std::ranges::views::join, std::ranges::join_view

template< ranges::input_range V >
requires ranges::view<V> and
ranges::input_range<ranges::range_reference_t<V>>
class join_view
: public ranges::view_interface<join_view<V>>

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

Call signature

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

#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';
}