I recently had the need of "reflecting" multipleenum class constructs in order to get their elements' names asstd::string objects or their element count. I came up with a C++11 variadic macro solution:

namespace ssvu{    namespace Internal    {        inline std::vector<std::string> getSplittedEnumVarArgs(const std::string& mEnumVarArgs)        {            std::vector<std::string> result;            // getSplit returns a collection of substrings split at a certain token            // Example: "a,b,c" -> {"a", "b", "c"}            // getTrimmedStrLR removes whitespace from the beginning and the end of a string            // Example: "  abc " -> "abc"            for(const auto& s : getSplit(mEnumVarArgs, ','))                 result.emplace_back(getTrimmedStrLR(std::string(std::begin(s), find(s, '='))));            return result;        }        template<typename> struct ReflectedEnumImpl;        template<template<typename> class T, typename TEnum> struct ReflectedEnumImpl<T<TEnum>>        {            using EnumType = T<TEnum>;            inline static const std::vector<std::string>& getElementsAsStrings() noexcept            {                static std::vector<std::string> result(getSplittedEnumVarArgs(EnumType::getEnumString()));                return result;            }            inline static std::size_t getElementCount() noexcept            {                return getElementsAsStrings().size();            }            inline static const std::string& getElementAsString(TEnum mElement) noexcept            {                // If the user changed the default enum values by using the `= ...'                // syntax, this function will return wrong values and possibly                // go out of bounds. Maybe this should throw an exception.                assert(!contains(EnumType::getEnumString(), '='));                return getElementsAsStrings()[std::size_t(mElement)];            }        };    }    #define SSVU_REFLECTED_ENUM_DEFINE_MANAGER(mName) template<typename> class mName    #define SSVU_REFLECTED_ENUM(mManagerName, mName, mUnderlying, ...) enum class mName : mUnderlying { __VA_ARGS__ }; \        template<> class mManagerName<mName> : public ssvu::Internal::ReflectedEnumImpl<mManagerName<mName>> \        { \            friend ssvu::Internal::ReflectedEnumImpl<mManagerName<mName>>; \            inline static const std::string& getEnumString(){ static std::string result{#__VA_ARGS__}; return result; } \        }}

Example usage:

SSVU_REFLECTED_ENUM_DEFINE_MANAGER(ReflectedEnum);SSVU_REFLECTED_ENUM(ReflectedEnum, Colors, int, Red, Yellow, Green);void tests(){    assert(int(Colors::Red) == 0);    assert(int(Colors::Yellow) == 1);    assert(int(Colors::Green) == 2);    assert(ReflectedEnum<Colors>::getElementAsString(Colors::Red) == "Red");    assert(ReflectedEnum<Colors>::getElementAsString(Colors::Yellow) == "Yellow");    assert(ReflectedEnum<Colors>::getElementAsString(Colors::Green) == "Green");}

What do you think?

Thoughts/questions:

• Consider the case where the user defines custom values for the enum elements:

  SSVU_REFLECTED_ENUM(ReflectedEnum, Test, int, A = -2, B = 15, C = 0);

  Getting element count would still be possible, as it's easy to count variadic macro elements. However, getting an element's name as a string would require using astd::map instead of an array. Should I figure out a way to detect if the enum has custom values? Or should I ditch the array for anstd::map altogheter?

  Or would an alternative syntax be better? Example:

  SSVU_REFLECTED_CUSTOM_ENUM(ReflectedEnum, Test, int, A, -2, B, 15, C, 0);

  Maybe this would be more flexible and easier to work with.

• I have macro variadic args iteration facilities in myssvu library. Do you think it's worthwhile figuring out a way to generate the enum string elements array at compile-time with macro metaprogramming? Or is the current solution good enough?

• \$\begingroup\$Abusing the preprocessor is a lot of fun\$\endgroup\$

  Vittorio Romeo

  – Vittorio Romeo

  2014-01-06 18:07:48 +00:00

  CommentedJan 6, 2014 at 18:07

1 Answer1

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