[range.transform]

# 25 Ranges library [[ranges]](./#ranges)

## 25.7 Range adaptors [[range.adaptors]](range.adaptors#range.transform)

### 25.7.9 Transform view [range.transform]

#### [25.7.9.1](#overview) Overview [[range.transform.overview]](range.transform.overview)

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5177)

transform_view presents
a view of an underlying sequence after
applying a transformation function to each element[.](#overview-1.sentence-1)

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5182)

The name views​::​transform denotes a
range adaptor object ([[range.adaptor.object]](range.adaptor.object "25.7.2 Range adaptor objects"))[.](#overview-2.sentence-1)

Given subexpressions E and F, the expressionviews​::​transform(E, F) is expression-equivalent totransform_view(E, F)[.](#overview-2.sentence-2)

[3](#overview-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5190)

[*Example [1](#overview-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](#view) Class template transform_view [[range.transform.view]](range.transform.view)

[🔗](#lib:transform_view)

namespace std::ranges {template<[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]") V, [move_constructible](concept.moveconstructible#concept:move_constructible "18.4.13 Concept move_­constructible [concept.moveconstructible]") F>requires [view](range.view#concept:view "25.4.5 Views [range.view]")<V> && is_object_v<F> &&[regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<F&, range_reference_t<V>> &&[*can-reference*](iterator.synopsis#concept:can-reference "24.2 Header <iterator>&nbsp;synopsis [iterator.synopsis]")<invoke_result_t<F&, range_reference_t<V>>>class transform_view : public view_interface<transform_view<V, F>> {private:// [[range.transform.iterator]](#iterator "25.7.9.3 Class template transform_­view​::​iterator"), class template transform_view​::​*iterator*template<bool> struct *iterator*; // *exposition only*// [[range.transform.sentinel]](#sentinel "25.7.9.4 Class template transform_­view​::​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](concept.default.init#concept:default_initializable "18.4.12 Concept default_­initializable [concept.default.init]")<V> && [default_initializable](concept.default.init#concept:default_initializable "18.4.12 Concept default_­initializable [concept.default.init]")<F> = default; constexpr explicit transform_view(V base, F fun); constexpr V base() const & requires [copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")<V> { return *base_*; }constexpr V base() && { return std::move(*base_*); }constexpr *iterator*<false> begin(); constexpr *iterator*<true> begin() constrequires [range](range.range#concept:range "25.4.2 Ranges [range.range]")<const V> &&[regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<const F&, range_reference_t<const V>>; constexpr *sentinel*<false> end(); constexpr *iterator*<false> end() requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<V>; constexpr *sentinel*<true> end() constrequires [range](range.range#concept:range "25.4.2 Ranges [range.range]")<const V> &&[regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<const F&, range_reference_t<const V>>; constexpr *iterator*<true> end() constrequires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<const V> &&[regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<const F&, range_reference_t<const V>>; constexpr auto size() requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<V> { return ranges::size(*base_*); }constexpr auto size() const requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V>{ return ranges::size(*base_*); }constexpr auto reserve_hint() requires [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")<V>{ return ranges::reserve_hint(*base_*); }constexpr auto reserve_hint() const requires [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")<const V>{ return ranges::reserve_hint(*base_*); }}; template<class R, class F> transform_view(R&&, F) -> transform_view<views::all_t<R>, F>;}

[🔗](#lib:transform_view,constructor)

`constexpr explicit transform_view(V base, F fun);
`

[1](#view-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5264)

*Effects*: Initializes *base_* with std​::​move(base) and*fun_* with std​::​move(fun)[.](#view-1.sentence-1)

[🔗](#lib:begin,transform_view)

`constexpr iterator<false> begin();
`

[2](#view-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5276)

*Effects*: Equivalent to:return *iterator*<false>{*this, ranges::begin(*base_*)};

[🔗](#lib:begin,transform_view_)

`constexpr iterator<true> begin() const
  requires [range](range.range#concept:range "25.4.2 Ranges [range.range]")<const V> &&
           [regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<const F&, range_reference_t<const V>>;
`

[3](#view-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5292)

*Effects*: Equivalent to:return *iterator*<true>{*this, ranges::begin(*base_*)};

[🔗](#lib:end,transform_view)

`constexpr sentinel<false> end();
`

[4](#view-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5306)

*Effects*: Equivalent to:return *sentinel*<false>{ranges::end(*base_*)};

[🔗](#lib:end,transform_view_)

`constexpr iterator<false> end() requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<V>;
`

[5](#view-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5320)

*Effects*: Equivalent to:return *iterator*<false>{*this, ranges::end(*base_*)};

[🔗](#lib:end,transform_view__)

`constexpr sentinel<true> end() const
  requires [range](range.range#concept:range "25.4.2 Ranges [range.range]")<const V> &&
           [regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<const F&, range_reference_t<const V>>;
`

[6](#view-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5336)

*Effects*: Equivalent to:return *sentinel*<true>{ranges::end(*base_*)};

[🔗](#lib:end,transform_view___)

`constexpr iterator<true> end() const
  requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<const V> &&
           [regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<const F&, range_reference_t<const V>>;
`

[7](#view-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5352)

*Effects*: Equivalent to:return *iterator*<true>{*this, ranges::end(*base_*)};

#### [25.7.9.3](#iterator) Class template transform_view​::​*iterator* [[range.transform.iterator]](range.transform.iterator)

[🔗](#lib:transform_view::iterator)

namespace std::ranges {template<[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]") V, [move_constructible](concept.moveconstructible#concept:move_constructible "18.4.13 Concept move_­constructible [concept.moveconstructible]") F>requires [view](range.view#concept:view "25.4.5 Views [range.view]")<V> && is_object_v<F> &&[regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<F&, range_reference_t<V>> &&[*can-reference*](iterator.synopsis#concept:can-reference "24.2 Header <iterator>&nbsp;synopsis [iterator.synopsis]")<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 presentusing 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](concept.default.init#concept:default_initializable "18.4.12 Concept default_­initializable [concept.default.init]")<iterator_t<*Base*>> = default; constexpr *iterator*(*Parent*& parent, iterator_t<*Base*> current); constexpr *iterator*(*iterator*<!Const> i)requires Const && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<iterator_t<V>, iterator_t<*Base*>>; constexpr const iterator_t<*Base*>& base() const & noexcept; constexpr iterator_t<*Base*> base() &&; constexpr decltype(auto) operator*() constnoexcept(noexcept(invoke(**parent_*->*fun_*, **current_*))) {return invoke(**parent_*->*fun_*, **current_*); }constexpr *iterator*& operator++(); constexpr void operator++(int); constexpr *iterator* operator++(int) requires [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; constexpr *iterator*& operator--() requires [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; constexpr *iterator* operator--(int) requires [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; constexpr *iterator*& operator+=(difference_type n)requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; constexpr *iterator*& operator-=(difference_type n)requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; constexpr decltype(auto) operator[](difference_type n) constrequires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*> {return invoke(**parent_*->*fun_*, *current_*[n]); }friend constexpr bool operator==(const *iterator*& x, const *iterator*& y)requires [equality_comparable](concept.equalitycomparable#concept:equality_comparable "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")<iterator_t<*Base*>>; friend constexpr bool operator<(const *iterator*& x, const *iterator*& y)requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; friend constexpr bool operator>(const *iterator*& x, const *iterator*& y)requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; friend constexpr bool operator<=(const *iterator*& x, const *iterator*& y)requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; friend constexpr bool operator>=(const *iterator*& x, const *iterator*& y)requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; friend constexpr auto operator<=>(const *iterator*& x, const *iterator*& y)requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*> && [three_way_comparable](cmp.concept#concept:three_way_comparable "17.12.4 Concept three_­way_­comparable [cmp.concept]")<iterator_t<*Base*>>; friend constexpr *iterator* operator+(*iterator* i, difference_type n)requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; friend constexpr *iterator* operator+(difference_type n, *iterator* i)requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; friend constexpr *iterator* operator-(*iterator* i, difference_type n)requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<*Base*>; friend constexpr difference_type operator-(const *iterator*& x, const *iterator*& y)requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<iterator_t<*Base*>, iterator_t<*Base*>>; };}

[1](#iterator-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5441)

*iterator*​::​iterator_concept is defined as follows:

- [(1.1)](#iterator-1.1)

  If *Base* models [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]"), theniterator_concept denotes random_access_iterator_tag[.](#iterator-1.1.sentence-1)

- [(1.2)](#iterator-1.2)

  Otherwise, if *Base* models [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]"), theniterator_concept denotes bidirectional_iterator_tag[.](#iterator-1.2.sentence-1)

- [(1.3)](#iterator-1.3)

  Otherwise, if *Base* models [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]"), theniterator_concept denotes forward_iterator_tag[.](#iterator-1.3.sentence-1)

- [(1.4)](#iterator-1.4)

  Otherwise, iterator_concept denotes input_iterator_tag[.](#iterator-1.4.sentence-1)

[2](#iterator-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5456)

The member [*typedef-name*](dcl.typedef#nt:typedef-name "9.2.4 The typedef specifier [dcl.typedef]") iterator_category is defined
if and only if *Base* models [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")[.](#iterator-2.sentence-1)

In that case,*iterator*​::​iterator_category is defined as follows:
Let C denote the typeiterator_traits<iterator_t<*Base*>>​::​iterator_category[.](#iterator-2.sentence-2)

- [(2.1)](#iterator-2.1)

  If is_reference_v<invoke_result_t<*maybe-const*<Const, F>&, range_reference_t<*Base*>>> is true, then
  * [(2.1.1)](#iterator-2.1.1)

if C models [derived_from](concept.derived#concept:derived_from "18.4.3 Concept derived_­from [concept.derived]")<contiguous_iterator_tag>,iterator_category denotes random_access_iterator_tag;

  * [(2.1.2)](#iterator-2.1.2)

otherwise,iterator_category denotes C[.](#iterator-2.1.sentence-1)

- [(2.2)](#iterator-2.2)

  Otherwise, iterator_category denotes input_iterator_tag[.](#iterator-2.2.sentence-1)

[🔗](#lib:iterator,transform_view::iterator)

`constexpr iterator(Parent& parent, iterator_t<Base> current);
`

[3](#iterator-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5487)

*Effects*: Initializes *current_* with std​::​move(current) and*parent_* with addressof(parent)[.](#iterator-3.sentence-1)

[🔗](#lib:transform_view::iterator,constructor)

`constexpr iterator(iterator<!Const> i)
  requires Const && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<iterator_t<V>, iterator_t<Base>>;
`

[4](#iterator-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5500)

*Effects*: Initializes *current_* with std​::​move(i.*current_*) and*parent_* with i.*parent_*[.](#iterator-4.sentence-1)

[🔗](#lib:base,transform_view::iterator)

`constexpr const iterator_t<Base>& base() const & noexcept;
`

[5](#iterator-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5512)

*Effects*: Equivalent to: return *current_*;

[🔗](#lib:base,transform_view::iterator_)

`constexpr iterator_t<Base> base() &&;
`

[6](#iterator-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5523)

*Effects*: Equivalent to: return std​::​move(*current_*);

[🔗](#lib:operator++,transform_view::iterator)

`constexpr iterator& operator++();
`

[7](#iterator-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5534)

*Effects*: Equivalent to:++*current_*;return *this;

[🔗](#lib:operator++,transform_view::iterator_)

`constexpr void operator++(int);
`

[8](#iterator-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5549)

*Effects*: Equivalent to ++*current_*[.](#iterator-8.sentence-1)

[🔗](#lib:operator++,transform_view::iterator__)

`constexpr iterator operator++(int) requires [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[9](#iterator-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5560)

*Effects*: Equivalent to:auto tmp = *this;++*this;return tmp;

[🔗](#lib:operator--,transform_view::iterator)

`constexpr iterator& operator--() requires [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[10](#iterator-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5576)

*Effects*: Equivalent to:--*current_*;return *this;

[🔗](#lib:operator--,transform_view::iterator_)

`constexpr iterator operator--(int) requires [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[11](#iterator-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5591)

*Effects*: Equivalent to:auto tmp = *this;--*this;return tmp;

[🔗](#lib:operator+=,transform_view::iterator)

`constexpr iterator& operator+=(difference_type n)
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[12](#iterator-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5608)

*Effects*: Equivalent to:*current_* += n;return *this;

[🔗](#lib:operator-=,transform_view::iterator)

`constexpr iterator& operator-=(difference_type n)
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[13](#iterator-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5624)

*Effects*: Equivalent to:*current_* -= n;return *this;

[🔗](#lib:operator==,transform_view::iterator)

`friend constexpr bool operator==(const iterator& x, const iterator& y)
  requires [equality_comparable](concept.equalitycomparable#concept:equality_comparable "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")<iterator_t<Base>>;
`

[14](#iterator-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5640)

*Effects*: Equivalent to: return x.*current_* == y.*current_*;

[🔗](#lib:operator%3c,transform_view::iterator)

`friend constexpr bool operator<(const iterator& x, const iterator& y)
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[15](#iterator-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5652)

*Effects*: Equivalent to: return x.*current_* < y.*current_*;

[🔗](#lib:operator%3e,transform_view::iterator)

`friend constexpr bool operator>(const iterator& x, const iterator& y)
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[16](#iterator-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5664)

*Effects*: Equivalent to: return y < x;

[🔗](#lib:operator%3c=,transform_view::iterator)

`friend constexpr bool operator<=(const iterator& x, const iterator& y)
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[17](#iterator-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5676)

*Effects*: Equivalent to: return !(y < x);

[🔗](#lib:operator%3e=,transform_view::iterator)

`friend constexpr bool operator>=(const iterator& x, const iterator& y)
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[18](#iterator-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5688)

*Effects*: Equivalent to: return !(x < y);

[🔗](#lib:operator%3c=%3e,transform_view::iterator)

`friend constexpr auto operator<=>(const iterator& x, const iterator& y)
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base> && [three_way_comparable](cmp.concept#concept:three_way_comparable "17.12.4 Concept three_­way_­comparable [cmp.concept]")<iterator_t<Base>>;
`

[19](#iterator-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5700)

*Effects*: Equivalent to: return x.*current_* <=> y.*current_*;

[🔗](#lib:operator+,transform_view::iterator)

`friend constexpr iterator operator+(iterator i, difference_type n)
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;
friend constexpr iterator operator+(difference_type n, iterator i)
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[20](#iterator-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5714)

*Effects*: Equivalent to: return *iterator*{*i.*parent_*, i.*current_* + n};

[🔗](#lib:operator-,transform_view::iterator)

`friend constexpr iterator operator-(iterator i, difference_type n)
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;
`

[21](#iterator-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5726)

*Effects*: Equivalent to: return *iterator*{*i.*parent_*, i.*current_* - n};

[🔗](#lib:operator-,transform_view::iterator_)

`friend constexpr difference_type operator-(const iterator& x, const iterator& y)
  requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<iterator_t<Base>, iterator_t<Base>>;
`

[22](#iterator-22)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5738)

*Effects*: Equivalent to: return x.*current_* - y.*current_*;

#### [25.7.9.4](#sentinel) Class template transform_view​::​*sentinel* [[range.transform.sentinel]](range.transform.sentinel)

[🔗](#lib:transform_view::sentinel)

namespace std::ranges {template<[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]") V, [move_constructible](concept.moveconstructible#concept:move_constructible "18.4.13 Concept move_­constructible [concept.moveconstructible]") F>requires [view](range.view#concept:view "25.4.5 Views [range.view]")<V> && is_object_v<F> &&[regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<F&, range_reference_t<V>> &&[*can-reference*](iterator.synopsis#concept:can-reference "24.2 Header <iterator>&nbsp;synopsis [iterator.synopsis]")<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](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<sentinel_t<V>, sentinel_t<*Base*>>; constexpr sentinel_t<*Base*> base() const; template<bool OtherConst>requires [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<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](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<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](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<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); };}

[🔗](#lib:transform_view::sentinel,constructor)

`constexpr explicit sentinel(sentinel_t<Base> end);
`

[1](#sentinel-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5791)

*Effects*: Initializes *end_* with end[.](#sentinel-1.sentence-1)

[🔗](#lib:transform_view::sentinel,constructor_)

`constexpr sentinel(sentinel<!Const> i)
  requires Const && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<sentinel_t<V>, sentinel_t<Base>>;
`

[2](#sentinel-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5803)

*Effects*: Initializes *end_* with std​::​move(i.*end_*)[.](#sentinel-2.sentence-1)

[🔗](#lib:base,transform_view::sentinel)

`constexpr sentinel_t<Base> base() const;
`

[3](#sentinel-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5814)

*Effects*: Equivalent to: return *end_*;

[🔗](#lib:operator==,transform_view::sentinel)

`template<bool OtherConst>
  requires [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
`

[4](#sentinel-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5827)

*Effects*: Equivalent to: return x.*current_* == y.*end_*;

[🔗](#lib:operator-,transform_view::sentinel)

`template<bool OtherConst>
  requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<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](#sentinel-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5841)

*Effects*: Equivalent to: return x.*current_* - y.*end_*;

[🔗](#lib:operator-,transform_view::sentinel_)

`template<bool OtherConst>
  requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<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](#sentinel-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L5855)

*Effects*: Equivalent to: return y.*end_* - x.*current_*;