NAME¶

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

Synopsis¶

Defined in header <ranges>
template< std::size_t N, class I, class S, ranges::subrange_kind K
>
(1) (since C++20)
requires ((N == 0 && std::copyable<I>) || N == 1)

constexpr auto get( const ranges::subrange<I, S, K>& r );
template< std::size_t N, class I, class S, ranges::subrange_kind K
>
(2) (since C++20)
requires (N < 2)

constexpr auto get( ranges::subrange<I, S, K>&& r );
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)
<< *get<1>(sub_a) << '\n'; // == *(end(a) - 1)

*get<0>(sub_a) = 42; // OK
// *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
the overload for const lvalue was it is removed from the
LWG 3589 C++20 ill-formed if N == 0 overload set
and I does not model copyable

See also¶

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