26 Ranges library [ranges]

26.1 General [ranges.general]

This Clause describes components for dealing with ranges of elements.
The following subclauses describe range and view requirements, and components for range primitives and range generators as summarized in Table 92.
Table 92: Ranges library summary [tab:range.summary]
Subclause
Header
Range access
<ranges>
Requirements
Range utilities
Range factories
Range adaptors
Range generators

26.2 Header <ranges> synopsis [ranges.syn]

#include <compare> // see [compare.syn] #include <initializer_list> // see [initializer.list.syn] #include <iterator> // see [iterator.synopsis] namespace std::ranges { inline namespace unspecified { // [range.access], range access inline constexpr unspecified begin = unspecified; // freestanding inline constexpr unspecified end = unspecified; // freestanding inline constexpr unspecified cbegin = unspecified; // freestanding inline constexpr unspecified cend = unspecified; // freestanding inline constexpr unspecified rbegin = unspecified; // freestanding inline constexpr unspecified rend = unspecified; // freestanding inline constexpr unspecified crbegin = unspecified; // freestanding inline constexpr unspecified crend = unspecified; // freestanding inline constexpr unspecified size = unspecified; // freestanding inline constexpr unspecified ssize = unspecified; // freestanding inline constexpr unspecified empty = unspecified; // freestanding inline constexpr unspecified data = unspecified; // freestanding inline constexpr unspecified cdata = unspecified; // freestanding } // [range.range], ranges template<class T> concept range = see below; // freestanding template<class T> constexpr bool enable_borrowed_range = false; // freestanding template<class T> concept borrowed_range = see below; // freestanding template<class T> using iterator_t = decltype(ranges::begin(declval<T&>())); // freestanding template<range R> using sentinel_t = decltype(ranges::end(declval<R&>())); // freestanding template<range R> using const_iterator_t = const_iterator<iterator_t<R>>; // freestanding template<range R> using const_sentinel_t = const_sentinel<sentinel_t<R>>; // freestanding template<range R> using range_difference_t = iter_difference_t<iterator_t<R>>; // freestanding template<sized_range R> using range_size_t = decltype(ranges::size(declval<R&>())); // freestanding template<range R> using range_value_t = iter_value_t<iterator_t<R>>; // freestanding template<range R> using range_reference_t = iter_reference_t<iterator_t<R>>; // freestanding template<range R> using range_const_reference_t = iter_const_reference_t<iterator_t<R>>; // freestanding template<range R> using range_rvalue_reference_t = iter_rvalue_reference_t<iterator_t<R>>; // freestanding template<range R> using range_common_reference_t = iter_common_reference_t<iterator_t<R>>; // freestanding // [range.sized], sized ranges template<class> constexpr bool disable_sized_range = false; // freestanding template<class T> concept sized_range = see below; // freestanding // [range.view], views template<class T> constexpr bool enable_view = see below; // freestanding struct view_base {}; // freestanding template<class T> concept view = see below; // freestanding // [range.refinements], other range refinements template<class R, class T> concept output_range = see below; // freestanding template<class T> concept input_range = see below; // freestanding template<class T> concept forward_range = see below; // freestanding template<class T> concept bidirectional_range = see below; // freestanding template<class T> concept random_access_range = see below; // freestanding template<class T> concept contiguous_range = see below; // freestanding template<class T> concept common_range = see below; // freestanding template<class T> concept viewable_range = see below; // freestanding template<class T> concept constant_range = see below; // freestanding // [view.interface], class template view_interface template<class D> requires is_class_v<D> && same_as<D, remove_cv_t<D>> class view_interface; // freestanding // [range.subrange], sub-ranges enum class subrange_kind : bool { unsized, sized }; // freestanding template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K = see below> requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>) class subrange; // freestanding template<class I, class S, subrange_kind K> constexpr bool enable_borrowed_range<subrange<I, S, K>> = true; // freestanding template<size_t N, class I, class S, subrange_kind K> requires ((N == 0 && copyable<I>) || N == 1) constexpr auto get(const subrange<I, S, K>& r); // freestanding template<size_t N, class I, class S, subrange_kind K> requires (N < 2) constexpr auto get(subrange<I, S, K>&& r); // freestanding } namespace std { using ranges::get; // freestanding } namespace std::ranges { // [range.dangling], dangling iterator handling struct dangling; // freestanding // [range.elementsof], class template elements_of template<range R, class Allocator = allocator<byte>> struct elements_of; template<range R> using borrowed_iterator_t = see below; // freestanding template<range R> using borrowed_subrange_t = see below; // freestanding // [range.utility.conv], range conversions template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args); // freestanding template<template<class...> class C, input_range R, class... Args> constexpr auto to(R&& r, Args&&... args); // freestanding template<class C, class... Args> requires (!view<C>) constexpr auto to(Args&&... args); // freestanding template<template<class...> class C, class... Args> constexpr auto to(Args&&... args); // freestanding // [range.empty], empty view template<class T> requires is_object_v<T> class empty_view; // freestanding template<class T> constexpr bool enable_borrowed_range<empty_view<T>> = true; // freestanding namespace views { template<class T> constexpr empty_view<T> empty{}; // freestanding } // [range.single], single view template<move_constructible T> requires is_object_v<T> class single_view; // freestanding namespace views { inline constexpr unspecified single = unspecified; } // freestanding template<bool Const, class T> using maybe-const = conditional_t<Const, const T, T>; // exposition only // [range.iota], iota view template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t> requires weakly-equality-comparable-with<W, Bound> && copyable<W> class iota_view; // freestanding template<class W, class Bound> constexpr bool enable_borrowed_range<iota_view<W, Bound>> = true; // freestanding namespace views { inline constexpr unspecified iota = unspecified; } // freestanding // [range.repeat], repeat view template<move_constructible T, semiregular Bound = unreachable_sentinel_t> requires see below class repeat_view; // freestanding namespace views { inline constexpr unspecified repeat = unspecified; } // freestanding // [range.istream], istream view template<movable Val, class CharT, class Traits = char_traits<CharT>> requires see below class basic_istream_view; template<class Val> using istream_view = basic_istream_view<Val, char>; template<class Val> using wistream_view = basic_istream_view<Val, wchar_t>; namespace views { template<class T> constexpr unspecified istream = unspecified; } // [range.adaptor.object], range adaptor objects template<class D> requires is_class_v<D> && same_as<D, remove_cv_t<D>> class range_adaptor_closure { }; // freestanding // [range.all], all view namespace views { inline constexpr unspecified all = unspecified; // freestanding template<viewable_range R> using all_t = decltype(all(declval<R>())); // freestanding } // [range.ref.view], ref view template<range R> requires is_object_v<R> class ref_view; // freestanding template<class T> constexpr bool enable_borrowed_range<ref_view<T>> = true; // freestanding // [range.owning.view], owning view template<range R> requires see below class owning_view; // freestanding template<class T> constexpr bool enable_borrowed_range<owning_view<T>> = // freestanding enable_borrowed_range<T>; // [range.as.rvalue], as rvalue view template<view V> requires input_range<V> class as_rvalue_view; // freestanding template<class T> constexpr bool enable_borrowed_range<as_rvalue_view<T>> = // freestanding enable_borrowed_range<T>; namespace views { inline constexpr unspecified as_rvalue = unspecified; } // freestanding // [range.filter], filter view template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class filter_view; // freestanding namespace views { inline constexpr unspecified filter = unspecified; } // freestanding // [range.transform], transform view template<input_range V, move_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> class transform_view; // freestanding namespace views { inline constexpr unspecified transform = unspecified; } // freestanding // [range.take], take view template<view> class take_view; // freestanding template<class T> constexpr bool enable_borrowed_range<take_view<T>> = // freestanding enable_borrowed_range<T>; namespace views { inline constexpr unspecified take = unspecified; } // freestanding // [range.take.while], take while view template<view V, class Pred> requires input_range<V> && is_object_v<Pred> && indirect_unary_predicate<const Pred, iterator_t<V>> class take_while_view; // freestanding namespace views { inline constexpr unspecified take_while = unspecified; } // freestanding // [range.drop], drop view template<view V> class drop_view; // freestanding template<class T> constexpr bool enable_borrowed_range<drop_view<T>> = // freestanding enable_borrowed_range<T>; namespace views { inline constexpr unspecified drop = unspecified; } // freestanding // [range.drop.while], drop while view template<view V, class Pred> requires input_range<V> && is_object_v<Pred> && indirect_unary_predicate<const Pred, iterator_t<V>> class drop_while_view; // freestanding template<class T, class Pred> constexpr bool enable_borrowed_range<drop_while_view<T, Pred>> = // freestanding enable_borrowed_range<T>; namespace views { inline constexpr unspecified drop_while = unspecified; } // freestanding // [range.join], join view template<input_range V> requires view<V> && input_range<range_reference_t<V>> class join_view; // freestanding namespace views { inline constexpr unspecified join = unspecified; } // freestanding // [range.join.with], join with view template<class R, class P> concept compatible-joinable-ranges = see below; // exposition only template<input_range V, forward_range Pattern> requires view<V> && input_range<range_reference_t<V>> && view<Pattern> && compatible-joinable-ranges<range_reference_t<V>, Pattern> class join_with_view; // freestanding namespace views { inline constexpr unspecified join_with = unspecified; } // freestanding // [range.lazy.split], lazy split view template<class R> concept tiny-range = see below; // exposition only template<input_range V, forward_range Pattern> requires view<V> && view<Pattern> && indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> && (forward_range<V> || tiny-range<Pattern>) class lazy_split_view; // freestanding // [range.split], split view template<forward_range V, forward_range Pattern> requires view<V> && view<Pattern> && indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> class split_view; // freestanding namespace views { inline constexpr unspecified lazy_split = unspecified; // freestanding inline constexpr unspecified split = unspecified; // freestanding } // [range.counted], counted view namespace views { inline constexpr unspecified counted = unspecified; } // freestanding // [range.common], common view template<view V> requires (!common_range<V> && copyable<iterator_t<V>>) class common_view; // freestanding template<class T> constexpr bool enable_borrowed_range<common_view<T>> = // freestanding enable_borrowed_range<T>; namespace views { inline constexpr unspecified common = unspecified; } // freestanding // [range.reverse], reverse view template<view V> requires bidirectional_range<V> class reverse_view; // freestanding template<class T> constexpr bool enable_borrowed_range<reverse_view<T>> = // freestanding enable_borrowed_range<T>; namespace views { inline constexpr unspecified reverse = unspecified; } // freestanding // [range.as.const], as const view template<input_range R> constexpr auto& possibly-const-range(R& r) { // exposition only if constexpr (constant_range<const R> && !constant_range<R>) { return const_cast<const R&>(r); } else { return r; } } template<view V> requires input_range<V> class as_const_view; // freestanding template<class T> constexpr bool enable_borrowed_range<as_const_view<T>> = // freestanding enable_borrowed_range<T>; namespace views { inline constexpr unspecified as_const = unspecified; } // freestanding // [range.elements], elements view template<input_range V, size_t N> requires see below class elements_view; // freestanding template<class T, size_t N> constexpr bool enable_borrowed_range<elements_view<T, N>> = // freestanding enable_borrowed_range<T>; template<class R> using keys_view = elements_view<R, 0>; // freestanding template<class R> using values_view = elements_view<R, 1>; // freestanding namespace views { template<size_t N> constexpr unspecified elements = unspecified; // freestanding inline constexpr auto keys = elements<0>; // freestanding inline constexpr auto values = elements<1>; // freestanding } // [range.enumerate], enumerate view template<input_range View> requires view<View> class enumerate_view; // freestanding template<class View> constexpr bool enable_borrowed_range<enumerate_view<View>> = // freestanding enable_borrowed_range<View>; namespace views { inline constexpr unspecified enumerate = unspecified; } // freestanding // [range.zip], zip view template<input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) class zip_view; // freestanding template<class... Views> constexpr bool enable_borrowed_range<zip_view<Views...>> = // freestanding (enable_borrowed_range<Views> && ...); namespace views { inline constexpr unspecified zip = unspecified; } // freestanding // [range.zip.transform], zip transform view template<move_constructible F, input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) && is_object_v<F> && regular_invocable<F&, range_reference_t<Views>...> && can-reference<invoke_result_t<F&, range_reference_t<Views>...>> class zip_transform_view; // freestanding namespace views { inline constexpr unspecified zip_transform = unspecified; } // freestanding // [range.adjacent], adjacent view template<forward_range V, size_t N> requires view<V> && (N > 0) class adjacent_view; // freestanding template<class V, size_t N> constexpr bool enable_borrowed_range<adjacent_view<V, N>> = // freestanding enable_borrowed_range<V>; namespace views { template<size_t N> constexpr unspecified adjacent = unspecified; // freestanding inline constexpr auto pairwise = adjacent<2>; // freestanding } // [range.adjacent.transform], adjacent transform view template<forward_range V, move_constructible F, size_t N> requires see below class adjacent_transform_view; // freestanding namespace views { template<size_t N> constexpr unspecified adjacent_transform = unspecified; // freestanding inline constexpr auto pairwise_transform = adjacent_transform<2>; // freestanding } // [range.chunk], chunk view template<view V> requires input_range<V> class chunk_view; // freestanding template<view V> requires forward_range<V> class chunk_view<V>; // freestanding template<class V> constexpr bool enable_borrowed_range<chunk_view<V>> = // freestanding forward_range<V> && enable_borrowed_range<V>; namespace views { inline constexpr unspecified chunk = unspecified; } // freestanding // [range.slide], slide view template<forward_range V> requires view<V> class slide_view; // freestanding template<class V> constexpr bool enable_borrowed_range<slide_view<V>> = enable_borrowed_range<V>; // freestanding namespace views { inline constexpr unspecified slide = unspecified; } // freestanding // [range.chunk.by], chunk by view template<forward_range V, indirect_binary_predicate<iterator_t<V>, iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class chunk_by_view; // freestanding namespace views { inline constexpr unspecified chunk_by = unspecified; } // freestanding // [range.stride], stride view template<input_range V> requires view<V> class stride_view; // freestanding template<class V> constexpr bool enable_borrowed_range<stride_view<V>> = // freestanding enable_borrowed_range<V>; namespace views { inline constexpr unspecified stride = unspecified; } // freestanding // [range.cartesian], cartesian product view template<input_range First, forward_range... Vs> requires (view<First> && ... && view<Vs>) class cartesian_product_view; // freestanding namespace views { inline constexpr unspecified cartesian_product = unspecified; } // freestanding } namespace std { namespace views = ranges::views; // freestanding template<class T> struct tuple_size; // freestanding template<size_t I, class T> struct tuple_element; // freestanding template<class I, class S, ranges::subrange_kind K> struct tuple_size<ranges::subrange<I, S, K>> // freestanding : integral_constant<size_t, 2> {}; template<class I, class S, ranges::subrange_kind K> struct tuple_element<0, ranges::subrange<I, S, K>> { // freestanding using type = I; // freestanding }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<1, ranges::subrange<I, S, K>> { // freestanding using type = S; // freestanding }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<0, const ranges::subrange<I, S, K>> { // freestanding using type = I; // freestanding }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<1, const ranges::subrange<I, S, K>> { // freestanding using type = S; // freestanding }; struct from_range_t { explicit from_range_t() = default; }; // freestanding inline constexpr from_range_t from_range{}; // freestanding }
Within this Clause, for an integer-like type X ([iterator.concept.winc]), make-unsigned-like-t<X> denotes make_unsigned_t<X> if X is an integer type; otherwise, it denotes a corresponding unspecified unsigned-integer-like type of the same width as X.
For an expression x of type X, to-unsigned-like(x) is x explicitly converted to make-unsigned-like-t<X>.
Also within this Clause, make-signed-like-t<X> for an integer-like type X denotes make_signed_t<X> if X is an integer type; otherwise, it denotes a corresponding unspecified signed-integer-like type of the same width as X.

26.3 Range access [range.access]

26.3.1 General [range.access.general]

In addition to being available via inclusion of the <ranges> header, the customization point objects in [range.access] are available when <iterator> is included.
Within [range.access], the reified object of a subexpression E denotes
  • the same object as E if E is a glvalue, or
  • the result of applying the temporary materialization conversion ([conv.rval]) to E otherwise.

26.3.2 ranges​::​begin [range.access.begin]

The name ranges​::​begin denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​begin(E) is ill-formed.
  • Otherwise, if T is an array type ([dcl.array]) and remove_all_extents_t<T> is an incomplete type, ranges​::​begin(E) is ill-formed with no diagnostic required.
  • Otherwise, if T is an array type, ranges​::​begin(E) is expression-equivalent to t + 0.
  • Otherwise, if auto(t.begin()) is a valid expression whose type models input_or_output_iterator, ranges​::​begin(E) is expression-equivalent to auto(t.begin()).
  • Otherwise, if T is a class or enumeration type and auto(begin(t)) is a valid expression whose type models input_or_output_iterator where the meaning of begin is established as-if by performing argument-dependent lookup only ([basic.lookup.argdep]), then ranges​::​begin(E) is expression-equivalent to that expression.
  • Otherwise, ranges​::​begin(E) is ill-formed.
[Note 1:
Diagnosable ill-formed cases above result in substitution failure when ranges​::​begin(E) appears in the immediate context of a template instantiation.
β€” end note]
[Note 2:
Whenever ranges​::​begin(E) is a valid expression, its type models input_or_output_iterator.
β€” end note]

26.3.3 ranges​::​end [range.access.end]

The name ranges​::​end denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​end(E) is ill-formed.
  • Otherwise, if T is an array type ([dcl.array]) and remove_all_extents_t<T> is an incomplete type, ranges​::​end(E) is ill-formed with no diagnostic required.
  • Otherwise, if T is an array of unknown bound, ranges​::​end(E) is ill-formed.
  • Otherwise, if T is an array, ranges​::​end(E) is expression-equivalent to t + extent_v<T>.
  • Otherwise, if auto(t.end()) is a valid expression whose type models sentinel_for<iterator_t<T>> then ranges​::​end(E) is expression-equivalent to auto(t.end()).
  • Otherwise, if T is a class or enumeration type and auto(end(t)) is a valid expression whose type models sentinel_for<iterator_t<T>> where the meaning of end is established as-if by performing argument-dependent lookup only ([basic.lookup.argdep]), then ranges​::​end(E) is expression-equivalent to that expression.
  • Otherwise, ranges​::​end(E) is ill-formed.
[Note 1:
Diagnosable ill-formed cases above result in substitution failure when ranges​::​end(E) appears in the immediate context of a template instantiation.
β€” end note]
[Note 2:
Whenever ranges​::​end(E) is a valid expression, the types S and I of ranges​::​end(E) and ranges​::​begin(E) model sentinel_for<S, I>.
β€” end note]

26.3.4 ranges​::​cbegin [range.access.cbegin]

The name ranges​::​cbegin denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​cbegin(E) is ill-formed.
  • Otherwise, let U be ranges​::​begin(possibly-const-range(t)).
    ranges​::​cbegin(E) is expression-equivalent to const_iterator<decltype(U)>(U).
[Note 1:
Whenever ranges​::​cbegin(E) is a valid expression, its type models input_or_output_iterator and constant-iterator.
β€” end note]

26.3.5 ranges​::​cend [range.access.cend]

The name ranges​::​cend denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​cend(E) is ill-formed.
  • Otherwise, let U be ranges​::​end(possibly-const-range(t)).
    ranges​::​cend(E) is expression-equivalent to const_sentinel<decltype(U)>(U).
[Note 1:
Whenever ranges​::​cend(E) is a valid expression, the types S and I of the expressions ranges​::​cend(E) and ranges​::​cbegin(E) model sentinel_for<S, I>.
If S models input_iterator, then S also models constant-iterator.
β€” end note]

26.3.6 ranges​::​rbegin [range.access.rbegin]

The name ranges​::​rbegin denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​rbegin(E) is ill-formed.
  • Otherwise, if T is an array type ([dcl.array]) and remove_all_extents_t<T> is an incomplete type, ranges​::​rbegin(E) is ill-formed with no diagnostic required.
  • Otherwise, if auto(t.rbegin()) is a valid expression whose type models input_or_output_iterator, ranges​::​rbegin(E) is expression-equivalent to auto(t.rbegin()).
  • Otherwise, if T is a class or enumeration type and auto(rbegin(t)) is a valid expression whose type models input_or_output_iterator where the meaning of rbegin is established as-if by performing argument-dependent lookup only ([basic.lookup.argdep]), then ranges​::​rbegin(E) is expression-equivalent to that expression.
  • Otherwise, if both ranges​::​begin(t) and ranges​::​end(t) are valid expressions of the same type which models bidirectional_iterator ([iterator.concept.bidir]), ranges​::​rbegin(E) is expression-equivalent to make_reverse_iterator(ranges​::​end(t)).
  • Otherwise, ranges​::​rbegin(E) is ill-formed.
[Note 1:
Diagnosable ill-formed cases above result in substitution failure when ranges​::​rbegin(E) appears in the immediate context of a template instantiation.
β€” end note]
[Note 2:
Whenever ranges​::​rbegin(E) is a valid expression, its type models input_or_output_iterator.
β€” end note]

26.3.7 ranges​::​rend [range.access.rend]

The name ranges​::​rend denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​rend(E) is ill-formed.
  • Otherwise, if T is an array type ([dcl.array]) and remove_all_extents_t<T> is an incomplete type, ranges​::​rend(E) is ill-formed with no diagnostic required.
  • Otherwise, if auto(t.rend()) is a valid expression whose type models sentinel_for<decltype(​ranges​::​rbegin(E))> then ranges​::​rend(E) is expression-equivalent to auto(t.rend()).
  • Otherwise, if T is a class or enumeration type and auto(rend(t)) is a valid expression whose type models sentinel_for<decltype(ranges​::​rbegin(E))> where the meaning of rend is established as-if by performing argument-dependent lookup only ([basic.lookup.argdep]), then ranges​::​rend(E) is expression-equivalent to that expression.
  • Otherwise, if both ranges​::​begin(t) and ranges​::​end(t) are valid expressions of the same type which models bidirectional_iterator ([iterator.concept.bidir]), then ranges​::​rend(E) is expression-equivalent to make_reverse_iterator(ranges​::​begin(t)).
  • Otherwise, ranges​::​rend(E) is ill-formed.
[Note 1:
Diagnosable ill-formed cases above result in substitution failure when ranges​::​rend(E) appears in the immediate context of a template instantiation.
β€” end note]
[Note 2:
Whenever ranges​::​rend(E) is a valid expression, the types S and I of the expressions ranges​::​rend(E) and ranges​::​rbegin(E) model sentinel_for<S, I>.
β€” end note]

26.3.8 ranges​::​crbegin [range.access.crbegin]

The name ranges​::​crbegin denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​crbegin(E) is ill-formed.
  • Otherwise, let U be ranges​::​rbegin(possibly-const-range(t)).
    ranges​::​crbegin(E) is expression-equivalent to const_iterator<decltype(U)>(U).
[Note 1:
Whenever ranges​::​crbegin(E) is a valid expression, its type models input_or_output_iterator and constant-iterator.
β€” end note]

26.3.9 ranges​::​crend [range.access.crend]

The name ranges​::​crend denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​crend(E) is ill-formed.
  • Otherwise, let U be ranges​::​rend(possibly-const-range(t)).
    ranges​::​crend(E) is expression-equivalent to const_sentinel<decltype(U)>(U).
[Note 1:
Whenever ranges​::​crend(E) is a valid expression, the types S and I of the expressions ranges​::​crend(E) and ranges​::​crbegin(E) model sentinel_for<S, I>.
If S models input_iterator, then S also models constant-iterator.
β€” end note]

26.3.10 ranges​::​size [range.prim.size]

The name ranges​::​size denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If T is an array of unknown bound ([dcl.array]), ranges​::​size(E) is ill-formed.
  • Otherwise, if T is an array type, ranges​::​size(E) is expression-equivalent to auto(extent_v<T>).
  • Otherwise, if disable_sized_range<remove_cv_t<T>> ([range.sized]) is false and auto(t.size()) is a valid expression of integer-like type ([iterator.concept.winc]), ranges​::​size(E) is expression-equivalent to auto(​t.size()).
  • Otherwise, if T is a class or enumeration type, disable_sized_range<remove_cv_t<T>> is false and auto(size(t)) is a valid expression of integer-like type where the meaning of size is established as-if by performing argument-dependent lookup only ([basic.lookup.argdep]), then ranges​::​size(E) is expression-equivalent to that expression.
  • Otherwise, if to-unsigned-like(ranges​::​end(t) - ranges​::​begin(t)) ([ranges.syn]) is a valid expression and the types I and S of ranges​::​begin(t) and ranges​::​end(t) (respectively) model both sized_sentinel_for<S, I> ([iterator.concept.sizedsentinel]) and forward_iterator<I>, then ranges​::​size(E) is expression-equivalent to to-unsigned-like(ranges​::​end(t) - ranges​::​begin(t)).
  • Otherwise, ranges​::​size(E) is ill-formed.
[Note 1:
Diagnosable ill-formed cases above result in substitution failure when ranges​::​size(E) appears in the immediate context of a template instantiation.
β€” end note]
[Note 2:
Whenever ranges​::​size(E) is a valid expression, its type is integer-like.
β€” end note]

26.3.11 ranges​::​ssize [range.prim.ssize]

The name ranges​::​ssize denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
If ranges​::​size(t) is ill-formed, ranges​::​ssize(E) is ill-formed.
Otherwise let D be make-signed-like-t<decltype(ranges​::​​size(t))>, or ptrdiff_t if it is wider than that type; ranges​::​ssize(E) is expression-equivalent to static_cast<D>(ranges​::​size(t)).

26.3.12 ranges​::​empty [range.prim.empty]

The name ranges​::​empty denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If T is an array of unknown bound ([dcl.array]), ranges​::​empty(E) is ill-formed.
  • Otherwise, if bool(t.empty()) is a valid expression, ranges​::​empty(E) is expression-equivalent to bool(t.empty()).
  • Otherwise, if (ranges​::​size(t) == 0) is a valid expression, ranges​::​empty(E) is expression-equivalent to (ranges​::​size(t) == 0).
  • Otherwise, if bool(ranges​::​begin(t) == ranges​::​end(t)) is a valid expression and the type of ranges​::​begin(t) models forward_iterator, ranges​::​empty(E) is expression-equivalent to bool(​ranges​::​begin(t) == ranges​::​end(t)).
  • Otherwise, ranges​::​empty(E) is ill-formed.
[Note 1:
Diagnosable ill-formed cases above result in substitution failure when ranges​::​empty(E) appears in the immediate context of a template instantiation.
β€” end note]
[Note 2:
Whenever ranges​::​empty(E) is a valid expression, it has type bool.
β€” end note]

26.3.13 ranges​::​data [range.prim.data]

The name ranges​::​data denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​data(E) is ill-formed.
  • Otherwise, if T is an array type ([dcl.array]) and remove_all_extents_t<T> is an incomplete type, ranges​::​data(E) is ill-formed with no diagnostic required.
  • Otherwise, if auto(t.data()) is a valid expression of pointer to object type, ranges​::​data(E) is expression-equivalent to auto(t.data()).
  • Otherwise, if ranges​::​begin(t) is a valid expression whose type models contiguous_iterator, ranges​::​data(E) is expression-equivalent to to_address(ranges​::​begin(t)).
  • Otherwise, ranges​::​data(E) is ill-formed.
[Note 1:
Diagnosable ill-formed cases above result in substitution failure when ranges​::​data(E) appears in the immediate context of a template instantiation.
β€” end note]
[Note 2:
Whenever ranges​::​data(E) is a valid expression, it has pointer to object type.
β€” end note]

26.3.14 ranges​::​cdata [range.prim.cdata]

template<class T> constexpr auto as-const-pointer(const T* p) { return p; } // exposition only
The name ranges​::​cdata denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​cdata(E) is ill-formed.
  • Otherwise, ranges​::​cdata(E) is expression-equivalent to as-const-pointer(ranges​::​data(possibly-const-range(t))).
[Note 1:
Whenever ranges​::​cdata(E) is a valid expression, it has pointer to constant object type.
β€” end note]

26.4 Range requirements [range.req]

26.4.1 General [range.req.general]

Ranges are an abstraction that allows a C++ program to operate on elements of data structures uniformly.
Calling ranges​::​begin on a range returns an object whose type models input_or_output_iterator ([iterator.concept.iterator]).
Calling ranges​::​end on a range returns an object whose type S, together with the type I of the object returned by ranges​::​begin, models sentinel_for<S, I>.
The library formalizes the interfaces, semantics, and complexity of ranges to enable algorithms and range adaptors that work efficiently on different types of sequences.
The range concept requires that ranges​::​begin and ranges​::​end return an iterator and a sentinel, respectively.
The sized_range concept refines range with the requirement that ranges​::​size be amortized .
The view concept specifies requirements on a range type to provide operations with predictable complexity.
Several refinements of range group requirements that arise frequently in concepts and algorithms.
Common ranges are ranges for which ranges​::​begin and ranges​::​end return objects of the same type.
Random access ranges are ranges for which ranges​::​begin returns a type that models random_access_iterator ([iterator.concept.random.access]).
(Contiguous, bidirectional, forward, input, and output ranges are defined similarly.)
Viewable ranges can be converted to views.

26.4.2 Ranges [range.range]

The range concept defines the requirements of a type that allows iteration over its elements by providing an iterator and sentinel that denote the elements of the range.
template<class T> concept range = requires(T& t) { ranges::begin(t); // sometimes equality-preserving (see below) ranges::end(t); };
The required expressions ranges​::​begin(t) and ranges​::​end(t) of the range concept do not require implicit expression variations ([concepts.equality]).
Given an expression t such that decltype((t)) is T&, T models range only if
  • [ranges​::​begin(t), ranges​::​end(t)) denotes a range ([iterator.requirements.general]),
  • both ranges​::​begin(t) and ranges​::​end(t) are amortized constant time and non-modifying, and
  • if the type of ranges​::​begin(t) models forward_iterator, ranges​::​begin(t) is equality-preserving.
[Note 1:
Equality preservation of both ranges​::​begin and ranges​::​end enables passing a range whose iterator type models forward_iterator to multiple algorithms and making multiple passes over the range by repeated calls to ranges​::​begin and ranges​::​end.
Since ranges​::​begin is not required to be equality-preserving when the return type does not model forward_iterator, it is possible for repeated calls to not return equal values or to not be well-defined.
β€” end note]
template<class T> concept borrowed_range = range<T> && (is_lvalue_reference_v<T> || enable_borrowed_range<remove_cvref_t<T>>);
Let U be remove_reference_t<T> if T is an rvalue reference type, and T otherwise.
Given a variable u of type U, T models borrowed_range only if the validity of iterators obtained from u is not tied to the lifetime of that variable.
[Note 2:
Since the validity of iterators is not tied to the lifetime of a variable whose type models borrowed_range, a function with a parameter of such a type can return iterators obtained from it without danger of dangling.
β€” end note]
template<class> constexpr bool enable_borrowed_range = false;
Remarks: Pursuant to [namespace.std], users may specialize enable_borrowed_range for cv-unqualified program-defined types.
Such specializations shall be usable in constant expressions ([expr.const]) and have type const bool.
[Example 1:
Each specialization S of class template subrange ([range.subrange]) models borrowed_range because
  • enable_borrowed_range<S> is specialized to have the value true, and
  • S's iterators do not have validity tied to the lifetime of an S object because they are β€œborrowed” from some other range.
β€” end example]

26.4.3 Sized ranges [range.sized]

The sized_range concept refines range with the requirement that the number of elements in the range can be determined in amortized constant time using ranges​::​size.
template<class T> concept sized_range = range<T> && requires(T& t) { ranges::size(t); };
Given an lvalue t of type remove_reference_t<T>, T models sized_range only if
  • ranges​::​size(t) is amortized , does not modify t, and is equal to ranges​::​distance(​ranges​::​begin(t), ranges​::​end(t)), and
  • if iterator_t<T> models forward_iterator, ranges​::​size(t) is well-defined regardless of the evaluation of ranges​::​begin(t).
    [Note 1:
    ranges​::​size(t) is otherwise not required to be well-defined after evaluating ranges​::​begin(t).
    For example, it is possible for ranges​::​size(t) to be well-defined for a sized_range whose iterator type does not model forward_iterator only if evaluated before the first call to ranges​::​begin(t).
    β€” end note]
template<class> constexpr bool disable_sized_range = false;
Remarks: Pursuant to [namespace.std], users may specialize disable_sized_range for cv-unqualified program-defined types.
Such specializations shall be usable in constant expressions ([expr.const]) and have type const bool.
[Note 2:
disable_sized_range allows use of range types with the library that satisfy but do not in fact model sized_range.
β€” end note]

26.4.4 Views [range.view]

The view concept specifies the requirements of a range type that has the semantic properties below, which make it suitable for use in constructing range adaptor pipelines ([range.adaptors]).
template<class T> concept view = range<T> && movable<T> && enable_view<T>;
T models view only if:
  • T has move construction; and
  • move assignment of an object of type T is no more complex than destruction followed by move construction; and
  • if N copies and/or moves are made from an object of type T that contained M elements, then those N objects have destruction; and
  • copy_constructible<T> is false, or T has copy construction; and
  • copyable<T> is false, or copy assignment of an object of type T is no more complex than destruction followed by copy construction.
[Note 1:
The constraints on copying and moving imply that a moved-from object of type T has destruction.
β€” end note]
[Example 1:
Examples of views are:
  • A range type that wraps a pair of iterators.
  • A range type that holds its elements by shared_ptr and shares ownership with all its copies.
  • A range type that generates its elements on demand.
A container such as vector<string> does not meet the semantic requirements of view since copying the container copies all of the elements, which cannot be done in constant time.
β€” end example]
Since the difference between range and view is largely semantic, the two are differentiated with the help of enable_view.
template<class T> constexpr bool is-derived-from-view-interface = see below; // exposition only template<class T> constexpr bool enable_view = derived_from<T, view_base> || is-derived-from-view-interface<T>;
For a type T, is-derived-from-view-interface<T> is true if and only if T has exactly one public base class view_interface<U> for some type U and T has no base classes of type view_interface<V> for any other type V.
Remarks: Pursuant to [namespace.std], users may specialize enable_view to true for cv-unqualified program-defined types which model view, and false for types which do not.
Such specializations shall be usable in constant expressions ([expr.const]) and have type const bool.

26.4.5 Other range refinements [range.refinements]

The output_range concept specifies requirements of a range type for which ranges​::​begin returns a model of output_iterator ([iterator.concept.output]).
template<class R, class T> concept output_range = range<R> && output_iterator<iterator_t<R>, T>; template<class T> concept input_range = range<T> && input_iterator<iterator_t<T>>; template<class T> concept forward_range = input_range<T> && forward_iterator<iterator_t<T>>; template<class T> concept bidirectional_range = forward_range<T> && bidirectional_iterator<iterator_t<T>>; template<class T> concept random_access_range = bidirectional_range<T> && random_access_iterator<iterator_t<T>>;
contiguous_range additionally requires that the ranges​::​data customization point object ([range.prim.data]) is usable with the range.
template<class T> concept contiguous_range = random_access_range<T> && contiguous_iterator<iterator_t<T>> && requires(T& t) { { ranges::data(t) } -> same_as<add_pointer_t<range_reference_t<T>>>; };
Given an expression t such that decltype((t)) is T&, T models contiguous_range only if to_address(​ranges​::​begin(t)) == ranges​::​data(t) is true.
The common_range concept specifies requirements of a range type for which ranges​::​begin and ranges​::​end return objects of the same type.
[Example 1:
The standard containers ([containers]) model common_range.
β€” end example]
template<class T> concept common_range = range<T> && same_as<iterator_t<T>, sentinel_t<T>>;
template<class R> constexpr bool is-initializer-list = see below; // exposition only
For a type R, is-initializer-list<R> is true if and only if remove_cvref_t<R> is a specialization of initializer_list.
The viewable_range concept specifies the requirements of a range type that can be converted to a view safely.
template<class T> concept viewable_range = range<T> && ((view<remove_cvref_t<T>> && constructible_from<remove_cvref_t<T>, T>) || (!view<remove_cvref_t<T>> && (is_lvalue_reference_v<T> || (movable<remove_reference_t<T>> && !is-initializer-list<T>))));
The constant_range concept specifies the requirements of a range type whose elements are not modifiable.
template<class T> concept constant_range = input_range<T> && constant-iterator<iterator_t<T>>;

26.5 Range utilities [range.utility]

26.5.1 General [range.utility.general]

The components in [range.utility] are general utilities for representing and manipulating ranges.

26.5.2 Helper concepts [range.utility.helpers]

Many of the types in subclause [range.utility] are specified in terms of the following exposition-only concepts: template<class R> concept simple-view = // exposition only view<R> && range<const R> && same_as<iterator_t<R>, iterator_t<const R>> && same_as<sentinel_t<R>, sentinel_t<const R>>; template<class I> concept has-arrow = // exposition only input_iterator<I> && (is_pointer_v<I> || requires(I i) { i.operator->(); }); template<class T, class U> concept different-from = // exposition only !same_as<remove_cvref_t<T>, remove_cvref_t<U>>; template<class R> concept range-with-movable-references = // exposition only input_range<R> && move_constructible<range_reference_t<R>> && move_constructible<range_rvalue_reference_t<R>>;

26.5.3 View interface [view.interface]

26.5.3.1 General [view.interface.general]

The class template view_interface is a helper for defining view-like types that offer a container-like interface.
It is parameterized with the type that is derived from it.
namespace std::ranges { template<class D> requires is_class_v<D> && same_as<D, remove_cv_t<D>> class view_interface { private: constexpr D& derived() noexcept { // exposition only return static_cast<D&>(*this); } constexpr const D& derived() const noexcept { // exposition only return static_cast<const D&>(*this); } public: constexpr bool empty() requires sized_range<D> || forward_range<D> { if constexpr (sized_range<D>) return ranges::size(derived()) == 0; else return ranges::begin(derived()) == ranges::end(derived()); } constexpr bool empty() const requires sized_range<const D> || forward_range<const D> { if constexpr (sized_range<const D>) return ranges::size(derived()) == 0; else return ranges::begin(derived()) == ranges::end(derived()); } constexpr auto cbegin() requires input_range<D> { return ranges::cbegin(derived()); } constexpr auto cbegin() const requires input_range<const D> { return ranges::cbegin(derived()); } constexpr auto cend() requires input_range<D> { return ranges::cend(derived()); } constexpr auto cend() const requires input_range<const D> { return ranges::cend(derived()); } constexpr explicit operator bool() requires requires { ranges::empty(derived()); } { return !ranges::empty(derived()); } constexpr explicit operator bool() const requires requires { ranges::empty(derived()); } { return !ranges::empty(derived()); } constexpr auto data() requires contiguous_iterator<iterator_t<D>> { return to_address(ranges::begin(derived())); } constexpr auto data() const requires range<const D> && contiguous_iterator<iterator_t<const D>> { return to_address(ranges::begin(derived())); } constexpr auto size() requires forward_range<D> && sized_sentinel_for<sentinel_t<D>, iterator_t<D>> { return to-unsigned-like(ranges::end(derived()) - ranges::begin(derived())); } constexpr auto size() const requires forward_range<const D> && sized_sentinel_for<sentinel_t<const D>, iterator_t<const D>> { return to-unsigned-like(ranges::end(derived()) - ranges::begin(derived())); } constexpr decltype(auto) front() requires forward_range<D>; constexpr decltype(auto) front() const requires forward_range<const D>; constexpr decltype(auto) back() requires bidirectional_range<D> && common_range<D>; constexpr decltype(auto) back() const requires bidirectional_range<const D> && common_range<const D>; template<random_access_range R = D> constexpr decltype(auto) operator[](range_difference_t<R> n) { return ranges::begin(derived())[n]; } template<random_access_range R = const D> constexpr decltype(auto) operator[](range_difference_t<R> n) const { return ranges::begin(derived())[n]; } }; }
The template parameter D for view_interface may be an incomplete type.
Before any member of the resulting specialization of view_interface other than special member functions is referenced, D shall be complete, and model both derived_from<view_interface<D>> and view.

26.5.3.2 Members [view.interface.members]

constexpr decltype(auto) front() requires forward_range<D>; constexpr decltype(auto) front() const requires forward_range<const D>;
Preconditions: !empty() is true.
Effects: Equivalent to: return *ranges​::​begin(derived());
constexpr decltype(auto) back() requires bidirectional_range<D> && common_range<D>; constexpr decltype(auto) back() const requires bidirectional_range<const D> && common_range<const D>;
Preconditions: !empty() is true.
Effects: Equivalent to: return *ranges​::​prev(ranges​::​end(derived()));

26.5.4 Sub-ranges [range.subrange]

26.5.4.1 General [range.subrange.general]

The subrange class template combines together an iterator and a sentinel into a single object that models the view concept.
Additionally, it models the sized_range concept when the final template parameter is subrange_kind​::​sized.
namespace std::ranges { template<class From, class To> concept uses-nonqualification-pointer-conversion = // exposition only is_pointer_v<From> && is_pointer_v<To> && !convertible_to<remove_pointer_t<From>(*)[], remove_pointer_t<To>(*)[]>; template<class From, class To> concept convertible-to-non-slicing = // exposition only convertible_to<From, To> && !uses-nonqualification-pointer-conversion<decay_t<From>, decay_t<To>>; template<class T, class U, class V> concept pair-like-convertible-from = // exposition only !range<T> && !is_reference_v<T> && pair-like<T> && constructible_from<T, U, V> && convertible-to-non-slicing<U, tuple_element_t<0, T>> && convertible_to<V, tuple_element_t<1, T>>; template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K = sized_sentinel_for<S, I> ? subrange_kind::sized : subrange_kind::unsized> requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>) class subrange : public view_interface<subrange<I, S, K>> { private: static constexpr bool StoreSize = // exposition only K == subrange_kind::sized && !sized_sentinel_for<S, I>; I begin_ = I(); // exposition only S end_ = S(); // exposition only make-unsigned-like-t<iter_difference_t<I>> size_ = 0; // exposition only; present only // if StoreSize is true public: subrange() requires default_initializable<I> = default; constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize); constexpr subrange(convertible-to-non-slicing<I> auto i, S s, make-unsigned-like-t<iter_difference_t<I>> n) requires (K == subrange_kind::sized); template<different-from<subrange> R> requires borrowed_range<R> && convertible-to-non-slicing<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S> constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>); template<borrowed_range R> requires convertible-to-non-slicing<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S> constexpr subrange(R&& r, make-unsigned-like-t<iter_difference_t<I>> n) requires (K == subrange_kind::sized) : subrange{ranges::begin(r), ranges::end(r), n} {} template<different-from<subrange> PairLike> requires pair-like-convertible-from<PairLike, const I&, const S&> constexpr operator PairLike() const; constexpr I begin() const requires copyable<I>; [[nodiscard]] constexpr I begin() requires (!copyable<I>); constexpr S end() const; constexpr bool empty() const; constexpr make-unsigned-like-t<iter_difference_t<I>> size() const requires (K == subrange_kind::sized); [[nodiscard]] constexpr subrange next(iter_difference_t<I> n = 1) const & requires forward_iterator<I>; [[nodiscard]] constexpr subrange next(iter_difference_t<I> n = 1) &&; [[nodiscard]] constexpr subrange prev(iter_difference_t<I> n = 1) const requires bidirectional_iterator<I>; constexpr subrange& advance(iter_difference_t<I> n); }; template<input_or_output_iterator I, sentinel_for<I> S> subrange(I, S) -> subrange<I, S>; template<input_or_output_iterator I, sentinel_for<I> S> subrange(I, S, make-unsigned-like-t<iter_difference_t<I>>) -> subrange<I, S, subrange_kind::sized>; template<borrowed_range R> subrange(R&&) -> subrange<iterator_t<R>, sentinel_t<R>, (sized_range<R> || sized_sentinel_for<sentinel_t<R>, iterator_t<R>>) ? subrange_kind::sized : subrange_kind::unsized>; template<borrowed_range R> subrange(R&&, make-unsigned-like-t<range_difference_t<R>>) -> subrange<iterator_t<R>, sentinel_t<R>, subrange_kind::sized>; }

26.5.4.2 Constructors and conversions [range.subrange.ctor]

constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize);
Preconditions: [i, s) is a valid range.
Effects: Initializes begin_ with std​::​move(i) and end_ with s.
constexpr subrange(convertible-to-non-slicing<I> auto i, S s, make-unsigned-like-t<iter_difference_t<I>> n) requires (K == subrange_kind::sized);
Preconditions: [i, s) is a valid range, and n == to-unsigned-like(ranges​::​distance(i, s)) is true.
Effects: Initializes begin_ with std​::​move(i) and end_ with s.
If StoreSize is true, initializes size_ with n.
[Note 1:
Accepting the length of the range and storing it to later return from size() enables subrange to model sized_range even when it stores an iterator and sentinel that do not model sized_sentinel_for.
β€” end note]
template<different-from<subrange> R> requires borrowed_range<R> && convertible-to-non-slicing<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S> constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);
Effects: Equivalent to:
  • If StoreSize is true, subrange(r, static_cast<decltype(size_)>(ranges​::​size(r))).
  • Otherwise, subrange(ranges​::​begin(r), ranges​::​end(r)).
template<different-from<subrange> PairLike> requires pair-like-convertible-from<PairLike, const I&, const S&> constexpr operator PairLike() const;
Effects: Equivalent to: return PairLike(begin_, end_);

26.5.4.3 Accessors [range.subrange.access]

constexpr I begin() const requires copyable<I>;
Effects: Equivalent to: return begin_;
[[nodiscard]] constexpr I begin() requires (!copyable<I>);
Effects: Equivalent to: return std​::​move(begin_);
constexpr S end() const;
Effects: Equivalent to: return end_;
constexpr bool empty() const;
Effects: Equivalent to: return begin_ == end_;
constexpr make-unsigned-like-t<iter_difference_t<I>> size() const requires (K == subrange_kind::sized);
Effects:
  • If StoreSize is true, equivalent to: return size_;
  • Otherwise, equivalent to: return to-unsigned-like(end_ - begin_);
[[nodiscard]] constexpr subrange next(iter_difference_t<I> n = 1) const & requires forward_iterator<I>;
Effects: Equivalent to: auto tmp = *this; tmp.advance(n); return tmp;
[[nodiscard]] constexpr subrange next(iter_difference_t<I> n = 1) &&;
Effects: Equivalent to: advance(n); return std::move(*this);
[[nodiscard]] constexpr subrange prev(iter_difference_t<I> n = 1) const requires bidirectional_iterator<I>;
Effects: Equivalent to: auto tmp = *this; tmp.advance(-n); return tmp;
constexpr subrange& advance(iter_difference_t<I> n);
Effects: Equivalent to: if constexpr (bidirectional_iterator<I>) { if (n < 0) { ranges::advance(begin_, n); if constexpr (StoreSize) size_ += to-unsigned-like(-n); return *this; } } auto d = n - ranges::advance(begin_, n, end_); if constexpr (StoreSize) size_ -= to-unsigned-like(d); return *this;
template<size_t N, class I, class S, subrange_kind K> requires ((N == 0 && copyable<I>) || N == 1) constexpr auto get(const subrange<I, S, K>& r); template<size_t N, class I, class S, subrange_kind K> requires (N < 2) constexpr auto get(subrange<I, S, K>&& r);
Effects: Equivalent to: if constexpr (N == 0) return r.begin(); else return r.end();

26.5.5 Dangling iterator handling [range.dangling]

The type dangling is used together with the template aliases borrowed_iterator_t and borrowed_subrange_t.
When an algorithm that typically returns an iterator into, or a subrange of, a range argument is called with an rvalue range argument that does not model borrowed_range ([range.range]), the return value possibly refers to a range whose lifetime has ended.
In such cases, the type dangling is returned instead of an iterator or subrange.
namespace std::ranges { struct dangling { constexpr dangling() noexcept = default; constexpr dangling(auto&&...) noexcept {} }; }
[Example 1: vector<int> f(); auto result1 = ranges::find(f(), 42); // #1 static_assert(same_as<decltype(result1), ranges::dangling>); auto vec = f(); auto result2 = ranges::find(vec, 42); // #2 static_assert(same_as<decltype(result2), vector<int>::iterator>); auto result3 = ranges::find(ranges::subrange{vec}, 42); // #3 static_assert(same_as<decltype(result3), vector<int>::iterator>);
The call to ranges​::​find at #1 returns ranges​::​dangling since f() is an rvalue vector; it is possible for the vector to be destroyed before a returned iterator is dereferenced.
However, the calls at #2 and #3 both return iterators since the lvalue vec and specializations of subrange model borrowed_range.
β€” end example]
For a type R that models range:
  • if R models borrowed_range, then borrowed_iterator_t<R> denotes iterator_t<R>, and borrowed_subrange_t<R> denotes subrange<iterator_t<R>>;
  • otherwise, both borrowed_iterator_t<R> and borrowed_subrange_t<R> denote dangling.

26.5.6 Class template elements_of [range.elementsof]

Specializations of elements_of encapsulate a range and act as a tag in overload sets to disambiguate when a range should be treated as a sequence rather than a single value.
[Example 1: template<bool YieldElements> generator<any> f(ranges::input_range auto&& r) { if constexpr (YieldElements) co_yield ranges::elements_of(r); // yield each element of r else co_yield r; // yield r as a single value } β€” end example]
namespace std::ranges { template<range R, class Allocator = allocator<byte>> struct elements_of { [[no_unique_address]] R range; [[no_unique_address]] Allocator allocator = Allocator(); }; template<class R, class Allocator = allocator<byte>> elements_of(R&&, Allocator = Allocator()) -> elements_of<R&&, Allocator>; }

26.5.7 Range conversions [range.utility.conv]

26.5.7.1 General [range.utility.conv.general]

The range conversion functions construct an object (usually a container) from a range, by using a constructor taking a range, a from_range_t tagged constructor, or a constructor taking a pair of iterators, or by inserting each element of the range into the default-constructed object.
ranges​::​to is applied recursively, allowing the conversion of a range of ranges.
[Example 1: string_view str = "the quick brown fox"; auto words = views::split(str, ' ') | to<vector<string>>(); // words is vector<string>{"the", "quick", "brown", "fox"} β€” end example]
Let reservable-container be defined as follows: template<class Container> constexpr bool reservable-container = // exposition only sized_range<Container> && requires(Container& c, range_size_t<Container> n) { c.reserve(n); { c.capacity() } -> same_as<decltype(n)>; { c.max_size() } -> same_as<decltype(n)>; };
Let container-insertable be defined as follows: template<class Container, class Ref> constexpr bool container-insertable = // exposition only requires(Container& c, Ref&& ref) { requires (requires { c.push_back(std::forward<Ref>(ref)); } || requires { c.insert(c.end(), std::forward<Ref>(ref)); }); };
Let container-inserter be defined as follows: template<class Ref, class Container> constexpr auto container-inserter(Container& c) { // exposition only if constexpr (requires { c.push_back(declval<Ref>()); }) return back_inserter(c); else return inserter(c, c.end()); }

26.5.7.2 ranges​::​to [range.utility.conv.to]

template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);
Mandates: C is a cv-unqualified class type.
Returns: An object of type C constructed from the elements of r in the following manner:
  • If C does not satisfy input_range or convertible_to<range_reference_t<R>, range_value_t<C>> is true:
  • Otherwise, if input_range<range_reference_t<R>> is true: to<C>(r | views::transform([](auto&& elem) { return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); }), std::forward<Args>(args)...);
  • Otherwise, the program is ill-formed.
template<template<class...> class C, input_range R, class... Args> constexpr auto to(R&& r, Args&&... args);
Let input-iterator be an exposition-only type: struct input-iterator { // exposition only using iterator_category = input_iterator_tag; using value_type = range_value_t<R>; using difference_type = ptrdiff_t; using pointer = add_pointer_t<range_reference_t<R>>; using reference = range_reference_t<R>; reference operator*() const; pointer operator->() const; input-iterator& operator++(); input-iterator operator++(int); bool operator==(const input-iterator&) const; };
[Note 1:
input-iterator meets the syntactic requirements of Cpp17InputIterator.
β€” end note]
Let DEDUCE_EXPR be defined as follows:
  • C(declval<R>(), declval<Args>()...) if that is a valid expression,
  • otherwise, C(from_range, declval<R>(), declval<Args>()...) if that is a valid expression,
  • otherwise, C(declval<input-iterator>(), declval<input-iterator>(), declval<Args>()...) if that is a valid expression,
  • otherwise, the program is ill-formed.
Returns: to<decltype(DEDUCE_EXPR)>(std​::​forward<R>(r), std​::​forward<Args>(args)...).

26.5.7.3 ranges​::​to adaptors [range.utility.conv.adaptors]

template<class C, class... Args> requires (!view<C>) constexpr auto to(Args&&... args); template<template<class...> class C, class... Args> constexpr auto to(Args&&... args);
Mandates: For the first overload, C is a cv-unqualified class type.
Returns: A range adaptor closure object ([range.adaptor.object]) f that is a perfect forwarding call wrapper ([func.require]) with the following properties:
  • It has no target object.
  • Its bound argument entities bound_args consist of objects of types decay_t<Args>... direct-non-list-initialized with std​::​forward<Args>(args)..., respectively.
  • Its call pattern is to<C>(r, bound_args...), where r is the argument used in a function call expression of f.

26.6 Range factories [range.factories]

26.6.1 General [range.factories.general]

Subclause [range.factories] defines range factories, which are utilities to create a view.
Range factories are declared in namespace std​::​ranges​::​views.

26.6.2 Empty view [range.empty]

26.6.2.1 Overview [range.empty.overview]

empty_view produces a view of no elements of a particular type.
[Example 1: auto e = views::empty<int>; static_assert(ranges::empty(e)); static_assert(0 == e.size()); β€” end example]

26.6.2.2 Class template empty_view [range.empty.view]

namespace std::ranges { template<class T> requires is_object_v<T> class empty_view : public view_interface<empty_view<T>> { public: static constexpr T* begin() noexcept { return nullptr; } static constexpr T* end() noexcept { return nullptr; } static constexpr T* data() noexcept { return nullptr; } static constexpr size_t size() noexcept { return 0; } static constexpr bool empty() noexcept { return true; } }; }

26.6.3 Single view [range.single]

26.6.3.1 Overview [range.single.overview]

single_view produces a view that contains exactly one element of a specified value.
The name views​::​single denotes a customization point object ([customization.point.object]).
Given a subexpression E, the expression views​::​single(E) is expression-equivalent to single_view<decay_t<decltype((E))>>(E).
[Example 1: for (int i : views::single(4)) cout << i; // prints 4 β€” end example]

26.6.3.2 Class template single_view [range.single.view]

namespace std::ranges { template<move_constructible T> requires is_object_v<T> class single_view : public view_interface<single_view<T>> { private: movable-box<T> value_; // exposition only (see [range.move.wrap]) public: single_view() requires default_initializable<T> = default; constexpr explicit single_view(const T& t) requires copy_constructible<T>; constexpr explicit single_view(T&& t); template<class... Args> requires constructible_from<T, Args...> constexpr explicit single_view(in_place_t, Args&&... args); constexpr T* begin() noexcept; constexpr const T* begin() const noexcept; constexpr T* end() noexcept; constexpr const T* end() const noexcept; static constexpr size_t size() noexcept; constexpr T* data() noexcept; constexpr const T* data() const noexcept; }; template<class T> single_view(T) -> single_view<T>; }
constexpr explicit single_view(const T& t) requires copy_constructible<T>;
Effects: Initializes value_ with t.
constexpr explicit single_view(T&& t);
Effects: Initializes value_ with std​::​move(t).
template<class... Args> requires constructible_from<T, Args...> constexpr explicit single_view(in_place_t, Args&&... args);
Effects: Initializes value_ as if by value_{in_place, std​::​forward<Args>(args)...}.
constexpr T* begin() noexcept; constexpr const T* begin() const noexcept;
Effects: Equivalent to: return data();
constexpr T* end() noexcept; constexpr const T* end() const noexcept;
Effects: Equivalent to: return data() + 1;
static constexpr size_t size() noexcept;
Effects: Equivalent to: return 1;
constexpr T* data() noexcept; constexpr const T* data() const noexcept;
Effects: Equivalent to: return value_.operator->();

26.6.4 Iota view [range.iota]

26.6.4.1 Overview [range.iota.overview]

iota_view generates a sequence of elements by repeatedly incrementing an initial value.
The name views​::​iota denotes a customization point object ([customization.point.object]).
Given subexpressions E and F, the expressions views​::​iota(E) and views​::​iota(E, F) are expression-equivalent to iota_view(E) and iota_view(E, F), respectively.
[Example 1: for (int i : views::iota(1, 10)) cout << i << ' '; // prints 1 2 3 4 5 6 7 8 9 β€” end example]

26.6.4.2 Class template iota_view [range.iota.view]

namespace std::ranges { template<class I> concept decrementable = see below; // exposition only template<class I> concept advanceable = see below; // exposition only template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t> requires weakly-equality-comparable-with<W, Bound> && copyable<W> class iota_view : public view_interface<iota_view<W, Bound>> { private: // [range.iota.iterator], class iota_view​::​iterator struct iterator; // exposition only // [range.iota.sentinel], class iota_view​::​sentinel struct sentinel; // exposition only W value_ = W(); // exposition only Bound bound_ = Bound(); // exposition only public: iota_view() requires default_initializable<W> = default; constexpr explicit iota_view(W value); constexpr explicit iota_view(type_identity_t<W> value, type_identity_t<Bound> bound); constexpr explicit iota_view(iterator first, see below last); constexpr iterator begin() const; constexpr auto end() const; constexpr iterator end() const requires same_as<W, Bound>; constexpr auto size() const requires see below; }; template<class W, class Bound> requires (!is-integer-like<W> || !is-integer-like<Bound> || (is-signed-integer-like<W> == is-signed-integer-like<Bound>)) iota_view(W, Bound) -> iota_view<W, Bound>; }
Let IOTA-DIFF-T(W) be defined as follows:
  • If W is not an integral type, or if it is an integral type and sizeof(iter_difference_t<W>) is greater than sizeof(W), then IOTA-DIFF-T(W) denotes iter_difference_t<W>.
  • Otherwise, IOTA-DIFF-T(W) is a signed integer type of width greater than the width of W if such a type exists.
  • Otherwise, IOTA-DIFF-T(W) is an unspecified signed-integer-like type ([iterator.concept.winc]) of width not less than the width of W.
    [Note 1:
    It is unspecified whether this type satisfies weakly_incrementable.
    β€” end note]
The exposition-only decrementable concept is equivalent to:
template<class I> concept decrementable = // exposition only incrementable<I> && requires(I i) { { --i } -> same_as<I&>; { i-- } -> same_as<I>; };
When an object is in the domain of both pre- and post-decrement, the object is said to be decrementable.
Let a and b be equal objects of type I.
I models decrementable only if
  • If a and b are decrementable, then the following are all true:
  • If a and b are incrementable, then bool(--(++a) == b).
The exposition-only advanceable concept is equivalent to:
template<class I> concept advanceable = // exposition only decrementable<I> && totally_ordered<I> && requires(I i, const I j, const IOTA-DIFF-T(I) n) { { i += n } -> same_as<I&>; { i -= n } -> same_as<I&>; I(j + n); I(n + j); I(j - n); { j - j } -> convertible_to<IOTA-DIFF-T(I)>; };
Let D be IOTA-DIFF-T(I).
Let a and b be objects of type I such that b is reachable from a after n applications of ++a, for some value n of type D.
I models advanceable only if
  • (a += n) is equal to b.
  • addressof(a += n) is equal to addressof(a).
  • I(a + n) is equal to (a += n).
  • For any two positive values x and y of type D, if I(a + D(x + y)) is well-defined, then I(a + D(x + y)) is equal to I(I(a + x) + y).
  • I(a + D(0)) is equal to a.
  • If I(a + D(n - 1)) is well-defined, then I(a + n) is equal to [](I c) { return ++c; }(I(a + D(n - 1))).
  • (b += -n) is equal to a.
  • (b -= n) is equal to a.
  • addressof(b -= n) is equal to addressof(b).
  • I(b - n) is equal to (b -= n).
  • D(b - a) is equal to n.
  • D(a - b) is equal to D(-n).
  • bool(a <= b) is true.
constexpr explicit iota_view(W value);
Preconditions: Bound denotes unreachable_sentinel_t or Bound() is reachable from value.
When W and Bound model totally_ordered_with, then bool(value <= Bound()) is true.
Effects: Initializes value_ with value.
constexpr explicit iota_view(type_identity_t<W> value, type_identity_t<Bound> bound);
Preconditions: Bound denotes unreachable_sentinel_t or bound is reachable from value.
When W and Bound model totally_ordered_with, then bool(value <= bound) is true.
Effects: Initializes value_ with value and bound_ with bound.
constexpr explicit iota_view(iterator first, see below last);
Effects: Equivalent to:
  • If same_as<W, Bound> is true, iota_view(first.value_, last.value_).
  • Otherwise, if Bound denotes unreachable_sentinel_t, iota_view(first.value_, last).
  • Otherwise, iota_view(first.value_, last.bound_).
Remarks: The type of last is:
constexpr iterator begin() const;
Effects: Equivalent to: return iterator{value_};
constexpr auto end() const;
Effects: Equivalent to: if constexpr (same_as<Bound, unreachable_sentinel_t>) return unreachable_sentinel; else return sentinel{bound_};
constexpr iterator end() const requires same_as<W, Bound>;
Effects: Equivalent to: return iterator{bound_};
constexpr auto size() const requires see below;
Effects: Equivalent to: if constexpr (is-integer-like<W> && is-integer-like<Bound>) return (value_ < 0) ? ((bound_ < 0) ? to-unsigned-like(-value_) - to-unsigned-like(-bound_) : to-unsigned-like(bound_) + to-unsigned-like(-value_)) : to-unsigned-like(bound_) - to-unsigned-like(value_); else return to-unsigned-like(bound_ - value_);
Remarks: The expression in the requires-clause is equivalent to: (same_as<W, Bound> && advanceable<W>) || (is-integer-like<W> && is-integer-like<Bound>) || sized_sentinel_for<Bound, W>

26.6.4.3 Class iota_view​::​iterator [range.iota.iterator]

namespace std::ranges { template<weakly_incrementable W, semiregular Bound> requires weakly-equality-comparable-with<W, Bound> && copyable<W> struct iota_view<W, Bound>::iterator { private: W value_ = W(); // exposition only public: using iterator_concept = see below; using iterator_category = input_iterator_tag; // present only if W models incrementable and // IOTA-DIFF-T(W) is an integral type using value_type = W; using difference_type = IOTA-DIFF-T(W); iterator() requires default_initializable<W> = default; constexpr explicit iterator(W value); constexpr W operator*() const noexcept(is_nothrow_copy_constructible_v<W>); constexpr iterator& operator++(); constexpr void operator++(int); constexpr iterator operator++(int) requires incrementable<W>; constexpr iterator& operator--() requires decrementable<W>; constexpr iterator operator--(int) requires decrementable<W>; constexpr iterator& operator+=(difference_type n) requires advanceable<W>; constexpr iterator& operator-=(difference_type n) requires advanceable<W>; constexpr W operator[](difference_type n) const requires advanceable<W>; friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<W>; friend constexpr bool operator<(const iterator& x, const iterator& y) requires totally_ordered<W>; friend constexpr bool operator>(const iterator& x, const iterator& y) requires totally_ordered<W>; friend constexpr bool operator<=(const iterator& x, const iterator& y) requires totally_ordered<W>; friend constexpr bool operator>=(const iterator& x, const iterator& y) requires totally_ordered<W>; friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires totally_ordered<W> && three_way_comparable<W>; friend constexpr iterator operator+(iterator i, difference_type n) requires advanceable<W>; friend constexpr iterator operator+(difference_type n, iterator i) requires advanceable<W>; friend constexpr iterator operator-(iterator i, difference_type n) requires advanceable<W>; friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires advanceable<W>; }; }
iterator​::​iterator_concept is defined as follows:
  • If W models advanceable, then iterator_concept is random_access_iterator_tag.
  • Otherwise, if W models decrementable, then iterator_concept is bidirectional_iterator_tag.
  • Otherwise, if W models incrementable, then iterator_concept is forward_iterator_tag.
  • Otherwise, iterator_concept is input_iterator_tag.
[Note 1:
Overloads for iter_move and iter_swap are omitted intentionally.
β€” end note]
constexpr explicit iterator(W value);
Effects: Initializes value_ with value.
constexpr W operator*() const noexcept(is_nothrow_copy_constructible_v<W>);
Effects: Equivalent to: return value_;
[Note 2:
The noexcept clause is needed by the default iter_move implementation.
β€” end note]
constexpr iterator& operator++();
Effects: Equivalent to: ++value_; return *this;
constexpr void operator++(int);
Effects: Equivalent to ++*this.
constexpr iterator operator++(int) requires incrementable<W>;
Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;
constexpr iterator& operator--() requires decrementable<W>;
Effects: Equivalent to: --value_; return *this;
constexpr iterator operator--(int) requires decrementable<W>;
Effects: Equivalent to: auto tmp = *this; --*this; return tmp;
constexpr iterator& operator+=(difference_type n) requires advanceable<W>;
Effects: Equivalent to: if constexpr (is-integer-like<W> && !is-signed-integer-like<W>) { if (n >= difference_type(0)) value_ += static_cast<W>(n); else value_ -= static_cast<W>(-n); } else { value_ += n; } return *this;
constexpr iterator& operator-=(difference_type n) requires advanceable<W>;
Effects: Equivalent to: if constexpr (is-integer-like<W> && !is-signed-integer-like<W>) { if (n >= difference_type(0)) value_ -= static_cast<W>(n); else value_ += static_cast<W>(-n); } else { value_ -= n; } return *this;
constexpr W operator[](difference_type n) const requires advanceable<W>;
Effects: Equivalent to: return W(value_ + n);
friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<W>;
Effects: Equivalent to: return x.value_ == y.value_;
friend constexpr bool operator<(const iterator& x, const iterator& y) requires totally_ordered<W>;
Effects: Equivalent to: return x.value_ < y.value_;
friend constexpr bool operator>(const iterator& x, const iterator& y) requires totally_ordered<W>;
Effects: Equivalent to: return y < x;
friend constexpr bool operator<=(const iterator& x, const iterator& y) requires totally_ordered<W>;
Effects: Equivalent to: return !(y < x);
friend constexpr bool operator>=(const iterator& x, const iterator& y) requires totally_ordered<W>;
Effects: Equivalent to: return !(x < y);
friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires totally_ordered<W> && three_way_comparable<W>;
Effects: Equivalent to: return x.value_ <=> y.value_;
friend constexpr iterator operator+(iterator i, difference_type n) requires advanceable<W>;
Effects: Equivalent to: i += n; return i;
friend constexpr iterator operator+(difference_type n, iterator i) requires advanceable<W>;
Effects: Equivalent to: return i + n;
friend constexpr iterator operator-(iterator i, difference_type n) requires advanceable<W>;
Effects: Equivalent to: i -= n; return i;
friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires advanceable<W>;
Effects: Equivalent to: using D = difference_type; if constexpr (is-integer-like<W>) { if constexpr (is-signed-integer-like<W>) return D(D(x.value_) - D(y.value_)); else return (y.value_ > x.value_) ? D(-D(y.value_ - x.value_)) : D(x.value_ - y.value_); } else { return x.value_ - y.value_; }

26.6.4.4 Class iota_view​::​sentinel [range.iota.sentinel]

namespace std::ranges { template<weakly_incrementable W, semiregular Bound> requires weakly-equality-comparable-with<W, Bound> && copyable<W> struct iota_view<W, Bound>::sentinel { private: Bound bound_ = Bound(); // exposition only public: sentinel() = default; constexpr explicit sentinel(Bound bound); friend constexpr bool operator==(const iterator& x, const sentinel& y); friend constexpr iter_difference_t<W> operator-(const iterator& x, const sentinel& y) requires sized_sentinel_for<Bound, W>; friend constexpr iter_difference_t<W> operator-(const sentinel& x, const iterator& y) requires sized_sentinel_for<Bound, W>; }; }
constexpr explicit sentinel(Bound bound);
Effects: Initializes bound_ with bound.
friend constexpr bool operator==(const iterator& x, const sentinel& y);
Effects: Equivalent to: return x.value_ == y.bound_;
friend constexpr iter_difference_t<W> operator-(const iterator& x, const sentinel& y) requires sized_sentinel_for<Bound, W>;
Effects: Equivalent to: return x.value_ - y.bound_;
friend constexpr iter_difference_t<W> operator-(const sentinel& x, const iterator& y) requires sized_sentinel_for<Bound, W>;
Effects: Equivalent to: return -(y - x);

26.6.5 Repeat view [range.repeat]

26.6.5.1 Overview [range.repeat.overview]

repeat_view generates a sequence of elements by repeatedly producing the same value.
The name views​::​repeat denotes a customization point object ([customization.point.object]).
Given subexpressions E and F, the expressions views​::​repeat(E) and views​::​repeat(E, F) are expression-equivalent to repeat_view(E) and repeat_view(E, F), respectively.
[Example 1: for (int i : views::repeat(17, 4)) cout << i << ' '; // prints 17 17 17 17 β€” end example]

26.6.5.2 Class template repeat_view [range.repeat.view]

namespace std::ranges { template<class T> concept integer-like-with-usable-difference-type = // exposition only is-signed-integer-like<T> || (is-integer-like<T> && weakly_incrementable<T>); template<move_constructible T, semiregular Bound = unreachable_sentinel_t> requires (is_object_v<T> && same_as<T, remove_cv_t<T>> && (integer-like-with-usable-difference-type<Bound> || same_as<Bound, unreachable_sentinel_t>)) class repeat_view : public view_interface<repeat_view<T, Bound>> { private: // [range.repeat.iterator], class repeat_view​::​iterator struct iterator; // exposition only movable-box<T> value_; // exposition only, see [range.move.wrap] Bound bound_ = Bound(); // exposition only public: repeat_view() requires default_initializable<T> = default; constexpr explicit repeat_view(const T& value, Bound bound = Bound()) requires copy_constructible<T>; constexpr explicit repeat_view(T&& value, Bound bound = Bound()); template<class... TArgs, class... BoundArgs> requires constructible_from<T, TArgs...> && constructible_from<Bound, BoundArgs...> constexpr explicit repeat_view(piecewise_construct_t, tuple<TArgs...> value_args, tuple<BoundArgs...> bound_args = tuple<>{}); constexpr iterator begin() const; constexpr iterator end() const requires (!same_as<Bound, unreachable_sentinel_t>); constexpr unreachable_sentinel_t end() const noexcept; constexpr auto size() const requires (!same_as<Bound, unreachable_sentinel_t>); }; template<class T, class Bound> repeat_view(T, Bound) -> repeat_view<T, Bound>; }
constexpr explicit repeat_view(const T& value, Bound bound = Bound()) requires copy_constructible<T>;
Preconditions: If Bound is not unreachable_sentinel_t, .
Effects: Initializes value_ with value and bound_ with bound.
constexpr explicit repeat_view(T&& value, Bound bound = Bound());
Preconditions: If Bound is not unreachable_sentinel_t, .
Effects: Initializes value_ with std​::​move(value) and bound_ with bound.
template<class... TArgs, class... BoundArgs> requires constructible_from<T, TArgs...> && constructible_from<Bound, BoundArgs...> constexpr explicit repeat_view(piecewise_construct_t, tuple<TArgs...> value_args, tuple<BoundArgs...> bound_args = tuple<>{});
Effects: Initializes value_ with make_from_tuple<T>(std​::​move(value_args)) and initializes
bound_ with make_from_tuple<Bound>(std​::​move(bound_args)).
The behavior is undefined if Bound is not unreachable_sentinel_t and bound_ is negative.
constexpr iterator begin() const;
Effects: Equivalent to: return iterator(addressof(*value_));
constexpr iterator end() const requires (!same_as<Bound, unreachable_sentinel_t>);
Effects: Equivalent to: return iterator(addressof(*value_), bound_);
constexpr unreachable_sentinel_t end() const noexcept;
Effects: Equivalent to: return unreachable_sentinel;
constexpr auto size() const requires (!same_as<Bound, unreachable_sentinel_t>);
Effects: Equivalent to: return to-unsigned-like(bound_);

26.6.5.3 Class repeat_view​::​iterator [range.repeat.iterator]

namespace std::ranges { template<move_constructible T, semiregular Bound> requires (is_object_v<T> && same_as<T, remove_cv_t<T>> && (integer-like-with-usable-difference-type<Bound> || same_as<Bound, unreachable_sentinel_t>)) class repeat_view<T, Bound>::iterator { private: using index-type = // exposition only conditional_t<same_as<Bound, unreachable_sentinel_t>, ptrdiff_t, Bound>; const T* value_ = nullptr; // exposition only index-type current_ = index-type(); // exposition only constexpr explicit iterator(const T* value, index-type b = index-type()); // exposition only public: using iterator_concept = random_access_iterator_tag; using iterator_category = random_access_iterator_tag; using value_type = T; using difference_type = see below; iterator() = default; constexpr const T& operator*() const noexcept; constexpr iterator& operator++(); constexpr iterator operator++(int); constexpr iterator& operator--(); constexpr iterator operator--(int); constexpr iterator& operator+=(difference_type n); constexpr iterator& operator-=(difference_type n); constexpr const T& operator[](difference_type n) const noexcept; friend constexpr bool operator==(const iterator& x, const iterator& y); friend constexpr auto operator<=>(const iterator& x, const iterator& y); friend constexpr iterator operator+(iterator i, difference_type n); friend constexpr iterator operator+(difference_type n, iterator i); friend constexpr iterator operator-(iterator i, difference_type n); friend constexpr difference_type operator-(const iterator& x, const iterator& y); }; }
If is-signed-integer-like<index-type> is true, the member typedef-name difference_type denotes index-type.
Otherwise, it denotes IOTA-DIFF-T(index-type) ([range.iota.view]).
constexpr explicit iterator(const T* value, index-type b = index-type());
Preconditions: If Bound is not unreachable_sentinel_t, .
Effects: Initializes value_ with value and current_ with b.
constexpr const T& operator*() const noexcept;
Effects: Equivalent to: return *value_;
constexpr iterator& operator++();
Effects: Equivalent to: ++current_; return *this;
constexpr iterator operator++(int);
Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;
constexpr iterator& operator--();
Preconditions: If Bound is not unreachable_sentinel_t, .
Effects: Equivalent to: --current_; return *this;
constexpr iterator operator--(int);
Effects: Equivalent to: auto tmp = *this; --*this; return tmp;
constexpr iterator& operator+=(difference_type n);
Preconditions: If Bound is not unreachable_sentinel_t, .
Effects: Equivalent to: current_ += n; return *this;
constexpr iterator& operator-=(difference_type n);
Preconditions: If Bound is not unreachable_sentinel_t, .
Effects: Equivalent to: current_ -= n; return *this;
constexpr const T& operator[](difference_type n) const noexcept;
Effects: Equivalent to: return *(*this + n);
friend constexpr bool operator==(const iterator& x, const iterator& y);
Effects: Equivalent to: return x.current_ == y.current_;
friend constexpr auto operator<=>(const iterator& x, const iterator& y);
Effects: Equivalent to: return x.current_ <=> y.current_;
friend constexpr iterator operator+(iterator i, difference_type n); friend constexpr iterator operator+(difference_type n, iterator i);
Effects: Equivalent to: i += n; return i;
friend constexpr iterator operator-(iterator i, difference_type n);
Effects: Equivalent to: i -= n; return i;
friend constexpr difference_type operator-(const iterator& x, const iterator& y);
Effects: Equivalent to: return static_cast<difference_type>(x.current_) - static_cast<difference_type>(y.current_);

26.6.6 Istream view [range.istream]

26.6.6.1 Overview [range.istream.overview]

basic_istream_view models input_range and reads (using operator>>) successive elements from its corresponding input stream.
The name views​::​istream<T> denotes a customization point object ([customization.point.object]).
Given a type T and a subexpression E of type U, if U models derived_from<basic_istream<typename U​::​char_type, typename U​::​traits_type>>, then the expression views​::​istream<T>(E) is expression-equivalent to basic_istream_view<T, typename U​::​char_type, typename U​::​traits_type>(E); otherwise, views​::​istream<T>(E) is ill-formed.
[Example 1: auto ints = istringstream{"0 1 2 3 4"}; ranges::copy(views::istream<int>(ints), ostream_iterator<int>{cout, "-"}); // prints 0-1-2-3-4- β€” end example]

26.6.6.2 Class template basic_istream_view [range.istream.view]

namespace std::ranges { template<class Val, class CharT, class Traits> concept stream-extractable = // exposition only requires(basic_istream<CharT, Traits>& is, Val& t) { is >> t; }; template<movable Val, class CharT, class Traits = char_traits<CharT>> requires default_initializable<Val> && stream-extractable<Val, CharT, Traits> class basic_istream_view : public view_interface<basic_istream_view<Val, CharT, Traits>> { public: constexpr explicit basic_istream_view(basic_istream<CharT, Traits>& stream); constexpr auto begin() { *stream_ >> value_; return iterator{*this}; } constexpr default_sentinel_t end() const noexcept; private: // [range.istream.iterator], class basic_istream_view​::​iterator struct iterator; // exposition only basic_istream<CharT, Traits>* stream_; // exposition only Val value_ = Val(); // exposition only }; }
constexpr explicit basic_istream_view(basic_istream<CharT, Traits>& stream);
Effects: Initializes stream_ with addressof(stream).
constexpr default_sentinel_t end() const noexcept;
Effects: Equivalent to: return default_sentinel;

26.6.6.3 Class basic_istream_view​::​iterator [range.istream.iterator]

namespace std::ranges { template<movable Val, class CharT, class Traits> requires default_initializable<Val> && stream-extractable<Val, CharT, Traits> class basic_istream_view<Val, CharT, Traits>::iterator { public: using iterator_concept = input_iterator_tag; using difference_type = ptrdiff_t; using value_type = Val; constexpr explicit iterator(basic_istream_view& parent) noexcept; iterator(const iterator&) = delete; iterator(iterator&&) = default; iterator& operator=(const iterator&) = delete; iterator& operator=(iterator&&) = default; iterator& operator++(); void operator++(int); Val& operator*() const; friend bool operator==(const iterator& x, default_sentinel_t); private: basic_istream_view* parent_; // exposition only }; }
constexpr explicit iterator(basic_istream_view& parent) noexcept;
Effects: Initializes parent_ with addressof(parent).
iterator& operator++();
Effects: Equivalent to: *parent_->stream_ >> parent_->value_; return *this;
void operator++(int);
Effects: Equivalent to ++*this.
Val& operator*() const;
Effects: Equivalent to: return parent_->value_;
friend bool operator==(const iterator& x, default_sentinel_t);
Effects: Equivalent to: return !*x.parent_->stream_;

26.7 Range adaptors [range.adaptors]

26.7.1 General [range.adaptors.general]

Subclause [range.adaptors] defines range adaptors, which are utilities that transform a range into a view with custom behaviors.
These adaptors can be chained to create pipelines of range transformations that evaluate lazily as the resulting view is iterated.
Range adaptors are declared in namespace std​::​ranges​::​views.
The bitwise or operator is overloaded for the purpose of creating adaptor chain pipelines.
The adaptors also support function call syntax with equivalent semantics.
[Example 1: vector<int> ints{0,1,2,3,4,5}; auto even = [](int i) { return 0 == i % 2; }; auto square = [](int i) { return i * i; }; for (int i : ints | views::filter(even) | views::transform(square)) { cout << i << ' '; // prints 0 4 16 } assert(ranges::equal(ints | views::filter(even), views::filter(ints, even))); β€” end example]

26.7.2 Range adaptor objects [range.adaptor.object]

A range adaptor closure object is a unary function object that accepts a range argument.
For a range adaptor closure object C and an expression R such that decltype((R)) models range, the following expressions are equivalent: C(R) R | C
Given an additional range adaptor closure object D, the expression C | D produces another range adaptor closure object E.
E is a perfect forwarding call wrapper ([func.require]) with the following properties:
  • Its target object is an object d of type decay_t<decltype((D))> direct-non-list-initialized with D.
  • It has one bound argument entity, an object c of type decay_t<decltype((C))> direct-non-list-initialized with C.
  • Its call pattern is d(c(arg)), where arg is the argument used in a function call expression of E.
The expression C | D is well-formed if and only if the initializations of the state entities of E are all well-formed.
Given an object t of type T, where
  • t is a unary function object that accepts a range argument,
  • T models derived_from<range_adaptor_closure<T>>,
  • T has no other base classes of type range_adaptor_closure<U> for any other type U, and
  • T does not model range
then the implementation ensures that t is a range adaptor closure object.
The template parameter D for range_adaptor_closure may be an incomplete type.
If an expression of type cv D is used as an operand to the | operator, D shall be complete and model derived_from<range_adaptor_closure<D>>.
The behavior of an expression involving an object of type cv D as an operand to the | operator is undefined if overload resolution selects a program-defined operator| function.
If an expression of type cv U is used as an operand to the | operator, where U has a base class of type range_adaptor_closure<T> for some type T other than U, the behavior is undefined.
The behavior of a program that adds a specialization for range_adaptor_closure is undefined.
A range adaptor object is a customization point object ([customization.point.object]) that accepts a viewable_range as its first argument and returns a view.
If a range adaptor object accepts only one argument, then it is a range adaptor closure object.
If a range adaptor object adaptor accepts more than one argument, then let range be an expression such that decltype((range)) models viewable_range, let args... be arguments such that adaptor(range, args...) is a well-formed expression as specified in the rest of subclause [range.adaptors], and let BoundArgs be a pack that denotes decay_t<decltype((args))>....
The expression adaptor(args...) produces a range adaptor closure object f that is a perfect forwarding call wrapper ([func.require]) with the following properties:
  • Its target object is a copy of adaptor.
  • Its bound argument entities bound_args consist of objects of types BoundArgs... direct-non-list-initialized with std​::​forward<decltype((args))>(args)..., respectively.
  • Its call pattern is adaptor(r, bound_args...), where r is the argument used in a function call expression of f.
The expression adaptor(args...) is well-formed if and only if the initialization of the bound argument entities of the result, as specified above, are all well-formed.

26.7.3 Movable wrapper [range.move.wrap]

Many types in this subclause are specified in terms of an exposition-only class template movable-box.
movable-box<T> behaves exactly like optional<T> with the following differences:
  • movable-box<T> constrains its type parameter T with move_constructible<T> && is_object_v<T>.
  • The default constructor of movable-box<T> is equivalent to: constexpr movable-box() noexcept(is_nothrow_default_constructible_v<T>) requires default_initializable<T> : movable-box{in_place} {}
  • If copyable<T> is not modeled, the copy assignment operator is equivalent to: constexpr movable-box& operator=(const movable-box& that) noexcept(is_nothrow_copy_constructible_v<T>) requires copy_constructible<T> { if (this != addressof(that)) { if (that) emplace(*that); else reset(); } return *this; }
  • If movable<T> is not modeled, the move assignment operator is equivalent to: constexpr movable-box& operator=(movable-box&& that) noexcept(is_nothrow_move_constructible_v<T>) { if (this != addressof(that)) { if (that) emplace(std::move(*that)); else reset(); } return *this; }
Recommended practice:
  • If copy_constructible<T> is true, movable-box<T> should store only a T if either T models copyable, or is_nothrow_move_constructible_v<T> && is_nothrow_copy_constructible_v<T> is true.
  • Otherwise, movable-box<T> should store only a T if either T models movable or is_nothrow_move_constructible_v<T> is true.

26.7.4 Non-propagating cache [range.nonprop.cache]

Some types in subclause [range.adaptors] are specified in terms of an exposition-only class template non-propagating-​cache.
non-propagating-cache<T> behaves exactly like optional<T> with the following differences:
  • non-propagating-cache<T> constrains its type parameter T with is_object_v<T>.
  • The copy constructor is equivalent to: constexpr non-propagating-cache(const non-propagating-cache&) noexcept {}
  • The move constructor is equivalent to: constexpr non-propagating-cache(non-propagating-cache&& other) noexcept { other.reset(); }
  • The copy assignment operator is equivalent to: constexpr non-propagating-cache& operator=(const non-propagating-cache& other) noexcept { if (addressof(other) != this) reset(); return *this; }
  • The move assignment operator is equivalent to: constexpr non-propagating-cache& operator=(non-propagating-cache&& other) noexcept { reset(); other.reset(); return *this; }
  • non-propagating-cache<T> has an additional member function template specified as follows:
    template<class I> constexpr T& emplace-deref(const I& i); // exposition only
    Mandates: The declaration T t(*i); is well-formed for some invented variable t.
    [Note 1:
    If *i is a prvalue of type cv T, there is no requirement that it is movable ([dcl.init.general]).
    β€” end note]
    Effects: Calls reset().
    Then direct-non-list-initializes the contained value with *i.
    Postconditions: *this contains a value.
    Returns: A reference to the new contained value.
    Throws: Any exception thrown by the initialization of the contained value.
    Remarks: If an exception is thrown during the initialization of T, *this does not contain a value, and the previous value (if any) has been destroyed.
[Note 2:
non-propagating-cache enables an input view to temporarily cache values as it is iterated over.
β€” end note]

26.7.5 Range adaptor helpers [range.adaptor.helpers]

namespace std::ranges { template<class F, class Tuple> constexpr auto tuple-transform(F&& f, Tuple&& t) { // exposition only return apply([&]<class... Ts>(Ts&&... elements) { return tuple<invoke_result_t<F&, Ts>...>(invoke(f, std::forward<Ts>(elements))...); }, std::forward<Tuple>(t)); } template<class F, class Tuple> constexpr void tuple-for-each(F&& f, Tuple&& t) { // exposition only apply([&]<class... Ts>(Ts&&... elements) { (static_cast<void>(invoke(f, std::forward<Ts>(elements))), ...); }, std::forward<Tuple>(t)); } template<class T> constexpr T& as-lvalue(T&& t) { // exposition only return static_cast<T&>(t); } }

26.7.6 All view [range.all]

26.7.6.1 General [range.all.general]

views​::​all returns a view that includes all elements of its range argument.
The name views​::​all denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E, the expression views​::​all(E) is expression-equivalent to:
  • decay-copy(E) if the decayed type of E models view.
  • Otherwise, ref_view{E} if that expression is well-formed.
  • Otherwise, owning_view{E}.

26.7.6.2 Class template ref_view [range.ref.view]

ref_view is a view of the elements of some other range.
namespace std::ranges { template<range R> requires is_object_v<R> class ref_view : public view_interface<ref_view<R>> { private: R* r_; // exposition only public: template<different-from<ref_view> T> requires see below constexpr ref_view(T&& t); constexpr R& base() const { return *r_; } constexpr iterator_t<R> begin() const { return ranges::begin(*r_); } constexpr sentinel_t<R> end() const { return ranges::end(*r_); } constexpr bool empty() const requires requires { ranges::empty(*r_); } { return ranges::empty(*r_); } constexpr auto size() const requires sized_range<R> { return ranges::size(*r_); } constexpr auto data() const requires contiguous_range<R> { return ranges::data(*r_); } }; template<class R> ref_view(R&) -> ref_view<R>; }
template<different-from<ref_view> T> requires see below constexpr ref_view(T&& t);
Effects: Initializes r_ with addressof(static_cast<R&>(std​::​forward<T>(t))).
Remarks: Let FUN denote the exposition-only functions void FUN(R&); void FUN(R&&) = delete;
The expression in the requires-clause is equivalent to: convertible_to<T, R&> && requires { FUN(declval<T>()); }

26.7.6.3 Class template owning_view [range.owning.view]

owning_view is a move-only view of the elements of some other range.
namespace std::ranges { template<range R> requires movable<R> && (!is-initializer-list<R>) // see [range.refinements] class owning_view : public view_interface<owning_view<R>> { private: R r_ = R(); // exposition only public: owning_view() requires default_initializable<R> = default; constexpr owning_view(R&& t); owning_view(owning_view&&) = default; owning_view& operator=(owning_view&&) = default; constexpr R& base() & noexcept { return r_; } constexpr const R& base() const & noexcept { return r_; } constexpr R&& base() && noexcept { return std::move(r_); } constexpr const R&& base() const && noexcept { return std::move(r_); } constexpr iterator_t<R> begin() { return ranges::begin(r_); } constexpr sentinel_t<R> end() { return ranges::end(r_); } constexpr auto begin() const requires range<const R> { return ranges::begin(r_); } constexpr auto end() const requires range<const R> { return ranges::end(r_); } constexpr bool empty() requires requires { ranges::empty(r_); } { return ranges::empty(r_); } constexpr bool empty() const requires requires { ranges::empty(r_); } { return ranges::empty(r_); } constexpr auto size() requires sized_range<R> { return ranges::size(r_); } constexpr auto size() const requires sized_range<const R> { return ranges::size(r_); } constexpr auto data() requires contiguous_range<R> { return ranges::data(r_); } constexpr auto data() const requires contiguous_range<const R> { return ranges::data(r_); } }; }
constexpr owning_view(R&& t);
Effects: Initializes r_ with std​::​move(t).

26.7.7 As rvalue view [range.as.rvalue]

26.7.7.1 Overview [range.as.rvalue.overview]

as_rvalue_view presents a view of an underlying sequence with the same behavior as the underlying sequence except that its elements are rvalues.
Some generic algorithms can be called with an as_rvalue_view to replace copying with moving.
The name views​::​as_rvalue denotes a range adaptor object ([range.adaptor.object]).
Let E be an expression and let T be decltype((E)).
The expression views​::​as_rvalue(E) is expression-equivalent to:
  • views​::​all(E) if same_as<range_rvalue_reference_t<T>, range_reference_t<T>> is true.
  • Otherwise, as_rvalue_view(E).
[Example 1: vector<string> words = {"the", "quick", "brown", "fox", "ate", "a", "pterodactyl"}; vector<string> new_words; ranges::copy(words | views::as_rvalue, back_inserter(new_words)); // moves each string from words into new_words β€” end example]

26.7.7.2 Class template as_rvalue_view [range.as.rvalue.view]

namespace std::ranges { template<view V> requires input_range<V> class as_rvalue_view : public view_interface<as_rvalue_view<V>> { V base_ = V(); // exposition only public: as_rvalue_view() requires default_initializable<V> = default; constexpr explicit as_rvalue_view(V base); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr auto begin() requires (!simple-view<V>) { return move_iterator(ranges::begin(base_)); } constexpr auto begin() const requires range<const V> { return move_iterator(ranges::begin(base_)); } constexpr auto end() requires (!simple-view<V>) { if constexpr (common_range<V>) { return move_iterator(ranges::end(base_)); } else { return move_sentinel(ranges::end(base_)); } } constexpr auto end() const requires range<const V> { if constexpr (common_range<const V>) { return move_iterator(ranges::end(base_)); } else { return move_sentinel(ranges::end(base_)); } } constexpr auto size() requires sized_range<V> { return ranges::size(base_); } constexpr auto size() const requires sized_range<const V> { return ranges::size(base_); } }; template<class R> as_rvalue_view(R&&) -> as_rvalue_view<views::all_t<R>>; }
constexpr explicit as_rvalue_view(V base);
Effects: Initializes base_ with std​::​move(base).

26.7.8 Filter view [range.filter]

26.7.8.1 Overview [range.filter.overview]

filter_view presents a view of the elements of an underlying sequence that satisfy a predicate.
The name views​::​filter denotes a range adaptor object ([range.adaptor.object]).
Given subexpressions E and P, the expression views​::​filter(E, P) is expression-equivalent to filter_view(E, P).
[Example 1: vector<int> is{ 0, 1, 2, 3, 4, 5, 6 }; auto evens = views::filter(is, [](int i) { return 0 == i % 2; }); for (int i : evens) cout << i << ' '; // prints 0 2 4 6 β€” end example]

26.7.8.2 Class template filter_view [range.filter.view]

namespace std::ranges { template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class filter_view : public view_interface<filter_view<V, Pred>> { private: V base_ = V(); // exposition only movable-box<Pred> pred_; // exposition only // [range.filter.iterator], class filter_view​::​iterator class iterator; // exposition only // [range.filter.sentinel], class filter_view​::​sentinel class sentinel; // exposition only public: filter_view() requires default_initializable<V> && default_initializable<Pred> = default; constexpr explicit filter_view(V base, Pred pred); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr const Pred& pred() const; constexpr iterator begin(); constexpr auto end() { if constexpr (common_range<V>) return iterator{*this, ranges::end(base_)}; else return sentinel{*this}; } }; template<class R, class Pred> filter_view(R&&, Pred) -> filter_view<views::all_t<R>, Pred>; }
constexpr explicit filter_view(V base, Pred pred);
Effects: Initializes base_ with std​::​move(base) and initializes pred_ with std​::​move(pred).
constexpr const Pred& pred() const;
Effects: Equivalent to: return *pred_;
constexpr iterator begin();
Preconditions: pred_.has_value() is true.
Returns: {*this, ranges​::​find_if(base_, ref(*pred_))}.
Remarks: In order to provide the amortized constant time complexity required by the range concept when filter_view models forward_range, this function caches the result within the filter_view for use on subsequent calls.

26.7.8.3 Class filter_view​::​iterator [range.filter.iterator]

namespace std::ranges { template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class filter_view<V, Pred>::iterator { private: iterator_t<V> current_ = iterator_t<V>(); // exposition only filter_view* parent_ = nullptr; // exposition only public: using iterator_concept = see below; using iterator_category = see below; // not always present using value_type = range_value_t<V>; using difference_type = range_difference_t<V>; iterator() requires default_initializable<iterator_t<V>> = default; constexpr iterator(filter_view& parent, iterator_t<V> current); constexpr const iterator_t<V>& base() const & noexcept; constexpr iterator_t<V> base() &&; constexpr range_reference_t<V> operator*() const; constexpr iterator_t<V> operator->() const requires has-arrow<iterator_t<V>> && copyable<iterator_t<V>>; constexpr iterator& operator++(); constexpr void operator++(int); constexpr iterator operator++(int) requires forward_range<V>; constexpr iterator& operator--() requires bidirectional_range<V>; constexpr iterator operator--(int) requires bidirectional_range<V>; friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<V>>; friend constexpr range_rvalue_reference_t<V> iter_move(const iterator& i) noexcept(noexcept(ranges::iter_move(i.current_))); friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.current_, y.current_))) requires indirectly_swappable<iterator_t<V>>; }; }
Modification of the element a filter_view​::​iterator denotes is permitted, but results in undefined behavior if the resulting value does not satisfy the filter predicate.
iterator​::​iterator_concept is defined as follows:
The member typedef-name iterator_category is defined if and only if V models forward_range.
In that case, iterator​::​iterator_category is defined as follows:
  • Let C denote the type iterator_traits<iterator_t<V>>​::​iterator_category.
  • If C models derived_from<bidirectional_iterator_tag>, then iterator_category denotes bidirectional_iterator_tag.
  • Otherwise, if C models derived_from<forward_iterator_tag>, then iterator_category denotes forward_iterator_tag.
  • Otherwise, iterator_category denotes C.
constexpr iterator(filter_view& parent, iterator_t<V> current);
Effects: Initializes current_ with std​::​move(current) and parent_ with addressof(parent).
constexpr const iterator_t<V>& base() const & noexcept;
Effects: Equivalent to: return current_;
constexpr iterator_t<V> base() &&;
Effects: Equivalent to: return std​::​move(current_);
constexpr range_reference_t<V> operator*() const;
Effects: Equivalent to: return *current_;
constexpr iterator_t<V> operator->() const requires has-arrow<iterator_t<V>> && copyable<iterator_t<V>>;
Effects: Equivalent to: return current_;
constexpr iterator& operator++();
Effects: Equivalent to: current_ = ranges::find_if(std::move(++current_), ranges::end(parent_->base_), ref(*parent_->pred_)); return *this;
constexpr void operator++(int);
Effects: Equivalent to ++*this.
constexpr iterator operator++(int) requires forward_range<V>;
Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;
constexpr iterator& operator--() requires bidirectional_range<V>;
Effects: Equivalent to: do --current_; while (!invoke(*parent_->pred_, *current_)); return *this;
constexpr iterator operator--(int) requires bidirectional_range<V>;
Effects: Equivalent to: auto tmp = *this; --*this; return tmp;
friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<V>>;
Effects: Equivalent to: return x.current_ == y.current_;
friend constexpr range_rvalue_reference_t<V> iter_move(const iterator& i) noexcept(noexcept(ranges::iter_move(i.current_)));
Effects: Equivalent to: return ranges​::​iter_move(i.current_);
friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.current_, y.current_))) requires indirectly_swappable<iterator_t<V>>;
Effects: Equivalent to ranges​::​iter_swap(x.current_, y.current_).

26.7.8.4 Class filter_view​::​sentinel [range.filter.sentinel]

namespace std::ranges { template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class filter_view<V, Pred>::sentinel { private: sentinel_t<V> end_ = sentinel_t<V>(); // exposition only public: sentinel() = default; constexpr explicit sentinel(filter_view& parent); constexpr sentinel_t<V> base() const; friend constexpr bool operator==(const iterator& x, const sentinel& y); }; }
constexpr explicit sentinel(filter_view& parent);
Effects: Initializes end_ with ranges​::​end(parent.base_).
constexpr sentinel_t<V> base() const;
Effects: Equivalent to: return end_;
friend constexpr bool operator==(const iterator& x, const sentinel& y);
Effects: Equivalent to: return x.current_ == y.end_;

26.7.9 Transform view [range.transform]

26.7.9.1 Overview [range.transform.overview]

transform_view presents a view of an underlying sequence after applying a transformation function to each element.
The name views​::​transform denotes a range adaptor object ([range.adaptor.object]).
Given subexpressions E and F, the expression views​::​transform(E, F) is expression-equivalent to transform_view(E, F).
[Example 1: vector<int> is{ 0, 1, 2, 3, 4 }; auto squares = views::transform(is, [](int i) { return i * i; }); for (int i : squares) cout << i << ' '; // prints 0 1 4 9 16 β€” end example]

26.7.9.2 Class template transform_view [range.transform.view]

namespace std::ranges { template<input_range V, move_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> class transform_view : public view_interface<transform_view<V, F>> { private: // [range.transform.iterator], class template transform_view​::​iterator template<bool> struct iterator; // exposition only // [range.transform.sentinel], class template transform_view​::​sentinel template<bool> struct sentinel; // exposition only V base_ = V(); // exposition only movable-box<F> fun_; // exposition only public: transform_view() requires default_initializable<V> && default_initializable<F> = default; constexpr explicit transform_view(V base, F fun); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr iterator<false> begin(); constexpr iterator<true> begin() const requires range<const V> && regular_invocable<const F&, range_reference_t<const V>>; constexpr sentinel<false> end(); constexpr iterator<false> end() requires common_range<V>; constexpr sentinel<true> end() const requires range<const V> && regular_invocable<const F&, range_reference_t<const V>>; constexpr iterator<true> end() const requires common_range<const V> && regular_invocable<const F&, range_reference_t<const V>>; constexpr auto size() requires sized_range<V> { return ranges::size(base_); } constexpr auto size() const requires sized_range<const V> { return ranges::size(base_); } }; template<class R, class F> transform_view(R&&, F) -> transform_view<views::all_t<R>, F>; }
constexpr explicit transform_view(V base, F fun);
Effects: Initializes base_ with std​::​move(base) and fun_ with std​::​move(fun).
constexpr iterator<false> begin();
Effects: Equivalent to: return iterator<false>{*this, ranges::begin(base_)};
constexpr iterator<true> begin() const requires range<const V> && regular_invocable<const F&, range_reference_t<const V>>;
Effects: Equivalent to: return iterator<true>{*this, ranges::begin(base_)};
constexpr sentinel<false> end();
Effects: Equivalent to: return sentinel<false>{ranges::end(base_)};
constexpr iterator<false> end() requires common_range<V>;
Effects: Equivalent to: return iterator<false>{*this, ranges::end(base_)};
constexpr sentinel<true> end() const requires range<const V> && regular_invocable<const F&, range_reference_t<const V>>;
Effects: Equivalent to: return sentinel<true>{ranges::end(base_)};
constexpr iterator<true> end() const requires common_range<const V> && regular_invocable<const F&, range_reference_t<const V>>;
Effects: Equivalent to: return iterator<true>{*this, ranges::end(base_)};

26.7.9.3 Class template transform_view​::​iterator [range.transform.iterator]

namespace std::ranges { template<input_range V, move_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> template<bool Const> class transform_view<V, F>::iterator { private: using Parent = maybe-const<Const, transform_view>; // exposition only using Base = maybe-const<Const, V>; // exposition only iterator_t<Base> current_ = iterator_t<Base>(); // exposition only Parent* parent_ = nullptr; // exposition only public: using iterator_concept = see below; using iterator_category = see below; // not always present using value_type = remove_cvref_t<invoke_result_t<maybe-const<Const, F>&, range_reference_t<Base>>>; using difference_type = range_difference_t<Base>; iterator() requires default_initializable<iterator_t<Base>> = default; constexpr iterator(Parent& parent, iterator_t<Base> current); constexpr iterator(iterator<!Const> i) requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>; constexpr const iterator_t<Base>& base() const & noexcept; constexpr iterator_t<Base> base() &&; constexpr decltype(auto) operator*() const noexcept(noexcept(invoke(*parent_->fun_, *current_))) { return invoke(*parent_->fun_, *current_); } constexpr iterator& operator++(); constexpr void operator++(int); constexpr iterator operator++(int) requires forward_range<Base>; constexpr iterator& operator--() requires bidirectional_range<Base>; constexpr iterator operator--(int) requires bidirectional_range<Base>; constexpr iterator& operator+=(difference_type n) requires random_access_range<Base>; constexpr iterator& operator-=(difference_type n) requires random_access_range<Base>; constexpr decltype(auto) operator[](difference_type n) const requires random_access_range<Base> { return invoke(*parent_->fun_, current_[n]); } friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<Base>>; friend constexpr bool operator<(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator>(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator<=(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator>=(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>; friend constexpr iterator operator+(iterator i, difference_type n) requires random_access_range<Base>; friend constexpr iterator operator+(difference_type n, iterator i) requires random_access_range<Base>; friend constexpr iterator operator-(iterator i, difference_type n) requires random_access_range<Base>; friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>; }; }
iterator​::​iterator_concept is defined as follows:
The member typedef-name iterator_category is defined if and only if Base models forward_range.
In that case, iterator​::​iterator_category is defined as follows: Let C denote the type iterator_traits<iterator_t<Base>>​::​iterator_category.
  • If is_reference_v<invoke_result_t<maybe-const<Const, F>&, range_reference_t<Base>>> is true, then
    • if C models derived_from<contiguous_iterator_tag>, iterator_category denotes random_access_iterator_tag;
    • otherwise, iterator_category denotes C.
  • Otherwise, iterator_category denotes input_iterator_tag.
constexpr iterator(Parent& parent, iterator_t<Base> current);
Effects: Initializes current_ with std​::​move(current) and parent_ with addressof(parent).
constexpr iterator(iterator<!Const> i) requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;
Effects: Initializes current_ with std​::​move(i.current_) and parent_ with i.parent_.
constexpr const iterator_t<Base>& base() const & noexcept;
Effects: Equivalent to: return current_;
constexpr iterator_t<Base> base() &&;
Effects: Equivalent to: return std​::​move(current_);
constexpr iterator& operator++();
Effects: Equivalent to: ++current_; return *this;
constexpr void operator++(int);
Effects: Equivalent to ++current_.
constexpr iterator operator++(int) requires forward_range<Base>;
Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;
constexpr iterator& operator--() requires bidirectional_range<Base>;
Effects: Equivalent to: --current_; return *this;
constexpr iterator operator--(int) requires bidirectional_range<Base>;
Effects: Equivalent to: auto tmp = *this; --*this; return tmp;
constexpr iterator& operator+=(difference_type n) requires random_access_range<Base>;
Effects: Equivalent to: current_ += n; return *this;
constexpr iterator& operator-=(difference_type n) requires random_access_range<Base>;
Effects: Equivalent to: current_ -= n; return *this;
friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<Base>>;
Effects: Equivalent to: return x.current_ == y.current_;
friend constexpr bool operator<(const iterator& x, const iterator& y) requires random_access_range<Base>;
Effects: Equivalent to: return x.current_ < y.current_;
friend constexpr bool operator>(const iterator& x, const iterator& y) requires random_access_range<Base>;
Effects: Equivalent to: return y < x;
friend constexpr bool operator<=(const iterator& x, const iterator& y) requires random_access_range<Base>;
Effects: Equivalent to: return !(y < x);
friend constexpr bool operator>=(const iterator& x, const iterator& y) requires random_access_range<Base>;
Effects: Equivalent to: return !(x < y);
friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;
Effects: Equivalent to: return x.current_ <=> y.current_;
friend constexpr iterator operator+(iterator i, difference_type n) requires random_access_range<Base>; friend constexpr iterator operator+(difference_type n, iterator i) requires random_access_range<Base>;
Effects: Equivalent to: return iterator{*i.parent_, i.current_ + n};
friend constexpr iterator operator-(iterator i, difference_type n) requires random_access_range<Base>;
Effects: Equivalent to: return iterator{*i.parent_, i.current_ - n};
friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
Effects: Equivalent to: return x.current_ - y.current_;

26.7.9.4 Class template transform_view​::​sentinel [range.transform.sentinel]

namespace std::ranges { template<input_range V, move_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> template<bool Const> class transform_view<V, F>::sentinel { private: using Parent = maybe-const<Const, transform_view>; // exposition only using Base = maybe-const<Const, V>; // exposition only sentinel_t<Base> end_ = sentinel_t<Base>(); // exposition only public: sentinel() = default; constexpr explicit sentinel(sentinel_t<Base> end); constexpr sentinel(sentinel<!Const> i) requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>; constexpr sentinel_t<Base> base() const; template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y); template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<maybe-const<OtherConst, V>> operator-(const iterator<OtherConst>& x, const sentinel& y); template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<maybe-const<OtherConst, V>> operator-(const sentinel& y, const iterator<OtherConst>& x); }; }
constexpr explicit sentinel(sentinel_t<Base> end);
Effects: Initializes end_ with end.
constexpr sentinel(sentinel<!Const> i) requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
Effects: Initializes end_ with std​::​move(i.end_).
constexpr sentinel_t<Base> base() const;
Effects: Equivalent to: return end_;
template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
Effects: Equivalent to: return x.current_ == y.end_;
template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<maybe-const<OtherConst, V>> operator-(const iterator<OtherConst>& x, const sentinel& y);
Effects: Equivalent to: return x.current_ - y.end_;
template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<maybe-const<OtherConst, V>> operator-(const sentinel& y, const iterator<OtherConst>& x);
Effects: Equivalent to: return y.end_ - x.current_;

26.7.10 Take view [range.take]

26.7.10.1 Overview [range.take.overview]

take_view produces a view of the first N elements from another view, or all the elements if the adapted view contains fewer than N.
The name views​::​take denotes a range adaptor object ([range.adaptor.object]).
Let E and F be expressions, let T be remove_cvref_t<decltype((E))>, and let D be range_difference_t<decltype((E))>.
If decltype((F)) does not model convertible_to<D>, views​::​take(E, F) is ill-formed.
Otherwise, the expression views​::​take(E, F) is expression-equivalent to:
  • If T is a specialization of empty_view ([range.empty.view]), then ((void)F, decay-copy(E)), except that the evaluations of E and F are indeterminately sequenced.
  • Otherwise, if T models random_access_range and sized_range and is a specialization of span ([views.span]), basic_string_view ([string.view]), or subrange ([range.subrange]), then U(ranges​::​begin(E), ranges​::​begin(E) + std​::​min<D>(ranges​::​distance(E), F)), except that E is evaluated only once, where U is a type determined as follows:
    • if T is a specialization of span, then U is span<typename T​::​element_type>;
    • otherwise, if T is a specialization of basic_string_view, then U is T;
    • otherwise, T is a specialization of subrange, and U is subrange<iterator_t<T>>;
  • otherwise, if T is a specialization of iota_view ([range.iota.view]) that models random_access_range and sized_range, then iota_view(*ranges​::​begin(E), *(ranges​::​begin(E) + std​::​
    min<D>(ranges​::​distance(E), F)))
    , except that E is evaluated only once.
  • Otherwise, if T is a specialization of repeat_view ([range.repeat.view]):
    • if T models sized_range, then views::repeat(*E.value_, std::min<D>(ranges::distance(E), F)) except that E is evaluated only once;
    • otherwise, views​::​repeat(*E.value_, static_cast<D>(F)).
  • Otherwise, take_view(E, F).
[Example 1: vector<int> is{0,1,2,3,4,5,6,7,8,9}; for (int i : is | views::take(5)) cout << i << ' '; // prints 0 1 2 3 4 β€” end example]

26.7.10.2 Class template take_view [range.take.view]

namespace std::ranges { template<view V> class take_view : public view_interface<take_view<V>> { private: V base_ = V(); // exposition only range_difference_t<V> count_ = 0; // exposition only // [range.take.sentinel], class template take_view​::​sentinel template<bool> class