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

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< 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

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)

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