std::ranges::views::enumerate,std::ranges::enumerate_view

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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)

empty_viewviews::empty

single_viewviews::single

basic_istream_viewviews::istream

iota_viewviews::iota

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
to_input_viewviews::to_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

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std::ranges::enumerate_view

Member functions
enumerate_view::enumerate_view
enumerate_view::base
enumerate_view::begin
enumerate_view::end
enumerate_view::size
enumerate_view::reserve_hint (C++26)
Deduction guides
Nested classes
Iterator
enumerate_view::iterator::iterator
enumerate_view::iterator::base
enumerate_view::iterator::index
enumerate_view::iterator::operator*
enumerate_view::iterator::operator[]
enumerate_view::iterator::operator++ enumerate_view::iterator::operator++(int) enumerate_view::iterator::operator-- enumerate_view::iterator::operator--(int) enumerate_view::iterator::operator+= enumerate_view::iterator::operator-=
operator==(enumerate_view::iterator) operator<=>(enumerate_view::iterator)
operator+(enumerate_view::iterator) operator-(enumerate_view::iterator)
iter_move(enumerate_view::iterator)
Sentinel
enumerate_view::sentinel::sentinel
enumerate_view::sentinel::base
operator==(enumerate_view::iterator,enumerate_view::sentinel)
operator-(enumerate_view::iterator,enumerate_view::sentinel)

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Defined in header`<ranges>`
template<ranges::view V> requires/range-with-movable-references/<V> class enumerate_view :publicranges::view_interface<enumerate_view<V>>	(1)	(since C++23)
namespace views{ inlineconstexpr/* unspecified / enumerate=/ unspecified */; }	(2)	(since C++23)
Call signature
template<ranges::viewable_range R> requires/* see below / constexpr/ see below */ enumerate( R&& r);		(since C++23)
Helper concepts
template<class R> concept/range-with-movable-references/= ranges::input_range<R>&& std::move_constructible<ranges::range_reference_t<R>>&& std::move_constructible<ranges::range_rvalue_reference_t<R>>;	(3)	(exposition only*)

1)enumerate_view is a range adaptor that takes aview and produces a view oftuples.i^th element (the tuple) of the resulting sequence holds:

the value equal toi, which is a zero-based index of the element of underlying sequence, and
the reference to the underlying element.

2) The nameviews::enumerate denotes aRangeAdaptorObject. Given a subexpressione, the expressionviews::enumerate(e) isexpression-equivalent toenumerate_view<views::all_t<decltype((e))>>(e) for any suitable subexpressione.

3) Ensures that the reference type of the underlying type can be moved.

enumerate_view models the conceptsrandom_access_range,bidirectional_range,forward_range,input_range,common_range, andsized_range when the underlying viewV models respective concepts.

[edit]Data members

Member	Description
`Vbase_`	an iterator to the underlying`view` (exposition-only member object*)

[edit]Member functions

(constructor)	constructs a`enumerate_view` (public member function)[edit]
base	returns a copy of the underlying (adapted) view (public member function)[edit]
begin	returns an iterator to the beginning (public member function)[edit]
end	returns an iterator or a sentinel to the end (public member function)[edit]
size	returns the number of elements, provided only if the underlying (adapted) range satisfies`sized_range` (public member function)[edit]
reserve_hint (C++26)	returns the approximate size of the resulting`approximately_sized_range` (public member function)[edit]
Inherited fromstd::ranges::view_interface
empty	returns whether the derived view is empty, provided only if it satisfies`sized_range` or`forward_range` (public member function of`std::ranges::view_interface<D>`)[edit]
cbegin (C++23)	returns a constant iterator to the beginning of the range (public member function of`std::ranges::view_interface<D>`)[edit]
cend (C++23)	returns a sentinel for the constant iterator of the range (public member function of`std::ranges::view_interface<D>`)[edit]
operator bool	returns whether the derived view is not empty, provided only ifranges::empty is applicable to it (public member function of`std::ranges::view_interface<D>`)[edit]
front	returns the first element in the derived view, provided if it satisfies`forward_range` (public member function of`std::ranges::view_interface<D>`)[edit]
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>`)[edit]
operator[]	returns the`n`^th element in the derived view, provided only if it satisfies`random_access_range` (public member function of`std::ranges::view_interface<D>`)[edit]

[edit]Deduction guides

[edit]Nested classes

iterator (C++23)	the iterator type (exposition-only member class template*)
sentinel (C++23)	the sentinel type (exposition-only member class template*)

[edit]Helper templates

template<class View> constexprbool enable_borrowed_range<ranges::enumerate_view<View>>= ranges::enable_borrowed_range<View>;		(since C++23)

This specialization ofranges::enable_borrowed_range makesenumerate_view satisfyborrowed_range when the underlying view satisfies it.

[edit]Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_ranges_enumerate`	`202302L`	(C++23)	`std::ranges::enumerate_view`

[edit]Example

Run this code

#include <initializer_list>#include <iostream>#include <map>#include <ranges>#include <vector> int main(){constexprstaticauto v={'A','B','C','D'}; for(autoconst[index, letter]: std::views::enumerate(v))std::cout<<'('<< index<<':'<< letter<<") ";std::cout<<'\n'; #if __cpp_lib_ranges_to_container// create a map using the position of each element as keyauto m= v| std::views::enumerate| std::ranges::to<std::map>(); for(autoconst[key, value]: m)std::cout<<'['<< key<<"]:"<< value<<' ';std::cout<<'\n';#endif std::vector<int> numbers{1,3,5,7}; // num is mutable even with const, which does not propagate to reference to// make it const, use `std::views::enumerate(numbers) | std::views::as_const`// or `std::views::enumerate(std::as_const(numbers))`for(autoconst[index, num]: std::views::enumerate(numbers)){++num;// the type is int&std::cout<< numbers[index]<<' ';}std::cout<<'\n';}

Possible output:

(0:A) (1:B) (2:C) (3:D)[0]:A [1]:B [2]:C [3]:D2 4 6 8

[edit]References

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

26.7.23 Enumerate view [range.enumerate]

[edit]See also

ranges::iota_viewviews::iota (C++20)	a`view` consisting of a sequence generated by repeatedly incrementing an initial value (class template)(customization point object)[edit]
ranges::zip_viewviews::zip (C++23)	a`view` consisting of tuples of references to corresponding elements of the adapted views (class template)(customization point object)[edit]
ranges::elements_viewviews::elements (C++20)	takes a`view` consisting of`tuple-like` values and a number N and produces a`view` of N^th element of each tuple (class template)(range adaptor object)[edit]