[range.transform.iterator]

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

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

### 25.7.9 Transform view [[range.transform]](range.transform#iterator)

#### 25.7.9.3 Class template transform_view​::​*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](#1)

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

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

- [(1.1)](#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[.](#1.1.sentence-1)

- [(1.2)](#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[.](#1.2.sentence-1)

- [(1.3)](#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[.](#1.3.sentence-1)

- [(1.4)](#1.4)

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

[2](#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]")[.](#2.sentence-1)

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

- [(2.1)](#2.1)

  If is_reference_v<invoke_result_t<*maybe-const*<Const, F>&, range_reference_t<*Base*>>> is true, then
  * [(2.1.1)](#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)](#2.1.2)

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

- [(2.2)](#2.2)

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

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

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

[3](#3)

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

*Effects*: Initializes *current_* with std​::​move(current) and*parent_* with addressof(parent)[.](#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](#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_*[.](#4.sentence-1)

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

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

[5](#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](#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](#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](#8)

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

*Effects*: Equivalent to ++*current_*[.](#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](#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](#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](#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](#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](#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](#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](#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](#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](#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](#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](#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](#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](#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](#22)

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

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