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

< cpp‎ | ranges
Defined in header <ranges>
template< ranges::input_range V >

  requires ranges::view<V> && ranges::input_range<ranges::range_reference_t<V>>

class join_view : public ranges::view_interface<join_view<V>>
(1) (since C++20)
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) Range adaptor object. The expression views::join(E) is expression-equivalent to join_view<views::all_t<decltype((E))>>{E} for any suitable subexpressions E.


[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] Member functions

constructs a join_view
(public member function) [edit]
returns a copy of the underlying (adapted) view
(public member function) [edit]
returns an iterator to the beginning
(public member function) [edit]
returns an iterator or a sentinel to the end
(public member function) [edit]

[edit] Deduction guides

[edit] Nested classes

the iterator type
(public member class)
the sentinel type
(public member class)

[edit] 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; })
 //          | std::views::join; // error before P2328R1
    return v | std::views::transform([](auto& p) -> std::string& { return; })
             | std::views::join; // OK

[edit] Example

#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';

1 2 3 4 5 6 7 8 9

[edit] 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) returns a copy of E when E is a join_view returns a nested join_view

[edit] See also

a view over the subranges obtained from splitting another view using a delimiter
(class template) (range adaptor object) [edit]