CODEWITHSUNDEEP
c++c++17type-traits
Croolman
Croolman

Reputation: 1123

Type trait for enum member value

I have a type trait to check whether or not the enum class contains a member called None.

template<typename T, typename = void>
    struct has_none : std::false_type
    {
    };

template<typename T>
struct has_none<T,
    std::void_t<decltype(T::None)>> : std::true_type {};

This check will be coupled with the std::is_enum_v. The question is, how would I create a type_trait which would check that the Enum::None has value of 0? Is it even possible when talking about type_traits?

Upvotes: 1

Views: 1609

Answers (2)

HolyBlackCat
HolyBlackCat

Reputation: 96043

Use std::enable_if:

template <typename T, typename = void>
struct has_none : std::false_type {};

template <typename T>
struct has_none<T, std::enable_if_t<T::None == T(0)>> : std::true_type {};

Upvotes: 6

rafix07
rafix07

Reputation: 20918

You could use conditional_t to select true_type/false_type as base class depending on the condition is true or not:

template<class E>
constexpr auto EnumToInt(E e) {
    return static_cast<std::underlying_type_t<E>>(e);
}
enum class Foo {
    None = 0
};
enum class Bar {
    None = 1
};
template<class T>
struct NoneAs0 : std::conditional_t< EnumToInt(T::None) == 0,
    std::true_type, std::false_type>
{};

int main() {
    std::cout << NoneAs0<Foo>::value << std::endl; // 1
    std::cout << NoneAs0<Bar>::value << std::endl; // 0

Upvotes: 3

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