constexpr map

constexpr_map.cpp

#include <utility>
#include <array>

template <auto...>
struct value_list
{};

template <class T, auto... Keys>
struct map_
{
    using data_type = T;
    static auto constexpr N = sizeof...(Keys);

    constexpr map_(value_list<Keys...>, std::array<T, N> arr)
    : arr_{ arr }
    {}

private:
    template <class Key, std::size_t... Is>
    auto constexpr at_impl(Key key, std::index_sequence<Is...>) const
    { return ((key == Keys ? Is + 1 : 0) + ...) - 1; }

public:
    template <class Key>
    auto constexpr at(Key const key) const
    { return arr_.at(at_impl(key, std::make_index_sequence<N>{})); }

    template <class Key>
    auto constexpr operator[](Key const i) const
    { return at(i); }

private:    
    std::array<T, N> arr_;
};

int main()
{
    auto constexpr seq = value_list<'a', 1>{};

    auto constexpr arr = std::array<int(*)(), 2>
    { []() { return 1; }, []() { return 2; } };
    auto constexpr map = map_{ seq, arr };

    auto constexpr fun = map['a'];
    auto constexpr val = fun();

    return val;
}