I am trying to wite type traits for types that can index into e.g. an std::vector
, which should include enum types because i can cast them to their underlying type.
I have written following traits so far.
#include <iostream>
#include <type_traits>
#include <cinttypes>
#include <vector>
#include <utility>
template<typename T>
struct is_unsigned_integral :
std::integral_constant<
bool,
std::is_integral<T>::value &&
std::is_unsigned<T>::value
> {};
template<typename T>
inline constexpr auto is_unsigned_integral_v =
is_unsigned_integral<T>::value;
template<typename, typename = void>
struct is_index : std::false_type {};
template<typename T>
struct is_index<
T,
std::enable_if_t<
is_unsigned_integral_v<
std::conditional_t<
std::is_enum_v<T>,
std::underlying_type_t<T>,
T
>
>
>
> : std::true_type {};
template<typename T>
inline constexpr auto is_index_v = is_index<T>::value;
enum class idx : unsigned int {};
int main() {
static_assert(is_index_v<unsigned int>, "");
static_assert(is_index_v<idx>, "");
return 0;
}
But im getting following error message
type_traits:2009:15: error:
only enumeration types have underlying types
typedef __underlying_type(_Tp) type;
I would expect the following
std::conditional_t<
std::is_enum_v<T>,
std::underlying_type_t<T>,
T
>
to evaluate eighter to T
or to underlying type is T
is an enum
.
How would i make this work?
CodePudding user response:
Substitution fails because there is no std::underlying_type_t<unsigned int>
.
You can specialize seperately:
#include <iostream>
#include <type_traits>
#include <cinttypes>
#include <vector>
#include <utility>
template<typename T>
struct is_unsigned_integral :
std::integral_constant<
bool,
std::is_integral<T>::value &&
std::is_unsigned<T>::value
> {};
template<typename T>
inline constexpr auto is_unsigned_integral_v =
is_unsigned_integral<T>::value;
template<typename, typename = void>
struct is_index : std::false_type {};
template<typename T>
struct is_index<
T,
std::enable_if_t< is_unsigned_integral_v<T> >
> : std::true_type {};
template <typename T>
struct is_index<T,std::enable_if_t<std::is_enum_v<T> >> : is_index<std::underlying_type_t<T> > {};
template<typename T>
inline constexpr auto is_index_v = is_index<T>::value;
enum class idx : unsigned int {};
int main() {
static_assert(is_index_v<unsigned int>, "");
static_assert(is_index_v<idx>, "");
return 0;
}
PS: From cppreference/std::underlying_type
If T is a complete enumeration (enum) type, provides a member typedef type that names the underlying type of T.
Otherwise, the behavior is undefined. (until C 20)
Otherwise, if T is not an enumeration type, there is no member type. Otherwise (T is an incomplete enumeration type), the program is ill-formed. (since C 20)
I have to admit that I am not sure how that undefined beahvior (before C 20) plays with SFINAE in your example. Though, note that it is not an issue in the above, because it only uses std::underlying_type_t<T>
when T
is actually an enum type.