std::ranges::views::iota, std::ranges::iota_view, std::ranges::views::indices

 

C++

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Feature test macros (C++20)

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Ranges library (C++20)

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Technical specifications

Symbols index

External libraries

 

Ranges library

Range access begin cbegin      end cend rbegin crbegin      rend crend size ssize      data cdata reserve_hint (C++26)         empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)         to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t      elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range      common_range	sized_range viewable_range      view	input_range output_range forward_range	bidirectional_range      random_access_range contiguous_range	approximately_sized_range (C++26) constant_range (C++23) sized-random-access-range (C++26)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota views::indices (C++26)	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all as_rvalue_viewviews::as_rvalue (C++23)(C++23)     filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while common_viewviews::common views::counted as_input_viewviews::as_input (C++26)(C++26)	ref_view drop_viewviews::drop drop_while_viewviews::drop_while lazy_split_viewviews::lazy_split split_viewviews::split join_viewviews::join join_with_viewviews::join_with (C++23)(C++23) concat_viewviews::concat (C++26)(C++26) cache_latest_viewviews::cache_latest (C++26)(C++26)	owning_view reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)    (C++23)	simple-view non-propagating-cache

 

std::ranges::iota_view

Member functions

iota_view::iota_view

iota_view::begin

iota_view::end

iota_view::empty

iota_view::size

Deduction guides

Nested classes

Iterator

Helper concepts

decrementable

advanceable

Member types

Member functions

iota_view::iterator::iterator

iota_view::iterator::operator*

iota_view::iterator::operator[]

iota_view::iterator::operator++ iota_view::iterator::operator++(int)

iota_view::iterator::operator-- iota_view::iterator::operator--(int)

iota_view::iterator::operator+=

iota_view::iterator::operator-=

Non-member functions

operator==(iota_view::iterator) operator<(iota_view::iterator) operator>(iota_view::iterator) operator<=(iota_view::iterator) operator>=(iota_view::iterator) operator<=>(iota_view::iterator)

operator+(iota_view::iterator)

operator-(iota_view::iterator)

Sentinel

Member functions

iota_view::sentinel::sentinel

Non-member functions

operator==(iota_view::iterator,iota_view::sentinel)

operator-(iota_view::iterator,iota_view::sentinel)

 

Defined in header <ranges>
template< std::weakly_incrementable W, std::semiregular Bound = std::unreachable_sentinel_t > requires /*weakly-equality-comparable-with*/<W, Bound> && std::copyable<W> class iota_view : public ranges::view_interface<iota_view<W, Bound>>	(1)	(since C++20)
namespace views { inline constexpr /* unspecified */ iota = /* unspecified */; }	(2)	(since C++20)
namespace views { inline constexpr /* unspecified */ indices = /* unspecified */; }	(3)	(since C++26)
Call signature
template< class W > requires /* see below */ constexpr /* see below */ iota( W&& value );		(since C++20)
template< class W, class Bound > requires /* see below */ constexpr /* see below */ iota( W&& value, Bound&& bound );		(since C++20)
template< class T > requires /*is-integer-like*/<T> constexpr /* see below */ indices( T bound );		(since C++26)

1) A range factory that generates a sequence of elements by repeatedly incrementing an initial value. Can be either bounded or unbounded (infinite).

2) views::iota(e) and views::iota(e, f) are expression-equivalent to iota_view<std::decay_t<decltype((e))>>(e) and iota_view(e, f) respectively for any suitable subexpressions e and f.

3) Let T be std::remove_cvref_t<decltype((e))>. Then views::indices(e) is expression-equivalent to views::iota(T(0), e) for any suitable subexpression e if /*is-integer-like*/<T> is true.

Customization point objects

See More

The name views::iota denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

The name views::indices denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

(since C++26)

Data members

Member	Definition
W value_	the beginning value (exposition-only member object*)
Bound bound_	the sentinel value, may be unreachable (exposition-only member object*)

Member functions

(constructor)	creates an iota_view (public member function)
begin	obtains the beginning iterator of an iota_view (public member function)
end	obtains the sentinel denoting the end of an iota_view (public member function)
empty	tests whether the iota_view is empty (i.e. the iterator and the sentinel compare equal) (public member function)
size (optional)	obtains the size of an iota_view (only provided if it is bounded) (public member function)
Inherited from ranges::view_interface
cbegin (C++23)	returns a constant iterator to the beginning of the range (public member function of std::ranges::view_interface<D>)
cend (C++23)	returns a sentinel for the constant iterator of the range (public member function of std::ranges::view_interface<D>)
operator bool	returns whether the derived view is not empty, provided only if ranges::empty is applicable to it (public member function of std::ranges::view_interface<D>)
front	returns the first element in the derived view, provided if it satisfies forward_range (public member function of std::ranges::view_interface<D>)
back	returns the last element in the derived view, provided only if it satisfies bidirectional_range and common_range (public member function of std::ranges::view_interface<D>)
operator[]	returns the nth element in the derived view, provided only if it satisfies random_access_range (public member function of std::ranges::view_interface<D>)

Deduction guides

Nested classes

iterator	the iterator type (exposition-only member class*)
sentinel	the sentinel type used when the iota_view is bounded and Bound and W are not the same type (exposition-only member class*)

Helper templates

template< std::weakly_incrementable W, std::semiregular Bound > constexpr bool ranges::enable_borrowed_range<ranges::iota_view<W, Bound>> = true;		(since C++20)

This specialization of ranges::enable_borrowed_range makes iota_view satisfy borrowed_range.

Possible implementation

See also the implementation in libstdc++.

views::indices (3)
namespace views { inline constexpr auto indices = []</*is-integer-like*/ I>(I n) { return views::iota(I{}, n); }; }

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_ranges_indices	202506L	(C++26)	ranges::views::indices

Example

Run this code

#include <algorithm>
#include <iostream>
#include <ranges>
    
struct Bound
{
    int bound;
    bool operator==(int x) const { return x == bound; }
};

int main()
{
    for (int i : std::ranges::iota_view{1, 10})
        std::cout << i << ' ';
    std::cout << '\n';
    
    for (int i : std::views::iota(1, 10))
        std::cout << i << ' ';
    std::cout << '\n';
    
    for (int i : std::views::iota(1, Bound{10}))
        std::cout << i << ' ';
    std::cout << '\n';
    
    for (int i : std::views::iota(1) | std::views::take(9))
        std::cout << i << ' ';
    std::cout << '\n';
    
    std::ranges::for_each(std::views::iota(1, 10),
                          [](int i){ std::cout << i << ' '; });
    std::cout << '\n';

#if __cpp_lib_ranges_indices
    constexpr int v[]{1, 2, 3, 4, 5, 6, 7, 8, 9};

    // auto broken_indices = views::iota(0, std::ranges::size(v));
    //     does not compile due to types (int VS size_t) mismatch

    auto good_indices = std::views::indices(std::ranges::size(v));

    for (auto i : good_indices)
        std::cout << v[i] << ' ';
    std::cout << '\n';
#endif
}

Output:

1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 4096	C++20	views::iota could copy an iota_view as-is	forbidden
P2325R3	C++20	iota_view required that W is semiregular as view required default_initializable	only requires that W is copyable

References

• C++26 standard (ISO/IEC 14882:2026):

• 25.6.4 Iota view [range.iota]

• C++23 standard (ISO/IEC 14882:2024):

• 26.6.4 Iota view [range.iota]

• C++20 standard (ISO/IEC 14882:2020):

• 24.6.4 Iota view [range.iota]

See also

iota (C++11)	fills a range with successive increments of the starting value (function template & algorithm function object)
ranges::iota (C++23)
ranges::iota (C++23)	fills a range with successive increments of the starting value (algorithm function object)
ranges::repeat_viewviews::repeat (C++23)	a view consisting of a generated sequence by repeatedly producing the same value (class template) (customization point object)
ranges::enumerate_viewviews::enumerate (C++23)	a view that maps each element of adapted sequence to a tuple of both the element's position and its value (class template) (range adaptor object)