[cfe-dev] Why is compiler support needed for std::underlying_type?

[Richard Smith]

On Mon, Oct 21, 2013 at 6:34 AM, Stephen Kelly <steveire at gmail.com> wrote:

>
> Hi,
>
> the libcxx source has a comment that compiler support is needed for
> std::underlying_type.
>
> Is there some reason that something like the following can't work?
>
>
>  #include <type_traits>
>
>  template <int> struct int_for_size;
>  template <>    struct int_for_size<1> { typedef signed char Signed;
> typedef
> unsigned char Unsigned; };
>  template <>    struct int_for_size<2> { typedef short Signed; typedef
> unsigned short Unsigned; };
>  template <>    struct int_for_size<4> { typedef int Signed; typedef
> unsigned int Unsigned; };
>  template <>    struct int_for_size<8> { typedef long long Signed; typedef
> long long Unsigned; };
>
>  template<typename T, bool>
>  struct ApplySign
>  {
>    typedef typename int_for_size<sizeof(T)>::Signed Type;
>  };
>  template<typename T>
>  struct ApplySign<T, false>
>  {
>    typedef typename int_for_size<sizeof(T)>::Unsigned Type;
>  };
>
>  template<typename T>
>  struct UnderLyingType
>  {
>      typedef typename ApplySign<T, std::is_signed<T>::value>::Type Type;
>  };
>
>  enum Something {
>    foo
>  };
>
>  int main()
>  {
>    UnderLyingType<Something>::Type s = 9;
>    return 0;
>  }
>

enum E1 : long {};
static_assert(std::is_same<std::underlying_type<E>::type, long>::value,
""); // ok
static_assert(std::is_same<UnderLyingType<E>::Type, long>::value, ""); //
fails

enum E2 : char {};
static_assert(std::is_same<std::underlying_type<E>::type, char>::value,
""); // ok
static_assert(std::is_same<UnderLyingType<E>::Type, char>::value, ""); //
fails


> Thanks,
>
> Steve.
>
>
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>
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