std::ranges::get(std::ranges::subrange)

 

C++

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Freestanding and hosted

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

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_reference_trange_rvalue_reference_t	range_value_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range	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	single_viewviews::single	iota_viewviews::iota	basic_istream_viewviews::istream
Adaptors
views::all_tviews::all ref_view owning_view common_viewviews::common reverse_viewviews::reverse views::counted zip_viewviews::zip (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)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

std::ranges::subrange

Member functions

subrange::subrange (C++20)

subrange::operator PairLike (C++20)

Observers

subrange::begin (C++20)

subrange::end (C++20)

subrange::empty (C++20)

subrange::size (C++20)

Iterator operations

subrange::advance (C++20)

subrange::next (C++20)

subrange::prev (C++20)

Deduction guides

Non-member functions

get (C++20)

Helper types

subrange_kind (C++20)

tuple_size (C++20)

tuple_element (C++20)

 

Defined in header <ranges>
template< std::size_t N, class I, class S, ranges::subrange_kind K >     requires ((N == 0 && std::copyable<I>) || N == 1) constexpr auto get( const ranges::subrange<I, S, K>& r );	(1)	(since C++20)
template< std::size_t N, class I, class S, ranges::subrange_kind K >     requires (N < 2) constexpr auto get( ranges::subrange<I, S, K>&& r );	(2)	(since C++20)
namespace std { using ranges::get; }	(3)	(since C++20)

1) Obtains the iterator or sentinel from a subrange lvalue (or a const rvalue) when N == 0 or N == 1, respectively. It is mainly provided for structured binding support.

2) Same as (1), except that it takes a non-const subrange rvalue.

3) (1-2) are imported into namespace std, which simplifies their usage and makes every subrange with a copyable iterator a pair-like type.

Parameters

r

-

a subrange

Return value

1) An iterator or sentinel copy constructed from the stored one when N == 0 or N == 1, respectively.

2) Same as (1), except that the iterator is move constructed if N == 0 and I does not satisfy copyable.

Possible implementation

template<std::size_t N, class I, class S, std::ranges::subrange_kind K> requires ((N == 0 && std::copyable<I>) || N == 1) constexpr auto get(const std::ranges::subrange<I, S, K>& r) { if constexpr (N == 0) return r.begin(); else return r.end(); }   template<std::size_t N, class I, class S, std::ranges::subrange_kind K> requires (N < 2) constexpr auto get(std::ranges::subrange<I, S, K>&& r) { if constexpr (N == 0) return r.begin(); // may perform move construction else return r.end(); }

Example

Run this code

#include <array>
#include <iostream>
#include <iterator>
#include <ranges>
 
int main()
{
    std::array a{1, -2, 3, -4};
 
    std::ranges::subrange sub_a{ std::next(a.begin()), std::prev(a.end()) };
 
    std::cout << *std::ranges::get<0>(sub_a) << ' '  // == *(begin(a) + 1)
              << *std::get<1>(sub_a) << '\n';        // == *(end(a) - 1)
 
    *std::get<0>(sub_a) = 42; // OK
//  *std::get<2>(sub_a) = 13; // hard error: index can only be 0 or 1
}

Output:

-2 -4

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 3589	C++20	the overload for const lvalue was ill-formed if N == 0 and I does not model copyable	it is removed from the overload set

See also

Structured binding (C++17)	binds the specified names to sub-objects or tuple elements of the initializer
std::get(std::tuple) (C++11)	tuple accesses specified element (function template)
std::get(std::pair) (C++11)	accesses an element of a pair (function template)
std::get(std::array) (C++11)	accesses an element of an array (function template)