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, 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.

Member functions

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

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

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) 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

Compiler support
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Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Technical specifications
Symbols index
External libraries

Member functions
join_view::join_view (C++20)
join_view::base (C++20)
join_view::begin (C++20)
join_view::end (C++20)
Deduction guides
Iterator
join_view::iterator::iterator (C++20)
join_view::iterator::operator* join_view::iterator::operator-> (C++20)(C++20)
join_view::iterator::operator++ join_view::iterator::operator++(int) join_view::iterator::operator-- join_view::iterator::operator--(int) (C++20)(C++20)(C++20)(C++20)
join_view::iterator::satisfy (C++20)
operator==(join_view::iterator) (C++20)
iter_move(join_view::iterator) (C++20)
iter_swap(join_view::iterator) (C++20)
Sentinel
join_view::sentinel::sentinel (C++20)
operator==(join_view::iterator,join_view::sentinel) (C++20)

ranges::split_viewviews::split (C++20)	a `view` over the subranges obtained from splitting another `view` using a delimiter (class template) (range adaptor object)
ranges::lazy_split_viewviews::lazy_split (C++20)	a `view` over the subranges obtained from splitting another `view` using a delimiter (class template) (range adaptor object)