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

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C++
Ranges library
Range adaptors
std::ranges::enumerate_view
Member functions
Deduction guides
Nested classes
Iterator
Sentinel
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 tuple s. 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.

Data members

Member name Definition
base_ (private) An iterator to the underlying sequence of type V .
( exposition-only member object* )

Member functions

constructs a enumerate_view
(public member function)
returns a copy of the underlying (adapted) view
(public member function)
returns an iterator to the beginning
(public member function)
returns an iterator or a sentinel to the end
(public member function)
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range .
(public member function)
Inherited from std::ranges::view_interface
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> )
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> )
returns the first element in the derived view. Provided if it satisfies forward_range .
(public member function of std::ranges::view_interface<D> )
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> )
returns the n th 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 >> =

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

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 14882:2024):
  • 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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