[const.iterators]

# 24 Iterators library [[iterators]](./#iterators)

## 24.5 Iterator adaptors [[predef.iterators]](predef.iterators#const.iterators)

### 24.5.3 Constant iterators and sentinels [const.iterators]

#### [24.5.3.1](#general) General [[const.iterators.general]](const.iterators.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4179)

Class template basic_const_iterator is an iterator adaptor
with the same behavior as the underlying iterator
except that its indirection operator implicitly converts
the value returned by the underlying iterator's indirection operator
to a type such that the adapted iterator is
a constant iterator ([[iterator.requirements]](iterator.requirements "24.3 Iterator requirements"))[.](#general-1.sentence-1)

Some generic algorithms can be called with constant iterators to avoid mutation[.](#general-1.sentence-2)

[2](#general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4188)

Specializations of basic_const_iterator are constant iterators[.](#general-2.sentence-1)

#### [24.5.3.2](#alias) Alias templates [[const.iterators.alias]](const.iterators.alias)

[🔗](#concept:constant-iterator)

`template<[indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]") It>
  using [iter_const_reference_t](#lib:iter_const_reference_t "24.5.3.2 Alias templates [const.iterators.alias]") =
    common_reference_t<const iter_value_t<It>&&, iter_reference_t<It>>;

template<class It>
  concept [constant-iterator](#concept:constant-iterator "24.5.3.2 Alias templates [const.iterators.alias]") =                                                   // exposition only
    [input_iterator](iterator.concept.input#concept:input_iterator "24.3.4.9 Concept input_­iterator [iterator.concept.input]")<It> && [same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<iter_const_reference_t<It>, iter_reference_t<It>>;

template<[input_iterator](iterator.concept.input#concept:input_iterator "24.3.4.9 Concept input_­iterator [iterator.concept.input]") I>
  using [const_iterator](#lib:const_iterator "24.5.3.2 Alias templates [const.iterators.alias]") = see below;
`

[1](#alias-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4207)

*Result*: If I models [*constant-iterator*](#concept:constant-iterator "24.5.3.2 Alias templates [const.iterators.alias]"), I[.](#alias-1.sentence-1)

Otherwise, basic_const_iterator<I>[.](#alias-1.sentence-2)

[🔗](#alias-itemdecl:2)

`template<[semiregular](concepts.object#concept:semiregular "18.6 Object concepts [concepts.object]") S>
  using [const_sentinel](#lib:const_sentinel "24.5.3.2 Alias templates [const.iterators.alias]") = see below;
`

[2](#alias-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4219)

*Result*: If S models [input_iterator](iterator.concept.input#concept:input_iterator "24.3.4.9 Concept input_­iterator [iterator.concept.input]"), const_iterator<S>[.](#alias-2.sentence-1)

Otherwise, S[.](#alias-2.sentence-2)

#### [24.5.3.3](#iterator) Class template basic_const_iterator [[const.iterators.iterator]](const.iterators.iterator)

namespace std {template<class I>concept [*not-a-const-iterator*](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") = *see below*; // *exposition only*template<[indirectly_readable](iterator.concept.readable#concept:indirectly_readable "24.3.4.2 Concept indirectly_­readable [iterator.concept.readable]") I>using *iter-const-rvalue-reference-t* = // *exposition only* common_reference_t<const iter_value_t<I>&&, iter_rvalue_reference_t<I>>; template<[input_iterator](iterator.concept.input#concept:input_iterator "24.3.4.9 Concept input_­iterator [iterator.concept.input]") Iterator>class [basic_const_iterator](#lib:basic_const_iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") { Iterator *current_* = Iterator(); // *exposition only*using *reference* = iter_const_reference_t<Iterator>; // *exposition only*using *rvalue-reference* = // *exposition only**iter-const-rvalue-reference-t*<Iterator>; public:using iterator_concept = *see below*; using iterator_category = *see below*; // not always presentusing value_type = iter_value_t<Iterator>; using difference_type = iter_difference_t<Iterator>;

 basic_const_iterator() requires [default_initializable](concept.default.init#concept:default_initializable "18.4.12 Concept default_­initializable [concept.default.init]")<Iterator> = default; constexpr basic_const_iterator(Iterator current); template<[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<Iterator> U>constexpr basic_const_iterator(basic_const_iterator<U> current); template<[*different-from*](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> T>requires [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<T, Iterator>constexpr basic_const_iterator(T&& current); constexpr const Iterator& base() const & noexcept; constexpr Iterator base() &&; constexpr *reference* operator*() const; constexpr const auto* operator->() constrequires is_lvalue_reference_v<iter_reference_t<Iterator>> &&[same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<remove_cvref_t<iter_reference_t<Iterator>>, value_type>; constexpr basic_const_iterator& operator++(); constexpr void operator++(int); constexpr basic_const_iterator operator++(int) requires [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11 Concept forward_­iterator [iterator.concept.forward]")<Iterator>; constexpr basic_const_iterator& operator--() requires [bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]")<Iterator>; constexpr basic_const_iterator operator--(int) requires [bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]")<Iterator>; constexpr basic_const_iterator& operator+=(difference_type n)requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr basic_const_iterator& operator-=(difference_type n)requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr *reference* operator[](difference_type n) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; template<[sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<Iterator> S>constexpr bool operator==(const S& s) const; template<[*not-a-const-iterator*](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") CI>requires [*constant-iterator*](#concept:constant-iterator "24.5.3.2 Alias templates [const.iterators.alias]")<CI> && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<Iterator const&, CI>constexpr operator CI() const &; template<[*not-a-const-iterator*](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") CI>requires [*constant-iterator*](#concept:constant-iterator "24.5.3.2 Alias templates [const.iterators.alias]")<CI> && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<Iterator, CI>constexpr operator CI() &&; constexpr bool operator<(const basic_const_iterator& y) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr bool operator>(const basic_const_iterator& y) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr bool operator<=(const basic_const_iterator& y) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr bool operator>=(const basic_const_iterator& y) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; constexpr auto operator<=>(const basic_const_iterator& y) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [three_way_comparable](cmp.concept#concept:three_way_comparable "17.12.4 Concept three_­way_­comparable [cmp.concept]")<Iterator>; template<[*different-from*](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I>constexpr bool operator<(const I& y) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[*different-from*](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I>constexpr bool operator>(const I& y) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[*different-from*](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I>constexpr bool operator<=(const I& y) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[*different-from*](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I>constexpr bool operator>=(const I& y) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[*different-from*](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I>constexpr auto operator<=>(const I& y) constrequires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I> &&[three_way_comparable_with](cmp.concept#concept:three_way_comparable_with "17.12.4 Concept three_­way_­comparable [cmp.concept]")<Iterator, I>; template<[*not-a-const-iterator*](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I>friend constexpr bool operator<(const I& x, const basic_const_iterator& y)requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[*not-a-const-iterator*](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I>friend constexpr bool operator>(const I& x, const basic_const_iterator& y)requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[*not-a-const-iterator*](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I>friend constexpr bool operator<=(const I& x, const basic_const_iterator& y)requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; template<[*not-a-const-iterator*](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I>friend constexpr bool operator>=(const I& x, const basic_const_iterator& y)requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>; friend constexpr basic_const_iterator operator+(const basic_const_iterator& i,
 difference_type n)requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; friend constexpr basic_const_iterator operator+(difference_type n, const basic_const_iterator& i)requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; friend constexpr basic_const_iterator operator-(const basic_const_iterator& i,
 difference_type n)requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>; template<[sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<Iterator> S>constexpr difference_type operator-(const S& y) const; template<[*not-a-const-iterator*](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") S>requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<S, Iterator>friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y); friend constexpr *rvalue-reference* iter_move(const basic_const_iterator& i)noexcept(noexcept(static_cast<*rvalue-reference*>(ranges::iter_move(i.*current_*)))){return static_cast<*rvalue-reference*>(ranges::iter_move(i.*current_*)); }};}

[1](#iterator-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4353)

Given some type I,
the concept [*not-a-const-iterator*](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") is defined as false if I is a specialization of basic_const_iterator andtrue otherwise[.](#iterator-1.sentence-1)

#### [24.5.3.4](#types) Member types [[const.iterators.types]](const.iterators.types)

[1](#types-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4361)

basic_const_iterator<Iterator>​::​iterator_concept is defined as follows:

- [(1.1)](#types-1.1)

  If Iterator models [contiguous_iterator](iterator.concept.contiguous#concept:contiguous_iterator "24.3.4.14 Concept contiguous_­iterator [iterator.concept.contiguous]"),
then iterator_concept denotes contiguous_iterator_tag[.](#types-1.1.sentence-1)

- [(1.2)](#types-1.2)

  Otherwise, if Iterator models [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]"),
then iterator_concept denotes random_access_iterator_tag[.](#types-1.2.sentence-1)

- [(1.3)](#types-1.3)

  Otherwise, if Iterator models [bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]"),
then iterator_concept denotes bidirectional_iterator_tag[.](#types-1.3.sentence-1)

- [(1.4)](#types-1.4)

  Otherwise, if Iterator models [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11 Concept forward_­iterator [iterator.concept.forward]"),
then iterator_concept denotes forward_iterator_tag[.](#types-1.4.sentence-1)

- [(1.5)](#types-1.5)

  Otherwise, iterator_concept denotes input_iterator_tag[.](#types-1.5.sentence-1)

[2](#types-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4380)

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 Iterator models [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11 Concept forward_­iterator [iterator.concept.forward]")[.](#types-2.sentence-1)

In that case,basic_const_iterator<Iterator>​::​iterator_category denotes
the type iterator_traits<​Iterator>​::​iterator_category[.](#types-2.sentence-2)

#### [24.5.3.5](#ops) Operations [[const.iterators.ops]](const.iterators.ops)

[🔗](#lib:basic_const_iterator,constructor)

`constexpr basic_const_iterator(Iterator current);
`

[1](#ops-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4395)

*Effects*: Initializes *current_* with std​::​move(current)[.](#ops-1.sentence-1)

[🔗](#lib:basic_const_iterator,constructor_)

`template<[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<Iterator> U>
  constexpr basic_const_iterator(basic_const_iterator<U> current);
`

[2](#ops-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4407)

*Effects*: Initializes *current_* with std​::​move(current.*current_*)[.](#ops-2.sentence-1)

[🔗](#lib:basic_const_iterator,constructor__)

`template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> T>
  requires [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<T, Iterator>
  constexpr basic_const_iterator(T&& current);
`

[3](#ops-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4421)

*Effects*: Initializes *current_* with std​::​forward<T>(current)[.](#ops-3.sentence-1)

[🔗](#lib:base,basic_const_iterator)

`constexpr const Iterator& base() const & noexcept;
`

[4](#ops-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4432)

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

[🔗](#lib:base,basic_const_iterator_)

`constexpr Iterator base() &&;
`

[5](#ops-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4443)

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

[🔗](#lib:operator*,basic_const_iterator)

`constexpr reference operator*() const;
`

[6](#ops-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4454)

*Effects*: Equivalent to: return static_cast<*reference*>(**current_*);

[🔗](#lib:operator-%3e,basic_const_iterator)

`constexpr const auto* operator->() const
  requires is_lvalue_reference_v<iter_reference_t<Iterator>> &&
           [same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<remove_cvref_t<iter_reference_t<Iterator>>, value_type>;
`

[7](#ops-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4467)

*Returns*: If Iterator models [contiguous_iterator](iterator.concept.contiguous#concept:contiguous_iterator "24.3.4.14 Concept contiguous_­iterator [iterator.concept.contiguous]"),to_address(*current_*);
otherwise, addressof(**current_*)[.](#ops-7.sentence-1)

[🔗](#lib:operator++,basic_const_iterator)

`constexpr basic_const_iterator& operator++();
`

[8](#ops-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4480)

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

[🔗](#lib:operator++,basic_const_iterator_)

`constexpr void operator++(int);
`

[9](#ops-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4495)

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

[🔗](#lib:operator++,basic_const_iterator__)

`constexpr basic_const_iterator operator++(int) requires [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11 Concept forward_­iterator [iterator.concept.forward]")<Iterator>;
`

[10](#ops-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4506)

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

[🔗](#lib:operator--,basic_const_iterator)

`constexpr basic_const_iterator& operator--() requires [bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]")<Iterator>;
`

[11](#ops-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4522)

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

[🔗](#lib:operator--,basic_const_iterator_)

`constexpr basic_const_iterator operator--(int) requires [bidirectional_iterator](iterator.concept.bidir#concept:bidirectional_iterator "24.3.4.12 Concept bidirectional_­iterator [iterator.concept.bidir]")<Iterator>;
`

[12](#ops-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4537)

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

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

`constexpr basic_const_iterator& operator+=(difference_type n)
  requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;
constexpr basic_const_iterator& operator-=(difference_type n)
  requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;
`

[13](#ops-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4557)

Let *op* be the operator[.](#ops-13.sentence-1)

[14](#ops-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4560)

*Effects*: Equivalent to:*current_* *op* n;return *this;

[🔗](#lib:operator%5b%5d,basic_const_iterator)

`constexpr reference operator[](difference_type n) const requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>
`

[15](#ops-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4575)

*Effects*: Equivalent to: return static_cast<*reference*>(*current_*[n]);

[🔗](#lib:operator==,basic_const_iterator)

`template<[sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<Iterator> S>
  constexpr bool operator==(const S& s) const;
`

[16](#ops-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4587)

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

[🔗](#ops-itemdecl:16)

`template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") CI>
  requires [constant-iterator](#concept:constant-iterator "24.5.3.2 Alias templates [const.iterators.alias]")<CI> && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<Iterator const&, CI>
constexpr operator CI() const &;
`

[17](#ops-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4599)

*Returns*: *current_*[.](#ops-17.sentence-1)

[🔗](#ops-itemdecl:17)

`template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") CI>
  requires [constant-iterator](#concept:constant-iterator "24.5.3.2 Alias templates [const.iterators.alias]")<CI> && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<Iterator, CI>
constexpr operator CI() &&;
`

[18](#ops-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4611)

*Returns*: std​::​move(*current_*)[.](#ops-18.sentence-1)

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

`constexpr bool operator<(const basic_const_iterator& y) const
  requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;
constexpr bool operator>(const basic_const_iterator& y) const
  requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;
constexpr bool operator<=(const basic_const_iterator& y) const
  requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;
constexpr bool operator>=(const basic_const_iterator& y) const
  requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;
constexpr auto operator<=>(const basic_const_iterator& y) const
  requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [three_way_comparable](cmp.concept#concept:three_way_comparable "17.12.4 Concept three_­way_­comparable [cmp.concept]")<Iterator>;
`

[19](#ops-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4635)

Let *op* be the operator[.](#ops-19.sentence-1)

[20](#ops-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4638)

*Effects*: Equivalent to:return *current_* *op* *y.current_*;

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

`template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I>
  constexpr bool operator<(const I& y) const
    requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>;
template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I>
  constexpr bool operator>(const I& y) const
    requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>;
template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I>
  constexpr bool operator<=(const I& y) const
    requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>;
template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I>
  constexpr bool operator>=(const I& y) const
    requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>;
template<[different-from](range.utility.helpers#concept:different-from "25.5.2 Helper concepts [range.utility.helpers]")<basic_const_iterator> I>
  constexpr auto operator<=>(const I& y) const
    requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I> &&
             [three_way_comparable_with](cmp.concept#concept:three_way_comparable_with "17.12.4 Concept three_­way_­comparable [cmp.concept]")<Iterator, I>;
`

[21](#ops-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4669)

Let *op* be the operator[.](#ops-21.sentence-1)

[22](#ops-22)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4672)

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

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

`template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I>
  friend constexpr bool operator<(const I& x, const basic_const_iterator& y)
    requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>;
template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I>
  friend constexpr bool operator>(const I& x, const basic_const_iterator& y)
    requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>;
template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I>
  friend constexpr bool operator<=(const I& x, const basic_const_iterator& y)
    requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>;
template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") I>
  friend constexpr bool operator>=(const I& x, const basic_const_iterator& y)
    requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator> && [totally_ordered_with](concept.totallyordered#concept:totally_ordered_with "18.5.5 Concept totally_­ordered [concept.totallyordered]")<Iterator, I>;
`

[23](#ops-23)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4697)

Let *op* be the operator[.](#ops-23.sentence-1)

[24](#ops-24)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4700)

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

[🔗](#lib:operator+,basic_const_iterator)

`friend constexpr basic_const_iterator operator+(const basic_const_iterator& i, difference_type n)
  requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;
friend constexpr basic_const_iterator operator+(difference_type n, const basic_const_iterator& i)
  requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;
`

[25](#ops-25)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4714)

*Effects*: Equivalent to: return basic_const_iterator(i.*current_* + n);

[🔗](#lib:operator-,basic_const_iterator)

`friend constexpr basic_const_iterator operator-(const basic_const_iterator& i, difference_type n)
  requires [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13 Concept random_­access_­iterator [iterator.concept.random.access]")<Iterator>;
`

[26](#ops-26)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4726)

*Effects*: Equivalent to: return basic_const_iterator(i.*current_* - n);

[🔗](#lib:operator-,basic_const_iterator_)

`template<[sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<Iterator> S>
  constexpr difference_type operator-(const S& y) const;
`

[27](#ops-27)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4738)

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

[🔗](#lib:operator-,basic_const_iterator__)

`template<[not-a-const-iterator](#concept:not-a-const-iterator "24.5.3.3 Class template basic_­const_­iterator [const.iterators.iterator]") S>
  requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<S, Iterator>
  friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y);
`

[28](#ops-28)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iterators.tex#L4751)

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