draconisplusplus/include/rfl/TaggedUnion.hpp

#ifndef RFL_TAGGEDUNION_HPP_
#define RFL_TAGGEDUNION_HPP_

#include <variant>

#include "define_literal.hpp"
#include "internal/Getter.hpp"
#include "internal/StringLiteral.hpp"
#include "internal/tag_t.hpp"

namespace rfl {

  // https://serde.rs/enum-representations.html
  template <internal::StringLiteral _discriminator, class... Ts>
  struct TaggedUnion {
    static constexpr internal::StringLiteral discrimininator_ = _discriminator;

    /// The type of the underlying variant.
    using VariantType = std::variant<Ts...>;

    /// A literal containing all the tags that are possible
    using PossibleTags = define_literal_t<internal::tag_t<_discriminator, Ts>...>;

    TaggedUnion(const VariantType& _variant) : variant_(_variant) {}

    TaggedUnion(VariantType&& _variant) noexcept : variant_(std::move(_variant)) {}

    TaggedUnion(const TaggedUnion<_discriminator, Ts...>& _tagged_union) = default;

    TaggedUnion(TaggedUnion<_discriminator, Ts...>&& _tagged_union) noexcept = default;

    template <
      class T,
      typename std::enable_if<std::is_convertible_v<T, VariantType>, bool>::type = true>
    TaggedUnion(const T& _t) : variant_(_t) {}

    template <
      class T,
      typename std::enable_if<std::is_convertible_v<T, VariantType>, bool>::type = true>
    TaggedUnion(T&& _t) noexcept : variant_(std::forward<T>(_t)) {}

    ~TaggedUnion() = default;

    /// Assigns the underlying object.
    TaggedUnion<_discriminator, Ts...>& operator=(const VariantType& _variant) {
      variant_ = _variant;
      return *this;
    }

    /// Assigns the underlying object.
    TaggedUnion<_discriminator, Ts...>& operator=(VariantType&& _variant) {
      variant_ = std::move(_variant);
      return *this;
    }

    /// Assigns the underlying object.
    template <
      class T,
      typename std::enable_if<std::is_convertible_v<T, VariantType>, bool>::type = true>
    TaggedUnion<_discriminator, Ts...>& operator=(T&& _variant) {
      variant_ = std::forward<T>(_variant);
      return *this;
    }

    /// Assigns the underlying object.
    template <
      class T,
      typename std::enable_if<std::is_convertible_v<T, VariantType>, bool>::type = true>
    TaggedUnion<_discriminator, Ts...>& operator=(const T& _variant) {
      variant_ = _variant;
      return *this;
    }

    /// Assigns the underlying object.
    TaggedUnion<_discriminator, Ts...>& operator=(const TaggedUnion<_discriminator, Ts...>& _other
    ) = default;

    /// Assigns the underlying object.
    TaggedUnion<_discriminator, Ts...>& operator=(TaggedUnion<_discriminator, Ts...>&& _other
    ) = default;

    /// Returns the underlying variant.
    VariantType& variant() { return variant_; }

    /// Returns the underlying variant.
    const VariantType& variant() const { return variant_; }

    static_assert(
      !PossibleTags::has_duplicates(),
      "Duplicate tags are not allowed inside tagged unions."
    );

    /// The underlying variant - a TaggedUnion is a thin wrapper
    /// around a variant that is mainly used for parsing.
    VariantType variant_;
  };

  namespace internal {

    template <StringLiteral _discriminator, class... NamedTupleTypes>
    struct Getter<TaggedUnion<_discriminator, NamedTupleTypes...>> {
     public:
      /// Retrieves the indicated value from the tuple.
      template <int _index>
      static inline auto& get(TaggedUnion<_discriminator, NamedTupleTypes...>& _tu) {
        return Getter<std::variant<NamedTupleTypes...>>::template get<_index>(_tu.variant_);
      }

      /// Gets a field by name.
      template <StringLiteral _field_name>
      static inline auto& get(TaggedUnion<_discriminator, NamedTupleTypes...>& _tu) {
        return Getter<std::variant<NamedTupleTypes...>>::template get<_field_name>(_tu.variant_);
      }

      /// Gets a field by the field type.
      template <class Field>
      static inline auto& get(TaggedUnion<_discriminator, NamedTupleTypes...>& _tu) {
        return Getter<std::variant<NamedTupleTypes...>>::template get<Field>(_tu.variant_);
      }

      /// Retrieves the indicated value from the tuple.
      template <int _index>
      static inline const auto& get_const(const TaggedUnion<_discriminator, NamedTupleTypes...>& _tu
      ) {
        return Getter<std::variant<NamedTupleTypes...>>::template get_const<_index>(_tu.variant_);
      }

      /// Gets a field by name.
      template <StringLiteral _field_name>
      static inline const auto& get_const(const TaggedUnion<_discriminator, NamedTupleTypes...>& _tu
      ) {
        return Getter<std::variant<NamedTupleTypes...>>::template get_const<_field_name>(
          _tu.variant_
        );
      }

      /// Gets a field by the field type.
      template <class Field>
      static inline const auto& get_const(const TaggedUnion<_discriminator, NamedTupleTypes...>& _tu
      ) {
        return Getter<std::variant<NamedTupleTypes...>>::template get_const<Field>(_tu.variant_);
      }
    };

  } // namespace internal

} // namespace rfl

#endif // RFL_TAGGEDUNION_HPP_