std::ranges::views::join,std::ranges::join_view(3)

NAME
std::ranges::views::join,std::ranges::join_view - std::ranges::views::join,std::ranges::join_view

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.

Member functions
   constructor constructs a join_view
   (C++20) (public member function)
   base returns a copy of the underlying (adapted) view
   (C++20) (public member function)
   begin returns an iterator to the beginning
   (C++20) (public member function)
   end returns an iterator or a sentinel to the end
   (C++20) (public member function)
         Inherited from std::ranges::view_interface
   empty returns whether the derived view is empty. Provided if it satisfies
   (C++20) sized_range or forward_range.
                 (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
   (C++20) ranges::empty is applicable to it.
                 (public member function of std::ranges::view_interface<D>)
   front returns the first element in the derived view. Provided if it
   (C++20) satisfies forward_range.
                 (public member function of std::ranges::view_interface<D>)
   back returns the last element in the derived view. Provided if it satisfies
   (C++20) bidirectional_range and common_range.
                 (public member function of std::ranges::view_interface<D>)
   Deduction guides
   Nested classes
   iterator the iterator type
   (C++20) (exposition-only member class template*)
   sentinel the sentinel type
   (C++20) (exposition-only member class template*)

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
 }

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
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3474 C++20 views::join(e) returned a copy of e when e is returns a nested
                       a join_view join_view
   P2328R1 C++20 non-view range prvalues could not be joined by made joinable
                       join_view

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