NAME¶

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

Synopsis¶

Defined in header <ranges>
template< ranges::view V >

requires /*range-with-movable-references*/<V> (1) (since C++23)
class enumerate_view

: public ranges::view_interface<enumerate_view<V>>
namespace views {

inline constexpr /* unspecified */ enumerate = /* unspecified (2) (since C++23)
*/;

}
Call signature
template< ranges::viewable_range R >

requires /* see below */ (since C++23)

constexpr /* see below */ enumerate( R&& r );
Helper concepts
template< class R >

concept /*range-with-movable-references*/ =
ranges::input_range<R> && (3) (exposition
std::move_constructible<ranges::range_reference_t<R>> && only*)

std::move_constructible<ranges::range_rvalue_reference_t<R>>;

1) enumerate_view is a range adaptor that takes a view and produces a view of
tuples. i^th element (the tuple) of the resulting sequence holds:
* the value equal to i, which is a zero-based index of the element of underlying
sequence, and
* the reference to the underlying element.
2) The name views::enumerate denotes a RangeAdaptorObject. Given a subexpression e,
the expression views::enumerate(e) is expression-equivalent to
enumerate_view<views::all_t<decltype((e))>>(e) for any suitable subexpression e.
3) Ensures that the reference type of the underlying type can be moved.

enumerate_view models the concepts random_access_range, bidirectional_range,
forward_range, input_range, common_range, and sized_range when the underlying view V
models respective concepts.

Member functions¶

constructor constructs a enumerate_view
(C++23) (public member function)
base returns a copy of the underlying (adapted) view
(C++23) (public member function)
begin returns an iterator to the beginning
(C++23) (public member function)
end returns an iterator or a sentinel to the end
(C++23) (public member function)
size returns the number of elements. Provided only if the underlying
(C++23) (adapted) range satisfies sized_range.
(public member function)
Inherited from std::ranges::view_interface
empty returns whether the derived view is empty. Provided if it satisfies
(C++20) sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
cbegin returns a constant iterator to the beginning of the range.
(C++23) (public member function of std::ranges::view_interface<D>)
cend returns a sentinel for the constant iterator of the range.
(C++23) (public member function of std::ranges::view_interface<D>)
operator bool returns whether the derived view is not empty. Provided if
(C++20) 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
(C++20) satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
back returns the last element in the derived view. Provided if it satisfies
(C++20) bidirectional_range and common_range.
(public member function of std::ranges::view_interface<D>)
operator[] returns the n^th element in the derived view. Provided if it satisfies
(C++20) random_access_range.
(public member function of std::ranges::view_interface<D>)

Deduction guides

Nested classes

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

Helper templates

template< class View >

constexpr bool enable_borrowed_range<ranges::enumerate_view<View>> = (since C++23)

std::ranges::enable_borrowed_range<View>;

This specialization of std::ranges::enable_borrowed_range makes enumerate_view
satisfy borrowed_range when the underlying view satisfies it.

Notes¶

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

Example¶

// Run this code

#include <iostream>
#include <map>
#include <ranges>
#include <vector>

int main()
{
constexpr static auto v = {'A', 'B', 'C', 'D'};

for (auto const [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 key
auto m = v | std::views::enumerate | std::ranges::to<std::map>();

for (auto const [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 (auto const [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]:D
2 4 6 8

References¶

* C++23 standard (ISO/IEC 14882:2023):

* 26.7.23 Enumerate view [range.enumerate]

See also¶

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