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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

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)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

std::ranges::join_view

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)

 

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

(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 sized_range or 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>)

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

See also

ranges::join_with_viewviews::join_with (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)
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)