std::ranges...umerate_view(3)

✨ std::ranges...umerate_view(3) ✨

okay so bestie, welcome to the C++ Standard Libary manual page for std::ranges...umerate_view(3) — yes the title is doing the absolute most, and we are gonna get through every single detail together. 🫶

🌸 NAME 🌸

• std::ranges::views::enumerate,std::ranges::enumerate_view — literally the two names this page is about. The NAME line repeats them exactly: std::ranges::views::enumerate,std::ranges::enumerate_view. No cap, that's the whole name section. 💅

🦋 Synopsis 🦋

Defined in header <ranges>:

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

That's number (1), since C++23. ✨

namespace views {
    inline constexpr /* unspecified */ enumerate = /* unspecified */;
}

That's number (2), since C++23. 🎀

📞 Call signature 📞

template< ranges::viewable_range R >
    requires /* see below */
constexpr /* see below */ enumerate( R&& r );

That's 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>>;

That's number (3), and it's exposition only*. It ensures that the reference type of the underlying type can be moved. 🔥

1. enumerate_view is a range adaptor that takes a view and produces a view of tuples. The 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 the 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 (adapted) range satisfies sized_range. (C++23) (public member function)

🌈 Inherited from std::ranges::view_interface 🌈

• empty — returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range. (C++20) (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 ranges::empty is applicable to it. (C++20) (public member function of std::ranges::view_interface<D>)
• front — returns the first element in the derived view. Provided if it satisfies forward_range. (C++20) (public member function of std::ranges::view_interface<D>)
• back — returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range. (C++20) (public member function of std::ranges::view_interface<D>)
• operator[] — returns the n^th element in the derived view. Provided if it satisfies random_access_range. (C++20) (public member function of std::ranges::view_interface<D>)

🎀 Deduction guides 🎀

Bestie, the page says Deduction guides and then gives no extra details. That's it. Don't invent, okay? 💀

🌀 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>> =
    std::ranges::enable_borrowed_range<View>;

That's 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: __cpp_lib_ranges_enumerate
• Value: 202302L
• Std: (C++23)
• Feature: 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 / views::iota — a view consisting of a sequence generated by repeatedly incrementing an initial value. (C++20) (class template) (customization point object)
• ranges::zip_view / views::zip — a view consisting of tuples of references to corresponding elements of the adapted views. (C++23) (class template) (customization point object)
• ranges::elements_view / views::elements — takes a view consisting of tuple-like values and a number N and produces a view of N^th element of each tuple. (C++20) (class template) (range adaptor object)

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