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

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.

Expression-equivalent

Expression e is expression-equivalent to expression f, if.

  • e and f have the same effects, and
  • either both are constant subexpressions or else neither is a constant subexpression, and
  • either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

Member functions

(C++20)
constructs a join_view
(public member function)
(C++20)
returns a copy of the underlying (adapted) view
(public member function)
(C++20)
returns an iterator to the beginning
(public member function)
(C++20)
returns an iterator or a sentinel to the end
(public member function)
Inherited from std::ranges::view_interface
(C++20)
Returns whether the derived view is empty. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
(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>)
(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>)
(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

Nested classes

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

#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) returns a copy of e when e is a join_view returns a nested join_view
P2328R1 C++20 non-view range prvalues can't be joined by join_view made joinable

See also

(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)
(C++20)
a view over the subranges obtained from splitting another view using a delimiter
(class template) (range adaptor object)
(C++20)
a view over the subranges obtained from splitting another view using a delimiter
(class template) (range adaptor object)

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