std::ranges...s::join_view(3)

💖 NAME

okay so bestie, this is the C++ Standard Libary manual page for std::ranges::views::join and std::ranges::join_view — page-title vibes: std::ranges...s::join_view(3). We folded the header in instead of copy-pasting it, because we're cute like that 💅

• std::ranges::views::join, std::ranges::join_view — std::ranges::views::join, std::ranges::join_view (yes, it's listed twice in the original NAME line — iconic duplication, no notes)

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✨ Synopsis

Defined in header <ranges>:

template< ranges::input_range V >

    requires ranges::view<V> and
             ranges::input_range<ranges::range_reference_t<V>>   (1)
(since C++20)
class join_view

    : public ranges::view_interface<join_view<V>>
namespace views {

    inline constexpr /* unspecified */ join = /* unspecified */; (2)
(since C++20)

}

1. A range adaptor that represents view consisting of the sequence obtained from flattening a view of ranges.
2. RangeAdaptorObject (and also RangeAdaptorClosureObject). The expression views::join(e) is expression-equivalent to join_view<views::all_t<decltype((e))>>{e} for any suitable subexpressions e.

join_view models input_range.

join_view models forward_range when:

• ranges::range_reference_t<V> is a reference type, and
• V and ranges::range_reference_t<V> each model forward_range.

join_view models bidirectional_range when:

• ranges::range_reference_t<V> is a reference type,
• V models bidirectional_range, and
• ranges::range_reference_t<V> models both bidirectional_range and common_range.

join_view models common_range when:

• ranges::range_reference_t<V> is a reference type, and
• V and ranges::range_reference_t<V> each model forward_range and common_range.

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🎀 Member functions

• constructor (C++20) — constructs a join_view (public member function)
• base (C++20) — returns a copy of the underlying (adapted) view (public member function)
• begin (C++20) — returns an iterator to the beginning (public member function)
• end (C++20) — returns an iterator or a sentinel to the end (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>)

Deduction guides

(That's the heading on the page; no extra details listed here, bestie.)

Nested classes

• iterator (C++20) — the iterator type (exposition-only member class template*)
• sentinel (C++20) — the sentinel type (exposition-only member class template*)

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🌸 Notes

Before P2328R1 was adopted, the inner range type (ranges::range_reference_t<V>) cannot be a container type (but can be reference to container). For example, it was not allowed to join a transform_view of std::string prvalue.

struct Person { int age; std::string name; };

auto f(std::vector<Person>& v) {
//  return v | std::views::transform([](auto& p){ return p.name; })
//           | std::views::join; // error before P2328R1
    return v | std::views::transform([](auto& p) -> std::string& {
return p.name; })
             | std::views::join; // OK
}

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🍓 Example

// Run this code

#include <iostream>
#include <ranges>
#include <string_view>
#include <vector>

int main()
{
    using namespace std::literals;

    const auto bits = {"https:"sv, "//"sv, "cppreference"sv, "."sv,
"com"sv};
    for (char const c : bits | std::views::join)
        std::cout << c;
    std::cout << '\n';

    const std::vector<std::vector<int>> v{{1, 2}, {3, 4, 5}, {6}, {7,
8, 9}};
    auto jv = std::ranges::join_view(v);
    for (int const e : jv)
        std::cout << e << ' ';
    std::cout << '\n';
}

💻 Output:

https://cppreference.com
1 2 3 4 5 6 7 8 9

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💀 Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• LWG 3474 — applied to C++20; behavior as published: views::join(e) returned a copy of e when e is a join_view; correct behavior: returns a nested join_view.
• P2328R1 — applied to C++20; behavior as published: non-view range prvalues could not be joined by join_view; correct behavior: made joinable.

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🌈 See also

• ranges::join_with_view, views::join_with (C++23) — a view consisting of the sequence obtained from flattening a view of ranges, with the delimiter in between elements (class template) (range adaptor object)
• ranges::split_view, views::split (C++20) — a view over the subranges obtained from splitting another view using a delimiter (class template) (range adaptor object)
• ranges::lazy_split_view, views::lazy_split (C++20) — a view over the subranges obtained from splitting another view using a delimiter (class template) (range adaptor object)

💖 Cute sign-off: source http://cppreference.com, date/version 2024.06.10, page std::ranges...s::join_view(3) — byeeee! 🫶