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std::ranges::get(std::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

See also

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

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