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std::ranges::views::join, std::ranges::join_view - cppreference.com

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Defined in header <ranges>

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>>

 (1) (since C++20)

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

 (2) (since C++20)

Call signature

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

 (since C++20)

1) A range adaptor that represents view consisting of the sequence obtained from flattening a view of ranges.

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:

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

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]
Inherited from ranges::view_interface
returns whether the derived view is empty, provided only if it satisfies sized_range or forward_range
(public member function of std::ranges::view_interface<D>) [edit]

(C++23)

 returns a constant iterator to the beginning of the range
(public member function of std::ranges::view_interface<D>) [edit]

(C++23)

 returns a sentinel for the constant iterator of the range
(public member function of std::ranges::view_interface<D>) [edit]
returns whether the derived view is not empty, provided only if ranges::empty is applicable to it
(public member function of std::ranges::view_interface<D>) [edit]
returns the first element in the derived view, provided if it satisfies forward_range
(public member function of std::ranges::view_interface<D>) [edit]
returns the last element in the derived view, provided only if it satisfies bidirectional_range and common_range
(public member function of std::ranges::view_interface<D>) [edit]

Deduction guides

Nested classes

the iterator type
(exposition-only member class template*)
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) returned a copy of e when e is a join_view returns a nested join_view
P2328R1 C++20 non-view range prvalues could not be joined by join_view made joinable

See also