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

 

C++

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Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

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Numerics library

Localizations library

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Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_trange_value_trange_reference_t	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)	elements_of (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Range factories
empty_viewviews::empty single_viewviews::single	iota_viewviews::iota repeat_viewviews::repeat (C++23)(C++23)	basic_istream_viewviews::istream
Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23) filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23) lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)
Range generators
std::generator (C++23)
Range adaptor objects
Range adaptor closure objects
range_adaptor_closure (C++23)
Helper items
copyable-box movable-box (until C++23)(C++23)	simple-view non-propagating-cache

 

std::ranges::enumerate_view

Member functions

enumerate_view::enumerate_view (C++23)

enumerate_view::base (C++23)

enumerate_view::begin (C++23)

enumerate_view::end (C++23)

enumerate_view::size (C++23)

Deduction guides

Nested classes

Iterator

enumerate_view::iterator::iterator (C++23)

enumerate_view::iterator::base (C++23)

enumerate_view::iterator::index (C++23)

enumerate_view::iterator::operator* (C++23)

enumerate_view::iterator::operator[] (C++23)

enumerate_view::iterator::operator++ enumerate_view::iterator::operator++(int) enumerate_view::iterator::operator-- enumerate_view::iterator::operator--(int) enumerate_view::iterator::operator+= enumerate_view::iterator::operator-= (C++23)(C++23)(C++23)(C++23)(C++23)(C++23)

operator==(enumerate_view::iterator) operator<=>(enumerate_view::iterator) (C++23)(C++23)

operator+(enumerate_view::iterator) operator-(enumerate_view::iterator) (C++23)(C++23)

iter_move(enumerate_view::iterator) (C++23)

Sentinel

enumerate_view::sentinel::sentinel (C++23)

enumerate_view::sentinel::base (C++23)

operator==(enumerate_view::iterator,enumerate_view::sentinel) (C++23)

operator-(enumerate_view::iterator,enumerate_view::sentinel) (C++23)

 

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 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.

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

(constructor) (C++23)	constructs a enumerate_view (public member function)
base (C++23)	returns a copy of the underlying (adapted) view (public member function)
begin (C++23)	returns an iterator to the beginning (public member function)
end (C++23)	returns an iterator or a sentinel to the end (public member function)
size (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
empty (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>)
cbegin (C++23)	returns a constant iterator to the beginning of the range. (public member function of std::ranges::view_interface<D>)
cend (C++23)	returns a sentinel for the constant iterator of the range. (public member function of std::ranges::view_interface<D>)
operator bool (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>)
front (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>)
back (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>)
operator[] (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>)

Nested classes

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

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';
 
    // 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

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