# std::ranges::constant_range

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< class T > concept constant_range = ranges::input_range<T> && /*constant-iterator*/<ranges::iterator_t<T>>; | (1) | (since C++23)
Helper concepts |  |
template< class T > concept /*constant-iterator*/ = std::input_iterator<T> && std::same_as<std::iter_const_reference_t<T>, std::iter_reference_t<T>>; | (2) | (exposition only*)

1) The `constant_range` concept is a refinement of `range` for which `ranges::begin` returns a constant iterator.
2) The exposition-only concept /*constant-iterator*/<T> is satisfied when the result of the indirection operation of the input iterator is its const reference type which implies read-only.

### Example

Run this code

    #include <ranges>
    #include <span>
    #include <string_view>
    #include <vector>

    // mechanisms for ensuring the parameter is a constant range
    // 1) an overload set where the mutable one defers to the constant one
    template<std::ranges::constant_range R>
    void takes_any_range1(R&& r)
    {
        // R is definitely a constant range
    }

    template<std::ranges::range R>
    void takes_any_range1(R&& r)
    {
        takes_any_range1(std::views::as_const(std::forward<R>(r)));
    }

    // 2) one function template that shadows its parameter
    template<std::ranges::range R>
    void takes_any_range2(R&& _r)
    {
        auto r = std::views::as_const(std::forward<R>(_r));

        // r is definitely a constant range
        // never use _r again
    }

    // 3) one function template that recursively invokes itself
    template<std::ranges::range R>
    void takes_any_range3(R&& r)
    {
        if constexpr (std::ranges::constant_range<R>)
        {
            // R is definitely a constant range
            // put implementation here
        }
        else
            takes_any_range3(std::views::as_const(std::forward<R>(r)));
    }

    static_assert
    (
            std::ranges::constant_range<const std::vector<int>> and
        not std::ranges::constant_range<std::vector<int>> and
            std::ranges::constant_range<std::string_view> and
        not std::ranges::constant_range<std::span<int>> and
            std::ranges::constant_range<std::span<const int>> and
        not std::ranges::constant_range<const std::span<int>>
    );

    int main() {}

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