Standard library header <iterator>

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Standard library headers

Language support <cfloat> <climits> <compare> (C++20) <coroutine> (C++20) <csetjmp> <csignal> <cstdarg> <cstddef> <cstdint> (C++11) <cstdlib> <exception> <initializer_list> (C++11)    <limits> <new> <source_location> (C++20) <stdfloat> (C++23) <typeinfo> <version> (C++20) Concepts <concepts> (C++20) Diagnostics <cassert> <cerrno> <debugging> (C++26) <stacktrace> (C++23) <stdexcept> <system_error> (C++11) Memory management <memory> <memory_resource> (C++17) <scoped_allocator> (C++11) Metaprogramming <type_traits> (C++11) <ratio> (C++11)

General utilities <any> (C++17) <bitset> <bit> (C++20) <charconv> (C++17) <expected> (C++23) <format> (C++20) <functional> <optional> (C++17) <tuple> (C++11) <typeindex> (C++11) <utility> <variant> (C++17) Containers <array> (C++11) <deque> <flat_map> (C++23) <flat_set> (C++23) <forward_list> (C++11) <inplace_vector> (C++26)    <list> <map> <mdspan> (C++23) <queue> <set> <span> (C++20) <stack> <unordered_map> (C++11) <unordered_set> (C++11) <vector> Iterators <iterator> Ranges <generator> (C++23) <ranges> (C++20)

Algorithms <algorithm> <numeric> Strings <cctype> <cstring> <cuchar> (C++11) <cwchar> <cwctype> <string_view> (C++17) <string> Text processing <clocale> <codecvt> (C++11/17/26*) <locale> <regex> (C++11) <text_encoding> (C++26)    Numerics <cfenv> (C++11) <cmath> <complex> <linalg> (C++26) <numbers> (C++20) <random> (C++11) <valarray> Time <chrono> (C++11) <ctime> C compatibility <ccomplex> (C++11/17/20*) <ciso646> (until C++20) <cstdalign> (C++11/17/20*) <cstdbool> (C++11/17/20*) <ctgmath> (C++11/17/20*)

Input/output <cinttypes> (C++11) <cstdio> <filesystem> (C++17) <fstream> <iomanip> <iosfwd> <iostream> <ios> <istream> <ostream> <print> (C++23) <spanstream> (C++23) <sstream> <streambuf> <strstream> (C++98/26*) <syncstream> (C++20) Concurrency support <atomic> (C++11) <barrier> (C++20) <condition_variable> (C++11) <future> (C++11) <hazard_pointer> (C++26) <latch> (C++20) <mutex> (C++11) <rcu> (C++26) <semaphore> (C++20) <shared_mutex> (C++14) <stdatomic.h> (C++23) <stop_token> (C++20) <thread> (C++11) Execution support <execution> (C++17)

 

This header is part of the iterator library.

This header is a partial freestanding header. Everything inside this header is freestanding beside stream iterators.

(since C++23)

Concepts
Iterator concepts
indirectly_readable (C++20)	specifies that a type is indirectly readable by applying operator * (concept)
indirectly_writable (C++20)	specifies that a value can be written to an iterator's referenced object (concept)
weakly_incrementable (C++20)	specifies that a semiregular type can be incremented with pre- and post-increment operators (concept)
incrementable (C++20)	specifies that the increment operation on a weakly_incrementable type is equality-preserving and that the type is equality_comparable (concept)
input_or_output_iterator (C++20)	specifies that objects of a type can be incremented and dereferenced (concept)
sentinel_for (C++20)	specifies a type is a sentinel for an input_or_output_iterator type (concept)
sized_sentinel_for (C++20)	specifies that the - operator can be applied to an iterator and a sentinel to calculate their difference in constant time (concept)
input_iterator (C++20)	specifies that a type is an input iterator, that is, its referenced values can be read and it can be both pre- and post-incremented (concept)
output_iterator (C++20)	specifies that a type is an output iterator for a given value type, that is, values of that type can be written to it and it can be both pre- and post-incremented (concept)
forward_iterator (C++20)	specifies that an input_iterator is a forward iterator, supporting equality comparison and multi-pass (concept)
bidirectional_iterator (C++20)	specifies that a forward_iterator is a bidirectional iterator, supporting movement backwards (concept)
random_access_iterator (C++20)	specifies that a bidirectional_iterator is a random-access iterator, supporting advancement in constant time and subscripting (concept)
contiguous_iterator (C++20)	specifies that a random_access_iterator is a contiguous iterator, referring to elements that are contiguous in memory (concept)
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)	specifies that a callable type can be invoked with the result of dereferencing an indirectly_readable type (concept)
indirect_unary_predicate (C++20)	specifies that a callable type, when invoked with the result of dereferencing an indirectly_readable type, satisfies predicate (concept)
indirect_binary_predicate (C++20)	specifies that a callable type, when invoked with the result of dereferencing two indirectly_readable types, satisfies predicate (concept)
indirect_equivalence_relation (C++20)	specifies that a callable type, when invoked with the result of dereferencing two indirectly_readable types, satisfies equivalence_relation (concept)
indirect_strict_weak_order (C++20)	specifies that a callable type, when invoked with the result of dereferencing two indirectly_readable types, satisfies strict_weak_order (concept)
Common algorithm requirements
indirectly_movable (C++20)	specifies that values may be moved from an indirectly_readable type to an indirectly_writable type (concept)
indirectly_movable_storable (C++20)	specifies that values may be moved from an indirectly_readable type to an indirectly_writable type and that the move may be performed via an intermediate object (concept)
indirectly_copyable (C++20)	specifies that values may be copied from an indirectly_readable type to an indirectly_writable type (concept)
indirectly_copyable_storable (C++20)	specifies that values may be copied from an indirectly_readable type to an indirectly_writable type and that the copy may be performed via an intermediate object (concept)
indirectly_swappable (C++20)	specifies that the values referenced by two indirectly_readable types can be swapped (concept)
indirectly_comparable (C++20)	specifies that the values referenced by two indirectly_readable types can be compared (concept)
permutable (C++20)	specifies the common requirements of algorithms that reorder elements in place (concept)
mergeable (C++20)	specifies the requirements of algorithms that merge sorted sequences into an output sequence by copying elements (concept)
sortable (C++20)	specifies the common requirements of algorithms that permute sequences into ordered sequences (concept)
Classes
Algorithm utilities
indirect_result_t (C++20)	computes the result of invoking a callable object on the result of dereferencing some set of indirectly_readable types (alias template)
projected (C++20)	helper template for specifying the constraints on algorithms that accept projections (class template)
projected_value_t (C++26)	computes the value type of an indirectly_readable type by projection (alias template)
Associated types
incrementable_traits (C++20)	computes the difference type of a weakly_incrementable type (class template)
indirectly_readable_traits (C++20)	computes the value type of an indirectly_readable type (class template)
iter_value_titer_reference_titer_const_reference_titer_difference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++23)(C++20)(C++20)(C++20)	computes the associated types of an iterator (alias template)
Primitives
iterator_traits	provides uniform interface to the properties of an iterator (class template)
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20)	empty class types used to indicate iterator categories (class)
iterator (deprecated in C++17)	base class to ease the definition of required types for simple iterators (class template)
Adaptors
reverse_iterator	iterator adaptor for reverse-order traversal (class template)
move_iterator (C++11)	iterator adaptor which dereferences to an rvalue (class template)
move_sentinel (C++20)	sentinel adaptor for std::move_iterator (class template)
basic_const_iterator (C++23)	iterator adaptor that converts an iterator into a constant iterator (class template)
const_iterator (C++23)	computes a constant iterator type for a given type (alias template)
const_sentinel (C++23)	computes a sentinel type to be used with constant iterators (alias template)
common_iterator (C++20)	adapts an iterator type and its sentinel into a common iterator type (class template)
default_sentinel_t (C++20)	default sentinel for use with iterators that know the bound of their range (class)
counted_iterator (C++20)	iterator adaptor that tracks the distance to the end of the range (class template)
unreachable_sentinel_t (C++20)	sentinel that always compares unequal to any weakly_incrementable type (class)
back_insert_iterator	iterator adaptor for insertion at the end of a container (class template)
front_insert_iterator	iterator adaptor for insertion at the front of a container (class template)
insert_iterator	iterator adaptor for insertion into a container (class template)
Stream Iterators
istream_iterator	input iterator that reads from std::basic_istream (class template)
ostream_iterator	output iterator that writes to std::basic_ostream (class template)
istreambuf_iterator	input iterator that reads from std::basic_streambuf (class template)
ostreambuf_iterator	output iterator that writes to std::basic_streambuf (class template)
Customization point objects
Defined in namespace std::ranges
iter_move (C++20)	casts the result of dereferencing an object to its associated rvalue reference type (customization point object)
iter_swap (C++20)	swaps the values referenced by two dereferenceable objects (customization point object)
Constants
unreachable_sentinel (C++20)	an object of type unreachable_sentinel_t that always compares unequal to any weakly_incrementable type (constant)
default_sentinel (C++20)	an object of type default_sentinel_t used with iterators that know the bound of their range (constant)
Functions
Adaptors
make_reverse_iterator (C++14)	creates a std::reverse_iterator of type inferred from the argument (function template)
make_move_iterator (C++11)	creates a std::move_iterator of type inferred from the argument (function template)
make_const_iterator (C++23)	creates a std::const_iterator of type inferred from the argument (function template)
make_const_sentinel (C++23)	creates a std::const_sentinel of type inferred from the argument (function template)
front_inserter	creates a std::front_insert_iterator of type inferred from the argument (function template)
back_inserter	creates a std::back_insert_iterator of type inferred from the argument (function template)
inserter	creates a std::insert_iterator of type inferred from the argument (function template)
Non-member operators
operator==operator!=operator<operator<=operator>operator>=operator<=> (C++11)(C++11)(removed in C++20)(C++11)(C++11)(C++11)(C++11)(C++20)	compares the underlying iterators (function template)
operator+ (C++11)	advances the iterator (function template)
operator- (C++11)	computes the distance between two iterator adaptors (function template)
operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)	compares the underlying iterators (function template)
operator+	advances the iterator (function template)
operator-	computes the distance between two iterator adaptors (function template)
operator==operator<=> (C++20)	compares the distances to the end (function template)
operator+ (C++20)	advances the iterator (function template)
operator- (C++20)	computes the distance between two iterator adaptors (function template)
operator==operator!= (removed in C++20)	compares two istream_iterators (function template)
operator==operator!= (removed in C++20)	compares two istreambuf_iterators (function template)
Operations
advance	advances an iterator by given distance (function template)
distance	returns the distance between two iterators (function template)
next (C++11)	increment an iterator (function template)
prev (C++11)	decrement an iterator (function template)
ranges::advance (C++20)	advances an iterator by given distance or to a given bound (niebloid)
ranges::distance (C++20)	returns the distance between an iterator and a sentinel, or between the beginning and end of a range (niebloid)
ranges::next (C++20)	increment an iterator by a given distance or to a bound (niebloid)
ranges::prev (C++20)	decrement an iterator by a given distance or to a bound (niebloid)
Range access
begincbegin (C++11)(C++14)	returns an iterator to the beginning of a container or array (function template)
endcend (C++11)(C++14)	returns an iterator to the end of a container or array (function template)
rbegincrbegin (C++14)	returns a reverse iterator to the beginning of a container or array (function template)
rendcrend (C++14)	returns a reverse end iterator for a container or array (function template)
sizessize (C++17)(C++20)	returns the size of a container or array (function template)
empty (C++17)	checks whether the container is empty (function template)
data (C++17)	obtains the pointer to the underlying array (function template)

Synopsis

#include <compare>
#include <concepts>
 
namespace std {
  template<class T> using /* with-reference */ = T&;  // exposition only
  template<class T> concept /* can-reference */       // exposition only
    = requires { typename /* with-reference */<T>; };
  template<class T> concept /* dereferenceable */     // exposition only
    = requires(T& t) {
      { *t } -> /* can-reference */;  // not required to be equality-preserving
    };
 
  // associated types
  // incrementable traits
  template<class> struct incrementable_traits;
  template<class T>
    using iter_difference_t = /* see description */;
 
  // indirectly readable traits
  template<class> struct indirectly_readable_traits;
  template<class T>
    using iter_value_t = /* see description */;
 
  // iterator traits
  template<class I> struct iterator_traits;
  template<class T> requires is_object_v<T> struct iterator_traits<T*>;
 
  template</* dereferenceable */ T>
    using iter_reference_t = decltype(*declval<T&>());
 
  namespace ranges {
    // customization point objects
    inline namespace /* unspecified */ {
      // ranges::iter_move
      inline constexpr /* unspecified */ iter_move = /* unspecified */;
 
      // ranges::iter_swap
      inline constexpr /* unspecified */ iter_swap = /* unspecified */;
    }
  }
 
  template</* dereferenceable */ T>
    requires requires(T& t) {
      { ranges::iter_move(t) } -> /* can-reference */;
    }
  using iter_rvalue_reference_t
    = decltype(ranges::iter_move(declval<T&>()));
 
  // iterator concepts
  // concept indirectly_readable
  template<class In>
    concept indirectly_readable = /* see description */;
 
  template<indirectly_readable T>
    using iter_common_reference_t =
      common_reference_t<iter_reference_t<T>, iter_value_t<T>&>;
 
  // concept indirectly_writable
  template<class Out, class T>
    concept indirectly_writable = /* see description */;
 
  // concept weakly_incrementable
  template<class I>
    concept weakly_incrementable = /* see description */;
 
  // concept incrementable
  template<class I>
    concept incrementable = /* see description */;
 
  // concept input_or_output_iterator
  template<class I>
    concept input_or_output_iterator = /* see description */;
 
  // concept sentinel_for
  template<class S, class I>
    concept sentinel_for = /* see description */;
 
  // concept sized_sentinel_for
  template<class S, class I>
    inline constexpr bool disable_sized_sentinel_for = false;
 
  template<class S, class I>
    concept sized_sentinel_for = /* see description */;
 
  // concept input_iterator
  template<class I>
    concept input_iterator = /* see description */;
 
  // concept output_iterator
  template<class I, class T>
    concept output_iterator = /* see description */;
 
  // concept forward_iterator
  template<class I>
    concept forward_iterator = /* see description */;
 
  // concept bidirectional_iterator
  template<class I>
    concept bidirectional_iterator = /* see description */;
 
  // concept random_access_iterator
  template<class I>
    concept random_access_iterator = /* see description */;
 
  // concept contiguous_iterator
  template<class I>
    concept contiguous_iterator = /* see description */;
 
  // indirect callable requirements
  // indirect callables
  template<class F, class I>
    concept indirectly_unary_invocable = /* see description */;
 
  template<class F, class I>
    concept indirectly_regular_unary_invocable = /* see description */;
 
  template<class F, class I>
    concept indirect_unary_predicate = /* see description */;
 
  template<class F, class I1, class I2>
    concept indirect_binary_predicate = /* see description */;
 
  template<class F, class I1, class I2 = I1>
    concept indirect_equivalence_relation = /* see description */;
 
  template<class F, class I1, class I2 = I1>
    concept indirect_strict_weak_order = /* see description */;
 
  template<class F, class... Is>
    requires (indirectly_readable<Is> && ...) && invocable<F, iter_reference_t<Is>...>
      using indirect_result_t = invoke_result_t<F, iter_reference_t<Is>...>;
 
  // projected
  template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj>
    struct projected;
 
  template<weakly_incrementable I, class Proj>
    struct incrementable_traits<projected<I, Proj>>;
 
  template<indirectly_­readable I, indirectly_­regular_­unary_­invocable<I> Proj>
    using projected_value_t = remove_cvref_t<invoke_result_t<Proj&, iter_value_t<I>&>>;
 
  // common algorithm requirements
  // concept indirectly_movable
  template<class In, class Out>
    concept indirectly_movable = /* see description */;
 
  template<class In, class Out>
    concept indirectly_movable_storable = /* see description */;
 
  // concept indirectly_copyable
  template<class In, class Out>
    concept indirectly_copyable = /* see description */;
 
  template<class In, class Out>
    concept indirectly_copyable_storable = /* see description */;
 
  // concept indirectly_swappable
  template<class I1, class I2 = I1>
    concept indirectly_swappable = /* see description */;
 
  // concept indirectly_comparable
  template<class I1, class I2, class R, class P1 = identity, class P2 = identity>
    concept indirectly_comparable = /* see description */;
 
  // concept permutable
  template<class I>
    concept permutable = /* see description */;
 
  // concept mergeable
  template<class I1, class I2, class Out,
      class R = ranges::less, class P1 = identity, class P2 = identity>
    concept mergeable = /* see description */;
 
  // concept sortable
  template<class I, class R = ranges::less, class P = identity>
    concept sortable = /* see description */;
 
  // primitives
  // iterator tags
  struct input_iterator_tag { };
  struct output_iterator_tag { };
  struct forward_iterator_tag: public input_iterator_tag { };
  struct bidirectional_iterator_tag: public forward_iterator_tag { };
  struct random_access_iterator_tag: public bidirectional_iterator_tag { };
  struct contiguous_iterator_tag: public random_access_iterator_tag { };
 
  // iterator operations
  template<class InputIt, class Distance>
    constexpr void advance(InputIt& i, Distance n);
  template<class InputIt>
    constexpr typename iterator_traits<InputIt>::difference_type
      distance(InputIt first, InputIt last);
  template<class InputIt>
    constexpr InputIt
      next(InputIt x, typename iterator_traits<InputIt>::difference_type n = 1);
  template<class BidirIt>
    constexpr BidirIt
      prev(BidirIt x, typename iterator_traits<BidirIt>::difference_type n = 1);
 
  // range iterator operations
  namespace ranges {
    // ranges::advance
    template<input_or_output_iterator I>
      constexpr void advance(I& i, iter_difference_t<I> n);
    template<input_or_output_iterator I, sentinel_for<I> S>
      constexpr void advance(I& i, S bound);
    template<input_or_output_iterator I, sentinel_for<I> S>
      constexpr iter_difference_t<I> advance(I& i, iter_difference_t<I> n, S bound);
 
    // ranges::distance
    template<class I, sentinel_for<I> S>
      requires (!sized_sentinel_for<S, I>)
      constexpr iter_difference_t<I> distance(I first, S last);
    template<class I, sized_sentinel_for<decay_t<I>> S>
      constexpr iter_difference_t<decay_t<I>> distance(I&& first, S last);
    template<range R>
      constexpr range_difference_t<R> distance(R&& r);
 
    // ranges::next
    template<input_or_output_iterator I>
      constexpr I next(I x);
    template<input_or_output_iterator I>
      constexpr I next(I x, iter_difference_t<I> n);
    template<input_or_output_iterator I, sentinel_for<I> S>
      constexpr I next(I x, S bound);
    template<input_or_output_iterator I, sentinel_for<I> S>
      constexpr I next(I x, iter_difference_t<I> n, S bound);
 
    // ranges::prev
    template<bidirectional_iterator I>
      constexpr I prev(I x);
    template<bidirectional_iterator I>
      constexpr I prev(I x, iter_difference_t<I> n);
    template<bidirectional_iterator I>
      constexpr I prev(I x, iter_difference_t<I> n, I bound);
  }
 
  // predefined iterators and sentinels
  // reverse iterators
  template<class It> class reverse_iterator;
 
  template<class It1, class It2>
    constexpr bool operator==(const reverse_iterator<It1>& x,
                              const reverse_iterator<It2>& y);
  template<class It1, class It2>
    constexpr bool operator!=(const reverse_iterator<It1>& x,
                              const reverse_iterator<It2>& y);
  template<class It1, class It2>
    constexpr bool operator<(const reverse_iterator<It1>& x,
                             const reverse_iterator<It2>& y);
  template<class It1, class It2>
    constexpr bool operator>(const reverse_iterator<It1>& x,
                             const reverse_iterator<It2>& y);
  template<class It1, class It2>
    constexpr bool operator<=(const reverse_iterator<It1>& x,
                              const reverse_iterator<It2>& y);
  template<class It1, class It2>
    constexpr bool operator>=(const reverse_iterator<It1>& x,
                              const reverse_iterator<It2>& y);
  template<class It1, three_way_comparable_with<It1> It2>
    constexpr compare_three_way_result_t<It1, It2>
      operator<=>(const reverse_iterator<It1>& x, const reverse_iterator<It2>& y);
 
  template<class It1, class It2>
    constexpr auto operator-(const reverse_iterator<It1>& x,
                             const reverse_iterator<It2>& y)
      -> decltype(y.base() - x.base());
  template<class It>
    constexpr reverse_iterator<It> operator+(iter_difference_t<It> n,
                                             const reverse_iterator<It>& x);
 
  template<class It>
    constexpr reverse_iterator<It> make_reverse_iterator(It i);
 
  template<class It1, class It2>
      requires (!sized_sentinel_for<It1, It2>)
    inline constexpr bool disable_sized_sentinel_for<reverse_iterator<It1>,
                                                     reverse_iterator<It2>> = true;
 
  // insert iterators
  template<class Container> class back_insert_iterator;
  template<class Container>
    constexpr back_insert_iterator<Container> back_inserter(Container& x);
 
  template<class Container> class front_insert_iterator;
  template<class Container>
    constexpr front_insert_iterator<Container> front_inserter(Container& x);
 
  template<class Container> class insert_iterator;
  template<class Container>
    constexpr insert_iterator<Container>
      inserter(Container& x, ranges::iterator_t<Container> i);
 
  // move iterators and sentinels
  template<class It> class move_iterator;
 
  template<class It1, class It2>
    constexpr bool operator==(const move_iterator<It1>& x, const move_iterator<It2>& y);
  template<class It1, class It2>
    constexpr bool operator<(const move_iterator<It1>& x, const move_iterator<It2>& y);
  template<class It1, class It2>
    constexpr bool operator>(const move_iterator<It1>& x, const move_iterator<It2>& y);
  template<class It1, class It2>
    constexpr bool operator<=(const move_iterator<It1>& x, const move_iterator<It2>& y);
  template<class It1, class It2>
    constexpr bool operator>=(const move_iterator<It1>& x, const move_iterator<It2>& y);
  template<class It1, three_way_comparable_with<It1> It2>
    constexpr compare_three_way_result_t<It1, It2>
      operator<=>(const move_iterator<It1>& x, const move_iterator<It2>& y);
 
  template<class It1, class It2>
    constexpr auto operator-(const move_iterator<It1>& x, const move_iterator<It2>& y)
      -> decltype(x.base() - y.base());
  template<class It>
    constexpr move_iterator<It>
      operator+(iter_difference_t<It> n, const move_iterator<It>& x);
 
  template<class It>
    constexpr move_iterator<It> make_move_iterator(It i);
 
  template<semiregular S> class move_sentinel;
 
  // common iterators
  template<input_or_output_iterator I, sentinel_for<I> S>
    requires (!same_as<I, S> && copyable<I>)
      class common_iterator;
 
  template<class I, class S>
    struct incrementable_traits<common_iterator<I, S>>;
 
  template<input_iterator I, class S>
    struct iterator_traits<common_iterator<I, S>>;
 
  // default sentinel
  struct default_sentinel_t;
  inline constexpr default_sentinel_t default_sentinel{};
 
  // counted iterators
  template<input_or_output_iterator I> class counted_iterator;
 
  template<input_iterator I>
    requires /* see description */
    struct iterator_traits<counted_iterator<I>>;
 
  // unreachable sentinel
  struct unreachable_sentinel_t;
  inline constexpr unreachable_sentinel_t unreachable_sentinel{};
 
  // stream iterators
  template<class T, class CharT = char, class Traits = char_traits<CharT>,
           class Distance = ptrdiff_t>
  class istream_iterator;
  template<class T, class CharT, class Traits, class Distance>
    bool operator==(const istream_iterator<T, CharT, Traits, Distance>& x,
                    const istream_iterator<T, CharT, Traits, Distance>& y);
 
  template<class T, class CharT = char, class traits = char_traits<CharT>>
      class ostream_iterator;
 
  template<class CharT, class Traits = char_traits<CharT>>
    class istreambuf_iterator;
  template<class CharT, class Traits>
    bool operator==(const istreambuf_iterator<CharT, Traits>& a,
                    const istreambuf_iterator<CharT, Traits>& b);
 
  template<class CharT, class Traits = char_traits<CharT>>
    class ostreambuf_iterator;
 
  // range access
  template<class C> constexpr auto begin(C& c) -> decltype(c.begin());
  template<class C> constexpr auto begin(const C& c) -> decltype(c.begin());
  template<class C> constexpr auto end(C& c) -> decltype(c.end());
  template<class C> constexpr auto end(const C& c) -> decltype(c.end());
  template<class T, size_t N> constexpr T* begin(T (&a)[N]) noexcept;
  template<class T, size_t N> constexpr T* end(T (&a)[N]) noexcept;
  template<class C> constexpr auto cbegin(const C& c) noexcept(noexcept(std::begin(c)))
    -> decltype(std::begin(c));
  template<class C> constexpr auto cend(const C& c) noexcept(noexcept(std::end(c)))
    -> decltype(std::end(c));
  template<class C> constexpr auto rbegin(C& c) -> decltype(c.rbegin());
  template<class C> constexpr auto rbegin(const C& c) -> decltype(c.rbegin());
  template<class C> constexpr auto rend(C& c) -> decltype(c.rend());
  template<class C> constexpr auto rend(const C& c) -> decltype(c.rend());
  template<class T, size_t N> constexpr reverse_iterator<T*> rbegin(T (&a)[N]);
  template<class T, size_t N> constexpr reverse_iterator<T*> rend(T (&a)[N]);
  template<class E> constexpr reverse_iterator<const E*> rbegin(initializer_list<E> il);
  template<class E> constexpr reverse_iterator<const E*> rend(initializer_list<E> il);
  template<class C> constexpr auto crbegin(const C& c) -> decltype(std::rbegin(c));
  template<class C> constexpr auto crend(const C& c) -> decltype(std::rend(c));
 
  template<class C> constexpr auto size(const C& c) -> decltype(c.size());
  template<class T, size_t N> constexpr size_t size(const T (&a)[N]) noexcept;
  template<class C> constexpr auto ssize(const C& c)
    -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;
  template<class T, ptrdiff_t N> constexpr ptrdiff_t ssize(const T (&a)[N]) noexcept;
  template<class C> constexpr auto empty(const C& c) -> decltype(c.empty());
  template<class T, size_t N> constexpr bool empty(const T (&a)[N]) noexcept;
  template<class E> constexpr bool empty(initializer_list<E> il) noexcept;
  template<class C> constexpr auto data(C& c) -> decltype(c.data());
  template<class C> constexpr auto data(const C& c) -> decltype(c.data());
  template<class T, size_t N> constexpr T* data(T (&a)[N]) noexcept;
  template<class E> constexpr const E* data(initializer_list<E> il) noexcept;
}

Concept indirectly_readable

namespace std {
  template<class In>
    concept __indirectlyReadableImpl = // exposition only
      requires(const In in) {
        typename iter_value_t<In>;
        typename iter_reference_t<In>;
        typename iter_rvalue_reference_t<In>;
        { *in } -> same_as<iter_reference_t<In>>
        { iter_move(in) } -> same_as<iter_rvalue_reference_t<In>>
      } &&
      common_reference_with<iter_reference_t<In>&&, iter_value_t<In>&> &&
      common_reference_with<iter_reference_t<In>&&, iter_rvalue_reference_t<In>&&> &&
      common_reference_with<iter_rvalue_reference_t<In>&&, const iter_value_t<In>&>;
 
  template<class In>
    concept indirectly_readable =
      __indirectlyReadableImpl<remove_cvref_t<In>>
}

Concept indirectly_writable

namespace std {
  template<class Out, class T>
    concept indirectly_writable =
      requires(Out&& o, T&& t) {
        *o = std::forward<T>(t); // not required to be equality-preserving
        *std::forward<Out>(o) = std::forward<T>(t);
        // not required to be equality-preserving
        const_cast<const iter_reference_t<Out>&&>(*o) =
        std::forward<T>(t); // not required to be equality-preserving
        const_cast<const iter_reference_t<Out>&&>(*std::forward<Out>(o)) =
        std::forward<T>(t); // not required to be equality-preserving
      };
}

Concept weakly_incrementable

namespace std {
  template<class T>
    inline constexpr bool __is_integer_like = /* see description */; // exposition only
 
  template<class T>
    inline constexpr bool __is_signed_integer_like =  // exposition only
      /* see description */;
 
  template<class I>
    concept weakly_incrementable =
      default_initializable<I> && movable<I> &&
      requires(I i) {
        typename iter_difference_t<I>;
        requires __is_signed_integer_like<iter_difference_t<I>>;
        { ++i } -> same_as<I&>;   // not required to be equality-preserving
        i++;                      // not required to be equality-preserving
      };
}

Concept incrementable

namespace std {
  template<class I>
    concept incrementable =
      regular<I> &&
      weakly_incrementable<I> &&
      requires(I i) {
        { i++ } -> same_as<I>;
      };
}

Concept input_or_output_iterator

namespace std {
  template<class I>
    concept input_or_output_iterator =
      requires(I i) {
        { *i } -> can-reference;
      } &&
      weakly_incrementable<I>;
}

Concept sentinel_for

namespace std {
  template<class S, class I>
    concept sentinel_for =
      semiregular<S> &&
      input_or_output_iterator<I> &&
      __WeaklyEqualityComparableWith<S, I>;
}

Concept sized_sentinel_for

namespace std {
  template<class S, class I>
    concept sized_sentinel_for =
      sentinel_for<S, I> &&
      !disable_sized_sentinel<remove_cv_t<S>, remove_cv_t<I>> &&
      requires(const I& i, const S& s) {
        { s - i } -> same_as<iter_difference_t<I>>;
        { i - s } -> same_as<iter_difference_t<I>>;
      };
}

Concept input_iterator

namespace std {
  template<class I>
    concept input_iterator =
      input_or_output_iterator<I> &&
      indirectly_readable<I> &&
      requires { typename /* ITER_CONCEPT */(I); } &&
      derived_from</* ITER_CONCEPT */(I), input_iterator_tag>;
}

Concept output_iterator

namespace std {
  template<class I, class T>
    concept output_iterator =
      input_or_output_iterator<I> &&
      indirectly_writable<I, T> &&
      requires(I i, T&& t) {
        *i++ = std::forward<T>(t); // not required to be equality-preserving
      };
}

Concept forward_iterator

namespace std {
  template<class I>
    concept forward_iterator =
      input_iterator<I> &&
      derived_from</* ITER_CONCEPT */(I), forward_iterator_tag> &&
      incrementable<I> &&
      sentinel_for<I, I>;
}

Concept bidirectional_iterator

namespace std {
  template<class I>
    concept bidirectional_iterator =
      forward_iterator<I> &&
      derived_from</* ITER_CONCEPT */(I), bidirectional_iterator_tag> &&
      requires(I i) {
        { --i } -> same_as<I&>;
        { i-- } -> same_as<I>;
      };
}

Concept random_access_iterator

namespace std {
  template<class I>
    concept random_access_iterator =
      bidirectional_iterator<I> &&
      derived_from</* ITER_CONCEPT */(I), random_access_iterator_tag> &&
      totally_ordered<I> &&
      sized_sentinel_for<I, I> &&
      requires(I i, const I j, const iter_difference_t<I> n) {
        { i += n } -> same_as<I&>;
        { j +  n } -> same_as<I>;
        { n +  j } -> same_as<I>;
        { i -= n } -> same_as<I&>;
        { j -  n } -> same_as<I>;
        {  j[n]  } -> same_as<iter_reference_t<I>>;
      };
}

Concept contiguous_iterator

namespace std {
  template<class I>
    concept contiguous_iterator =
      random_access_iterator<I> &&
      derived_from</* ITER_CONCEPT */(I), contiguous_iterator_tag> &&
      is_lvalue_reference_v<iter_reference_t<I>> &&
      same_as<iter_value_t<I>, remove_cvref_t<iter_reference_t<I>>> &&
      requires(const I& i) {
        { to_address(i) } -> same_as<add_pointer_t<iter_reference_t<I>>>;
      };
}

Concept indirectly_unary_invocable

namespace std {
  template<class F, class I>
    concept indirectly_unary_invocable =
      indirectly_readable<I> &&
      copy_constructible<F> &&
      invocable<F&, iter_value_t<I>&> &&
      invocable<F&, iter_reference_t<I>> &&
      common_reference_with<
        invoke_result_t<F&, iter_value_t<I>&>,
        invoke_result_t<F&, iter_reference_t<I>>>;
}

Concept indirectly_regular_unary_invocable

namespace std {
  template<class F, class I>
    concept indirectly_regular_unary_invocable =
      indirectly_readable<I> &&
      copy_constructible<F> &&
      regular_invocable<F&, iter_value_t<I>&> &&
      regular_invocable<F&, iter_reference_t<I>> &&
      common_reference_with<
        invoke_result_t<F&, iter_value_t<I>&>,
        invoke_result_t<F&, iter_reference_t<I>>>;
}

Concept indirect_unary_predicate

namespace std {
  template<class F, class I>
    concept indirect_unary_predicate =
      indirectly_readable<I> &&
      copy_constructible<F> &&
      predicate<F&, iter_value_t<I>&> &&
      predicate<F&, iter_reference_t<I>>;
}

Concept indirect_binary_predicate

namespace std {
  template<class F, class I1, class I2 = I1>
    concept indirect_binary_predicate =
      indirectly_readable<I1> && indirectly_readable<I2> &&
      copy_constructible<F> &&
      predicate<F&, iter_value_t<I1>&, iter_value_t<I2>&> &&
      predicate<F&, iter_value_t<I1>&, iter_reference_t<I2>> &&
      predicate<F&, iter_reference_t<I1>, iter_value_t<I2>&> &&
      predicate<F&, iter_reference_t<I1>, iter_reference_t<I2>>;
}

Concept indirect_equivalence_relation

namespace std {
  template<class F, class I1, class I2 = I1>
    concept indirect_equivalence_relation =
      indirectly_readable<I1> && indirectly_readable<I2> &&
      copy_constructible<F> &&
      equivalence_relation<F&, iter_value_t<I1>&, iter_value_t<I2>&> &&
      equivalence_relation<F&, iter_value_t<I1>&, iter_reference_t<I2>> &&
      equivalence_relation<F&, iter_reference_t<I1>, iter_value_t<I2>&> &&
      equivalence_relation<F&, iter_reference_t<I1>, iter_reference_t<I2>>;
}

Concept indirect_strict_weak_order

namespace std {
  template<class F, class I1, class I2 = I1>
    concept indirect_strict_weak_order =
      indirectly_readable<I1> && indirectly_readable<I2> &&
      copy_constructible<F> &&
      strict_weak_order<F&, iter_value_t<I1>&, iter_value_t<I2>&> &&
      strict_weak_order<F&, iter_value_t<I1>&, iter_reference_t<I2>> &&
      strict_weak_order<F&, iter_reference_t<I1>, iter_value_t<I2>&> &&
      strict_weak_order<F&, iter_reference_t<I1>, iter_reference_t<I2>>;
}

Concept indirectly_movable

namespace std {
  template<class In, class Out>
    concept indirectly_movable =
      indirectly_readable<In> &&
      indirectly_writable<Out, iter_rvalue_reference_t<In>>;
}

Concept indirectly_movable_storable

namespace std {
  template<class In, class Out>
    concept indirectly_movable_storable =
      indirectly_movable<In, Out> &&
      indirectly_writable<Out, iter_value_t<In>> &&
      movable<iter_value_t<In>> &&
      constructible_from<iter_value_t<In>, iter_rvalue_reference_t<In>> &&
      assignable_from<iter_value_t<In>&, iter_rvalue_reference_t<In>>;
}

Concept indirectly_copyable

namespace std {
  template<class In, class Out>
    concept indirectly_copyable =
      indirectly_readable<In> &&
      indirectly_writable<Out, iter_reference_t<In>>;
}

Concept indirectly_copyable_storable

namespace std {
  template<class In, class Out>
    concept indirectly_copyable_storable =
      indirectly_copyable<In, Out> &&
      indirectly_writable<Out, iter_value_t<In>&> &&
      indirectly_writable<Out, const iter_value_t<In>&> &&
      indirectly_writable<Out, iter_value_t<In>&&> &&
      indirectly_writable<Out, const iter_value_t<In>&&> &&
      copyable<iter_value_t<In>> &&
      constructible_from<iter_value_t<In>, iter_reference_t<In>> &&
      assignable_from<iter_value_t<In>&, iter_reference_t<In>>;
}

Concept indirectly_swappable

namespace std {
  template<class I1, class I2 = I1>
    concept indirectly_swappable =
      indirectly_readable<I1> && indirectly_readable<I2> &&
      requires(const I1 i1, const I2 i2) {
        ranges::iter_swap(i1, i1);
        ranges::iter_swap(i2, i2);
        ranges::iter_swap(i1, i2);
        ranges::iter_swap(i2, i1);
      };
}

Concept indirectly_comparable

namespace std {
  template<class I1, class I2, class R, class P1 = identity, class P2 = identity>
    concept indirectly_comparable =
      indirect_predicate<R, projected<I1, P1>, projected<I2, P2>>;
}

Concept permutable

namespace std {
  template<class I>
    concept permutable =
      forward_iterator<I> &&
      indirectly_movable_storable<I, I> &&
      indirectly_swappable<I, I>;
}

Concept mergeable

namespace std {
  template<class I1, class I2, class Out, class R = ranges::less,
           class P1 = identity, class P2 = identity>
    concept mergeable =
      input_iterator<I1> &&
      input_iterator<I2> &&
      weakly_incrementable<Out> &&
      indirectly_copyable<I1, Out> &&
      indirectly_copyable<I2, Out> &&
      indirect_strict_weak_order<R, projected<I1, P1>, projected<I2, P2>>;
}

Concept sortable

namespace std {
  template<class I, class R = ranges::less, class P = identity>
    concept sortable =
      permutable<I> &&
      indirect_strict_weak_order<R, projected<I, P>>;
}

Class template std::incrementable_traits

namespace std {
  template<class> struct incrementable_traits { };
 
  template<class T>
    requires is_object_v<T>
  struct incrementable_traits<T*> {
    using difference_type = ptrdiff_t;
  };
 
  template<class I>
  struct incrementable_traits<const I>
    : incrementable_traits<I> { };
 
  template<class T>
    requires requires { typename T::difference_type; }
  struct incrementable_traits<T> {
    using difference_type = typename T::difference_type;
  };
 
  template<class T>
    requires (!requires { typename T::difference_type; } &&
              requires(const T& a, const T& b) { { a - b } -> integral; })
  struct incrementable_traits<T> {
    using difference_type = make_signed_t<decltype(declval<T>() - declval<T>())>;
  };
 
  template<class T>
    using iter_difference_t = /* see description */;
}

Class template std::indirectly_readable_traits

namespace std {
  template<class> struct __cond_value_type { };   // exposition only
  template<class T>
    requires is_object_v<T>
  struct __cond_value_type {
    using value_type = remove_cv_t<T>;
  };
 
  template<class> struct indirectly_readable_traits { };
 
  template<class T>
  struct indirectly_readable_traits<T*>
    : __cond_value_type<T> { };
 
  template<class I>
    requires is_array_v<I>
  struct indirectly_readable_traits<I> {
    using value_type = remove_cv_t<remove_extent_t<I>>;
  };
 
  template<class I>
  struct indirectly_readable_traits<const I>
    : indirectly_readable_traits<I> { };
 
  template<class T>
    requires requires { typename T::value_type; }
  struct indirectly_readable_traits<T>
    : __cond_value_type<typename T::value_type> { };
 
  template<class T>
    requires requires { typename T::element_type; }
  struct indirectly_readable_traits<T>
    : __cond_value_type<typename T::element_type> { };
}

Class template std::projected

namespace std {
  template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj>
  struct projected {
    using value_type = remove_cvref_t<indirect_result_t<Proj&, I>>;
    indirect_result_t<Proj&, I> operator*() const; // not defined
  };
 
  template<weakly_incrementable I, class Proj>
  struct incrementable_traits<projected<I, Proj>> {
    using difference_type = iter_difference_t<I>;
  };
}

Class template std::iterator_traits

namespace std {
  template<class I>
  struct iterator_traits {
    using iterator_category = /* see description */;
    using value_type        = /* see description */;
    using difference_type   = /* see description */;
    using pointer           = /* see description */;
    using reference         = /* see description */;
  };
 
  template<class T>
    requires is_object_v<T>
  struct iterator_traits<T*> {
    using iterator_concept  = contiguous_iterator_tag;
    using iterator_category = random_access_iterator_tag;
    using value_type        = remove_cv_t<T>;
    using difference_type   = ptrdiff_t;
    using pointer           = T*;
    using reference         = T&;
  };
}

Iterator tags

namespace std {
  struct input_iterator_tag { };
  struct output_iterator_tag { };
  struct forward_iterator_tag: public input_iterator_tag { };
  struct bidirectional_iterator_tag: public forward_iterator_tag { };
  struct random_access_iterator_tag: public bidirectional_iterator_tag { };
  struct contiguous_iterator_tag: public random_access_iterator_tag { };
}

Class template std::reverse_iterator

namespace std {
  template<class Iter>
  class reverse_iterator {
  public:
    using iterator_type     = Iter;
    using iterator_concept  = /* see description */;
    using iterator_category = /* see description */;
    using value_type        = iter_value_t<Iter>;
    using difference_type   = iter_difference_t<Iter>;
    using pointer           = typename iterator_traits<Iter>::pointer;
    using reference         = iter_reference_t<Iter>;
 
    constexpr reverse_iterator();
    constexpr explicit reverse_iterator(Iter x);
    template<class U> constexpr reverse_iterator(const reverse_iterator<U>& u);
    template<class U> constexpr reverse_iterator& operator=(const reverse_iterator<U>& u);
 
    constexpr Iter base() const;
    constexpr reference operator*() const;
    constexpr pointer   operator->() const requires /* see description */;
 
    constexpr reverse_iterator& operator++();
    constexpr reverse_iterator  operator++(int);
    constexpr reverse_iterator& operator--();
    constexpr reverse_iterator  operator--(int);
 
    constexpr reverse_iterator  operator+ (difference_type n) const;
    constexpr reverse_iterator& operator+=(difference_type n);
    constexpr reverse_iterator  operator- (difference_type n) const;
    constexpr reverse_iterator& operator-=(difference_type n);
    constexpr /* unspecified */ operator[](difference_type n) const;
 
    friend constexpr iter_rvalue_reference_t<Iter>
      iter_move(const reverse_iterator& i) noexcept(/* see description */);
    template<indirectly_swappable<Iter> Iter2>
      friend constexpr void
        iter_swap(const reverse_iterator& x,
                  const reverse_iterator<Iter2>& y) noexcept(/* see description */);
 
  protected:
    Iter current;
  };
}

Class template std::back_insert_iterator

namespace std {
  template<class Container>
  class back_insert_iterator {
  protected:
    Container* container = nullptr;
 
  public:
    using iterator_category = output_iterator_tag;
    using value_type        = void;
    using difference_type   = ptrdiff_t;
    using pointer           = void;
    using reference         = void;
    using container_type    = Container;
 
    constexpr back_insert_iterator() noexcept = default;
    constexpr explicit back_insert_iterator(Container& x);
    constexpr back_insert_iterator& operator=(const typename Container::value_type& value);
    constexpr back_insert_iterator& operator=(typename Container::value_type&& value);
 
    constexpr back_insert_iterator& operator*();
    constexpr back_insert_iterator& operator++();
    constexpr back_insert_iterator  operator++(int);
  };
}

Class template std::front_insert_iterator

namespace std {
  template<class Container>
  class front_insert_iterator {
  protected:
    Container* container = nullptr;
 
  public:
    using iterator_category = output_iterator_tag;
    using value_type        = void;
    using difference_type   = ptrdiff_t;
    using pointer           = void;
    using reference         = void;
    using container_type    = Container;
 
    constexpr front_insert_iterator(Container& x) noexcept = default;
    constexpr explicit front_insert_iterator(Container& x);
    constexpr front_insert_iterator&
      operator=(const typename Container::value_type& value);
    constexpr front_insert_iterator& operator=(typename Container::value_type&& value);
 
    constexpr front_insert_iterator& operator*();
    constexpr front_insert_iterator& operator++();
    constexpr front_insert_iterator  operator++(int);
  };
}

Class template std::insert_iterator

namespace std {
  template<class Container>
  class insert_iterator {
  protected:
    Container* container = nullptr;
    ranges::iterator_t<Container> iter = ranges::iterator_t<Container>();
 
  public:
    using iterator_category = output_iterator_tag;
    using value_type        = void;
    using difference_type   = ptrdiff_t;
    using pointer           = void;
    using reference         = void;
    using container_type    = Container;
 
    insert_iterator() = default;
    constexpr insert_iterator(Container& x, ranges::iterator_t<Container> i);
    constexpr insert_iterator& operator=(const typename Container::value_type& value);
    constexpr insert_iterator& operator=(typename Container::value_type&& value);
 
    constexpr insert_iterator& operator*();
    constexpr insert_iterator& operator++();
    constexpr insert_iterator& operator++(int);
  };
}

Class template std::move_iterator

namespace std {
  template<class Iter>
  class move_iterator {
  public:
    using iterator_type     = Iter;
    using iterator_concept  = /* see description */;
    using iterator_category = /* see description */;
    using value_type        = iter_value_t<Iter>;
    using difference_type   = iter_difference_t<Iter>;
    using pointer           = Iter;
    using reference         = iter_rvalue_reference_t<Iter>;
 
    constexpr move_iterator();
    constexpr explicit move_iterator(Iter i);
    template<class U> constexpr move_iterator(const move_iterator<U>& u);
    template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);
 
    constexpr iterator_type base() const &;
    constexpr iterator_type base() &&;
    constexpr reference operator*() const;
    constexpr pointer operator->() const;
 
    constexpr move_iterator& operator++();
    constexpr auto operator++(int);
    constexpr move_iterator& operator--();
    constexpr move_iterator operator--(int);
 
    constexpr move_iterator operator+(difference_type n) const;
    constexpr move_iterator& operator+=(difference_type n);
    constexpr move_iterator operator-(difference_type n) const;
    constexpr move_iterator& operator-=(difference_type n);
    constexpr reference operator[](difference_type n) const;
 
    template<sentinel_for<Iter> S>
      friend constexpr bool
        operator==(const move_iterator& x, const move_sentinel<S>& y);
    template<sized_sentinel_for<Iter> S>
      friend constexpr iter_difference_t<Iter>
        operator-(const move_sentinel<S>& x, const move_iterator& y);
    template<sized_sentinel_for<Iter> S>
      friend constexpr iter_difference_t<Iter>
        operator-(const move_iterator& x, const move_sentinel<S>& y);
    friend constexpr iter_rvalue_reference_t<Iter>
      iter_move(const move_iterator& i)
        noexcept(noexcept(ranges::iter_move(i.current)));
    template<indirectly_swappable<Iter> Iter2>
      friend constexpr void
        iter_swap(const move_iterator& x, const move_iterator<Iter2>& y)
          noexcept(noexcept(ranges::iter_swap(x.current, y.current)));
 
  private:
    Iter current;     // exposition only
  };
}

Class template std::move_sentinel

namespace std {
  template<semiregular S>
  class move_sentinel {
  public:
    constexpr move_sentinel();
    constexpr explicit move_sentinel(S s);
    template<class S2>
      requires convertible_to<const S2&, S>
        constexpr move_sentinel(const move_sentinel<S2>& s);
    template<class S2>
      requires assignable_from<S&, const S2&>
        constexpr move_sentinel& operator=(const move_sentinel<S2>& s);
 
    constexpr S base() const;
  private:
    S last;     // exposition only
  };
}

Class template std::common_iterator

namespace std {
  template<input_or_output_iterator I, sentinel_for<I> S>
    requires (!same_as<I, S> && copyable<I>)
  class common_iterator {
  public:
    constexpr common_iterator() = default;
    constexpr common_iterator(I i);
    constexpr common_iterator(S s);
    template<class I2, class S2>
      requires convertible_to<const I2&, I> && convertible_to<const S2&, S>
        constexpr common_iterator(const common_iterator<I2, S2>& x);
 
    template<class I2, class S2>
      requires convertible_to<const I2&, I> && convertible_to<const S2&, S> &&
               assignable_from<I&, const I2&> && assignable_from<S&, const S2&>
        common_iterator& operator=(const common_iterator<I2, S2>& x);
 
    decltype(auto) operator*();
    decltype(auto) operator*() const
      requires dereferenceable<const I>;
    decltype(auto) operator->() const
      requires /* see description */;
 
    common_iterator& operator++();
    decltype(auto) operator++(int);
 
    template<class I2, sentinel_for<I> S2>
      requires sentinel_for<S, I2>
    friend bool operator==(
      const common_iterator& x, const common_iterator<I2, S2>& y);
    template<class I2, sentinel_for<I> S2>
      requires sentinel_for<S, I2> && equality_comparable_with<I, I2>
    friend bool operator==(
      const common_iterator& x, const common_iterator<I2, S2>& y);
 
    template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
      requires sized_sentinel_for<S, I2>
    friend iter_difference_t<I2> operator-(
      const common_iterator& x, const common_iterator<I2, S2>& y);
 
    friend constexpr decltype(auto) iter_move(const common_iterator& i)
      noexcept(noexcept(ranges::iter_move(declval<const I&>())))
        requires input_iterator<I>;
    template<indirectly_swappable<I> I2, class S2>
      friend void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y)
        noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));
 
  private:
    variant<I, S> v_;   // exposition only
  };
 
  template<class I, class S>
  struct incrementable_traits<common_iterator<I, S>> {
    using difference_type = iter_difference_t<I>;
  };
 
  template<input_iterator I, class S>
  struct iterator_traits<common_iterator<I, S>> {
    using iterator_concept = /* see description */;
    using iterator_category = /* see description */;
    using value_type = iter_value_t<I>;
    using difference_type = iter_difference_t<I>;
    using pointer = /* see description */;
    using reference = iter_reference_t<I>;
  };
}

Class std::default_sentinel_t

namespace std {
  struct default_sentinel_t { };
}

Class template std::counted_iterator

namespace std {
  template<input_or_output_iterator I>
  class counted_iterator {
  public:
    using iterator_type = I;
 
    constexpr counted_iterator() = default;
    constexpr counted_iterator(I x, iter_difference_t<I> n);
    template<class I2>
      requires convertible_to<const I2&, I>
        constexpr counted_iterator(const counted_iterator<I2>& x);
 
    template<class I2>
      requires assignable_from<I&, const I2&>
        constexpr counted_iterator& operator=(const counted_iterator<I2>& x);
 
    constexpr I base() const & requires copy_constructible<I>;
    constexpr I base() &&;
    constexpr iter_difference_t<I> count() const noexcept;
    constexpr decltype(auto) operator*();
    constexpr decltype(auto) operator*() const
      requires dereferenceable<const I>;
    constexpr auto operator->() const noexcept
      requires contiguous_iterator<I>;
 
    constexpr counted_iterator& operator++();
    decltype(auto) operator++(int);
    constexpr counted_iterator operator++(int)
      requires forward_iterator<I>;
    constexpr counted_iterator& operator--()
      requires bidirectional_iterator<I>;
    constexpr counted_iterator operator--(int)
      requires bidirectional_iterator<I>;
 
    constexpr counted_iterator operator+(iter_difference_t<I> n) const
      requires random_access_iterator<I>;
    friend constexpr counted_iterator operator+(
      iter_difference_t<I> n, const counted_iterator& x)
        requires random_access_iterator<I>;
    constexpr counted_iterator& operator+=(iter_difference_t<I> n)
      requires random_access_iterator<I>;
 
    constexpr counted_iterator operator-(iter_difference_t<I> n) const
      requires random_access_iterator<I>;
    template<common_with<I> I2>
      friend constexpr iter_difference_t<I2> operator-(
        const counted_iterator& x, const counted_iterator<I2>& y);
    friend constexpr iter_difference_t<I> operator-(
      const counted_iterator& x, default_sentinel_t);
    friend constexpr iter_difference_t<I> operator-(
      default_sentinel_t, const counted_iterator& y);
    constexpr counted_iterator& operator-=(iter_difference_t<I> n)
      requires random_access_iterator<I>;
 
    constexpr decltype(auto) operator[](iter_difference_t<I> n) const
      requires random_access_iterator<I>;
 
    template<common_with<I> I2>
      friend constexpr bool operator==(
        const counted_iterator& x, const counted_iterator<I2>& y);
    friend constexpr bool operator==(
      const counted_iterator& x, default_sentinel_t);
 
    template<common_with<I> I2>
      friend constexpr strong_ordering operator<=>(
        const counted_iterator& x, const counted_iterator<I2>& y);
 
    friend constexpr decltype(auto) iter_move(const counted_iterator& i)
      noexcept(noexcept(ranges::iter_move(i.current)))
        requires input_iterator<I>;
    template<indirectly_swappable<I> I2>
      friend constexpr void iter_swap(const counted_iterator& x,
                                      const counted_iterator<I2>& y)
        noexcept(noexcept(ranges::iter_swap(x.current, y.current)));
 
  private:
    I current = I();                    // exposition only
    iter_difference_t<I> length = 0;    // exposition only
  };
 
  template<input_iterator I>
  struct iterator_traits<counted_iterator<I>> : iterator_traits<I> {
    using pointer = void;
  };
}

Class std::unreachable_sentinel_t

namespace std {
  struct unreachable_sentinel_t {
    template<weakly_incrementable I>
      friend constexpr bool operator==(unreachable_sentinel_t, const I&) noexcept
      { return false; }
  };
}

Class template std::istream_iterator

namespace std {
  template<class T, class CharT = char, class Traits = char_traits<CharT>,
           class Distance = ptrdiff_t>
  class istream_iterator {
  public:
    using iterator_category = input_iterator_tag;
    using value_type        = T;
    using difference_type   = Distance;
    using pointer           = const T*;
    using reference         = const T&;
    using char_type         = CharT;
    using traits_type       = Traits;
    using istream_type      = basic_istream<CharT, Traits>;
 
    constexpr istream_iterator();
    constexpr istream_iterator(default_sentinel_t);
    istream_iterator(istream_type& s);
    istream_iterator(const istream_iterator& x) = default;
    ~istream_iterator() = default;
    istream_iterator& operator=(const istream_iterator&) = default;
 
    const T& operator*() const;
    const T* operator->() const;
    istream_iterator& operator++();
    istream_iterator  operator++(int);
 
    friend bool operator==(const istream_iterator& i, default_sentinel_t);
 
  private:
    basic_istream<CharT, Traits>* in_stream; // exposition only
    T value;                                 // exposition only
  };
}

Class template std::ostream_iterator

namespace std {
  template<class T, class CharT = char, classTraits = char_traits<CharT>>
  class ostream_iterator {
  public:
    using iterator_category = output_iterator_tag;
    using value_type        = void;
    using difference_type   = ptrdiff_t;
    using pointer           = void;
    using reference         = void;
    using char_type         = CharT;
    using traits_type       = Traits;
    using ostream_type      = basic_ostream<CharT, Traits>;
 
    constexpr ostreambuf_iterator() noexcept = default;
    ostream_iterator(ostream_type& s);
    ostream_iterator(ostream_type& s, const CharT* delimiter);
    ostream_iterator(const ostream_iterator& x);
    ~ostream_iterator();
    ostream_iterator& operator=(const ostream_iterator&) = default;
    ostream_iterator& operator=(const T& value);
 
    ostream_iterator& operator*();
    ostream_iterator& operator++();
    ostream_iterator& operator++(int);
 
  private:
    basic_ostream<CharT, Traits>* out_stream = nullptr;          // exposition only
    const CharT* delim = nullptr;                                // exposition only
  };
}

Class template std::istreambuf_iterator

namespace std {
  template<class CharT, class Traits = char_traits<CharT>>
  class istreambuf_iterator {
  public:
    using iterator_category = input_iterator_tag;
    using value_type        = CharT;
    using difference_type   = typename Traits::off_type;
    using pointer           = /* unspecified */;
    using reference         = CharT;
    using char_type         = CharT;
    using traits_type       = Traits;
    using int_type          = typename Traits::int_type;
    using streambuf_type    = basic_streambuf<CharT, Traits>;
    using istream_type      = basic_istream<CharT, Traits>;
 
    class proxy;                          // exposition only
 
    constexpr istreambuf_iterator() noexcept;
    constexpr istreambuf_iterator(default_sentinel_t) noexcept;
    istreambuf_iterator(const istreambuf_iterator&) noexcept = default;
    ~istreambuf_iterator() = default;
    istreambuf_iterator(istream_type& s) noexcept;
    istreambuf_iterator(streambuf_type* s) noexcept;
    istreambuf_iterator(const proxy& p) noexcept;
    istreambuf_iterator& operator=(const istreambuf_iterator&) noexcept = default;
    CharT operator*() const;
    istreambuf_iterator& operator++();
    proxy operator++(int);
    bool equal(const istreambuf_iterator& b) const;
 
    friend bool operator==(const istreambuf_iterator& i, default_sentinel_t s);
 
  private:
    streambuf_type* sbuf_;                // exposition only
  };
 
  template<class CharT, class Traits>
  class istreambuf_iterator<CharT, Traits>::proxy { // exposition only
    CharT keep_;
    basic_streambuf<CharT, Traits>* sbuf_;
    proxy(CharT c, basic_streambuf<CharT, Traits>* sbuf)
      : keep_(c), sbuf_(sbuf) { }
  public:
    CharT operator*() { return keep_; }
  };
}

Class template std::ostreambuf_iterator

namespace std {
  template<class CharT, class Traits = char_traits<CharT>>
  class ostreambuf_iterator {
  public:
    using iterator_category = output_iterator_tag;
    using value_type        = void;
    using difference_type   = ptrdiff_t;
    using pointer           = void;
    using reference         = void;
    using char_type         = CharT;
    using traits_type       = Traits;
    using streambuf_type    = basic_streambuf<CharT, Traits>;
    using ostream_type      = basic_ostream<CharT, Traits>;
 
    constexpr ostreambuf_iterator() noexcept = default;
    ostreambuf_iterator(ostream_type& s) noexcept;
    ostreambuf_iterator(streambuf_type* s) noexcept;
    ostreambuf_iterator& operator=(CharT c);
 
    ostreambuf_iterator& operator*();
    ostreambuf_iterator& operator++();
    ostreambuf_iterator& operator++(int);
    bool failed() const noexcept;
 
  private:
    streambuf_type* sbuf_ = nullptr;    // exposition only
  };
}

Class template std::iterator

namespace std {
  template<class Category, class T, class Distance = ptrdiff_t,
           class Pointer = T*, class Reference = T&>
  struct iterator {
    typedef Category  iterator_category;
    typedef T         value_type;
    typedef Distance  difference_type;
    typedef Pointer   pointer;
    typedef Reference reference;
  };
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 349	C++98	the exposition-only member delim of std::ostream_iterator had type const char*	corrected to const CharT*