std::ranges::get(std::ranges::subrange)(3)

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)