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)