void pop_front();
void pop_back();

Effects: An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements.

An erase operation that erases the first element of a deque but not the last element invalidates only iterators and references to the erased elements.

An erase operation that erases neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque.

[ Note

pop_front and pop_back are erase operations.

— end note

]

Complexity: The number of calls to the destructor of T is the same as the number of elements erased, but the number of calls to the assignment operator of T is no more than the lesser of the number of elements before the erased elements and the number of elements after the erased elements.

Throws: Nothing unless an exception is thrown by the assignment operator of T.

22.3.8.5 Erasure [deque.erasure]

🔗

template<class T, class Allocator, class U>
  typename deque<T, Allocator>::size_type
    erase(deque<T, Allocator>& c, const U& value);

Effects: Equivalent to:

auto it = remove(c.begin(), c.end(), value);
auto r = distance(it, c.end());
c.erase(it, c.end());
return r;

🔗

template<class T, class Allocator, class Predicate>
  typename deque<T, Allocator>::size_type
    erase_if(deque<T, Allocator>& c, Predicate pred);

Effects: Equivalent to:

auto it = remove_if(c.begin(), c.end(), pred);
auto r = distance(it, c.end());
c.erase(it, c.end());
return r;

22.3.9 Class template forward_list [forwardlist]

22.3.9.1 Overview [forwardlist.overview]

A forward_list is a container that supports forward iterators and allows constant time insert and erase operations anywhere within the sequence, with storage management handled automatically.

Fast random access to list elements is not supported.

[ Note

It is intended that forward_list have zero space or time overhead relative to a hand-written C-style singly linked list.

Features that would conflict with that goal have been omitted.

— end note

]

A forward_list meets all of the requirements of a container (Table 73), except that the size() member function is not provided and operator== has linear complexity.

A forward_list also meets all of the requirements for an allocator-aware container (Table 76).

In addition, a forward_list provides the assign member functions (Table 77) and several of the optional container requirements (Table 78).

Descriptions are provided here only for operations on forward_list that are not described in that table or for operations where there is additional semantic information.

[ Note

Modifying any list requires access to the element preceding the first element of interest, but in a forward_list there is no constant-time way to access a preceding element.

For this reason, ranges that are modified, such as those supplied to erase and splice, must be open at the beginning.

— end note

]

namespace std {
  template<class T, class Allocator = allocator<T>>
  class forward_list {
  public:
    // types
    using value_type      = T;
    using allocator_type  = Allocator;
    using pointer         = typename allocator_traits<Allocator>::pointer;
    using const_pointer   = typename allocator_traits<Allocator>::const_pointer;
    using reference       = value_type&;
    using const_reference = const value_type&;
    using size_type       = implementation-defined; // see [container.requirements]
    using difference_type = implementation-defined; // see [container.requirements]
    using iterator        = implementation-defined; // see [container.requirements]
    using const_iterator  = implementation-defined; // see [container.requirements]

    // [forwardlist.cons], construct/copy/destroy
    forward_list() : forward_list(Allocator()) { }
    explicit forward_list(const Allocator&);
    explicit forward_list(size_type n, const Allocator& = Allocator());
    forward_list(size_type n, const T& value, const Allocator& = Allocator());
    template<class InputIterator>
      forward_list(InputIterator first, InputIterator last, const Allocator& = Allocator());
    forward_list(const forward_list& x);
    forward_list(forward_list&& x);
    forward_list(const forward_list& x, const Allocator&);
    forward_list(forward_list&& x, const Allocator&);
    forward_list(initializer_list<T>, const Allocator& = Allocator());
    ~forward_list();
    forward_list& operator=(const forward_list& x);
    forward_list& operator=(forward_list&& x)
      noexcept(allocator_traits<Allocator>::is_always_equal::value);
    forward_list& operator=(initializer_list<T>);
    template<class InputIterator>
      void assign(InputIterator first, InputIterator last);
    void assign(size_type n, const T& t);
    void assign(initializer_list<T>);
    allocator_type get_allocator() const noexcept;

    // [forwardlist.iter], iterators
    iterator before_begin() noexcept;
    const_iterator before_begin() const noexcept;
    iterator begin() noexcept;
    const_iterator begin() const noexcept;
    iterator end() noexcept;
    const_iterator end() const noexcept;

    const_iterator cbegin() const noexcept;
    const_iterator cbefore_begin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type max_size() const noexcept;

    // [forwardlist.access], element access
    reference front();
    const_reference front() const;

    // [forwardlist.modifiers], modifiers
    template<class... Args> reference emplace_front(Args&&... args);
    void push_front(const T& x);
    void push_front(T&& x);
    void pop_front();

    template<class... Args> iterator emplace_after(const_iterator position, Args&&... args);
    iterator insert_after(const_iterator position, const T& x);
    iterator insert_after(const_iterator position, T&& x);

    iterator insert_after(const_iterator position, size_type n, const T& x);
    template<class InputIterator>
      iterator insert_after(const_iterator position, InputIterator first, InputIterator last);
    iterator insert_after(const_iterator position, initializer_list<T> il);

    iterator erase_after(const_iterator position);
    iterator erase_after(const_iterator position, const_iterator last);
    void swap(forward_list&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value);

    void resize(size_type sz);
    void resize(size_type sz, const value_type& c);
    void clear() noexcept;

    // [forwardlist.ops], forward_list operations
    void splice_after(const_iterator position, forward_list& x);
    void splice_after(const_iterator position, forward_list&& x);
    void splice_after(const_iterator position, forward_list& x, const_iterator i);
    void splice_after(const_iterator position, forward_list&& x, const_iterator i);
    void splice_after(const_iterator position, forward_list& x,
                      const_iterator first, const_iterator last);
    void splice_after(const_iterator position, forward_list&& x,
                      const_iterator first, const_iterator last);

    size_type remove(const T& value);
    template<class Predicate> size_type remove_if(Predicate pred);

    size_type unique();
    template<class BinaryPredicate> size_type unique(BinaryPredicate binary_pred);

    void merge(forward_list& x);
    void merge(forward_list&& x);
    template<class Compare> void merge(forward_list& x, Compare comp);
    template<class Compare> void merge(forward_list&& x, Compare comp);

    void sort();
    template<class Compare> void sort(Compare comp);

    void reverse() noexcept;
  };

  template<class InputIterator, class Allocator = allocator<iter-value-type<InputIterator>>>
    forward_list(InputIterator, InputIterator, Allocator = Allocator())
      -> forward_list<iter-value-type<InputIterator>, Allocator>;

  // swap
  template<class T, class Allocator>
    void swap(forward_list<T, Allocator>& x, forward_list<T, Allocator>& y)
      noexcept(noexcept(x.swap(y)));
}

An incomplete type T may be used when instantiating forward_list if the allocator meets the allocator completeness requirements .

T shall be complete before any member of the resulting specialization of forward_list is referenced.

22.3.9.2 Constructors, copy, and assignment [forwardlist.cons]

🔗

explicit forward_list(const Allocator&);

Effects: Constructs an empty forward_list object using the specified allocator.

Complexity: Constant.

🔗

explicit forward_list(size_type n, const Allocator& = Allocator());

Preconditions: T is Cpp17DefaultInsertable into *this.

Effects: Constructs a forward_list object with n default-inserted elements using the specified allocator.

Complexity: Linear in n.

🔗

forward_list(size_type n, const T& value, const Allocator& = Allocator());

Preconditions: T is Cpp17CopyInsertable into *this.

Effects: Constructs a forward_list object with n copies of value using the specified allocator.

Complexity: Linear in n.

🔗

template<class InputIterator>
  forward_list(InputIterator first, InputIterator last, const Allocator& = Allocator());

Effects: Constructs a forward_list object equal to the range [first, last).

Complexity: Linear in distance(first, last).

22.3.9.3 Iterators [forwardlist.iter]

🔗

iterator before_begin() noexcept;
const_iterator before_begin() const noexcept;
const_iterator cbefore_begin() const noexcept;

Returns: A non-dereferenceable iterator that, when incremented, is equal to the iterator returned by begin().

Effects: cbefore_begin() is equivalent to const_cast<forward_list const&>(*this).before_begin().

Remarks: before_begin() == end() shall equal false.

22.3.9.4 Element access [forwardlist.access]

🔗

reference front();
const_reference front() const;

Returns: *begin()

22.3.9.5 Modifiers [forwardlist.modifiers]

None of the overloads of insert_after shall affect the validity of iterators and references, and erase_after shall invalidate only iterators and references to the erased elements.

If an exception is thrown during insert_after there shall be no effect.

Inserting n elements into a forward_list is linear in n, and the number of calls to the copy or move constructor of T is exactly equal to n.

Erasing n elements from a forward_list is linear in n and the number of calls to the destructor of type T is exactly equal to n.

🔗

template<class... Args> reference emplace_front(Args&&... args);

Effects: Inserts an object of type value_type constructed with value_type(std::forward<Args>(args)...) at the beginning of the list.

🔗

void push_front(const T& x);
void push_front(T&& x);

Effects: Inserts a copy of x at the beginning of the list.

🔗

void pop_front();

Effects: As if by erase_after(before_begin()).

🔗

iterator insert_after(const_iterator position, const T& x);
iterator insert_after(const_iterator position, T&& x);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts a copy of x after position.

Returns: An iterator pointing to the copy of x.

🔗

iterator insert_after(const_iterator position, size_type n, const T& x);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts n copies of x after position.

Returns: An iterator pointing to the last inserted copy of x or position if n == 0.

🔗

template<class InputIterator>
  iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Neither first nor last are iterators in *this.

Effects: Inserts copies of elements in [first, last) after position.

Returns: An iterator pointing to the last inserted element or position if first == last.

🔗

iterator insert_after(const_iterator position, initializer_list<T> il);

Effects: insert_after(p, il.begin(), il.end()).

Returns: An iterator pointing to the last inserted element or position if il is empty.

🔗

template<class... Args>
  iterator emplace_after(const_iterator position, Args&&... args);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

Effects: Inserts an object of type value_type constructed with value_type(std::forward<Args>(args)...) after position.

Returns: An iterator pointing to the new object.

🔗

iterator erase_after(const_iterator position);

Preconditions: The iterator following position is dereferenceable.

Effects: Erases the element pointed to by the iterator following position.

Returns: An iterator pointing to the element following the one that was erased, or end() if no such element exists.

Throws: Nothing.

🔗

iterator erase_after(const_iterator position, const_iterator last);

Preconditions: All iterators in the range (position, last) are dereferenceable.

Effects: Erases the elements in the range (position, last).

Returns: last.

Throws: Nothing.

🔗

void resize(size_type sz);

Preconditions: T is Cpp17DefaultInsertable into *this.

Effects: If sz < distance(begin(), end()), erases the last distance(begin(), end()) - sz elements from the list.

Otherwise, inserts sz - distance(begin(), end()) default-inserted elements at the end of the list.

🔗

void resize(size_type sz, const value_type& c);

Preconditions: T is Cpp17CopyInsertable into *this.

Effects: If sz < distance(begin(), end()), erases the last distance(begin(), end()) - sz elements from the list.

Otherwise, inserts sz - distance(begin(), end()) copies of c at the end of the list.

🔗

void clear() noexcept;

Effects: Erases all elements in the range [begin(), end()).

Remarks: Does not invalidate past-the-end iterators.

22.3.9.6 Operations [forwardlist.ops]

In this subclause, arguments for a template parameter named Predicate or BinaryPredicate shall meet the corresponding requirements in [algorithms.requirements].

For merge and sort, the definitions and requirements in [alg.sorting] apply.

🔗

void splice_after(const_iterator position, forward_list& x);
void splice_after(const_iterator position, forward_list&& x);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

get_allocator() == x.get_allocator() is true.

addressof(x) != this is true.

Effects: Inserts the contents of x after position, and x becomes empty.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity:

O (distance(x.begin(), x.end()))

🔗

void splice_after(const_iterator position, forward_list& x, const_iterator i);
void splice_after(const_iterator position, forward_list&& x, const_iterator i);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

The iterator following i is a dereferenceable iterator in x.

get_allocator() == x.get_allocator() is true.

Effects: Inserts the element following i into *this, following position, and removes it from x.

The result is unchanged if position == i or position == ++i.

Pointers and references to *++i continue to refer to the same element but as a member of *this.

Iterators to *++i continue to refer to the same element, but now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity:

O (1)

🔗

void splice_after(const_iterator position, forward_list& x,
                  const_iterator first, const_iterator last);
void splice_after(const_iterator position, forward_list&& x,
                  const_iterator first, const_iterator last);

Preconditions: position is before_begin() or is a dereferenceable iterator in the range [begin(), end()).

(first, last) is a valid range in x, and all iterators in the range (first, last) are dereferenceable.

position is not an iterator in the range (first, last).

get_allocator() == x.get_allocator() is true.

Effects: Inserts elements in the range (first, last) after position and removes the elements from x.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Complexity:

O (distance(first, last))

🔗

size_type remove(const T& value);
template<class Predicate> size_type remove_if(Predicate pred);

Effects: Erases all the elements in the list referred to by a list iterator i for which the following conditions hold: *i == value (for remove()), pred(*i) is true (for remove_if()).

Invalidates only the iterators and references to the erased elements.

Returns: The number of elements erased.

Throws: Nothing unless an exception is thrown by the equality comparison or the predicate.

Remarks: Stable .

Complexity: Exactly distance(begin(), end()) applications of the corresponding predicate.

🔗

size_type unique();
template<class BinaryPredicate> size_type unique(BinaryPredicate pred);

Effects: Erases all but the first element from every consecutive group of equal elements referred to by the iterator i in the range [first + 1, last) for which *i == *(i-1) (for the version with no arguments) or pred(*i, *(i - 1)) (for the version with a predicate argument) holds.

Invalidates only the iterators and references to the erased elements.

Returns: The number of elements erased.

Throws: Nothing unless an exception is thrown by the equality comparison or the predicate.

Complexity: If the range [first, last) is not empty, exactly (last - first) - 1 applications of the corresponding predicate, otherwise no applications of the predicate.

🔗

void merge(forward_list& x);
void merge(forward_list&& x);
template<class Compare> void merge(forward_list& x, Compare comp);
template<class Compare> void merge(forward_list&& x, Compare comp);

Preconditions: *this and x are both sorted with respect to the comparator operator< (for the first two overloads) or comp (for the last two overloads), and get_allocator() == x.get_allocator() is true.

Effects: Merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()).

x is empty after the merge.

If an exception is thrown other than by a comparison there are no effects.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Remarks: Stable ([algorithm.stable]).

Complexity: At most distance(begin(), end()) + distance(x.begin(), x.end()) - 1 comparisons.

🔗

void sort();
template<class Compare> void sort(Compare comp);

Effects: Sorts the list according to the operator< or the comp function object.

If an exception is thrown, the order of the elements in *this is unspecified.

Does not affect the validity of iterators and references.

Remarks: Stable .

Complexity: Approximately

N log N

comparisons, where N is distance(begin(), end()).

🔗

void reverse() noexcept;

Effects: Reverses the order of the elements in the list.

Does not affect the validity of iterators and references.

Complexity: Linear time.

22.3.9.7 Erasure [forward.list.erasure]

🔗

template<class T, class Allocator, class U>
  typename forward_list<T, Allocator>::size_type
    erase(forward_list<T, Allocator>& c, const U& value);

Effects: Equivalent to: return erase_if(c, [&](auto& elem) { return elem == value; });

🔗

template<class T, class Allocator, class Predicate>
  typename forward_list<T, Allocator>::size_type
    erase_if(forward_list<T, Allocator>& c, Predicate pred);

Effects: Equivalent to: return c.remove_if(pred);

22.3.10 Class template list [list]

22.3.10.1 Overview [list.overview]

A list is a sequence container that supports bidirectional iterators and allows constant time insert and erase operations anywhere within the sequence, with storage management handled automatically.

Unlike vectors and deques, fast random access to list elements is not supported, but many algorithms only need sequential access anyway.

A list meets all of the requirements of a container, of a reversible container (given in two tables in [container.requirements]), of a sequence container, including most of the optional sequence container requirements ([sequence.reqmts]), and of an allocator-aware container (Table 76).

The exceptions are the operator[] and at member functions, which are not provided.225

Descriptions are provided here only for operations on list that are not described in one of these tables or for operations where there is additional semantic information.

namespace std {
  template<class T, class Allocator = allocator<T>>
  class list {
  public:
    // types
    using value_type             = T;
    using allocator_type         = Allocator;
    using pointer                = typename allocator_traits<Allocator>::pointer;
    using const_pointer          = typename allocator_traits<Allocator>::const_pointer;
    using reference              = value_type&;
    using const_reference        = const value_type&;
    using size_type              = implementation-defined; // see [container.requirements]
    using difference_type        = implementation-defined; // see [container.requirements]
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

    // [list.cons], construct/copy/destroy
    list() : list(Allocator()) { }
    explicit list(const Allocator&);
    explicit list(size_type n, const Allocator& = Allocator());
    list(size_type n, const T& value, const Allocator& = Allocator());
    template<class InputIterator>
      list(InputIterator first, InputIterator last, const Allocator& = Allocator());
    list(const list& x);
    list(list&& x);
    list(const list&, const Allocator&);
    list(list&&, const Allocator&);
    list(initializer_list<T>, const Allocator& = Allocator());
    ~list();
    list& operator=(const list& x);
    list& operator=(list&& x)
      noexcept(allocator_traits<Allocator>::is_always_equal::value);
    list& operator=(initializer_list<T>);
    template<class InputIterator>
      void assign(InputIterator first, InputIterator last);
    void assign(size_type n, const T& t);
    void assign(initializer_list<T>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator               begin() noexcept;
    const_iterator         begin() const noexcept;
    iterator               end() noexcept;
    const_iterator         end() const noexcept;
    reverse_iterator       rbegin() noexcept;
    const_reverse_iterator rbegin() const noexcept;
    reverse_iterator       rend() noexcept;
    const_reverse_iterator rend() const noexcept;

    const_iterator         cbegin() const noexcept;
    const_iterator         cend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;

    // [list.capacity], capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;
    void      resize(size_type sz);
    void      resize(size_type sz, const T& c);

    // element access
    reference       front();
    const_reference front() const;
    reference       back();
    const_reference back() const;

    // [list.modifiers], modifiers
    template<class... Args> reference emplace_front(Args&&... args);
    template<class... Args> reference emplace_back(Args&&... args);
    void push_front(const T& x);
    void push_front(T&& x);
    void pop_front();
    void push_back(const T& x);
    void push_back(T&& x);
    void pop_back();

    template<class... Args> iterator emplace(const_iterator position, Args&&... args);
    iterator insert(const_iterator position, const T& x);
    iterator insert(const_iterator position, T&& x);
    iterator insert(const_iterator position, size_type n, const T& x);
    template<class InputIterator>
      iterator insert(const_iterator position, InputIterator first, InputIterator last);
    iterator insert(const_iterator position, initializer_list<T> il);

    iterator erase(const_iterator position);
    iterator erase(const_iterator position, const_iterator last);
    void     swap(list&) noexcept(allocator_traits<Allocator>::is_always_equal::value);
    void     clear() noexcept;

    // [list.ops], list operations
    void splice(const_iterator position, list& x);
    void splice(const_iterator position, list&& x);
    void splice(const_iterator position, list& x, const_iterator i);
    void splice(const_iterator position, list&& x, const_iterator i);
    void splice(const_iterator position, list& x, const_iterator first, const_iterator last);
    void splice(const_iterator position, list&& x, const_iterator first, const_iterator last);

    size_type remove(const T& value);
    template<class Predicate> size_type remove_if(Predicate pred);

    size_type unique();
    template<class BinaryPredicate>
      size_type unique(BinaryPredicate binary_pred);

    void merge(list& x);
    void merge(list&& x);
    template<class Compare> void merge(list& x, Compare comp);
    template<class Compare> void merge(list&& x, Compare comp);

    void sort();
    template<class Compare> void sort(Compare comp);

    void reverse() noexcept;
  };

  template<class InputIterator, class Allocator = allocator<iter-value-type<InputIterator>>>
    list(InputIterator, InputIterator, Allocator = Allocator())
      -> list<iter-value-type<InputIterator>, Allocator>;

  // swap
  template<class T, class Allocator>
    void swap(list<T, Allocator>& x, list<T, Allocator>& y)
      noexcept(noexcept(x.swap(y)));
}

An incomplete type T may be used when instantiating list if the allocator meets the allocator completeness requirements .

T shall be complete before any member of the resulting specialization of list is referenced.

225)

These member functions are only provided by containers whose iterators are random access iterators.

⮥

22.3.10.2 Constructors, copy, and assignment [list.cons]

🔗

explicit list(const Allocator&);

Effects: Constructs an empty list, using the specified allocator.

Complexity: Constant.

🔗

explicit list(size_type n, const Allocator& = Allocator());

Preconditions: T is Cpp17DefaultInsertable into *this.

Effects: Constructs a list with n default-inserted elements using the specified allocator.

Complexity: Linear in n.

🔗

list(size_type n, const T& value, const Allocator& = Allocator());

Preconditions: T is Cpp17CopyInsertable into *this.

Effects: Constructs a list with n copies of value, using the specified allocator.

Complexity: Linear in n.

🔗

template<class InputIterator>
  list(InputIterator first, InputIterator last, const Allocator& = Allocator());

Effects: Constructs a list equal to the range [first, last).

Complexity: Linear in distance(first, last).

22.3.10.3 Capacity [list.capacity]

🔗

void resize(size_type sz);

Preconditions: T is Cpp17DefaultInsertable into *this.

Effects: If size() < sz, appends sz - size() default-inserted elements to the sequence.

If sz <= size(), equivalent to:

list<T>::iterator it = begin();
advance(it, sz);
erase(it, end());

🔗

void resize(size_type sz, const T& c);

Preconditions: T is Cpp17CopyInsertable into *this.

Effects: As if by:

if (sz > size())
  insert(end(), sz-size(), c);
else if (sz < size()) {
  iterator i = begin();
  advance(i, sz);
  erase(i, end());
}
else
  ;                 // do nothing

22.3.10.4 Modifiers [list.modifiers]

🔗

iterator insert(const_iterator position, const T& x);
iterator insert(const_iterator position, T&& x);
iterator insert(const_iterator position, size_type n, const T& x);
template<class InputIterator>
  iterator insert(const_iterator position, InputIterator first,
                  InputIterator last);
iterator insert(const_iterator position, initializer_list<T>);

template<class... Args> reference emplace_front(Args&&... args);
template<class... Args> reference emplace_back(Args&&... args);
template<class... Args> iterator emplace(const_iterator position, Args&&... args);
void push_front(const T& x);
void push_front(T&& x);
void push_back(const T& x);
void push_back(T&& x);

Remarks: Does not affect the validity of iterators and references.

If an exception is thrown there are no effects.

Complexity: Insertion of a single element into a list takes constant time and exactly one call to a constructor of T.

Insertion of multiple elements into a list is linear in the number of elements inserted, and the number of calls to the copy constructor or move constructor of T is exactly equal to the number of elements inserted.

🔗

iterator erase(const_iterator position);
iterator erase(const_iterator first, const_iterator last);

void pop_front();
void pop_back();
void clear() noexcept;

Effects: Invalidates only the iterators and references to the erased elements.

Throws: Nothing.

Complexity: Erasing a single element is a constant time operation with a single call to the destructor of T.

Erasing a range in a list is linear time in the size of the range and the number of calls to the destructor of type T is exactly equal to the size of the range.

22.3.10.5 Operations [list.ops]

Since lists allow fast insertion and erasing from the middle of a list, certain operations are provided specifically for them.226

In this subclause, arguments for a template parameter named Predicate or BinaryPredicate shall meet the corresponding requirements in [algorithms.requirements].

For merge and sort, the definitions and requirements in [alg.sorting] apply.

list provides three splice operations that destructively move elements from one list to another.

The behavior of splice operations is undefined if get_allocator() != x.get_allocator().

🔗

void splice(const_iterator position, list& x);
void splice(const_iterator position, list&& x);

Preconditions: addressof(x) != this is true.

Effects: Inserts the contents of x before position and x becomes empty.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity: Constant time.

🔗

void splice(const_iterator position, list& x, const_iterator i);
void splice(const_iterator position, list&& x, const_iterator i);

Preconditions: i is a valid dereferenceable iterator of x.

Effects: Inserts an element pointed to by i from list x before position and removes the element from x.

The result is unchanged if position == i or position == ++i.

Pointers and references to *i continue to refer to this same element but as a member of *this.

Iterators to *i (including i itself) continue to refer to the same element, but now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity: Constant time.

🔗

void splice(const_iterator position, list& x, const_iterator first,
            const_iterator last);
void splice(const_iterator position, list&& x, const_iterator first,
            const_iterator last);

Preconditions: [first, last) is a valid range in x.

position is not an iterator in the range [first, last).

Effects: Inserts elements in the range [first, last) before position and removes the elements from x.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Throws: Nothing.

Complexity: Constant time if addressof(x) == this; otherwise, linear time.

🔗

size_type remove(const T& value);
template<class Predicate> size_type remove_if(Predicate pred);

Effects: Erases all the elements in the list referred to by a list iterator i for which the following conditions hold: *i == value, pred(*i) != false.

Invalidates only the iterators and references to the erased elements.

Returns: The number of elements erased.

Throws: Nothing unless an exception is thrown by *i == value or pred(*i) != false.

Remarks: Stable .

Complexity: Exactly size() applications of the corresponding predicate.

🔗

size_type unique();
template<class BinaryPredicate> size_type unique(BinaryPredicate binary_pred);

Effects: Erases all but the first element from every consecutive group of equal elements referred to by the iterator i in the range [first + 1, last) for which *i == *(i-1) (for the version of unique with no arguments) or pred(*i, *(i - 1)) (for the version of unique with a predicate argument) holds.

Invalidates only the iterators and references to the erased elements.

Returns: The number of elements erased.

Throws: Nothing unless an exception is thrown by *i == *(i-1) or pred(*i, *(i - 1))

Complexity: If the range [first, last) is not empty, exactly (last - first) - 1 applications of the corresponding predicate, otherwise no applications of the predicate.

🔗

void merge(list& x);
void merge(list&& x);
template<class Compare> void merge(list& x, Compare comp);
template<class Compare> void merge(list&& x, Compare comp);

Preconditions: Both the list and the argument list shall be sorted with respect to the comparator operator< (for the first two overloads) or comp (for the last two overloads), and get_allocator() == x.get_allocator() is true.

Effects: If addressof(x) == this, does nothing; otherwise, merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()).

The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by comp; that is, for every iterator i, in the range other than the first, the condition comp(*i, *(i - 1)) will be false.

Pointers and references to the moved elements of x now refer to those same elements but as members of *this.

Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

Remarks: Stable ([algorithm.stable]).

If addressof(x) != this, the range [x.begin(), x.end()) is empty after the merge.

No elements are copied by this operation.

Complexity: At most size() + x.size() - 1 applications of comp if addressof(x) != this; otherwise, no applications of comp are performed.

If an exception is thrown other than by a comparison there are no effects.

🔗

void reverse() noexcept;

Effects: Reverses the order of the elements in the list.

Does not affect the validity of iterators and references.

Complexity: Linear time.

🔗

void sort();
template<class Compare> void sort(Compare comp);

Effects: Sorts the list according to the operator< or a Compare function object.

If an exception is thrown, the order of the elements in *this is unspecified.

Does not affect the validity of iterators and references.

Remarks: Stable .

Complexity: Approximately

N log N

comparisons, where N == size().

226)

As specified in [allocator.requirements], the requirements in this Clause apply only to lists whose allocators compare equal.

⮥

22.3.10.6 Erasure [list.erasure]

🔗

template<class T, class Allocator, class U>
  typename list<T, Allocator>::size_type
    erase(list<T, Allocator>& c, const U& value);

Effects: Equivalent to: return erase_if(c, [&](auto& elem) { return elem == value; });

🔗

template<class T, class Allocator, class Predicate>
  typename list<T, Allocator>::size_type
    erase_if(list<T, Allocator>& c, Predicate pred);

Effects: Equivalent to: return c.remove_if(pred);

22.3.11 Class template vector [vector]

22.3.11.1 Overview [vector.overview]

A vector is a sequence container that supports (amortized) constant time insert and erase operations at the end; insert and erase in the middle take linear time.

Storage management is handled automatically, though hints can be given to improve efficiency.

A vector meets all of the requirements of a container and of a reversible container (given in two tables in [container.requirements]), of a sequence container, including most of the optional sequence container requirements ([sequence.reqmts]), of an allocator-aware container (Table 76), and, for an element type other than bool, of a contiguous container .

The exceptions are the push_front, pop_front, and emplace_front member functions, which are not provided.

Descriptions are provided here only for operations on vector that are not described in one of these tables or for operations where there is additional semantic information.

The types iterator and const_iterator meet the constexpr iterator requirements ([iterator.requirements.general]).

namespace std {
  template<class T, class Allocator = allocator<T>>
  class vector {
  public:
    // types
    using value_type             = T;
    using allocator_type         = Allocator;
    using pointer                = typename allocator_traits<Allocator>::pointer;
    using const_pointer          = typename allocator_traits<Allocator>::const_pointer;
    using reference              = value_type&;
    using const_reference        = const value_type&;
    using size_type              = implementation-defined; // see [container.requirements]
    using difference_type        = implementation-defined; // see [container.requirements]
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

    // [vector.cons], construct/copy/destroy
    constexpr vector() noexcept(noexcept(Allocator())) : vector(Allocator()) { }
    constexpr explicit vector(const Allocator&) noexcept;
    constexpr explicit vector(size_type n, const Allocator& = Allocator());
    constexpr vector(size_type n, const T& value, const Allocator& = Allocator());
    template<class InputIterator>
      constexpr vector(InputIterator first, InputIterator last, const Allocator& = Allocator());
    constexpr vector(const vector& x);
    constexpr vector(vector&&) noexcept;
    constexpr vector(const vector&, const Allocator&);
    constexpr vector(vector&&, const Allocator&);
    constexpr vector(initializer_list<T>, const Allocator& = Allocator());
    constexpr ~vector();
    constexpr vector& operator=(const vector& x);
    constexpr vector& operator=(vector&& x)
      noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
               allocator_traits<Allocator>::is_always_equal::value);
    constexpr vector& operator=(initializer_list<T>);
    template<class InputIterator>
      constexpr void assign(InputIterator first, InputIterator last);
    constexpr void assign(size_type n, const T& u);
    constexpr void assign(initializer_list<T>);
    constexpr allocator_type get_allocator() const noexcept;

    // iterators
    constexpr iterator               begin() noexcept;
    constexpr const_iterator         begin() const noexcept;
    constexpr iterator               end() noexcept;
    constexpr const_iterator         end() const noexcept;
    constexpr reverse_iterator       rbegin() noexcept;
    constexpr const_reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator       rend() noexcept;
    constexpr const_reverse_iterator rend() const noexcept;

    constexpr const_iterator         cbegin() const noexcept;
    constexpr const_iterator         cend() const noexcept;
    constexpr const_reverse_iterator crbegin() const noexcept;
    constexpr const_reverse_iterator crend() const noexcept;

    // [vector.capacity], capacity
    [[nodiscard]] constexpr bool empty() const noexcept;
    constexpr size_type size() const noexcept;
    constexpr size_type max_size() const noexcept;
    constexpr size_type capacity() const noexcept;
    constexpr void      resize(size_type sz);
    constexpr void      resize(size_type sz, const T& c);
    constexpr void      reserve(size_type n);
    constexpr void      shrink_to_fit();

    // element access
    constexpr reference       operator[](size_type n);
    constexpr const_reference operator[](size_type n) const;
    constexpr const_reference at(size_type n) const;
    constexpr reference       at(size_type n);
    constexpr reference       front();
    constexpr const_reference front() const;
    constexpr reference       back();
    constexpr const_reference back() const;

    // [vector.data], data access
    constexpr T*       data() noexcept;
    constexpr const T* data() const noexcept;

    // [vector.modifiers], modifiers
    template<class... Args> constexpr reference emplace_back(Args&&... args);
    constexpr void push_back(const T& x);
    constexpr void push_back(T&& x);
    constexpr void pop_back();

    template<class... Args> constexpr iterator emplace(const_iterator position, Args&&... args);
    constexpr iterator insert(const_iterator position, const T& x);
    constexpr iterator insert(const_iterator position, T&& x);
    constexpr iterator insert(const_iterator position, size_type n, const T& x);
    template<class InputIterator>
      constexpr iterator insert(const_iterator position,
                                InputIterator first, InputIterator last);
    constexpr iterator insert(const_iterator position, initializer_list<T> il);
    constexpr iterator erase(const_iterator position);
    constexpr iterator erase(const_iterator first, const_iterator last);
    constexpr void     swap(vector&)
      noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value ||
               allocator_traits<Allocator>::is_always_equal::value);
    constexpr void     clear() noexcept;
  };

  template<class InputIterator, class Allocator = allocator<iter-value-type<InputIterator>>>
    vector(InputIterator, InputIterator, Allocator = Allocator())
      -> vector<iter-value-type<InputIterator>, Allocator>;

  // swap
  template<class T, class Allocator>
    constexpr void swap(vector<T, Allocator>& x, vector<T, Allocator>& y)
      noexcept(noexcept(x.swap(y)));
}

An incomplete type T may be used when instantiating vector if the allocator meets the allocator completeness requirements .

T shall be complete before any member of the resulting specialization of vector is referenced.

22.3.11.2 Constructors, copy, and assignment [vector.cons]

🔗

constexpr explicit vector(const Allocator&) noexcept;

Effects: Constructs an empty vector, using the specified allocator.

Complexity: Constant.

🔗

constexpr explicit vector(size_type n, const Allocator& = Allocator());

Preconditions: T is Cpp17DefaultInsertable into *this.

Effects: Constructs a vector with n default-inserted elements using the specified allocator.

Complexity: Linear in n.

🔗

constexpr vector(size_type n, const T& value,
                 const Allocator& = Allocator());

Preconditions: T is Cpp17CopyInsertable into *this.

Effects: Constructs a vector with n copies of value, using the specified allocator.

Complexity: Linear in n.

🔗

template<class InputIterator>
  constexpr vector(InputIterator first, InputIterator last,
                   const Allocator& = Allocator());

Effects: Constructs a vector equal to the range [first, last), using the specified allocator.

Complexity: Makes only N calls to the copy constructor of T (where N is the distance between first and last) and no reallocations if iterators first and last are of forward, bidirectional, or random access categories.

It makes order N calls to the copy constructor of T and order logN reallocations if they are just input iterators.

22.3.11.3 Capacity [vector.capacity]

🔗

constexpr size_type capacity() const noexcept;

Returns: The total number of elements that the vector can hold without requiring reallocation.

Complexity: Constant time.

🔗

constexpr void reserve(size_type n);

Preconditions: T is Cpp17MoveInsertable into *this.

Effects: A directive that informs a vector of a planned change in size, so that it can manage the storage allocation accordingly.

After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise.

Reallocation happens at this point if and only if the current capacity is less than the argument of reserve().

If an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable type, there are no effects.

Complexity: It does not change the size of the sequence and takes at most linear time in the size of the sequence.

Throws: length_error if n > max_size().227

Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator.

[ Note

If no reallocation happens, they remain valid.

— end note

]

No reallocation shall take place during insertions that happen after a call to reserve() until an insertion would make the size of the vector greater than the value of capacity().

🔗

constexpr void shrink_to_fit();

Preconditions: T is Cpp17MoveInsertable into *this.

Effects: shrink_to_fit is a non-binding request to reduce capacity() to size().

[ Note

The request is non-binding to allow latitude for implementation-specific optimizations.

— end note

]

It does not increase capacity(), but may reduce capacity() by causing reallocation.

If an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T there are no effects.

Complexity: If reallocation happens, linear in the size of the sequence.

Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator.

[ Note

If no reallocation happens, they remain valid.

— end note

]

🔗

constexpr void swap(vector& x)
  noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value ||
           allocator_traits<Allocator>::is_always_equal::value);

Effects: Exchanges the contents and capacity() of *this with that of x.

Complexity: Constant time.

🔗

constexpr void resize(size_type sz);

Preconditions: T is Cpp17MoveInsertable and Cpp17DefaultInsertable into *this.

Effects: If sz < size(), erases the last size() - sz elements from the sequence.

Otherwise, appends sz - size() default-inserted elements to the sequence.

Remarks: If an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T there are no effects.

🔗

constexpr void resize(size_type sz, const T& c);

Preconditions: T is Cpp17CopyInsertable into *this.

Effects: If sz < size(), erases the last size() - sz elements from the sequence.

Otherwise, appends sz - size() copies of c to the sequence.

Remarks: If an exception is thrown there are no effects.

227)

reserve() uses Allocator::allocate() which may throw an appropriate exception.

⮥

22.3.11.4 Data [vector.data]

🔗

constexpr T*         data() noexcept;
constexpr const T*   data() const noexcept;

Returns: A pointer such that [data(), data() + size()) is a valid range.

For a non-empty vector, data() == addressof(front()).

Complexity: Constant time.

22.3.11.5 Modifiers [vector.modifiers]

🔗

constexpr iterator insert(const_iterator position, const T& x);
constexpr iterator insert(const_iterator position, T&& x);
constexpr iterator insert(const_iterator position, size_type n, const T& x);
template<class InputIterator>
  constexpr iterator insert(const_iterator position, InputIterator first, InputIterator last);
constexpr iterator insert(const_iterator position, initializer_list<T>);

template<class... Args> constexpr reference emplace_back(Args&&... args);
template<class... Args> constexpr iterator emplace(const_iterator position, Args&&... args);
constexpr void push_back(const T& x);
constexpr void push_back(T&& x);

Remarks: Causes reallocation if the new size is greater than the old capacity.

Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator.

If no reallocation happens, then references, pointers, and iterators before the insertion point remain valid but those at or after the insertion point, including the past-the-end iterator, are invalidated.

If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of T or by any InputIterator operation there are no effects.

If an exception is thrown while inserting a single element at the end and T is Cpp17CopyInsertable or is_nothrow_move_constructible_v<T> is true, there are no effects.

Otherwise, if an exception is thrown by the move constructor of a non-Cpp17CopyInsertable T, the effects are unspecified.

Complexity: If reallocation happens, linear in the number of elements of the resulting vector; otherwise, linear in the number of elements inserted plus the distance to the end of the vector.

🔗

constexpr iterator erase(const_iterator position);
constexpr iterator erase(const_iterator first, const_iterator last);
constexpr void pop_back();

Effects: Invalidates iterators and references at or after the point of the erase.

Complexity: The destructor of T is called the number of times equal to the number of the elements erased, but the assignment operator of T is called the number of times equal to the number of elements in the vector after the erased elements.

Throws: Nothing unless an exception is thrown by the assignment operator or move assignment operator of T.

22.3.11.6 Erasure [vector.erasure]

🔗

template<class T, class Allocator, class U>
  constexpr typename vector<T, Allocator>::size_type
    erase(vector<T, Allocator>& c, const U& value);

Effects: Equivalent to:

auto it = remove(c.begin(), c.end(), value);
auto r = distance(it, c.end());
c.erase(it, c.end());
return r;

🔗

template<class T, class Allocator, class Predicate>
  constexpr typename vector<T, Allocator>::size_type
    erase_if(vector<T, Allocator>& c, Predicate pred);

Effects: Equivalent to:

auto it = remove_if(c.begin(), c.end(), pred);
auto r = distance(it, c.end());
c.erase(it, c.end());
return r;

22.3.12 Class vector<bool> [vector.bool]

To optimize space allocation, a specialization of vector for bool elements is provided:

namespace std {
  template<class Allocator>
  class vector<bool, Allocator> {
  public:
    // types
    using value_type             = bool;
    using allocator_type         = Allocator;
    using pointer                = implementation-defined;
    using const_pointer          = implementation-defined;
    using const_reference        = bool;
    using size_type              = implementation-defined; // see [container.requirements]
    using difference_type        = implementation-defined; // see [container.requirements]
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

    // bit reference
    class reference {
      friend class vector;
      constexpr reference() noexcept;
    public:
      constexpr reference(const reference&) = default;
      constexpr ~reference();
      constexpr operator bool() const noexcept;
      constexpr reference& operator=(const bool x) noexcept;
      constexpr reference& operator=(const reference& x) noexcept;
      constexpr void flip() noexcept;   // flips the bit
    };

    // construct/copy/destroy
    constexpr vector() : vector(Allocator()) { }
    constexpr explicit vector(const Allocator&);
    constexpr explicit vector(size_type n, const Allocator& = Allocator());
    constexpr vector(size_type n, const bool& value, const Allocator& = Allocator());
    template<class InputIterator>
      constexpr vector(InputIterator first, InputIterator last, const Allocator& = Allocator());
    constexpr vector(const vector& x);
    constexpr vector(vector&& x);
    constexpr vector(const vector&, const Allocator&);
    constexpr vector(vector&&, const Allocator&);
    constexpr vector(initializer_list<bool>, const Allocator& = Allocator()));
    constexpr ~vector();
    constexpr vector& operator=(const vector& x);
    constexpr vector& operator=(vector&& x);
    constexpr vector& operator=(initializer_list<bool>);
    template<class InputIterator>
      constexpr void assign(InputIterator first, InputIterator last);
    constexpr void assign(size_type n, const bool& t);
    constexpr void assign(initializer_list<bool>);
    constexpr allocator_type get_allocator() const noexcept;

    // iterators
    constexpr iterator               begin() noexcept;
    constexpr const_iterator         begin() const noexcept;
    constexpr iterator               end() noexcept;
    constexpr const_iterator         end() const noexcept;
    constexpr reverse_iterator       rbegin() noexcept;
    constexpr const_reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator       rend() noexcept;
    constexpr const_reverse_iterator rend() const noexcept;

    constexpr const_iterator         cbegin() const noexcept;
    constexpr const_iterator         cend() const noexcept;
    constexpr const_reverse_iterator crbegin() const noexcept;
    constexpr const_reverse_iterator crend() const noexcept;

    // capacity
    [[nodiscard]] constexpr bool empty() const noexcept;
    constexpr size_type size() const noexcept;
    constexpr size_type max_size() const noexcept;
    constexpr size_type capacity() const noexcept;
    constexpr void      resize(size_type sz, bool c = false);
    constexpr void      reserve(size_type n);
    constexpr void      shrink_to_fit();

    // element access
    constexpr reference       operator[](size_type n);
    constexpr const_reference operator[](size_type n) const;
    constexpr const_reference at(size_type n) const;
    constexpr reference       at(size_type n);
    constexpr reference       front();
    constexpr const_reference front() const;
    constexpr reference       back();
    constexpr const_reference back() const;

    // modifiers
    template<class... Args> constexpr reference emplace_back(Args&&... args);
    constexpr void push_back(const bool& x);
    constexpr void pop_back();
    template<class... Args> constexpr iterator emplace(const_iterator position, Args&&... args);
    constexpr iterator insert(const_iterator position, const bool& x);
    constexpr iterator insert(const_iterator position, size_type n, const bool& x);
    template<class InputIterator>
      constexpr iterator insert(const_iterator position,
                                InputIterator first, InputIterator last);
    constexpr iterator insert(const_iterator position, initializer_list<bool> il);

    constexpr iterator erase(const_iterator position);
    constexpr iterator erase(const_iterator first, const_iterator last);
    constexpr void swap(vector&);
    constexpr static void swap(reference x, reference y) noexcept;
    constexpr void flip() noexcept;     // flips all bits
    constexpr void clear() noexcept;
  };
}

Unless described below, all operations have the same requirements and semantics as the primary vector template, except that operations dealing with the bool value type map to bit values in the container storage and allocator_traits::construct is not used to construct these values.

There is no requirement that the data be stored as a contiguous allocation of bool values.

A space-optimized representation of bits is recommended instead.

reference is a class that simulates the behavior of references of a single bit in vector<bool>.

The conversion function returns true when the bit is set, and false otherwise.

The assignment operator sets the bit when the argument is (convertible to) true and clears it otherwise.

flip reverses the state of the bit.

🔗

constexpr void flip() noexcept;

Effects: Replaces each element in the container with its complement.

🔗

constexpr static void swap(reference x, reference y) noexcept;

Effects: Exchanges the contents of x and y as if by:

bool b = x;
x = y;
y = b;

🔗

template<class Allocator> struct hash<vector<bool, Allocator>>;

The specialization is enabled ([unord.hash]).

22.4 Associative containers [associative]

22.4.1 In general [associative.general]

The header <map> defines the class templates map and multimap; the header <set> defines the class templates set and multiset.

The following exposition-only alias templates may appear in deduction guides for associative containers:

template<class InputIterator>
  using iter-value-type =
    typename iterator_traits<InputIterator>::value_type;                // exposition only
template<class InputIterator>
  using iter-key-type = remove_const_t<
    typename iterator_traits<InputIterator>::value_type::first_type>;   // exposition only
template<class InputIterator>
  using iter-mapped-type =
    typename iterator_traits<InputIterator>::value_type::second_type;   // exposition only
template<class InputIterator>
  using iter-to-alloc-type = pair<
    add_const_t<typename iterator_traits<InputIterator>::value_type::first_type>,
    typename iterator_traits<InputIterator>::value_type::second_type>;  // exposition only

22.4.2 Header <map> synopsis [associative.map.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  // [map], class template map
  template<class Key, class T, class Compare = less<Key>,
           class Allocator = allocator<pair<const Key, T>>>
    class map;

  template<class Key, class T, class Compare, class Allocator>
    bool operator==(const map<Key, T, Compare, Allocator>& x,
                    const map<Key, T, Compare, Allocator>& y);
  template<class Key, class T, class Compare, class Allocator>
    synth-three-way-result<pair<const Key, T>>
      operator<=>(const map<Key, T, Compare, Allocator>& x,
                  const map<Key, T, Compare, Allocator>& y);

  template<class Key, class T, class Compare, class Allocator>
    void swap(map<Key, T, Compare, Allocator>& x,
              map<Key, T, Compare, Allocator>& y)
      noexcept(noexcept(x.swap(y)));

  template<class Key, class T, class Compare, class Allocator, class Predicate>
    typename map<Key, T, Compare, Allocator>::size_type
      erase_if(map<Key, T, Compare, Allocator>& c, Predicate pred);

  // [multimap], class template multimap
  template<class Key, class T, class Compare = less<Key>,
           class Allocator = allocator<pair<const Key, T>>>
    class multimap;

  template<class Key, class T, class Compare, class Allocator>
    bool operator==(const multimap<Key, T, Compare, Allocator>& x,
                    const multimap<Key, T, Compare, Allocator>& y);
  template<class Key, class T, class Compare, class Allocator>
    synth-three-way-result<pair<const Key, T>>
      operator<=>(const multimap<Key, T, Compare, Allocator>& x,
                  const multimap<Key, T, Compare, Allocator>& y);

  template<class Key, class T, class Compare, class Allocator>
    void swap(multimap<Key, T, Compare, Allocator>& x,
              multimap<Key, T, Compare, Allocator>& y)
      noexcept(noexcept(x.swap(y)));

  template<class Key, class T, class Compare, class Allocator, class Predicate>
    typename multimap<Key, T, Compare, Allocator>::size_type
      erase_if(multimap<Key, T, Compare, Allocator>& c, Predicate pred);

  namespace pmr {
    template<class Key, class T, class Compare = less<Key>>
      using map = std::map<Key, T, Compare,
                           polymorphic_allocator<pair<const Key, T>>>;

    template<class Key, class T, class Compare = less<Key>>
      using multimap = std::multimap<Key, T, Compare,
                                     polymorphic_allocator<pair<const Key, T>>>;
  }
}

22.4.3 Header <set> synopsis [associative.set.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  // [set], class template set
  template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>>
    class set;

  template<class Key, class Compare, class Allocator>
    bool operator==(const set<Key, Compare, Allocator>& x,
                    const set<Key, Compare, Allocator>& y);
  template<class Key, class Compare, class Allocator>
    synth-three-way-result<Key> operator<=>(const set<Key, Compare, Allocator>& x,
                                            const set<Key, Compare, Allocator>& y);

  template<class Key, class Compare, class Allocator>
    void swap(set<Key, Compare, Allocator>& x,
              set<Key, Compare, Allocator>& y)
      noexcept(noexcept(x.swap(y)));

  template<class Key, class Compare, class Allocator, class Predicate>
    typename set<Key, Compare, Allocator>::size_type
      erase_if(set<Key, Compare, Allocator>& c, Predicate pred);

  // [multiset], class template multiset
  template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>>
    class multiset;

  template<class Key, class Compare, class Allocator>
    bool operator==(const multiset<Key, Compare, Allocator>& x,
                    const multiset<Key, Compare, Allocator>& y);
  template<class Key, class Compare, class Allocator>
    synth-three-way-result<Key> operator<=>(const multiset<Key, Compare, Allocator>& x,
                                            const multiset<Key, Compare, Allocator>& y);

  template<class Key, class Compare, class Allocator>
    void swap(multiset<Key, Compare, Allocator>& x,
              multiset<Key, Compare, Allocator>& y)
      noexcept(noexcept(x.swap(y)));

  template<class Key, class Compare, class Allocator, class Predicate>
    typename multiset<Key, Compare, Allocator>::size_type
      erase_if(multiset<Key, Compare, Allocator>& c, Predicate pred);

  namespace pmr {
    template<class Key, class Compare = less<Key>>
      using set = std::set<Key, Compare, polymorphic_allocator<Key>>;

    template<class Key, class Compare = less<Key>>
      using multiset = std::multiset<Key, Compare, polymorphic_allocator<Key>>;
  }
}

22.4.4 Class template map [map]

22.4.4.1 Overview [map.overview]

A map is an associative container that supports unique keys (contains at most one of each key value) and provides for fast retrieval of values of another type T based on the keys.

The map class supports bidirectional iterators.

A map meets all of the requirements of a container, of a reversible container ([container.requirements]), of an associative container ([associative.reqmts]), and of an allocator-aware container (Table 76).

A map also provides most operations described in [associative.reqmts] for unique keys.

This means that a map supports the a_uniq operations in [associative.reqmts] but not the a_eq operations.

For a map<Key,T> the key_type is Key and the value_type is pair<const Key,T>.

Descriptions are provided here only for operations on map that are not described in one of those tables or for operations where there is additional semantic information.

namespace std {
  template<class Key, class T, class Compare = less<Key>,
           class Allocator = allocator<pair<const Key, T>>>
  class map {
  public:
    // types
    using key_type               = Key;
    using mapped_type            = T;
    using value_type             = pair<const Key, T>;
    using key_compare            = Compare;
    using allocator_type         = Allocator;
    using pointer                = typename allocator_traits<Allocator>::pointer;
    using const_pointer          = typename allocator_traits<Allocator>::const_pointer;
    using reference              = value_type&;
    using const_reference        = const value_type&;
    using size_type              = implementation-defined; // see [container.requirements]
    using difference_type        = implementation-defined; // see [container.requirements]
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    using node_type              = unspecified;
    using insert_return_type     = insert-return-type<iterator, node_type>;

    class value_compare {
      friend class map;
    protected:
      Compare comp;
      value_compare(Compare c) : comp(c) {}
    public:
      bool operator()(const value_type& x, const value_type& y) const {
        return comp(x.first, y.first);
      }
    };

    // [map.cons], construct/copy/destroy
    map() : map(Compare()) { }
    explicit map(const Compare& comp, const Allocator& = Allocator());
    template<class InputIterator>
      map(InputIterator first, InputIterator last,
          const Compare& comp = Compare(), const Allocator& = Allocator());
    map(const map& x);
    map(map&& x);
    explicit map(const Allocator&);
    map(const map&, const Allocator&);
    map(map&&, const Allocator&);
    map(initializer_list<value_type>,
      const Compare& = Compare(),
      const Allocator& = Allocator());
    template<class InputIterator>
      map(InputIterator first, InputIterator last, const Allocator& a)
        : map(first, last, Compare(), a) { }
    map(initializer_list<value_type> il, const Allocator& a)
      : map(il, Compare(), a) { }
    ~map();
    map& operator=(const map& x);
    map& operator=(map&& x)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Compare>);
    map& operator=(initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator               begin() noexcept;
    const_iterator         begin() const noexcept;
    iterator               end() noexcept;
    const_iterator         end() const noexcept;

    reverse_iterator       rbegin() noexcept;
    const_reverse_iterator rbegin() const noexcept;
    reverse_iterator       rend() noexcept;
    const_reverse_iterator rend() const noexcept;

    const_iterator         cbegin() const noexcept;
    const_iterator         cend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // [map.access], element access
    mapped_type& operator[](const key_type& x);
    mapped_type& operator[](key_type&& x);
    mapped_type&       at(const key_type& x);
    const mapped_type& at(const key_type& x) const;

    // [map.modifiers], modifiers
    template<class... Args> pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    pair<iterator, bool> insert(const value_type& x);
    pair<iterator, bool> insert(value_type&& x);
    template<class P> pair<iterator, bool> insert(P&& x);
    iterator insert(const_iterator position, const value_type& x);
    iterator insert(const_iterator position, value_type&& x);
    template<class P>
      iterator insert(const_iterator position, P&&);
    template<class InputIterator>
      void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    insert_return_type insert(node_type&& nh);
    iterator           insert(const_iterator hint, node_type&& nh);

    template<class... Args>
      pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
    template<class... Args>
      pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
    template<class... Args>
      iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
    template<class... Args>
      iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
    template<class M>
      pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
    template<class M>
      pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
    template<class M>
      iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
    template<class M>
      iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(map&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_swappable_v<Compare>);
    void      clear() noexcept;

    template<class C2>
      void merge(map<Key, T, C2, Allocator>& source);
    template<class C2>
      void merge(map<Key, T, C2, Allocator>&& source);
    template<class C2>
      void merge(multimap<Key, T, C2, Allocator>& source);
    template<class C2>
      void merge(multimap<Key, T, C2, Allocator>&& source);

    // observers
    key_compare key_comp() const;
    value_compare value_comp() const;

    // map operations
    iterator       find(const key_type& x);
    const_iterator find(const key_type& x) const;
    template<class K> iterator       find(const K& x);
    template<class K> const_iterator find(const K& x) const;

    size_type      count(const key_type& x) const;
    template<class K> size_type count(const K& x) const;

    bool           contains(const key_type& x) const;
    template<class K> bool contains(const K& x) const;

    iterator       lower_bound(const key_type& x);
    const_iterator lower_bound(const key_type& x) const;
    template<class K> iterator       lower_bound(const K& x);
    template<class K> const_iterator lower_bound(const K& x) const;

    iterator       upper_bound(const key_type& x);
    const_iterator upper_bound(const key_type& x) const;
    template<class K> iterator       upper_bound(const K& x);
    template<class K> const_iterator upper_bound(const K& x) const;

    pair<iterator, iterator>               equal_range(const key_type& x);
    pair<const_iterator, const_iterator>   equal_range(const key_type& x) const;
    template<class K>
      pair<iterator, iterator>             equal_range(const K& x);
    template<class K>
      pair<const_iterator, const_iterator> equal_range(const K& x) const;
  };

  template<class InputIterator, class Compare = less<iter-key-type<InputIterator>>,
           class Allocator = allocator<iter-to-alloc-type<InputIterator>>>
    map(InputIterator, InputIterator, Compare = Compare(), Allocator = Allocator())
      -> map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Compare, Allocator>;

  template<class Key, class T, class Compare = less<Key>,
           class Allocator = allocator<pair<const Key, T>>>
    map(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator())
      -> map<Key, T, Compare, Allocator>;

  template<class InputIterator, class Allocator>
    map(InputIterator, InputIterator, Allocator)
      -> map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
             less<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    map(initializer_list<pair<Key, T>>, Allocator) -> map<Key, T, less<Key>, Allocator>;

  // swap
  template<class Key, class T, class Compare, class Allocator>
    void swap(map<Key, T, Compare, Allocator>& x,
              map<Key, T, Compare, Allocator>& y)
      noexcept(noexcept(x.swap(y)));
}

22.4.4.2 Constructors, copy, and assignment [map.cons]

🔗

explicit map(const Compare& comp, const Allocator& = Allocator());

Effects: Constructs an empty map using the specified comparison object and allocator.

Complexity: Constant.

🔗

template<class InputIterator>
  map(InputIterator first, InputIterator last,
      const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty map using the specified comparison object and allocator, and inserts elements from the range [first, last).

Complexity: Linear in N if the range [first, last) is already sorted using comp and otherwise

N log N

, where N is last - first.

22.4.4.3 Element access [map.access]

🔗

mapped_type& operator[](const key_type& x);

Effects: Equivalent to: return try_emplace(x).first->second;

🔗

mapped_type& operator[](key_type&& x);

Effects: Equivalent to: return try_emplace(move(x)).first->second;

🔗

mapped_type&       at(const key_type& x);
const mapped_type& at(const key_type& x) const;

Returns: A reference to the mapped_type corresponding to x in *this.

Throws: An exception object of type out_of_range if no such element is present.

Complexity: Logarithmic.

22.4.4.4 Modifiers [map.modifiers]

🔗

template<class P>
  pair<iterator, bool> insert(P&& x);
template<class P>
  iterator insert(const_iterator position, P&& x);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: The first form is equivalent to return emplace(std::forward(x)).

The second form is equivalent to return emplace_hint(position, std::forward(x)).

🔗

template<class... Args>
  pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);

Preconditions: value_type is Cpp17EmplaceConstructible into map from piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise inserts an object of type value_type constructed with piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

template<class... Args>
  pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

Preconditions: value_type is Cpp17EmplaceConstructible into map from piecewise_construct, forward_as_tuple(std::move(k)), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise inserts an object of type value_type constructed with piecewise_construct, forward_as_tuple(std::move(k)), forward_as_tuple(std::forward<Args>(args)...).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

template<class M>
  pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into map from k, forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise inserts an object of type value_type constructed with k, std::forward<M>(obj).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

template<class M>
  pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into map from move(k), forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise inserts an object of type value_type constructed with std::move(k), std::forward<M>(obj).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

22.4.4.5 Erasure [map.erasure]

🔗

template<class Key, class T, class Compare, class Allocator, class Predicate>
  typename map<Key, T, Compare, Allocator>::size_type
    erase_if(map<Key, T, Compare, Allocator>& c, Predicate pred);

Effects: Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

22.4.5 Class template multimap [multimap]

22.4.5.1 Overview [multimap.overview]

A multimap is an associative container that supports equivalent keys (possibly containing multiple copies of the same key value) and provides for fast retrieval of values of another type T based on the keys.

The multimap class supports bidirectional iterators.

A multimap meets all of the requirements of a container and of a reversible container ([container.requirements]), of an associative container ([associative.reqmts]), and of an allocator-aware container (Table 76).

A multimap also provides most operations described in [associative.reqmts] for equal keys.

This means that a multimap supports the a_eq operations in [associative.reqmts] but not the a_uniq operations.

For a multimap<Key,T> the key_type is Key and the value_type is pair<const Key,T>.

Descriptions are provided here only for operations on multimap that are not described in one of those tables or for operations where there is additional semantic information.

namespace std {
  template<class Key, class T, class Compare = less<Key>,
           class Allocator = allocator<pair<const Key, T>>>
  class multimap {
  public:
    // types
    using key_type               = Key;
    using mapped_type            = T;
    using value_type             = pair<const Key, T>;
    using key_compare            = Compare;
    using allocator_type         = Allocator;
    using pointer                = typename allocator_traits<Allocator>::pointer;
    using const_pointer          = typename allocator_traits<Allocator>::const_pointer;
    using reference              = value_type&;
    using const_reference        = const value_type&;
    using size_type              = implementation-defined; // see [container.requirements]
    using difference_type        = implementation-defined; // see [container.requirements]
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    using node_type              = unspecified;

    class value_compare {
      friend class multimap;
    protected:
      Compare comp;
      value_compare(Compare c) : comp(c) { }
    public:
      bool operator()(const value_type& x, const value_type& y) const {
        return comp(x.first, y.first);
      }
    };

    // [multimap.cons], construct/copy/destroy
    multimap() : multimap(Compare()) { }
    explicit multimap(const Compare& comp, const Allocator& = Allocator());
    template<class InputIterator>
      multimap(InputIterator first, InputIterator last,
               const Compare& comp = Compare(),
               const Allocator& = Allocator());
    multimap(const multimap& x);
    multimap(multimap&& x);
    explicit multimap(const Allocator&);
    multimap(const multimap&, const Allocator&);
    multimap(multimap&&, const Allocator&);
    multimap(initializer_list<value_type>,
      const Compare& = Compare(),
      const Allocator& = Allocator());
    template<class InputIterator>
      multimap(InputIterator first, InputIterator last, const Allocator& a)
        : multimap(first, last, Compare(), a) { }
    multimap(initializer_list<value_type> il, const Allocator& a)
      : multimap(il, Compare(), a) { }
    ~multimap();
    multimap& operator=(const multimap& x);
    multimap& operator=(multimap&& x)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Compare>);
    multimap& operator=(initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator               begin() noexcept;
    const_iterator         begin() const noexcept;
    iterator               end() noexcept;
    const_iterator         end() const noexcept;

    reverse_iterator       rbegin() noexcept;
    const_reverse_iterator rbegin() const noexcept;
    reverse_iterator       rend() noexcept;
    const_reverse_iterator rend() const noexcept;

    const_iterator         cbegin() const noexcept;
    const_iterator         cend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // [multimap.modifiers], modifiers
    template<class... Args> iterator emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    iterator insert(const value_type& x);
    iterator insert(value_type&& x);
    template<class P> iterator insert(P&& x);
    iterator insert(const_iterator position, const value_type& x);
    iterator insert(const_iterator position, value_type&& x);
    template<class P> iterator insert(const_iterator position, P&& x);
    template<class InputIterator>
      void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    iterator insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(multimap&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_swappable_v<Compare>);
    void      clear() noexcept;

    template<class C2>
      void merge(multimap<Key, T, C2, Allocator>& source);
    template<class C2>
      void merge(multimap<Key, T, C2, Allocator>&& source);
    template<class C2>
      void merge(map<Key, T, C2, Allocator>& source);
    template<class C2>
      void merge(map<Key, T, C2, Allocator>&& source);

    // observers
    key_compare key_comp() const;
    value_compare value_comp() const;

    // map operations
    iterator       find(const key_type& x);
    const_iterator find(const key_type& x) const;
    template<class K> iterator       find(const K& x);
    template<class K> const_iterator find(const K& x) const;

    size_type      count(const key_type& x) const;
    template<class K> size_type count(const K& x) const;

    bool           contains(const key_type& x) const;
    template<class K> bool contains(const K& x) const;

    iterator       lower_bound(const key_type& x);
    const_iterator lower_bound(const key_type& x) const;
    template<class K> iterator       lower_bound(const K& x);
    template<class K> const_iterator lower_bound(const K& x) const;

    iterator       upper_bound(const key_type& x);
    const_iterator upper_bound(const key_type& x) const;
    template<class K> iterator       upper_bound(const K& x);
    template<class K> const_iterator upper_bound(const K& x) const;

    pair<iterator, iterator>               equal_range(const key_type& x);
    pair<const_iterator, const_iterator>   equal_range(const key_type& x) const;
    template<class K>
      pair<iterator, iterator>             equal_range(const K& x);
    template<class K>
      pair<const_iterator, const_iterator> equal_range(const K& x) const;
  };

  template<class InputIterator, class Compare = less<iter-key-type<InputIterator>>,
           class Allocator = allocator<iter-to-alloc-type<InputIterator>>>
    multimap(InputIterator, InputIterator, Compare = Compare(), Allocator = Allocator())
      -> multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                  Compare, Allocator>;

  template<class Key, class T, class Compare = less<Key>,
           class Allocator = allocator<pair<const Key, T>>>
    multimap(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator())
      -> multimap<Key, T, Compare, Allocator>;

  template<class InputIterator, class Allocator>
    multimap(InputIterator, InputIterator, Allocator)
      -> multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                  less<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    multimap(initializer_list<pair<Key, T>>, Allocator)
      -> multimap<Key, T, less<Key>, Allocator>;

  // swap
  template<class Key, class T, class Compare, class Allocator>
    void swap(multimap<Key, T, Compare, Allocator>& x,
              multimap<Key, T, Compare, Allocator>& y)
      noexcept(noexcept(x.swap(y)));
}

22.4.5.2 Constructors [multimap.cons]

🔗

explicit multimap(const Compare& comp, const Allocator& = Allocator());

Effects: Constructs an empty multimap using the specified comparison object and allocator.

Complexity: Constant.

🔗

template<class InputIterator>
  multimap(InputIterator first, InputIterator last,
           const Compare& comp = Compare(),
           const Allocator& = Allocator());

Effects: Constructs an empty multimap using the specified comparison object and allocator, and inserts elements from the range [first, last).

Complexity: Linear in N if the range [first, last) is already sorted using comp and otherwise

N log N

, where N is last - first.

22.4.5.3 Modifiers [multimap.modifiers]

🔗

template<class P> iterator insert(P&& x);
template<class P> iterator insert(const_iterator position, P&& x);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: The first form is equivalent to return emplace(std::forward(x)).

The second form is equivalent to return emplace_hint(position, std::forward(x)).

22.4.5.4 Erasure [multimap.erasure]

🔗

template<class Key, class T, class Compare, class Allocator, class Predicate>
  typename multimap<Key, T, Compare, Allocator>::size_type
    erase_if(multimap<Key, T, Compare, Allocator>& c, Predicate pred);

Effects: Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

22.4.6 Class template set [set]

22.4.6.1 Overview [set.overview]

A set is an associative container that supports unique keys (contains at most one of each key value) and provides for fast retrieval of the keys themselves.

The set class supports bidirectional iterators.

A set meets all of the requirements of a container, of a reversible container ([container.requirements]), of an associative container ([associative.reqmts]), and of an allocator-aware container (Table 76).

A set also provides most operations described in [associative.reqmts] for unique keys.

This means that a set supports the a_uniq operations in [associative.reqmts] but not the a_eq operations.

For a set<Key> both the key_type and value_type are Key.

Descriptions are provided here only for operations on set that are not described in one of these tables and for operations where there is additional semantic information.

namespace std {
  template<class Key, class Compare = less<Key>,
           class Allocator = allocator<Key>>
  class set {
  public:
    // types
    using key_type               = Key;
    using key_compare            = Compare;
    using value_type             = Key;
    using value_compare          = Compare;
    using allocator_type         = Allocator;
    using pointer                = typename allocator_traits<Allocator>::pointer;
    using const_pointer          = typename allocator_traits<Allocator>::const_pointer;
    using reference              = value_type&;
    using const_reference        = const value_type&;
    using size_type              = implementation-defined; // see [container.requirements]
    using difference_type        = implementation-defined; // see [container.requirements]
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    using node_type              = unspecified;
    using insert_return_type     = insert-return-type<iterator, node_type>;

    // [set.cons], construct/copy/destroy
    set() : set(Compare()) { }
    explicit set(const Compare& comp, const Allocator& = Allocator());
    template<class InputIterator>
      set(InputIterator first, InputIterator last,
          const Compare& comp = Compare(), const Allocator& = Allocator());
    set(const set& x);
    set(set&& x);
    explicit set(const Allocator&);
    set(const set&, const Allocator&);
    set(set&&, const Allocator&);
    set(initializer_list<value_type>, const Compare& = Compare(),
        const Allocator& = Allocator());
    template<class InputIterator>
      set(InputIterator first, InputIterator last, const Allocator& a)
        : set(first, last, Compare(), a) { }
    set(initializer_list<value_type> il, const Allocator& a)
      : set(il, Compare(), a) { }
    ~set();
    set& operator=(const set& x);
    set& operator=(set&& x)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Compare>);
    set& operator=(initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator               begin() noexcept;
    const_iterator         begin() const noexcept;
    iterator               end() noexcept;
    const_iterator         end() const noexcept;

    reverse_iterator       rbegin() noexcept;
    const_reverse_iterator rbegin() const noexcept;
    reverse_iterator       rend() noexcept;
    const_reverse_iterator rend() const noexcept;

    const_iterator         cbegin() const noexcept;
    const_iterator         cend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    pair<iterator,bool> insert(const value_type& x);
    pair<iterator,bool> insert(value_type&& x);
    iterator insert(const_iterator position, const value_type& x);
    iterator insert(const_iterator position, value_type&& x);
    template<class InputIterator>
      void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    insert_return_type insert(node_type&& nh);
    iterator           insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(set&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_swappable_v<Compare>);
    void      clear() noexcept;

    template<class C2>
      void merge(set<Key, C2, Allocator>& source);
    template<class C2>
      void merge(set<Key, C2, Allocator>&& source);
    template<class C2>
      void merge(multiset<Key, C2, Allocator>& source);
    template<class C2>
      void merge(multiset<Key, C2, Allocator>&& source);

    // observers
    key_compare key_comp() const;
    value_compare value_comp() const;

    // set operations
    iterator       find(const key_type& x);
    const_iterator find(const key_type& x) const;
    template<class K> iterator       find(const K& x);
    template<class K> const_iterator find(const K& x) const;

    size_type      count(const key_type& x) const;
    template<class K> size_type count(const K& x) const;

    bool           contains(const key_type& x) const;
    template<class K> bool contains(const K& x) const;

    iterator       lower_bound(const key_type& x);
    const_iterator lower_bound(const key_type& x) const;
    template<class K> iterator       lower_bound(const K& x);
    template<class K> const_iterator lower_bound(const K& x) const;

    iterator       upper_bound(const key_type& x);
    const_iterator upper_bound(const key_type& x) const;
    template<class K> iterator       upper_bound(const K& x);
    template<class K> const_iterator upper_bound(const K& x) const;

    pair<iterator, iterator>               equal_range(const key_type& x);
    pair<const_iterator, const_iterator>   equal_range(const key_type& x) const;
    template<class K>
      pair<iterator, iterator>             equal_range(const K& x);
    template<class K>
      pair<const_iterator, const_iterator> equal_range(const K& x) const;
  };

  template<class InputIterator,
           class Compare = less<iter-value-type<InputIterator>>,
           class Allocator = allocator<iter-value-type<InputIterator>>>
    set(InputIterator, InputIterator,
        Compare = Compare(), Allocator = Allocator())
      -> set<iter-value-type<InputIterator>, Compare, Allocator>;

  template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>>
    set(initializer_list<Key>, Compare = Compare(), Allocator = Allocator())
      -> set<Key, Compare, Allocator>;

  template<class InputIterator, class Allocator>
    set(InputIterator, InputIterator, Allocator)
      -> set<iter-value-type<InputIterator>,
             less<iter-value-type<InputIterator>>, Allocator>;

  template<class Key, class Allocator>
    set(initializer_list<Key>, Allocator) -> set<Key, less<Key>, Allocator>;

  // swap
  template<class Key, class Compare, class Allocator>
    void swap(set<Key, Compare, Allocator>& x,
              set<Key, Compare, Allocator>& y)
      noexcept(noexcept(x.swap(y)));
}

22.4.6.2 Constructors, copy, and assignment [set.cons]

🔗

explicit set(const Compare& comp, const Allocator& = Allocator());

Effects: Constructs an empty set using the specified comparison objects and allocator.

Complexity: Constant.

🔗

template<class InputIterator>
  set(InputIterator first, InputIterator last,
      const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty set using the specified comparison object and allocator, and inserts elements from the range [first, last).

Complexity: Linear in N if the range [first, last) is already sorted using comp and otherwise

N log N

, where N is last - first.

22.4.6.3 Erasure [set.erasure]

🔗

template<class Key, class Compare, class Allocator, class Predicate>
  typename set<Key, Compare, Allocator>::size_type
    erase_if(set<Key, Compare, Allocator>& c, Predicate pred);

Effects: Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

22.4.7 Class template multiset [multiset]

22.4.7.1 Overview [multiset.overview]

A multiset is an associative container that supports equivalent keys (possibly contains multiple copies of the same key value) and provides for fast retrieval of the keys themselves.

The multiset class supports bidirectional iterators.

A multiset meets all of the requirements of a container, of a reversible container ([container.requirements]), of an associative container ([associative.reqmts]), and of an allocator-aware container (Table 76).

multiset also provides most operations described in [associative.reqmts] for duplicate keys.

This means that a multiset supports the a_eq operations in [associative.reqmts] but not the a_uniq operations.

For a multiset<Key> both the key_type and value_type are Key.

Descriptions are provided here only for operations on multiset that are not described in one of these tables and for operations where there is additional semantic information.

namespace std {
  template<class Key, class Compare = less<Key>,
           class Allocator = allocator<Key>>
  class multiset {
  public:
    // types
    using key_type               = Key;
    using key_compare            = Compare;
    using value_type             = Key;
    using value_compare          = Compare;
    using allocator_type         = Allocator;
    using pointer                = typename allocator_traits<Allocator>::pointer;
    using const_pointer          = typename allocator_traits<Allocator>::const_pointer;
    using reference              = value_type&;
    using const_reference        = const value_type&;
    using size_type              = implementation-defined; // see [container.requirements]
    using difference_type        = implementation-defined; // see [container.requirements]
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    using node_type              = unspecified;

    // [multiset.cons], construct/copy/destroy
    multiset() : multiset(Compare()) { }
    explicit multiset(const Compare& comp, const Allocator& = Allocator());
    template<class InputIterator>
      multiset(InputIterator first, InputIterator last,
               const Compare& comp = Compare(), const Allocator& = Allocator());
    multiset(const multiset& x);
    multiset(multiset&& x);
    explicit multiset(const Allocator&);
    multiset(const multiset&, const Allocator&);
    multiset(multiset&&, const Allocator&);
    multiset(initializer_list<value_type>, const Compare& = Compare(),
             const Allocator& = Allocator());
    template<class InputIterator>
      multiset(InputIterator first, InputIterator last, const Allocator& a)
        : multiset(first, last, Compare(), a) { }
    multiset(initializer_list<value_type> il, const Allocator& a)
      : multiset(il, Compare(), a) { }
    ~multiset();
    multiset& operator=(const multiset& x);
    multiset& operator=(multiset&& x)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Compare>);
    multiset& operator=(initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator               begin() noexcept;
    const_iterator         begin() const noexcept;
    iterator               end() noexcept;
    const_iterator         end() const noexcept;

    reverse_iterator       rbegin() noexcept;
    const_reverse_iterator rbegin() const noexcept;
    reverse_iterator       rend() noexcept;
    const_reverse_iterator rend() const noexcept;

    const_iterator         cbegin() const noexcept;
    const_iterator         cend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> iterator emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    iterator insert(const value_type& x);
    iterator insert(value_type&& x);
    iterator insert(const_iterator position, const value_type& x);
    iterator insert(const_iterator position, value_type&& x);
    template<class InputIterator>
      void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    iterator insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(multiset&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_swappable_v<Compare>);
    void      clear() noexcept;

    template<class C2>
      void merge(multiset<Key, C2, Allocator>& source);
    template<class C2>
      void merge(multiset<Key, C2, Allocator>&& source);
    template<class C2>
      void merge(set<Key, C2, Allocator>& source);
    template<class C2>
      void merge(set<Key, C2, Allocator>&& source);

    // observers
    key_compare key_comp() const;
    value_compare value_comp() const;

    // set operations
    iterator       find(const key_type& x);
    const_iterator find(const key_type& x) const;
    template<class K> iterator       find(const K& x);
    template<class K> const_iterator find(const K& x) const;

    size_type      count(const key_type& x) const;
    template<class K> size_type count(const K& x) const;

    bool           contains(const key_type& x) const;
    template<class K> bool contains(const K& x) const;

    iterator       lower_bound(const key_type& x);
    const_iterator lower_bound(const key_type& x) const;
    template<class K> iterator       lower_bound(const K& x);
    template<class K> const_iterator lower_bound(const K& x) const;

    iterator       upper_bound(const key_type& x);
    const_iterator upper_bound(const key_type& x) const;
    template<class K> iterator       upper_bound(const K& x);
    template<class K> const_iterator upper_bound(const K& x) const;

    pair<iterator, iterator>               equal_range(const key_type& x);
    pair<const_iterator, const_iterator>   equal_range(const key_type& x) const;
    template<class K>
      pair<iterator, iterator>             equal_range(const K& x);
    template<class K>
      pair<const_iterator, const_iterator> equal_range(const K& x) const;
  };

  template<class InputIterator,
           class Compare = less<iter-value-type<InputIterator>>,
           class Allocator = allocator<iter-value-type<InputIterator>>>
    multiset(InputIterator, InputIterator,
             Compare = Compare(), Allocator = Allocator())
      -> multiset<iter-value-type<InputIterator>, Compare, Allocator>;

  template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>>
    multiset(initializer_list<Key>, Compare = Compare(), Allocator = Allocator())
      -> multiset<Key, Compare, Allocator>;

  template<class InputIterator, class Allocator>
    multiset(InputIterator, InputIterator, Allocator)
      -> multiset<iter-value-type<InputIterator>,
                  less<iter-value-type<InputIterator>>, Allocator>;

  template<class Key, class Allocator>
    multiset(initializer_list<Key>, Allocator) -> multiset<Key, less<Key>, Allocator>;

  // swap
  template<class Key, class Compare, class Allocator>
    void swap(multiset<Key, Compare, Allocator>& x,
              multiset<Key, Compare, Allocator>& y)
      noexcept(noexcept(x.swap(y)));
}

22.4.7.2 Constructors [multiset.cons]

🔗

explicit multiset(const Compare& comp, const Allocator& = Allocator());

Effects: Constructs an empty multiset using the specified comparison object and allocator.

Complexity: Constant.

🔗

template<class InputIterator>
  multiset(InputIterator first, InputIterator last,
           const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty multiset using the specified comparison object and allocator, and inserts elements from the range [first, last).

Complexity: Linear in N if the range [first, last) is already sorted using comp and otherwise

N log N

, where N is last - first.

22.4.7.3 Erasure [multiset.erasure]

🔗

template<class Key, class Compare, class Allocator, class Predicate>
  typename multiset<Key, Compare, Allocator>::size_type
    erase_if(multiset<Key, Compare, Allocator>& c, Predicate pred);

Effects: Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

22.5 Unordered associative containers [unord]

22.5.1 In general [unord.general]

The header <unordered_map> defines the class templates unordered_map and unordered_multimap; the header <unordered_set> defines the class templates unordered_set and unordered_multiset.

The exposition-only alias templates iter-value-type, iter-key-type, iter-mapped-type, and iter-to-alloc-type defined in [associative.general] may appear in deduction guides for unordered containers.

22.5.2 Header <unordered_map> synopsis [unord.map.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  // [unord.map], class template unordered_map
  template<class Key,
           class T,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Alloc = allocator<pair<const Key, T>>>
    class unordered_map;

  // [unord.multimap], class template unordered_multimap
  template<class Key,
           class T,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Alloc = allocator<pair<const Key, T>>>
    class unordered_multimap;

  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_map<Key, T, Hash, Pred, Alloc>& a,
                    const unordered_map<Key, T, Hash, Pred, Alloc>& b);

  template<class Key, class T, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& a,
                    const unordered_multimap<Key, T, Hash, Pred, Alloc>& b);

  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
              unordered_map<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
              unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class K, class T, class H, class P, class A, class Predicate>
    typename unordered_map<K, T, H, P, A>::size_type
      erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred);

  template<class K, class T, class H, class P, class A, class Predicate>
    typename unordered_multimap<K, T, H, P, A>::size_type
      erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred);

  namespace pmr {
    template<class Key,
             class T,
             class Hash = hash<Key>,
             class Pred = equal_to<Key>>
      using unordered_map =
        std::unordered_map<Key, T, Hash, Pred,
                           polymorphic_allocator<pair<const Key, T>>>;
    template<class Key,
             class T,
             class Hash = hash<Key>,
             class Pred = equal_to<Key>>
      using unordered_multimap =
        std::unordered_multimap<Key, T, Hash, Pred,
                                polymorphic_allocator<pair<const Key, T>>>;

  }
}

22.5.3 Header <unordered_set> synopsis [unord.set.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  // [unord.set], class template unordered_set
  template<class Key,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Alloc = allocator<Key>>
    class unordered_set;

  // [unord.multiset], class template unordered_multiset
  template<class Key,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Alloc = allocator<Key>>
    class unordered_multiset;

  template<class Key, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_set<Key, Hash, Pred, Alloc>& a,
                    const unordered_set<Key, Hash, Pred, Alloc>& b);

  template<class Key, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_multiset<Key, Hash, Pred, Alloc>& a,
                    const unordered_multiset<Key, Hash, Pred, Alloc>& b);

  template<class Key, class Hash, class Pred, class Alloc>
    void swap(unordered_set<Key, Hash, Pred, Alloc>& x,
              unordered_set<Key, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class Key, class Hash, class Pred, class Alloc>
    void swap(unordered_multiset<Key, Hash, Pred, Alloc>& x,
              unordered_multiset<Key, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template<class K, class H, class P, class A, class Predicate>
    typename unordered_set<K, H, P, A>::size_type
      erase_if(unordered_set<K, H, P, A>& c, Predicate pred);

  template<class K, class H, class P, class A, class Predicate>
    typename unordered_multiset<K, H, P, A>::size_type
      erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred);

  namespace pmr {
    template<class Key,
             class Hash = hash<Key>,
             class Pred = equal_to<Key>>
      using unordered_set = std::unordered_set<Key, Hash, Pred,
                                               polymorphic_allocator<Key>>;

    template<class Key,
             class Hash = hash<Key>,
             class Pred = equal_to<Key>>
      using unordered_multiset = std::unordered_multiset<Key, Hash, Pred,
                                                         polymorphic_allocator<Key>>;
  }
}

22.5.4 Class template unordered_map [unord.map]

22.5.4.1 Overview [unord.map.overview]

An unordered_map is an unordered associative container that supports unique keys (an unordered_map contains at most one of each key value) and that associates values of another type mapped_type with the keys.

The unordered_map class supports forward iterators.

An unordered_map meets all of the requirements of a container, of an unordered associative container, and of an allocator-aware container (Table 76).

It provides the operations described in the preceding requirements table for unique keys; that is, an unordered_map supports the a_uniq operations in that table, not the a_eq operations.

For an unordered_map<Key, T> the key type is Key, the mapped type is T, and the value type is pair<const Key, T>.

Subclause [unord.map] only describes operations on unordered_map that are not described in one of the requirement tables, or for which there is additional semantic information.

namespace std {
  template<class Key,
           class T,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Allocator = allocator<pair<const Key, T>>>
  class unordered_map {
  public:
    // types
    using key_type             = Key;
    using mapped_type          = T;
    using value_type           = pair<const Key, T>;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename allocator_traits<Allocator>::pointer;
    using const_pointer        = typename allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = implementation-defined; // see [container.requirements]
    using difference_type      = implementation-defined; // see [container.requirements]

    using iterator             = implementation-defined; // see [container.requirements]
    using const_iterator       = implementation-defined; // see [container.requirements]
    using local_iterator       = implementation-defined; // see [container.requirements]
    using const_local_iterator = implementation-defined; // see [container.requirements]
    using node_type            = unspecified;
    using insert_return_type   = insert-return-type<iterator, node_type>;

    // [unord.map.cnstr], construct/copy/destroy
    unordered_map();
    explicit unordered_map(size_type n,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l,
                    size_type n = see below,
                    const hasher& hf = hasher(),
                    const key_equal& eql = key_equal(),
                    const allocator_type& a = allocator_type());
    unordered_map(const unordered_map&);
    unordered_map(unordered_map&&);
    explicit unordered_map(const Allocator&);
    unordered_map(const unordered_map&, const Allocator&);
    unordered_map(unordered_map&&, const Allocator&);
    unordered_map(initializer_list<value_type> il,
                  size_type n = see below,
                  const hasher& hf = hasher(),
                  const key_equal& eql = key_equal(),
                  const allocator_type& a = allocator_type());
    unordered_map(size_type n, const allocator_type& a)
      : unordered_map(n, hasher(), key_equal(), a) { }
    unordered_map(size_type n, const hasher& hf, const allocator_type& a)
      : unordered_map(n, hf, key_equal(), a) { }
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
        : unordered_map(f, l, n, hasher(), key_equal(), a) { }
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                    const allocator_type& a)
        : unordered_map(f, l, n, hf, key_equal(), a) { }
    unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a)
      : unordered_map(il, n, hasher(), key_equal(), a) { }
    unordered_map(initializer_list<value_type> il, size_type n, const hasher& hf,
                  const allocator_type& a)
      : unordered_map(il, n, hf, key_equal(), a) { }
    ~unordered_map();
    unordered_map& operator=(const unordered_map&);
    unordered_map& operator=(unordered_map&&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Hash> &&
               is_nothrow_move_assignable_v<Pred>);
    unordered_map& operator=(initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // [unord.map.modifiers], modifiers
    template<class... Args> pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    pair<iterator, bool> insert(const value_type& obj);
    pair<iterator, bool> insert(value_type&& obj);
    template<class P> pair<iterator, bool> insert(P&& obj);
    iterator       insert(const_iterator hint, const value_type& obj);
    iterator       insert(const_iterator hint, value_type&& obj);
    template<class P> iterator insert(const_iterator hint, P&& obj);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    insert_return_type insert(node_type&& nh);
    iterator           insert(const_iterator hint, node_type&& nh);

    template<class... Args>
      pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
    template<class... Args>
      pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
    template<class... Args>
      iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
    template<class... Args>
      iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
    template<class M>
      pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
    template<class M>
      pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
    template<class M>
      iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
    template<class M>
      iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_map&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_swappable_v<Hash> &&
               is_nothrow_swappable_v<Pred>);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // map operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    template<class K>
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    pair<iterator, iterator>               equal_range(const key_type& k);
    pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // [unord.map.elem], element access
    mapped_type& operator[](const key_type& k);
    mapped_type& operator[](key_type&& k);
    mapped_type& at(const key_type& k);
    const mapped_type& at(const key_type& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;
    size_type max_bucket_count() const noexcept;
    size_type bucket_size(size_type n) const;
    size_type bucket(const key_type& k) const;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  template<class InputIterator,
           class Hash = hash<iter-key-type<InputIterator>>,
           class Pred = equal_to<iter-key-type<InputIterator>>,
           class Allocator = allocator<iter-to-alloc-type<InputIterator>>>
    unordered_map(InputIterator, InputIterator, typename see below::size_type = see below,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash, Pred,
                       Allocator>;

  template<class Key, class T, class Hash = hash<Key>,
           class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>>
    unordered_map(initializer_list<pair<Key, T>>,
                  typename see below::size_type = see below, Hash = Hash(),
                  Pred = Pred(), Allocator = Allocator())
      -> unordered_map<Key, T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_map(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                       hash<iter-key-type<InputIterator>>,
                       equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_map(InputIterator, InputIterator, Allocator)
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                       hash<iter-key-type<InputIterator>>,
                       equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_map(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator)
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                       equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_map(initializer_list<pair<Key, T>>, typename see below::size_type,
                  Allocator)
      -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_map(initializer_list<pair<Key, T>>, Allocator)
      -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>;

  template<class Key, class T, class Hash, class Allocator>
    unordered_map(initializer_list<pair<Key, T>>, typename see below::size_type, Hash,
                  Allocator)
      -> unordered_map<Key, T, Hash, equal_to<Key>, Allocator>;

  // swap
  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
              unordered_map<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
}

A size_type parameter type in an unordered_map deduction guide refers to the size_type member type of the type deduced by the deduction guide.

22.5.4.2 Constructors [unord.map.cnstr]

🔗

unordered_map() : unordered_map(size_type(see below)) { }
explicit unordered_map(size_type n,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_map using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

For the default constructor, the number of buckets is implementation-defined.

max_load_factor() returns 1.0.

Complexity: Constant.

🔗

template<class InputIterator>
  unordered_map(InputIterator f, InputIterator l,
                size_type n = see below,
                const hasher& hf = hasher(),
                const key_equal& eql = key_equal(),
                const allocator_type& a = allocator_type());
unordered_map(initializer_list<value_type> il,
              size_type n = see below,
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_map using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

If n is not provided, the number of buckets is implementation-defined.

Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form.

max_load_factor() returns 1.0.

Complexity: Average case linear, worst case quadratic.

22.5.4.3 Element access [unord.map.elem]

🔗

mapped_type& operator[](const key_type& k);

Effects: Equivalent to: return try_emplace(k).first->second;

🔗

mapped_type& operator[](key_type&& k);

Effects: Equivalent to: return try_emplace(move(k)).first->second;

🔗

mapped_type& at(const key_type& k);
const mapped_type& at(const key_type& k) const;

Returns: A reference to x.second, where x is the (unique) element whose key is equivalent to k.

Throws: An exception object of type out_of_range if no such element is present.

22.5.4.4 Modifiers [unord.map.modifiers]

🔗

template<class P>
  pair<iterator, bool> insert(P&& obj);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: Equivalent to: return emplace(std::forward(obj));

🔗

template<class P>
  iterator insert(const_iterator hint, P&& obj);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: Equivalent to: return emplace_hint(hint, std::forward(obj));

🔗

template<class... Args>
  pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_map from piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise inserts an object of type value_type constructed with piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

template<class... Args>
  pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_map from piecewise_construct, forward_as_tuple(std::move(k)), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise inserts an object of type value_type constructed with piecewise_construct, forward_as_tuple(std::move(k)), forward_as_tuple(std::forward<Args>(args)...).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

template<class M>
  pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_map from k, std::forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise inserts an object of type value_type constructed with k, std::forward<M>(obj).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

template<class M>
  pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into unordered_map from std::move(k), std::forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise inserts an object of type value_type constructed with std::move(k), std::forward<M>(obj).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

22.5.4.5 Erasure [unord.map.erasure]

🔗

template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_map<K, T, H, P, A>::size_type
    erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred);

Effects: Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

22.5.5 Class template unordered_multimap [unord.multimap]

22.5.5.1 Overview [unord.multimap.overview]

An unordered_multimap is an unordered associative container that supports equivalent keys (an instance of unordered_multimap may contain multiple copies of each key value) and that associates values of another type mapped_type with the keys.

The unordered_multimap class supports forward iterators.

An unordered_multimap meets all of the requirements of a container, of an unordered associative container, and of an allocator-aware container (Table 76).

It provides the operations described in the preceding requirements table for equivalent keys; that is, an unordered_multimap supports the a_eq operations in that table, not the a_uniq operations.

For an unordered_multimap<Key, T> the key type is Key, the mapped type is T, and the value type is pair<const Key, T>.

Subclause [unord.multimap] only describes operations on unordered_multimap that are not described in one of the requirement tables, or for which there is additional semantic information.

namespace std {
  template<class Key,
           class T,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Allocator = allocator<pair<const Key, T>>>
  class unordered_multimap {
  public:
    // types
    using key_type             = Key;
    using mapped_type          = T;
    using value_type           = pair<const Key, T>;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename allocator_traits<Allocator>::pointer;
    using const_pointer        = typename allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = implementation-defined; // see [container.requirements]
    using difference_type      = implementation-defined; // see [container.requirements]

    using iterator             = implementation-defined; // see [container.requirements]
    using const_iterator       = implementation-defined; // see [container.requirements]
    using local_iterator       = implementation-defined; // see [container.requirements]
    using const_local_iterator = implementation-defined; // see [container.requirements]
    using node_type            = unspecified;

    // [unord.multimap.cnstr], construct/copy/destroy
    unordered_multimap();
    explicit unordered_multimap(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l,
                         size_type n = see below,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    unordered_multimap(const unordered_multimap&);
    unordered_multimap(unordered_multimap&&);
    explicit unordered_multimap(const Allocator&);
    unordered_multimap(const unordered_multimap&, const Allocator&);
    unordered_multimap(unordered_multimap&&, const Allocator&);
    unordered_multimap(initializer_list<value_type> il,
                       size_type n = see below,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_multimap(size_type n, const allocator_type& a)
      : unordered_multimap(n, hasher(), key_equal(), a) { }
    unordered_multimap(size_type n, const hasher& hf, const allocator_type& a)
      : unordered_multimap(n, hf, key_equal(), a) { }
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
        : unordered_multimap(f, l, n, hasher(), key_equal(), a) { }
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a)
        : unordered_multimap(f, l, n, hf, key_equal(), a) { }
    unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a)
      : unordered_multimap(il, n, hasher(), key_equal(), a) { }
    unordered_multimap(initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a)
      : unordered_multimap(il, n, hf, key_equal(), a) { }
    ~unordered_multimap();
    unordered_multimap& operator=(const unordered_multimap&);
    unordered_multimap& operator=(unordered_multimap&&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Hash> &&
               is_nothrow_move_assignable_v<Pred>);
    unordered_multimap& operator=(initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // [unord.multimap.modifiers], modifiers
    template<class... Args> iterator emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    iterator insert(const value_type& obj);
    iterator insert(value_type&& obj);
    template<class P> iterator insert(P&& obj);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class P> iterator insert(const_iterator hint, P&& obj);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    iterator insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_multimap&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_swappable_v<Hash> &&
               is_nothrow_swappable_v<Pred>);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // map operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    pair<iterator, iterator>               equal_range(const key_type& k);
    pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;
    size_type max_bucket_count() const noexcept;
    size_type bucket_size(size_type n) const;
    size_type bucket(const key_type& k) const;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  template<class InputIterator,
           class Hash = hash<iter-key-type<InputIterator>>,
           class Pred = equal_to<iter-key-type<InputIterator>>,
           class Allocator = allocator<iter-to-alloc-type<InputIterator>>>
    unordered_multimap(InputIterator, InputIterator,
                       typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            Hash, Pred, Allocator>;

  template<class Key, class T, class Hash = hash<Key>,
           class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>>
    unordered_multimap(initializer_list<pair<Key, T>>,
                       typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multimap<Key, T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            hash<iter-key-type<InputIterator>>,
                            equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multimap(InputIterator, InputIterator, Allocator)
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            hash<iter-key-type<InputIterator>>,
                            equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Hash,
                       Allocator)
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                            equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_multimap(initializer_list<pair<Key, T>>, typename see below::size_type,
                       Allocator)
      -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_multimap(initializer_list<pair<Key, T>>, Allocator)
      -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>;

  template<class Key, class T, class Hash, class Allocator>
    unordered_multimap(initializer_list<pair<Key, T>>, typename see below::size_type,
                       Hash, Allocator)
      -> unordered_multimap<Key, T, Hash, equal_to<Key>, Allocator>;

  // swap
  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
              unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
}

A size_type parameter type in an unordered_multimap deduction guide refers to the size_type member type of the type deduced by the deduction guide.

22.5.5.2 Constructors [unord.multimap.cnstr]

🔗

unordered_multimap() : unordered_multimap(size_type(see below)) { }
explicit unordered_multimap(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_multimap using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

For the default constructor, the number of buckets is implementation-defined.

max_load_factor() returns 1.0.

Complexity: Constant.

🔗

template<class InputIterator>
  unordered_multimap(InputIterator f, InputIterator l,
                     size_type n = see below,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());
unordered_multimap(initializer_list<value_type> il,
                   size_type n = see below,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_multimap using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

If n is not provided, the number of buckets is implementation-defined.

Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form.

max_load_factor() returns 1.0.

Complexity: Average case linear, worst case quadratic.

22.5.5.3 Modifiers [unord.multimap.modifiers]

🔗

template<class P>
  iterator insert(P&& obj);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: Equivalent to: return emplace(std::forward(obj));

🔗

template<class P>
  iterator insert(const_iterator hint, P&& obj);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: Equivalent to: return emplace_hint(hint, std::forward(obj));

22.5.5.4 Erasure [unord.multimap.erasure]

🔗

template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_multimap<K, T, H, P, A>::size_type
    erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred);

Effects: Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

22.5.6 Class template unordered_set [unord.set]

22.5.6.1 Overview [unord.set.overview]

An unordered_set is an unordered associative container that supports unique keys (an unordered_set contains at most one of each key value) and in which the elements' keys are the elements themselves.

The unordered_set class supports forward iterators.

An unordered_set meets all of the requirements of a container, of an unordered associative container, and of an allocator-aware container (Table 76).

It provides the operations described in the preceding requirements table for unique keys; that is, an unordered_set supports the a_uniq operations in that table, not the a_eq operations.

For an unordered_set<Key> the key type and the value type are both Key.

The iterator and const_iterator types are both constant iterator types.

It is unspecified whether they are the same type.

Subclause [unord.set] only describes operations on unordered_set that are not described in one of the requirement tables, or for which there is additional semantic information.

namespace std {
  template<class Key,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Allocator = allocator<Key>>
  class unordered_set {
  public:
    // types
    using key_type             = Key;
    using value_type           = Key;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename allocator_traits<Allocator>::pointer;
    using const_pointer        = typename allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = implementation-defined; // see [container.requirements]
    using difference_type      = implementation-defined; // see [container.requirements]

    using iterator             = implementation-defined; // see [container.requirements]
    using const_iterator       = implementation-defined; // see [container.requirements]
    using local_iterator       = implementation-defined; // see [container.requirements]
    using const_local_iterator = implementation-defined; // see [container.requirements]
    using node_type            = unspecified;
    using insert_return_type   = insert-return-type<iterator, node_type>;

    // [unord.set.cnstr], construct/copy/destroy
    unordered_set();
    explicit unordered_set(size_type n,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_set(InputIterator f, InputIterator l,
                    size_type n = see below,
                    const hasher& hf = hasher(),
                    const key_equal& eql = key_equal(),
                    const allocator_type& a = allocator_type());
    unordered_set(const unordered_set&);
    unordered_set(unordered_set&&);
    explicit unordered_set(const Allocator&);
    unordered_set(const unordered_set&, const Allocator&);
    unordered_set(unordered_set&&, const Allocator&);
    unordered_set(initializer_list<value_type> il,
                  size_type n = see below,
                  const hasher& hf = hasher(),
                  const key_equal& eql = key_equal(),
                  const allocator_type& a = allocator_type());
    unordered_set(size_type n, const allocator_type& a)
      : unordered_set(n, hasher(), key_equal(), a) { }
    unordered_set(size_type n, const hasher& hf, const allocator_type& a)
      : unordered_set(n, hf, key_equal(), a) { }
    template<class InputIterator>
      unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
        : unordered_set(f, l, n, hasher(), key_equal(), a) { }
    template<class InputIterator>
      unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                    const allocator_type& a)
      : unordered_set(f, l, n, hf, key_equal(), a) { }
    unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a)
      : unordered_set(il, n, hasher(), key_equal(), a) { }
    unordered_set(initializer_list<value_type> il, size_type n, const hasher& hf,
                  const allocator_type& a)
      : unordered_set(il, n, hf, key_equal(), a) { }
    ~unordered_set();
    unordered_set& operator=(const unordered_set&);
    unordered_set& operator=(unordered_set&&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Hash> &&
               is_nothrow_move_assignable_v<Pred>);
    unordered_set& operator=(initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    pair<iterator, bool> insert(const value_type& obj);
    pair<iterator, bool> insert(value_type&& obj);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    insert_return_type insert(node_type&& nh);
    iterator           insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_set&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_swappable_v<Hash> &&
               is_nothrow_swappable_v<Pred>);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // set operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    pair<iterator, iterator>               equal_range(const key_type& k);
    pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;
    size_type max_bucket_count() const noexcept;
    size_type bucket_size(size_type n) const;
    size_type bucket(const key_type& k) const;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  template<class InputIterator,
           class Hash = hash<iter-value-type<InputIterator>>,
           class Pred = equal_to<iter-value-type<InputIterator>>,
           class Allocator = allocator<iter-value-type<InputIterator>>>
    unordered_set(InputIterator, InputIterator, typename see below::size_type = see below,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_set<iter-value-type<InputIterator>,
                       Hash, Pred, Allocator>;

  template<class T, class Hash = hash<T>,
           class Pred = equal_to<T>, class Allocator = allocator<T>>
    unordered_set(initializer_list<T>, typename see below::size_type = see below,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_set<T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_set(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_set<iter-value-type<InputIterator>,
                       hash<iter-value-type<InputIterator>>,
                       equal_to<iter-value-type<InputIterator>>,
                       Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_set(InputIterator, InputIterator, typename see below::size_type,
                  Hash, Allocator)
      -> unordered_set<iter-value-type<InputIterator>, Hash,
                       equal_to<iter-value-type<InputIterator>>,
                       Allocator>;

  template<class T, class Allocator>
    unordered_set(initializer_list<T>, typename see below::size_type, Allocator)
      -> unordered_set<T, hash<T>, equal_to<T>, Allocator>;

  template<class T, class Hash, class Allocator>
    unordered_set(initializer_list<T>, typename see below::size_type, Hash, Allocator)
      -> unordered_set<T, Hash, equal_to<T>, Allocator>;

  // swap
  template<class Key, class Hash, class Pred, class Alloc>
    void swap(unordered_set<Key, Hash, Pred, Alloc>& x,
              unordered_set<Key, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
}

A size_type parameter type in an unordered_set deduction guide refers to the size_type member type of the type deduced by the deduction guide.

22.5.6.2 Constructors [unord.set.cnstr]

🔗

unordered_set() : unordered_set(size_type(see below)) { }
explicit unordered_set(size_type n,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_set using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

For the default constructor, the number of buckets is implementation-defined.

max_load_factor() returns 1.0.

Complexity: Constant.

🔗

template<class InputIterator>
  unordered_set(InputIterator f, InputIterator l,
                size_type n = see below,
                const hasher& hf = hasher(),
                const key_equal& eql = key_equal(),
                const allocator_type& a = allocator_type());
unordered_set(initializer_list<value_type> il,
              size_type n = see below,
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_set using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

If n is not provided, the number of buckets is implementation-defined.

Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form.

max_load_factor() returns 1.0.

Complexity: Average case linear, worst case quadratic.

22.5.6.3 Erasure [unord.set.erasure]

🔗

template<class K, class H, class P, class A, class Predicate>
  typename unordered_set<K, H, P, A>::size_type
    erase_if(unordered_set<K, H, P, A>& c, Predicate pred);

Effects: Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

22.5.7 Class template unordered_multiset [unord.multiset]

22.5.7.1 Overview [unord.multiset.overview]

An unordered_multiset is an unordered associative container that supports equivalent keys (an instance of unordered_multiset may contain multiple copies of the same key value) and in which each element's key is the element itself.

The unordered_multiset class supports forward iterators.

An unordered_multiset meets all of the requirements of a container, of an unordered associative container, and of an allocator-aware container (Table 76).

It provides the operations described in the preceding requirements table for equivalent keys; that is, an unordered_multiset supports the a_eq operations in that table, not the a_uniq operations.

For an unordered_multiset<Key> the key type and the value type are both Key.

The iterator and const_iterator types are both constant iterator types.

It is unspecified whether they are the same type.

Subclause [unord.multiset] only describes operations on unordered_multiset that are not described in one of the requirement tables, or for which there is additional semantic information.

namespace std {
  template<class Key,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Allocator = allocator<Key>>
  class unordered_multiset {
  public:
    // types
    using key_type             = Key;
    using value_type           = Key;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename allocator_traits<Allocator>::pointer;
    using const_pointer        = typename allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = implementation-defined; // see [container.requirements]
    using difference_type      = implementation-defined; // see [container.requirements]

    using iterator             = implementation-defined; // see [container.requirements]
    using const_iterator       = implementation-defined; // see [container.requirements]
    using local_iterator       = implementation-defined; // see [container.requirements]
    using const_local_iterator = implementation-defined; // see [container.requirements]
    using node_type            = unspecified;

    // [unord.multiset.cnstr], construct/copy/destroy
    unordered_multiset();
    explicit unordered_multiset(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_multiset(InputIterator f, InputIterator l,
                         size_type n = see below,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    unordered_multiset(const unordered_multiset&);
    unordered_multiset(unordered_multiset&&);
    explicit unordered_multiset(const Allocator&);
    unordered_multiset(const unordered_multiset&, const Allocator&);
    unordered_multiset(unordered_multiset&&, const Allocator&);
    unordered_multiset(initializer_list<value_type> il,
                       size_type n = see below,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_multiset(size_type n, const allocator_type& a)
      : unordered_multiset(n, hasher(), key_equal(), a) { }
    unordered_multiset(size_type n, const hasher& hf, const allocator_type& a)
      : unordered_multiset(n, hf, key_equal(), a) { }
    template<class InputIterator>
      unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
        : unordered_multiset(f, l, n, hasher(), key_equal(), a) { }
    template<class InputIterator>
      unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a)
      : unordered_multiset(f, l, n, hf, key_equal(), a) { }
    unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a)
      : unordered_multiset(il, n, hasher(), key_equal(), a) { }
    unordered_multiset(initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a)
      : unordered_multiset(il, n, hf, key_equal(), a) { }
    ~unordered_multiset();
    unordered_multiset& operator=(const unordered_multiset&);
    unordered_multiset& operator=(unordered_multiset&&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Hash> &&
               is_nothrow_move_assignable_v<Pred>);
    unordered_multiset& operator=(initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> iterator emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    iterator insert(const value_type& obj);
    iterator insert(value_type&& obj);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    iterator insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_multiset&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_swappable_v<Hash> &&
               is_nothrow_swappable_v<Pred>);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // set operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    pair<iterator, iterator>               equal_range(const key_type& k);
    pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;
    size_type max_bucket_count() const noexcept;
    size_type bucket_size(size_type n) const;
    size_type bucket(const key_type& k) const;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  template<class InputIterator,
           class Hash = hash<iter-value-type<InputIterator>>,
           class Pred = equal_to<iter-value-type<InputIterator>>,
           class Allocator = allocator<iter-value-type<InputIterator>>>
    unordered_multiset(InputIterator, InputIterator, see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multiset<iter-value-type<InputIterator>,
                            Hash, Pred, Allocator>;

  template<class T, class Hash = hash<T>,
           class Pred = equal_to<T>, class Allocator = allocator<T>>
    unordered_multiset(initializer_list<T>, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multiset<T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multiset(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_multiset<iter-value-type<InputIterator>,
                            hash<iter-value-type<InputIterator>>,
                            equal_to<iter-value-type<InputIterator>>,
                            Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_multiset(InputIterator, InputIterator, typename see below::size_type,
                       Hash, Allocator)
      -> unordered_multiset<iter-value-type<InputIterator>, Hash,
                            equal_to<iter-value-type<InputIterator>>,
                            Allocator>;

  template<class T, class Allocator>
    unordered_multiset(initializer_list<T>, typename see below::size_type, Allocator)
      -> unordered_multiset<T, hash<T>, equal_to<T>, Allocator>;

  template<class T, class Hash, class Allocator>
    unordered_multiset(initializer_list<T>, typename see below::size_type, Hash, Allocator)
      -> unordered_multiset<T, Hash, equal_to<T>, Allocator>;

  // swap
  template<class Key, class Hash, class Pred, class Alloc>
    void swap(unordered_multiset<Key, Hash, Pred, Alloc>& x,
              unordered_multiset<Key, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
}

A size_type parameter type in an unordered_multiset deduction guide refers to the size_type member type of the type deduced by the deduction guide.

22.5.7.2 Constructors [unord.multiset.cnstr]

🔗

unordered_multiset() : unordered_multiset(size_type(see below)) { }
explicit unordered_multiset(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_multiset using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

For the default constructor, the number of buckets is implementation-defined.

max_load_factor() returns 1.0.

Complexity: Constant.

🔗

template<class InputIterator>
  unordered_multiset(InputIterator f, InputIterator l,
                     size_type n = see below,
                     const hasher& hf = hasher(),
                     const key_equal& eql = key_equal(),
                     const allocator_type& a = allocator_type());
unordered_multiset(initializer_list<value_type> il,
                   size_type n = see below,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());

Effects: Constructs an empty unordered_multiset using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

If n is not provided, the number of buckets is implementation-defined.

Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form.

max_load_factor() returns 1.0.

Complexity: Average case linear, worst case quadratic.

22.5.7.3 Erasure [unord.multiset.erasure]

🔗

template<class K, class H, class P, class A, class Predicate>
  typename unordered_multiset<K, H, P, A>::size_type
    erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred);

Effects: Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

22.6 Container adaptors [container.adaptors]

22.6.1 In general [container.adaptors.general]

The headers <queue> and <stack> define the container adaptors queue, priority_queue, and stack.

The container adaptors each take a Container template parameter, and each constructor takes a Container reference argument.

This container is copied into the Container member of each adaptor.

If the container takes an allocator, then a compatible allocator may be passed in to the adaptor's constructor.

Otherwise, normal copy or move construction is used for the container argument.

The first template parameter T of the container adaptors shall denote the same type as Container::value_type.

For container adaptors, no swap function throws an exception unless that exception is thrown by the swap of the adaptor's Container or Compare object (if any).

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

(4.1)
It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.
(4.2)
It has a Compare template parameter and a type that qualifies as an allocator is deduced for that parameter.
(4.3)
It has a Container template parameter and a type that qualifies as an allocator is deduced for that parameter.
(4.4)
It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.
(4.5)
It has both Container and Allocator template parameters, and uses_allocator_v<Container, Allocator> is false.

22.6.2 Header <queue> synopsis [queue.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  template<class T, class Container = deque<T>> class queue;

  template<class T, class Container>
    bool operator==(const queue<T, Container>& x, const queue<T, Container>& y);
  template<class T, class Container>
    bool operator!=(const queue<T, Container>& x, const queue<T, Container>& y);
  template<class T, class Container>
    bool operator< (const queue<T, Container>& x, const queue<T, Container>& y);
  template<class T, class Container>
    bool operator> (const queue<T, Container>& x, const queue<T, Container>& y);
  template<class T, class Container>
    bool operator<=(const queue<T, Container>& x, const queue<T, Container>& y);
  template<class T, class Container>
    bool operator>=(const queue<T, Container>& x, const queue<T, Container>& y);
  template<class T, three_way_comparable Container>
    compare_three_way_result_t<Container>
      operator<=>(const queue<T, Container>& x, const queue<T, Container>& y);

  template<class T, class Container>
    void swap(queue<T, Container>& x, queue<T, Container>& y) noexcept(noexcept(x.swap(y)));
  template<class T, class Container, class Alloc>
    struct uses_allocator<queue<T, Container>, Alloc>;

  template<class T, class Container = vector<T>,
           class Compare = less<typename Container::value_type>>
    class priority_queue;

  template<class T, class Container, class Compare>
    void swap(priority_queue<T, Container, Compare>& x,
              priority_queue<T, Container, Compare>& y) noexcept(noexcept(x.swap(y)));
  template<class T, class Container, class Compare, class Alloc>
    struct uses_allocator<priority_queue<T, Container, Compare>, Alloc>;
}

22.6.3 Header <stack> synopsis [stack.syn]

#include <compare>              // see [compare.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {
  template<class T, class Container = deque<T>> class stack;

  template<class T, class Container>
    bool operator==(const stack<T, Container>& x, const stack<T, Container>& y);
  template<class T, class Container>
    bool operator!=(const stack<T, Container>& x, const stack<T, Container>& y);
  template<class T, class Container>
    bool operator< (const stack<T, Container>& x, const stack<T, Container>& y);
  template<class T, class Container>
    bool operator> (const stack<T, Container>& x, const stack<T, Container>& y);
  template<class T, class Container>
    bool operator<=(const stack<T, Container>& x, const stack<T, Container>& y);
  template<class T, class Container>
    bool operator>=(const stack<T, Container>& x, const stack<T, Container>& y);
  template<class T, three_way_comparable Container>
    compare_three_way_result_t<Container>
      operator<=>(const stack<T, Container>& x, const stack<T, Container>& y);

  template<class T, class Container>
    void swap(stack<T, Container>& x, stack<T, Container>& y) noexcept(noexcept(x.swap(y)));
  template<class T, class Container, class Alloc>
    struct uses_allocator<stack<T, Container>, Alloc>;
}

22.6.4 Class template queue [queue]

22.6.4.1 Definition [queue.defn]

Any sequence container supporting operations front(), back(), push_back() and pop_front() can be used to instantiate queue.

In particular, list and deque can be used.

namespace std {
  template<class T, class Container = deque<T>>
  class queue {
  public:
    using value_type      = typename Container::value_type;
    using reference       = typename Container::reference;
    using const_reference = typename Container::const_reference;
    using size_type       = typename Container::size_type;
    using container_type  =          Container;

  protected:
    Container c;

  public:
    queue() : queue(Container()) {}
    explicit queue(const Container&);
    explicit queue(Container&&);
    template<class Alloc> explicit queue(const Alloc&);
    template<class Alloc> queue(const Container&, const Alloc&);
    template<class Alloc> queue(Container&&, const Alloc&);
    template<class Alloc> queue(const queue&, const Alloc&);
    template<class Alloc> queue(queue&&, const Alloc&);

    [[nodiscard]] bool empty() const    { return c.empty(); }
    size_type         size()  const     { return c.size(); }
    reference         front()           { return c.front(); }
    const_reference   front() const     { return c.front(); }
    reference         back()            { return c.back(); }
    const_reference   back() const      { return c.back(); }
    void push(const value_type& x)      { c.push_back(x); }
    void push(value_type&& x)           { c.push_back(std::move(x)); }
    template<class... Args>
      decltype(auto) emplace(Args&&... args)
        { return c.emplace_back(std::forward<Args>(args)...); }
    void pop()                          { c.pop_front(); }
    void swap(queue& q) noexcept(is_nothrow_swappable_v<Container>)
      { using std::swap; swap(c, q.c); }
  };

  template<class Container>
    queue(Container) -> queue<typename Container::value_type, Container>;

  template<class Container, class Allocator>
    queue(Container, Allocator) -> queue<typename Container::value_type, Container>;

  template<class T, class Container>
    void swap(queue<T, Container>& x, queue<T, Container>& y) noexcept(noexcept(x.swap(y)));

  template<class T, class Container, class Alloc>
    struct uses_allocator<queue<T, Container>, Alloc>
      : uses_allocator<Container, Alloc>::type { };
}

22.6.4.2 Constructors [queue.cons]

🔗

explicit queue(const Container& cont);

Effects: Initializes c with cont.

🔗

explicit queue(Container&& cont);

Effects: Initializes c with std::move(cont).

22.6.4.3 Constructors with allocators [queue.cons.alloc]

If uses_allocator_v<container_type, Alloc> is false the constructors in this subclause shall not participate in overload resolution.

🔗

template<class Alloc> explicit queue(const Alloc& a);

Effects: Initializes c with a.

🔗

template<class Alloc> queue(const container_type& cont, const Alloc& a);

Effects: Initializes c with cont as the first argument and a as the second argument.

🔗

template<class Alloc> queue(container_type&& cont, const Alloc& a);

Effects: Initializes c with std::move(cont) as the first argument and a as the second argument.

🔗

template<class Alloc> queue(const queue& q, const Alloc& a);

Effects: Initializes c with q.c as the first argument and a as the second argument.

🔗

template<class Alloc> queue(queue&& q, const Alloc& a);

Effects: Initializes c with std::move(q.c) as the first argument and a as the second argument.

22.6.4.4 Operators [queue.ops]

🔗

template<class T, class Container>
  bool operator==(const queue<T, Container>& x, const queue<T, Container>& y);

Returns: x.c == y.c.

🔗

template<class T, class Container>
  bool operator!=(const queue<T, Container>& x,  const queue<T, Container>& y);

Returns: x.c != y.c.

🔗

template<class T, class Container>
  bool operator< (const queue<T, Container>& x, const queue<T, Container>& y);

Returns: x.c < y.c.

🔗

template<class T, class Container>
  bool operator> (const queue<T, Container>& x, const queue<T, Container>& y);

Returns: x.c > y.c.

🔗

template<class T, class Container>
  bool operator<=(const queue<T, Container>& x, const queue<T, Container>& y);

Returns: x.c <= y.c.

🔗

template<class T, class Container>
    bool operator>=(const queue<T, Container>& x,
                    const queue<T, Container>& y);

Returns: x.c >= y.c.

🔗

template<class T, three_way_comparable Container>
  compare_three_way_result_t<Container>
    operator<=>(const queue<T, Container>& x, const queue<T, Container>& y);

Returns: x.c <=> y.c.

22.6.4.5 Specialized algorithms [queue.special]

🔗

template<class T, class Container>
  void swap(queue<T, Container>& x, queue<T, Container>& y) noexcept(noexcept(x.swap(y)));

Constraints: is_swappable_v<Container> is true.

Effects: As if by x.swap(y).

22.6.5 Class template priority_queue [priority.queue]

22.6.5.1 Overview [priqueue.overview]

Any sequence container with random access iterator and supporting operations front(), push_back() and pop_back() can be used to instantiate priority_queue.

In particular, vector and deque can be used.

Instantiating priority_queue also involves supplying a function or function object for making priority comparisons; the library assumes that the function or function object defines a strict weak ordering .

namespace std {
  template<class T, class Container = vector<T>,
           class Compare = less<typename Container::value_type>>
  class priority_queue {
  public:
    using value_type      = typename Container::value_type;
    using reference       = typename Container::reference;
    using const_reference = typename Container::const_reference;
    using size_type       = typename Container::size_type;
    using container_type  = Container;
    using value_compare   = Compare;

  protected:
    Container c;
    Compare comp;

  public:
    priority_queue() : priority_queue(Compare()) {}
    explicit priority_queue(const Compare& x) : priority_queue(x, Container()) {}
    priority_queue(const Compare& x, const Container&);
    priority_queue(const Compare& x, Container&&);
    template<class InputIterator>
      priority_queue(InputIterator first, InputIterator last, const Compare& x,
                     const Container&);
    template<class InputIterator>
      priority_queue(InputIterator first, InputIterator last,
                     const Compare& x = Compare(), Container&& = Container());
    template<class Alloc> explicit priority_queue(const Alloc&);
    template<class Alloc> priority_queue(const Compare&, const Alloc&);
    template<class Alloc> priority_queue(const Compare&, const Container&, const Alloc&);
    template<class Alloc> priority_queue(const Compare&, Container&&, const Alloc&);
    template<class Alloc> priority_queue(const priority_queue&, const Alloc&);
    template<class Alloc> priority_queue(priority_queue&&, const Alloc&);

    [[nodiscard]] bool empty() const { return c.empty(); }
    size_type size()  const          { return c.size(); }
    const_reference   top() const    { return c.front(); }
    void push(const value_type& x);
    void push(value_type&& x);
    template<class... Args> void emplace(Args&&... args);
    void pop();
    void swap(priority_queue& q) noexcept(is_nothrow_swappable_v<Container> &&
                                          is_nothrow_swappable_v<Compare>)
      { using std::swap; swap(c, q.c); swap(comp, q.comp); }
  };

  template<class Compare, class Container>
    priority_queue(Compare, Container)
      -> priority_queue<typename Container::value_type, Container, Compare>;

  template<class InputIterator,
           class Compare = less<typename iterator_traits<InputIterator>::value_type>,
           class Container = vector<typename iterator_traits<InputIterator>::value_type>>
    priority_queue(InputIterator, InputIterator, Compare = Compare(), Container = Container())
      -> priority_queue<typename iterator_traits<InputIterator>::value_type, Container, Compare>;

  template<class Compare, class Container, class Allocator>
    priority_queue(Compare, Container, Allocator)
      -> priority_queue<typename Container::value_type, Container, Compare>;

  // no equality is provided

  template<class T, class Container, class Compare>
    void swap(priority_queue<T, Container, Compare>& x,
              priority_queue<T, Container, Compare>& y) noexcept(noexcept(x.swap(y)));

  template<class T, class Container, class Compare, class Alloc>
    struct uses_allocator<priority_queue<T, Container, Compare>, Alloc>
      : uses_allocator<Container, Alloc>::type { };
}

22.6.5.2 Constructors [priqueue.cons]

🔗

priority_queue(const Compare& x, const Container& y);
priority_queue(const Compare& x, Container&& y);

Preconditions: x defines a strict weak ordering ([alg.sorting]).

Effects: Initializes comp with x and c with y (copy constructing or move constructing as appropriate); calls make_heap(c.begin(), c.end(), comp).

🔗

template<class InputIterator>
  priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container& y);
template<class InputIterator>
  priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(),
                 Container&& y = Container());

Preconditions: x defines a strict weak ordering ([alg.sorting]).

Effects: Initializes comp with x and c with y (copy constructing or move constructing as appropriate); calls c.insert(c.end(), first, last); and finally calls make_heap(c.begin(), c.end(), comp).

22.6.5.3 Constructors with allocators [priqueue.cons.alloc]

If uses_allocator_v<container_type, Alloc> is false the constructors in this subclause shall not participate in overload resolution.

🔗

template<class Alloc> explicit priority_queue(const Alloc& a);

Effects: Initializes c with a and value-initializes comp.

🔗

template<class Alloc> priority_queue(const Compare& compare, const Alloc& a);

Effects: Initializes c with a and initializes comp with compare.

🔗

template<class Alloc>
  priority_queue(const Compare& compare, const Container& cont, const Alloc& a);

Effects: Initializes c with cont as the first argument and a as the second argument, and initializes comp with compare; calls make_heap(c.begin(), c.end(), comp).

🔗

template<class Alloc>
  priority_queue(const Compare& compare, Container&& cont, const Alloc& a);

Effects: Initializes c with std::move(cont) as the first argument and a as the second argument, and initializes comp with compare; calls make_heap(c.begin(), c.end(), comp).

🔗

template<class Alloc> priority_queue(const priority_queue& q, const Alloc& a);

Effects: Initializes c with q.c as the first argument and a as the second argument, and initializes comp with q.comp.

🔗

template<class Alloc> priority_queue(priority_queue&& q, const Alloc& a);

Effects: Initializes c with std::move(q.c) as the first argument and a as the second argument, and initializes comp with std::move(q.comp).

22.6.5.4 Members [priqueue.members]

🔗

void push(const value_type& x);

Effects: As if by:

c.push_back(x);
push_heap(c.begin(), c.end(), comp);

🔗

void push(value_type&& x);

Effects: As if by:

c.push_back(std::move(x));
push_heap(c.begin(), c.end(), comp);

🔗

template<class... Args> void emplace(Args&&... args);

Effects: As if by:

c.emplace_back(std::forward<Args>(args)...);
push_heap(c.begin(), c.end(), comp);

🔗

void pop();

Effects: As if by:

pop_heap(c.begin(), c.end(), comp);
c.pop_back();

22.6.5.5 Specialized algorithms [priqueue.special]

🔗

template<class T, class Container, class Compare>
  void swap(priority_queue<T, Container, Compare>& x,
            priority_queue<T, Container, Compare>& y) noexcept(noexcept(x.swap(y)));

Constraints: is_swappable_v<Container> is true and is_swappable_v<Compare> is true.

Effects: As if by x.swap(y).

22.6.6 Class template stack [stack]

Any sequence container supporting operations back(), push_back() and pop_back() can be used to instantiate stack.

In particular, vector, list and deque can be used.

22.6.6.1 Definition [stack.defn]

namespace std {
  template<class T, class Container = deque<T>>
  class stack {
  public:
    using value_type      = typename Container::value_type;
    using reference       = typename Container::reference;
    using const_reference = typename Container::const_reference;
    using size_type       = typename Container::size_type;
    using container_type  = Container;

  protected:
    Container c;

  public:
    stack() : stack(Container()) {}
    explicit stack(const Container&);
    explicit stack(Container&&);
    template<class Alloc> explicit stack(const Alloc&);
    template<class Alloc> stack(const Container&, const Alloc&);
    template<class Alloc> stack(Container&&, const Alloc&);
    template<class Alloc> stack(const stack&, const Alloc&);
    template<class Alloc> stack(stack&&, const Alloc&);

    [[nodiscard]] bool empty() const    { return c.empty(); }
    size_type size()  const             { return c.size(); }
    reference         top()             { return c.back(); }
    const_reference   top() const       { return c.back(); }
    void push(const value_type& x)      { c.push_back(x); }
    void push(value_type&& x)           { c.push_back(std::move(x)); }
    template<class... Args>
      decltype(auto) emplace(Args&&... args)
        { return c.emplace_back(std::forward<Args>(args)...); }
    void pop()                          { c.pop_back(); }
    void swap(stack& s) noexcept(is_nothrow_swappable_v<Container>)
      { using std::swap; swap(c, s.c); }
  };

  template<class Container>
    stack(Container) -> stack<typename Container::value_type, Container>;

  template<class Container, class Allocator>
    stack(Container, Allocator) -> stack<typename Container::value_type, Container>;

  template<class T, class Container, class Alloc>
    struct uses_allocator<stack<T, Container>, Alloc>
      : uses_allocator<Container, Alloc>::type { };
}

22.6.6.2 Constructors [stack.cons]

🔗

explicit stack(const Container& cont);

Effects: Initializes c with cont.

🔗

explicit stack(Container&& cont);

Effects: Initializes c with std::move(cont).

22.6.6.3 Constructors with allocators [stack.cons.alloc]

If uses_allocator_v<container_type, Alloc> is false the constructors in this subclause shall not participate in overload resolution.

🔗

template<class Alloc> explicit stack(const Alloc& a);

Effects: Initializes c with a.

🔗

template<class Alloc> stack(const container_type& cont, const Alloc& a);

Effects: Initializes c with cont as the first argument and a as the second argument.

🔗

template<class Alloc> stack(container_type&& cont, const Alloc& a);

Effects: Initializes c with std::move(cont) as the first argument and a as the second argument.

🔗

template<class Alloc> stack(const stack& s, const Alloc& a);

Effects: Initializes c with s.c as the first argument and a as the second argument.

🔗

template<class Alloc> stack(stack&& s, const Alloc& a);

Effects: Initializes c with std::move(s.c) as the first argument and a as the second argument.

22.6.6.4 Operators [stack.ops]

🔗

template<class T, class Container>
  bool operator==(const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c == y.c.

🔗

template<class T, class Container>
  bool operator!=(const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c != y.c.

🔗

template<class T, class Container>
  bool operator< (const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c < y.c.

🔗

template<class T, class Container>
  bool operator> (const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c > y.c.

🔗

template<class T, class Container>
  bool operator<=(const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c <= y.c.

🔗

template<class T, class Container>
    bool operator>=(const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c >= y.c.

🔗

template<class T, three_way_comparable Container>
  compare_three_way_result_t<Container>
    operator<=>(const stack<T, Container>& x, const stack<T, Container>& y);

Returns: x.c <=> y.c.

22.6.6.5 Specialized algorithms [stack.special]

🔗

template<class T, class Container>
  void swap(stack<T, Container>& x, stack<T, Container>& y) noexcept(noexcept(x.swap(y)));

Constraints: is_swappable_v<Container> is true.

Effects: As if by x.swap(y).

22.7 Views [views]

22.7.1 General [views.general]

The header defines the view span.

22.7.2 Header synopsis [span.syn]

namespace std {
  // constants
  inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();

  // [views.span], class template span
  template<class ElementType, size_t Extent = dynamic_extent>
    class span;

  template<class ElementType, size_t Extent>
    inline constexpr bool ranges::enable_view<span<ElementType, Extent>> =
      Extent == 0 || Extent == dynamic_extent;
  template<class ElementType, size_t Extent>
    inline constexpr bool ranges::enable_borrowed_range<span<ElementType, Extent>> = true;

  // [span.objectrep], views of object representation
  template<class ElementType, size_t Extent>
    span<const byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
      as_bytes(span<ElementType, Extent> s) noexcept;

  template<class ElementType, size_t Extent>
    span<byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
      as_writable_bytes(span<ElementType, Extent> s) noexcept;
}

22.7.3 Class template span [views.span]

22.7.3.1 Overview [span.overview]

A span is a view over a contiguous sequence of objects, the storage of which is owned by some other object.

All member functions of span have constant time complexity.

namespace std {
  template<class ElementType, size_t Extent = dynamic_extent>
  class span {
  public:
    // constants and types
    using element_type = ElementType;
    using value_type = remove_cv_t<ElementType>;
    using size_type = size_t;
    using difference_type = ptrdiff_t;
    using pointer = element_type*;
    using const_pointer = const element_type*;
    using reference = element_type&;
    using const_reference = const element_type&;
    using iterator = implementation-defined;        // see [span.iterators]
    using reverse_iterator = std::reverse_iterator<iterator>;
    static constexpr size_type extent = Extent;

    // [span.cons], constructors, copy, and assignment
    constexpr span() noexcept;
    template<class It>
      constexpr explicit(extent != dynamic_extent) span(It first, size_type count);
    template<class It, class End>
      constexpr explicit(extent != dynamic_extent) span(It first, End last);
    template<size_t N>
      constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;
    template<class T, size_t N>
      constexpr span(array<T, N>& arr) noexcept;
    template<class T, size_t N>
      constexpr span(const array<T, N>& arr) noexcept;
    template<class R>
      constexpr explicit(extent != dynamic_extent) span(R&& r);
    constexpr span(const span& other) noexcept = default;
    template<class OtherElementType, size_t OtherExtent>
      constexpr explicit(see below) span(const span<OtherElementType, OtherExtent>& s) noexcept;

    ~span() noexcept = default;

    constexpr span& operator=(const span& other) noexcept = default;

    // [span.sub], subviews
    template<size_t Count>
      constexpr span<element_type, Count> first() const;
    template<size_t Count>
      constexpr span<element_type, Count> last() const;
    template<size_t Offset, size_t Count = dynamic_extent>
      constexpr span<element_type, see below> subspan() const;

    constexpr span<element_type, dynamic_extent> first(size_type count) const;
    constexpr span<element_type, dynamic_extent> last(size_type count) const;
    constexpr span<element_type, dynamic_extent> subspan(
      size_type offset, size_type count = dynamic_extent) const;

    // [span.obs], observers
    constexpr size_type size() const noexcept;
    constexpr size_type size_bytes() const noexcept;
    [[nodiscard]] constexpr bool empty() const noexcept;

    // [span.elem], element access
    constexpr reference operator[](size_type idx) const;
    constexpr reference front() const;
    constexpr reference back() const;
    constexpr pointer data() const noexcept;

    // [span.iterators], iterator support
    constexpr iterator begin() const noexcept;
    constexpr iterator end() const noexcept;
    constexpr reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator rend() const noexcept;

  private:
    pointer data_;              // exposition only
    size_type size_;            // exposition only
  };

  template<class It, class EndOrSize>
    span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>>;
  template<class T, size_t N>
    span(T (&)[N]) -> span<T, N>;
  template<class T, size_t N>
    span(array<T, N>&) -> span<T, N>;
  template<class T, size_t N>
    span(const array<T, N>&) -> span<const T, N>;
  template<class R>
    span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>;
}

ElementType is required to be a complete object type that is not an abstract class type.

22.7.3.2 Constructors, copy, and assignment [span.cons]

🔗

constexpr span() noexcept;

Constraints: Extent == dynamic_extent || Extent == 0 is true.

Postconditions: size() == 0 && data() == nullptr.

🔗

template<class It>
  constexpr explicit(extent != dynamic_extent) span(It first, size_type count);

Constraints: Let U be remove_reference_t<iter_reference_t<It>>.

(3.1)
It satisfies contiguous_iterator.
(3.2)
is_convertible_v<U(*)[], element_type(*)[]> is true.

[ Note
:
The intent is to allow only qualification conversions of the iterator reference type to element_type.
— end note
]

Preconditions:

(4.1)
[first, first + count) is a valid range.
(4.2)
It models contiguous_iterator.
(4.3)
If extent is not equal to dynamic_extent, then count is equal to extent.

Effects: Initializes data_ with to_address(first) and size_ with count.

Throws: Nothing.

🔗

template<class It, class End>
  constexpr explicit(extent != dynamic_extent) span(It first, End last);

Constraints: Let U be remove_reference_t<iter_reference_t<It>>.

(7.1)
is_convertible_v<U(*)[], element_type(*)[]> is true.

[ Note
:
The intent is to allow only qualification conversions of the iterator reference type to element_type.
— end note
]
(7.2)
It satisfies contiguous_iterator.
(7.3)
End satisfies sized_sentinel_for<It>.
(7.4)
is_convertible_v<End, size_t> is false.

Preconditions:

(8.1)
If extent is not equal to dynamic_extent, then last - first is equal to extent.
(8.2)
[first, last) is a valid range.
(8.3)
It models contiguous_iterator.
(8.4)
End models sized_sentinel_for<It>.

Effects: Initializes data_ with to_address(first) and size_ with last - first.

Throws: When and what last - first throws.

🔗

template<size_t N> constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;
template<class T, size_t N> constexpr span(array<T, N>& arr) noexcept;
template<class T, size_t N> constexpr span(const array<T, N>& arr) noexcept;

Constraints: Let U be remove_pointer_t<decltype(data(arr))>.

(11.1)
extent == dynamic_extent || N == extent is true, and
(11.2)
is_convertible_v<U(*)[], element_type(*)[]> is true.

[ Note
:
The intent is to allow only qualification conversions of the array element type to element_type.
— end note
]

Effects: Constructs a span that is a view over the supplied array.

[ Note

type_identity_t affects class template argument deduction.

— end note

]

Postconditions: size() == N && data() == data(arr) is true.

🔗

template<class R> constexpr explicit(extent != dynamic_extent) span(R&& r);

Constraints: Let U be remove_reference_t<ranges::range_reference_t<R>>.

(14.1)
R satisfies ranges::contiguous_range and ranges::sized_range.
(14.2)
Either R satisfies ranges::borrowed_range or is_const_v<element_type> is true.
(14.3)
remove_cvref_t<R> is not a specialization of span.
(14.4)
remove_cvref_t<R> is not a specialization of array.
(14.5)
is_array_v<remove_cvref_t<R>> is false.
(14.6)
is_convertible_v<U(*)[], element_type(*)[]> is true.

[ Note
:
The intent is to allow only qualification conversions of the range reference type to element_type.
— end note
]

Preconditions:

(15.1)
If extent is not equal to dynamic_extent, then ranges::size(r) is equal to extent.
(15.2)
R models ranges::contiguous_range and ranges::sized_range.
(15.3)
If is_const_v<element_type> is false, R models ranges::borrowed_range.

Effects: Initializes data_ with ranges::data(r) and size_ with ranges::size(r).

Throws: What and when ranges::data(r) and ranges::size(r) throw.

🔗

constexpr span(const span& other) noexcept = default;

Postconditions: other.size() == size() && other.data() == data().

🔗

template<class OtherElementType, size_t OtherExtent>
  constexpr explicit(see below) span(const span<OtherElementType, OtherExtent>& s) noexcept;

Constraints:

(19.1)
extent == dynamic_extent || OtherExtent == dynamic_extent || extent == OtherExtent is true, and
(19.2)
is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true.
[ Note
: The intent is to allow only qualification conversions of the OtherElementType to element_type. — end note
]

Preconditions: If extent is not equal to dynamic_extent, then s.size() is equal to extent.

Effects: Constructs a span that is a view over the range [s.data(), s.data() + s.size()).

Postconditions: size() == s.size() && data() == s.data().

Remarks: The expression inside explicit is equivalent to:

extent != dynamic_extent && OtherExtent == dynamic_extent

🔗

constexpr span& operator=(const span& other) noexcept = default;

Postconditions: size() == other.size() && data() == other.data().

22.7.3.3 Deduction guides [span.deduct]

🔗

template<class It, class EndOrSize>
  span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>>;

Constraints: It satisfies contiguous_iterator.

🔗

template<class R>
  span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>;

Constraints: R satisfies ranges::contiguous_range.

22.7.3.4 Subviews [span.sub]

🔗

template<size_t Count> constexpr span<element_type, Count> first() const;

Mandates: Count <= Extent is true.

Preconditions: Count <= size() is true.

Effects: Equivalent to: return R{data(), Count}; where R is the return type.

🔗

template<size_t Count> constexpr span<element_type, Count> last() const;

Mandates: Count <= Extent is true.

Preconditions: Count <= size() is true.

Effects: Equivalent to: return R{data() + (size() - Count), Count}; where R is the return type.

🔗

template<size_t Offset, size_t Count = dynamic_extent>
  constexpr span<element_type, see below> subspan() const;

Mandates:

Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset)

is true.

Preconditions:

Offset <= size() && (Count == dynamic_extent || Count <= size() - Offset)

is true.

Effects: Equivalent to:

return span<ElementType, see below>(
  data() + Offset, Count != dynamic_extent ? Count : size() - Offset);

Remarks: The second template argument of the returned span type is:

Count != dynamic_extent ? Count
                        : (Extent != dynamic_extent ? Extent - Offset
                                                    : dynamic_extent)

🔗

constexpr span<element_type, dynamic_extent> first(size_type count) const;

Preconditions: count <= size() is true.

Effects: Equivalent to: return {data(), count};

🔗

constexpr span<element_type, dynamic_extent> last(size_type count) const;

Preconditions: count <= size() is true.

Effects: Equivalent to: return {data() + (size() - count), count};

🔗

constexpr span<element_type, dynamic_extent> subspan(
  size_type offset, size_type count = dynamic_extent) const;

Preconditions:

offset <= size() && (count == dynamic_extent || count <= size() - offset)

is true.

Effects: Equivalent to:

return {data() + offset, count == dynamic_extent ? size() - offset : count};

22.7.3.5 Observers [span.obs]

🔗

constexpr size_type size() const noexcept;

Effects: Equivalent to: return size_;

🔗

constexpr size_type size_bytes() const noexcept;

Effects: Equivalent to: return size() * sizeof(element_type);

🔗

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

Effects: Equivalent to: return size() == 0;

22.7.3.6 Element access [span.elem]

🔗

constexpr reference operator[](size_type idx) const;

Preconditions: idx < size() is true.

Effects: Equivalent to: return *(data() + idx);

🔗

constexpr reference front() const;

Preconditions: empty() is false.

Effects: Equivalent to: return *data();

🔗

constexpr reference back() const;

Preconditions: empty() is false.

Effects: Equivalent to: return *(data() + (size() - 1));

🔗

constexpr pointer data() const noexcept;

Effects: Equivalent to: return data_;

22.7.3.7 Iterator support [span.iterators]

🔗

using iterator = implementation-defined;

The type models contiguous_iterator ([iterator.concept.contiguous]), meets the Cpp17RandomAccessIterator requirements ([random.access.iterators]), and meets the requirements for constexpr iterators ([iterator.requirements.general]).

All requirements on container iterators ([container.requirements]) apply to span::iterator as well.

🔗

constexpr iterator begin() const noexcept;

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

If empty() is true, then it returns the same value as end().

🔗

constexpr iterator end() const noexcept;

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

🔗

constexpr reverse_iterator rbegin() const noexcept;

Effects: Equivalent to: return reverse_iterator(end());

🔗

constexpr reverse_iterator rend() const noexcept;

Effects: Equivalent to: return reverse_iterator(begin());

22.7.3.8 Views of object representation [span.objectrep]

🔗

template<class ElementType, size_t Extent>
  span<const byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
    as_bytes(span<ElementType, Extent> s) noexcept;

Effects: Equivalent to: return R{reinterpret_cast<const byte*>(s.data()), s.size_bytes()}; where R is the return type.

🔗

template<class ElementType, size_t Extent>
  span<byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
    as_writable_bytes(span<ElementType, Extent> s) noexcept;

Constraints: is_const_v<ElementType> is false.

Effects: Equivalent to: return R{reinterpret_cast<byte*>(s.data()), s.size_bytes()}; where R is the return type.

22 Containers library [containers]

22.1 General [containers.general]

22.2 Container requirements [container.requirements]

22.2.1 General container requirements [container.requirements.general]

22.2.2 Container data races [container.requirements.dataraces]

22.2.3 Sequence containers [sequence.reqmts]

22.2.4 Node handles [container.node]

22.2.4.1 Overview [container.node.overview]

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

22.2.4.3 Destructor [container.node.dtor]

22.2.4.4 Observers [container.node.observers]

22.2.4.5 Modifiers [container.node.modifiers]

22.2.5 Insert return type [container.insert.return]

22.2.6 Associative containers [associative.reqmts]

22.2.6.1 Exception safety guarantees [associative.reqmts.except]

22.2.7 Unordered associative containers [unord.req]

22.2.7.1 Exception safety guarantees [unord.req.except]

22.3 Sequence containers [sequences]

22.3.1 In general [sequences.general]

22.3.2 Header <array> synopsis [array.syn]

22.3.3 Header <deque> synopsis [deque.syn]

22.3.4 Header <forward_­list> synopsis [forward.list.syn]

22.3.5 Header <list> synopsis [list.syn]

22.3.6 Header <vector> synopsis [vector.syn]

22.3.7 Class template array [array]

22.3.7.1 Overview [array.overview]

22.3.7.2 Constructors, copy, and assignment [array.cons]

22.3.7.3 Member functions [array.members]

22.3.7.4 Specialized algorithms [array.special]

22.3.7.5 Zero-sized arrays [array.zero]

22.3.7.6 Array creation functions [array.creation]

22.3.7.7 Tuple interface [array.tuple]

22.3.8 Class template deque [deque]

22.3.8.1 Overview [deque.overview]

22.3.8.2 Constructors, copy, and assignment [deque.cons]

22.3.8.3 Capacity [deque.capacity]

22.3.8.4 Modifiers [deque.modifiers]

22.3.8.5 Erasure [deque.erasure]

22.3.9 Class template forward_­list [forwardlist]

22.3.9.1 Overview [forwardlist.overview]

22.3.9.2 Constructors, copy, and assignment [forwardlist.cons]

22.3.9.3 Iterators [forwardlist.iter]

22.3.9.4 Element access [forwardlist.access]

22.3.9.5 Modifiers [forwardlist.modifiers]

22.3.9.6 Operations [forwardlist.ops]

22.3.9.7 Erasure [forward.list.erasure]

22.3.10 Class template list [list]

22.3.10.1 Overview [list.overview]

22.3.10.2 Constructors, copy, and assignment [list.cons]

22.3.10.3 Capacity [list.capacity]

22.3.10.4 Modifiers [list.modifiers]

22.3.10.5 Operations [list.ops]

22.3.10.6 Erasure [list.erasure]

22.3.11 Class template vector [vector]

22.3.11.1 Overview [vector.overview]

22.3.11.2 Constructors, copy, and assignment [vector.cons]

22.3.11.3 Capacity [vector.capacity]

22.3.11.4 Data [vector.data]

22.3.11.5 Modifiers [vector.modifiers]

22.3.11.6 Erasure [vector.erasure]

22.3.12 Class vector<bool> [vector.bool]

22.4 Associative containers [associative]

22.4.1 In general [associative.general]

22.4.2 Header <map> synopsis [associative.map.syn]

22.4.3 Header <set> synopsis [associative.set.syn]

22.4.4 Class template map [map]

22.4.4.1 Overview [map.overview]

22.4.4.2 Constructors, copy, and assignment [map.cons]

22.4.4.3 Element access [map.access]

22.4.4.4 Modifiers [map.modifiers]

22.4.4.5 Erasure [map.erasure]

22.4.5 Class template multimap [multimap]

22.4.5.1 Overview [multimap.overview]

22.4.5.2 Constructors [multimap.cons]

22.4.5.3 Modifiers [multimap.modifiers]

22.4.5.4 Erasure [multimap.erasure]

22.4.6 Class template set [set]

22.4.6.1 Overview [set.overview]

22.4.6.2 Constructors, copy, and assignment [set.cons]

22.4.6.3 Erasure [set.erasure]

22.3.4 Header <forward_list> synopsis [forward.list.syn]

22.3.9 Class template forward_list [forwardlist]

22.5.2 Header <unordered_map> synopsis [unord.map.syn]

22.5.3 Header <unordered_set> synopsis [unord.set.syn]

22.5.4 Class template unordered_map [unord.map]

22.5.5 Class template unordered_multimap [unord.multimap]

22.5.6 Class template unordered_set [unord.set]

22.5.7 Class template unordered_multiset [unord.multiset]

22.6.5 Class template priority_queue [priority.queue]