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 std::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();
    }
  };
 
} // namespace std
 
#endif