variant_builder.hpp

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#ifndef KAGOME_CORE_COMMON_DYNAMIC_VARIANT_HPP
#define KAGOME_CORE_COMMON_DYNAMIC_VARIANT_HPP

#include <functional>
#include <type_traits>
#include <vector>

#include <boost/mpl/at.hpp>
#include <boost/mpl/for_each.hpp>
#include <boost/mpl/int.hpp>
#include <boost/variant.hpp>

namespace kagome::common {

  namespace dynamic_variant {
    template <typename Concrete, typename Variant>
    void init_variant(Variant &v) {
      v = Concrete{};
    }

    template <typename Variant>
    struct functors_vector_builder {
      std::vector<std::function<void(Variant &)>> *vec;
      template <typename Element>
      void operator()(Element) {
        vec->push_back(&init_variant<Element, Variant>);
      }
    };

    template <typename>
    struct is_boost_variant : std::false_type {};

    template <typename... Ts>
    struct is_boost_variant<boost::variant<Ts...>> : std::true_type {};
  }  // namespace dynamic_variant

  template <typename Variant, typename Types = typename Variant::types>
  class VariantBuilder {
    Variant &v_;
    std::vector<std::function<void(Variant &)>> funcs_;

   public:
    explicit VariantBuilder(Variant &v) : v_(v) {
      static_assert(dynamic_variant::is_boost_variant<Variant>::value);
      dynamic_variant::functors_vector_builder<Variant> builder = {&funcs_};
      boost::mpl::for_each<Types>(builder);
    };

    void init(size_t index) {
      funcs_[index](v_);
    }
  };

}  // namespace kagome::common

#endif  // KAGOME_CORE_COMMON_DYNAMIC_VARIANT_HPP