std::ranges::views::enumerate,std::ranges::enumerate_view(3)

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)