25 Ranges library [ranges]

25.1 General [ranges.general]

1
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This Clause describes components for dealing with ranges of elements.
2
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The following subclauses describe range and view requirements, and components for range primitives and range generators as summarized in Table 79.
Table 79: Ranges library summary [tab:range.summary]
Subclause
Header
Range access
<ranges>
Requirements
Range utilities
Range factories
Range adaptors
Range generators

25.2 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 = decltype(ranges::cbegin(declval<R&>())); // freestanding template<range R> using const_sentinel_t = decltype(ranges::cend(declval<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<input_range V, forward_range Pattern> requires see below 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.concat], concat view template<input_range... Views> requires see below class concat_view; // freestanding namespace views { inline constexpr unspecified concat = 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) noexcept { // 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<view V> requires see below 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>> = // freestanding enable_borrowed_range<V>; 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; }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<1, ranges::subrange<I, S, K>> { // freestanding using type = S; }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<0, const ranges::subrange<I, S, K>> { // freestanding using type = I; }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<1, const ranges::subrange<I, S, K>> { // freestanding using type = S; }; struct from_range_t { explicit from_range_t() = default; }; // freestanding inline constexpr from_range_t from_range{}; // freestanding }
1
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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>.
2
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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.

25.3 Range access [range.access]

25.3.1 General [range.access.general]

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In addition to being available via inclusion of the <ranges> header, the customization point objects in [range.access] are available when the header <iterator> is included.
2
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Within [range.access], the reified object of a subexpression E denotes
  • (2.1)
    the same object as E if E is a glvalue, or
  • (2.2)
    the result of applying the temporary materialization conversion ([conv.rval]) to E otherwise.

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

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The name ranges​::​begin denotes a customization point object ([customization.point.object]).
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Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • (2.1)
    If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​begin(E) is ill-formed.
  • (2.2)
    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.
  • (2.3)
    Otherwise, if T is an array type, ranges​::​begin(E) is expression-equivalent to t + 0.
  • (2.4)
    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()).
  • (2.5)
    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.
  • (2.6)
    Otherwise, ranges​::​begin(E) is ill-formed.
3
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[Note 1: 
Diagnosable ill-formed cases above result in substitution failure when ranges​::​begin(E) appears in the immediate context of a template instantiation.
— end note]
4
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[Note 2: 
Whenever ranges​::​begin(E) is a valid expression, its type models input_or_output_iterator.
— end note]

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

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The name ranges​::​end denotes a customization point object ([customization.point.object]).
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Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • (2.1)
    If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​end(E) is ill-formed.
  • (2.2)
    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.
  • (2.3)
    Otherwise, if T is an array of unknown bound, ranges​::​end(E) is ill-formed.
  • (2.4)
    Otherwise, if T is an array, ranges​::​end(E) is expression-equivalent to t + extent_v<T>.
  • (2.5)
    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()).
  • (2.6)
    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.
  • (2.7)
    Otherwise, ranges​::​end(E) is ill-formed.
3
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[Note 1: 
Diagnosable ill-formed cases above result in substitution failure when ranges​::​end(E) appears in the immediate context of a template instantiation.
— end note]
4
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[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>.
— end note]

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

1
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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:
  • (1.1)
    If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​cbegin(E) is ill-formed.
  • (1.2)
    Otherwise, let U be ranges​::​begin(possibly-const-range(t)).
    ranges​::​cbegin(E) is expression-equivalent to const_iterator<decltype(U)>(U).
2
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[Note 1: 
Whenever ranges​::​cbegin(E) is a valid expression, its type models input_or_output_iterator and constant-iterator.
— end note]

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

1
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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:
  • (1.1)
    If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​cend(E) is ill-formed.
  • (1.2)
    Otherwise, let U be ranges​::​end(possibly-const-range(t)).
    ranges​::​cend(E) is expression-equivalent to const_sentinel<decltype(U)>(U).
2
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[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.
— end note]

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

1
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The name ranges​::​rbegin denotes a customization point object ([customization.point.object]).
2
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Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • (2.1)
    If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​rbegin(E) is ill-formed.
  • (2.2)
    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.
  • (2.3)
    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()).
  • (2.4)
    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.
  • (2.5)
    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)).
  • (2.6)
    Otherwise, ranges​::​rbegin(E) is ill-formed.
3
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[Note 1: 
Diagnosable ill-formed cases above result in substitution failure when ranges​::​rbegin(E) appears in the immediate context of a template instantiation.
— end note]
4
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[Note 2: 
Whenever ranges​::​rbegin(E) is a valid expression, its type models input_or_output_iterator.
— end note]

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

1
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The name ranges​::​rend denotes a customization point object ([customization.point.object]).
2
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Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • (2.1)
    If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​rend(E) is ill-formed.
  • (2.2)
    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.
  • (2.3)
    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()).
  • (2.4)
    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.
  • (2.5)
    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)).
  • (2.6)
    Otherwise, ranges​::​rend(E) is ill-formed.
3
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[Note 1: 
Diagnosable ill-formed cases above result in substitution failure when ranges​::​rend(E) appears in the immediate context of a template instantiation.
— end note]
4
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[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>.
— end note]

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

1
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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:
  • (1.1)
    If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​crbegin(E) is ill-formed.
  • (1.2)
    Otherwise, let U be ranges​::​rbegin(possibly-const-range(t)).
    ranges​::​crbegin(E) is expression-equivalent to const_iterator<decltype(U)>(U).
2
#
[Note 1: 
Whenever ranges​::​crbegin(E) is a valid expression, its type models input_or_output_iterator and constant-iterator.
— end note]

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

1
#
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:
  • (1.1)
    If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​crend(E) is ill-formed.
  • (1.2)
    Otherwise, let U be ranges​::​rend(possibly-const-range(t)).
    ranges​::​crend(E) is expression-equivalent to const_sentinel<decltype(U)>(U).
2
#
[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.
— end note]

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

1
#
The name ranges​::​size denotes a customization point object ([customization.point.object]).
2
#
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • (2.1)
    If T is an array of unknown bound ([dcl.array]), ranges​::​size(E) is ill-formed.
  • (2.2)
    Otherwise, if T is an array type, ranges​::​size(E) is expression-equivalent to auto(extent_v<T>).
  • (2.3)
    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()).
  • (2.4)
    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.
  • (2.5)
    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)).
  • (2.6)
    Otherwise, ranges​::​size(E) is ill-formed.
3
#
[Note 1: 
Diagnosable ill-formed cases above result in substitution failure when ranges​::​size(E) appears in the immediate context of a template instantiation.
— end note]
4
#
[Note 2: 
Whenever ranges​::​size(E) is a valid expression, its type is integer-like.
— end note]

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

1
#
The name ranges​::​ssize denotes a customization point object ([customization.point.object]).
2
#
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)).

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

1
#
The name ranges​::​empty denotes a customization point object ([customization.point.object]).
2
#
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • (2.1)
    If T is an array of unknown bound ([dcl.array]), ranges​::​empty(E) is ill-formed.
  • (2.2)
    Otherwise, if bool(t.empty()) is a valid expression, ranges​::​empty(E) is expression-equivalent to bool(t.empty()).
  • (2.3)
    Otherwise, if (ranges​::​size(t) == 0) is a valid expression, ranges​::​empty(E) is expression-equivalent to (ranges​::​size(t) == 0).
  • (2.4)
    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)).
  • (2.5)
    Otherwise, ranges​::​empty(E) is ill-formed.
3
#
[Note 1: 
Diagnosable ill-formed cases above result in substitution failure when ranges​::​empty(E) appears in the immediate context of a template instantiation.
— end note]
4
#
[Note 2: 
Whenever ranges​::​empty(E) is a valid expression, it has type bool.
— end note]

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

1
#
The name ranges​::​data denotes a customization point object ([customization.point.object]).
2
#
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • (2.1)
    If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​data(E) is ill-formed.
  • (2.2)
    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.
  • (2.3)
    Otherwise, if auto(t.data()) is a valid expression of pointer to object type, ranges​::​data(E) is expression-equivalent to auto(t.data()).
  • (2.4)
    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)).
  • (2.5)
    Otherwise, ranges​::​data(E) is ill-formed.
3
#
[Note 1: 
Diagnosable ill-formed cases above result in substitution failure when ranges​::​data(E) appears in the immediate context of a template instantiation.
— end note]
4
#
[Note 2: 
Whenever ranges​::​data(E) is a valid expression, it has pointer to object type.
— end note]

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

template<class T> constexpr auto as-const-pointer(const T* p) noexcept { return p; } // exposition only
1
#
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:
  • (1.1)
    If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​cdata(E) is ill-formed.
  • (1.2)
    Otherwise, ranges​::​cdata(E) is expression-equivalent to as-const-pointer(ranges​::​data(possibly-const-range(t))).
2
#
[Note 1: 
Whenever ranges​::​cdata(E) is a valid expression, it has pointer to constant object type.
— end note]

25.4 Range requirements [range.req]

25.4.1 General [range.req.general]

1
#
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.
2
#
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.
3
#
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.

25.4.2 Ranges [range.range]

1
#
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); };
2
#
Given an expression t such that decltype((t)) is T&, T models range only if
  • (2.1)
    [ranges​::​begin(t), ranges​::​end(t)) denotes a range ([iterator.requirements.general]),
  • (2.2)
    both ranges​::​begin(t) and ranges​::​end(t) are amortized constant time and non-modifying, and
  • (2.3)
    if the type of ranges​::​begin(t) models forward_iterator, ranges​::​begin(t) is equality-preserving.
3
#
[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.
— end note]
🔗
template<class T> concept borrowed_range = range<T> && (is_lvalue_reference_v<T> || enable_borrowed_range<remove_cvref_t<T>>);
4
#
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.
5
#
[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.
— end note]
🔗
template<class> constexpr bool enable_borrowed_range = false;
6
#
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.
7
#
[Example 1: 
Each specialization S of class template subrange ([range.subrange]) models borrowed_range because
  • (7.1)
    enable_borrowed_range<S> is specialized to have the value true, and
  • (7.2)
    S's iterators do not have validity tied to the lifetime of an S object because they are “borrowed” from some other range.
— end example]

25.4.3 Sized ranges [range.sized]

1
#
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); };
2
#
Given an lvalue t of type remove_reference_t<T>, T models sized_range only if
  • (2.1)
    ranges​::​size(t) is amortized , does not modify t, and is equal to ranges​::​distance(​ranges​::​begin(t), ranges​::​end(t)), and
  • (2.2)
    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).
    — end note]
🔗
template<class> constexpr bool disable_sized_range = false;
3
#
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.
4
#
[Note 2: 
disable_sized_range allows use of range types with the library that satisfy but do not in fact model sized_range.
— end note]

25.4.4 Views [range.view]

1
#
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>;
2
#
T models view only if
  • (2.1)
    T has move construction; and
  • (2.2)
    move assignment of an object of type T is no more complex than destruction followed by move construction; and
  • (2.3)
    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
  • (2.4)
    copy_constructible<T> is false, or T has copy construction; and
  • (2.5)
    copyable<T> is false, or copy assignment of an object of type T is no more complex than destruction followed by copy construction.
3
#
[Note 1: 
The constraints on copying and moving imply that a moved-from object of type T has destruction.
— end note]
4
#
[Example 1: 
Examples of views are:
  • (4.1)
    A range type that wraps a pair of iterators.
  • (4.2)
    A range type that holds its elements by shared_ptr and shares ownership with all its copies.
  • (4.3)
    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.
— end example]
5
#
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>;
6
#
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.
7
#
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.

25.4.5 Other range refinements [range.refinements]

1
#
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>>;
2
#
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>>>; };
3
#
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.
4
#
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.
— 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
5
#
For a type R, is-initializer-list<R> is true if and only if remove_cvref_t<R> is a specialization of initializer_list.
6
#
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>))));
7
#
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>>;

25.5 Range utilities [range.utility]

25.5.1 General [range.utility.general]

1
#
The components in [range.utility] are general utilities for representing and manipulating ranges.

25.5.2 Helper concepts [range.utility.helpers]

1
#
Many of the types in [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>>;

25.5.3 View interface [view.interface]

25.5.3.1 General [view.interface.general]

1
#
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]; } }; }
2
#
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.

25.5.3.2 Members [view.interface.members]

🔗
constexpr decltype(auto) front() requires forward_range<D>; constexpr decltype(auto) front() const requires forward_range<const D>;
1
#
Preconditions: !empty() is true.
2
#
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>;
3
#
Preconditions: !empty() is true.
4
#
Effects: Equivalent to: return *ranges​::​prev(ranges​::​end(derived()));

25.5.4 Sub-ranges [range.subrange]

25.5.4.1 General [range.subrange.general]

1
#
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>; 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); constexpr subrange next(iter_difference_t<I> n = 1) const & requires forward_iterator<I>; constexpr subrange next(iter_difference_t<I> n = 1) &&; 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>; }

25.5.4.2 Constructors and conversions [range.subrange.ctor]

🔗
constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize);
1
#
Preconditions: [i, s) is a valid range.
2
#
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);
3
#
Preconditions: [i, s) is a valid range, and n == to-unsigned-like(ranges​::​distance(i, s)) is true.
4
#
Effects: Initializes begin_ with std​::​move(i) and end_ with s.
If StoreSize is true, initializes size_ with n.
5
#
[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.
— 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>);
6
#
Effects: Equivalent to:
  • (6.1)
    If StoreSize is true, subrange(r, static_cast<decltype(size_)>(ranges​::​size(r))).
  • (6.2)
    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;
7
#
Effects: Equivalent to: return PairLike(begin_, end_);

25.5.4.3 Accessors [range.subrange.access]

🔗
constexpr I begin() const requires copyable<I>;
1
#
Effects: Equivalent to: return begin_;
🔗
constexpr I begin() requires (!copyable<I>);
2
#
Effects: Equivalent to: return std​::​move(begin_);
🔗
constexpr S end() const;
3
#
Effects: Equivalent to: return end_;
🔗
constexpr bool empty() const;
4
#
Effects: Equivalent to: return begin_ == end_;
🔗
constexpr make-unsigned-like-t<iter_difference_t<I>> size() const requires (K == subrange_kind::sized);
5
#
Effects:
  • (5.1)
    If StoreSize is true, equivalent to: return size_;
  • (5.2)
    Otherwise, equivalent to: return to-unsigned-like(end_ - begin_);
🔗
constexpr subrange next(iter_difference_t<I> n = 1) const & requires forward_iterator<I>;
6
#
Effects: Equivalent to: auto tmp = *this; tmp.advance(n); return tmp;
🔗
constexpr subrange next(iter_difference_t<I> n = 1) &&;
7
#
Effects: Equivalent to: advance(n); return std::move(*this);
🔗
constexpr subrange prev(iter_difference_t<I> n = 1) const requires bidirectional_iterator<I>;
8
#
Effects: Equivalent to: auto tmp = *this; tmp.advance(-n); return tmp;
🔗
constexpr subrange& advance(iter_difference_t<I> n);
9
#
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);
10
#
Effects: Equivalent to: if constexpr (N == 0) return r.begin(); else return r.end();

25.5.5 Dangling iterator handling [range.dangling]

1
#
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 {} }; }
2
#
[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.
— end example]
3
#
For a type R that models range:
  • (3.1)
    if R models borrowed_range, then borrowed_iterator_t<R> denotes iterator_t<R>, and borrowed_subrange_t<R> denotes subrange<iterator_t<R>>;
  • (3.2)
    otherwise, both borrowed_iterator_t<R> and borrowed_subrange_t<R> denote dangling.

25.5.6 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 } — 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>; }

25.5.7 Range conversions [range.utility.conv]

25.5.7.1 General [range.utility.conv.general]

1
#
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.
2
#
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"} — end example]
3
#
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)>; };
4
#
Let container-appendable be defined as follows: template<class Container, class Ref> constexpr bool container-appendable = // exposition only requires(Container& c, Ref&& ref) { requires (requires { c.emplace_back(std::forward<Ref>(ref)); } || requires { c.push_back(std::forward<Ref>(ref)); } || requires { c.emplace(c.end(), std::forward<Ref>(ref)); } || requires { c.insert(c.end(), std::forward<Ref>(ref)); }); };
5
#
Let container-append be defined as follows: template<class Container> constexpr auto container-append(Container& c) { // exposition only return [&c]<class Ref>(Ref&& ref) { if constexpr (requires { c.emplace_back(declval<Ref>()); }) c.emplace_back(std::forward<Ref>(ref)); else if constexpr (requires { c.push_back(declval<Ref>()); }) c.push_back(std::forward<Ref>(ref)); else if constexpr (requires { c.emplace(c.end(), declval<Ref>()); }) c.emplace(c.end(), std::forward<Ref>(ref)); else c.insert(c.end(), std::forward<Ref>(ref)); }; }

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

🔗
template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);
1
#
Mandates: C is a cv-unqualified class type.
2
#
Returns: An object of type C constructed from the elements of r in the following manner:
  • (2.1)
    If C does not satisfy input_range or convertible_to<range_reference_t<R>, range_value_t<C>> is true:
  • (2.2)
    Otherwise, if input_range<range_reference_t<R>> is true: to<C>(ref_view(r) | views::transform([](auto&& elem) { return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); }), std::forward<Args>(args)...);
  • (2.3)
    Otherwise, the program is ill-formed.
🔗
template<template<class...> class C, input_range R, class... Args> constexpr auto to(R&& r, Args&&... args);
3
#
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.
— end note]
4
#
Let DEDUCE_EXPR be defined as follows:
  • (4.1)
    C(declval<R>(), declval<Args>()...) if that is a valid expression,
  • (4.2)
    otherwise, C(from_range, declval<R>(), declval<Args>()...) if that is a valid expression,
  • (4.3)
    otherwise, C(declval<input-iterator>(), declval<input-iterator>(), declval<Args>()...) if that is a valid expression,
  • (4.4)
    otherwise, the program is ill-formed.
5
#
Returns: to<decltype(DEDUCE_EXPR)>(std​::​forward<R>(r), std​::​forward<Args>(args)...).

25.5.7.3 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);
1
#
Mandates: For the first overload, C is a cv-unqualified class type.
2
#
Returns: A range adaptor closure object ([range.adaptor.object]) f that is a perfect forwarding call wrapper ([func.require]) with the following properties:
  • (2.1)
    It has no target object.
  • (2.2)
    Its bound argument entities bound_args consist of objects of types decay_t<Args>... direct-non-list-initialized with std​::​forward<Args>(args)..., respectively.
  • (2.3)
    Its call pattern is to<C>(r, bound_args...), where r is the argument used in a function call expression of f.

25.6 Range factories [range.factories]

25.6.1 General [range.factories.general]

1
#
Subclause [range.factories] defines range factories, which are utilities to create a view.
2
#
Range factories are declared in namespace std​::​ranges​::​views.

25.6.2 Empty view [range.empty]

25.6.2.1 Overview [range.empty.overview]

1
#
empty_view produces a view of no elements of a particular type.
2
#
[Example 1: auto e = views::empty<int>; static_assert(ranges::empty(e)); static_assert(0 == e.size()); — end example]

25.6.2.2 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; } }; }

25.6.3 Single view [range.single]

25.6.3.1 Overview [range.single.overview]

1
#
single_view produces a view that contains exactly one element of a specified value.
2
#
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).
3
#
[Example 1: for (int i : views::single(4)) cout << i; // prints 4 — end example]

25.6.3.2 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 bool empty() 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>;
1
#
Effects: Initializes value_ with t.
🔗
constexpr explicit single_view(T&& t);
2
#
Effects: Initializes value_ with std​::​move(t).
🔗
template<class... Args> requires constructible_from<T, Args...> constexpr explicit single_view(in_place_t, Args&&... args);
3
#
Effects: Initializes value_ as if by value_{in_place, std​::​forward<Args>(args)...}.
🔗
constexpr T* begin() noexcept; constexpr const T* begin() const noexcept;
4
#
Effects: Equivalent to: return data();
🔗
constexpr T* end() noexcept; constexpr const T* end() const noexcept;
5
#
Effects: Equivalent to: return data() + 1;
🔗
static constexpr bool empty() noexcept;
6
#
Effects: Equivalent to: return false;
🔗
static constexpr size_t size() noexcept;
7
#
Effects: Equivalent to: return 1;
🔗
constexpr T* data() noexcept; constexpr const T* data() const noexcept;
8
#
Effects: Equivalent to: return value_.operator->();

25.6.4 Iota view [range.iota]

25.6.4.1 Overview [range.iota.overview]

1
#
iota_view generates a sequence of elements by repeatedly incrementing an initial value.
2
#
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<decay_t<decltype((E))>>(E) and iota_view(E, F), respectively.
3
#
[Example 1: for (int i : views::iota(1, 10)) cout << i << ' '; // prints 1 2 3 4 5 6 7 8 9 — end example]

25.6.4.2 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 bool empty() const; 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>; }
1
#
Let IOTA-DIFF-T(W) be defined as follows:
  • (1.1)
    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>.
  • (1.2)
    Otherwise, IOTA-DIFF-T(W) is a signed integer type of width greater than the width of W if such a type exists.
  • (1.3)
    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.
    — end note]
2
#
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>; };
3
#
When an object is in the domain of both pre- and post-decrement, the object is said to be decrementable.
4
#
Let a and b be equal objects of type I.
I models decrementable only if
  • (4.1)
    If a and b are decrementable, then the following are all true:
  • (4.2)
    If a and b are incrementable, then bool(--(++a) == b).
5
#
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);
6
#
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.
7
#
Effects: Initializes value_ with value.
🔗
constexpr explicit iota_view(type_identity_t<W> value, type_identity_t<Bound> bound);
8
#
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.
9
#
Effects: Initializes value_ with value and bound_ with bound.
🔗
constexpr explicit iota_view(iterator first, see below last);
10
#
Effects: Equivalent to:
  • (10.1)
    If same_as<W, Bound> is true, iota_view(first.value_, last.value_).
  • (10.2)
    Otherwise, if Bound denotes unreachable_sentinel_t, iota_view(first.value_, last).
  • (10.3)
    Otherwise, iota_view(first.value_, last.bound_).
11
#
Remarks: The type of last is:
🔗
constexpr iterator begin() const;
12
#
Effects: Equivalent to: return iterator{value_};
🔗
constexpr auto end() const;
13
#
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>;
14
#
Effects: Equivalent to: return iterator{bound_};
🔗
constexpr bool empty() const;
15
#
Effects: Equivalent to: return value_ == bound_;
🔗
constexpr auto size() const requires see below;
16
#
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_);
17
#
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>

25.6.4.3 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>; }; }
1
#
iterator​::​iterator_concept is defined as follows:
  • (1.1)
    If W models advanceable, then iterator_concept is random_access_iterator_tag.
  • (1.2)
    Otherwise, if W models decrementable, then iterator_concept is bidirectional_iterator_tag.
  • (1.3)
    Otherwise, if W models incrementable, then iterator_concept is forward_iterator_tag.
  • (1.4)
    Otherwise, iterator_concept is input_iterator_tag.
2
#
[Note 1: 
Overloads for iter_move and iter_swap are omitted intentionally.
— end note]
🔗
constexpr explicit iterator(W value);
3
#
Effects: Initializes value_ with value.
🔗
constexpr W operator*() const noexcept(is_nothrow_copy_constructible_v<W>);
4
#
Effects: Equivalent to: return value_;
5
#
[Note 2: 
The noexcept clause is needed by the default iter_move implementation.
— end note]
🔗
constexpr iterator& operator++();
6
#
Effects: Equivalent to: ++value_; return *this;
🔗
constexpr void operator++(int);
7
#
Effects: Equivalent to ++*this.
🔗
constexpr iterator operator++(int) requires incrementable<W>;
8
#
Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;
🔗
constexpr iterator& operator--() requires decrementable<W>;
9
#
Effects: Equivalent to: --value_; return *this;
🔗
constexpr iterator operator--(int) requires decrementable<W>;
10
#
Effects: Equivalent to: auto tmp = *this; --*this; return tmp;
🔗
constexpr iterator& operator+=(difference_type n) requires advanceable<W>;
11
#
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>;
12
#
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>;
13
#
Effects: Equivalent to: return W(value_ + n);
🔗
friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<W>;
14
#
Effects: Equivalent to: return x.value_ == y.value_;
🔗
friend constexpr bool operator<(const iterator& x, const iterator& y) requires totally_ordered<W>;
15
#
Effects: Equivalent to: return x.value_ < y.value_;
🔗
friend constexpr bool operator>(const iterator& x, const iterator& y) requires totally_ordered<W>;
16
#
Effects: Equivalent to: return y < x;
🔗
friend constexpr bool operator<=(const iterator& x, const iterator& y) requires totally_ordered<W>;
17
#
Effects: Equivalent to: return !(y < x);
🔗
friend constexpr bool operator>=(const iterator& x, const iterator& y) requires totally_ordered<W>;
18
#
Effects: Equivalent to: return !(x < y);
🔗
friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires totally_ordered<W> && three_way_comparable<W>;
19
#
Effects: Equivalent to: return x.value_ <=> y.value_;
🔗
friend constexpr iterator operator+(iterator i, difference_type n) requires advanceable<W>;
20
#
Effects: Equivalent to: i += n; return i;
🔗
friend constexpr iterator operator+(difference_type n, iterator i) requires advanceable<W>;
21
#
Effects: Equivalent to: return i + n;
🔗
friend constexpr iterator operator-(iterator i, difference_type n) requires advanceable<W>;
22
#
Effects: Equivalent to: i -= n; return i;
🔗
friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires advanceable<W>;
23
#
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_; }

25.6.4.4 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);
1
#
Effects: Initializes bound_ with bound.
🔗
friend constexpr bool operator==(const iterator& x, const sentinel& y);
2
#
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>;
3
#
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>;
4
#
Effects: Equivalent to: return -(y - x);

25.6.5 Repeat view [range.repeat]

25.6.5.1 Overview [range.repeat.overview]

1
#
repeat_view generates a sequence of elements by repeatedly producing the same value.
2
#
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<decay_t<decltype((E))>>(E) and repeat_view(E, F), respectively.
3
#
[Example 1: for (int i : views::repeat(17, 4)) cout << i << ' '; // prints 17 17 17 17 — end example]

25.6.5.2 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 = unreachable_sentinel_t> repeat_view(T, Bound = Bound()) -> repeat_view<T, Bound>; }
🔗
constexpr explicit repeat_view(const T& value, Bound bound = Bound()) requires copy_constructible<T>;
1
#
Preconditions: If Bound is not unreachable_sentinel_t, bound  ≥ 0.
2
#
Effects: Initializes value_ with value and bound_ with bound.
🔗
constexpr explicit repeat_view(T&& value, Bound bound = Bound());
3
#
Preconditions: If Bound is not unreachable_sentinel_t, bound  ≥ 0.
4
#
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<>{});
5
#
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;
6
#
Effects: Equivalent to: return iterator(addressof(*value_));
🔗
constexpr iterator end() const requires (!same_as<Bound, unreachable_sentinel_t>);
7
#
Effects: Equivalent to: return iterator(addressof(*value_), bound_);
🔗
constexpr unreachable_sentinel_t end() const noexcept;
8
#
Effects: Equivalent to: return unreachable_sentinel;
🔗
constexpr auto size() const requires (!same_as<Bound, unreachable_sentinel_t>);
9
#
Effects: Equivalent to: return to-unsigned-like(bound_);

25.6.5.3 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); }; }
1
#
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());
2
#
Preconditions: If Bound is not unreachable_sentinel_t, b  ≥ 0.
3
#
Effects: Initializes value_ with value and current_ with b.
🔗
constexpr const T& operator*() const noexcept;
4
#
Effects: Equivalent to: return *value_;
🔗
constexpr iterator& operator++();
5
#
Effects: Equivalent to: ++current_; return *this;
🔗
constexpr iterator operator++(int);
6
#
Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;
🔗
constexpr iterator& operator--();
7
#
Preconditions: If Bound is not unreachable_sentinel_t, .
8
#
Effects: Equivalent to: --current_; return *this;
🔗
constexpr iterator operator--(int);
9
#
Effects: Equivalent to: auto tmp = *this; --*this; return tmp;
🔗
constexpr iterator& operator+=(difference_type n);
10
#
Preconditions: If Bound is not unreachable_sentinel_t, .
11
#
Effects: Equivalent to: current_ += n; return *this;
🔗
constexpr iterator& operator-=(difference_type n);
12
#
Preconditions: If Bound is not unreachable_sentinel_t, .
13
#
Effects: Equivalent to: current_ -= n; return *this;
🔗
constexpr const T& operator[](difference_type n) const noexcept;
14
#
Effects: Equivalent to: return *(*this + n);
🔗
friend constexpr bool operator==(const iterator& x, const iterator& y);
15
#
Effects: Equivalent to: return x.current_ == y.current_;
🔗
friend constexpr auto operator<=>(const iterator& x, const iterator& y);
16
#
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);
17
#
Effects: Equivalent to: i += n; return i;
🔗
friend constexpr iterator operator-(iterator i, difference_type n);
18
#
Effects: Equivalent to: i -= n; return i;
🔗
friend constexpr difference_type operator-(const iterator& x, const iterator& y);
19
#
Effects: Equivalent to: return static_cast<difference_type>(x.current_) - static_cast<difference_type>(y.current_);

25.6.6 Istream view [range.istream]

25.6.6.1 Overview [range.istream.overview]

1
#
basic_istream_view models input_range and reads (using operator>>) successive elements from its corresponding input stream.
2
#
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.
3
#
[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- — end example]

25.6.6.2 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);
1
#
Effects: Initializes stream_ with addressof(stream).
🔗
constexpr default_sentinel_t end() const noexcept;
2
#
Effects: Equivalent to: return default_sentinel;

25.6.6.3 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;
1
#
Effects: Initializes parent_ with addressof(parent).
🔗
iterator& operator++();
2
#
Effects: Equivalent to: *parent_->stream_ >> parent_->value_; return *this;
🔗
void operator++(int);
3
#
Effects: Equivalent to ++*this.
🔗
Val& operator*() const;
4
#
Effects: Equivalent to: return parent_->value_;
🔗
friend bool operator==(const iterator& x, default_sentinel_t);
5
#
Effects: Equivalent to: return !*x.parent_->stream_;

25.7 Range adaptors [range.adaptors]

25.7.1 General [range.adaptors.general]

1
#
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.
2
#
Range adaptors are declared in namespace std​::​ranges​::​views.
3
#
The bitwise or operator is overloaded for the purpose of creating adaptor chain pipelines.
The adaptors also support function call syntax with equivalent semantics.
4
#
[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))); — end example]

25.7.2 Range adaptor objects [range.adaptor.object]

1
#
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:
  • (1.1)
    Its target object is an object d of type decay_t<decltype((D))> direct-non-list-initialized with D.
  • (1.2)
    It has one bound argument entity, an object c of type decay_t<decltype((C))> direct-non-list-initialized with C.
  • (1.3)
    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.
2
#
Given an object t of type T, where
  • (2.1)
    t is a unary function object that accepts a range argument,
  • (2.2)
    T models derived_from<range_adaptor_closure<T>>,
  • (2.3)
    T has no other base classes of type range_adaptor_closure<U> for any other type U, and
  • (2.4)
    T does not model range
then the implementation ensures that t is a range adaptor closure object.
3
#
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.
4
#
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.
5
#
The behavior of a program that adds a specialization for range_adaptor_closure is undefined.
6
#
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.
7
#
If a range adaptor object accepts only one argument, then it is a range adaptor closure object.
8
#
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:
  • (8.1)
    Its target object is a copy of adaptor.
  • (8.2)
    Its bound argument entities bound_args consist of objects of types BoundArgs... direct-non-list-initialized with std​::​forward<decltype((args))>(args)..., respectively.
  • (8.3)
    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.

25.7.3 Movable wrapper [range.move.wrap]

1
#
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:
  • (1.1)
    movable-box<T> constrains its type parameter T with move_constructible<T> && is_object_v<T>.
  • (1.2)
    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} {}
  • (1.3)
    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; }
  • (1.4)
    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; }
2
#
Recommended practice:
  • (2.1)
    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.
  • (2.2)
    Otherwise, movable-box<T> should store only a T if either T models movable or is_nothrow_move_constructible_v<T> is true.

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

1
#
Some types in [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:
  • (1.1)
    non-propagating-cache<T> constrains its type parameter T with is_object_v<T>.
  • (1.2)
    The copy constructor is equivalent to: constexpr non-propagating-cache(const non-propagating-cache&) noexcept {}
  • (1.3)
    The move constructor is equivalent to: constexpr non-propagating-cache(non-propagating-cache&& other) noexcept { other.reset(); }
  • (1.4)
    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; }
  • (1.5)
    The move assignment operator is equivalent to: constexpr non-propagating-cache& operator=(non-propagating-cache&& other) noexcept { reset(); other.reset(); return *this; }
  • (1.6)
    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]).
    — 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.
2
#
[Note 2: 
non-propagating-cache enables an input view to temporarily cache values as it is iterated over.
— end note]

25.7.5 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); } template<bool Const, class... Views> concept all-random-access = // exposition only (random_access_range<maybe-const<Const, Views>> && ...); template<bool Const, class... Views> concept all-bidirectional = // exposition only (bidirectional_range<maybe-const<Const, Views>> && ...); template<bool Const, class... Views> concept all-forward = // exposition only (forward_range<maybe-const<Const, Views>> && ...); }

25.7.6 All view [range.all]

25.7.6.1 General [range.all.general]

1
#
views​::​all returns a view that includes all elements of its range argument.
2
#
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:
  • (2.1)
    decay-copy(E) if the decayed type of E models view.
  • (2.2)
    Otherwise, ref_view{E} if that expression is well-formed.
  • (2.3)
    Otherwise, owning_view{E}.

25.7.6.2 Class template ref_view [range.ref.view]

1
#
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);
2
#
Effects: Initializes r_ with addressof(static_cast<R&>(std​::​forward<T>(t))).
3
#
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>()); }

25.7.6.3 Class template owning_view [range.owning.view]

1
#
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);
2
#
Effects: Initializes r_ with std​::​move(t).

25.7.7 As rvalue view [range.as.rvalue]

25.7.7.1 Overview [range.as.rvalue.overview]

1
#
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.
2
#
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:
  • (2.1)
    views​::​all(E) if T models input_range and same_as<range_rvalue_reference_t<T>, range_reference_t<T>> is true.
  • (2.2)
    Otherwise, as_rvalue_view(E).
3
#
[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 — end example]

25.7.7.2 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>>; }
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constexpr explicit as_rvalue_view(V base);
1
#
Effects: Initializes base_ with std​::​move(base).

25.7.8 Filter view [range.filter]

25.7.8.1 Overview [range.filter.overview]

1
#
filter_view presents a view of the elements of an underlying sequence that satisfy a predicate.
2
#
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).
3
#
[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 — end example]

25.7.8.2 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>; }
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constexpr explicit filter_view(V base, Pred pred);
1
#
Effects: Initializes base_ with std​::​move(base) and initializes pred_ with std​::​move(pred).
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constexpr const Pred& pred() const;
2
#
Effects: Equivalent to: return *pred_;
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constexpr iterator begin();
3
#
Preconditions: pred_.has_value() is true.
4
#
Returns: {*this, ranges​::​find_if(base_, ref(*pred_))}.
5
#
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.

25.7.8.3 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>>; }; }
1
#
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.
2
#
iterator​::​iterator_concept is defined as follows:
3
#
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:
  • (3.1)
    Let C denote the type iterator_traits<iterator_t<V>>​::​iterator_category.
  • (3.2)
    If C models derived_from<bidirectional_iterator_tag>, then iterator_category denotes bidirectional_iterator_tag.
  • (3.3)
    Otherwise, if C models derived_from<forward_iterator_tag>, then iterator_category denotes forward_iterator_tag.
  • (3.4)
    Otherwise, iterator_category denotes C.
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constexpr iterator(filter_view& parent, iterator_t<V> current);
4
#
Effects: Initializes current_ with std​::​move(current) and parent_ with addressof(parent).
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constexpr const iterator_t<V>& base() const & noexcept;
5
#
Effects: Equivalent to: return current_;
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constexpr iterator_t<V> base() &&;
6
#
Effects: Equivalent to: return std​::​move(current_);
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constexpr range_reference_t<V> operator*() const;
7
#
Effects: Equivalent to: return *current_;
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constexpr iterator_t<V> operator->() const requires has-arrow<iterator_t<V>> && copyable<iterator_t<V>>;
8
#
Effects: Equivalent to: return current_;
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constexpr iterator& operator++();
9
#
Effects: Equivalent to: current_ = ranges::find_if(std::move(++current_), ranges::end(parent_->base_), ref(*parent_->pred_)); return *this;
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constexpr void operator++(int);
10
#
Effects: Equivalent to ++*this.
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constexpr iterator operator++(int) requires forward_range<V>;
11
#
Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;
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constexpr iterator& operator--() requires bidirectional_range<V>;
12
#
Effects: Equivalent to: do --current_; while (!invoke(*parent_->pred_, *current_)); return *this;
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constexpr iterator operator--(int) requires bidirectional_range<V>;
13
#
Effects: Equivalent to: auto tmp = *this; --*this; return tmp;
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friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<V>>;
14
#
Effects: Equivalent to: return x.current_ == y.current_;
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friend constexpr range_rvalue_reference_t<V> iter_move(const iterator& i) noexcept(noexcept(ranges::iter_move(i.current_)));
15
#
Effects: Equivalent to: return ranges​::​iter_move(i.current_);
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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>>;
16
#
Effects: Equivalent to ranges​::​iter_swap(x.current_, y.current_).

25.7.8.4 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);
1
#
Effects: Initializes end_ with ranges​::​end(parent.base_).
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constexpr sentinel_t<V> base() const;
2
#
Effects: Equivalent to: return end_;
🔗
friend constexpr bool operator==(const iterator& x, const sentinel& y);
3
#
Effects: Equivalent to: return x.current_ == y.end_;

25.7.9 Transform view [range.transform]

25.7.9.1 Overview [range.transform.overview]

1
#
transform_view presents a view of an underlying sequence after applying a transformation function to each element.
2
#
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).
3
#
[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 — end example]

25.7.9.2 Class template transform_view [range.transform.view]

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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);
1
#
Effects: Initializes base_ with std​::​move(base) and fun_ with std​::​move(fun).
🔗
constexpr iterator<false> begin();
2
#
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>>;
3
#
Effects: Equivalent to: return iterator<true>{*this, ranges::begin(base_)};
🔗
constexpr sentinel<false> end();
4
#
Effects: Equivalent to: return sentinel<false>{ranges::end(base_)};
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constexpr iterator<false> end() requires common_range<V>;
5
#
Effects: Equivalent to: return iterator<false>{*this, ranges::end(base_)};
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constexpr sentinel<true> end() const requires range<const V> && regular_invocable<const F&, range_reference_t<const V>>;
6
#
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>>;
7
#
Effects: Equivalent to: return iterator<true>{*this, ranges::end(base_)};

25.7.9.3 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>>; }; }
1
#
iterator​::​iterator_concept is defined as follows:
2
#
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.
  • (2.1)
    If is_reference_v<invoke_result_t<maybe-const<Const, F>&, range_reference_t<Base>>> is true, then
    • (2.1.1)
      if C models derived_from<contiguous_iterator_tag>, iterator_category denotes random_access_iterator_tag;
    • (2.1.2)
      otherwise, iterator_category denotes C.
  • (2.2)
    Otherwise, iterator_category denotes input_iterator_tag.
🔗
constexpr iterator(Parent& parent, iterator_t<Base> current);
3
#
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>>;
4
#
Effects: Initializes current_ with std​::​move(i.current_) and parent_ with i.parent_.
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constexpr const iterator_t<Base>& base() const & noexcept;
5
#
Effects: Equivalent to: return current_;
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constexpr iterator_t<Base> base() &&;
6
#
Effects: Equivalent to: return std​::​move(current_);
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constexpr iterator& operator++();
7
#
Effects: Equivalent to: ++current_; return *this;
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constexpr void operator++(int);
8
#
Effects: Equivalent to ++current_.
🔗
constexpr iterator operator++(int) requires forward_range<Base>;
9
#
Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;
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constexpr iterator& operator--() requires bidirectional_range<Base>;
10
#
Effects: Equivalent to: --current_; return *this;
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constexpr iterator operator--(int) requires bidirectional_range<Base>;
11
#
Effects: Equivalent to: auto tmp = *this; --*this; return tmp;
🔗
constexpr iterator& operator+=(difference_type n) requires random_access_range<Base>;
12
#
Effects: Equivalent to: current_ += n; return *this;
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constexpr iterator& operator-=(difference_type n) requires random_access_range<Base>;
13
#
Effects: Equivalent to: current_ -= n; return *this;
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friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<Base>>;
14
#
Effects: Equivalent to: return x.current_ == y.current_;
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friend constexpr bool operator<(const iterator& x, const iterator& y) requires random_access_range<Base>;
15
#
Effects: Equivalent to: return x.current_ < y.current_;
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friend constexpr bool operator>(const iterator& x, const iterator& y) requires random_access_range<Base>;
16
#
Effects: Equivalent to: return y < x;
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friend constexpr bool operator<=(const iterator& x, const iterator& y) requires random_access_range<Base>;
17
#
Effects: Equivalent to: return !(y < x);
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friend constexpr bool operator>=(const iterator& x, const iterator& y) requires random_access_range<Base>;
18
#
Effects: Equivalent to: return !(x < y);
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friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;
19
#
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>;
20
#
Effects: Equivalent to: return iterator{*i.parent_, i.current_ + n};
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friend constexpr iterator operator-(iterator i, difference_type n) requires random_access_range<Base>;
21
#
Effects: Equivalent to: return iterator{*i.parent_, i.current_ - n};
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friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
22
#
Effects: Equivalent to: return x.current_ - y.current_;

25.7.9.4 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); }; }
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constexpr explicit sentinel(sentinel_t<Base> end);
1
#
Effects: Initializes end_ with end.
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constexpr sentinel(sentinel<!Const> i) requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
2
#
Effects: Initializes end_ with std​::​move(i.end_).
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constexpr sentinel_t<Base> base() const;
3
#
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);
4
#
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);
5
#
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);
6
#
Effects: Equivalent to: return y.end_ - x.current_;

25.7.10 Take view [range.take]

25.7.10.1 Overview [range.take.overview]

1
#
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.
2
#
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:
  • (2.1)
    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.
  • (2.2)
    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:
    • (2.2.1)
      if T is a specialization of span, then U is span<typename T​::​element_type>;
    • (2.2.2)
      otherwise, if T is a specialization of basic_string_view, then U is T;
    • (2.2.3)
      otherwise, T is a specialization of subrange, and U is subrange<iterator_t<T>>;