Get index by run-time value.

ct_value_index_only.cpp

#include <iostream>
#include <utility>

template <size_t I, class F>
inline auto ct_value(F&& f)
{
    return std::forward<F>(f)(std::integral_constant<size_t, I>());
}

template <class F, size_t... I>
inline auto ct_value(size_t v, F&& f, std::index_sequence<I...>)
{
    using R = std::invoke_result_t<F, std::integral_constant<size_t, 0>>;
    static constexpr R (*vtable[])(F&&) = { &ct_value<I>... };

    if (v >= sizeof...(I))
        return R();
    else
        return vtable[v](std::forward<F>(f));
}

template <size_t N, class F>
inline auto ct_value(size_t v, F&& f)
{
    return ct_value(v, std::forward<F>(f), std::make_index_sequence<N>());
}


int main()
{
    int x;
    std::cin >> x;

    ct_value<1024>(x, [](auto I) {
        std::cout << "found: " << I << '\n';
    });
}