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

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>>

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

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

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

Deduction guides

Nested classes

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';
}

https://cppreference.com
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Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also