22 Containers library [containers]

22.3 Sequence containers [sequences]

22.3.1 In general [sequences.general]

1
#
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.
2
#
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]

1
#
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.
2
#
An array is an aggregate that can be list-initialized with up to N elements whose types are convertible to T.
3
#
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.
4
#
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.
5
#
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]

1
#
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)>;
2
#
Mandates: (is_­same_­v<T, U> && ...) is true.

22.3.7.3 Member functions [array.members]

🔗
constexpr size_type size() const noexcept;
1
#
Returns: N.
🔗
constexpr T* data() noexcept; constexpr const T* data() const noexcept;
2
#
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);
3
#
Effects: As if by fill_­n(begin(), N, u).
🔗
constexpr void swap(array& y) noexcept(is_nothrow_swappable_v<T>);
4
#
Effects: Equivalent to swap_­ranges(begin(), end(), y.begin()).
5
#
[ 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)));
1
#
Constraints: N == 0 or is_­swappable_­v<T> is true.
2
#
Effects: As if by x.swap(y).
3
#
Complexity: Linear in N.

22.3.7.5 Zero-sized arrays [array.zero]

1
#
array shall provide support for the special case N == 0.
2
#
In the case that N == 0, begin() == end() == unique value.
The return value of data() is unspecified.
3
#
The effect of calling front() or back() for a zero-sized array is undefined.
4
#
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]);
1
#
Mandates: is_­array_­v<T> is false and is_­constructible_­v<T, T&> is true.
2
#
Preconditions: T meets the Cpp17CopyConstructible requirements.
3
#
Returns: {{ a[0], …, a[N - 1] }}.
🔗
template<class T, size_t N> constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);
4
#
Mandates: is_­array_­v<T> is false and is_­move_­constructible_­v<T> is true.
5
#
Preconditions: T meets the Cpp17MoveConstructible requirements.
6
#
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; };
1
#
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;
2
#
Mandates: I < N is true.
3
#
Returns: A reference to the element of a, where indexing is zero-based.

22.3.8 Class template deque [deque]

22.3.8.1 Overview [deque.overview]

1
#
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.
2
#
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&);
1
#
Effects: Constructs an empty deque, using the specified allocator.
2
#
Complexity: Constant.
🔗
explicit deque(size_type n, const Allocator& = Allocator());
3
#
Effects: Constructs a deque with n default-inserted elements using the specified allocator.
4
#
Preconditions: T is Cpp17DefaultInsertable into *this.
5
#
Complexity: Linear in n.
🔗
deque(size_type n, const T& value, const Allocator& = Allocator());
6
#
Effects: Constructs a deque with n copies of value, using the specified allocator.
7
#
Preconditions: T is Cpp17CopyInsertable into *this.
8
#
Complexity: Linear in n.
🔗
template<class InputIterator> deque(InputIterator first, InputIterator last, const Allocator& = Allocator());
9
#
Effects: Constructs a deque equal to the range [first, last), using the specified allocator.
10
#
Complexity: Linear in distance(first, last).

22.3.8.3 Capacity [deque.capacity]

🔗
void resize(size_type sz);
1
#
Preconditions: T is Cpp17MoveInsertable and Cpp17DefaultInsertable into *this.
2
#
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);
3
#
Preconditions: T is Cpp17CopyInsertable into *this.
4
#
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();
5
#
Preconditions: T is Cpp17MoveInsertable into *this.
6
#
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.
7
#
Complexity: If the size is not equal to the old capacity, linear in the size of the sequence; otherwise constant.
8
#
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);
1
#
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.
2
#
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.
3
#
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();
4
#
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
 ]
5
#
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.
6
#
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);
1
#
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);
2
#
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]

1
#
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
 ]
2
#
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.
3
#
[ 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)));
}
4
#
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&);
1
#
Effects: Constructs an empty forward_­list object using the specified allocator.
2
#
Complexity: Constant.
🔗
explicit forward_list(size_type n, const Allocator& = Allocator());
3
#
Preconditions: T is Cpp17DefaultInsertable into *this.
4
#
Effects: Constructs a forward_­list object with n default-inserted elements using the specified allocator.
5
#
Complexity: Linear in n.
🔗
forward_list(size_type n, const T& value, const Allocator& = Allocator());
6
#
Preconditions: T is Cpp17CopyInsertable into *this.
7
#
Effects: Constructs a forward_­list object with n copies of value using the specified allocator.
8
#
Complexity: Linear in n.
🔗
template<class InputIterator> forward_list(InputIterator first, InputIterator last, const Allocator& = Allocator());
9
#
Effects: Constructs a forward_­list object equal to the range [first, last).
10
#
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;
1
#
Returns: A non-dereferenceable iterator that, when incremented, is equal to the iterator returned by begin().
2
#
Effects: cbefore_­begin() is equivalent to const_­cast<forward_­list const&>(*this).before_­begin().
3
#
Remarks: before_­begin() == end() shall equal false.

22.3.9.4 Element access [forwardlist.access]

🔗
reference front(); const_reference front() const;
1
#
Returns: *begin()

22.3.9.5 Modifiers [forwardlist.modifiers]

1
#
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);
2
#
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);
3
#
Effects: Inserts a copy of x at the beginning of the list.
🔗
void pop_front();
4
#
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);
5
#
Preconditions: position is before_­begin() or is a dereferenceable iterator in the range [begin(), end()).
6
#
Effects: Inserts a copy of x after position.
7
#
Returns: An iterator pointing to the copy of x.
🔗
iterator insert_after(const_iterator position, size_type n, const T& x);
8
#
Preconditions: position is before_­begin() or is a dereferenceable iterator in the range [begin(), end()).
9
#
Effects: Inserts n copies of x after position.
10
#
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);
11
#
Preconditions: position is before_­begin() or is a dereferenceable iterator in the range [begin(), end()).
Neither first nor last are iterators in *this.
12
#
Effects: Inserts copies of elements in [first, last) after position.
13
#
Returns: An iterator pointing to the last inserted element or position if first == last.
🔗
iterator insert_after(const_iterator position, initializer_list<T> il);
14
#
Effects: insert_­after(p, il.begin(), il.end()).
15
#
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);
16
#
Preconditions: position is before_­begin() or is a dereferenceable iterator in the range [begin(), end()).
17
#
Effects: Inserts an object of type value_­type constructed with value_­type(std​::​forward<Args>(​args)...) after position.
18
#
Returns: An iterator pointing to the new object.
🔗
iterator erase_after(const_iterator position);
19
#
Preconditions: The iterator following position is dereferenceable.
20
#
Effects: Erases the element pointed to by the iterator following position.
21
#
Returns: An iterator pointing to the element following the one that was erased, or end() if no such element exists.
22
#
Throws: Nothing.
🔗
iterator erase_after(const_iterator position, const_iterator last);
23
#
Preconditions: All iterators in the range (position, last) are dereferenceable.
24
#
Effects: Erases the elements in the range (position, last).
25
#
Returns: last.
26
#
Throws: Nothing.
🔗
void resize(size_type sz);
27
#
Preconditions: T is Cpp17DefaultInsertable into *this.
28
#
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);
29
#
Preconditions: T is Cpp17CopyInsertable into *this.
30
#
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;
31
#
Effects: Erases all elements in the range [begin(), end()).
32
#
Remarks: Does not invalidate past-the-end iterators.

22.3.9.6 Operations [forwardlist.ops]

1
#
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);
2
#
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.
3
#
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.
4
#
Throws: Nothing.
5
#
Complexity:
🔗
void splice_after(const_iterator position, forward_list& x, const_iterator i); void splice_after(const_iterator position, forward_list&& x, const_iterator i);
6
#
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.
7
#
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.
8
#
Throws: Nothing.
9
#
Complexity:
🔗
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);
10
#
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.
11
#
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.
12
#
Complexity:
🔗
size_type remove(const T& value); template<class Predicate> size_type remove_if(Predicate pred);
13
#
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.
14
#
Returns: The number of elements erased.
15
#
Throws: Nothing unless an exception is thrown by the equality comparison or the predicate.
16
#
Remarks: Stable.
17
#
Complexity: Exactly distance(begin(), end()) applications of the corresponding predicate.
🔗
size_type unique(); template<class BinaryPredicate> size_type unique(BinaryPredicate pred);
18
#
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.
19
#
Returns: The number of elements erased.
20
#
Throws: Nothing unless an exception is thrown by the equality comparison or the predicate.
21
#
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);
22
#
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.
23
#
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.
24
#
Remarks: Stable ([algorithm.stable]).
25
#
Complexity: At most distance(begin(), end()) + distance(x.begin(), x.end()) - 1 comparisons.
🔗
void sort(); template<class Compare> void sort(Compare comp);
26
#
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.
27
#
Remarks: Stable.
28
#
Complexity: Approximately comparisons, where N is distance(begin(), end()).
🔗
void reverse() noexcept;
29
#
Effects: Reverses the order of the elements in the list.
Does not affect the validity of iterators and references.
30
#
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);
1
#
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);
2
#
Effects: Equivalent to: return c.remove_­if(pred);

22.3.10 Class template list [list]

22.3.10.1 Overview [list.overview]

1
#
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.
2
#
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)));
}
3
#
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&);
1
#
Effects: Constructs an empty list, using the specified allocator.
2
#
Complexity: Constant.
🔗
explicit list(size_type n, const Allocator& = Allocator());
3
#
Preconditions: T is Cpp17DefaultInsertable into *this.
4
#
Effects: Constructs a list with n default-inserted elements using the specified allocator.
5
#
Complexity: Linear in n.
🔗
list(size_type n, const T& value, const Allocator& = Allocator());
6
#
Preconditions: T is Cpp17CopyInsertable into *this.
7
#
Effects: Constructs a list with n copies of value, using the specified allocator.
8
#
Complexity: Linear in n.
🔗
template<class InputIterator> list(InputIterator first, InputIterator last, const Allocator& = Allocator());
9
#
Effects: Constructs a list equal to the range [first, last).
10
#
Complexity: Linear in distance(first, last).

22.3.10.3 Capacity [list.capacity]

🔗
void resize(size_type sz);
1
#
Preconditions: T is Cpp17DefaultInsertable into *this.
2
#
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);
3
#
Preconditions: T is Cpp17CopyInsertable into *this.
4
#
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);
1
#
Remarks: Does not affect the validity of iterators and references.
If an exception is thrown there are no effects.
2
#
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;
3
#
Effects: Invalidates only the iterators and references to the erased elements.
4
#
Throws: Nothing.
5
#
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]

1
#
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.
2
#
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);
3
#
Preconditions: addressof(x) != this is true.
4
#
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.
5
#
Throws: Nothing.
6
#
Complexity: Constant time.
🔗
void splice(const_iterator position, list& x, const_iterator i); void splice(const_iterator position, list&& x, const_iterator i);
7
#
Preconditions: i is a valid dereferenceable iterator of x.
8
#
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.
9
#
Throws: Nothing.
10
#
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);
11
#
Preconditions: [first, last) is a valid range in x.
position is not an iterator in the range [first, last).
12
#
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.
13
#
Throws: Nothing.
14
#
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);
15
#
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.
16
#
Returns: The number of elements erased.
17
#
Throws: Nothing unless an exception is thrown by *i == value or pred(*i) != false.
18
#
Remarks: Stable.
19
#
Complexity: Exactly size() applications of the corresponding predicate.
🔗
size_type unique(); template<class BinaryPredicate> size_type unique(BinaryPredicate binary_pred);
20
#
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.
21
#
Returns: The number of elements erased.
22
#
Throws: Nothing unless an exception is thrown by *i == *(i-1) or pred(*i, *(i - 1))
23
#
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);
24
#
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.
25
#
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.
26
#
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.
27
#
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;
28
#
Effects: Reverses the order of the elements in the list.
Does not affect the validity of iterators and references.
29
#
Complexity: Linear time.
🔗
void sort(); template<class Compare> void sort(Compare comp);
30
#
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.
31
#
Remarks: Stable.
32
#
Complexity: Approximately 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);
1
#
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);
2
#
Effects: Equivalent to: return c.remove_­if(pred);

22.3.11 Class template vector [vector]

22.3.11.1 Overview [vector.overview]

1
#
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.
2
#
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.
3
#
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)));
}
4
#
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;
1
#
Effects: Constructs an empty vector, using the specified allocator.
2
#
Complexity: Constant.
🔗
constexpr explicit vector(size_type n, const Allocator& = Allocator());
3
#
Preconditions: T is Cpp17DefaultInsertable into *this.
4
#
Effects: Constructs a vector with n default-inserted elements using the specified allocator.
5
#
Complexity: Linear in n.
🔗
constexpr vector(size_type n, const T& value, const Allocator& = Allocator());
6
#
Preconditions: T is Cpp17CopyInsertable into *this.
7
#
Effects: Constructs a vector with n copies of value, using the specified allocator.
8
#
Complexity: Linear in n.
🔗
template<class InputIterator> constexpr vector(InputIterator first, InputIterator last, const Allocator& = Allocator());
9
#
Effects: Constructs a vector equal to the range [first, last), using the specified allocator.
10
#
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;
1
#
Returns: The total number of elements that the vector can hold without requiring reallocation.
2
#
Complexity: Constant time.
🔗
constexpr void reserve(size_type n);
3
#
Preconditions: T is Cpp17MoveInsertable into *this.
4
#
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.
5
#
Complexity: It does not change the size of the sequence and takes at most linear time in the size of the sequence.
6
#
Throws: length_­error if n > max_­size().227
7
#
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();
8
#
Preconditions: T is Cpp17MoveInsertable into *this.
9
#
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.
10
#
Complexity: If reallocation happens, linear in the size of the sequence.
11
#
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);
12
#
Effects: Exchanges the contents and capacity() of *this with that of x.
13
#
Complexity: Constant time.
🔗
constexpr void resize(size_type sz);
14
#
Preconditions: T is Cpp17MoveInsertable and Cpp17DefaultInsertable into *this.
15
#
Effects: If sz < size(), erases the last size() - sz elements from the sequence.
Otherwise, appends sz - size() default-inserted elements to the sequence.
16
#
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);
17
#
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.
18
#
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;
1
#
Returns: A pointer such that [data(), data() + size()) is a valid range.
For a non-empty vector, data() == addressof(front()).
2
#
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);
1
#
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.
2
#
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();
3
#
Effects: Invalidates iterators and references at or after the point of the erase.
4
#
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.
5
#
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);
1
#
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);
2
#
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]

1
#
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;
  };
}
2
#
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.
3
#
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.
4
#
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;
5
#
Effects: Replaces each element in the container with its complement.
🔗
constexpr static void swap(reference x, reference y) noexcept;
6
#
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>>;
7
#
The specialization is enabled ([unord.hash]).