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

< cpp‎ | ranges‎ | subrange

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

#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