variant_overloaded.hpp

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// Copyright (c) 2021 MIT Digital Currency Initiative,
//                    Federal Reserve Bank of Boston
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
#ifndef OPENCBDC_TX_SRC_COMMON_VARIANT_OVERLOADED_H_
#define OPENCBDC_TX_SRC_COMMON_VARIANT_OVERLOADED_H_
 
#include <array>
#include <cassert>
#include <cstddef>
#include <variant>
 
namespace cbdc {
    template<class... Ts>
    struct overloaded : Ts... {
        using Ts::operator()...;
    };
 
    template<class... Ts>
    overloaded(Ts...) -> overloaded<Ts...>;
 
    template<typename... Ts>
    [[nodiscard]] auto expand_type(size_t i) -> std::variant<Ts...> {
        static_assert((std::is_default_constructible_v<Ts> && ...));
        using T = typename std::variant<Ts...>;
        assert(i < std::variant_size_v<T>);
        static constexpr auto t = std::array{+[]() {
            return T{Ts{}};
        }...};
        // TODO: deserialization error handling for variant indexes.
        return t.at(i)();
    }
}
 
#endif // OPENCBDC_TX_SRC_COMMON_VARIANT_OVERLOADED_H_