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std::ranges::views::enumerate, std::ranges::enumerate_view - cppreference.com

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Defined in header <ranges>

template< ranges::view V > requires /*range-with-movable-references*/<V> class enumerate_view : public ranges::view_interface<enumerate_view<V>>

 (1) (since C++23)

namespace views { inline constexpr /* 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 a view and produces a view of std::tuples. ith 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.

Data members

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

Member functions

constructs a enumerate_view
(public member function) [edit]
returns a copy of the underlying (adapted) view
(public member function) [edit]
returns an iterator to the beginning
(public member function) [edit]
returns an iterator or a sentinel to the end
(public member function) [edit]
returns the number of elements, provided only if the underlying (adapted) range satisfies sized_range
(public member function) [edit]
returns the approximate size of the resulting approximately_sized_range
(public member function) [edit]
Inherited from ranges::view_interface
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]

(C++23)

 returns a constant iterator to the beginning of the range
(public member function of std::ranges::view_interface<D>) [edit]

(C++23)

 returns a sentinel for the constant iterator of the range
(public member function of std::ranges::view_interface<D>) [edit]
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>) [edit]
returns the first element in the derived view, provided if it satisfies forward_range
(public member function of std::ranges::view_interface<D>) [edit]
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]
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>) [edit]

Deduction guides

Nested classes

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

Helper templates

template< class View > constexpr bool enable_borrowed_range<ranges::enumerate_view<View>> = ranges::enable_borrowed_range<View>;

 (since C++23)

This specialization of 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

#include <initializer_list>
#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 14882:2024):
  • 26.7.23 Enumerate view [range.enumerate]

See also