Add to_underlying_type to cast enum to underlying

[JoeLoser]

Summary:

• It is verbose at the call sites to cast an enum or enum class to its
  underlying type.
• Introduce to_underlying_type, which, given an enum or enum class,
  returns the value from static casting to the underlying type.

[yfeldblum]

Looks handy!

[JoeLoser]

Thanks for the review, @yfeldblum. I've fixed up your comments.

What do you think about extending to_underlying_type return the value if a user passes in an integral type? This would make https://github.com/facebook/folly/blob/master/folly/DynamicConverter.h#L213 also eligible for to_underlying_type. Something like

template <class E>
constexpr std::underlying_type_t<E> to_underlying_type(E e) noexcept {
   static_assert(std::is_enum<E>::value || std::is_integral<E>::value, "not an enum or integral type");
   if (std::is_enum<E>::value)
   {
       return static_cast<std::underlying_type_t<E>>(e);
   }
   else if (std::is_integral<E>::value)
   {
        return e;
   }
 }

This would let callers use this utility in more generic programming contexts where you know you are working a regular value type, but not sure whether it is wrapped in an enum or enum class. It eliminates the need to do SFINAE machinery (such as checking if the type is an enum) to not force a hard error in a caller using to_underlying_type.

[yfeldblum]

Maybe, but that sounds like a completely different function.

E.g. to_signed just wraps static_cast<make_signed_t<...>> (effectively).

Then you run into questions like, why not work on arithmetic but non-integral types, etc.?

std::underlying_type is only defined over complete enumeration type until C++20, and is only defined over complete types and only has a member type alias type for complete enumeration types since C++20. I'd just go with to_underlying_type being a wrapper for static_cast<underlying_type_t<...>> with param type deduction.

[facebook-github-bot]

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