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

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*/ =     // exposition only
        ranges::input_range<R> &&
        std::move_constructible<ranges::range_reference_t<R>> &&
        std::move_constructible<ranges::range_rvalue_reference_t<R>>;
 (3) (since C++23)
1) enumerate_view is a range adaptor that takes a view and produces a view of 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

Typical implementations of enumerate_view hold only one non-static data member:

  • base_ of type V. The name is for exposition only.

Member functions

(C++23)
 constructs a enumerate_view
(public member function)
(C++23)
 returns a copy of the underlying (adapted) view
(public member function)
(C++23)
 returns an iterator to the beginning
(public member function)
(C++23)
 returns an iterator or a sentinel to the end
(public member function)
(C++23)
 returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range.
(public member function)
Inherited from std::ranges::view_interface
(C++20)
 returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
(C++23)
 returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>)
(C++23)
 returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>)
(C++20)
 returns whether the derived view is not empty. Provided if ranges::empty is applicable to it.
(public member function of std::ranges::view_interface<D>)
(C++20)
 returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
(C++20)
 returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range.
(public member function of std::ranges::view_interface<D>)
(C++20)
 returns the nth element in the derived view. Provided if it satisfies random_access_range.
(public member function of std::ranges::view_interface<D>)

Deduction guides

Nested classes

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

Helper templates

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

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 Comment
__cpp_lib_ranges_enumerate 202302L (C++23) std::ranges::enumerate_view

Example

#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';
 
    // 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';
 
 
    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';
}

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

(C++20)
 a view consisting of a sequence generated by repeatedly incrementing an initial value
(class template) (customization point object)
(C++23)
 a view consisting of tuples of references to corresponding elements of the adapted views
(class template) (customization point object)
(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)

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