EnumWrapper.h

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#ifndef WXMAXIMA_ENUMWRAPPER_H
#define WXMAXIMA_ENUMWRAPPER_H
 
#include <cstddef>
#include <functional>
#include <type_traits>
 
template <typename Enum, typename Storage, Enum defaultValue = Enum{},
          typename std::enable_if<std::is_enum<Enum>::value &&
                                      std::is_integral<Storage>::value,
                                  bool>::type = true>
class EnumWrapper {
  Storage value = Storage(defaultValue);
 
  // We can at least make sure that the default value fits
  static_assert(
      Enum(Storage(defaultValue)) == defaultValue,
      "The default value doesn't survive a roundtrip through this wrapper.");
  // There's no point in using this type if it doesn't save space.
  static_assert(
      sizeof(Storage) < sizeof(Enum),
      "EnumWrapper should be used only to save space."
      "When removing it, don't forget to take care of default value!");
 
public:
  constexpr EnumWrapper() = default;
  constexpr EnumWrapper(Storage) = delete;
  // cppcheck-suppress noExplicitConstructor
  constexpr EnumWrapper(Enum value) noexcept : value(value) {}
  constexpr operator Enum() const noexcept { return Enum(value); }
  constexpr operator Storage() const = delete;
  constexpr size_t hash() const { return std::hash<Storage>()(value); }
 
  constexpr bool operator==(EnumWrapper o) const { return value == o.value;  }
  constexpr bool operator==(Enum o) const { return value == o;  }
  constexpr bool operator!=(EnumWrapper o) const { return value != o.value;  }
  constexpr bool operator!=(Enum o) const { return value != o;  }
};
 
template <typename E, typename S, E defVal>
constexpr bool operator==(E a, EnumWrapper<E,S,defVal> b) { return b == a;  }
 
template <typename E, typename S, E defVal>
constexpr bool operator!=(E a, EnumWrapper<E,S,defVal> b) { return b != a;  }
 
namespace std {
 
template <typename Enum, typename Storage, Enum defaultValue>
struct hash<EnumWrapper<Enum, Storage, defaultValue>> {
  constexpr auto
  operator()(EnumWrapper<Enum, Storage, defaultValue> value) const {
    return value.hash();
  }
};
 
}
 
#endif