On Mon, 15 Dec 2025, Jonathan Wakely wrote:
> Replace the O(n) definitions using __is_one_of with constant-time
> checks that look for a static member in the __is_integral_helper class
> template. That class template is already specialized for every signed
> and unsigned integer type, so we don't need to define any additional
> specializations. We can just add a static data member that says whether
> the type is a signed integer type, an unsigned integer type, or neither.
> The __is_signed_integer and __is_unsigned_integer traits can then
> inspect that value.
>
> The new enum type could be extended in future to distinguish the
> character types (char, wchar_t, char8_t, char16_t, and char32_t) and
> bool from non-integer types, but that isn't needed for now.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/type_traits (_Integer_kind): New enum type.
> (__is_integral_helper::_S_kind): New static data member in
> primary template and each explicit specialization.
> (__is_signed_integer, __is_unsigned_integer): Use _S_kind
> instead of O(n) disjunction with is_same.
LGTM
> ---
>
> Tested x86_64-linux.
>
> libstdc++-v3/include/std/type_traits | 158 +++++++++++++--------------
> 1 file changed, 79 insertions(+), 79 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/type_traits
> b/libstdc++-v3/include/std/type_traits
> index 3f0bcc4e77d2..b9a7b12d7e6e 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -348,78 +348,102 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> : public true_type { };
>
> /// @cond undocumented
> +
> + // Every integral type is either one of the character types, one of the
> + // signed integer types, one of the unsigned integer types, or bool,
> + // or a cv-qualified version of one of those types ([basic.fundamental]).
> + // For now we only need to distinguish the signed/unsigned integer types.
> + enum class _Integer_kind { _Signed, _Unsigned, _None = -1 };
> +
> template<typename>
> struct __is_integral_helper
> - : public false_type { };
> + : public false_type
> + { static constexpr auto _S_kind = _Integer_kind::_None; };
>
> template<>
> struct __is_integral_helper<bool>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_None; };
>
> template<>
> struct __is_integral_helper<char>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_None; };
>
> template<>
> struct __is_integral_helper<signed char>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Signed; };
>
> template<>
> struct __is_integral_helper<unsigned char>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Unsigned; };
>
> // We want is_integral<wchar_t> to be true (and make_signed/unsigned to
> work)
> // even when libc doesn't provide working <wchar.h> and related functions,
> // so don't check _GLIBCXX_USE_WCHAR_T here.
> template<>
> struct __is_integral_helper<wchar_t>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_None; };
>
> #ifdef _GLIBCXX_USE_CHAR8_T
> template<>
> struct __is_integral_helper<char8_t>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_None; };
> #endif
>
> template<>
> struct __is_integral_helper<char16_t>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_None; };
>
> template<>
> struct __is_integral_helper<char32_t>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_None; };
>
> template<>
> struct __is_integral_helper<short>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Signed; };
>
> template<>
> struct __is_integral_helper<unsigned short>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Unsigned; };
>
> template<>
> struct __is_integral_helper<int>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Signed; };
>
> template<>
> struct __is_integral_helper<unsigned int>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Unsigned; };
>
> template<>
> struct __is_integral_helper<long>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Signed; };
>
> template<>
> struct __is_integral_helper<unsigned long>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Unsigned; };
>
> template<>
> struct __is_integral_helper<long long>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Signed; };
>
> template<>
> struct __is_integral_helper<unsigned long long>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Unsigned; };
>
> // Conditionalizing on __STRICT_ANSI__ here will break any port that
> // uses one of these types for size_t.
> @@ -427,59 +451,87 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __extension__
> template<>
> struct __is_integral_helper<__GLIBCXX_TYPE_INT_N_0>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Signed; };
>
> __extension__
> template<>
> struct __is_integral_helper<unsigned __GLIBCXX_TYPE_INT_N_0>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Unsigned; };
> #endif
> #if defined(__GLIBCXX_TYPE_INT_N_1)
> __extension__
> template<>
> struct __is_integral_helper<__GLIBCXX_TYPE_INT_N_1>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Signed; };
>
> __extension__
> template<>
> struct __is_integral_helper<unsigned __GLIBCXX_TYPE_INT_N_1>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Unsigned; };
> #endif
> #if defined(__GLIBCXX_TYPE_INT_N_2)
> __extension__
> template<>
> struct __is_integral_helper<__GLIBCXX_TYPE_INT_N_2>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Signed; };
>
> __extension__
> template<>
> struct __is_integral_helper<unsigned __GLIBCXX_TYPE_INT_N_2>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Unsigned; };
> #endif
> #if defined(__GLIBCXX_TYPE_INT_N_3)
> __extension__
> template<>
> struct __is_integral_helper<__GLIBCXX_TYPE_INT_N_3>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Signed; };
>
> __extension__
> template<>
> struct __is_integral_helper<unsigned __GLIBCXX_TYPE_INT_N_3>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Unsigned; };
> #endif
>
> #if defined __SIZEOF_INT128__ && defined __STRICT_ANSI__
> __extension__
> template<>
> struct __is_integral_helper<__int128>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Signed; };
>
> __extension__
> template<>
> struct __is_integral_helper<unsigned __int128>
> - : public true_type { };
> + : public true_type
> + { static constexpr auto _S_kind = _Integer_kind::_Unsigned; };
> #endif
>
> + // Check if a type is one of the signed integer types.
> + template<typename _Tp>
> + using __is_signed_integer
> + = __bool_constant<__is_integral_helper<_Tp>::_S_kind
> + == _Integer_kind::_Signed>;
> +
> + // Check if a type is one of the unsigned integer types.
> + template<typename _Tp>
> + using __is_unsigned_integer
> + = __bool_constant<__is_integral_helper<_Tp>::_S_kind
> + == _Integer_kind::_Unsigned>;
> +
> + // Check if a type is one of the signed or unsigned integer types.
> + // i.e. an integral type except bool, char, wchar_t, and charN_t.
> + template<typename _Tp>
> + using __is_signed_or_unsigned_integer
> + = __or_<__is_signed_integer<_Tp>, __is_unsigned_integer<_Tp>>;
> +
> /// @endcond
>
> /// is_integral
> @@ -823,58 +875,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> template<typename _Tp, typename... _Types>
> using __is_one_of = __or_<is_same<_Tp, _Types>...>;
>
> - // Check if a type is one of the signed integer types.
> - __extension__
> - template<typename _Tp>
> - using __is_signed_integer = __is_one_of<_Tp,
> - signed char, signed short, signed int, signed long,
> - signed long long
> -#if defined(__GLIBCXX_TYPE_INT_N_0)
> - , signed __GLIBCXX_TYPE_INT_N_0
> -#endif
> -#if defined(__GLIBCXX_TYPE_INT_N_1)
> - , signed __GLIBCXX_TYPE_INT_N_1
> -#endif
> -#if defined(__GLIBCXX_TYPE_INT_N_2)
> - , signed __GLIBCXX_TYPE_INT_N_2
> -#endif
> -#if defined(__GLIBCXX_TYPE_INT_N_3)
> - , signed __GLIBCXX_TYPE_INT_N_3
> -#endif
> -#if defined __STRICT_ANSI__ && defined __SIZEOF_INT128__
> - , signed __int128
> -#endif
> - >;
> -
> - // Check if a type is one of the unsigned integer types.
> - __extension__
> - template<typename _Tp>
> - using __is_unsigned_integer = __is_one_of<_Tp,
> - unsigned char, unsigned short, unsigned int, unsigned long,
> - unsigned long long
> -#if defined(__GLIBCXX_TYPE_INT_N_0)
> - , unsigned __GLIBCXX_TYPE_INT_N_0
> -#endif
> -#if defined(__GLIBCXX_TYPE_INT_N_1)
> - , unsigned __GLIBCXX_TYPE_INT_N_1
> -#endif
> -#if defined(__GLIBCXX_TYPE_INT_N_2)
> - , unsigned __GLIBCXX_TYPE_INT_N_2
> -#endif
> -#if defined(__GLIBCXX_TYPE_INT_N_3)
> - , unsigned __GLIBCXX_TYPE_INT_N_3
> -#endif
> -#if defined __STRICT_ANSI__ && defined __SIZEOF_INT128__
> - , unsigned __int128
> -#endif
> - >;
> -
> - // Check if a type is one of the signed or unsigned integer types.
> - // i.e. an integral type except bool, char, wchar_t, and charN_t.
> - template<typename _Tp>
> - using __is_signed_or_unsigned_integer
> - = __or_<__is_signed_integer<_Tp>, __is_unsigned_integer<_Tp>>;
> -
> // __void_t (std::void_t for C++11)
> template<typename...> using __void_t = void;
> /// @endcond
> --
> 2.52.0
>
>
