22 Containers library [containers]

22.3 Sequence containers [sequences]

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]).